REVIEWS
THE EVOLUTION AND GENETICS
OF INNATE IMMUNITY
Deborah A. Kimbrell* and Bruce Beutler‡
The immune system provides protection from a wide range of pathogens. One component
of immunity, the phylogenetically ancient innate immune response, fights infections from the
moment of first contact and is the fundamental defensive weapon of multicellular organisms.
The Toll family of receptors has a crucial role in immune defence. Studies in fruitflies and in
mammals reveal that the defensive strategies of invertebrates and vertebrates are highly
conserved at the molecular level, which raises the exciting prospects of an increased
understanding of innate immunity.
CLONAL EXPANSION
The proliferation of a
lymphocyte clone bearing an
antibody or T-cell receptor that is
specific for a particular antigen.
CLONAL ELIMINATION
The removal, generally in the
thymus, of a T cell bearing a
receptor that recognizes molecules
within the host. Such cells, if not
eliminated, would otherwise cause
autoimmune disease.
GRANULOCYTES
White blood cells, encompassing
neutrophils, eosinophils and
basophils, which are dedicated
to the ingestion and destruction
of microorganisms (bacteria,
for example).
*Department of Molecular
and Cellular Biology,
University of California,
1 Shields Avenue, Davis,
California 95616-8535,
USA. ‡Department of
Immunology, The Scripps
Research Institute, 10,550
N. Torrey Pines Road,
La Jolla, California 92037,
USA. Correspondence to
D.A.K. e-mail:
[email protected]
256
The need for defence against microbial invasion poses
two fundamental problems for the host. First, there are
many would-be pathogens. How does the host recognize and destroy all of them? Second, how does the host
discriminate between the constituents of the external
world and the constituents of ‘self ’? In vertebrates,
defence against microbial invasion is subserved by two
distinct cellular systems: the acquired (‘specific’ or
‘adaptive’) immune system, and the innate immune system. In different ways, each system has solved both fundamental problems.
Acquired immunity is mediated by lymphocytes
(white blood cells), which have evolved to express an
enormous array of recombinant receptors — the
immunoglobulins and T-cell receptors — that are capable of recognizing any pathogen that the host might ever
encounter. Through CLONAL EXPANSION, cells bearing reactive receptors that are appropriate to the circumstance
are called into service; through CLONAL ELIMINATION, selftolerance is enforced.
The acquired immune response has arisen only
recently; it was built atop the phylogenetically older
innate immune system, by which it is controlled and
assisted. In the absence of an innate immune system,
the acquired immune response offers weak protection
— a circumstance that is all too evident in those
patients that have inadequate numbers of circulating
GRANULOCYTES. Innate immunity developed before the
separation of vertebrates and invertebrates, and most
| APRIL 2001 | VOLUME 2
multicellular organisms exclusively depend on it. It is
a system that acts effectively without previous exposure to a pathogen. Moreover, it confers broad protection against pathogens. Whereas the acquired
immune response takes days or weeks to develop
maximum efficacy, the innate immune response is
essentially instantaneous.
Although both immune systems were discovered at
around the same time, insight into the mechanisms used
by acquired immunity quickly outpaced a comparable
understanding of innate immunity. Ehrlich’s ‘antitoxins’1
(antibodies) were, after all, abundant proteins that were
known to circulate in the bloodstream, which made
them accessible to measurement and chemical analysis
from the time that they were first detected. The purification of antibodies, the determination of their primary
and tertiary structure and, ultimately, the triumphant
understanding of the rearrangements that occur to yield
antibody diversity, had partly been achieved by the early
1980s. By contrast, the ‘eyes’ of Metchnikoff ’s
phagocytes2 in the innate system have evaded detection
for a much longer time. The elucidation and insight into
innate immunity required very different methods and
different perspectives, drawn from two separate domains
of biology that involve insects and mammals.
The last common ancestor of insects and mammals
is thought to have lived more than half a billion years
ago. During that time, the physiology of insects and
mammals has diverged considerably: innate immune
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REVIEWS
a
b
Microglial
cell
Circulating
antimicrobial
peptides
Digestive
tract
Melanotic
cluster
Trachea
Alveolar
macrophage
Fat body
Lungs
Kupffer cell
Spleen
Liver
Stomach
Reproductive
system
Clotted
wound
Lymph
node
Needle
Haemocyte
Microbe inactivated by
antimicrobial peptides
Phagocytosed
microbe
Natural
killer cell
Figure 1 | The immune response in Drosophila and in mammals. a | In
Circulation
Drosophila, the hallmark of the humoral immune response is the induction of
antimicrobial peptides. A systemic infection induces the transcription of
antimicrobial peptides, mainly in the fat body of the fruitfly (blue), which is
analogous to the liver. These peptides are transported into the haemolymph
Granulocytes
(blood), where they accumulate to high concentrations and circulate
54
throughout the body . Some tissues respond directly to localized sites of
infection23,27. For example, the trachea (orange) — the respiratory system —
produces antimicrobial peptides in response to airway infections and the
reproductive system (yellow) produces antimicrobial peptides23,27,53. The
Monocyte
cellular immune response is characterized by the presence of haemocytes
(blood cells), which circulate or attach themselves to organs. A subset of
haemocytes produces antimicrobial peptides in response to infection.
Haemocytes mobilize throughout the body to phagocytose invading
microbes and to produce melanotic clusters, which are composed of melanized haemocytes that surround other cells. Melanotic clusters are formed when:
haemocytes encapsulate the egg of a parasitic wasp that has been injected into the HAEMOCOEL of a larva95; haemocytes form nodules around masses of
bacteria; abnormal haemocytes or haematopoietic organs form melanotic tumours, which if not lethal persist in the adults78; haemocytes respond to and
melanize around abnormal self tissue, which is generally not lethal and found in larvae and adults78–79,96. A systemic infection can be instigated in the
laboratory by puncture with a septic needle (as indicated in the figure) or in nature by a septic wound. In both cases, the site of wounding clots with a melanincontaining seal. Infection can also be instigated in nature through internal organs, such as bacterial infection through the larval gut by Erwinia carotovora97.
b | Macrophages are distributed throughout the mammalian body and, although macrophage cells are morphologically diverse, they fulfil similar functions.
The liver and spleen on the one hand, and the lymph nodes on the other, are particularly important stations for the interception of pathogens that circulate in
the blood or lymph, respectively. Macrophages engulf and destroy microbes upon first encounter, and also secrete cytokines that orchestrate both the innate
and adaptive immune responses. The dendritic cell, a specialized relative of the macrophage, presents antigens to lymphoid cells to stimulate adaptive
immunity. Neutrophils patrol the blood to detect pathogens, but rapidly attach to the walls of small blood vessels and leave the circulation by extravasation
when the body is threatened by an extravascular infection. Many cells can produce antimicrobial peptides, which guard epithelial surfaces against invasion
and also help to ensure the sterility of the plasma. In addition to the indicated epithelial surfaces, antimicrobial peptides also protect additional sites such as
the skin and the reproductive tracts.
CYTOKINES
A wide array of proteins,
functionally similar to classical
endocrine hormones, that
mediate signalling between cells.
Cytokines have a vital role in
communication between
different cells of the immune
system. Although usually
secreted, they might
occasionally be anchored to cell
surfaces. They often act at close
range, but also circulate and
exert their effects at a distance.
defence based on antimicrobial peptides predominates
in insects, whereas CYTOKINE-mediated inflammation
seems to hold sway in mammals. Was there reason to
suspect that they would have much in common? In fact,
innate defence is so fundamental that vertebrates, invertebrates and plants have many similarities. Genetic studies in Drosophila and mice were central to the appreciation of such similarities. In this review, we examine
common themes in the defensive strategies used by
Drosophila and mammals, and focus in particular on
Toll-receptor signalling.
NATURE REVIEWS | GENETICS
How is an invader recognized?
Invasive microbes of all types must be recognized,
regardless of the route by which they enter the host.
Where does such recognition take place, and what is the
crucial interface between the pathogen and the immune
system of the host? A multilaminar array of defences is
the rule in multicellular organisms and, in most, the first
tier of defence is a physical barrier (for example, the skin
in mammals and the exoskeleton in insects). Once this
layer of defence has been breached, various specialized
cells and tissues are responsible for the containment of
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REVIEWS
HAEMOCOEL
The haemocoel is the blood
space of arthropods and
molluscs. It is a very large,
blood-filled cavity, which
occupies most or all of the body.
HUMORAL IMMUNITY
B-cell-mediated immunity in
mammals that fights bacteria
and viruses in body fluids with
antibodies that circulate in
blood plasma and lymph, fluids
formerly called humours. In
insects, humoral immunity
refers to the immune response
that produces antimicrobial
peptides, particularly at high
concentrations in the
haemolymph (blood).
MACROPHAGES
Phagocytic cells that respond to
non-self material (for example,
bacteria, protozoa or tumour
cells) to release substances that
stimulate other cells of the
immune system. They are also
involved in antigen presentation
and are derived from
monocytes, which circulate in
the blood.
GRAM-NEGATIVE
Bacteria that fail to take up
Gram stains during histological
preparation for identification:
typically bacteria present in the
bowel rather than the throat
and respiratory tissues.
Examples of infections caused
by this class of bacteria include
the plague, tularemia, cholera
and typhoid fever.
REL ONCOGENE
The Rel oncogene was
originally found in an avian
reticuloendotheliosis virus;
it is the prototype of a family
of transcription factors that
includes NF-κB, c-rel, Relish,
Dif and Dorsal.
GRAM-POSITIVE
Bacteria that take up Gram
stains during histological
preparation for identification:
typically bacteria present in the
throat and respiratory tissues.
Infections caused by this class
of bacteria include anthrax
and listeriosis.
258
infection, and constitute the next tier of defence (FIG. 1).
In Drosophila, for instance, haemocytes (blood cells) are
the bulwark of the cellular response, and the fat body
(analogous to the liver) is the main site of the HUMORAL
IMMUNE RESPONSE, which produces abundant antimicrobial proteins. In mammals, MACROPHAGES and other
phagocytic cells patrol the blood and interstitial tissues,
and are found at their highest density in specialized
lymphoreticular organs (the lymph nodes and spleen).
Recognition of infection by this second tier of
defence lies at the centre of innate immunity. Here, as
in most biological systems, recognition implies the
existence of specific receptors. Microorganisms have
various features that distinguish them from multicellular organisms — features that have been exploited by
the innate defence system to recognize and combat
pathogens. These features are known as ‘microbial patterns’ and their detectors are defined as ‘pattern-recognition receptors’3. Examples of microbial patterns
include β-1,3-glucan of fungi, peptidoglycan and
lipopolysaccharide (LPS) of bacteria, and phosphoglycan of parasites.
Many pattern-recognition receptors have been identified in vertebrates and in invertebrates. For example,
the β2 integrin CR3 (also known as ITGB2 in humans)
is a broad-range pattern-recognition receptor4.
Scavenger receptors, known in mammals as a family of
proteins with diverse ligands, have many counterparts
in Drosophila5,6. In insects other than Drosophila, the
peptidoglycan-recognition protein (PGRP) has been
identified in Trichoplusia ni 7, and the GRAM-NEGATIVEbinding protein (GNBP) has been isolated from both
the silkmoth Bombyx mori 8 and the mosquito
Anopheles gambiae 9. In Drosophila, a family of PGRP
genes and one GNBP gene have been identified10,11.
Lectins (the mannose-binding protein, for example),
which have carbohydrate-recognition domains, are also
common in vertebrates and are increasingly being
found in invertebrates12. The Drosophila genome project has revealed many other genes of these types13, but
none has been shown to directly mediate an immune
response. The best candidates include the PGRP family
and GNBP, as some of the PGRP proteins bind peptidoglycan10 and GNBP confers a response in Drosophila
cells in tissue culture11.
Pattern-recognition receptors are not necessarily cell
associated. The mammalian LPS-binding protein (LBP)
is one example. This circulating protein, produced by
the liver, delivers LPS to the surface of macrophages,
permitting the quick detection of a Gram-negative
infection. But it does not directly alert cells. Mannosebinding protein, and soluble versions of CD14 (a lipidanchored membrane protein), are other examples of
‘peripheral’ sensors that detect host invasion.
Drosophila Toll protein
The most striking paralogous system for innate-immunity sensing is the array of Toll family receptors, of
which Drosophila Toll is the prototype. Toll is a singlepass transmembrane receptor with an ectodomain
marked by leucine-rich repeat motifs. The Toll gene
| APRIL 2001 | VOLUME 2
was originally identified for its essential role in development as the determinant of dorsoventral polarity in the
fruitfly embryo (BOX 1). In addition, Toll was found to
have a crucial role in immune defence. This second
function of Toll was discovered by investigating the
transcriptional regulation of antimicrobial genes that
are induced by infection. Subsequently, nuclear factorκB (NF-κB) immune signalling, first identified in
mammals, was suggested to occur in insects because
NF-κB-binding motifs were identified in promoters of
immunoresponsive genes in the silkmoth (Hyalophora
cecropia) and in Drosophila14. Several additional common features further supported the possibility of a NFκB-related system in Drosophila. For example, Dorsal
and Cactus, which are downstream members of the
Toll dorsoventral cascade, are homologous to mammalian NF-κB and to its inhibitor IκB, respectively.
Cactus and Dorsal also associate with one another in a
manner similar to that of NF-κB and IκB, to produce a
latent transcription factor. Subsequently, Toll and the
entire dorsoventral regulatory gene cassette were tested
directly for involvement in the Drosophila immune
response and found to control the response to fungal
infection15. Flies deficient in Toll were so severely
immunocompromised that a fungal infection was
lethal and correlated to the lack of proper induction of
the antifungal protein gene Drosomycin. By contrast,
Toll mutants showed normal rates of survival when
exposed to bacterial infection, although they showed a
reduced induction of antibacterial protein genes.
Therefore, the regulation of the genes induced by infection seemed to be quite complex15.
At least part of this complex regulation could be
understood from the results of previous studies on the
gene imd (immune deficiency)16. Flies that were mutant
for imd had an immunocompromised response to bacteria, but had normal resistance to fungal infection16. So,
at least two pathways of immune response were established: one mediated by Toll and the other by Imd.
In addition to dorsal, two more genes with homology to the NF-κB/REL ONCOGENE were identified. These
are Dif (Dorsal-related immunity factor) and Rel
(Relish)17,18. In parallel to NF-κB/REL proteins in
mammals, Dorsal, Dif and Rel seem to function as
homodimers and heterodimers in the regulation of the
immune response19. Altogether, the results indicate
largely separate but partially interrelated pathways for
signalling that involve Toll and Imd, respectively, converging upon REL-domain-containing proteins. More
specifically, the Toll dorsoventral regulatory cassette
and Dif mainly respond to fungal and GRAM-POSITIVE
bacterial infections, whereas Imd and Rel mainly
respond to Gram-negative bacterial infections15–19.
This has been confirmed and extended by the findings
that Dif mutants are sensitive to fungal infection20, and
that homologues of the mammalian IκB kinase (IKK)
complex regulate Rel in the antibacterial response21,22
(see figure in BOX 1). Additionally, the response of
antimicrobial peptide-encoding genes is tissue-specific, and in particular imd, rather than Toll, regulates
Drosomycin in the respiratory tract23.
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REVIEWS
Box 1 | Toll and Toll-like receptor signal transduction
a | Drosophila development and immunity. Toll was first identified in the
landmark genetic screens of Christiane Nusslein-Volhard and Eric Weischaus for
mutations that alter embryonic patterning in Drosophila51,52. The original signal
Serine
for dorsoventral polarity is maternally derived and is relayed by a signalNec
protease
transduction pathway into the embryo to produce a nuclear morphogen
Imd
gradient51,52 (the members of the classical signal-transduction pathway are
or
Spätzle
indicated in red type in part a). Toll is the cell membrane component of the
IKK
system
and, as such, is centrally positioned in the cascade of events that lead to
Toll
dorsoventral axis formation. Downstream from Toll are Pelle, a protein kinase,
β γ
receptor
Cell
and Tube, both of which have domains characteristic of cell-death signalling
membrane
proteins (indicated as ‘d’ for death domain). The last member of the pathway is
Tube d d Pelle
encoded by the dorsal gene. Upon Toll signalling, Dorsal (a Rel-homology
Rel
domain protein) dissociates from the Dorsal–Cactus complex. This leads to the
degradation of Cactus and the translocation of Dorsal into the nucleus, where it
X
X
induces the transcription of zygotic genes. Ultimately, the nuclear gradient of
Dorsal across the embryo sets the dorsoventral pattern of embryonic structures.
These, plus additional genes (black type), are also used in immunity to direct the
Cactus Dorsal Dif Cactus
expression of immune-response target genes.
In fact, the signalling pathways involved in the immune response are much more
complicated, as the Imd and Rel pathways have also been found to mediate the
DNA
Nucleus
immune response. The canonical Toll dorsoventral regulatory cassette and that
Immune response target genes
mediated through the Rel-oncogene domain protein Dif mainly respond to fungal
Developmental target genes
and Gram-positive bacterial infections, whereas Imd and Rel (also a Rel-oncogene
domain protein) mainly respond to Gram-negative bacterial infections15–19.
b
b | Signal transduction. The immune pathways of Drosophila (blue in part b) that
Ligand/
Lipopeptides
LPS
were shown in panel a are compared with the pathways in mammals (green). The
inducer:
Peptidoglycan
Gram-negative bacteria
Fungi
term ligand refers to the molecules that bind to the mammalian receptors, whereas
Gram-positive bacteria
inducer refers to the the molecules that activate Drosophila receptors because
Spätzle (which binds to Toll-1) is the only confirmed ligand in Drosophila. It is
commonly asserted that Toll-like receptors (TLRs) are dimeric proteins, although
Receptors: TLR2
TLR4
it is uncertain whether they are all homodimers, or whether some heterodimers
Toll1
?
exist. Enforced dimerization of TLR4 leads to transduction of a signal in vitro29, as
do polyclonal antibodies against TLR485. Moreover, a multimeric structure of
Signalling: MyD88
MyD88
TLR4 would account for the dominant-negative phenotype of a Tlr4 mutation in
IRAK
IRAK
the C3H/HeJ strain of mice. It is also widely believed that MYD88 transduces the
TRAF6
TRAF6
signal from all ten mammalian TLR family members, although this has not been
NIK/IKK
NIK/IKK
Tube/Pelle
Imd
tested for all of the receptors, nor can it be until their specific ligands have been
IKKB/IKKγ
identified. In mammals, MYD88 is presumed to associate with the interleukin-1
receptor-associated kinase (IRAK), which is homologous to Drosophila Pelle.
IRAK might be recruited to the MYD88–TLR complex through the action of
Targets:
IκB/NF-κB
IκB/NF-κB
another protein, known as Tollip86. The signal from IRAK is carried, perhaps in
Dorsal/Dif/Cactus
Rel
conjunction with TRAF6, to TAK1, NIK and finally, IκB. Degradation of IκB
permits the nuclear translocation of NF-κB (a homologue of the Drosophila Dorsal protein), and ultimately, the transcription of many genes that are
NF-κB dependent. Among these are genes that encode cytokines, which mediate inflammation and signal the activation of adaptive immunity. The
sequence of events as outlined is incomplete because it is clear that some activating events can occur in the absence of MYD88 (REF. 87) and of IRAK.
Other, unknown, proteins might function as part of the activation complex for individual TLRs, or perhaps for all TLRs collectively. In fact, a human
mutation (in an uncloned locus) seems to abrogate signalling pathways as diverse as those initiated by TLRs, such as the TLR4 and the IL-1 receptor88.
(Dif, Dorsal-related immunity factor; IκB, nuclear factor (NF)-κB inhibitor; IKK, IκB kinase; Imd, immune deficiency; Nec, Necrotic (a
Drosophila serine protease inhibitor); NIK, NF-κB-inducing kinase; Rel, Relish; TAK1, TGF-β-activated kinase 1; TRAF6, tumour-necrosisfactor-receptor-associated factor 6.)
a
Fungi
Gram-positive
bacteria
Gram-negative
bacteria
Surprisingly, the ligand for Toll in the immune
response is not a microbial product, and so Toll is not
a pattern-recognition receptor24. Instead, Toll is activated by the same ligand in the antimicrobial
response as in the determination of dorsoventral
polarity24. This ligand is the proteolytically cleaved
product of spätzle, which encodes a growth factor of
the cysteine knot family. Interestingly, the processing
of Spätzle by an extracellular serine protease is nega-
NATURE REVIEWS | GENETICS
tively regulated by a serine protease inhibitor of the
serpin family, encoded by Spn43Ac at the nec locus
(necrotic)24. In Spn43Ac-deficient mutants, the antifungal protein gene Drosomycin is constitutively
active, unless the mutant is also deficient for spätzle or
Toll. Toll signalling thus involves negative regulation
by this inhibitor24, indicating that the microbial activation of Toll signalling might also proceed by the
inactivation of Spn43Ac.
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REVIEWS
The Drosophila Toll receptor is one of a family of Toll
homologues, which includes Toll (Toll-1), 18 wheeler
(18w or Toll-2) and Toll receptors 3–9 (REFS 25, 26). The
18w gene functions in development and in the response
to bacterial infection25. Out of Tolls 1–8, only Toll-1 and
Toll-5 can induce antimicrobial protein genes in a cellculture assay26, and these receptors only induce an antifungal response. Furthermore, Toll-5 seems to signal
through the Toll dorsoventral signalling pathway. In
contrast to systemic fungal infection, which uses the
Toll-1 pathway, local fungal infections do not depend on
Toll-1 or Spätzle signalling27. In principle, Toll-5 might
mediate the local response, but would not depend on
Spätzle for this purpose. As such, Toll-5, unlike Toll-1,
might be a pattern-recognition receptor. An alternative
hypothesis might be that an antifungal response signals
through a pathway that does not involve the Toll family.
Out of the remaining Toll genes, Toll-6, Toll-7 and Toll-8
are present at high levels during embryogenesis and
moulting, which indicates that these, like Toll and 18w,
might have a role in development26. As Tolls 3–9 have
only been identified recently, mutants are not yet available to test the respective functions of these receptors.
Mammalian Toll-like receptors
The first mammalian protein that was shown to have
homology to Toll was the interleukin-1 receptor (IL1R)28. This receptor signals through the IL-1R-associated kinase (IRAK), a homologue of the Drosophila
dorsoventral protein kinase Pelle (BOX 1). IRAK activates
NF-κB translocation in the same way that Toll activates
a REL family member (later shown to be Dif 20). In spite
of this, and although IL-1R has an undoubted role in
immunity, it does not act as a primary sensor of infection. Instead, IL-1 is produced by macrophages that
have previously sensed infection. Moreover, in structural terms, IL-1R and its close homologues in the mammalian genome (IL-18R and ST2) differ from Toll in
their ectodomains, which are derived from the
immunoglobulin superfamily and do not contain
leucine-rich repeat motifs.
In the late 1990s, interest in the immune function
of Toll in Drosophila inspired a directed search for
other mammalian genes that are related to Toll. This
search was facilitated by the advent of expressed
sequence tag (EST) databases, which permitted
homologous genes to be quickly identified and cloned.
By the beginning of 1998, five Toll-like receptors
(TLRs) had been cloned from mammalian cDNAs on
the basis of the observed homology between ESTs and
Drosophila Toll. However, the function of these TLRs
was not clear. More than 500 million years of divergence between Drosophila and mammals has drastically altered the function of many proteins and eliminated many others from one or the other species.
However, in the realm of innate immunity, it might
rightly have been pointed out that some things had not
changed. The antimicrobial peptides of Drosophila were,
after all, present in mammals (porcine cecropin, for
example) and, among its many functions, NF-κB was
well known to be important in mammalian innate
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immunity, especially insofar as it assisted in the transcription of many cytokine genes. Moreover, one representative of the leucine-rich Toll-like receptor family (hToll; later called TLR4) elicits NF-κB translocation29.
This reaffirmed that the Toll-like receptors might participate in the inflammatory response, but again, gave no
clue as to their precise function.
Attempts to establish function by expressing Toll-like
receptors at high levels in mammalian cells gave the
wrong impression of their function. Indeed, it was
claimed (incorrectly) that mammalian Toll-like receptor
2 (TLR2) could transduce the LPS signal30,31, a proposition that was ultimately rejected on the basis of knockout and positional cloning studies in mice (see below).
Hence, the TLRs remained orphans and, ultimately, the
primary sensors of innate immunity in mammals were
identified using a purely genetic approach.
This approach grew from a biological puzzle of long
standing, related to the responses of whole animals to
infection by Gram-negative bacteria, and more precisely, to the response to LPS. LPS was widely known as an
activator of innate immunity par excellence, and had
been structurally defined from several species of Gramnegative bacteria32. When administered to most mammalian species, LPS causes the prompt development of
fever, disturbances in the clotting of blood (hypercoagulability in some parts of the vascular tree and haemorrhage elsewhere), hypotension and shock33. All these
effects are mediated by the activation of
macrophages34,35, which in turn release toxic cytokines,
notably tumour necrosis factor (TNF). This factor is
known to provoke the release of terminal constituents
of the inflammatory cascade36, and is considered to be
largely responsible for LPS-induced lethality.
In mice, and by implication perhaps in all mammals,
genetic evidence favoured the existence of a single pathway to transduce the LPS signal37,38. So, spontaneous
mutations at the Lps locus in C3H/HeJ mice and in
C57BL/10ScCr mice entirely abolish responses to LPS.
Although such animals survive the administration of
any dose of LPS, they are highly susceptible to infection
by Gram-negative bacteria, succumbing over a period of
days to the injection of small numbers of Salmonella
typhimurium, which seem to grow without restriction
in the liver and spleen39. It therefore seems that LPS
sensing is of crucial importance for innate immune
recognition of Gram-negative infection.
The pathway through which LPS signals has been
investigated extensively at a biochemical level. The
LPS is conveyed to the surface of cells by the acutephase reactant LBP. There, it binds to the glycosylphosphoinositol-anchored membrane protein
CD14. However, as CD14 lacks a transmembrane
domain, how the LPS signal might be transduced
across the membrane remained unclear. The genetic
lesions of C3H/HeJ and C57BL/10ScCr mice seemed
to offer a clue to the puzzle.
The Lps mutations of these mice were identified by a
strict positional cloning approach and were found to
affect the Tlr4 gene40,41. In C3H/HeJ mice, a point mutation (P712H) modified the cytoplasmic domain of the
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Figure 2 | Evolution of the Toll interleukin-1-receptor domain. Two great divisions
in the evolution of the Toll interleukin-1-receptor (TIR) domain are immediately evident
in the phylogenetic tree, as Metazoa (brown) and Viridiplantae (green) show only weak
TIR homology at the protein level. TIR domains are found in most plant and animal
taxa. In early metazoans, at least three lines of TIR became established. These gave
rise to most of the Drosophila Tolls (1–8; a relatively primitive offshoot), to the IL-1R/IL18R/ST2/MYD88/SIGIRR cluster, and to the mammalian TLR cluster, in which
Drosophila Toll-9 is also represented. Estimates of divergence dates include the
mammalian radiation (~120 Myr); the divergence of birds and mammals (~310 Myr);
the divergence of insects and mammals (~600 Myr); and the divergence of plants and
animals (~1,800 Myr). Most sequences for plant disease resistance proteins (green)
came from Arabidopsis thaliana (AT). Also included are sequences (obtained from
GenBank and SMART) from Linum usitatissimum (LU), Solanum tuberosum (ST) and
Nicotiana tabacum (NT). The animal sequences (brown) used were derived from Mus
musculus (MUS), Homo sapiens (HUM), Equus caballus (EQU), Gallus gallus (GAL),
Drosophila melanogaster (DM), Rattus norvegicus (RAT), Drosophila pseudoobscura
(DP) and Schistocerca americana (SA). The plot was made using the MAXIMUM
PARSIMONY method, and the phylogram is linear with respect to the time of divergence.
Calculations were performed using GCG software.
AT-O81747.-3
AT-F16620.210
AT-T1308.20
AT-AC00418
AT-F23E13.40
AT-F24J7.90
AT-O23001
AT-T8F5.18
AT-AC002342
AT-F16G20.210
AT-RPP1
AT-Q9ZSN4
AT-Q9ZSN5
AT-RPP1
AT-T22B4.150
AT-AC005167
AT-Z97342
AT-O04264
AT-AL161545
AT-Z97342
AT-Z97342
AT-Z97342
AT-T24H24.18
AT-F24J7.60
AT-A71438
AT-F23E13.30
AT-F24J7.80
AT-AC002354
AT-T7N9.24
AT-T7N9.23
AT-T7N9.24
AT-004565-2
LU-L6TR
LU-U73916
ST-NL25
ST-NL27
NT-U15605
AT-O49469
AT-F24J7.60
MUSTLR9
HUMTLR9
MUSTLR8
HUMTLR8
MUSTLR7
HUMTLR7
MUSTLR5
HUMTLR5
HUMTLR3
HUMTLR4
EQUTLR4
MUSTLR4
MUSTLR2
HUMTLR2
GALTLR2
MUSTLR6
HUMTLR6
HUMTLR1
HUMTLR10
DMTOLL9
RATFIT1M
MUSST2
MUSIL1RAP
HUMIL1RAP
RATL1RAP
MUSIL1RRP
HUMIL1RRP
RATIL1R
MUSIL1R
HUMIL1R
RATL1RRP2
HUMIL1RRP2
GALIL1R
MUSIL18RAP
HUMIL18RAP
HUMSIGIRR
HUMMYD88
DMMYD88
DMTOLL7
DMTOLL2
DMTOLL6
DMTOLL4
SATOLL
DMTOLL3
DPTOLL
DMTOLL1
DMTOLL5
DMTOLL8
MAXIMUM PARSIMONY
A mathematical method
for determining the
evolutionary relationship
between proteins, wherein
account is taken of the
minimum number of
mutations that are required
to effect transition from
one member of the family
to another.
Last common ancestor of:
Plants/animals
2,000 Myr ago
NATURE REVIEWS | GENETICS
1,500 Myr ago
Insects/mammals
1,000 Myr ago
Birds/mammals
500 Myr ago
Mammalian
TLR cluster
IL-1/IL-18/
ST2/SIGIRR/
MYD88 cluster
Drosophila
Toll cluster
Mice/human
Present
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Tlr4 protein, creating a dominant-negative effect42. In
C57BL/10ScCr mice, the locus was deleted entirely43,
leading to a recessive failure of the LPS response. Later, a
knockout mutation of Tlr4 (REF. 44) proved to be an
excellent phenocopy of the C57BL/10ScCr mouse in
terms of LPS response, confirming that the gene is
required for LPS signal transduction.
In further genetic studies of LPS responses, it was
demonstrated that the species origin of Tlr4 was
entirely responsible for species-specific responses to
certain LPS partial structures45,46. For example,
although a tetra-acylated form of lipid A (the toxic
centre of LPS) elicits a strong response from mouse or
hamster macrophages, it behaves as a pure antagonist
of LPS when applied to human cells. Correspondingly,
mouse and hamster Tlr4 is capable of responding to
tetra-acylated lipid A, whereas human TLR4 is not
and, indeed, is blocked by tetra-acylated lipid A in its
ability to respond to LPS. This observation effectively
proves that Tlr4 has direct contact with LPS.
During 1999 and 2000, knockout mutations of Tlr2
and Tlr9 revealed that the former acts as a specific
transducer for bacterial lipopeptide and peptidoglycan
signals47, whereas the latter acts to detect bacterial
DNA48. The general impression of TLR function,
therefore, is one in which each of the TLRs recognizes
a discrete subset of those molecules that are widely
shared by microbial pathogens. In this way, ten TLRs
collectively provide protection against an immense
number of microorganisms.
Evolution of Toll-like receptors
BILATERIA
Members of the animal
kingdom that possess bilateral
symmetry — the property of
having two similar sides, with
definite upper and lower
surfaces, and anterior and
posterior ends.
PROTHROMBIN
(clotting factor II). One of the
13 chemical components of the
blood that create the clotting
mechanism. Prothrombin is a
blood plasma protein and is
synthesized in the liver.
COMPLEMENT
Group of blood proteins that
circulate and reside in the
tissues, the actions of which
‘complement’ the work of
antibodies. They ‘burst’ bacteria
by creating pores in the
bacteria’s membrane.
Complement proteins can also
cover the surfaces of bacteria
and act as flags for phagocytes.
Proteolytic cleavage fragments
of complement proteins act as
local signals for inflammation.
262
The protein motif shared by Toll and IL-1R, or TIR, is
evident in plant proteins as well as in animal
proteins49,50, so this motif might be traceable to the origins of eukaryotic life, that is, between one and two billion years ago. In plants, the TIR motifs fulfil well-established but mechanistically undefined defensive
functions, protecting plants from infection by fungal,
viral or bacterial pathogens. Plant proteins that bear TIR
domains are cytoplasmic, and the TIR domain is generally located at the amino-terminal end of the polypeptide chain, rather than at the carboxy-terminal end, in
contrast to the arrangement of TIR domains in animal
proteins. Plant TIRs are often present in conjunction
with a nucleotide-binding domain or, alternatively, with
leucine-rich repeat motifs.
In animals, the modern descendants of the TIR
superfamily were established within the BILATERIA,
which includes the last common ancestors of humans
and flies. Most mammalian TIR domains are a part of
membrane-anchored proteins (the transducer MYD88
(myeloid differentiation primary response gene 88)
being the exception). The fact that a close relative of
MYD88 (SIGIRR — single immunoglobulin (Ig) IL1R-related molecule) is a transmembrane protein
(although one that has acquired Ig-type repeats) might
be evidence that transducers have, even in recent
times, been able to acquire constitutive membrane
localization. MYD88 and SIGIRR share a proximal
common ancestor with the IL-1R/IL-18R/ST2 family
| APRIL 2001 | VOLUME 2
of TIR-bearing receptors. In Drosophila, two lines of
leucine-rich Toll receptors are represented: Tolls 1–8
and Toll-9. The Toll-9 receptor resembles the mammalian TLRs more closely than do any of the other
Drosophila Tolls. Among mammals, only a single type
of TLR survives (FIG. 2).
There is a fundamental difference in the use of Tolls
(by insects) and TLRs (by mammals). On the basis of
present evidence, it is known that some Tolls respond
to an endogenous protein ligand that is generated by
proteolytic cleavage, as stated above for Toll-1, and
have no direct contact with microbial molecules,
whereas mammalian TLRs are pattern-recognition
receptors. A final discrepancy between the Tolls and
the TLRs relates to the dual function ascribed to the
former proteins. As described above, Toll and 18w have
developmental as well as immune functions, and other
Drosophila Toll genes might be similar in this respect.
By contrast, no representative of the TIR family is
believed to be essential for development in mammals.
Although the archetypal function of TIRs seems to be
a defensive one, in the sense that both plants and animals broadly apply the TIR for this function,
Drosophila has adopted the TLR for developmental
tasks as well. Further studies will discern whether
mammalian TLRs also have developmental functions,
and if this current discrepancy between insects and
mammals is meaningful.
An intriguing question is raised by this last observation. To permit the normal development of
dorsoventral polarity, the Toll ligand Spätzle is
cleaved from pro-Spätzle in the extracellular space by
the active form of the protease Easter 51,52. Easter is
activated by Snake, which in turn is activated by Gd
(Gastrulation defective). Gd, which is activated by
Pipe, Nudel and Windbeutel, has the strongest
homology to mammalian PROTHROMBIN, and Snake
and Easter resemble several proteins known to participate in the coagulation and COMPLEMENT cascades and,
therefore, in the inflammatory response. Do development and inflammation then share a genuine functional ancestry? Insofar as proteins might be ‘captured’ to discharge tasks wholly different from those
that they originally evolved to serve, no clear answer
can be given.
What is the response to infection?
Insects have three general innate responses (FIG. 1): the
synthesis of antimicrobial peptides; phagocytosis or
encapsulation of invading organisms by haemocytes;
and the initiation of proteolytic cascades, leading to
clotting and melanization. In Drosophila, several signalling pathways other than those that involve the Toll
family and imd genes have been implicated in the
response to infection, for example the JNK (jun
kinase) and JAK/STAT (janus kinase/signal transducers and activators of transcription) pathways (see
below and TABLE 1).
The main effectors of the insect humoral immune
response are the antimicrobial proteins, the study of
which led to the first area of insect immunity to be
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Box 2 | Apoptosis and the immune response
Cell death represents a threat to an organism because of the potential release of
harmful cellular components. There is mounting evidence of a similarity between
the mechanisms for dealing with this type of internal threat and that of the external
threat towards which innate immunity is directed. Because both immunity and
apoptosis involve engulfment of cells by macrophages, the connection is logical but
highlights the problem of self versus non-self recognition. The consideration of
response to internal and external threats as part of the same overall process of
defending the organism has been termed the ‘danger model’89.
In Drosophila, the transduction of cell-death signals and the induction of an
antimicrobial response has recently been connected by the dual function of the
CASPASE Dredd (Death-related ced-3/Nedd2-like protein), which was first shown to
be an effector of apoptosis90, and now has been identified as a participant in the
immune response91,92. More specifically, Dredd is a regulator that is essential for
resistance to Gram-negative bacterial infections92. Additionally, dredd, imd
(immune deficiency) and Rel (Relish) might define a pathway that is required for
resistance to Gram-negative bacteria92.
In mammals, the caspase cascade is activated by interactions between plasma
membrane receptors and cytoplasmic proteins such as FADD (Fas-associated
death-domain protein) — sometimes itself activated through interaction with
another death-domain protein, TRADD, which associates with the tumour necrosis
factor (TNF)-receptor. FADD triggers the activation of the procaspase 8 (FLICE)
directly or through Apaf1, which triggers activation of procaspase 9. Both a
Drosophila homologue of FADD (REF. 93) and a Drosophila homologue of Apaf1,
known as Dark (REF. 94), are reported to engage Dredd, thereby triggering apoptosis.
Therefore, a system with similarity to the Fas/TNF/LT (lymphotoxin) signalling
cascades is present in flies and can be tested further for connections between immunity
and apoptosis.
CASPASE
Enzymes that are responsible
for the breakdown of the cell
during apoptosis by cleaving
numerous cellular proteins.
They are synthesized as inactive
procaspases that are later
activated by proteolytic cleavage
into active caspases.
PUPARIUM
A case formed by the hardening
of the last larval skin, in which
the pupa is formed.
identified molecularly. The path of discovery in insect
immunity has generally been from effector to sensor.
The first studies found and characterized antibacterial
proteins; then followed the identification of their corresponding genes and studies that focused on the regulation of induction of antimicrobial genes; finally,
came the studies on immune recognition. All these
arenas of inquiry have now broadened and branched
out into new areas.
The Drosophila antimicrobial proteins are
Cecropin, Diptericin, Defensin, Attacin, Drosocin,
Drosomycin and Metchnikowin, with the latter two
being the most strongly antifungal and the remaining
mainly antibacterial. In addition, Andropin is a constitutively expressed male-specific antibacterial peptide, and Lysozyme is produced in abundance.
Cecropin, Diptericin and Drosomycin are encoded by
two or more genes in a cluster, whereas Defensin,
Drosocin and Metchnikowin are encoded by single
genes, Andropin is encoded by a single gene in the
Cecropin cluster, Lysozyme is encoded in unlinked
clusters and Attacin is encoded by a pair and a single
copy on the same chromosome arm13,53,54. Genes that
encode Drosophila antimicrobial protein effectors are
typically induced in response to infection, which
leads to an accumulation of the corresponding
mRNA. So, too, are genes that encode proteins with
separate functions in insect innate immunity. For
example, Toll and other genes in the signalling cascades, and recognition-receptor genes such as
galactin and some members of the Pgrp family, are
also induced by infection10,15,25,55.
NATURE REVIEWS | GENETICS
As in Drosophila, mammalian antimicrobial peptides protect epithelial surfaces and fluid compartments and, in some instances, also reside within the
intracellular compartment. The emplacement of such
defensive proteins, many of which have structural
similarity across broad taxa, bespeaks a conserved
evolutionary strategy 23,27,54,56,57. In general, mammalian antimicrobial proteins are produced constitutively, whereas insect antimicrobial proteins are typically induced by infection. One of the exceptions in
mammals is bovine lung defensin, which is not only
induced by infection but also has a promoter with
NF-κB and Il-1 binding sites similar to the canonical
promoters of insect antimicrobial protein genes 58.
Antimicrobial proteins cause stasis or lysis of the target organisms59. Importantly, antimicrobial proteins
can have additional effects, in particular a role in
feedback regulation of cytokine responses, which
makes antimicrobial proteins even more important
as therapeutic agents56,59.
In vertebrates, the inflammatory response is the
hallmark of an innate immune reaction. Inflammation
is an extremely complex phenomenon, which in the
classical description entails swelling due to the extravasation of fluid (hyperaemia), the recruitment and activation of leukocytes, and the destruction and remodelling of tissues. It is orchestrated by cytokines, which are
produced by host cells (chiefly tissue macrophages)
that encounter infectious organisms. There is a broad
consensus that inflammation limits the spread of
infectious organisms, although a precise understanding of how it achieves this end remains elusive.
Therefore, agents that prevent inflammation (for
example, glucocorticoid hormones), and even selective
inhibitors of individual cytokines60,61 might lead to
overwhelming infection.
In Drosophila, haemocytes are the hallmark of the
cellular immune response. Two general types of
haemocyte mediate the cellular response in flies: crystal
cells and plasmatocytes5,62,63. Crystal cells release their
crystalline contents, for example to provide substrates
for melanin, blood clotting or tanning of the PUPARIUM.
The plasmatocyte lineage functions as phagocytic cells
that are comparable to vertebrate macrophages and
natural killer cells. In contrast to embryonic haemocytes, larval haemocytes originate from the
haematopoietic organ (lymph glands), and it is thought
that adult haemocytes are carried over through metamorphosis. During the immune response, haemocytes
mobilize to phagocytose cells, to produce antimicrobial
proteins and to form melanotic clusters (FIG. 1). The
phenotype of melanotic clusters, or tumours, is shared
by mutations in various genes involved in immune
function. Well-studied examples of these are Tumorous
lethal (Tum) and lethal(1)aberrant immune response8
(RpS6air8). Tum is an oncogenic mutation in the janus
kinase gene hop (hopscotch) that results in a leukaemia
phenotype (hopTum )64. Further, in hopTum mutants the
complement-like TepI (thiolester-containing protein I)
gene is constitutively activated65. RpS6air8 is a tumoursuppressor mutation in the Drosophila homologue of
© 2001 Macmillan Magazines Ltd
VOLUME 2 | APRIL 2001 | 2 6 3
REVIEWS
the human ribosomal protein S6 (RpS6), which also
causes haematopoietic system overgrowth66. In addition, mutations in genes that encode proteins that are
known to support developmental processes and immunity, such as Toll (REF. 67), are also often found to cause
the formation of small melanotic tumours. Analysis of
specific melanotic tumour mutations and of the
process of melanotic cluster formation, offers clues for
understanding haemocyte function and immune
recognition and response.
Table 1 | Immune-related genes in Drosophila and mammals: homologies and analogies
Haematopoietic
determinants
Transcription factors
Receptors
Scavenger receptors (SR)
Gram-negative-binding
proteins (GNBP)
Peptidoglycan-recognition
proteins (PGRP)
Lectins
Toll family
Drosophila
Mammals*
References§
lozenge
serpent
gcm
AML1
Gata family
Gcm family
98
98
98
epithelial membrane protein
SR-Cl
GNBP family
CD36
Domains of SRC1 have homology
to various mammalian proteins;
see also SR family
Not identified
PGRP family
Pglyrp family
galactin
See text
Lgals family
See text
dorsal, Dif, Rel
cactus
spätzle
pelle
snake and easter
IKK and kenny
NF-κB family
17–19,101
IκB family
102
Bdnf; homology is to cysteine
103
knot nerve growth factors
Spi17 and others that encode serine
24
protease inhibitors of the serpin family
F2/prothrombin and others that
104,105
encode clotting factors
IRAK1
106
Prss7 and Klk/kallikrein family
105
IKK family
21,22
hopscotch
Stat92E/marelle
raf
basket
p38b
JAK family
STAT family
Raf1
JNK family
p38
Various in Drosophila, but
only reported insect immune
related is hemolin of silkmoth
Antimicrobial protein
gene families
malvolio
Transferrin
Other matrix metalloproteinase
members, for example, kuzbanian
Thor
lethal(1)aberrant
immune response8
Reactive intermediates of
oxygen and nitrogen
Immunoglobulin superfamily,
including some TlR-related genes
110
Antimicrobial protein gene families
54,56,57,59
croquemort
dredd
CD36
CASP family
6
90–92
Tep family
Various from genome
annotation
C3 and A2M
For example, genes that encode
C1 and fibrinogen domains
65,72
13,84
99
5,100
11
10
55
See text
Toll signalling components
Spn43Ac of necrotic locus
gastrulation defective
Signalling pathways
other than Toll
Effectors
Immunoglobulin motif
Antimicrobial proteins
Metal ion accessibility
and metalloproteinase
regulation
Translation related
Cytotoxic molecules
64
107
108
109
Nramp/Scf11
Transferrin
Mmp7/matrilysin
111
112
113
4E-binding protein family
ribosomal protein S6
114
66
Reactive intermediates of
oxygen and nitrogen
115
Apoptosis and immunity
Additional components
*Mammalian gene list shows representative members, for example, a human homologue might be listed but there might also be mouse
and rat homologues. See links for direct accessing of homologous gene and protein sequences, for example, Flybase and Jackson Lab.
§
References are starting points to obtain fuller information. See Flybase and Jackson lab links for additional references linked directly to
gene and protein sequences.
(A2M, α2 macroglobulin; AML1, acute myeloid leukaemia 1; Dif, Dorsal-related immunity factor; Gata, GATA-binding protein; gcm, glial
cells missing; IKK, IκB kinase; IRAK1, interleukin-1-receptor-associated kinase; JAK, janus kinase; JNK, jun kinase; Prss7, protease serine
7; Rel, Relish; Scf11, solute carrier family 11; STAT, signal transducers and activators of transcription.)
264
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IATROGENIC
Disease caused by the
physician in the course
of treating the patient.
Interestingly, many of the receptors involved in
immune recognition also mediate apoptosis — the
mammalian cytokine receptors of the TNF superfamily, for example68. Immunity and apoptosis are increasingly found to have a variety of connections (BOX 2).
Many additional homologies and parallels to vertebrate
innate immunity can be observed in Drosophila (TABLE
1). Some similar features have previously been well
studied in other invertebrates (see, for example, REFS
69–71). In terms of evolutionary conservation, the α2macroglobulin family of conserved protease inhibitors
might represent a prototypical immune response, in
that it is a protein that envelops its molecular prey72. As
only a limited set of topics can be covered here, some
other mechanisms are also listed in TABLE 1.
Undoubtedly, many other features and responses have
not yet been recognized.
Perspectives
Striking similarities across phyla in innate immunity
extend from initiation of infection through to mechanisms of infection clearance. In some respects, the similarities should not be surprising, as the characteristics of
the infecting organisms have common themes and the
demand for survival of infection is inescapably shared.
Conversely, many questions remain, such as the recognition and activation of different Toll receptors. Clearly
the study of many different organisms contributes to a
greater understanding of immunity. Furthermore, this
understanding provides both indirect and direct means
of approach to deal with infection and disease in
humans. For example, antimicrobial proteins from vertebrate and invertebrate sources can be used pharmaceutically to treat infections. It is also believed that
mutations that affect the human TLRs and/or the associated proteins that transduce microbial signals might
predispose to infection73. Many human diseases result
from a failure of innate immunity, whether primary,
secondary or IATROGENIC. For example, in cystic fibrosis
the primary defect of transmembrane conductance secondarily leads to a high salt concentration in the surface
fluid of the lung airway. This inactivates antimicrobial
defensin, which in turn permits colonization by microbial pathogens74.
A continuing challenge in immunity is the intersection of mammals and insects, through parasites that
have life cycles in both. At times, a single pathogen
might dwell within evolutionarily divergent hosts,
assuming different forms in each, as suits its developmental strategy. Such is the case in malaria, leishmaniasis and many other parasitic diseases that are transmitted to mammals by way of an insect vector. Studies in
1.
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Ehrlich, P. The Croonian lecture: on immunity. Proc. R.
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Metchnikoff, E. Immunity in the Infectious Diseases
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Janeway, C. J. Approaching the asymptote? Evolution and
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NATURE REVIEWS | GENETICS
5.
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insects, parasites and humans can all lead to disease
control, as targets for disruption of disease occur at all
steps in the transmission cycle75–77.
The multifaceted power of genomics is being brought
to bear on all aspects of innate immunity. With the
genomes of several species (human, mouse, Drosophila,
Plasmodium, Leishmania and others) either completed
or nearing completion, common themes in innate
immunity are cast in sharp relief. Many new options
have already been looked at and further opportunities
have increased markedly. In Drosophila, genetic screens
have been used to analyse immunity78–82, and the need
for more screens has increased now that the genome has
been sequenced. In particular, new genetic approaches
and techniques for mutational analysis83 are needed, for
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functions, whether in Drosophila, humans or other
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also yield intriguing clues. Genome projects so far have
shown that about one-third of the predicted proteins of
each organism cannot be grouped either with those of
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Links
DATABASE LINKS CR3 | Pgrp | Gnbp | Toll | Dorsal |
Cactus | Drosomycin | imd | Dif | Rel | IKK | spätzle | nec |
18 wheeler | Toll-6 | Toll-7 | IRAK | Pelle | IL-18R | ST2 |
TLR4 | MYD88 | SIGIRR | Toll-9 | Easter | Snake | Gd |
Pipe | Nudel | Windbeutel | Cecropin | Diptericin |
Defensin | Attacin | Drosocin | Metchnikowin |
Andropin | Lysozyme | RpS6air8 | hop | hopTum | TepI |
RpS6 | TRAF6 | TAK1 | NIK | Dredd | FADD | TRADD |
FLICE | Apaf1 | Dark
FURTHER INFORMATION Toll signalling in Drosophila |
Flybase | Jackson lab
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Immune mechanisms against extracellular pathogens |
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Crucial demonstration of the immune function of Toll
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Acknowledgements
D.A.K.’s lab is supported by the National Institutes of Health. B.B. is
an investigator of the Howard Hughes Medical Institute.
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