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Cutting Edge |



*
Immunology Division, Walter and Eliza Hall Institute of Medical Research, Royal Melbourne Hospital, and
Department of Microbiology and Immunology, University of Melbourne, Parkville, Victoria, Australia; and
Cooperative Research Center for Vaccine Technology, Queensland Institute of Medical Research, Royal Brisbane Hospital, Herston, Queensland, Australia
| Abstract |
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| Introduction |
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Since identification of the CD154/CD40-helper pathway for CTL induction, other alternative pathways have been reported. Several groups have provided evidence that RANK (TRANCE receptor, OPG) and RANKL can function in a similar way to CD154/CD40 for licensing dendritic cells (15). Furthermore, Pardoll and colleagues (16) provided in vitro evidence for a soluble helper factor that was not IL-2. To what degree each of these and other unidentified molecular interactions contribute to CD4 T cell-mediated help for CTL induction is unclear, but probably depends on the circumstances in which priming takes place.
To better understand the requirements for help during CTL induction, we began examining the fate of CTL when cross-primed by OVA-coated spleen cells (17) in the absence of help. To do this, we used the OVA-specific CD8 T cells from OT-I transgenic mice (18, 19) to follow naive precursors primed in the absence of help. To our surprise, these precursors were able to respond normally to priming in the absence of CD4 T cells, leading us to discover that the CTL precursor frequency dramatically affected the dependence on CD4 T cell help.
| Materials and Methods |
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All mice used were between 6 and 12 wk old, and were bred and maintained at the Walter and Eliza Hall Institute for Medical Research (Parkville, Victoria, Australia). Rag-1-deficient OT-I mice (20) and class II-deficient mice (21) have been described. Rag-1-deficient OT-I mice have been back-crossed to C57BL/6 eight generations. CD154-deficient OT-I mice were generated by intercrossing OT-I and CD154-deficient mice. These mice were three to nine generations back-crossed to C57BL/6. Ly5.1 congenic OT-I mice were generated by intercrossing OT-I and Ly5.1 congenic C57BL/6 mice.
Priming of mice
For priming mice with cell-associated OVA (17), bm1 spleen cells (2 x 108/ml) were incubated with 10 mg/ml OVA for 10 min at 37°C. Cells were washed, irradiated for 1000 rad, and after two more washes, 20 x 106 cells were injected i.v. into mice. The cells used to prime were referred to as OVA-coated spleen cells.
Preparation of OT-Is for adoptive transfer
OT-I cells were prepared as described (22, 23).
Briefly, OT-I cells were derived from the spleen and lymph nodes of
Rag-1-deficient OT-I mice. Cells were treated with RL172 (anti-CD4)
and J11d (anti-heat stable Ag) for 30 min on ice,
centrifuged, and then depleted by treatment with rabbit complement for
20 min at 37°C. Prepared cells were examined by flow cytometry for
the proportion of V
2+CD8+ cells to determine
the number of OT-I cells for adoptive transfer.
Fluorescent labeling of OT-I cells
CFSE-labeling was performed as previously described (24). Briefly, semipurified OT-I cells were resuspended in PBS containing 0.1% BSA (Sigma Aldrich, St. Louis, MO) at 107 cells/ml. For fluorescence-labeling, 1 µl of a CFSE (Molecular Probes, Eugene, OR) stock solution (5 mM in DMSO) was incubated with 107 cells for 10 min at 37°C.
Assessment of OT-I numbers
Cells from pooled lymph node and spleen were stained using the
following mAbs: PE-conjugated anti-CD8 (YTS 169.4; Caltag
Labortatories, San Francisco, CA), FITC-conjugated anti-V
5.1/2
TCR (MR9-4; Ref. 24), and biotinylated anti-V
2 TCR
(B20.1; Ref. 25). mAbs were conjugated to biotin or FITC
using standard protocols. Biotin-labeled mAbs were detected with
streptavidin-Tricolor (Caltag Laboratories). Analysis was performed on
a FACScan (BD Biosciences, Mountain View, CA). Live gates were set on
lymphocytes by forward and side scatter profiles. A total of
10,00020,000 live cells were collected for analysis. OT-I T cells
were identified as CD8+, V
2+, and
V
5+. This proportion in a noninjected recipient was
<2%. CD8+, V
2+, and V
5+ T
cells above this background were considered to be OT-I T cells.
OVA-specific CTL generation
OVA-specific CTL were generated as previously described (3). Briefly, B6 mice were primed with OVA-coated spleen cells. After 7 days, spleens were removed and single cells were cultured with 108 1500 rad-irradiated OVA257264-coated B6 spleen cells for 6 days. Cytotoxicity was assessed in a conventional 51Cr-release assay using the H-2b cell line EL4 with and without OVA-peptide-coating as targets. The percentage of OVA-specific lysis represents the lysis of peptide-coated EL4 minus the lysis of EL4 alone, i.e., peptide-dependent lysis. Lysis of unlabeled EL4 targets was <10% in all experiments. Lytic units were calculated by determining the minimum number of effectors required to generate 20% OVA-specific lysis and then dividing this into the total number of effectors generated per responder spleen.
Intracellular IFN-
staining
Staining for intracellular IFN-
was performed as
previously described (26).
| Results and Discussion |
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As previously reported (17), B6 mice can generate
OVA-specific CTL when primed with irradiated spleen cells either
osmotically loaded or coated with whole OVA. This response is mediated
by cross-priming, where host APC capture donor cells and cross-present
associated OVA on both class I and class II molecules. Generation of
CTL by this type of priming is CD4 dependent, and therefore cannot be
achieved in class II-deficient mice (Fig. 1
; Ref. 13). This
observation prompted us to examine the fate of precursor CTL primed in
the absence of help.
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The ability of OT-I T cells to proliferate in response to Ag,
independently of CD4 help, prompted us to examine whether this was
accompanied by an expansion in number. To do this, 2 x
106 OT-I cells were transferred into class II-deficient
recipients and the fate of the cells determined following
cross-priming. The experiments shown measured the number of OT-I cells
present at 2 wk after priming. However, similar results were also
observed at 1 or 6 wk. Fig. 3
, A and B, indicates that both normal and
class II-deficient hosts showed an equivalent increase in the number of
OT-I cells after cross-priming. The expansion of OT-I cells in class
II-deficient mice was surprising, and had two alternative explanations:
either 1) expansion was not accompanied by differentiation into
effector CTL and the mice were still functionally unprimed, or 2)
expansion was indicative of priming, and the behavior of these OT-I
cells was different from that of a normal repertoire.
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Large numbers of OT-I cells are required to allow priming in the absence of help
The above data indicated that adoptively transferred OT-I cells
could be cross-primed in the absence of CD4 T cell help. There were two
main explanations for why these transgenic cells were no longer helper
dependent: either 1) OT-I cells were helper independent because they
represented a very high affinity CTL, or 2) the presence of a large
number of CTL precursors converted a helper-dependent response into a
helper-independent response. To address this issue, class II-deficient
mice were injected with 10-fold decreasing numbers of OT-I cells, from
106 to 104 cells, and then primed and assessed
for CTL activity (Fig. 4
). As shown,
class II-deficient mice only generated CTL immunity when reconstituted
with 106 OT-I cells. Because 105 OT-I cells are
detectable by flow cytometry using tetramer-staining, but we cannot
detect OVA-specific cells in a normal naive repertoire (data not
shown), this number of OT-I cells must vastly out-number the normal
frequency of precursors. Failure to detect priming at this frequency
indicates OT-I cells do not have a special property that allows them to
be primed in the absence of help, but rather it is the high precursor
frequency achieved when 106 cells are transferred that
enabled helper-independent priming.
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As we and others have reported, CD4 T cells provide help for CTL
via the expression of CD154, which signals CD40 on the cross-priming
APC, converting it into a cell capable of CTL priming (5, 13, 14). We wondered whether large numbers of OT-I cells might
provide their own help via expression of CD154, particularly since
CD154 has been shown to be expressed by a minor population of CD8 T
cells (27, 28, 29). This has been demonstrated to contribute
to CD8 T cell expansion in mucosal tissues (30). To test
this possibility, OT-I mice were backcrossed to CD154-deficient mice,
and their ability to generate CTL immunity was examined. Like wild-type
OT-I cells, high numbers of CD154 knockout OT-I cells were able to
generate helper-independent CTL responses (Fig. 5
). Therefore, the ability of OT-I cells
to overcome the requirement for help at high doses is not mediated via
CD154.
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The data presented above support a model where the helper
dependence of CTL responses can be strongly influenced by the frequency
of naive CTL precursors. When frequencies are low, help is critical,
but as the number of precursors increase, CTL immunity becomes helper
independent. The ability of CD8 T cells to provide their own help at
high precursor frequencies is consistent with a report that CD8 T cells
can cause DC maturation (31), and the observation that CD8
T cells can boost the response of other CD8 T cells under some
circumstances (32). In this study, we do not observe an
essential role for a CD154 signal in the mechanism by which CD8 T cells
help themselves. Perhaps the simplest explanation is that while single
CD8 T cells are unable to make sufficient cytokines to facilitate their
own expansion, multiple clones together might achieve local threshold
concentrations. Whatever the reason for helper independence at high
frequencies, our report provides a potential explanation for some of
the variability in the helper dependence of CTL priming with different
immunogens. If an immunogen is complicated, and therefore contains
multiple CTL epitopes, then the frequency of CTL precursors may be
high, and the response helper independent. This could, for example,
apply to some anti-viral responses that appear to be helper
independent (7, 8, 9). In contrast, if the immunogen is
simple and contains few CTL epitopes, such as the case for OVA, then
responses will most likely require CD4 T cell help (Fig. 1
and Ref.
13).
The data presented in this report highlight the dramatic effect of frequency on the helper dependence of CTL. This report also highlights the issue that examination of naive CTL using high frequencies of TCR transgenic T cells must consider the potential contribution of increased frequency.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. William R. Heath, Immunology Division, The Walter and Eliza Hall Institute, P. O. Royal Melbourne Hospital, Parkville 3050, Victoria, Australia. E-mail address: heath{at}wehi.edu.au ![]()
Received for publication August 21, 2001. Accepted for publication November 26, 2001.
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a haplotypes: determination with a V
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