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Program in Immunology and Virology, and
Department of Pathology, University of Massachusetts Medical Center, Worcester, MA 01655
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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revealed that the LCMV-specific CD4+
Thp frequency remains relatively stable following multiple heterologous
virus infections or protein Ag immunizations, even under conditions
that dramatically reduce the LCMV-specific CD8+ CTL
precursor frequency. These data indicate that the CD4+ and
CD8+ memory T cell pools are regulated independently and
that the loss in CD8+ T cell memory following heterologous
virus infections is not a consequence of a parallel loss in the memory
CD4+ T cell population. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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following virus peptide stimulation have shown high frequencies of
virus-specific CD8+ T cells during the acute
infection and have revealed that about 10% of the
CD8+ T cells are LCMV-specific in the memory
state after resolution of an acute LCMV infection (7, 8, 9, 10, 11, 12).
This accounts for an even greater proportion (>30%) of
CD8+ T cells defined by memory markers
(11). This skewing of the memory T cell pool with
CD8+ T cells specific for a single pathogen can
be so great as to create shelf-space problems for the accommodation of
memory T cells should other pathogens induce similar T cell memory
levels. This CD8+ T cell shelf-space problem is
ameliorated by individual T cell clones cross-reacting with more than
one pathogen and by the fact that many memory
CD8+ T cells specific to one pathogen are lost
after infection with subsequent pathogens (3, 13, 14, 15).
CD8+ T cell memory is thus very stable in the
absence of other antigenic challenges (1, 2, 3, 16), but
memory CD8+ T cells are driven toward extinction
by subsequent infections with heterologous agents (3, 12, 14). CD4+ T cell memory to LCMV and other agents is also very stable in the absence of subsequent infections (4, 5, 6, 17, 18, 19), but the resistance of the CD4+ T cells to deletion by stimulation with heterologous viruses has not been evaluated. Some have proposed that CD4+ T cells are needed for the maintenance of CD8+ T cell responses (20), suggesting the possibility that the loss in the CD8+ T cell memory could be secondary to a loss in CD4+ T cell memory. Although potentially interactive, CD4+ and CD8+ cells appear to represent separately controlled T cell pools, as the CD4/CD8 ratio is usually about 2:1 before and after infection but can be inverted to 1:2 or 1:3 during an acute viral infection. Because the CD8+ T cells expand more vigorously than the CD4+ T cells during the acute virus infection (4), there is a higher selective pressure on the CD8+ T cells once the virus is cleared and the T cell numbers decline by apoptosis as the immune system returns to homeostasis (14).
We have recently demonstrated that the LCMV-specific
CD4+ Th precursor (Thp) frequency is extremely
stable into long-term immunity and, using intracellular staining for
MHC class II peptide-specific IFN--producing cells, that as many as
2% of the CD4+ T cells are LCMV-specific into
memory (4, 5). In the present study we asked the following
questions: 1) Does the CD4 T cell memory pool remain stable after
strong challenges of the immune system with heterologous viruses? 2) Is
the loss in CD8+ T cell memory observed after a
series of virus infections secondary to a loss in
CD4+ T cell memory? 3) Do factors that greatly
modulate the CD8+ T cell memory pool also
modulate the CD4+ T cell memory pool, or are
these two memory T cell pools regulated independently? We questioned
whether heterologous virus infections and soluble protein Ags would
perturb the stable LCMV-specific memory CD4+ Thp
pool and demonstrate in this study that memory
CD4+ T cells can resist deletion after a series
of heterologous viral infections, even under conditions resulting in
considerable memory CD8+ T cell loss (3, 12, 14). This argues that the size and specificity of the
CD4+ and CD8+ memory T cell
pools are regulated independently and that the loss in virus-specific
CD8+ T cell memory is not simply a consequence of
a concurrent attrition of the memory CD4+ T cell
population.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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C57BL/6 (H-2b) mice were purchased from The Jackson Laboratory (Bar Harbor, ME). Male mice were used at 2–10 mo of age for most experiments.
Viruses, protein Ags, and inoculations of mice
The LCMV Armstrong strain and the AN3739 strain of Pichinde virus (PV) were propagated in baby hamster kidney cells (BHK21), as described previously (21). The WR strain of vaccinia virus (VV) was grown in L929 cells (22). Murine CMV (MCMV), strain Smith, was obtained from the salivary glands of infected BALB/c mice (23). For acute virus infections, mice were inoculated i.p. with 5 x 104 PFU LCMV, 4 x 106 PFU PV, 1 x 106 PFU VV, or 1 x 104 PFU MCMV. To prevent CD4+ T cell activity against cell debris or FBS Ags present in the virus stock, the virus preparations were either directly diluted in PBS (LCMV and MCMV) or purified over sucrose gradients and diluted in PBS (PV and VV), as described previously (12, 15). Mice receiving multiple virus infections were immunized with a sublethal dose of one virus, rested for at least 8 wk to allow the immune system to return to homeostasis, and then challenged with the next virus. Immune mice used in all memory experiments were infected with the last virus no less than 8 wk previously. For protein Ag stimulation, mice were injected i.p. with either 100 µg keyhole limpet hemocyanin (KLH) (Sigma, St. Louis, MO) in emulsified CFA (Difco, Detroit, MI) or 100 µg OVA (Sigma) in emulsified TiterMax Gold (TMG) adjuvant (Sigma).
Cell preparations and flow cytometry
Mice were sacrificed by cervical dislocation, and spleens were
removed aseptically. Splenic leukocytes were obtained by preparing
single-cell suspensions from spleens and treating them with 0.84%
NH4Cl to lyse the erythrocytes, as described
previously (24). For multicolor FACS analysis, 
1
x 106 cells were stained with PE-conjugated
anti-CD4 (clone H129.19) and either anti-CD44 (clone IM7) or
anti-CD62 ligand (CD62L; clone MEL-14) in PBS supplemented with 2%
FBS and 0.02% NaN3 (staining buffer). All mAb
used in FACS staining were purchased from BD PharMingen (San Diego,
CA). Stained cells were resuspended in PBS, fixed with 2%
paraformaldehyde, washed, resuspended in staining buffer, and analyzed
in two-color mode using a Becton Dickinson (San Jose, CA) FACScan flow
cytometer. Single-color controls were used in all multiparameter FACS
analyses for electronic compensation settings on the flow cytometer,
and between 10,000 and 20,000 events were acquired from each
preparation. Lymphocyte populations were first gated based on forward
scatter and 90° side-scatter and then analyzed using CellQuest
software (Becton Dickinson).
Intracellular IFN- staining
Intracellular cytokine staining was performed as described in
detail previously (5). Briefly, 2 x
106 cells were stimulated for 5 h in the
presence of 10 U/ml of recombinant human IL-2 (BD PharMingen) and 10
µg/ml of brefeldin A (Sigma) in the presence (5 µg/ml) or absence
of one of the two LCMV MHC class II-restricted peptides, GP61–80
(GLNGPDIYKGVYQFKSVEFD) and NP309–328 (SGEGWPYIACRTSIVGRAWE)
(25). The cells were subsequently washed in staining
buffer, blocked with purified anti-FcRII/III mAb (clone 2.4G2;
BD PharMingen), and stained with FITC-conjugated anti-CD4. After
fixation with 2% formaldehyde in PBS, the cells were washed in
permeabilization buffer (staining buffer containing 0.5% saponin;
Sigma) and stained with PE-conjugated anti-mouse IFN- (clone
XMG1.2; BD PharMingen) or a rat IgG1 isotype control mAb (clone R3-34;
BD PharMingen). At least 60,000 events were acquired from each sample.
The data were analyzed using CellQuest software (Becton Dickinson).
Intracellular IFN- staining for virus peptide-specific
CD8+ T cells was performed as described above
using the MHC class I-restricted GP33–43 (KAVYNFATCGI) or NP396–404
(FQPQNGQFI) peptides.
LDA of virus-specific precursors
LDA to detect LCMV-specific CD4+ Thp were performed as described previously (4). To determine the frequency of peptide-specific CD4+ Thp, peritoneal exudate cells were pulsed with 5 µg/ml of one of the two known LCMV MHC class II-restricted peptides, GP61–80 and NP309–328. For determinations of KLH or OVA-specific CD4+ Thp, peritoneal exudate cells were pulsed with 100 µg/ml of the appropriate protein Ag. The frequencies were corrected for the purity of the sorted CD4+ populations. LDA to detect LCMV-specific CD8+ CTL precursor (CTLp) were performed as described previously (3, 13).
Statistical analysis
CD4+ Thp and CD8+
CTLp frequencies were calculated using 
2
analysis according to the method of Taswell (26) on a
computer program kindly provided by Dr. Richard Miller (University of
Michigan, Ann Arbor, MI).
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Fig. 1
shows the percentage of
CD4+ and CD8+ T cells in
the spleen of C57BL/6 mice acutely infected with either LCMV, VV, PV,
or MCMV. Also shown is the increase in the total leukocyte number in
the spleen that occurs during each of these virus infections
(3). In agreement with previous work (4, 24),
LCMV infection results in a 2-fold increase in the total leukocyte
number in the spleen and a conversion of the CD4 to CD8 ratio from 2:1
to 1:2–3. Each of the other viruses also induced an increase in the
total leukocyte number in the spleen as well as a conversion of the CD4
to CD8 ratio (Fig. 1), but none of the other viruses induced these
changes to the extent LCMV infection does. Correlating with the
inversion of the CD4 to CD8 ratio, each of the viruses also induced a
marked increase in the total number of CD8+ T
cells. Fold-increases in the total number of CD8+
T cells per spleen at day 9 p.i. were as follows: LCMV, 5.6; VV,
1.7; PV, 2.7; and MCMV, 2.2. In contrast, LCMV, VV, and PV infection
induced no increase in the total number of CD4+ T
cells in the spleen, whereas MCMV infection induced only a slight
1.3-fold (i.e., 30%) increase. It has been reported that there is an
increase in the proportion of CD4+ T cells
expressing increased amounts of CD44 and lower amounts of CD62L
following an acute LCMV infection (4, 6, 27, 28). Fig. 2 shows that each of these virus
infections similarly induces the modulation of the activation markers
CD44 and CD62L on gated CD4+ T cells, consistent
with an activated cell phenotype (29, 30). To further
examine the extent to which each of these viruses activate
CD4+ T cells, we performed intracellular IFN-
staining on PMA- and ionomycin-stimulated CD4+ T
cells isolated from naive mice or mice acutely infected with either
LCMV, VV, PV, or MCMV. As can be seen in Fig. 3, each of the viruses stimulates
increased frequencies of IFN--producing CD4+ T
cells. This, combined with the up-regulation of activation markers
discussed above, suggests that all of these viruses are capable of
inducing an activated CD4+ T cell response, even
though the total number of CD4+ T cells in the
spleen remains relatively constant.
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LCMV-specific CD4+ Thp and
CD8+ CTLp frequencies remain quite stable in
long-term immunity (1, 2, 3, 4, 5, 6). However, heterologous virus
infections have a profound impact on the memory
CD8+ T cell pool by causing reductions in the
CD8+ CTLp frequency to earlier virus infections
(3, 12, 14). In this study, we examined the stability of
the LCMV-specific CD4+ Thp frequency following
multiple heterologous virus infections to determine whether these
heterologous viruses could also induce a similar perturbation in the
memory CD4+ T cell compartment. We focused on
experiments examining LCMV-immune mice that had been challenged with
1–3 heterologous viruses. Fig. 4 shows
that the LCMV-specific CD4+ Thp does not decline
(within the 95% confidence limits) following three heterologous virus
infections (PV, VV, and MCMV) in three individual experiments using a
pool of three mice per group. A CD8 LDA was performed in one of these
assays (Fig. 4, bottom group) to show that within the same
mice, the LCMV-specific CD8+ CTLp frequency does
decline, as we have previously reported (3). The
CD8+ CTLp frequency declined 6-fold in LCMV- and
PV-immune mice and 18-fold in LCMV-, PV-, VV-, and MCMV-immune mice as
compared with LCMV-immune mice. This decline in CTL memory is more
dramatic than the usual 4-fold reduction we usually see with this virus
combination, but we presented this in this study to show that in some
experiments major declines in CD8+ T cell memory
can occur under conditions of CD4+ T cell
stability. Experiments on the stability of LCMV-specific T cell memory
were also done in mice infected first with PV and then sequentially
with LCMV, VV, and MCMV. Fig. 5 shows
three experiments indicating that the LCMV-specific
CD4+ Thp frequency did not decline after two
additional infections. In contrast, CD8+ CTLp to
LCMV declined by more than 2-fold (Fig. 5, bottom), a result
consistent with our previous studies (12).
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shows two independent
LDA experiments, using a pool of three mice per group, in which there
is no significant decline in the LCMV-specific
CD4+ Thp frequency to whole virus or to either of
the two LCMV MHC class II-restricted CD4 peptides in LCMV-, PV-, VV-,
and MCMV-immune mice. Thus, in a total of five independent LDA
experiments (using a pool of three mice per group) shown in Table I and
Fig. 4
, there was <2-fold decrease in the LCMV-specific
CD4+ Thp frequency to whole virus and in two
experiments (using a pool of three mice per group) there was <2-fold
decrease in the LCMV-specific CD4+ Thp frequency
to the GP61–80 epitope. The NP309–328 epitope represents a much
smaller proportion of the IL-2-producing LCMV-specific
CD4+ T cells detectable in LDA and is so low that
sometimes the frequency of NP309–328 peptide-specific
CD4+ Thp is not significantly different (within
the 95% confidence limits) from sham-immunized controls. This was the
case in the first of the two experiments performed in Table I examining
LCMV-, PV-, VV-, and MCMV-immune mice. In the second experiment the
frequencies were considerably above background, and there was no
decline (within the 95% confidence limits) in the frequency of
NP309–328 peptide-specific CD4+ Thp following
three heterologous virus infections.
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-producing
CD4+ T cells following multiple heterologous virus
infections
LCMV-specific memory CD4+ and
CD8+ T cells can be detected following
stimulation with peptide and IL-2 in the presence of brefeldin A and
staining for intracellular IFN-. In this study we questioned whether
the frequency of LCMV-specific memory CD4+ T
cells remained stable by this assay, as it did using the IL-2-based CD4
LDA. Fig. 6 shows data from one of four
experiments comparing the frequencies of LCMV peptide-specific
CD4+ and CD8+ T cells in
mice receiving only LCMV or else receiving subsequent infections with
three additional viruses. In these experiments, each group represents a
pool of splenic lymphocytes from three mice per group.
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A shows, along with controls, intracellular IFN-
flow cytometry plots of CD4+ T cells exposed to
GP61–80 peptide and CD8+ T cells exposed to the
MHC class I immunodominant NP396–404 peptide. Of note is that
virtually no signal was found in peptide-treated cells from uninfected
mice, and background IFN- production by T cells from immune mice in
the absence of peptide was low. Detectable and relatively constant
levels of GP61–80 peptide-specific CD4+ T cells
were seen in mice immune to the four viruses or only to LCMV. However,
in these same mice, substantial losses in NP396–404 peptide-specific
CD8+ T cell memory occurred after the additional
virus infections.
Fig. 6B plots the levels of Ag-specific T cells as a
function of each virus infection. There is some experimental variation,
and in this experiment NP396–404 peptide-specific
CD8+ T cell frequencies declined with each
successive infection; GP33–43 peptide-specific
CD8+ T cell frequencies showed an overall
decline, though in this case there was a modest rebound after the
fourth infection. In contrast, the GP61–80 peptide-specific CD4 memory
remained stable after four viruses. There was a small dip in the
GP61–80 peptide-specific CD4+ T cells after the
third virus, VV, in this and two other experiments, but it is
noteworthy that after the fourth infection the GP61–80 frequencies
returned to normal. It is clear that the dynamics in shifting
frequencies of the CD4+ and
CD8+ T cell populations seem very independent of
each other.
Table II shows additional data with both
GP61–80 and NP309–328 peptide-specific CD4+ T
cells in a group of mice immunized 2 years previously to the group
presented in Fig. 6. In this study, CD4+ T cell
frequencies were analyzed from individual mice and are presented as the
means ± SDs. In the experiment shown in Table II, upper panel,
the mice had a relatively high NP309–328 response, but, like the
GP61–80 response, it did not decline after three additional virus
infections. Compared with this are CD8+ T cell
frequencies to the MHC class I GP33–43 peptide from this earlier group
of mice. These data, which represent the mean of two separate
experiments, each with a pool of three mice per group, show a 5-fold
decline in the GP33–43 peptide-specific CD8+ T
cell memory, as we have previously published (12). In a
separate series of experiments displayed in Table II, lower panel,
LCMV-specific memory CD4+ T cell responses were
quantified in mice that first received a PV infection and then were
infected with LCMV, VV, and MCMV. Again, in two separate experiments
with a total of six individual mice per group, both GP61–80 and
NP309–328 peptide-specific memory CD4+ T cell
responses remained relatively stable. These experiments collectively
show, at the single-cell level, that the CD4+ and
CD8+ memory T cell pools are independently
regulated after subsequent infections and that the
CD4+ T cell pool appears to be more stable under
these conditions of infection.
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One reason why there is no dramatic decline in the
CD4+ Thp frequency following multiple
heterologous virus infections may be that all of these virus infections
induce stronger CD8+ than
CD4+ T cell responses (Fig. 1). In an attempt to
induce a CD4+ T cell response under conditions
that do not preferentially expand CD8+ T cells,
we examined the LCMV-specific CD4+ Thp frequency
in LCMV-immune mice challenged with one or two consecutive protein Ags
in adjuvant. Table III shows in three
separate experiments, using a pool of three mice per group, that in
LCMV-immune mice challenged with the complex protein Ag KLH in CFA that
there was <2-fold decrease in the LCMV-specific
CD4+ Thp frequency to whole virus and there was
<2-fold decrease in the LCMV-specific CD4+ Thp
frequency to the NP309–328 epitope. The LCMV-specific
CD4+ Thp frequency to the GP61–80 epitope
declined 3.3-fold in one of three experiments, but this drop was not
consistent, as the other two experiments demonstrated no such
significant decline (within the 95% confidence limits, and average
decline in the three experiments was 1.2-fold). Likewise, challenge of
LCMV-immune mice with CFA alone resulted in <2-fold reduction in the
LCMV-specific CD4+ Thp frequency to whole virus
and the NP309–328 epitope, whereas there was a 2.2-fold decrease to
the GP61–80 epitope in only one of the three experiments. Finally, two
additional experiments were performed in which LCMV-immune mice were
challenged with KLH in CFA followed by OVA in a different adjuvant
(TiterMax Gold). In each of these experiments there was <2-fold
decrease in the LCMV-specific CD4+ Thp frequency
to whole virus or to either of the two MHC class II-restricted peptides
(Table III). A measurable protein Ag-specific
CD4+ Thp frequency was detectable in all of these
experiments, demonstrating that each of these immunizations efficiently
stimulated a CD4+ T cell response (see Table III). These results demonstrate that regardless of the
CD4+ T cell stimuli, the LCMV-specific
CD4+ Thp frequency remains relatively stable,
even though modest declines are sometimes observed. However, none of
the Ags tested could stimulate CD4+ T cell
responses to the magnitude of virus-induced CD8+
T cell responses.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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-producing cells.
In contrast, heterologous viral infections reproducibly cause
significant decreases in otherwise stable virus-specific
CD8+ T cell memory (3, 12, 14).
These virus-induced decreases in CD8+ T cell
memory have now been shown by LDA for CTLp (3), by flow
cytometry for IFN--producing cells in response to MHC class I
peptides (Ref. 12 , Table II, and Fig. 6), and by frequency
analyses of CD8+ T cells binding chimeric IgG-MHC
molecules loaded with MHC class I viral peptides (12).
This dichotomy, which is illustrated well in Table II, Fig. 6, and by
the third experiment in Fig. 4, indicates that
CD4+ and CD8+ memory T cell
pools are regulated independently of each other. This means that the
loss in CD8+ T cell memory is not secondary to a
loss in CD4+ T cells, and it also indicates that
whatever factors drive the loss of memory CD8+ T
cells do not cause attrition of the CD4+ T cell
memory population.
The differences in the stabilities of the
CD4+ and CD8+ memory T cell
pools may not be due to any inherent differences in their regulation
but instead may be a consequence of the apparently higher frequency of
virus-specific CD8+ than
CD4+ T cells within the memory pool (1, 2, 3, 4, 5, 6, 8, 11). Work in the LCMV system has revealed that >10% of the
CD8+ T cells in LCMV-immune mice are
LCMV-specific (8, 11, 12). If memory
CD8+ T cells for each pathogen a host experiences
were preserved at a similar frequency, the CD8+ T
cell memory pool would quickly fill to capacity. These memory T cells
could be accommodated by a continual expansion of the pool, but, even
though the memory pool does moderately increase in size with age,
homeostatic regulation limits this expansion and the size of lymphoid
organs. Therefore, accommodation of new memory cells should require a
deletion of some of the memory cells pre-existing in the pool.
Assessments of the frequency of LCMV-specific
CD4+ memory T cells suggest that they may be
substantially lower than CD8+ memory T cells
(Refs. 3, 4, 5 , Table II, and Fig. 6).
Another factor relevant to memory pool "shelf-space" issues relates
to the different dynamics of the CD4+ and
CD8+ T cell responses during acute viral
infections, as illustrated in Fig. 1. Viruses induce major expansions
in the number of CD8+ T cells but not
CD4+ T cells (3, 4). During that
expansion period CD8+ T cells not specific for
the virus do not increase in number and, therefore, are diluted out
(31). After clearance of virus, the cells in the immune
system undergo apoptosis and return to preinfection levels
(32). The contraction of the CD8+ T
cell response is much more dramatic than that of the
CD4+ T cell response, and competition for
protective niches in the homeostatic spleen would be much more vigorous
for the CD8+ than for the
CD4+ T cells. Such a competition might be
disadvantageous for the survival of pre-existing memory
CD8+ T cells. Should the
CD4+ T cell number expand as dramatically as the
CD8+ T cells do during viral infection, there
might be similar attrition of the CD4+ T cells.
However, few pathogens or protein Ags stimulate
CD4+ T cell responses to the levels that
CD8+ T cells get stimulated by viral
infections.
A third possible reason for more selective deletion of CD8+ than CD4+ T cell memory may rest on differential sensitivity of these memory populations to apoptotic events. We have recently found that pre-existing memory T cells not specific for an infecting virus undergo apoptosis and decline in number and that such apoptosis is more dramatic in the CD8+ than in the CD4+ T cell compartment (33). In this model there would be an active deletion of memory cells exposed to cytokines in the absence of Ag stimulation, in contrast to a more passive dilution and competition model as described above.
A fourth factor that may be of significance in the preservation of memory T cell populations is whether or not they display cross-reactivity to subsequently encountered Ags. It is well established that any given TCR can see more than one ligand (34, 35, 36, 37, 38), and in some cases these cross-reactive interactions can lead to altered T cell responses and effector functions (39, 40, 41). We have observed in our experiments on the decline of CD8+ T cell memory after heterologous viral infections that cross-reactive memory T cells may be preserved at the expense of the non-cross-reactive ones, which are deleted (12). It is thus possible that CD4+ T cell memory is more stable because there is a higher level of CD4+ T cell cross-reactivity between heterologous Ags or even endogenous self-Ags. In LCMV-immune mice challenged with VV there is a very rapid and profound CD4+ and CD8+ memory-like T cell response that includes CD8+ CTL cross-reactive between LCMV and VV (15). If cross-reactivity at the CD4+ T cell level exists, it may be more difficult to reduce the CD4+ T cell memory by heterologous infections. However, if cross-reactivity were to explain memory CD4+ T cell stability, it would have to be dramatic and very wide-spread, as the stability was observed in epitopes expressed on different proteins, and even diverse Ags such as KLH and OVA, in their respective adjuvants, failed to reproducibly induce a decline in CD4+ T cell memory.
Persistence of the original Ag could also potentially influence the relative stabilities of CD4+ vs CD8+ T cell memory. Soluble protein Ags have been shown to persist in vivo in the form of immune complexes associated with follicular dendritic cells (42, 43). This might serve as a reservoir for CD4+ T cell Ags to exogenously access the class II Ag-presenting pathway. The preservation of CD8+ T cell Ags would most likely require a continued live virus infection of cells to engage the endogenous MHC class I pathway.
The resistance of the LCMV-specific memory CD4+ T cell pool to deletion following heterologous virus infections may play a role in preserving the remaining virus-specific CD8+ CTLp. In support of this, recent work in the influenza virus model has demonstrated that depletion of CD4+ T cells from Ig-/- mice results in diminished influenza virus-specific memory CD8+ T cell responses (44). Immunity against each of the viruses used in this study is primarily dependent on MHC class I-restricted CD8+ CTL (3), which can often function independently of CD4+ T cells to mount a protective response (45, 46, 47, 48, 49). However, recent work has suggested that CD4+ T cells may play a role in the long-term maintenance of memory CD8+ CTL to viruses such as LCMV (20). There is a continually cycling population of LCMV-specific CD8+ CTLp that are capable of mediating cytolysis ex vivo against highly sensitive target cells (3, 4, 50, 51), and the continued presence of CD4+ T cells may help maintain the continued cycling of a small frequency of virus-specific memory CD8+ CTLp, possibly through the production of cytokines such as IL-2. Thus, the maintenance of the LCMV-specific CD4+ Thp frequency may help to maintain the remaining virus-specific CD8+ CTLp to ensure long-term protective immunity.
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Acknowledgments |
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Footnotes |
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2 Current address: Beirne B. Carter Center for Immunology Research, University of Virginia Health Sciences Center, MR-4 Building, 400 Lane Road, Box 801386, Charlottesville, VA 22908.
3 Address correspondence and reprint requests to Dr. Raymond M. Welsh, Department of Pathology, University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, MA 01655.
4 Abbreviations used in this paper: LDA, limiting dilution assays; LCMV, lymphocytic choriomeningitis virus; Thp, Th precursor; PV, Pichinde virus; VV, vaccinia virus; MCMV, murine CMV; KLH, keyhole limpet hemocyanin; CD62L, CD62 ligand; CTLp, CTL precursor.
Received for publication October 27, 2000. Accepted for publication November 1, 2000.
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 B. Combadiere, A. Boissonnas, G. Carcelain, E. Lefranc, A. Samri, F. Bricaire, P. Debre, and B. Autran Distinct Time Effects of Vaccination on Long-Term Proliferative and IFN-{gamma}-producing T Cell Memory to Smallpox in Humans J. Exp. Med., June 7, 2004; 199(11): 1585 - 1593. [Abstract] [Full Text] [PDF]
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R. M. Kondrack, J. Harbertson, J. T. Tan, M. E. McBreen, C. D. Surh, and L. M. Bradley Interleukin 7 Regulates the Survival and Generation of Memory CD4 Cells J. Exp. Med., December 15, 2003; 198(12): 1797 - 1806. [Abstract] [Full Text] [PDF]
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 M. Nascimbeni, E. Mizukoshi, M. Bosmann, M. E. Major, K. Mihalik, C. M. Rice, S. M. Feinstone, and B. Rehermann Kinetics of CD4+ and CD8+ Memory T-Cell Responses during Hepatitis C Virus Rechallenge of Previously Recovered Chimpanzees J. Virol., April 15, 2003; 77(8): 4781 - 4793. [Abstract] [Full Text] [PDF]
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D. K. Smith, R. Dudani, J. A. Pedras-Vasconcelos, Y. Chapdelaine, H. van Faassen, and S. Sad Cross-Reactive Antigen Is Required to Prevent Erosion of Established T Cell Memory and Tumor Immunity: A Heterologous Bacterial Model of Attrition J. Immunol., August 1, 2002; 169(3): 1197 - 1206. [Abstract] [Full Text] [PDF]
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M. Sester, U. Sester, B. Gartner, B. Kubuschok, M. Girndt, A. Meyerhans, and H. Kohler Sustained High Frequencies of Specific CD4 T Cells Restricted to a Single Persistent Virus J. Virol., March 19, 2002; 76(8): 3748 - 3755. [Abstract] [Full Text] [PDF]
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 C. Dercamp, V. Sanchez, J. Barrier, E. Trannoy, and B. Guy Depletion of Human NK and CD8 Cells prior to In Vitro H1N1 Flu Vaccine Stimulation Increases the Number of Gamma Interferon-Secreting Cells Compared to the Initial Undepleted Population in an ELISPOT Assay Clin. Vaccine Immunol., March 1, 2002; 9(2): 230 - 235. [Abstract] [Full Text] [PDF]
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 L. R. Wedderburn, A. Patel, H. Varsani, and P. Woo Divergence in the degree of clonal expansions in inflammatory T cell subpopulations mirrors HLA-associated risk alleles in genetically and clinically distinct subtypes of childhood arthritis Int. Immunol., December 1, 2001; 13(12): 1541 - 1550. [Abstract] [Full Text] [PDF]
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