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T Cell-Intrinsic Factors Contribute to the
Differential Ability of CD8 + T Cells To
Rapidly Secrete IFN- γ in the Absence of
Antigen
Elsa N. Bou Ghanem, Christina C. Nelson and Sarah E. F.
D'Orazio
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The Journal of Immunology is published twice each month by
The American Association of Immunologists, Inc.,
1451 Rockville Pike, Suite 650, Rockville, MD 20852
Copyright © 2011 by The American Association of
Immunologists, Inc. All rights reserved.
Print ISSN: 0022-1767 Online ISSN: 1550-6606.
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J Immunol 2011; 186:1703-1712; Prepublished online 29
December 2010;
doi: 10.4049/jimmunol.1001960
http://www.jimmunol.org/content/186/3/1703
The Journal of Immunology
T Cell-Intrinsic Factors Contribute to the Differential Ability
of CD8+ T Cells To Rapidly Secrete IFN-g in the Absence of
Antigen
Elsa N. Bou Ghanem, Christina C. Nelson, and Sarah E. F. D’Orazio
I
nterferon-g is a multifunctional cytokine that is critical for
the clearance of intracellular pathogens such as Listeria
monocytogenes (1–4). Although NK cells were long thought
to be the primary source of IFN-g during the innate phase of
the immune response, recent studies demonstrated that other cell
types, including both CD4+ and CD8+ T cells, NKT cells, neutrophils, and some subsets of macrophages and dendritic cells
(DCs) can also rapidly secrete IFN-g within 24 h of L. monocytogenes infection (5–10). In one study, the number of IFN-g–
secreting CD8+ T cells approximately equaled the number of IFNg+ NK cells in the spleen 16 h after L. monocytogenes infection of
C57BL/6/J (B6) mice (5), suggesting that CD8+ T cells may also
be a primary source of early IFN-g production.
In vitro, incubation with IL-12 alone is sufficient to trigger NK
cells to rapidly produce IFN-g, or to drive naive T cells to differentiate into Th1-type cells that can secrete IFN-g within several
days. However, if both IL-12 and IL-18 are present, a subset of
splenic CD8+ T cells can also rapidly secrete IFN-g in the absence
of specific Ag (5, 11). IL-15 can increase T cell sensitivity to the
IL-12– and IL-18–driven pathway of IFN-g production, although
it is not required for bystander activation of CD8+ T cells (5, 12,
13). In vivo, this rapid IFN-g response is triggered by infection
with bacterial pathogens that replicate in the host cell cytosol such
as L. monocytogenes or Burkholderia pseudomallei (5, 14–16) duDepartment of Microbiology, Immunology, and Molecular Genetics, University of
Kentucky, Lexington, KY 40536
Received for publication June 11, 2010. Accepted for publication November 30,
2010.
Address correspondence and reprint requests to Dr. Sarah E.F. D’Orazio, Department
of Microbiology, Immunology, and Molecular Genetics, University of Kentucky, 800
Rose Street-MS415, Lexington, KY 40536. E-mail address: [email protected]
Abbreviations used in this article: B6, C57BL/6/J; BALB/c, BALB/c/By/J; C.B10,
C.B10-H2b/LilMcd/J; DC, dendritic cell; hpi, hours postinfection; ICS, intracellular
cytokine staining; LLO, listeriolysin O; RP-10, RPMI 1640 medium supplemented
with 10% FBS; RT, reverse transcription; TCM, central memory; TEM, effector memory.
Copyright Ó 2011 by The American Association of Immunologists, Inc. 0022-1767/11/$16.00
www.jimmunol.org/cgi/doi/10.4049/jimmunol.1001960
ring certain acute viral infections (11, 17) or following the injection of a large dose of LPS (18). Berg et al. (11) showed that
adoptive transfer of CD8+ T cells capable of rapidly secreting
IFN-g could partially protect naive mice against L. monocytogenes
challenge, regardless of the Ag specificity of the T cells. This
suggests that cytokine-driven bystander activation of CD8+ T cells
could be an important part of the innate immune response against
intracellular pathogens.
Memory phenotype CD8+ T cells are thought to be able to respond to IL-12 and IL-18 rapidly because they express higher
levels of the receptors for these cytokines than found on the surface of naive cells. The IL-12R is composed of two chains: IL12Rb1 and IL-12Rb2. b1 binds the p40 subunit of IL-12, a
common chain shared by both IL-12 and IL-23, and b2 provides specificity to the IL-12R by binding the p35 subunit of IL-12
(19). Ligation of both IL-12R subunits is required for signal
transduction into the cytosol (20). The IL-18R is also a heterodimer that consists of a binding subunit (IL-18Ra) and a signaling
peptide (IL-18Rb) that are required to form a high-affinity heterotrimeric complex with IL-18 (21–23). Expression of all four
subunits is typically low on resting T cells, but the receptors can
be reciprocally upregulated in the presence of IL-12 and IL-18
(24–27).
Rapid, Ag-independent secretion of IFN-g by CD8+ T cells was
originally described in B6 mice, a strain that is relatively resistant to L. monocytogenes infection. We recently showed that not
all mouse strains have CD8+ T cells capable of generating a robust rapid IFN-g response during L. monocytogenes infection (7).
Mouse strains with a weak or absent IFN-g response (such as
BALB/c/By/J [BALB/c] mice) were generally more susceptible to
L. monocytogenes infection than those that are rapid IFN-g responders. One explanation for the differential IFN-g response in
BALB/c and B6 mice could be varying levels of IL-12 and/or IL18 secretion by DCs and macrophages during the first few hours
after L. monocytogenes infection. In support of this idea, Liu et al.
(28) showed that DC isolated from L. monocytogenes-infected
B6 mice expressed significantly more IL-12 and IL-15 than did
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A subset of CD44hiCD8+ T cells isolated from C57BL/6/J (B6) mice, but not BALB/c/By/J (BALB/c) mice, rapidly secrete IFN-g
within 16 h of infection with Listeria monocytogenes. This Ag-independent response requires the presence of both IL-12 and IL-18.
Previous studies showed that dendritic cells from B6 mice produced more Th1-type cytokines such as IL-12 than did those from
BALB/c mice in response to L. monocytogenes infection. In this report, we demonstrate that the microenvironment in L. monocytogenes-infected BALB/c mice is sufficient to induce responsive B6 CD8+ T cells to rapidly secrete IFN-g. Furthermore, BALB/c
CD8+ T cells did not rapidly secrete IFN-g even when they were exposed to high concentrations of IL-12 plus IL-18 in vitro. In the
presence of IL-12 and IL-18, B6 CD44hiCD8+ T cells upregulated expression of the receptor subunits for these cytokines more
rapidly than did BALB/c T cells. In comparing particular subsets of memory phenotype CD8+ T cells, we found that virtual
memory cells, rather than true Ag-experienced cells, had the greatest level of impairment in BALB/c mice. These data suggest that
the degree of cytokine-driven bystander activation of CD8+ T cells that occurs during infection depends on both APCs and T cellintrinsic properties that can vary among mouse strains. The Journal of Immunology, 2011, 186: 1703–1712.
1704
CD8+ T CELLS DIFFER IN THEIR ABILITY TO RAPIDLY SECRETE IFN-g
BALB/c DCs. Likewise, other studies demonstrated that macrophages from B6 mice produced more IL-12 than did BALB/c
macrophages after stimulation with purified TLR ligands (29, 30).
However, CD8+ T cells do not rapidly secrete IFN-g unless IL-18
is also present, and it is not known whether L. monocytogenes infection also induces a greater IL-18 response in B6 mice.
In this study, we investigated the differences between BALB/c
and B6 mice that contribute to the differential rapid IFN-g response observed during L. monocytogenes infection. We found
that only IL-12, and not IL-15 or IL-18, was significantly increased in B6 mice at the earliest stages of infection. Surprisingly,
however, the cytokine microenvironment in the B6 spleen was not
sufficient to overcome intrinsic defects in BALB/c CD8+ T cells.
The data presented in this article suggest that the impaired response in BALB/c mice is primarily due to deficiencies in memory
phenotype cells that arise via homeostatic proliferation rather than
true Ag-experienced memory CD8+ T cells.
Mice
Female BALB/c, B6, and C.B10-H2b/LilMcd/J (C.B10) mice were purchased from The Jackson Laboratory. Thy1.1/luciferase-expressing B6
mice (31) were provided by Robert Negrin (Stanford University). All mice
were maintained in a specific pathogen-free facility at the University of
Kentucky, and all procedures were performed in accordance with Institutional Animal Care and Use Committee guidelines.
L. monocytogenes infection
L. monocytogenes 10403s were grown in brain heart infusion broth (Difco)
shaking at 37˚C until early stationary phase and aliquots were frozen at
280˚C. Prior to infection of mice, a bacterial aliquot was thawed on ice
and grown to early exponential phase shaking at 37˚C in brain heart infusion broth. The bacteria were washed once and then diluted to 5 3 106
CFU/ml in PBS prior to i.v. injection of 200 ml in the lateral tail vein.
ELISA
Whole spleens were injected with 100 U/ml type IV collagenase (Invitrogen), minced, and digested for 30 min at 37˚C in 7% CO2 to release DCs.
Single-cell suspensions of splenocytes were washed in HBSS and then
incubated in RPMI 1640 medium supplemented with 10% FBS media (RP10) at 37˚C in 7% CO2 for either 6 or 24 h. Supernatants were collected
and stored at 280˚C. The amounts of secreted IL-12 (p70) or IL-15/IL15R complexes were measured using Ready-Set-Go kits from eBioscience.
IL-18 concentrations were determined by ELISA using capture Ab (clone
74) at 4 mg/ml, a biotin-labeled detection Ab (clone 93-10C) at a 1:2000
dilution, and rIL-18 in the range of 25–1600 pg/ml, all from MBL.
Flow cytometry
Anti-CD4 (clone GK1.5), CD8a (53-6.7), CD49b (DX5), CD44 (IM7),
CD62L (MEL-14), CD127 (A7R34), CCR7 (4B12), CD49d (R1-2)
CD90.1 (Thy1.1; clone HIS51), CD90.2 (Thy1.2; clone 53-2.1), and
IFN-g (XMG1.2) Abs were purchased from eBioscience. Ab specific for
TCRb (clone H57-597) and CD122 (TM-b1) were purchased from BD
Biosciences. For receptor staining experiments, cells were harvested, fixed,
and then surface stained for IL-18Ra (clone 112614; R&D Systems), IL12Rb1 (clone 114), or IL-18Rb (clone TC30-28E3) from BD Biosciences,
or permeabilized and stained with hamster anti-mouse IL-12Rb2 Ab
(clone HAM10B9) followed by mouse anti-hamster IgG Ab (clone G9490.5), both from BD Biosciences. For detection of phosphorylated STAT4,
cells were fixed using PhosFlow Lyse/Fix buffer according to the manufacturer’s instructions and permeabilized with Perm III buffer (BD Biosciences) for 18 min prior to staining with PE-conjugated anti-STAT4p and
allophycocyanin-conjugated Abs specific for CD4, CD8, or CD44. Fluorescence intensities of live lymphocytes (at least 25,000 events as determined by forward and side scatter gating) were measured on a
FACSCalibur and analyzed using CellQuest (BD Biosciences).
IFN-g intracellular cytokine staining
Intracellular cytokine staining (ICS) was performed using the Cytofix/
Cytoperm kit (BD Biosciences). Splenocytes from infected or uninfected
CD8+ T cell isolation
CD8+ T cells were enriched from murine spleens by negative selection
using IMag beads according to the manufacturer’s instructions (BD Biosciences). CD8+ cells were either used directly ex vivo or cultured in RP10 media with or without the addition of murine rIL-12 and rIL-18. The
experiments shown in Fig. 3 were also performed with cells obtained by
positive selection using allophycocyanin-conjugated anti-CD8a Abs followed by anti-allophycocyanin-coated magnetic beads (data not shown)
and similar results were obtained.
Adoptive transfer
Unlabeled CD8-enriched cells (2 3 106, average purity of 92%) were
injected into each recipient mouse. For transfer of C.B10 T cells into
C.B10 mice, the T cells were labeled with 2.5 mM CFSE. Two hours later,
groups of mice were either infected with 1 3 106 CFU L. monocytogenes
or given PBS as a control. Single-cell suspensions of splenocytes were
prepared 16 hours postinfection (hpi) and passed over nylon wool columns
to enrich for T cells. Cells were maintained in media containing GolgiPlug
throughout the enrichment procedure and were incubated for a total of 4 h
ex vivo prior to ICS.
Cytokine receptor mRNA expression
Total RNA was extracted from CD8+-enriched cells using TRIzol reagent
(Invitrogen). cDNA was obtained using SuperScript III (Invitrogen) with
random hexamer primers. Targets were amplified from either cDNA or
control samples processed without reverse transcription (RT) using Taq
DNA polymerase (Invitrogen) for 25 cycles with the following primers:
IL-12Rb1, 59-TGCACCCCTGAGGACT TCCCGGAG-39 and 59-CTGTG
ACCCCAGCCAAGGACGCTG-39; IL-12Rb2, 59-GCTACCT ACGGATAATCTCCTGATG-39 and 59-ACCTATGGTGAAGCTAACTTTATGGAG-39; IL-18Ra, 59-TTACCTGACTAACG GAGCCAGGCGTG-39 and 59GCCAGCGGTTCTCAACC TTCCTAATGC-39; IL-18Rb, 59-TCAATGGACCCCGTGTCTTTG-39 and 59-GCTCTGT GTCTGTTCCAGGAAC-39;
b-actin sense, 59-TGTGATGGTGGGAATGGGTCAGAA-39, anti-sense:
59-TGCCACAGGATTCCAT ACCCAAGA-39. For each of the RNA
samples used in this study, the no RT control did not yield any PCR
products. The amplified PCR products were separated by agarose gel
electrophoresis, stained with ethidium bromide, and images of each gel
were captured using a Kodak Gel Logic 2200 imaging system. Band intensities were analyzed using Kodak Molecular Imaging Software version
4.0.
Statistical analysis
Statistical analysis was performed as indicated using Prism software for
Macintosh (version 4.0b; GraphPad Software). A p value ,0.05 was
considered significant.
Results
CD8+ T cells are a primary source of early IFN-g during
L. monocytogenes infection of B6, but not BALB/c mice
We previously showed that a subset of memory phenotype CD8+
T cells from B6, but not BALB/c mice, could rapidly secrete IFNg within 16 h of infection with L. monocytogenes (7). Because
other cell types such as NK cells and NKT cells can serve as
source of early IFN-g in BALB/c mice, the physiological relevance of an additional subset of rapid IFN-g–producing cells
in B6 mice was not clear. To identify the total number of rapid
IFN-g–secreting lymphocytes in each mouse strain, we harvested
splenocytes from BALB/c and B6 mice 16 h after L. monocytogenes infection. ICS was performed directly ex vivo to determine the number of CD4+ T cells, CD8+ T cells, NK cells, and
NKT cells that were actively secreting IFN-g during infection.
In BALB/c mice, the largest subset of cells in the spleen that
rapidly secreted IFN-g during L. monocytogenes infection was NK
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Materials and Methods
mice were incubated in RP-10 medium (32) containing GolgiPlug for 4 h
directly ex vivo (without any further in vitro stimulation) prior to Ab
staining. For in vitro assays, CD8+-enriched cells were incubated with
rIL-12 (eBioscience) plus rIL-18 (MBL) for the time indicated, Golgi Plug
was added, and ICS was performed 4 h later.
The Journal of Immunology
L. monocytogenes-infected B6 and BALB/c splenocytes differ
in IL-12, but not IL-15 or IL-18, secretion directly ex vivo
vested from L. monocytogenes-infected BALB/c and B6 mice.
Whole splenocytes were isolated 15 hpi, and the cytokines being
actively secreted by these cells were allowed to accumulate in the
culture media for 6 h. As shown in Fig. 2A, there was a striking
difference in the amount of IL-12 secreted by infected splenocytes, a result that is consistent with the previous study by Liu
et al. (28). In contrast, there was no significant difference in the
amount of IL-15 secreted by BALB/c or B6 splenocytes (Fig. 2B).
However, when the cells were incubated for a prolonged period
of time in vitro (24 h, as previously reported by Liu et al.), we did
see significantly greater secretion of IL-15 with B6 splenocytes.
Surprisingly, we found only a slight increase in IL-18 secretion for
infected splenocytes compared with uninfected cells and no significant differences between the mouse strains after 6 h of cytokine accumulation (Fig. 2C). After 24 h of in vitro culture, B6
splenocytes secreted significantly more IL-18 than did BALB/c
cells, and they showed a greater induction compared with uninfected splenocytes. These results suggest that IL-12 may be the
only cytokine that is present at a significantly higher concentration
in the spleens of B6 mice compared with BALB mice during the
first 15–21 h after L. monocytogenes infection.
Ag-independent bystander activation of CD8+ T cells requires the
presence of both IL-12 and IL-18 and can be enhanced by the
presence of IL-15 (5). Liu et al. (28) showed that splenic DCs
isolated from L. monocytogenes-infected B6 mice produced more
IL-12 and IL-15 than did DCs isolated from BALB/c mice. However, because the rapid IFN-g response only occurs in vivo during infection with cytosolic pathogens, we postulated that the
ability to rapidly trigger IL-18 secretion could be the rate-limiting
step that determines how quickly CD8+ T cells will undergo bystander activation. To test this, we measured the amount of IL-12,
IL-15, and IL-18 produced directly ex vivo by splenocytes har-
FIGURE 1. CD8+ T cells are a primary source of early IFN-g in B6 but
not BALB/c mice. Groups of mice (n = 3) were infected (i.v.) with 1 3 106
CFU L. monocytogenes 10403s (gray bars) or given PBS (naive; open
bars). Splenocytes were harvested 16 hpi and IFN-g ICS was performed
directly ex vivo. A, Mean values 6 SD for the total number of IFN-g+ cells
per BALB/c or B6 spleen are shown. B, Mean values 6 SD for % IFN-g+
cells in each BALB/c or B6 subset are shown. The p values for unpaired t
tests are shown above each data set. *One-way ANOVA indicated that the
mean value for B6 CD8+ T cells was significantly different from the mean
for either CD4+ or NKT cells, but was not significantly different from NK
cells. For BALB/c mice, the mean value for NK cells was significantly
different from the mean for all other cell types. Data from one of two
independent experiments are shown.
FIGURE 2. BALB/c mice secrete sufficient IL-12/IL-18 during L.
monocytogenes infection to induce highly responsive subsets of cells to
rapidly secrete IFN-g. Mice were either infected i.v with L. monocytogenes
(n = 5) or given PBS (n = 3), and spleens were harvested 15 h later.
Collagenase-treated single-cell suspensions were incubated at 37˚C in 7%
CO2 for either 6 or 24 h. The amount of IL-12 (A), IL-15/15R complex (B),
or IL-18 (C) accumulated in each supernatant was quantified by ELISA.
ND, not detected. *p # 0.05, **p = 0.0004 using Student t test; ns, mean
values not significantly different. D, Reciprocal transfers of CD8+ T cells
from Thy1.1+ B6 and Thy1.2+ C.B10 mice were performed prior to i.v.
injection with PBS (open bars) or L. monocytogenes (filled bars). Mean
values 6 SE for pooled data from two experiments using at least three
infected mice per group are shown.
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cells (Fig. 1A). In contrast, in B6 mice, the largest numbers of
IFN-g+ cells in the spleen were CD8+ T cells. Overall, the total
number of IFN-g–secreting lymphocytes in B6 spleens was ∼4fold greater than the number found in BALB/c spleens. As expected, an increased percentage of IFN-g+ cells was observed in
B6 mice compared with BALB/c mice for all four cell types examined (Fig. 1B). These data were consistent with the hypothesis
that the increased levels of IL-12 found in B6 mice resulted in
a more complete induction of IFN-g–secreting cells. However,
a closer examination revealed that the strain-specific differences in IFN-g production were much greater for all of the T cell
subsets than for NK cells. For example, L. monocytogenes infection of BALB/c mice resulted in only a 4-fold increase in
IFN-g–secreting CD8+ T cells, CD4+ T cells, or NKT cells, but
infection of B6 mice led to 22-, 14-, and 11-fold increases, respectively, of the same T cell subsets (Fig. 1B). In contrast, the
increase in IFN-g+ NK cells after L. monocytogenes infection
was proportional (∼200-fold greater) in each mouse strain. These
data indicated that bystander activation of T cells was more efficient in B6 mice than in BALB/c mice.
1705
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CD8+ T CELLS DIFFER IN THEIR ABILITY TO RAPIDLY SECRETE IFN-g
Adoptively transferred B6 CD8+ T cells can rapidly secrete
IFN-g in L. monocytogenes-infected BALB/c mice
Resting CD8+ T cells do not differentially express IL-12R or
IL-18R subunits
Because B6 T cells appeared to respond more efficiently to the
presence of IL-12 and IL-18, we next considered the possibility that
resting CD8+ T cells from B6 mice expressed a higher level of the
receptor for either IL-12 (IL-12R) or IL-18 (IL-18R). Splenic
CD8+ T cells were isolated from naive mice, total RNA was
extracted, and the mRNA levels for each receptor subunit were
determined using RT-PCR. As shown in Fig. 4A, expression of the
two IL-18R subunits (IL-18Ra and IL-18Rb) and the b2 subunit
of the IL-12R was similar in both mouse strains. We were unable
to detect IL-12Rb1 expression in resting CD8+ T cells isolated
BALB/c CD8+ T cells do not respond to rIL-12 plus rIL-18 as
efficiently as do B6 CD8+ T cells
To examine IL-12/IL-18 responsiveness directly, we incubated
CD8+ T cells isolated from either B6 or BALB/c mice with
varying concentrations of rIL-12 plus rIL-18 and identified IFN-g+
cells 15 h later. For B6 T cells, the maximal IFN-g response was
observed following incubation with 5 ng/ml rIL-12 plus rIL-18
(Fig. 3A). Incubation of BALB/c CD8+ T cells with the same
concentration of cytokines failed to induce a significant increase
in IFN-g expression. Even when treated with 10-fold greater (50
ng/ml) rIL-12 plus rIL-18, there was still only a 4-fold increase in
IFN-g+ T cells, similar to the results we observed in L. monocytogenes-infected BALB/c mice (Fig. 1A). However, we did observe a robust IFN-g response by CD8+ T cells when we incubated naive splenocytes from either BALB/c or B6 mice with
FIGURE 3. B6 CD8+ T cells respond to the presence of rIL-12 plus
rIL-18 faster than BALB/c CD8+ T cells. Splenic CD8+ T cells (A) or
whole splenocytes (B) harvested from either a naive BALB/c or B6 mouse
were incubated with rIL-12 plus rIL-18 (each cytokine at the indicated
final concentration) or in media alone. ICS was performed 15 (A) or 72 h
(B) later to determine the total number of IFN-g+ cells. Dot plots shown
are gated on CD8+ T cells; numbers in the corners indicate the percentage
of IFN-g+ cells shown in the small boxes. Data from one of three separate
experiments are shown.
FIGURE 4. B6 mice upregulate IL-12R and IL-18R expression in CD8+
T cells more efficiently than do BALB/c mice. CD8+ T cells from BALB/c
(BA) or B6 mice were analyzed either directly ex vivo or after incubation
with 50 ng/ml rIL-12 plus rIL-18. RT-PCR targets were amplified (25
cycles) from (A) either genomic DNA or cDNA prepared directly ex vivo
or (B) from cells incubated in IL-12 plus IL-18 for the indicated times or in
media alone (M) for 8 h. Data from one of two independent experiments
are shown. Percentages of receptor subunit-positive CD8+ T cells (C) or
CD44hiCD8+ T cells (D) as measured by flow cytometry are shown as
mean values 6 SD for triplicate samples from BALB/c (open bars) and B6
(filled bars) mice. E, Mean fluorescence intensities for receptor subunit
staining on CD44hiCD8+ T cells. Representative data from one of three
separate experiments (n = 3 mice) are shown. *p # 0.002 using Student t
test.
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To confirm that the cytokines secreted in the spleens of BALB/c
mice during L. monocytogenes infection were insufficient to induce CD8 + T cells to rapidly produce IFN-g, we adoptively
transferred CD8+ T cells harvested from Thy1.1-expressing B6
mice into naive wild-type (Thy1.2+) B6 mice or C.B10 (BALB/c
congenic; H-2b) mice. We previously showed that the rapid IFN-g
response by CD8+ T cells was not linked to MHC haplotype and
that CD8+ T cells from C.B10 mice did not rapidly secrete IFN-g
during L. monocytogenes infection (7). The recipient mice were
then infected with L. monocytogenes or given PBS as a control,
and the number of IFN-g–secreting CD8+ T cells was determined
directly ex vivo 14 h later. As expected, a significant increase in
IFN-g+ endogenous (Thy1.2+) CD8+ T cells was observed in B6
mice but not in C.B10 recipient mice (Fig. 2D). Surprisingly,
analysis of the transferred Thy1.1+ population revealed a strong
IFN-g response in either host. Thus, the cytokine environment
in C.B10 mice was sufficient to induce the transferred B6 CD8+
T cells, but not the endogenous C.B10 CD8+ T cells, to rapidly
secrete IFN-g. Transfer of the Thy1.2+ C.B10 T cells into Thy1.1expressing B6 mice did not significantly increase the number of
IFN-g+ cells (Fig. 2D), suggesting that the local microenvironment could not overcome T cell-intrinsic defects in the C.B10
CD8+ T cells. Taken together, these results indicated that cytokinedriven bystander activation of B6 T cells was highly efficient, and
that BALB/c CD8+ T cells either did not colocalize with APCs
producing high levels of IL-12 and IL-18 or they were unable to
respond as efficiently to the presence of these cytokines during
infection.
rIL-12 plus rIL-18 for 3 d (Fig. 3B), a result that is consistent
with previous reports in the literature. These data indicated that
BALB/c CD8+ T cells could respond to the presence of IL-12 plus
IL-18; however, an additional 48 h was required before high levels
of IFN-g–secreting cells could be detected. We conclude from
these studies that there are T cell-intrinsic factors that define the
differential ability of BALB/c and B6 CD8+ T cells to rapidly
undergo cytokine-driven bystander activation.
The Journal of Immunology
from either mouse strain. As a control, we also tested our primers
(which were derived from the B6 genome) on chromosomal DNA
and confirmed that the genes for all four receptor subunits could
be amplified equally well in BALB/c mice (Fig. 4A). We concluded from these experiments that expression of IL-12R and IL18R in resting T cells was not likely to account for the increased
responsiveness of B6 CD8+ T cells.
Differences in IL-12R and IL-18R upregulation following
incubation with IL-12 plus IL-18
CD8+ T cells was the ability to quickly increase and maintain
expression of the receptors that bind IL-12 and IL-18. Analysis
of the mean fluorescence intensities for the most abundantly
expressed subunits (IL-12Rb2 and IL-18Ra) indicated that treatment with rIL-12 plus rIL-18 resulted in an increase in receptorpositive cells and did not change the receptor subunit density on
each cell (Fig. 4E). Further analysis of the ligand interactions and
cell surface stability of these cytokine receptors will require the
generation of better affinity reagents to detect each subunit.
STAT4 activation in CD8+ T cells exposed to rIL-12 plus
rIL-18
Signaling through the TYK2/STAT4 pathway is required for both
the IL-12–dependent upregulation of IL-18R expression and Agindependent IFN-g secretion by CD8+ T cells (34–36). Kuroda
et al. (37) recently showed that there were mouse strain-specific
differences in STAT4 expression in murine macrophages. In that
study, there was no differential expression of STAT4 in T cells;
however, the use of bulk T cell lysates could have masked a difference found specifically in just one subset of memory phenotype
CD8+ T cells. To test whether differential activation of STAT4 is
one of the T cell-intrinsic properties that determines IL-12/IL-18
responsiveness, we incubated whole splenocytes with 10 ng/ml
IL-12 and/or IL-18 for 30 min and then measured the amount of
phosphorylated STAT4 present in single T cells using flow cytometry.
As shown in Fig. 5, IL-12 treatment induced phosphorylation of
STAT4 in ∼6% of splenocytes in both BALB/c and B6 mice.
Stimulation with rIL-18 alone had no effect on the levels of activated STAT4 found in whole splenocytes. Surprisingly, treatment with both rIL-12 and rIL-18 resulted in a lower number of
STAT4p+ cells (∼4.5%) than seen after stimulation with rIL-12
alone in both strains of mice (Fig. 5A). The vast majority of cells
detected with the STAT4p-specific Ab were neither CD4+ nor
FIGURE 5. IL-12 induces phosphorylation of STAT4 in CD8+ T cells
isolated from either BALB/c or B6 mice. Whole splenocytes from naive
mice were incubated with the indicated cytokines (final concentration of
each was 50 ng/ml) for 30 min, and then the cells were fixed and STAT4p
ICS was performed. A, Representative dot plots from one of three independent experiments are shown. B, Pooled data (n = 5 mice) were used
to calculate mean values 6 SD. Mean values for B6 and BALB/c cells
exposed to rIL-12 were significantly different by unpaired t test.
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An alternate explanation for the differential IFN-g response could
be that B6 mice were more efficient than BALB/c mice at upregulating receptor subunit expression in CD8+ T cells following
IL-12 and IL-18 exposure. To test this, CD8+ T cells isolated from
the spleens of naive mice were incubated with 50 ng/ml rIL-12
and rIL-18 or in media alone, and RNA was harvested either 4 or
8 h later. After 8 h in media, expression of all four cytokine receptor subunits had decreased to barely detectable levels (Fig. 4B).
However, after incubation with IL-12 plus IL-18, a significant
increase in mRNA expression was observed in both BALB/c and
B6 CD8+ T cells.
For IL-12R, the most highly expressed chain was IL-12Rb2 (Fig.
4B), and B6 mice appeared to express more transcripts than did
BALB/c mice. Because IL-12Rb2 binds the p35 subunit of IL-12
and provides specificity to the receptor (19), a difference in IL12Rb2 expression could provide a mechanism for increased responsiveness to IL-12. To find out whether this potential increase
in mRNA expression resulted in increased protein expression, we
incubated CD8+ T cells with rIL-12 plus rIL-18 for up to 10 h and
then stained the cells with subunit-specific mAbs. Treatment with
rIL-12 and rIL-18 induced a significantly greater increase in IL12Rb2 protein expression in B6 mice compared with BALB/c
mice for both bulk CD8+ T cells (Fig. 4C) and the CD44hi subset of CD8+ T cells (Fig. 4D). We were unable to detect IL-12Rb1
protein on the surface of CD8+ T cells (data not shown), a result
that was consistent with the low mRNA expression we observed
(Fig. 4B).
The increase in IL-18Ra mRNA expression was comparable in
B6 and BALB/c CD8+ T cells incubated with rIL-12 plus rIL-18
(Fig. 4B). Likewise, we did not observe a significant difference in
the cell surface expression of IL-18Ra when bulk CD8+ T cells
isolated from B6 or BALB/c mice were compared (Fig. 4C).
However, when we gated on CD44hiCD8+ T cells (the IFN-g–
secreting subset), there was a 3-fold increase in IL-18Ra+ cells in
B6 mice, whereas the number of BALB/c IL-18Ra+ cells did not
change significantly (Fig. 4D). Interestingly, IL-18Rb expression
was not sustained in BALB/c mice, and by 8 h post-treatment, we
could no longer detect mRNA in CD8+ T cells (Fig. 4B). Similar
results were reported by Neumann and Martin (25), who showed
that IL-18Rb mRNA could be detected in B6 but not in BALB/c
thymocytes 24 h after incubation with IL-12. IL-18Rb expression
is needed to promote a high-affinity binding interaction with IL-18
(22), and a lack of sustained expression in BALB/c mice could
partially explain why CD8+ T cells had an impaired rapid IFN-g
response. Unfortunately, we were unable to determine whether
the intriguing mRNA expression pattern observed for IL-18Rb
was maintained at the protein level because we could not detect
any specific staining on CD8+ T cells using the only commercially
available anti-murine IL-18Rb Ab (clone TC30-28E3). TC3028E3 was previously shown to bind IL-18Rb on the surface of
transfected cells (33), but it has not been demonstrated to stain
primary cells.
Collectively, our data suggested that one of the T cell-intrinsic
factors that contributed to the differential rapid IFN-g response by
1707
1708
CD8+ T CELLS DIFFER IN THEIR ABILITY TO RAPIDLY SECRETE IFN-g
CD8+ T cells, a finding that is consistent with expression primarily
in macrophages or DCs at this time point. However, when we
compared the ability of rIL-12 to activate STAT4 in CD8+ T cells,
we observed that a slightly larger fraction of total CD8+ T cells
expressed STAT4p in B6 mice compared with BALB/c mice (Fig.
5B). We also performed one experiment in which CD8+ T cells
were enriched from whole splenocytes by negative selection prior
to stimulation with IL-12 and were then stained for STAT4p and
CD44. The results of that experiment showed that most STAT4p+
CD8+ T cells expressed a high level of CD44 (data not shown).
Taken together, these results suggest that the ability to quickly
activate STAT4 signaling in CD8+ T cells may also contribute to
strain-specific differences in rapid IFN-g production, but it is
not likely to be the major factor that determines IL-12/IL-18 responsiveness.
The rapid IL-12– and IL-18–dependent IFN-g response is not
restricted to one particular subset of memory phenotype CD8+
T cells
were approximately equally divided between the TEM and TCM
subsets (Fig. 6B). These results are consistent with a previous
study by Berg et al. (5), which showed that the rapid IFNg–responding CD8+ T cells in B6 mice were CD44hi and Ly6Chi,
and they expressed variable levels of CD62L. Taken together, these
observations suggest that cytokine-induced bystander activation of
CD8+ T cells is not limited to either the TEM or TCM subset.
Next, we analyzed memory phenotype CD8+ T cells based on
surface expression of CD122, a cytokine receptor subunit that is
shared by the IL-2 and IL-15 receptors. Most memory T cells
express high levels of CD122 on the cell surface, and the longterm maintenance of these cells is thought to require homeostatic
proliferation triggered by the binding of IL-15 (39). However,
a subset of IL-15–independent memory T cells has also been
described in B6 mice. These cells express lower levels of CD122
and appear to have a more activated phenotype than do resting
memory T cells that are CD122hi (40–42). Based on these observations, we predicted that most of the rapid IFN-g–secreting cells
would belong to the CD122lo subset of memory phenotype cells.
Naive splenocytes harvested from either B6 or BALB/c mice
were stimulated with rIL-12 plus rIL-18 and 18 h later CD8+ T cells
were analyzed for surface expression of both CD44 and CD122
and the intracellular accumulation of IFN-g. As predicted, we
found that most of IFN-g+CD44hiCD8+ T cells expressed low
levels of CD122 on the cell surface in both strains of mice (Fig.
7A). However, we noticed that only 21% (in BALB/c) or 30% (in
B6) of the CD44hiCD8+ T cells expressed high levels of CD122 at
the end of the assay (Fig. 7B). These data were not consistent with
previous reports in the literature that used freshly isolated T cells
(39, 42, 43), and they suggested that surface expression of CD122
might be downregulated during the course of the 18 h in vitro
incubation used in this assay. Additional staining experiments using either freshly isolated splenocytes or cells that were incubated in tissue culture media for various lengths of time revealed
that as little as 4 h of in vitro incubation resulted in a significant
decrease in CD122 surface expression (data not shown). To
overcome this limitation, we harvested splenocytes from both
BALB/c and B6 mice and sorted the CD44hiCD8+ T cells into
CD122lo and CD122hi populations. As shown in Fig. 7C, ∼30%
of the memory phenotype cells were CD122lo in both strains of
mice, a result that is consistent with previous studies (44). The
sorted cells were then incubated with rIL-12 plus rIL-18 for
14 h in vitro before performing IFN-g ICS.
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Typically, only 2–6% of splenic CD8+ T cells in B6 mice are IFNg+ by ICS analysis 16 h after L. monocytogenes infection (5, 7,
32), despite the fact that up to 15% of CD8+ T cells in an unimmunized mouse can have a memory-like (CD44hi) phenotype.
When these responses are measured directly ex vivo, this observation is not surprising, since it is highly likely that CD44hiCD8+
T cells in the spleen are not uniformly exposed to the same local concentrations of IL-12 and IL-18 derived from L. monocytogenes-infected macrophages or DCs. However, in the
experiments described above, all of the CD8+ T cells in the culture
wells were exposed to 50 ng/ml rIL-12 plus rIL-18, yet only
a fraction of the CD44hi cells was capable of rapidly secreting
IFN-g, even in responsive B6 mice. This suggests that a particular
subset of memory phenotype CD8+ T cells may be the only cells
that can rapidly produce IFN-g in an IL-12– and IL-18–dependent
manner.
To test whether the rapid IFN-g response was restricted to
a particular subset of memory phenotype T cells in B6 mice and
whether BALB/c mice had the same distribution of memory
phenotype cells, splenic CD8+ T cells were purified and incubated
in vitro with IL-12 plus IL-18, and the number of CD44hi T cells
that secreted IFN-g within 16 h was measured. Consistent with
previous studies, CD44 expression varied considerably on B6
CD8+ T cells, with ∼12.5% of the cells demonstrating a CD44hi
phenotype (Fig. 6A). In contrast, most CD8+ T cells from BALB/c
mice expressed relatively high levels of CD44, and when we gated
on the subset of cells with the highest expression of CD44, we
found a slightly higher percentage of cells (13.9%) in BALB/c
mice. Because BALB/c spleens are typically larger than spleens
from B6 mice of the same age, this corresponded to one and a half
times as many CD44hiCD8+ T cells in the spleens of BALB/c mice
(mean, 2.4 3 106 6 0.65 3 106 cells/spleen) compared with B6
mice (mean, 1.53 3 106 6 0.11 3 106 cells/spleen). These results
indicated that the diminished rapid IFN-g response in BALB/c
mice was not due simply to an overall lower number of T cells
expressing high levels of CD44 in BALB/c spleen.
Because effector memory (TEM) cells are known to display
effector functions such as cytokine secretion more rapidly than do
central memory (TCM) cells after activation through the TCR (38),
it would be reasonable to predict that the CD44hi T cells capable
of secreting IFN-g within 16 h of exposure to IL-12 plus IL-18
would have predominantly a TEM phenotype. B6 and BALB/c
mice had similar percentages of TEM (CD44hiCD62Llow) and
TCM (CD44hiCD62Lhi) cells in the spleen (data not shown);
however, contrary to our expectation, the IFN-g+CD8+ T cells
FIGURE 6. The rapid IFN-g response is not restricted to either the TEM
or TCM subset of CD44hiCD8+ T cells. Splenic CD8+ T cells were incubated with 50 ng/ml rIL-12 plus rIL-18 for 14–16 h and then IFN-g ICS
was performed. A, Representative histograms reveal CD44 staining patterns on CD8+ T cells in BALB/c versus B6 mice. B, Dot plots are gated on
CD8+CD44hi T cells and show the TEM (CD62Llow) and TCM (CD62Lhi)
cells that were secreting IFN-g. For B6 mice, 47% of the IFN-g+ cells were
TEM and 53% were TCM. For BALB/c mice, 58% of the IFN-g+ cells were
TEM and 42% were TCM. Representative data from one of four mice are
shown.
The Journal of Immunology
1709
FIGURE 7. Cytokine-driven bystander activation occurs in both the
CD122lo and CD122hi subsets of memory phenotype CD8+ T cells. A,
Naive splenocytes were incubated for 14 h with 50 ng/ml IL-12 plus IL-18
or media alone and IFN-g ICS was performed. B, Representative histograms show the CD122 expression levels at the end of the assay described
in A. Data from one of five mice used in three different experiments are
shown. C, Gating strategies for sorting CD122lo and CD122hi populations
of CD44hiCD8+ T cells are shown. D, The sorted T cells were incubated
with or without IL-12/IL-18, and ICS was performed as described above.
As expected, the overall number of IFN-g+ cells was lower for
BALB/c T cells compared with B6 T cells (Fig. 7D). For B6 mice,
both the CD122lo and the CD122hi cells were equally responsive
to the presence of IL-12 and IL-18, with ∼4.5% of each population rapidly secreting IFN-g. Thus, the rapid IFN-g–responding
cells were not restricted to either the CD122hi or CD122lo subset
of memory phenotype cells in these mice. However, in BALB/c
mice, the CD122lo population of cells appeared to be more responsive to the presence of IL-12 plus IL-18, with ∼4-fold more
CD122lo cells secreting IFN-g compared with the CD122hi cells
(Fig. 7D). Taken together, these data confirm that BALB/c CD8+
T cells have a generalized defect that limits rapid IFN-g secretion
in response to IL-12 plus IL-18, and they suggest that the CD122hi
subset of memory cells in BALB/c mice may have additional
T cell-intrinsic properties that render the cells less responsive to
bystander activation than do B6 T cells.
Most rapid IFN-g–responding CD8+ T cells in both B6 and
BALB/c mice are not true Ag-experienced cells
In a recent report, Haluszczak et al. (45) described another way to
categorize CD8+ T cells with a CD44hi phenotype as either “true
FIGURE 8. Memory phenotype CD8+ T cells in BALB/c mice display
a greater deficit in rapid IFN-g secretion than do true Ag-experienced cells.
CD8+ T cells enriched from naive splenocytes were incubated for 14 h in
50 ng/ml rIL-12 plus IL-18 or media alone. A, Representative histograms
(n = 3) show the CD49d expression pattern on CD44hiCD8+ T cells. B,
The percentage of IFN-g+ cells that were Ag-experienced memory cells
(CD49dhi) or had a memory-like phenotype (CD49dlo) in naive BALB/c
versus B6 mice (n = 3) 6 SD is shown. C, The proportion of each CD44hi
CD8+ T cell subset that was actively secreting IFN-g is indicated. D,
Splenocytes from either naive or L. monocytogenes-immune mice (infected
with 1 3 103 CFU L. monocytogenes 10403s 4 wk earlier) were incubated
for 14 h in the presence of either L. monocytogenes, LLO91–99 peptide (100
nM final concentration; Bio-Synthesis), or media alone, and then IFN-g
ICS was performed. Representative dot plots from one of four mice
are shown. Each experiment was performed twice. E, LLO91–99-specific
CD44hiCD8+ T cells from D that rapidly secreted IFN-g were analyzed for
CD49d expression. Mean values 6 SD for n = 3 mice are shown.
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memory” cells that have previously encountered Ag and been
activated through the TCR, or “virtual memory” cells that develop
by undergoing homeostatic proliferation in a normal (not lymphopenic) host. The only surface marker that could be used to
distinguish the two cell types in that study was CD49d (a4 integrin), which was present at significantly higher levels on true
Ag-experienced memory cells. Because Haluszczak et al. used
only B6 mice, we considered the possibility that BALB/c mice
had fewer virtual memory cells than did B6 mice, and that deficiencies in this particular subset contributed significantly to a diminished rapid IFN-g response. Representative histograms for the
level of CD49d staining on CD44hiCD8+ T cells in either BALB/c
or B6 spleen are shown in Fig. 8A. There was no significant difference in the absolute number of memory phenotype cells expressing the highest level of CD49d in any of the age-matched
mice we tested. In both strains of mice, the vast majority of the
memory phenotype CD8+ T cells that rapidly secreted IFN-g expressed low levels of CD49d on the cell surface (Fig. 8B). Because the mice used for these experiments were only 6 wk old
and were previously unimmunized, it was not surprising to find
that very few cells had an Ag-experienced phenotype.
1710
CD8+ T CELLS DIFFER IN THEIR ABILITY TO RAPIDLY SECRETE IFN-g
Discussion
In many infection models, B6 mice are considered Th1 responders
because they mount a strong IFN-g–secreting T cell response, and
BALB/c mice are thought of as Th2 responders that typically
produce more IL-10–secreting T cells (46–48). In these models,
the differential ability of APCs to secrete IL-4 and IL-12 following pathogen encounter is thought to create polarized microenvironments that drive the differentiation of naive CD4+ T cells
into either Th1 or Th2 cells (30, 49). The mouse model of L.
monocytogenes infection represents an exception to this paradigm
since L. monocytogenes can induce IL-12 production that leads to
a strong Th1-type immune response in both strains of mice within
5–7 d of infection (50). In fact, coinfection with L. monocytogenes
increased the resistance of BALB/c mice to Leishmania major,
a pathogen that typically elicits a nonprotective Th2 response in
BALB/c mice (51). However, cytokine-driven bystander activation
of CD8+ T cells is significantly delayed in BALB/c mice, a phenotype that could be explained by reduced IL-12 secretion (7). In
this article, we show that BALB/c CD8+ T cells are less responsive
to cytokine stimulation both in vitro and in vivo, even when the
cells are incubated in the same local microenvironment as B6
T cells. Thus, the ability of CD8+ T cells to rapidly secrete IFN-g
during L. monocytogenes infection depends on both an “APC
effect” (how much IL-12/IL-18 is produced by macrophages and
DCs) and the intrinsic responsiveness of individual T cells.
Although L. monocytogenes infection induces a significantly
greater IL-12 response in B6 mice compared with BALB/c mice,
we did not observe a difference in the amount of IL-15 or IL-18
produced directly ex vivo. Our results differ from a previous study
by Liu et al. (28) because we examined cytokine production by
whole splenocytes rather than just DCs, and we collected supernatants after just 6 h of ex vivo incubation, a period corresponding
to 15–21 h after L. monocytogenes infection. Prolonged in vitro
incubation (24 h) did result in an increased production of IL-15
and IL-18 in B6 mice, similar to that observed by Liu et al. for IL15. However, a robust rapid IFN-g response can be detected in B6
mice within 16–18 h of L. monocytogenes infection, so the necessary stimuli must be present in vivo within this time frame. It
is not currently possible to measure the concentration of a given
cytokine in the local microenvironment between two cells in the
spleen, so there could be localized differences in IL-15 or IL-18
secretion in vivo that influence the ability of CD8+ T cells to
rapidly secrete IFN-g. However, our data suggest that IL-12 is the
critical cytokine needed to prime memory phenotype CD8+ T cells
to be able to respond to available levels of IL-18 that are secreted
during L. monocytogenes infection.
In B6 mice, only 29% of the CD44hiCD8+ T cells in the spleen
(corresponding to ∼5% of total splenic CD8+ T cells) were capable of rapidly producing IFN-g, even when incubated under optimal conditions in vitro, with most memory phenotype T cells
requiring up to 3 d to begin secreting IFN-g. Surprisingly, the
CD8+ T cells in naive B6 mice that were IFN-g+ within 18 h of
exposure to IL-12 plus IL-18 were not limited to any previously
defined subset of memory phenotype cells. Instead, our results
suggest that IL-12 and IL-18 responsiveness is a property that
varies among individual CD44hiCD8+ T cells.
In naive BALB/c mice, very few CD44hiCD8+ T cells appear to
be primed for rapid IFN-g secretion. Although we did not find
significant differences in the representation of any memory phenotype subsets in the spleens of BALB/c mice compared with B6
mice, we did find a greater IFN-g secretion defect in BALB/c
T cells with a virtual memory phenotype compared with true
Ag-experienced cells. This suggests that signaling through the
TCR results in comparable activation of BALB/c and B6 memory
CD8 + T cells, but that homeostatic proliferation induced by
growth factors or interactions with self MHC class I results in
a less complete activation status in BALB/c T cells. Because the
vast majority of studies describing CD8+ cells with a memory-like
phenotype have used only B6 mice, it is possible that the pathways
leading to a partially activated phenotype differ in other mouse
strains.
A key finding of this study was that T cell-intrinsic factors are
critical for determining whether an individual CD8+ T cell can
rapidly produce IFN-g. The most obvious feature that could explain a differential response to cytokine stimulation would be
differing levels of expression of the receptors for IL-12 and IL-18.
We did not observe such differences in resting CD8+ T cells;
however, the cells that rapidly secreted IFN-g were able to quickly
upregulate surface expression of both IL-12R and IL-18R. In
B6 mice, the percentage of CD44hiCD8+ cells that expressed
IL-12Rb2 (∼25%) or IL-18Ra (∼28%) within 10 h of stimulation with IL-12 plus IL-18 approximated the percentage of cells
that expressed detectable levels of IFN-g 6–10 h later. These data
suggest that rapid assembly of IL-12R and IL-18R complexes on
the cell surface is a major factor governing the response time for
IFN-g secretion in individual B6 memory phenotype T cells. In
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After 18 h incubation with rIL-12 plus rIL-18, a third of the Agexperienced (CD49dhi) memory phenotype (CD44hi) CD8+ T cells
in naive B6 spleen were IFN-g+ (Fig. 8C). BALB/c mice displayed a decreased but comparable response, with nearly 20% of
the CD49dhi cells rapidly secreting IFN-g. In contrast, a much
greater defect was observed for CD49dlo virtual memory cells,
with only 3% of naive BALB/c T cells producing IFN-g compared
with 22% of the B6 T cells (Fig. 8C). These data suggest that
memory phenotype CD8+ T cells that became partially activated
through homeostatic proliferation account for most of the difference we observed in rapid IFN-g production in B6 versus BALB/c
mice. In fact, CD44hiCD8+ T cells from L. monocytogenesimmune BALB/c mice did rapidly secrete IFN-g after secondary in vitro exposure to either L. monocytogenes or the immunodominant peptide listeriolysin O (LLO)91–99 (Fig. 8D), a result
consistent with previous work by Raué et al. (17), who examined
rapid IFN-g expression in lymphocytic choriomeningitis virusimmune BALB/c mice. More than 95% of the BALB/c CD44hi
CD8+ T cells that responded to peptide stimulation within 18 h
expressed high levels of CD49d on the cell surface, a finding that
confirms this molecule as a marker of Ag-experienced cells (Fig.
8E).
These results indicated that prior signaling through the TCR
could prime CD44hiCD8+ T cells to induce rapid IFN-g secretion
via a pathway that can readily be activated in either BALB/c or B6
mice. To further demonstrate this point, we tested the ability of
CD44hiCD8+ T cells isolated from BALB/c mice that had been
previously immunized with a sublethal dose of L. monocytogenes
to rapidly secrete IFN-g after incubation with IL-12 plus IL-18.
As shown in Fig 8C, a third of the CD49dhi T cells were IFN-g+
18 h later, a response that was comparable to that seen in naive
B6 mice. In contrast, there was no significant difference in the
response of the CD49dlo cells in naive mice compared with
L. monocytogenes-immune mice, and the number of IFN-g+ cells
in either group was much lower than observed in naive B6
mice. Collectively, these results suggest that T cell-intrinsic differences in memory phenotype or virtual memory cells, rather
than true Ag-experienced cells, determine the overall magnitude
of cytokine-driven bystander activation of CD44hiCD8+ T cells
during infection.
The Journal of Immunology
pattern, with most of the T cells expressing relatively high levels
of CD44. However, cells that rapidly secreted IFN-g in either
mouse strain consistently displayed the highest levels of CD44 on
the cell surface, allowing us to discriminate a CD44hi population
in both BALB/c and B6 mice. Activated isoforms of CD44 can
bind hyaluronic acid, and it is thought that high-level expression
of CD44 on memory CD8+ T cells promotes migration of the cells
into inflamed tissues (60). However, given the differences in regulation of CD44 expression between B6 and BALB/c mice, it is
likely that as yet undefined conformational changes upon activation of CD44 are more important than the absolute levels of protein expressed on the cell surface. Note that the CD44 staining pattern on BALB/c CD8+ T cells is very similar to CD44 expression levels on CD8+ T cells isolated from human PBMCs (E.N.
Bou Ghanem and S.E.F. D’Orazio, unpublished observations).
Preliminary data from our laboratory indicate that human CD8+
T cells also display a differential ability to rapidly secrete IFN-g,
and that individuals express a wide range of phenotypes with
regard to IL-12 plus IL-18 responsiveness (E.N. Bou Ghanem and
S.E.F. D’Orazio, manuscript in preparation). Thus, BALB/c and
B6 mice represent the two extremes of a spectrum of responses
that are observed in humans. Because the ability of CD8+ T cells
to rapidly secrete IFN-g varies considerably among individuals,
cytokine-driven bystander activation may be a critical component
of the innate immune responses that define overall host resistance
to infection with intracellular pathogens.
Acknowledgments
We thank Greg Baumann and J. Scott Bryson for technical assistance and
Yasuhiro Suzuki for critical reading of this manuscript.
Disclosures
The authors have no financial conflicts of interest.
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contrast, up to 10% of BALB/c CD44hiCD8+ T cells expressed
IL-18Ra on the cell surface 10 h poststimulation, but only a third
of those cells were able to rapidly secrete IFN-g. This suggests
that there are T cell-intrinsic properties downstream of receptor
expression that also play a role in the weaker IFN-g response
of BALB/c memory phenotype T cells.
The IL-12–dependent pathway to IFN-g production in T cells is
well characterized, but it has been studied mainly in the context
of naive CD4+ T cells differentiating into Th1 cells after TCR
stimulation, and it is not yet clear how this will apply to bystander
activation of CD8+ T cells, which has a strict requirement for both
IL-12 and IL-18. After ligand binding, the IL-12R complex activates the Jak/STAT pathway, resulting in a signaling complex
that contains both phosphorylated Tyk2/Jak2 and phosphorylated
STAT4, while signaling through the IL-18R activates the MEK
and NF-kB pathways and results in activation of the transcription
factor AP-1 (20). Activated STAT4 directly binds to a region of
the IFN-g promoter (2180 to 2280 bp) that is known to be responsive to IL-12 and not TCR signaling; however, Nakahira et al.
(52, 53) showed that the STAT4/AP-1 complex induced by IL12 plus IL-18 has a higher affinity for binding the IFN-g promoter. STAT4 can also indirectly promote IFN-g expression by
increasing the transcription of both T-bet (54) and ERM (55),
transcription factors that also have binding sites in the IFN-g
promoter. Using a flow cytometric technique to detect activated
STAT4 at the single cell level, we showed in this study that a
slightly higher percentage of B6 CD8+ T cells contained phosphorylated STAT4 after incubation with IL-12 compared with
BALB/c CD8+ T cells. Thus, the ability to quickly activate STAT4
could be another T cell-intrinsic factor that contributes to overall
IL-12/IL-18 responsiveness.
It is likely that other T cell-intrinsic properties also contribute to
the variable responses of memory phenotype CD8+ T cells. For
example, IL-18 may induce higher levels of GADD45 proteins in
B6 T cells, which would allow for greater activation of the MAPK
pathway via MEKK4 (56). Alternatively, B6 CD8+ T cells may
express higher levels of key signaling intermediates in either the
MEK/NF-kB or TYK2/Jak2 pathways such that a lower level of
stimulus is needed because the preformed cytosolic proteins can
readily be phosphorylated. If this were true, then BALB/c T cells
may need sustained stimulation to upregulate expression of these
signaling intermediates enough to then trigger expression of IFNg. Such a signaling threshold model was proposed by Chandok
et al. (57), who noted that elevated levels of ZAP70 protein were
needed to see rapid production of IFN-g by memory CD4+ T cells
in BALB/c mice. Epigenetic modifications may also determine
whether an individual CD8+ T cell can rapidly secrete IFN-g.
Two recent studies showed that the IFN-g promoter and enhancer
regions in most, but not all, B6 memory CD8+ T cells were
demethylated and exhibited an increase in acetylated histones (58,
59). These studies suggest that the extent of chromatin modifications at the IFN-g locus may ultimately determine the response time needed for an individual T cell to secrete IFN-g
following IL-12 plus IL-18 stimulation. Epigenetic changes in the
loci encoding IL-12R and IL-18R subunits have not yet been
studied in CD8+ T cells, but such differences could partially explain why only a portion of the CD8+ T cells exposed to IL-12
plus IL-18 could quickly upregulate expression of the receptors
for these cytokines.
CD44 is commonly used as a surface marker to identify murine
memory phenotype CD8+ T cells. Typically, B6 CD8+ T cells have
a broad CD44 staining pattern encompassing low (naive), intermediate, and high (memory) expression levels. We showed in
this study that BALB/c CD8+ cells have a less diverse staining
1711
1712
CD8+ T CELLS DIFFER IN THEIR ABILITY TO RAPIDLY SECRETE IFN-g
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