CD4+ T helper cell response is required for memory in CD8+ T lymphocytes induced by a poly(I:C)-adjuvanted MHC I-restricted peptide epitope

It was known that double-stranded RNA (dsRNA) has vaccine adjuvant activity by acting through the dsRNA/toll-like receptor (TLR)3 signaling. More recently, it was reported that immunization of mice with a poly(I:C)-adjuvanted major histocompatibility (MHC) class I-restricted peptide epitope derived from a tumor-specific antigen protein induced strong tumor-killing CD8 CTL responses. However, it remains unclear whether the poly(I:C) can help an MHC class I-restricted peptide epitope to induce a strong and functional memory CD8 CTL response in the absence of a T helper (TH) response. By using in vivo CTL and direct tumor challenging assays, we demonstrated that, although poly(I:C) as a vaccine adjuvant significantly enhanced the immune responses induced by an MHC I-restricted peptide epitope, MHC class II-restricted TH cell responses were still required to generate memory CTL responses. The MHC class II-restricted TH epitopes can be cognate, noncognate, or even from the necrotic bodies of un-related tumor cells. Thus, future efforts of using synthetic dsRNA as an adjuvant to induce specific and long-lasting memory CTL immune responses should include the provision of an appropriate TH cell response.     

MHC class II-restricted tumor antigens recognized by CD4+ T cells: new strategies for cancer vaccine design

The adoptive transfer of tumor-infiltrating lymphocytes (TIL) can mediate tumor regression in patients with melanoma. This finding has led to the identification and characterization of tumor-associated antigens recognized by CD8+ TIL. Several clinical trials based on the genes recognized by these CD8+ T cells have been attempted, but with only limited success. Meanwhile, increasing evidence has demonstrated that CD4+ T cells play important roles in generating and maintaining antitumor immune responses in animal models. These data suggest that it may be necessary to engage both CD4+ and CD8+ T cells for more effective antitumor immunotherapy. In this report, we review emerging molecular approaches in cloning major histocompatibility complex (MHC) class II restricted tumor antigens recognized by CD4+ T cells as well as approaches to identify new MHC class II-restricted epitopes from known tumor antigens recognized by CD8+ cytotoxic T lymphocytes and/or antibodies. Progress made in this field has shed light on the roles of tumor antigen-specific CD4+ T cells in humans; it has also provided new insights into the understanding of tumor genesis and the interaction between tumor and the immune system. More importantly, the discovery of MHC class II-restricted tumor antigens has provided opportunities for developing a new generation of cancer vaccines aimed at eliciting both CD4+ and CD8+ T-cell responses against tumor.     

Antagonist Peptide Selects Thymocytes Expressing a Class II Major Histocompatibility Complex–restricted T Cell Receptor into the CD8 Lineage

CD4/CD8 lineage decision is an important event during T cell maturation in the thymus. CD8 T cell differentiation usually requires corecognition of major histocompatibility complex (MHC) class I by the T cell receptor (TCR) and CD8, whereas CD4 T cells differentiate as a consequence of MHC class II recognition by the TCR and CD4. The involvement of specific peptides in the selection of T cells expressing a particular TCR could be demonstrated so far for the CD8 lineage only. We used mice transgenic for an MHC class II-restricted TCR to investigate the role of antagonistic peptides in CD4 T cell differentiation. Interestingly, antagonists blocked the development of CD4⁺ cells that normally differentiate in thymus organ culture from those mice, and they induced the generation of CD8⁺ cells in thymus organ culture from mice impaired in CD4⁺ cell development (invariant chain–deficient mice). These results are in line with recent observations that antagonistic signals direct differentiation into the CD8 lineage, regardless of MHC specificity.     

CD4+8- thymocytes bearing major histocompatibility complex class I-restricted T cell receptors: evidence for homeostatic control of early stages of CD4/CD8 lineage development.

During thymus development CD4+ CD8+ precursor cells differentiate into mature CD4+ and CD8+ T cells expressing T cell receptors (TCR) that recognize foreign antigens in association with major histocompatibility complex (MHC) class II or I molecules, respectively. Studies with TCR transgenic mice have shown that the accumulation of mature CD4+ and CD8+ thymocytes is strongly skewed by the MHC restriction specificity of the TCR, thus suggesting that commitment of CD4+ CD8+ precursors to the CD4 or CD8 lineage is a direct consequence of TCR/MHC interactions. However, we show here that CD4+ cells expressing an inappropriate (MHC class I-specific) TCR appear transiently in the neonatal thymus of TCR transgenic mice and can also be found in the periphery of adult TCR transgenic recombination-deficient SCID mice. These data argue that the early stages of CD4 and CD8 lineage development in the thymus are (at least in part) controlled by homeostatic mechanisms independent of appropriate TCR/MHC interactions.     

Depletion of CD8+ cells in human thymic medulla results in selective immune deficiency

CD8 molecules expressed on the surface of a subset of T cells participate in the selection of class I MHC antigen-restricted T cells in the thymus, and in MHC-restricted immune responses of mature class I MHC antigen-restricted T cells. Here we describe an immune-deficient patient with lack of CD8+ peripheral blood cells. The patient presented with Pneumocystis carinii pneumonia and was unable to reject an allogeneic skin graft, but had normal primary and secondary antibody responses. Examination of the patient's thymus revealed that the loss of CD8+ cells occurred during intrathymic differentiation: the patient's immature cortical thymocytes included both CD4+ and CD8+ cells while the mature medullary cells expressed the CD4 but not the CD8 protein on their surface. Northern blot and polymerase chain reaction analyses revealed the presence of CD8 alpha and beta mRNA in the patient's thymus but not in the peripheral blood. Both class I MHC antigen expression and the expressed TCR V beta repertoire are normal in this patient. These data are consistent with an impaired selection of CD8+ cells in the patient's thymus and support the role of the CD8 surface protein in thymic selection previously characterized in genetically manipulated and inbred mice.     

Maturational arrest from CD4+8+ to CD4+8- thymocytes in a mutant strain (LEC) of rat

A mutant strain (LEC) of rats was found to have a novel defect in T cell maturation, that is, arrest of differentiation from CD4+8+ to CD4+8- but not to CD4-8+ thymocytes. FACS analyses demonstrated a deficiency in the CD4+8- T cell subset in the thymus and a marked decrease in CD4+ T cells in peripheral lymphoid organs. Expression of the T cell receptor (TCR)/CD3 complex in CD4+8+ and CD4-8+ thymocytes of LEC rats was normal. Expression of class II major histocompatibility complex (MHC) in the thymus of LEC rats was also the same as that of normal rats. These results indicate that maturational arrest occurs only in the transition pathway from CD4+8+ to CD4+8- thymocytes, and that this mutation can not be attributed to the default of expression of either TCR/CD3, CD4, or class II MHC antigen. Consequently, dysfunction of helper T cells was observed in LEC rats, while killer T cells and B cells functioned normally. Although the complete identification of the origin of this mutation requires further studies, it is hoped that such investigations will throw light on the mechanism of positive selection.     

Redirecting human CD4+ T lymphocytes to the MHC class I-restricted melanoma antigen MAGE-A1 by TCR alphabeta gene transfer requires CD8alpha

Adoptive immunotherapy involving the transfer of autologous tumor or virus-reactive T lymphocytes has demonstrated its effectiveness in the eradication of cancer and virally infected cells. Clinical trails and in vitro studies have focused on CD8+ cytotoxic T-cell receptor (TCR) alphabeta lymphocytes since these cells directly kill virally infected- and tumor cells after antigen-specific recognition via their TCR alphabeta. However, increasing evidence suggests that induction of sustained immunity against cancer and viral infections depends on the presence of tumor- or virus-specific CD4+ T lymphocytes, which are restricted by MHC class II. Here, we show that these MHC class II-restricted CD4+ T lymphocytes can efficiently be redirected to MHC class I-restricted tumor cells by retroviral introduction of an HLA-A1/MAGE-A1-specific chimeric two-chain TCR ValphaCalphazeta/VbetaCbetazeta (tcTCR/zeta). However, TCR-transduced CD4+ T lymphocytes were only able to specifically bind to HLA-A1/MAGE-A1 complexes and respond to HLA-A1+/MAGE-A1+ melanoma cells when the CD8alpha gene was cointroduced. These CD4+/CD8alpha+/TCR(POS) T lymphocytes produce IFN-gamma, TNFalpha and IL-2 when specifically stimulated via the introduced TCR with immobilized HLA-A1/MAGE-A1 complexes or HLA-A1+/MAGE-A1+ melanoma cells. Furthermore, introduction of the CD8alpha gene into TCR(POS) T lymphocytes rendered these T lymphocytes cytotoxic for HLA-A1+/MAGE-A1+ melanoma cells. These results demonstrate that human CD4+ T lymphocytes when genetically grafted with an HLA-A1/MAGE-A1-specific TCR and CD8alpha are induced to kill and produce cytokines upon specific interaction with the relevant melanoma cells. Hence, CD4+ T lymphocytes, in addition to CD8+ T lymphocytes, may be critical effector cells for adoptive immuno-gene therapy to generate a sustained tumor-specific immune response in cancer patients.     

Vaccination of rhesus monkeys with synthetic peptide in a fusogenic proteoliposome elicits simian immunodeficiency virus-specific CD8+ cytotoxic T lymphocytes.

An effective vaccine against the human immunodeficiency virus should be capable of eliciting both an antibody and a cytotoxic T lymphocyte (CTL) response. However, when viral proteins and peptides are formulated with traditional immunological adjuvants and inoculated via a route acceptable for use in humans, they have not been successful at eliciting virus-specific, major histocompatibility complex (MHC) class I-restricted CTL. We have designed a novel viral subunit vaccine by encapsulating a previously defined synthetic peptide CTL epitope of the simian immunodeficiency virus (SIV) gag protein within a proteoliposome capable of attaching to and fusing with plasma membranes. Upon fusing, the encapsulated contents of this proteoliposome can enter the MHC class I processing pathway through the cytoplasm. In this report, we show that after a single intramuscular vaccination, rhesus monkeys develop a CD8+ cell-mediated, MHC class I-restricted CTL response that recognizes the synthetic peptide immunogen. The induced CTL also demonstrate antiviral immunity by recognizing SIV gag protein endogenously processed by target cells infected with SIV/vaccinia recombinant virus. These results demonstrate that virus-specific, MHC class I-restricted, CD8+ CTL can be elicited by a safe, nonreplicating viral subunit vaccine in a primate model for acquired immune deficiency syndrome. Moreover, the proteoliposome vaccine formation described can include multiple synthetic peptide epitopes, and, thus, offers a simple means of generating antiviral cell-mediated immunity in a genetically heterogeneous population.     

Evidence for the presentation of major histocompatibility complex class I-restricted Epstein-Barr virus nuclear antigen 1 peptides to CD8+ T lymphocytes

The Epstein-Barr virus (EBV)-encoded nuclear antigen 1 (EBNA1) is expressed in all EBV-associated tumors, making it an important target for immunotherapy. However, evidence for major histocompatibility complex (MHC) class I-restricted EBNA1 peptides endogenously presented by EBV-transformed B and tumor cells remains elusive. Here we describe for the first time the identification of an endogenously processed human histocompatibility leukocyte antigen (HLA)-B8-restricted EBNA1 peptide that is recognized by CD8+ T cells. T cell recognition could be inhibited by the treatment of target cells with proteasome inhibitors that block the MHC class I antigen processing pathway, but not by an inhibitor (chloroquine) of MHC class II antigen processing. We also demonstrate that new protein synthesis is required for the generation of the HLA-B8 epitope for T cell recognition, suggesting that defective ribosomal products (DRiPs) are the major source of T cell epitopes. Experiments with protease inhibitors indicate that some serine proteases may participate in the degradation of EBNA1 DRiPs before they are further processed by proteasomes. These findings not only provide the first evidence of the presentation of an MHC class I-restricted EBNA1 epitope to CD8+ T cells, but also offer new insight into the molecular mechanisms involved in the processing and presentation of EBNA1.     

Complementary dendritic cell-activating function of CD8+ and CD4+ T cells: helper role of CD8+ T cells in the development of T helper type 1 responses

Dendritic cells (DCs) activated by CD40L-expressing CD4+ T cells act as mediators of "T helper (Th)" signals for CD8+ T lymphocytes, inducing their cytotoxic function and supporting their long-term activity. Here, we show that the optimal activation of DCs, their ability to produce high levels of bioactive interleukin (IL)-12p70 and to induce Th1-type CD4+ T cells, is supported by the complementary DC-activating signals from both CD4+ and CD8+ T cells. Cord blood- or peripheral blood-isolated naive CD8+ T cells do not express CD40L, but, in contrast to naive CD4+ T cells, they are efficient producers of IFN-gamma at the earliest stages of the interaction with DCs. Naive CD8+ T cells cooperate with CD40L-expressing naive CD4+ T cells in the induction of IL-12p70 in DCs, promoting the development of primary Th1-type CD4+ T cell responses. Moreover, the recognition of major histocompatibility complex class I-presented epitopes by antigen-specific CD8+ T cells results in the TNF-alpha- and IFN-gamma-dependent increase in the activation level of DCs and in the induction of type-1 polarized mature DCs capable of producing high levels of IL-12p70 upon a subsequent CD40 ligation. The ability of class I-restricted CD8+ T cells to coactivate and polarize DCs may support the induction of Th1-type responses against class I-presented epitopes of intracellular pathogens and contact allergens, and may have therapeutical implications in cancer and chronic infections.     

Identification of NY-ESO-1 epitopes presented by human histocompatibility antigen (HLA)-DRB4*0101-0103 and recognized by CD4(+) T lymphocytes of patients with NY-ESO-1-expressing melanoma

NY-ESO-1 is a member of the cancer-testis family of tumor antigens that elicits strong humoral and cellular immune responses in patients with NY-ESO-1-expressing cancers. Since CD4(+) T lymphocytes play a critical role in generating antigen-specific cytotoxic T lymphocyte and antibody responses, we searched for NY-ESO-1 epitopes presented by histocompatibility leukocyte antigen (HLA) class II molecules. Autologous monocyte-derived dendritic cells of cancer patients were incubated with recombinant NY-ESO-1 protein and used in enzyme-linked immunospot (ELISPOT) assays to detect NY-ESO-1-specific CD4(+) T lymphocyte responses. To identify possible epitopes presented by distinct HLA class II alleles, overlapping 18-mer peptides derived from NY-ESO-1 were synthetized and tested for recognition by CD4(+) T lymphocytes in autologous settings. We identified three NY-ESO-1-derived peptides presented by DRB4*0101-0103 and recognized by CD4(+) T lymphocytes of two melanoma patients sharing these HLA class II alleles. Specificity of recognition was confirmed by proliferation assays. The characterization of HLA class II-restricted epitopes will be useful for the assessment of spontaneous and vaccine-induced immune responses of cancer patients against defined tumor antigens. Further, the therapeutic efficacy of active immunization using antigenic HLA class I-restricted peptides may be improved by adding HLA class II-presented epitopes.     

Both L3T4+ and Lyt-2+ helper T cells initiate cytotoxic T lymphocyte responses against allogenic major histocompatibility antigens but not against trinitrophenyl-modified self

This study characterizes the T helper (Th) cells that initiate primary cytotoxic T lymphocyte (CTL) responses against allogeneic and trinitrophenyl (TNP)-modified self class I major histocompatibility (MHC) determinants. We show that two distinct Th cell subsets participate in allospecific CTL responses: (a) an L3T4+,Lyt-2- class II-restricted Th cell population, and (b) an L3T4-,Lyt-2+ class I-restricted Th cell population. Both of these T cell subpopulations were shown to function in allospecific CTL responses as helper cells by their ability to show synergy with allospecific CTL precursors. Thus, primary class I allospecific CTL responses represent an immune response involving not only L3T4+ Th cells, but Lyt-2+ Th cells as well. One of the necessary functions performed by both L3T4+ and Lyt-2+ Th cell populations in allospecific CTL responses was found to be the secretion of interleukin 2. Finally, despite the many similarities between anti-allo- and anti-TNP-CTL responses, anti-TNP-CTL responses were found to be mediated by only L3T4+ Th cells, not by Lyt-2+ Th cells. Consequently, Lyt-2+ Th cells appear to be a helper cell population that is primarily involved in MHC-specific immune responses.     

An early CD4+ T cell-dependent immunoglobulin A response to influenza infection in the absence of key cognate T-B interactions

Contact-mediated interactions between CD4+ T cells and B cells are considered crucial for T cell-dependent B cell responses. To investigate the ability of activated CD4+ T cells to drive in vivo B cell responses in the absence of key cognate T-B interactions, we constructed radiation bone marrow chimeras in which CD4+ T cells would be activated by wild-type (WT) dendritic cells, but would interact with B cells that lacked expression of either major histocompatibility complex class II (MHC II) or CD40. B cell responses were assessed after influenza virus infection of the respiratory tract, which elicits a vigorous, CD4+ T cell-dependent antibody response in WT mice. The influenza-specific antibody response was strongly reduced in MHC II knockout and CD40 knockout mice. MHC II-deficient and CD40-deficient B cells in the chimera environment also produced little virus-specific immunoglobulin (Ig)M and IgG, but generated a strong virus-specific IgA response with virus-neutralizing activity. The IgA response was entirely influenza specific, in contrast to the IgG2a response, which had a substantial nonvirus-specific component. Our study demonstrates a CD4+ T cell-dependent, antiviral IgA response that is generated in the absence of B cell signaling via MHC II or CD40, and is restricted exclusively to virus-specific B cells.     

The CD4 molecule is not always required for the T cell response to bacterial enterotoxins

T cells respond in a V beta-restricted fashion to bacterial enterotoxins bound to major histocompatibility complex (MHC) class II molecules. The requirement for CD4 in MHC class II-restricted T cell responses is very well established. We have assessed the role of CD4 in the T cell response to the bacterial enterotoxins Staphylococcal enterotoxin A (SEA), SEB, and toxic shock syndrome toxin 1. Three CD4- murine T cell hybridomas were transfected with the human CD4 molecule and assayed for interleukin 2 production in the presence of accessory cells bearing human MHC class II molecules and of the appropriate enterotoxin. The results clearly indicate that CD4- cells responded even to suboptimal concentrations of enterotoxin(s) equally well as CD4+ cells. Furthermore, expression of CD4 did not result in the acquisition of previously undetectable reactivity to enterotoxins. These results suggest that unlike the case with antigen-specific responses, formation of a T cell receptor-CD3/CD4 supramolecular complex is not always essential for T cell activation by bacterial enterotoxins.     

Immunization of patients with metastatic melanoma using both class I- and class II-restricted peptides from melanoma-associated antigens

Cancer vaccines targeting CD8+ T cells have been successful in eliciting immunologic responses but disappointing in inducing clinical responses. Strong evidence supports the importance of CD4+ T cells in "helping" cytotoxic CD8+ cells in antitumor immunity. We report here on two consecutive clinical trials evaluating the impact of immunization with both human leukocyte antigen class I- and class II-restricted peptides from the gp100 melanoma antigen. In Protocol 1, 22 patients with metastatic melanoma were immunized with two modified class I A*0201-restricted peptides, gp100:209-217(210M) and MART-1:26-35(27L). In Protocol 2, 19 patients received the same class I-restricted peptides in combination with a class II DRB1*0401-restricted peptide, gp100:44-59. As assessed by in vitro sensitization assays using peripheral blood mononuclear cells (PBMC) against the native gp100:209-217 peptide, 95% of patients in Protocol 1 were successfully immunized after two vaccinations in contrast to 50% of patients in Protocol 2 (P(2) < 0.005). Furthermore, the degree of sensitization was significantly lower in patients in Protocol 2 (P = 0.01). Clinically, one patient in Protocol 2 had an objective response, and none did in Protocol 1. Thus, the addition of the class II-restricted peptide gp100:44-59 did not improve clinical response but might have diminished the immunologic response of circulating PBMC to the class I-restricted peptide gp100:209-217. The reasons for this decreased immune reactivity are unclear but may involve increased CD4+CD25+ regulatory T-cell activity, increased apoptosis of activated CD8+ T cells, or the trafficking of sensitized CD8+ reactive cells out of the peripheral blood. Moreover, the sequential, nonrandomized nature of patient enrollment for the two trials may account for the differences in immunologic response.     

Reactivity of V beta 17a+ CD8+ T cell hybrids. Analysis using a new CD8+ T cell fusion partner

Tolerance to IE molecules leads to deletion of V beta 17a-bearing T cells. Both, the CD4+ as well as the CD8+ T cell subsets are affected. A large percentage of CD4+ V beta 17a+ T cell hybrids recognize IE molecules. We now have investigated the reactivity for IE antigens of CD8+ V beta 17a+ T cell hybrids. Using a transfection approach, we have introduced the murine CD8 molecule into different V beta 17a+ T cell hybrids. Furthermore, the CD8 cDNA was transfected into the BW5147 alpha-beta- fusion partner. This allowed us to generate a large number of V beta 17a+ T cell hybrids by fusion with the appropriate T cells. Only 6% of T cell hybrids were stimulated to produce IL-2 upon incubation with IE+ cells. However, in those, the CD8 molecule seemed not to contribute to the IE reactivity of the hybrid, since mAbs against the CD8 molecule failed to inhibit their reactivity. This low percentage of V beta 17a+ CD8+ IE-reactive T cell hybrids contrasts with the strong reduction of CD8+ V beta 17a+ T cells in IE+ mice, strongly suggesting that elimination of such cells in the thymus occurs when they are coexpressing CD4 and CD8. This view was confirmed by the occasional expression of CD4 in some hybrids in which case IE reactivity was detected. Furthermore, we demonstrated the functional integrity of the introduced CD8 molecule by: (a) reconstitution of the IL-2 response in a class I-restricted TNP-specific T cell hybrid; and (b) by generation of alloreactive class I-restricted T cell hybrids using the new CD8+ fusion cell line. This CD8+ fusion partner, BWLyt2-4, should prove useful to study antigen processing and antigen presentation requirements of class I-restricted T cells.     

Cellular basis of skin allograft rejection across a class I major histocompatibility barrier in mice depleted of CD8+ T cells in vivo.

The present study was undertaken to define the cellular mechanisms involved in the rejection of major histocompatibility complex (MHC) class I disparate skin grafts by mice depleted of CD8+ T cells in vivo. Mice were effectively depleted of CD8+ T cells by adult thymectomy followed by in vivo administration of anti-CD8 monoclonal antibody (mAb) and then engrafted with allogeneic skin. We found that CD8 depleted mice did reject MHC class I disparate skin grafts, but only when the grafts also expressed additional alloantigens. Despite the marked depletion of CD8+ T cells in these mice, we found that their rejection of MHC class I disparate grafts was mediated by CD8+ cytolytic T lymphocyte (CTL) effectors that had escaped depletion. These CD8+ CTL effectors were unique in that: (a) their generation was dependent upon the injected anti-CD8 mAb and upon exposure to class I MHC alloantigens expressed on the engrafted skin, and (b) their effector function was resistant to blockade by anti-CD8 mAb. We observed that the additional alloantigens coexpressed on MHC class I disparate grafts that triggered graft rejection in CD8-depleted mice could be MHC-linked or not and that they functioned in these rejection responses to activate third party specific CD4+ T helper (Th) cells to provide helper signals for the generation of CD8+ anti-CD8 resistant CTL effector cells. Thus, mice depleted of CD8+ T cells by thymectomy and in vivo administration of anti-CD8 mAb harbor a unique population of anti-CD8 resistant, CD8+ effector cells that mediate anti-MHC class I responses in vivo and in vitro, but require help from third party specific Th cells to do so.     

Differential requirements for the chemokine receptor CCR7 in T cell activation during Listeria monocytogenes infection

Effective priming of T cell responses depends on cognate interactions between naive T cells and professional antigen-presenting cells (APCs). This contact is the result of highly coordinated migration processes, in which the chemokine receptor CCR7 and its ligands, CCL19 and CCL21, play a central role. We used the murine Listeria monocytogenes infection model to characterize the role of the CCR7/CCR7 ligand system in the generation of T cell responses during bacterial infection. We demonstrate that efficient priming of naive major histocompatibility complex (MHC) class Ia-restricted CD8+ T cells requires CCR7. In contrast, MHC class Ib-restricted CD8+ T cells and MHC class II-restricted CD4+ T cells seem to be less dependent on CCR7; memory T cell responses are independent of CCR7. Infection experiments with bone marrow chimeras or mice reconstituted with purified T cell populations indicate that CCR7 has to be expressed on CD8+ T cells and professional APCs to promote efficient MHC class Ia-restricted T cell priming. Thus, different T cell subtypes and maturation stages have discrete requirements for CCR7.     

CD8+ T cells respond clonally to Mls-1a-encoded determinants

T cell responses to the product of the minor lymphocyte stimulatory locus Mls-1a involve the selective use of TCR V beta domains (especially V beta 6 and V beta 8.1) and are generally considered to be restricted to the CD4+ mature subset. We show here that CD8+ (presumably MHC class I-restricted) T cells bearing V beta 6 or V beta 8.1 also respond preferentially to Mls-1a determinants either in vitro (in mixed leukocyte cultures) or in vivo (in an adoptive transfer system). In vitro responses of both CD4+ V beta 6+ and CD8+ V beta 6+ cells to Mls-1a were dependent upon the MHC haplotype of the stimulator cells, with I-E+ (H-2d or H-2k) alleles being much more stimulatory than I-E- (H-2q). These data strengthen the analogy between Mls gene products and other MHC class II-dependent superantigens such as the bacterial enterotoxins.     

An MHC class Ib-restricted CD8 T cell response confers antiviral immunity

Although immunity against intracellular pathogens is primarily provided by CD8 T lymphocytes that recognize pathogen-derived peptides presented by major histocompatibility complex (MHC) class Ia molecules, MHC class Ib-restricted CD8 T cells have been implicated in antiviral immunity. Using mouse polyoma virus (PyV), we found that MHC class Ia-deficient (K(b-/-)D(b-/-)) mice efficiently control this persistently infecting mouse pathogen. CD8 T cell depletion mitigates clearance of PyV in K(b-/-)D(b-/-) mice. We identified the ligand for PyV-specific CD8 T cells in K(b-/-)D(b-/-) mice as a nonamer peptide from the VP2 capsid protein presented by Q9, a member of the beta(2) microglobulin-associated Qa-2 family. Using Q9-VP2 tetramers, we monitored delayed but progressive expansion of these antigen-specific CD8alphabeta T cells in K(b-/-)D(b-/-) mice. Importantly, we demonstrate that Q9-VP2-specific CD8 T cells more effectively clear wild-type PyV than a VP2 epitope(null) mutant PyV. Finally, we show that wild-type mice also generate Q9-restricted VP2 epitope-specific CD8 T cells to PyV infection. To our knowledge, this is the first evidence for a defined MHC class Ib-restricted antiviral CD8 T cell response that contributes to host defense. This study motivates efforts to uncover MHC class Ib-restricted CD8 T cell responses in other viral infections, and given the limited polymorphism of MHC class Ib molecules, it raises the possibility of developing peptide-based viral vaccines having broad coverage across MHC haplotypes.