2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
Annu. Rev. Microbiol. 2001. 55:743–72
c 2001 by Annual Reviews. All rights reserved
Copyright �
ASPECTS OF FUNGAL PATHOGENESIS IN HUMANS
Jo-Anne H. van Burik1 and Paul T. Magee2
1
Department of Medicine, Division of Infectious Diseases, School of Medicine,
University of Minnesota, Minneapolis, Minnesota 55455; e-mail: [email protected]
2
Department of Genetics, Cell Biology, and Development, College of Biological Sciences,
University of Minnesota, St. Paul, Minnesota 55108; e-mail: [email protected]
Key Words virulence factors, primary fungal pathogens, opportunistic fungal
pathogens, mucosal fungal infections, systemic fungal infections
■ Abstract Fungal diseases have become increasingly important in the past few
years. Because few fungi are professional pathogens, fungal pathogenic mechanisms
tend to be highly complex, arising in large part from adaptations of preexisting characteristics of the organisms’ nonparasitic lifestyles. In the past few years, genetic
approaches have elucidated many fungal virulence factors, and increasing knowledge
of host reactions has also clarified much about fungal diseases. The literature on fungal
pathogenesis has grown correspondingly; this review, therefore, will not attempt to
provide comprehensive coverage of fungal disease but focuses on properties of the
infecting fungus and interactions with the host. These topics have been chosen to make
the review most useful to two kinds of readers: fungal geneticists and molecular biologists who are interested in learning about the biological problems posed by infectious
diseases, and physicians who want to know the kinds of basic approaches available to
study fungal virulence.
CONTENTS
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
THE INFECTING AGENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Primary Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opportunistic Pathogens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
POTENTIAL VIRULENCE FACTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Growth at Elevated Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adherence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Penetration and Dissemination Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Nutritional and Metabolic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Necrotic Factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Adaptations of Specific Organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
THE HOST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Immunocompetence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Infection Routes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0066-4227/01/1001-0743$14.00
744
744
745
745
745
746
746
748
748
749
751
754
756
757
758
743
2 Aug 2001
11:11
744
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
Immune and Other Host Defenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CLASSES OF INFECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mucosal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Systemic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
759
761
761
762
763
INTRODUCTION
Virulence is a complex interrelationship between the infecting organism and the
host. Pathogenesis involves interaction (and sometimes modification) of factors on
both sides. This is particularly true of fungal pathogenesis. There is no single factor
that causes or permits these organisms to be agents of diseases that range from
superficial through invasive human infection (32, 64). This review focuses on the
pathogenesis of systemic fungal infections in humans. However, we pay attention
where appropriate to localized infections caused by fungi that are important systematic pathogens. An example is thrush in AIDS patients, a limited manifestation
of infection by the major fungal yeast pathogen of humans, Candida albicans. We
divide our discussion into two parts, the first concentrating on properties of the
infecting fungus, and the second on the interactions with the host.
THE INFECTING AGENT
Fungal pathogens can be divided into two general classes, primary pathogens and
opportunistic pathogens. Fungi in the former class usually have an environmental
reservoir and infect individuals who have either been exposed to a large dose
or who are immunologically naive to the fungus. Opportunistic pathogens take
advantage of debilitated or immunocompromised hosts to cause infection. They
may have an environmental reservoir (e.g., Cryptococcus neoformans, Aspergillus
fumigatus) or exist as commensals in healthy organisms (e.g., Candida species).
The mechanisms of fungal pathogenesis are much less-well understood than
are those of bacterial pathogens. In contrast to bacteria, few fungi are professional
pathogens. For the most part, they exist either as saprophytes in nonanimal settings
or as commensals, coexisting with the host without any negative consequences.
Several are capable of infecting healthy hosts and causing severe systemic disease.
Model examples of such primary pathogens include Coccidioides immitis and
Histoplasma capsulatum, whose characteristic diseases have been known for many
years. Opportunistic human fungal pathogens have become increasingly important
over the past 20 years, paradoxically because the success of modern medical
practice has led to the survival of debilitated and immunosuppressed patients.
Such patients are highly susceptible to infections by opportunistic pathogens such
as Candida species, C. neoformans, A. fumigatus and other Aspergillus species,
and the zygomycetes.
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
745
The fact that the condition of the patient is a major factor in a great many fungal infections makes evident the fact that fungal disease often involves a complex
interplay between host and pathogen. Indeed, it seems clear that fungal diseases,
like all infectious disease, are dynamic processes. With at least two of the opportunistic pathogens, C. albicans and C. neoformans, there is evidence of change
among the population of the invading microorganisms, with a subset of individual organisms being selected during the process of infection and invasion. Thus,
in this case at least, the genetic composition of the pathogen population may be
different at the end of the infection compared with the beginning. Whether there
are corresponding pathogen-specific changes (which would have to be phenotypic
rather than genotypic) in the host is unclear, although the standard immunologicalresponse pathways are of course induced by infection and play important roles in
both resistance and pathogenesis.
Primary Pathogens
We have chosen to consider primary pathogens as those fungi that cause disease
in noncompromised patients. This distinction is necessarily a gray one, because
C. neoformans, a model opportunist, sometimes causes disease in healthy individuals, and the primary pathogens, such as C. immitis, are much more virulent
in immunocompromised patients. Furthermore, infection by a primary pathogen
often leads to subclinical disease. However, the distinction is worth making in the
effort to understand general mechanisms of pathogenesis.
Opportunistic Pathogens
Opportunistic pathogens incite disease in hosts whose local or systemic immune
attributes have been impaired, damaged, or are innately dysfunctional. The pathogenesis of opportunistic infections involves production of virulence factors that
allow individual organisms to be commensals during times when humans have
normal immune systems. Then, as the immune system wanes, the organism takes
its opportunity of being in the right place at the right time to continue growth. This
unregulated growth then leads to invasive infection.
POTENTIAL VIRULENCE FACTORS
Because of the complex nature of the host-fungus interaction, there are few factors that are absolutely required for fungal virulence. However, some properties
are frequently associated with pathogenesis across the fungal kingdom, and some
have been found to be important for specific pathogens. We discuss examples
of both kinds. We favor evidence based upon differential virulence in isogenic
strains (strains that differ at one genetic locus) differing in the property, rather
than correlatory studies. Construction of isogenic strains has become possible in
most of the pathogenic fungi in the past several years. In addition, it is important
2 Aug 2001
11:11
746
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
to remember that because pathogenesis is complex, possession of a single putative virulence factor is not likely to render a fungus pathogenic; a complex mix
of properties is usually required. The most convincing evidence for the importance of a potential virulence factor is to show simultaneous loss of the factor
and loss of virulence, together with the regain of virulence when the property is
restored.
Growth at Elevated Temperatures
The ability to grow at body temperature, 37◦ C, and within the fever range, 38◦ –
42◦ C, of the human host is clearly an important requirement for systemic infection. This property is not a simple genetic one; experiments with Saccharomyces
cerevisiae variants able to grow at physiological temperatures suggest that it is a
multigenic trait (96). S. cerevisiae is normally not pathogenic, but McCusker and
coworkers have found that strains, which are able to grow at 42◦ C and form pseudohyphae, are able to infect and persist in mice. This property is highly correlated
with the high-temperature growth phenotype.
In C. neoformans, the Ca2+-dependent protein phosphatase calcineurin is required for growth at 37◦ C, as shown by the sensitivity of the organism to cyclosporin and FK506 at 37◦ C and not at 24◦ C and by the failure of a strain with
a disruption in the catalytic subunit of calcineurin to grow at 37◦ C. Laboratory
strains in which the calcineurin gene has been disrupted are avirulent in an animal
model of cryptococcal meningitis (110, 111).
Adherence
For most fungal infections, the ability of the host to resist the physical clearing
of the infectious agent is important. For example, the lungs have effective means
of clearing foreign particles, but C. immitis, Aspergillus species, H. capsulatum,
and C. neoformans all infect via the bronchial route and must avoid clearance.
C. albicans also must adhere to various host surfaces both as a commensal to
avoid being washed out of its various niches and as a pathogen during the onset of
hematogenous infections.
A large number of studies have demonstrated the importance of adherence
in various pathogens. A 120-kDa cell wall adhesin, WI-1, which contains 34
copies of a 25–amino acid tandem repeat, has been isolated from the surface of
all Blastomyces dermatitidis examined. This adhesin mediates attachment to human monocyte-derived macrophages mainly through binding complement type
3 receptors. This protein is both released into the growth medium and found on
the cell wall; free WI-1 seems to be recaptured by the fungus and binds with the
cell wall via covalent and noncovalent interactions (16). The disruption of the
gene prevents binding to and infection of macrophages, diminishes adhesion, and
attenuates the virulence of the fungus (17, 76). Hence, in blastomycosis a single molecule affects adhesion both at the site of entry and during later stages of
infection.
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
747
Adhesion in C. albicans has been the focus of much investigation, and several
gene products have been implicated. Three of the most intensely studied adhesion
mechanisms involve the HWP1 gene product, the ALS gene family, and the INT1
gene product.
The HWP1 gene was identified by Staab et al as a partial cDNA fragment from
a library of fungal-specific transcripts containing tandem repeats of glutamine and
proline residues (141). The complete gene was cloned and then shown to mediate
binding to buccal epithelial cells by a novel mechanism involving transglutamination (140, 142, 149). The virulence of a strain with both alleles disrupted was
greatly attenuated (155).
The ALS gene family has at least four genes. The ALS1 (agglutinin-like
sequence) gene of C. albicans encodes a protein similar to alpha-agglutinin, a
cell-surface adhesion glycoprotein involved in mating of S. cerevisiae, except that
ALS1 has a central domain consisting of a tandemly repeated 36-amino acid sequence. Genomic Southern blots from several C. albicans isolates indicate that
the number of copies of the tandem repeat element in ALS1 differs across strains
and, in some cases, between ALS1 alleles in the same strain, suggesting a straindependent variability in ALS1 protein size (69). The ALA1 gene was identified
in a screen of C. albicans genes able to confer adhesion on S. cerevisiae; ALA1
is also a member of the ALS gene family, because it has homology to the C. albicans ALS1 protein and the S. cerevisiae alpha-agglutinin protein (53). When
transformed into a nonadherent strain of S. cerevisiae, the agglutinin-like cell
surface proteins induced a flocculation phenotype (48, 50). The ALS2 and ALS4
genes in the growing ALS family of C. albicans have been isolated by polymerase chain reaction (PCR) screening of fosmids. ALS4 expression was correlated
with the growth phase of the culture; ALS2 expression was not observed under
many different in vitro growth conditions (68). Northern blot analysis demonstrates that ALS1 expression varies, depending on which C. albicans strain is
examined, and that ALS3 is hyphal specific; both are cell surface glycoproteins
(67).
The existence of integrin-like proteins in C. albicans has been postulated because monoclonal antibodies to the human leukocyte integrins alpha M and alpha
X bind to blastospores and germ tubes, recognize a candidal surface protein of
approximately 185 kDa, and inhibit candidal adhesion to human epithelium. The
gene alpha INT1 was isolated from a library of C. albicans genomic DNA by
screening with a cDNA probe from the transmembrane domain of human alpha
M (52). Disruption of INT1 in C. albicans suppresses hyphal growth, adhesion
to epithelial cells, and virulence in mice. Thus, INT1 links adhesion, filamentous
growth, and pathogenicity in C. albicans, and Int1p may be an attractive target for
the development of antifungal therapy (51).
A genomic screen, using insertion mutagenesis, of Candida glabrata yielded
a mutant that was unable to adhere to cultured laryngeal cells. The mutant was
not affected in virulence, suggesting that there are multiple adhesins involved in
pathogenesis in this yeast (28a).
2 Aug 2001
11:11
748
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
Penetration and Dissemination Factors
The first step in fungal infection is introduction of the agent to the host. Infections
may be limited to portal of entry or they may become systemic, disseminating either
via hematogenous or contiguous routes. Movement from the infecting surface into
the bloodstream requires tissue damage. This damage can be preexisting or can
occur either by mechanical penetration or new tissue necrosis. Hence, the ability of
fungi to penetrate host cells is crucial for progression of infection in the setting of
intact skin or gut barriers. For Candida, it is the ability of hyphae to grow through
host cell walls that is proposed to account for the importance of polymorphism in
virulence. A. fumigatus and other true molds are able to penetrate blood vessels
and grow along the vessel lumen as they invade tissue (12) and can use the same or
different virulence factors in that process to cross layers of tissue, unlike bacteria
whose infections conform to tissue planes.
Hyphae respond thigmotropically (movement toward or away from a touch
stimulus) and morphologically to cues such as the presence of a surface, pores,
grooves, and ridges. Growth on some firm surfaces elicits a helical growth response. Hyphae follow grooves and ridges of inert substrates and penetrate pores
of filtration membranes. Thus, thigmotropism may enhance the ability of a hypha
to invade epithelia of a host at sites of weakened integrity or to follow vasculature
(60).
Fungi may also spread from the site of infection throughout the host by such
mechanisms as host phagocytosis. C. albicans invades endothelial cells through
being phagocytosed (46). H. capsulatum is phagocytosed by macrophages but does
not seem to be killed and multiplies within the phagosome (42), a characteristic
shared by the closely related fungus Blastomyces dermatitidis.
Nutritional and Metabolic Factors
In order to flourish in the host, fungi need to be able to carry out biosynthetic
reactions while concentrating relatively scarce nutrients like Ca2+ and Fe2+. Experiments with auxotrophic mutants of C. albicans (auxotrophs require a nutrient
that the parent organism, the prototroph, does not require) have shown that the inability to synthesize purines, pyrimidines, or heme de novo significantly diminishes
virulence (75). In fact, auxotrophy for almost any amino acid affects C. albicans
virulence (B.B. Magee & J. Becker, unpublished data). An uracil auxotroph of
Histoplasma capsulatum is virulent in both a cultured macrophage and a mouse
pulmonary colonization model (131a). Strains of C. neoformans unable to synthesize adenine are avirulent and will not produce meningitidis in the rabbit model
(115).
The ability to synthesize fatty acids has also been shown to be essential for
C. albicans both in a systemic mouse model and in a rat model of oropharyngeal
disease. Deletion of the FAS2 gene led to an avirulent Candida strain that was
an auxotroph for several fatty acids. The heterozygote was also diminished in its
virulence (171, 172).
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
749
Glycolytic enzymes constitute a group of C. albicans proteins that are immunogenic during oral and esophageal infections (151). Changes in glycolytic
gene expression accompany the dimorphic transition in C. albicans and reflect
the underlying physiological status of the cells during morphogenesis (150). Also,
there is a broad spectrum of variability in the expression of genes controlling the
utilization of alternative carbon and nitrogen sources (125).
Urease production has been shown to be a virulence factor in C. neoformans
in mouse intravenous and inhalation models but not in rabbit meningitis models
(30a). The authors suggest that this enzyme may not be needed for maintenance
in the cerebrospinal fluid but may be essential for establishment or maintenance
of a disseminated model of infection.
Necrotic Factors
Necrotic factors are vehicles of virulence because they allow the fungus to overcome structural barriers that the human host uses to prevent invasive infection.
Most necrotic factors are enzymes. Because the majority of fungal pathogens are
opportunists, these enzymes may have evolved for saprophytic purposes and might
be considered nutritional factors, but it seems more likely that their major role in
infection is degradation of host tissue. Cellular and tissue damage at the site of the
organism are characteristic of many fungal infections. Among the factors that are
thought to contribute to this damage are extracellular degradative enzymes, such
as proteinases, phosphatases, and DNAses. The evidence adduced for the role of
these enzymes is usually diminished virulence in a mutant lacking the activity.
Although such findings are persuasive, it is important to bear in mind that a failure to demonstrate involvement of a particular activity is not conclusive because
many of these genes are members of families, and several of the products may be
involved in pathogenesis.
The earliest identification of a potential necrotic factor was the extracellular
proteinase of C. albicans, first identified by Staib (143). Further work, largely by
Ruchel and coworkers, showed that this enzyme was usually found at infection
sites (122, 123). Because this was the first identifiable potential virulence factor,
it has been the subject of much investigation and has been a dominant influence
on thinking about fungal pathogenesis. Several different names were originally
given to the gene for this enzyme, such as Opa1 (104) and EPR (65), but all
correspond to members of the secreted aspartyl proteinase (SAP) gene family.
Early experiments from two laboratories provided evidence for the essential role
of this activity in infection. McDonald & Odds showed that a variant of C. albicans lacking the activity was avirulent in a mouse model (92), and Kwon-Chung
et al, using a different variant, repeated the results and found that revertants had
regained their virulence (81). Recent studies by Hube and coworkers have shown
that there are at least seven and probably nine different genes in the SAP gene
family; therefore the earlier experiments are hard to interpret, because neither
MacDonald nor Kwon-Chung could have had a true structural gene mutant. It
2 Aug 2001
11:11
750
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
is possible that both Sap variants were blocked in secretion, not synthesis, of an
active proteinase.
Experiments designed to study the role of SAP in virulence illustrate the problems associated with analysis of gene families. Of the SAP family members that
have been studied, two (SAP1 and SAP3) are regulated during phenotypic switching between the white and opaque forms of WO-1. SAP2 is dominant in the yeast
form. The SAP4, SAP5, and SAP6 genes are observed only at neutral pH during
the serum-induced yeast-to-hyphal transition (70). The group of Sap4p, Sap5p,
and Sap6p isoenzymes plays an important role in the process of induction of SAP2
and thus in the normal progression of systemic infection by C. albicans (126).
Hence, the various members of the SAP gene family may have distinct roles in the
colonization and invasion of the host. SAP multigene families also exist in other
Candida species, such as Candida tropicalis, Candida parapsilosis, and Candida
guillermondii (101).
A mutant lacking all nine SAP genes has not been isolated, but the role of several genes has been deduced using individual gene mutants. Six of the SAP genes
have been disrupted. Single disruptions of SAP1, SAP2, or SAP3 each caused diminished virulence (71), whereas a triple disruption of SAP4-6, which seem to
be coexpressed, also diminished but did not ablate virulence (126). These experiments cannot be considered wholly conclusive, because in neither case did the
investigators test the virulence of the disruptant in which the missing gene(s) was
replaced.
A proteinase associated with C. immitis may function in the compartmentalization of the spherules into endospores, and the endospores have proteinase activity
associated with them. This activity may function to break down lung tissue, the
primary site of initial infection, and allow dissemination of the fungus. Cole et al
and Yu et al have isolated the genes for a proteinase and a urease, both of which
may be important in pathogenesis (29, 168).
A. fumigatus secretes at least two proteinases, both of which are active on
elastin, a protein that constitutes about 30% of lung tissue. One of these elastases
is a serine proteinase; the other is a metalloproteinase. Both have been cloned;
the serine proteinase was disrupted independently in two laboratories, yielding
a mutant with no detectable extracellular elastase activity but comparable to the
wild type in virulence in mice (100, 135, 152). A third laboratory, using chemical
mutagenesis, reported that loss of the activity led to diminished virulence in an
immunosuppressed mouse model (77). Whether the difference in these results is
due to the method of mutant construction or the model of infection has not been
resolved.
C. albicans is known to secrete phospholipases and to possess a phospholipase
gene family. Extracellular phospholipases have a role in the pathogenicity of C.
albicans, as blood isolates produce significantly more extracellular phospholipase
activity than do commensal strains (72). Two different phospholipase B molecules
are found in culture supernatants, and the gene for each has been cloned. Disruptants of one gene (PLB1) are less virulent than the parent strain (88). It is obvious
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
751
that such enzymes ought to be important in breaching the membranes of host cells
during infection. Based on Southern blots, homologues of the PLB1 gene may also
be present in Candida parapsilosis, Candida tropicalis, and Candida krusei, but
not in Candida pseudotropicalis, Candida glabrata, or S. cerevisiae (49). However,
S. cerevisiae has several enzymes with the same activity; therefore presence of the
activity alone does not predispose to virulence (85, 165). C. neoformans, A. fumigatus, and Aspergillus flavus secrete phospholipases. Several of the corresponding
genes have been cloned (57).
Morphology
Almost all pathogenic fungi can grow in more
than one form. Aspergillus species, which are classical filamentous molds, form
conidia that are the infectious agent. The major exception is C. neoformans, which
apparently exists only in the yeast form in vivo. In vitro it also grows mostly as
a yeast; however, it does form filaments during the mating process, and the small
basidiospores have been proposed to be the agents of infection. Furthermore,
there is a strong correlation between MATα (mating-type alpha) and virulence in
Cryptococcus, and MATα cells, in contrast to MATa cells, can undergo haploid
fruiting, which involves formation of filaments and basidia, in the absence of a
sexual partner (24, 170).
MORPHOLOGICAL VERSATILITY
SPECIFIC PARASITIC CELL FORMS H. capsulatum, Blastomyces dermatitidis, and
several (but not all) species of Candida can grow both as yeasts and as hyphae.
In C. albicans, both the yeast and hyphal forms are found at the site of infection,
whereas in Histoplasma and Blastomyces, the yeast form seems to be the major,
if not the exclusive, parasitic form. The transition to the parasitic form in H.
capsulatum leads to a specific pattern of gene expression; this pattern facilitates
many of the steps important in infection, including blocking acidification of the
phagolysosome and synthesis of the calcium-binding protein, Cbp1p. In contrast
to C. albicans, the H. capsulatum hypha seems to play no role in vivo.
The classical example of a primary pathogen with a specialized parasitic form is
C. immitis, a soil fungus found in the dry southwest of the United States, especially
New Mexico, Arizona, Texas, parts of Utah and Nevada, and the central San Joaquin
Valley of California. The saprophytic phase grows in the soil as a septated mycelium
in which alternate cells in a hypha undergo cytolysis while their neighbors grow
heavy walls. The intact cells are called arthroconidia; they break away from the
lysed cells and are disseminated by wind. Arthroconidia are inhaled by mammalian
hosts and swell in the lungs to form large multinucleate cells called spherules. The
spherules are large, 20–100 µm, and segment internally. The nuclei are separated
into individual compartments and walled off, forming endospores. Eventually the
spherule ruptures, releasing hundreds of endospores, each capable of becoming
a new spherule. C. immitis is an organism of extreme virulence in part because
of the sheer numbers of organisms that can be produced in an infection. Because
2 Aug 2001
11:11
752
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
there is no genetic system available for this fungus, relatively little is known about
the factors required for pathogenesis. Because spherule formation seems to be
necessary for pathogenesis, any necessary gene whose product is required for
that part of the life cycle should be considered a virulence factor. In fact, a β1,3-glucanase, whose gene has been cloned, may be important for the transition
from spherule to endospores (78). However, in the absence of a way to create
strains lacking these proteins in order to test for virulence, the importance of these
activities remains unproven.
MULTIPLE PARASITIC CELL FORMS Dimorphic fungi regulate their cellular morphology in response to environmental conditions. For example, ellipsoidal single
cells of C. albicans (blastospores) predominate in rich media, whereas filaments
composed of elongated cells attached end-to-end form in response to starvation,
serum, and other conditions. A variety of environmental changes, including a shift
from an aerobic to a fermentative metabolism or growth on particular compounds
such as N-acetyl glucosamine, cause C. albicans to switch from yeast to filamentous growth. This change is accompanied by changes in carbohydrate metabolism
and an interruption of electron transfer within the cell (82). Both temperature (a
shift to 37◦ C) and pH can regulate C. albicans dimorphism (19). The complex
regulation of these pathways is reviewed in Brown & Gow (18).
The formation of filaments in C. albicans has been intensively studied in the
past few years. There are several pathways that control the cell morphology of this
fungus. Two of these are signal transduction pathways culminating in transcription factors with orthologues (genes of similar sequence with similar functions)
in S. cerevisiae. The gene CPH1 in C. albicans corresponds to STE12 in S. cerevisiae, and EFG1 is similar to PHD1. The pathway culminating in CPH1 is so
similar to that of S. cerevisiae that several of the C. albicans genes complement
the corresponding Saccharomyces mutants. Lo et al have presented evidence for
the importance of these last two filamentation pathways by showing that mutants
of C. albicans that were unable to form hyphae because they were deleted for both
the transcription factors were avirulent in a mouse model (89). However, such
mutants can form filaments in a gnotobiotic (germ-free) pig infection model and
under certain in vitro conditions (118). Deletions in INT1 (51) and CEK1 (31),
both of which block filamentation under some conditions, diminish virulence. In
summary, the evidence suggests that all the morphogenetic pathways in Candida
play a role in virulence; however, it seems likely that no one pathway is indispensable. Recent virulence studies of filamentation regulatory mutants argue that both
yeast and filamentous forms have roles in infection (99).
Several antifungal agents have morphological effects on C. albicans. Amphotericin B prevents filamentous growth (84). In the presence of the azole antifungals,
cells appear as aggregates or chains of swollen structures, many of them with a
bud-like formation (59). Pneumocandins can cause profound changes in hyphal
growth; light micrographs show abnormally swollen germ tubes, highly branched
hyphal tips, and many cells with distended balloon shapes. Electron micrographs
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
753
of Aspergillus demonstrate that lipopeptides produce changes in cell walls; drugtreated germlings show stubby growth with thick walls and a conspicuous dark
outer layer that is much thicker in the subapical regions. The rest of the hyphal
tip ultrastructure is unaffected, indicating considerable specificity of the drug for
the primary target. The drug-induced growth alteration produced compact clumps
in broth dilution wells, making it possible to score morphological effects macroscopically (79).
Many molds form conidia, or vegetative spores; scattered by wind or water, these
small, resistant cells serve as a mode of dissemination. In the case of aspergillosis,
conidia serve as the propagule that infects debilitated patients. Hydrophobicity is
thought to contribute to the efficacy of Aspergillus conidia, already an ideal size
for deposition into alveoli, to disperse in air. What makes the outermost cell wall
layer hydrophobic are interwoven fasicles of clustered proteinaceous microfibrils
called rodlets. A rodletless mutant of A. fumigatus is hydrophilic and has lost the
ability to disperse conidia, but has not lost morbidity in a murine model of invasive pulmonary aspergillosis (153). Although all humans are constantly exposed
to Aspergillus spores, the immune system is usually effective at preventing colonization and subsequent infection. Hence, the form of aspergillosis that otherwise
healthy people develop is related to allergy. Conidia bind to fibrinogen and laminin,
but the binding properties are lost as the conidia germinate and produce hyphae
(2, 30, 83, 90). Hence, this cell form may play a role more complicated than that
of a simple vector in establishing the infection.
PHENOTYPIC SWITCHING The capacity of fungi to undergo an epigenetic change
(regulation of expression of gene activity without alteration of genetic structure) in
colony morphology has come to be called phenotypic switching. This phenomenon
was often observed in C. albicans, but its significance was not understood until
Soll’s lab rediscovered it and set out to analyze its role in pathogenesis. Phenotypic
switching is characterized by a reversible change, usually occurring between 10−3
and 10−5 per cell division, in some property or properties of the cell. Slutsky
et al found that colonies arising from cells plated on Lee’s medium showed a
standard (smooth) phenotype in 99.99% of the cases, but the remaining 0.01% had a
wrinkled surface. Careful observation showed that there were a number of potential
colony phenotypes and that the cells could pass from any one of these phenotypes
to any other (133). A second paper described a particular strain, WO-1, that has
two major colony phenotypes, “white” and “opaque” (134). White colonies almost
exclusively contain the classical yeast cells. Opaque colonies contain cells that are
bean shaped and differ from white cells in a variety of ways, including surface
properties, gene expression, and temperature sensitivity (1, 58, 73). Opaque cells
switch to white cells within one generation at 37◦ C (9, 119). The white phenotype
is more virulent in a systemic mouse model of infection, whereas opaque cells
are more effective in colonization in a mouse cutaneous infection model (80). The
white-opaque transition is relatively rare among C. albicans isolates; therefore its
overall significance in disease is questionable (103).
2 Aug 2001
11:11
754
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
Colony phenotype switching is much more common. The various colony forms
probably result from different distributions of yeast, pseudohyphal, and hyphal
cells in parts of the colony. Cells of different colony phenotypes show different adhesion properties and levels of hypha formation when tested in vitro (156). Strains
isolated from patients often show high levels of switching, but the significance
of the phenomenon in disease is unclear (137). C. tropicalis is capable of highfrequency switching of colony morphology just as C. albicans is, and there is more
than one strain-specific switching repertoire in C. tropicalis (138).
Colony phenotype and genetic similarity, when assessed by comparing groups
of commensal and pathogenic strains of C. albicans collected from the oral cavities of individuals, demonstrate minor variant colony morphologies (other than
smooth) among 38% of pathogenic and 16% of commensal isolates. Commensal and pathogenic groups in the same geographical locale have common clonal
origins (63).
C. neoformans can also undergo phenotypic switching. A smooth (S) avirulent
variant gave rise to wrinkled (WR) and pseudohyphal (PH) phenotypes; they varied
in cell shape. Virulence varied in the order WR > PH > S (47).
H. capsulatum is an intracellular parasite that can survive in the phagolysosome.
The organism is capable of generating variants with diminished β-glucan in the
cell wall. Such variants survive in the phagolysosome, but they do not kill the cells
like their virulent parents. They are thought to contribute to latency and, much
later, to be capable of reactivation (44).
Adaptations of Specific Organisms
GROWTH AT DIFFERENT pHs
C. albicans can grow both at acid and at basic pHs,
a reflection of its ability to colonize several niches, ranging from the acid vagina to
the neutral oropharyngeal tract. This ability to tolerate a wide pH range is important
in several models of virulence. C. albicans has two pH-regulated genes, PHR2 and
PHR1, the former expressed at acid pH and the latter at neutral and basic pH
(106, 127). Deletion of one or the other leads to an inability to grow at the pH at
which the gene is expressed. DeBernardis and coworkers showed that virulence
of the disruptants was related to the niche: The PHR2 deletion was avirulent in
the vagina, whereas the PHR1 deletion showed reduced virulence in a systemic
model (34). Davis et al showed that deletions of the RIM101 gene, a pH-regulated
transcription factor, also reduced virulence in a systemic mouse model and in
an endothelial-cell–damage model, and an activated allele of the RIM101 gene
restored virulence (33).
TOXIN PRODUCTION A. fumigatus produces gliotoxin, but it is not known whether
clinically significant amounts of gliotoxin are produced in human disease. Gliotoxin
inhibits macrophage phagocytosis via DNA fragmentation and apoptosis, targets the neutrophil respiratory burst, and inhibits T-cell activation and proliferation (166, 167). Filamentous fungi also produce several ribotoxins, alpha-sarcin,
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
755
restrictocin, and mitogillin, but the biological function of these potent toxins is
unknown (94).
Melanins are scavengers of reactive oxygen intermediaries, making
organisms relatively resistant to leukocyte attack (20). Melanin synthesis is catalyzed by a membrane-bound phenoloxidase with a substrate specificity for phenolic compounds that contain hydroxyl or amino groups, such as L-DOPA and
dopamine (117). Melanin is deposited in the cell wall of C. neoformans (40, 108).
The brown color of this pigment can be revealed by growth on birdseed agar or by
the Masson-Fontana stain (74). Although much attention has been focused on the
role of melanin in cryptococcosis, this substance is constitutively produced in the
dematiaceous fungi, such as Cladosporium and Wangiella dermatitidis. The role
of melanin in W. dermatitidis has been extensively examined. Melanin seems to
be important in initiating an infection, because melanin-deficient strains are much
less infective on a colony-forming–unit basis but can cause the same neurological
symptoms once infection is established (37–39).
MELANIN
IRON AND CALCIUM Iron is an essential element for the growth and metabolism of
fungi. C. neoformans capsular polysaccharide synthesis is increased by limitation
of ferric iron (157). Most pathogenic microbes elaborate siderophores (molecules
that can bind iron) to mobilize iron from ferric ligands. One such siderophore,
deferoxamine, which is used as an iron chelator in medical practice, has a known
association with the development of zygomycete infections because zygomycetes
use the iron in deferoxamine efficiently, unlike C. albicans or A. fumigatus (13, 14).
Hereditary or acquired hemochromatosis, the human disease with progressive
iron overload leading to fibrosis and organ failure, poses an increased risk for the
development of fungal infection even in the absence of chelator usage. Multiple
transfusions have led to severe acquired hemochromatosis followed by the development of zygomycosis (54, 97). Over the past decade Stachybotrys, a fungus
almost never isolated from human pathologic material, was associated with infants
having acute pulmonary hemorrhage and hemosiderosis in households with major
water damage (158, 159).
The phagolysosome, in which H. capsulatum survives, is a calcium-poor environment. The yeast but not the hyphal phase of this fungus secretes a calciumbinding protein (CBP). This protein aids in the uptake of Ca2+ (7). Disruption of
CBP leads to avirulence both in vitro in cultured macrophages and in vivo in a
mouse pulmonary colonization model (131a).
SURFACE PROPERTIES The best known of the fungal immunoevasion systems is
the capsule of C. neoformans. A viscous polysaccharide capsule composed of
glucuronoxyomannan and other components, the capsule is believed to present a
surface not recognized by phagocytes, downregulate cytokine secretion, inhibit
leukocyte accumulation, induce suppressive T-cells, inhibit antigen presentation,
and inhibit lymphoproliferation. Hence, it serves as a barrier to host defenses in a
2 Aug 2001
11:11
756
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
variety of ways (93, 116). Its importance as a virulence factor is demonstrated by
a comprehensive series of analyses of acapsular mutants (21–24).
The opportunistic fungus Pneumocystis carinii, an obligate parasite most closely
related to Schizosaccharomyces pombe, can vary its surface glycoproteins in a manner closely analogous to Trypanosoma brucei. A large number of nonexpressed
structural genes for the surface glycoproteins exist; any of these can move to an
expression site by recombination. Because the genes are highly diverse, replacement in the expression site changes the antigenicity of the surface, providing a
defense against host antibodies (160–163).
MATING TYPE Virulence in C. neoformans is associated with the MATα genotype.
Most clinical isolates are serotype A, and this serotype seems to exist exclusively
in the MATα configuration. In pathogenic isolates of the other three serotypes, B
through D, MATα seems to predominate as well. In contrast to S. cerevisiae, the
MAT locus in C. neoformans contains the gene for the transcription factor STE12.
Its orthologue in S. cerevisiae is required for the hormonal response leading to
mating as well as for pseudohyphal growth (124). Recently two conflicting reports
about the mechanism of the MAT effect have appeared. Yue et al have found that a
disruptant of STE12alpha in serotype A is able to mate but not to undergo haploid
fruiting and that the disruptant is normally virulent (170). In contrast, Chang and
coworkers have shown that this transcription factor controls the expression of
several virulence factors, including melanin and capsule formation in serotype D
cells (24). C. neoformans serotype D cells deleted for STE12alpha are therefore
avirulent, and replacement of the gene restores virulence. Such cells are still able to
mate. The apparent contradictions in these two reports may be due to the difference
in serotypes; one of the disruptions may have left some function intact, or one of
the disruptions may have removed an adjacent gene. The results using serotype
D suggest that some parts of the sexual apparatus may be adapted in pathogenic
fungi for purposes related to virulence, an interesting idea because many of these
fungi seem either to lack a sexual cycle or to mate rarely (24).
THE HOST
Commensal fungi are virtually ubiquitous among humans. A yeast microflora is
acquired at or soon after birth and persists in many patients despite occasional or
routine use of antifungal agents. However, the composition of the flora can change.
Patients with AIDS have replacement of original commensal strains of Candida
species early in the manifestation of the disease (129). Strain replacement occurs
from women with vulvovaginal C. albicans infections to their male partners or
vice versa (130, 131), and infecting strains do not represent a group genetically
distinguishable from vaginal commensal isolates (130). The infecting strain can
exhibit minor genetic changes in each successive episode of Candida vaginitis;
therefore they are not genetically stable (131).
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
757
Immunocompetence
ESTROGEN-BINDING PROTEINS Estrogen-binding proteins may represent virulence
factors for those fungal organisms where men are more likely than women to experience disseminated infection. Exploratory work in this area has concentrated
on the organism Paracoccidioides brasiliensis, where overt disease following puberty is almost exclusively found among men (146). Specifically, 17β-estradiol
demonstrates its protective effect by inhibiting the phase transition from mycelial
to yeast forms (90). High-affinity, estrogen-binding sites are sensitive to temperatures above 37◦ C in yeast-form P. brasiliensis cytosol (147). Despite these
data, animal models have failed to consistently replicate this disease imbalance in
women.
Disseminated infection with C. immitis is more common among men and nonwhite persons, particularly those of Filipino ancestry (113). However, pregnant
women with coccidioidomycosis develop dissemination and serious disease more
frequently than does the general population (3). The relative resistance of women
to disseminated infection reverses because 17β-estradiol, which increases exponentially during pregnancy, stimulates coccidioidal growth in a dose-dependent
manner by accelerating the rate of spherule maturation and endospore release (41).
UNDERLYING DISEASE Patients with diabetes mellitus are prone to infection (154)
owing to immune system aberrations such as impaired opsonization and decreased
chemotactic activity of neutrophils and monocytes. Fungal infections that are common among diabetics include mucosal candidiasis syndromes and, among patients
with ketoacidosis, zygomycete infections and perhaps coccidioidomycosis (164).
However, diabetics do not appear to be at substantial risk for dissemination. Although vaginal colonization with Candida organisms was thought to be more
frequent among diabetic women, recent evidence indicates that the ∼20% rate of
vaginal C. albicans carriage is not higher in diabetic women than in nondiabetic
women (121). Uncontrolled diabetes predisposes to symptomatic vaginitis (8).
Pregnant women have an increased susceptibility to vaginal infection by Candida, resulting in both a higher prevalence of colonization and a higher rate of
symptomatic vaginitis. High levels of reproductive hormones increase the glycogen content in the vaginal tissue, providing an excellent carbon source for Candida
organisms (95). Estrogen may also enhance adherence of yeast cells to vaginal
mucosa. The rate of symptomatic vaginitis is maximally increased in the third
trimester (105). Symptomatic vaginitis recurrences are more common during pregnancy (35), and relapse episodes following antifungal therapy are common during
pregnancy (25).
Protein calorie malnutrition is a risk factor for gastrointestinal zygomycosis
(87, 102, 144). Approximately one third of all reported cases of gastrointestinal
zygomycosis occur among children (102).
Burn wound victims showed an overall 10-fold increase in fungal infections
between 1964 and 1970, probably owing to the introduction of topical antibacterial
2 Aug 2001
11:11
758
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
agents (107). The notable obvious risks are the elimination of bacterial symbiotic
organisms and the breach in cutaneous integrity. Also implicated in risk are the
persistent hyperglycemia and glucosuria that develop during a condition called
“burn stress pseudodiabetes” (145).
IATROGENIC FACTORS Some studies have shown an increase in either vaginal
colonization or symptomatic vaginitis episodes with Candida following the use of
high-estrogen dose oral contraceptives (55, 139). Other studies have found carriage
of yeasts to be increased among those individuals using intrauterine devices (114),
diaphragm (66), and nonoxynol 9 (6, 66) as contraception.
Broad-spectrum antibiotic use predisposes to yeast vaginitis by eliminating
the normal symbiosis between bacterial and yeast flora, with lower numbers of
lactobacilli documented in vaginal cultures among women with symptomatic yeast
vaginitis (4, 112). Lactobacilli are thought to maintain a protective effect by steric
interference for receptor sites of vaginal epithelial cells and by competing for
nutrients (136).
Infection Routes
ENDOGENOUS C. albicans is part of the normal human body flora, and it can
become pathogenic if it moves from the body compartment where it performs its
function as a normal commensal to a compartment that reacts to its presence. Other
Candida species may become the major endogenous gut yeast when immunocompromised individuals have received antifungal therapy that wipes out C. albicans.
A breakdown of gut mucosa, from oncologic chemotherapy, radiation, trauma,
concurrent viral ulcers, etc., allows a commensal Candida strain to relocate from
the gut to the bloodstream. Overgrowth of the strain at nonsterile commensal sites
such as in the mouth (thrush), esophagus, colon, pulmonary tree, etc., may cause
a localized infection that serves more as an irritant for the individual with the
problem rather than as a life-threatening illness.
Fungi that are environmental saprobes can cause invasive disease
in humans if they enter the human body. Usually such organisms are carried in
the air, inhaled into the pulmonary tree, and begin a localized invasive infection
that may or may not disseminate further in the body. Examples include the primary pathogen C. immitis among immunocompetent people, or the opportunistic
pathogens A. fumigatus or Pneumocystis carinii among immunocompromised people. Dissemination can occur either as contiguous extension (such as when sinus
infection erodes back through bone into the brain) or by hematogenous spread
once the organism erodes into the blood vessels. Some fungi, such as the primary
pathogen Sporothrix schenkii or the opportunistic molds Aspergillus, Fusarium,
and the zygomycetes, can be directly inoculated through intact skin as a route
of invasion. Yet a third but rare portal of entry into the body for fungal organisms is through ingestion. It is well known that malnutrition is a risk factor for
EXOGENOUS
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
759
zygomycete infection, particularly in third world countries, and Aspergillus disease of the intestines has led to fatal bleeding from the gastrointestinal tract among
heavily immunocompromised humans. For this reason, health food supplements
made from natural substances that are not quality-controlled to remove mold spores
are contraindicated among immunocompromised patients.
Immune and Other Host Defenses
After a microorganism penetrates one of the primary protective surface barriers (intact skin, mucous membranes, and pulmonary system) into the body, it
encounters the immune system, which provides defense against invasion of colonizing yeast flora or exogenously inhaled fungal organisms. Humoral immunity (antibodies, complement cascade) and cell-mediated immunity (neutrophils,
macrophages/monocytes, eosinophils, basophils, B- and T-lymphocytes, NK cells,
and cytokines) are the fundamental areas of host defense.
The humoral immune system consists of soluble substances
circulating in the blood. These can include neutralizing antibodies, the complement cascade, additional opsonic proteins (lipopolysaccharide-binding protein,
mannose-binding protein), and inflammatory mediators. Antibody-mediated immune responses have at least a partial role in defense against cryptococcosis and
candidiasis (61, 169). Phagocytosis experiments in the presence of plasma treated
in various ways to inhibit the complement pathways demonstrate that optimal
association of A. fumigatus conidia with neutrophils is dependent on an active
alternative complement pathway (148).
HUMORAL IMMUNITY
CELL-MEDIATED IMMUNITY Cells that destroy infectious agents through phagocytosis or cytotoxicity provide cell-mediated immunity. Cytokines, soluble mediators
secreted by some of these cells to regulate the proliferation, maturation, differentiation, and activation of other cells, are also considered part of this arm of the
immune system.
The neutrophil is an important phagocytic cell in the defense against fungal
infections. Although it is an abundant cell within the circulation, its lifespan of
6–10 h leads to constant replacement of this cell by the bone marrow. The risk of
infection in patients is directly related to the quantity and quality of circulating
neutrophils. For aspergillosis in particular, the risk of infection increases if the
duration of neutropenia (neutrophil count below 0.5 × 109 cells/liter) is greater than
21 days (56), although the risk of invasive tissue–mold infection and/or fungemia
is a continuum, with a greater risk developing as the duration and severity of
neutropenia progresses.
Part of the importance of neutrophils relates to the expression of a number
of receptors on their cell surface. Glycoproteins (L-selectin, CD11a/CD18, and
CD11b/CD18) mediate the adhesion of neutrophils to endothelium in the vascular cell wall. Neutrophils then migrate through the endothelium (diapedesis) into
2 Aug 2001
11:11
760
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
surrounding tissues, where the processes of chemotaxis and phagocytosis allow
destruction of ingested fungi. The elastolytic proteinase of A. fumigatus appears to
act as an inhibitory agent in vitro to the respiratory burst and chemotaxis activities
in a concentration-dependent manner in the human neutrophil (62). Aspergillus
conidia are effective inducers of host chemokine responses that play a central role
in the recruitment of neutrophils into the lung (128).
Macrophages are longer-lived phagocytes than neutrophils. At a site of infection, they appear several hours after neutrophils, persist at the site longer, and
can contribute to chronic granulomatous responses (with lymphocytes). Host tissue damage is more extensive with macrophages. Macrophage phagocytosis is
not dependent on complement. Interferon gamma increases the ability of tissue
macrophages to kill fungi (27, 28, 91). Degradation of phagocytosed microorganisms is carried out in the macrophage by at least two mechanisms: killing by active
oxygen intermediates and degradation by lysosomal enzymes. The enzymes have
pH optima in the acidic range, and the phagolysosome is highly acidic. The first
step in H. capsulatum infection is phagocytosis by macrophages. The organism
survives intracellularly first by failing to trigger the oxidative burst. The mechanism
of this avoidance procedure is not known (42). Second, by preventing host acidification of the lysosome, the fungus avoids the degradative action of the enzymes,
because they are much less active at neutral pH (43).
The monocyte is a circulating mononuclear phagocyte that matures into a
macrophage in tissues. Under resting conditions, monocytes and macrophages kill
ingested microorganisms by a variety of inflammatory mechanisms, but this microbicidal capacity is relatively limited when compared with neutrophils. Human peripheral blood monocytes produce and release the potent inflammatory molecule
tumor necrosis factor in a dose-dependent response when exposed to C. albicans
and other Candida yeasts (5). Tumor necrosis factor stimulates neutrophil-induced
damage of A. fumigatus hyphae (120). In models of myeloperoxidase deficiency
and chronic granulomatous disease, myeloperoxidase-independent oxidative and
nonoxidative mechanisms appear to be active in hyphal damage by monocytes
(36).
T-lymphocytes, named for their site of maturation in the thymus, are central cells
in lymphocyte-activated killing. To be recognized by a T-lymphocyte, foreign (fungal) antigens must be presented to the T-lymphocyte by antigen-presenting cells,
and both the T-lymphocyte and the antigen-presenting cell must have identical
genes for certain major histocompatibility complex proteins so the cell can distinguish self from nonself. Suppressed T-lymphocyte function can lead to fungal
infections such as aspergillosis pneumonia, Pneumocystis carinii pneumonia, or
cryptococcal meningitis.
The brain contains microglial cells, sessile mononuclear phagocytes, that participate in host response to infections. Microglia respond to practically all pathologic events in the central nervous system, aided by infiltrating hematogenous
macrophages. Human microglial cells are potent effector cells against C. neoformans in vitro in the presence of specific antibody. A murine monoclonal antibody to
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
761
the capsular glucuronoxylomannan can produce dose-dependent enhancement of
C. neoformans phagocytosis by microglia (86). An in vitro–generated microglial
cell line, transferred intracerebrally into syngeneic immunocompetent mice, resulted in local protection against a lethal dose of C. albicans (11).
One strategy for improving resistance to opportunistic pathogens is to upregulate, during the invasion process, host cellular responses that are relevant to host
defense mechanisms. Invasion of endothelial cells by C. albicans in vitro stimulates
the conversion of arachidonic acid into prostaglandins, upregulating the synthesis
of endothelial cell cyclooxygenase and increasing the activity of the endothelial
cell phospholipase, modulating the leukocyte response at the candida-leukocyteendothelial cell interface (45). The secretion of these prostaglandins has no effect
on the amount of endothelial cell injury induced by C. albicans.
PLATELETS Platelets have the potential to play an important role in normal host
defenses against invasive aspergillosis. Platelets, like neutrophils, attach to cell
walls of the hyphal form of A. fumigatus. Damage to the cell wall occurs, and
defined hyphal surface glycoproteins are released. Rapid appearance of the surface
antigen CD63 and release of markers of platelet degranulation are evidence for
platelet activation during attachment to hyphae (26).
CLASSES OF INFECTIONS
Mucosal
The mucosal immune system uses lymphocytes localized into three functional
compartments, intraepithelial (T-lymphocytes), lamina propria (B-lymphocytes),
and Peyer’s patches. These compartments incorporate cells into the epithelial cell
layer, the lamina immediately below, and, for Peyer’s patches found only in the
gut, loosely organized lymphoid tissue that are able to sample the antigenic milieu
of the lumen.
Infections of gut and vaginal mucosal surfaces are usually described by their location. Gastrointestinal infections are commonly referred to as thrush (oropharynx)
and esophagitis. The term thrush can be referenced to the latter half of the eighteenth century, when “countrey people taught us the virtues of the thrush-moss for
sore throats” (132).
The in vitro observations of morphological variation in Candida are mirrored
by the different clinical presentations in mucosal infection, both for the model
oral yeast Candida as well as other fungi that cause observable infection among
immunosuppressed patients. Typical manifestations of oropharyngeal candidiasis among the AIDS population are pseudomembranes (visible colony-forming
units on the mucosal surface), erythema (no visible colony-forming units, but
enough infection that inflammatory mediators or use of mucosal nutrients by
the yeast causes a reddened color change to the mucosal surface), and angular
2 Aug 2001
11:11
762
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
cheilitis (fissures at the corner of the mouth where yeast has no space to grow
visibly).
In non-AIDS–immunosuppressed patients, fungi such as the zygomycetes and
Fusarium, Aspergillus, and Trichosporon species are emerging as pathogens that
may invade the oral mucosa during periods of immunosuppression. These fungal infections of the oropharynx can produce a locally invasive leathery plaque
(vegetative hyphae from a tangle of true mold hyphae), an eschar, or a chronic or
irregular necrotic ulceration including progression to palatal perforation to form
oral-antral fistulae (tissue necrosis). The oral mucosa is a frequent site, in approximately 30%–50% of patients, of involvement during histoplasmosis. The oral
lesions are commonly confused with squamous cell carcinomas (15). A variety of
mucosal surface lesions can be the initial clue to diagnosis among patients having infection with paracoccidioidomycosis, cryptococcosis, blastomycosis, and
coccidioidomycosis (98).
Candida organisms gain access to the vaginal lumen and secretions predominantly from the adjacent perianal area (10). Longitudinal changes in yeast flora
occurred significantly more often in fecal samples than in oral samples, and significantly less often in sites colonized with C. albicans than in sites colonized with
other species (109).
Systemic
An interesting characteristic of tissue-invasive fungal infections is that some progress to systemic infection while others remain localized. In principle, progression
is a function of the degree of immunocompromise of the host and the virulence
factors produced by the fungus that has come into contact with that host. Although
a great deal is known about host responses to the first steps of fungal infection,
predicting whether a patient, with a predefined set of immune defect(s), will contain a localized infection is still so difficult that immediate antifungal treatment
is initiated. Some of these treatments are exaggerated and perhaps unnecessary.
Further understanding of interactions between the parasite and the host in this
situation might allow for more informed and evidence-based decisions regarding
diagnostic and therapeutic strategies.
The most common sites of localized nonmucosal fungal infection are the skin
with its underlying soft tissues and the lung. Identifying the cause of these infections can be difficult, because superficial cultures from nonsterile sites (surface
swab of infected wound, nose culture during sinusitis, sputum culture for an infection deep in the lungs) rarely discriminate between pathogens, commensal flora,
and environmental saprobes. Deep cultures from these nonsterile sites (swab following debridement of an infected wound, aspiration of pus from an air-fluid level
in the sinuses, bronchoalveolar lavage for pneumonia) are a better basis upon which
to treat with antifungal agents, although the uncertainty of pathogenicity is still
present. Culture and histologic examination of sterile tissues, such as skin punch
biopsy and open-lung biopsy specimens, are helpful in diagnosis.
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
763
When infection progresses from localized tissue involvement to widespread
disease, recognition of the infecting organism may not be any easier than for
contained infections. Disseminated hematogenous yeast fungemias are typically
diagnosed by examination of blood samples. Disease may also manifest with
embolic skin lesions in the setting of negative blood cultures. These fungal lesions
require examination by staining the roof of a blister or by examining a fungal stain
of a biopsy specimen while awaiting culture results. Embolic infections result from
a plug of organisms or an infected blood clot that occludes blood vessels. When
this occurs in the skin layers of the dermis and subcutis, an ischemic tissue cone
with necrotic tissue forms above. Therefore, a skin biopsy for a suspected fungal
lesion should be taken from the center of the lesion and go deep in order to reach
subcutaneous fat.
CONCLUSION
As this review demonstrates, the study of fungal pathogenesis is complex and
rapidly evolving. Because properties of the pathogen and of the host seem approximately equally important, and because multiple factors on both sides are involved,
the evolving genomic studies of the fungi and of the host have the greatest promise
of providing rapid understanding. Work on one gene or one property, no matter how
detailed and careful, will not be useful as the kind of comprehensive analysis now
available in humans as well as in several pathogenic fungi. Thus the global approach
will probably be the most fruitful way to study fungal pathogenesis in the future.
ACKNOWLEDGMENTS
In this review, the work from our laboratories was supported by NIH K08 grant
AI-01411 awarded to Dr. van Burik and by NIH R01 grant AI-46351 and contract
AI-05406 awarded to Dr. Magee.
Visit the Annual Reviews home page at www.AnnualReviews.org
LITERATURE CITED
1. Anderson JM, Soll DR. 1987. Unique
phenotype of opaque cells in the whiteopaque transition of Candida albicans. J.
Bacteriol. 169:5579–88
2. Annaix V, Bouchara JP, Larcher G,
Chabasse D, Tronchin G. 1992. Specific
binding of human fibrinogen fragment D
to Aspergillus fumigatus conidia. Infect.
Immun. 60:1747–55
3. Arsura EL, Kilgore WB, Ratnayake SN.
1998. Erythema nodosum in pregnant patients with coccidioidomycosis. Clin. Infect. Dis. 27:1201–3
4. Auger P, Joly J. 1980. Microbial flora
associated with Candida albicans vulvovaginitis. Obstet. Gynecol. 55:397–401
5. Aybay C, Imir T. 1996. Tumor necrosis
factor induction from monocyte/macrophages by Candida species. Immunobiology 196:363–74
2 Aug 2001
11:11
764
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
6. Barbone F, Austin H, Louv WC, Alexander WJ. 1990. A follow-up study of
methods of contraception, sexual activity, and rates of trichomoniasis, candidiasis, and bacterial vaginosis. Am.
J. Obstet. Gynecol. 163:510–14
7. Batanghari JW, Deepe GS Jr, Di Cera E,
Goldman WE. 1998. Histoplasma acquisition of calcium and expression of CBP1
during intracellular parasitism. Mol. Microbiol. 27:531–39
8. Berg AO, Heidrich FE, Fihn SD, Bergman JJ, Wood RW, et al. 1984. Establishing the cause of genitourinary symptoms
in women in a family practice. Comparison of clinical examination and comprehensive microbiology. JAMA 251:620–
25
9. Bergen MS, Voss E, Soll DR. 1990.
Switching at the cellular level in the whiteopaque transition of Candida albicans. J.
Gen. Microbiol. 136:1925–36
10. Bertholf ME, Stafford MJ. 1983. Colonization of Candida albicans in vagina,
rectum, and mouth. J. Fam. Pract. 16:
919–24
11. Blasi E, Mazzolla R, Barluzzi R, Mosci
P, Bartoli A, Bistoni F. 1991. Intracerebral transfer of an in vitro established
microglial cell line: local induction of a
protective state against lethal challenge
with Candida albicans. J. Neuroimmunol.
32:249–57
12. Bodey GP, Vartivarian S. 1989. Aspergillosis. Eur. J. Clin. Microbiol. Infect.
Dis. 8:413–37
13. Boelaert JR, de Locht M, Van Cutsem
J, Kerrels V, Cantinieaux B, et al. 1993.
Mucormycosis during deferoxamine therapy is a siderophore-mediated infection.
In vitro and in vivo animal studies. J. Clin.
Invest. 91:1979–86
14. Boelaert JR, Fenves AZ, Coburn JW.
1991. Deferoxamine therapy and mucormycosis in dialysis patients: report of
an international registry. Am. J. Kidney
Dis. 18:660–67
15. Bokmeyer TJ. 1980. Ulcerated lesion of
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
the alveolar ridge. J. Am. Dent. Assoc.
101:665–67
Brandhorst TT, Klein B. 2000. Cell
wall biogenesis of Blastomyces dermatitidis. Evidence for a novel mechanism of
cell surface localization of a virulenceassociated adhesin via extracellular release and reassociation with cell wall
chitin. J. Biol. Chem. 275:7925–34
Brandhorst TT, Wuthrich M, Warner T,
Klein B. 1999. Targeted gene disruption reveals an adhesin indispensable for
pathogenicity of Blastomyces dermatitidis. J. Exp. Med. 189:1207–16
Brown AJ, Gow NA. 1999. Regulatory
networks controlling Candida albicans
morphogenesis. Trends Microbiol. 7:
333–38
Buffo J, Herman MA, Soll DR. 1984.
A characterization of pH-regulated dimorphism in Candida albicans. Mycopathologia 85:21–30
Casadevall A, Rosas AL, Nosanchuk JD.
2000. Melanin and virulence in Cryptococcus neoformans. Curr. Opin. Microbiol. 3:354–58
Chang YC, Cherniak R, Kozel TR,
Granger DL, Morris LC, et al. 1997.
Structure and biological activities of acapsular Cryptococcus neoformans 602 complemented with the CAP64 gene. Infect.
Immun. 65:1584–92
Chang YC, Kwon-Chung KJ. 1998.
Isolation of the third capsule-associated
gene, CAP60, required for virulence in
Cryptococcus neoformans. Infect. Immun.
66:2230–36
Chang YC, Kwon-Chung KJ. 1999. Isolation, characterization, and localization
of a capsule-associated gene, CAP10, of
Cryptococcus neoformans. J. Bacteriol.
181:5636–43
Chang YC, Wickes BL, Miller GF,
Penoyer LA, Kwon-Chung KJ. 2000.
Cryptococcus neoformans STE12alpha
regulates virulence but is not essential for
mating. J. Exp. Med. 191:871–82
Christensen ES, Buch A. 1982. Vaginal
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
26.
27.
28.
28a.
29.
30.
30a.
31.
Candida albicans treated with three different concentrations of natamycin (Pimafucin) for 6 days. Acta Obstet. Gynecol.
Scand. 61:325–28
Christin L, Wysong DR, Meshulam T,
Hastey R, Simons ER, Diamond RD.
1998. Human platelets damage Aspergillus fumigatus hyphae and may supplement killing by neutrophils. Infect. Immun. 66:1181–89
Clemons KV, Darbonne WC, Curnutte
JT, Sobel RA, Stevens DA. 2000. Experimental histoplasmosis in mice treated
with anti-murine interferon-gamma antibody and in interferon-gamma gene
knockout mice. Microbes Infect. 2:997–
1001
Clemons KV, Stevens DA. 2000. Treatment of orogastrointestinal candidosis
in SCID mice with fluconazole alone
or in combination with recombinant
granulocyte colony-stimulating factor or
interferon-gamma. Med. Mycol. 38:213–
19
Cormack BP, Ghori N, Falkow S. 1999.
An adhesin of the yeast pathogen Candida
glabrata mediating adherence to human
epithelial cells. Science 285:578–82
Cole GT, Zhu SW, Hsu LL, Kruse D, Seshan KR, Wang F. 1992. Isolation and expression of a gene which encodes a wallassociated proteinase of Coccidioides immitis. Infect. Immun. 60:416–27
Coulot P, Bouchara JP, Renier G, Annaix
V, Planchenault C, et al. 1994. Specific
interaction of Aspergillus fumigatus with
fibrinogen and its role in cell adhesion.
Infect. Immun. 62:2169–77
Cox GM, Mukherjee J, Cole GT, Casadevall A, Perfect J. 2000. Urease as a virulence factor in experimental cryptococcosis. Infect. Immun. 68:443–48
Csank C, Schroppel K, Leberer E, Harcus D, Mohamed O, et al. 1998. Roles of
the Candida albicans mitogen-activated
protein kinase homolog, cek1p, in hyphal development and systemic candidiasis. Infect. Immun. 66:2713–21
765
32. Cutler JE. 1991. Putative virulence factors
of Candida albicans. Annu. Rev. Microbiol. 45:187–218
33. Davis D, Edwards JE Jr, Mitchell AP,
Ibrahim AS. 2000. Candida albicans
RIM101 pH response pathway is required
for host-pathogen interactions. Infect. Immun. 68:5953–59
34. De Bernardis F, Muhlschlegel FA, Cassone A, Fonzi WA. 1998. The pH of the
host niche controls gene expression in and
virulence of Candida albicans. Infect. Immun. 66:3317–25
35. de Louvois J, Hurley R, Stanley VC.
1975. Microbial flora of the lower genital tract during pregnancy: relationship
to morbidity. J. Clin. Pathol. 28:731–
35
36. Diamond RD, Huber E, Haudenschild
CC. 1983. Mechanisms of destruction
of Aspergillus fumigatus hyphae mediated by human monocytes. J. Infect. Dis.
147:474–83
37. Dixon DM, Migliozzi J, Cooper CR Jr,
Solis O, Breslin B, Szaniszlo PJ. 1992.
Melanized and non-melanized multicellular form mutants of Wangiella dermatitidis in mice: mortality and histopathology studies. Mycoses 35:17–21
38. Dixon DM, Polak A, Conner GW. 1989.
Mel-mutants of Wangiella dermatitidis in
mice: evaluation of multiple mouse and
fungal strains. J. Med. Vet. Mycol. 27:335–
41
39. Dixon DM, Polak A, Szaniszlo PJ. 1987.
Pathogenicity and virulence of wild-type
and melanin-deficient Wangiella dermatitidis. J. Med. Vet. Mycol. 25:97–106
40. Doering TL, Nosanchuk JD, Roberts
WK, Casadevall A. 1999. Melanin as a
potential cryptococcal defence against
microbicidal proteins. Med. Mycol. 37:
175–81
41. Drutz DJ, Huppert M, Sun SH, McGuire WL. 1981. Human sex hormones
stimulate the growth and maturation
of Coccidioides immitis. Infect. Immun.
32:897–907
2 Aug 2001
11:11
766
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
42. Eissenberg LG, Goldman WE. 1987.
Histoplasma capsulatum fails to trigger release of superoxide from macrophages. Infect. Immun. 55:29–34
43. Eissenberg LG, Goldman WE, Schlesinger PH. 1993. Histoplasma capsulatum modulates the acidification of phagolysosomes. J. Exp. Med. 177:1605–11
44. Eissenberg LG, Poirier S, Goldman WE.
1996. Phenotypic variation and persistence of Histoplasma capsulatum yeasts
in host cells. Infect. Immun. 64:5310–14
45. Filler SG, Ibe BO, Ibrahim AS, Ghannoum MA, Raj JU, Edwards JE Jr. 1994.
Mechanisms by which Candida albicans
induces endothelial cell prostaglandin
synthesis. Infect. Immun. 62:1064–69
46. Fratti RA, Ghannoum MA, Edwards
JE Jr, Filler SG. 1996. Gamma interferon protects endothelial cells from damage by Candida albicans by inhibiting
endothelial cell phagocytosis. Infect. Immun. 64:4714–18
47. Fries BC, Goldman DL, Cherniak R,
Ju R, Casadevall A. 1999. Phenotypic
switching in Cryptococcus neoformans
results in changes in cellular morphology
and glucuronoxylomannan structure. Infect. Immun. 67:6076–83
48. Fu Y, Filler SG, Spellberg BJ, Fonzi
W, Ibrahim AS, et al. 1998. Cloning and
characterization of CAD1/AAF1, a gene
from Candida albicans that induces adherence to endothelial cells after expression
in Saccharomyces cerevisiae. Infect. Immun. 66:2078–84
49. Fu Y, Ibrahim AS, Fonzi W, Zhou X,
Ramos CF, Ghannoum MA. 1997. Cloning and characterization of a gene
(LIP1) which encodes a lipase from the
pathogenic yeast Candida albicans. Microbiology 143:331–40
50. Fu Y, Rieg G, Fonzi WA, Belanger PH,
Edwards JE Jr, Filler SG. 1998. Expression of the Candida albicans gene ALS1
in Saccharomyces cerevisiae induces adherence to endothelial and epithelial cells.
Infect. Immun. 66:1783–86
51. Gale CA, Bendel CM, McClellan M,
Hauser M, Becker JM, et al. 1998. Linkage of adhesion, filamentous growth, and
virulence in Candida albicans to a single
gene, INT1. Science 279:1355–58
52. Gale CA, Finkel D, Tao NJ, Meinke M,
McClellan M, et al. 1996. Cloning and expression of a gene encoding an integrinlike protein in Candida albicans. Proc.
Natl. Acad. Sci. USA 93:357–61
53. Gaur NK, Klotz SA. 1997. Expression,
cloning, and characterization of a Candida albicans gene, ALA1, that confers adherence properties upon Saccharomyces
cerevisiae for extracellular matrix proteins. Infect. Immun. 65:5289–94
54. Gaziev D, Baronciani D, Galimberti M,
Polchi P, Angelucci E, et al. 1996. Mucormycosis after bone marrow transplantation: report of four cases in thalassemia
and review of the literature. Bone Marrow
Transplant. 17:409–14
55. Geiger AM, Foxman B. 1996. Risk factors for vulvovaginal candidiasis: a casecontrol study among university students.
Epidemiology 7:182–87
56. Gerson SL, Talbot GH, Hurwitz S,
Strom BL, Lusk EJ, Cassileth PA.
1984. Prolonged granulocytopenia: the
major risk factor for invasive pulmonary
aspergillosis in patients with acute
leukemia. Ann. Intern. Med. 100:345–51
57. Ghannoum MA. 2000. Potential role of
phospholipases in virulence and fungal pathogenesis. Clin. Microbiol. Rev.
13:122–43
58. Ghannoum MA, Swairjo I, Soll DR.
1990. Variation in lipid and sterol contents
in Candida albicans white and opaque
phenotypes. J. Med. Vet. Mycol. 28:103–
15
59. Gil C, Perez-Diaz R, Nombela C. 1994.
Inhibitory and morphological effects of
several antifungal agents on three types
of Candida albicans morphological mutants. J. Med. Vet. Mycol. 32:151–62
60. Gow NA, Perera TH, SherwoodHigham J, Gooday GW, Gregory DW,
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
61.
62.
63.
64.
65.
66.
67.
68.
69.
70.
Marshall D. 1994. Investigation of touchsensitive responses by hyphae of the human pathogenic fungus Candida albicans.
Scanning Microsc. 8:705–10
Han Y, Riesselman MH, Cutler JE. 2000.
Protection against candidiasis by an immunoglobulin G3 (IgG3) monoclonal antibody specific for the same mannotriose
as an IgM protective antibody. Infect. Immun. 68:1649–54
Hasegawa Y, Nikai T, Ogawa K, Sugihara H. 1999. The effect of an Aspergillus
fumigatus derived elastolytic proteinase
on human neutrophils. Nippon Ishinkin
Gakkai Zasshi 40:235–38
Hellstein J, Vawter-Hugart H, Fotos P,
Schmid J, Soll DR. 1993. Genetic similarity and phenotypic diversity of commensal and pathogenic strains of Candida
albicans isolated from the oral cavity. J.
Clin. Microbiol. 31:3190–99
Hogan LH, Klein BS, Levitz SM. 1996.
Virulence factors of medically important
fungi. Clin. Microbiol. Rev. 9:469–88
Homma M, Chibana H, Tanaka K. 1993.
Induction of extracellular proteinase in
Candida albicans. J. Gen. Microbiol.
139:1187–93
Hooton TM, Fihn SD, Johnson C, Roberts PL, Stamm WE. 1989. Association
between bacterial vaginosis and acute cystitis in women using diaphragms. Arch.
Intern. Med. 149:1932–36
Hoyer LL, Payne TL, Bell M, Myers
AM, Scherer S. 1998. Candida albicans
ALS3 and insights into the nature of the
ALS gene family. Curr. Genet. 33:451–59
Hoyer LL, Payne TL, Hecht JE. 1998.
Identification of Candida albicans ALS2
and ALS4 and localization of als proteins
to the fungal cell surface. J. Bacteriol.
180:5334–43
Hoyer LL, Scherer S, Shatzman AR,
Livi GP. 1995. Candida albicans ALS1:
domains related to a Saccharomyces cerevisiae sexual agglutinin separated by a repeating motif. Mol. Microbiol. 15:39–54
Hube B, Monod M, Schofield DA,
71.
72.
73.
74.
75.
76.
77.
78.
79.
767
Brown AJ, Gow NA. 1994. Expression of
seven members of the gene family encoding secretory aspartyl proteinases in Candida albicans. Mol. Microbiol. 14:87–99
Hube B, Sanglard D, Odds FC, Hess D,
Monod M, et al. 1997. Disruption of each
of the secreted aspartyl proteinase genes
SAP1, SAP2, and SAP3 of Candida albicans attenuates virulence. Infect. Immun.
65:3529–38
Ibrahim AS, Mirbod F, Filler SG, Banno
Y, Cole GT, et al. 1995. Evidence implicating phospholipase as a virulence factor of Candida albicans. Infect. Immun.
63:1993–98
Kennedy MJ, Rogers AL, Hanselmen
LR, Soll DR, Yancey RJ Jr. 1988. Variation in adhesion and cell surface hydrophobicity in Candida albicans white
and opaque phenotypes. Mycopathologia
102:149–56
Kimura M, McGinnis MR. 1998. Fontana-Masson–stained tissue from cultureproven mycoses. Arch. Pathol. Lab. Med.
122:1107–11
Kirsch DR, Whitney RR. 1991. Pathogenicity of Candida albicans auxotrophic mutants in experimental infections.
Infect. Immun. 59:3297–300
Klein BS. 2000. Molecular basis of
pathogenicity in Blastomyces dermatitidis: the importance of adhesion. Curr.
Opin. Microbiol. 3:339–43
Kolattukudy PE, Lee JD, Rogers LM,
Zimmerman P, Ceselski S, et al. 1993.
Evidence for possible involvement of an
elastolytic serine protease in aspergillosis.
Infect. Immun. 61:2357–68
Kruse D, Cole GT. 1992. A seroreactive 120-kilodalton beta-1,3-glucanase
of Coccidioides immitis which may
participate in spherule morphogenesis. Infect. Immun. 60:4350–63
Kurtz MB, Heath IB, Marrinan J, Dreikorn S, Onishi J, Douglas C. 1994. Morphological effects of lipopeptides against
Aspergillus fumigatus correlate with
activities against (1,3)-beta-D-glucan
2 Aug 2001
11:11
768
80.
81.
82.
83.
84.
85.
86.
87.
88.
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
synthase. Antimicrob. Agents Chemother.
38:1480–89
Kvaal C, Lachke SA, Srikantha T, Daniels K, McCoy J, Soll DR. 1999. Misexpression of the opaque-phase-specific
gene PEP1 (SAP1) in the white phase of
Candida albicans confers increased virulence in a mouse model of cutaneous infection. Infect. Immun. 67:6652–62
Kwon-Chung KJ, Lehman D, Good C,
Magee PT. 1985. Genetic evidence for
role of extracellular proteinase in virulence of Candida albicans. Infect. Immun.
49:571–75
Land GA, McDonald WC, Stjernholm
RL, Friedman L. 1975. Factors affecting filamentation in Candida albicans:
changes in respiratory activity of Candida
albicans during filamentation. Infect. Immun. 12:119–27
Larcher G, Bouchara JP, Annaix V,
Symoens F, Chabasse D, Tronchin G.
1992. Purification and characterization
of a fibrinogenolytic serine proteinase
from Aspergillus fumigatus culture filtrate. FEBS Lett. 308:65–69
Leberer E, Ziegelbauer K, Schmidt A,
Harcus D, Dignard D, et al. 1997. Virulence and hyphal formation of Candida
albicans require the Ste20p-like protein
kinase CaCla4p. Curr. Biol. 7:539–46
Lee KS, Patton JL, Fido M, Hines LK,
Kohlwein SD, et al. 1994. The Saccharomyces cerevisiae PLB1 gene encodes
a protein required for lysophospholipase
and phospholipase B activity. J. Biol.
Chem. 269:19725–30
Lee SC, Kress Y, Dickson DW, Casadevall A. 1995. Human microglia mediate
anti–Cryptococcus neoformans activity in
the presence of specific antibody. J. Neuroimmunol. 62:43–52
Leeming JG, Moss HA, Elliott TSJ. 1996.
Risk of tongue depressors to the immunocompromised. Lancet 348:889
Leidich SD, Orlean P. 1996. Gpi1, a Saccharomyces cerevisiae protein that participates in the first step in glycosylphos-
89.
90.
91.
92.
93.
94.
95.
96.
97.
98.
phatidylinositol anchor synthesis. J. Biol.
Chem. 271:27829–37
Lo HJ, Kohler JR, DiDomenico B,
Loebenberg D, Cacciapuoti A, Fink GR.
1997. Nonfilamentous C. albicans mutants are avirulent. Cell 90:939–49
Loose DS, Stover EP, Restrepo A,
Stevens DA, Feldman D. 1983. Estradiol binds to a receptor-like cytosol binding protein and initiates a biological response in Paracoccidioides brasiliensis.
Proc. Natl. Acad. Sci. USA 80:7659–63
Lutz JE, Clemons KV, Stevens DA.
2000. Enhancement of antifungal chemotherapy by interferon-gamma in experimental systemic cryptococcosis. J. Antimicrob. Chemother. 46:437–42
Macdonald F, Odds FC. 1983. Virulence
for mice of a proteinase-secreting strain
of Candida albicans and a proteinasedeficient mutant. J. Gen. Microbiol.
129:431–38
MacGill TC, MacGill RS, Casadevall
A, Kozel TR. 2000. Biological correlates of capsular (quellung) reactions of
Cryptococcus neoformans. J. Immunol.
164:4835–42
Martinez-Ruiz A, Kao R, Davies J, Martinez del Pozo A. 1999. Ribotoxins are a
more widespread group of proteins within
the filamentous fungi than previously believed. Toxicon 37:1549–63
McCourtie J, Douglas LJ. 1981. Relationship between cell surface composition of Candida albicans and adherence
to acrylic after growth on different carbon
sources. Infect. Immun. 32:1234–41
McCusker JH, Clemons KV, Stevens DA,
Davis RW. 1994. Genetic characterization
of pathogenic Saccharomyces cerevisiae
isolates. Genetics 136:1261–69
McNab AA, McKelvie P. 1997. Iron overload is a risk factor for zygomycosis. Arch.
Ophthalmol. 115:919–21
Meyer RD. 1986. Cutaneous and mucosal manifestations of the deep mycotic
infections. Acta Derm. Venereol. Suppl.
121:57–72
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
99. Mitchell AP. 1998. Dimorphism and
virulence in Candida albicans. Curr.
Opin. Microbiol. 1:687–92
100. Monod M, Paris S, Sanglard D, JatonOgay K, Bille J, Latge JP. 1993. Isolation
and characterization of a secreted metalloprotease of Aspergillus fumigatus. Infect.
Immun. 61:4099–104
101. Monod M, Togni G, Hube B, Sanglard
D. 1994. Multiplicity of genes encoding
secreted aspartic proteinases in Candida
species. Mol. Microbiol. 13:357–68
102. Mooney JE, Wanger A. 1993. Mucormycosis of the gastrointestinal tract in
children: report of a case and review of the
literature. Pediatr. Infect. Dis. J. 12:872–
76
103. Morrow B, Ramsey H, Soll DR. 1994.
Regulation of phase-specific genes in the
more general switching system of Candida albicans strain 3153A. J. Med. Vet.
Mycol. 32:287–94
104. Morrow B, Srikantha T, Soll DR. 1992.
Transcription of the gene for a pepsinogen, PEP1, is regulated by white-opaque
switching in Candida albicans. Mol. Cell.
Biol. 12:2997–3005
105. Morton RS, Rashid S. 1977. Candidal
vaginitis: natural history, predisposing
factors and prevention. Proc. R. Soc. Med.
70:3–6
106. Muhlschlegel FA, Fonzi WA. 1997.
PHR2 of Candida albicans encodes a
functional homolog of the pH-regulated gene PHR1 with an inverted pattern of pH-dependent expression. Mol.
Cell. Biol. 17:5960–67
107. Nash G, Foley FD, Goodwin MN Jr,
Bruck HM, Greenwald KA, Pruitt BA
Jr. 1971. Fungal burn wound infection.
JAMA 215:1664–66
108. Nosanchuk JD, Rudolph J, Rosas AL,
Casadevall A. 1999. Evidence that Cryptococcus neoformans is melanized in
pigeon excreta: implications for pathogenesis. Infect. Immun. 67:5477–79
109. Odds FC, Kibbler CC, Walker E, Bhamra A, Prentice HG, Noone P. 1989.
110.
111.
112.
113.
114.
115.
116.
117.
118.
769
Carriage of Candida species and C. albicans biotypes in patients undergoing
chemotherapy or bone marrow transplantation for haematological disease. J. Clin.
Pathol. 42:1259–66
Odom A, Del Poeta M, Perfect J, Heitman J. 1997. The immunosuppressant
FK506 and its nonimmunosuppressive
analog 685,818 are toxic to Cryptococcus neoformans by inhibition of a common target protein. Antimicrob. Agents
Chemother. 41:156–61
Odom A, Muir S, Lim E, Toffaletti DL,
Perfect J, Heitman J. 1997. Calcineurin
is required for virulence of Cryptococcus
neoformans. EMBO J. 16:2576–89
Oriel JD, Waterworth PM. 1975. Effects of minocycline and tetracycline on
the vaginal yeast flora. J. Clin. Pathol.
28:403–6
Pappagianis D. 1988. Epidemiology of
coccidioidomycosis. Curr. Top. Med. Mycol. 2:199–238
Parewijck W, Claeys G, Thiery M, van
Kets H. 1988. Candidiasis in women fitted
with an intrauterine contraceptive device.
Br. J. Obstet. Gynaecol. 95:408–10
Perfect JR, Toffaletti DL, Rude TH.
1993. The gene encoding phosphoribosylaminoimidazole carboxylase (ADE2) is
essential for growth of Cryptococcus neoformans in cerebrospinal fluid. Infect. Immun. 61:4446–51
Perfect JR, Wong B, Chang YC, KwonChung KJ, Williamson PR. 1998. Cryptococcus neoformans: virulence and host
defences. Med. Mycol. 36:79–86
Rhodes JC, Polacheck I, Kwon-Chung
KJ. 1982. Phenoloxidase activity and virulence in isogenic strains of Cryptococcus neoformans. Infect. Immun. 36:1175–
84
Riggle PJ, Andrutis KA, Chen X, Tzipori SR, Kumamoto CA. 1999. Invasive
lesions containing filamentous forms produced by a Candida albicans mutant that
is defective in filamentous growth in culture. Infect. Immun. 67:3649–52
2 Aug 2001
11:11
770
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
119. Rikkerink EH, Magee BB, Magee PT.
1988. Opaque-white phenotype transition: a programmed morphological transition in Candida albicans. J. Bacteriol.
170:895–99
120. Roilides E, Dimitriadou-Georgiadou A,
Sein T, Kadiltsoglou I, Walsh TJ. 1998.
Tumor necrosis factor alpha enhances
antifungal activities of polymorphonuclear and mononuclear phagocytes against Aspergillus fumigatus. Infect. Immun. 66:5999–6003
121. Rowe BR, Logan MN, Farrell I, Barnett
AH. 1990. Is candidiasis the true cause of
vulvovaginal irritation in women with diabetes mellitus? J. Clin. Pathol. 43:644–
45
122. Ruchel R. 1981. Properties of a purified
proteinase from the yeast Candida albicans. Biochim. Biophys. Acta 659:99–
113
123. Ruchel R, Boning B. 1983. Detection
of Candida proteinase by enzyme immunoassay and interaction of the enzyme with alpha-2-macroglobulin. J. Immunol. Methods 61:107–16
124. Rupp S, Summers E, Lo HJ, Madhani
H, Fink G. 1999. MAP kinase and
cAMP filamentation signaling pathways
converge on the unusually large promoter of the yeast FLO11 gene. EMBO
J. 18:1257–69
125. Rustchenko EP, Howard DH, Sherman
F. 1997. Variation in assimilating functions occurs in spontaneous Candida albicans mutants having chromosomal alterations. Microbiology 143:1765–78
126. Sanglard D, Hube B, Monod M, Odds
FC, Gow NA. 1997. A triple deletion
of the secreted aspartyl proteinase genes
SAP4, SAP5, and SAP6 of Candida albicans causes attenuated virulence. Infect.
Immun. 65:3539–46
127. Saporito-Irwin SM, Birse CE, Sypherd
PS, Fonzi WA. 1995. PHR1, a pHregulated gene of Candida albicans, is
required for morphogenesis. Mol. Cell.
Biol. 15:601–13
128. Schelenz S, Smith DA, Bancroft
GJ. 1999. Cytokine and chemokine
responses following pulmonary challenge with Aspergillus fumigatus: obligatory role of TNF-alpha and GM-CSF
in neutrophil recruitment. Med. Mycol.
37:183–94
129. Schmid J, Odds FC, Wiselka MJ,
Nicholson KG, Soll DR. 1992. Genetic
similarity and maintenance of Candida
albicans strains from a group of AIDS
patients, demonstrated by DNA fingerprinting. J. Clin. Microbiol. 30:935–41
130. Schmid J, Rotman M, Reed B, Pierson
CL, Soll DR. 1993. Genetic similarity of
Candida albicans strains from vaginitis
patients and their partners. J. Clin. Microbiol. 31:39–46
131. Schroppel K, Rotman M, Galask R,
Mac K, Soll DR. 1994. Evolution and
replacement of Candida albicans strains
during recurrent vaginitis demonstrated
by DNA fingerprinting. J. Clin. Microbiol. 32:2646–54
131a. Sebghati TS, Engle JT, Goldman WE.
2000. Intraceullar parasitism by Histoplasma capsulatum: fungal virulence
and calcium dependence. Science 290:
1368–71
132. Simpson JA, Weiner ESC, eds. 1989.
The Oxford English Dictionary, 18:26.
Oxford: Clarendon. 2nd ed.
133. Slutsky B, Buffo J, Soll DR. 1985. Highfrequency switching of colony morphology in Candida albicans. Science
230:666–69
134. Slutsky B, Staebell M, Anderson J,
Risen L, Pfaller M, Soll DR. 1987.
“White-opaque transition”: a second
high-frequency switching system in
Candida albicans. J. Bacteriol. 169:
189–97
135. Smith JM, Davies JE, Holden DW.
1993. Construction and pathogenicity of
Aspergillus fumigatus mutants that do
not produce the ribotoxin restrictocin.
Mol. Microbiol. 9:1071–77
136. Sobel JD, Myers PG, Kaye D, Levison
2 Aug 2001
11:11
AR
AR135-27.tex
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
FUNGAL PATHOGENESIS
137.
138.
139.
140.
141.
142.
143.
144.
145.
146.
147.
ME. 1981. Adherence of Candida albicans to human vaginal and buccal epithelial cells. J. Infect. Dis. 143:76–82
Soll DR. 1988. High-frequency switching in Candida albicans and its relations
to vaginal candidiasis. Am. J. Obstet. Gynecol. 158:997–1001
Soll DR, Staebell M, Langtimm C,
Pfaller M, Hicks J, Rao TV. 1988. Multiple Candida strains in the course of a
single systemic infection. J. Clin. Microbiol. 26:1448–59
Spinillo A, Capuzzo E, Nicola S, Baltaro F, Ferrari A, Monaco A. 1995. The
impact of oral contraception on vulvovaginal candidiasis. Contraception
51:293–97
Staab JF, Bradway SD, Fidel PL, Sundstrom P. 1999. Adhesive and mammalian transglutaminase substrate properties of Candida albicans Hwp1. Science
283:1535–38
Staab JF, Ferrer CA, Sundstrom P. 1996.
Developmental expression of a tandemly
repeated, proline- and glutamine-rich
amino acid motif on hyphal surfaces on
Candida albicans. J. Biol. Chem. 271:
6298–305
Staab JF, Sundstrom P. 1998. Genetic
organization and sequence analysis of
the hypha-specific cell wall protein gene
HWP1 of Candida albicans. Yeast 14:
681–86
Staib F. 1969. Proteolysis and pathogenicity of Candida albicans strains.
Mycopathol. Mycol. Appl. 37:345–48
Stein A, Schmaman A. 1965. Rupture
of the stomach due to mucormycosis. S.
Afr. J. Surg. 3:123–29
Stephens GL, Cronen MR. 1966. Burn
stress pseudodiabetes. J. Ky. Med. Assoc.
64:37–41
Stevens DA. 1989. The interface of mycology and endocrinology. J. Med. Vet.
Mycol. 27:133–40
Stover EP, Schar G, Clemons KV, Stevens DA, Feldman D. 1986. Estradiolbinding proteins from mycelial and
148.
149.
150.
151.
152.
153.
154.
155.
156.
157.
771
yeast-form cultures of Paracoccidioides
brasiliensis. Infect. Immun. 51:199–203
Sturtevant J, Latge JP. 1992. Participation of complement in the phagocytosis
of the conidia of Aspergillus fumigatus
by human polymorphonuclear cells. J.
Infect. Dis. 166:580–86
Sundstrom P. 1999. Adhesins in Candida
albicans. Curr. Opin. Microbiol. 2:353–
57
Swoboda RK, Bertram G, Delbruck S,
Ernst JF, Gow NA, et al. 1994. Fluctuations in glycolytic mRNA levels during
morphogenesis in Candida albicans reflect underlying changes in growth and
are not a response to cellular dimorphism. Mol. Microbiol. 13:663–72
Swoboda RK, Bertram G, Hollander H,
Greenspan D, Greenspan JS, et al. 1993.
Glycolytic enzymes of Candida albicans
are nonubiquitous immunogens during
candidiasis. Infect. Immun. 61:4263–
71
Tang CM, Smith JM, Arst HN Jr,
Holden DW. 1994. Virulence studies of
Aspergillus nidulans mutants requiring
lysine or p-aminobenzoic acid in invasive pulmonary aspergillosis. Infect. Immun. 62:5255–60
Thau N, Monod M, Crestani B, Rolland
C, Tronchin G, et al. 1994. Rodletless
mutants of Aspergillus fumigatus. Infect.
Immun. 62:4380–88
Thornton GF. 1971. Infections and diabetes. Med. Clin. North Am. 55:931–38
Tsuchimori N, Sharkey LL, Fonzi WA,
French SW, Edwards JE Jr, Filler SG.
2000. Reduced virulence of HWP1deficient mutants of Candida albicans
and their interactions with host cells. Infect. Immun. 68:1997–2002
Vargas K, Wertz PW, Drake D, Morrow
B, Soll DR. 1994. Differences in adhesion of Candida albicans 3153A cells
exhibiting switch phenotypes to buccal
epithelium and stratum corneum. Infect.
Immun. 62:1328–35
Vartivarian SE, Anaissie EJ, Cowart RE,
2 Aug 2001
11:11
772
158.
159.
160.
161.
162.
163.
164.
165.
AR
AR135-27.tex
VAN BURIK
�
AR135-27.SGM
ARv2(2001/05/10)
P1: GJB
MAGEE
Sprigg HA, Tingler MJ, Jacobson ES.
1993. Regulation of cryptococcal capsular polysaccharide by iron. J. Infect. Dis.
167:186–90
Vesper SJ, Dearborn DG, Elidemir O,
Haugland RA. 2000. Quantification of
siderophore and hemolysin from Stachybotrys chartarum strains, including a
strain isolated from the lung of a child
with pulmonary hemorrhage and hemosiderosis. Appl. Environ. Microbiol.
66:2678–81
Vesper SJ, Dearborn DG, Yike I, Sorenson WG, Haugland RA. 1999. Hemolysis, toxicity, and randomly amplified
polymorphic DNA analysis of Stachybotrys chartarum strains. Appl. Environ.
Microbiol. 65:3175–81
Wada M, Nakamura Y. 1996. Antigenic
variation by telomeric recombination
of major-surface–glycoprotein genes of
Pneumocystis carinii. J. Eukaryot. Microbiol. 43:S8
Wada M, Nakamura Y. 1996. Unique
telomeric expression site of majorsurface–glycoprotein genes of Pneumocystis carinii. DNA Res. 3:55–64
Wada M, Nakamura Y. 1997. cDNA
cloning and overexpression of cell surface subtilisin-like proteases (SSP) of
Pneumocystis carinii. J. Eukaryot. Microbiol. 44:S55–56
Wada M, Sunkin SM, Stringer JR, Nakamura Y. 1995. Antigenic variation by
positional control of major surface glycoprotein gene expression in Pneumocystis carinii. J. Infect. Dis. 171:1563–68
Westphal SA, Sarosi GA. 1994. Diabetic ketoacidosis associated with pulmonary coccidioidomycosis. Clin. Infect. Dis. 18:974–78
Witt W, Mertsching A, Konig E. 1984.
166.
167.
168.
169.
170.
171.
172.
Secretion of phospholipase B from Saccharomyces cerevisiae. Biochim. Biophys. Acta 795:117–24
Yamada A, Kataoka T, Nagai K.
2000. The fungal metabolite gliotoxin:
immunosuppressive activity on CTLmediated cytotoxicity. Immunol. Lett.
71:27–32
Yoshida LS, Abe S, Tsunawaki S.
2000. Fungal gliotoxin targets the onset
of superoxide-generating NADPH oxidase of human neutrophils. Biochem.
Biophys. Res. Commun. 268:716–23
Yu JJ, Smithson SL, Thomas PW,
Kirkland TN, Cole GT. 1997. Isolation
and characterization of the urease gene
(URE ) from the pathogenic fungus Coccidioides immitis. Gene 198:387–91
Yuan R, Casadevall A, Spira G, Scharff
MD. 1995. Isotype switching from IgG3
to IgG1 converts a nonprotective murine
antibody to Cryptococcus neoformans
into a protective antibody. J. Immunol.
154:1810–16
Yue C, Cavallo LM, Alspaugh JA,
Wang P, Cox GM, et al. 1999. The
STE12alpha homolog is required for
haploid filamentation but largely dispensable for mating and virulence in Cryptococcus neoformans. Genetics 153:1601–
15
Zhao XJ, McElhaney-Feser GE, Bowen
WH, Cole MF, Broedel SE Jr, Cihlar
RL. 1996. Requirement for the Candida
albicans FAS2 gene for infection in a rat
model of oropharyngeal candidiasis. Microbiology 142:2509–14
Zhao XJ, McElhaney-Feser GE, Sheridan MJ, Broedel SE Jr, Cihlar RL. 1997.
Avirulence of Candida albicans FAS2
mutants in a mouse model of systemic
candidiasis. Infect. Immun. 65:829–32