CD28 signal enhances apoptosis of CD8 T cells after strong TCR ligation

High avidity ligation of the TCR induces negative selection in the thymus and can also induce apoptosis of peripheral T cells. Costimulation through CD28 enhances T cell activation and facilitates negative selection in the thymus, but the role of CD28 in peripheral T cell deletional tolerance has not been investigated. We used 2C CD28 wild-type and 2C CD28-deficient strains to assess the effects of CD28 and TCR avidity on peripheral T cell expansion and apoptosis. We compared the activation, division, expansion, and apoptosis of CD28(+/+) and CD28(-/-) 2C cells in response to self-Ag (K(b)), alloantigens with intermediate (K(bm3)), high (L(d)), or very high (L(d) + QL9 peptide) avidity. With intermediate avidity alloantigen, the CD28 signal enhanced T cell activation and expansion. However, when T cells encountered high avidity alloantigen, the CD28 signal reduced T cell expansion and increased apoptosis. These results indicate that the CD28 signal can down-regulate peripheral T cell responses by increasing apoptosis when TCR ligation exceeds a critical threshold.     

Peptide length variants p2Ca and QL9 present distinct conformations to L(d)-specific T cells

Recent advances have provided insights into how the TCR interacts with MHC/peptide complexes and a rationale to predict optimal epitopes for MHC binding and T cell recognition. For example, peptides of nine residues are predicted to be optimal for binding to H2-L(d), although 8 mer epitopes have also been identified. It has been predicted that 8 mer and 9 mer length variant peptides bound to L(d) present identical epitopes to T cells. However, in contrast to this prediction, we demonstrate here that the 8 mer peptide p2Ca and its 9 mer length variant QL9, extended by an N-terminal glutamine, assume distinct conformations when bound to L(d). We generated self-L(d)-restricted CTL clones specific for p2Ca that recognize L(d)/QL9 poorly if at all. This result is in sharp contrast to what has been observed with L(d)-alloreactive T cells that possess a much higher affinity for L(d)/QL9 than for L(d)/p2Ca. Alanine substitutions of the N-terminal residues of the QL9 peptide rescue detection by these self-L(d)/p2Ca-specific T cells, but decrease recognition by the L(d)-alloreactive 2C T cell clone. In addition, 2C T cell recognition of the p2Ca peptide is affected by different alanine substitutions compared with 2C T cell recognition of the QL9 peptide. These data clearly demonstrate that the p2Ca and QL9 peptides assume distinct conformations when bound to L(d) and, furthermore, demonstrate that there is flexibility in peptide binding within the MHC class I cleft.     

Effective adoptive therapy of tap-deficient lymphoma using diverse high avidity alloreactive T cells

High avidity for antigen and diversity of T cell receptor (TCR) repertoire are essential for effective immunity against cancer. We have previously created a transgenic mouse strain with increased TCR avidity in a diverse T cell population. In this report, we show that strong alloreactive responses of transgenic T cells against targets with low MHC class I expression can be used for effective adoptive transfer of tumor immunity in vivo. Alloreactive transgenic T cells could be an effective therapeutic approach counteracting tumor evasion of the immune system.     

Trachea allograft class I molecules directly activate and retain CD8+ T cells that cause obliterative airways disease

Human T cells responding against transplanted allogeneic lung tissue have been implicated in late graft failure secondary to obliterative bronchiolitis. This obliterative airways disease (OAD) also develops in heterotopic murine tracheal allografts in association with graft infiltration by both CD8(+) and CD4(+) T cells. To date, there has been little evidence to suggest that directly alloreactive CD8(+) T cells either promote chronic rejection or lead to the development of OAD following airway allotransplantation. Using L(d)-specific TCR-Tg 2C CD8(+) T cells adoptively transferred into wild-type B6 (H-2(b)) mice and the transplantation of BALB/c (H-2(d)) tracheal allografts, we now show that the direct recognition of donor-specific class I MHC molecules by host CD8(+) T cells leads to their activation, clonal expansion within the graft, and differentiation to an effector phenotype with the capacity to induce airway fibrosis. In addition, these experiments demonstrate that ongoing direct alloantigen recognition within the transplanted airway tissue is necessary for the recruitment and retention of these directly alloreactive CD8(+) T cells. Thus, these experiments are the first to definitively show a role for directly alloreactive CD8(+) T cells in the chronic rejection that leads to OAD.     

Alloreactive and syngeneic CTL are comparably dependent on interaction with MHC class I alpha-helical residues.

The molecular basis for the difference in the strength of T cell responses to self vs alloantigens is unknown, but may reflect how T cells are selected in the thymus. Because T cells with a high affinity for foreign as opposed to self MHC molecules are able to mature, it has been proposed that alloreactive T cells may be more strongly dependent upon interaction with MHC residues than are self-restricted T cells. This study was undertaken to rigorously address this hypothesis. Whereas other studies have compared self vs alloantigen recognition of different MHC alleles by a single T cell clone, we have compared self vs alloantigen recognition of a single MHC allele, H-2Ld, by a large panel of self-restricted and alloreactive T cell clones. Target cells expressing Ld molecules mutated at several different potential TCR contact residues were analyzed to determine which residues are important for recognition by self-restricted vs alloreactive T cells. We unequivocally demonstrate that self-restricted and alloreactive T cells do not differ, but rather are comparably dependent on interaction with MHC residues. Importantly, both self-restricted and alloreactive T cells are dependent upon the same MHC residues as primary contacts and, in addition, share a common recognition pattern of Ld. Furthermore, our analysis enables us to provide a model for allotype-specific T cell recognition of Ld vs Kb class I molecules.     

Mutations in a dominant Nef epitope of simian immunodeficiency virus diminish TCR:epitope peptide affinity but not epitope peptide:MHC class I binding

Viruses like HIV and SIV escape from containment by CD8(+) T lymphocytes through generating mutations that interfere with epitope peptide:MHC class I binding. However, mutations in some viral epitopes are selected for that have no impact on this binding. We explored the mechanism underlying the evolution of such epitopes by studying CD8(+) T lymphocyte recognition of a dominant Nef epitope of SIVmac251 in infected Mamu-A*02(+) rhesus monkeys. Clonal analysis of the p199RY-specific CD8(+) T lymphocyte repertoire in these monkeys indicated that identical T cell clones were capable of recognizing wild-type (WT) and mutant epitope sequences. However, we found that the functional avidity of these CD8(+) T lymphocytes for the mutant peptide:Mamu-A*02 complex was diminished. Using surface plasmon resonance to measure the binding affinity of the p199RY-specific TCR repertoire for WT and mutant p199RY peptide:Mamu-A*02 monomeric complexes, we found that the mutant p199RY peptide:Mamu-A*02 complexes had a lower affinity for TCRs purified from CD8(+) T lymphocytes than did the WT p199RY peptide:Mamu-A*02 complexes. These studies demonstrated that differences in TCR affinity for peptide:MHC class I ligands can alter functional p199RY-specific CD8(+) T lymphocyte responses to mutated epitopes, decreasing the capacity of these cells to contain SIVmac251 replication.     

"Pruning" of alloreactive CD4+ T cells using 5- (and 6-)carboxyfluorescein diacetate succinimidyl ester prolongs skin allograft survival

Removal of alloreactive cells by either thymic deletion or deletion/anergy in the periphery is regarded as crucial to the development of tolerance. Dyes, such as CFSE, that allow monitoring of cell division suggest that in vitro proliferation could be a used as a way of "pruning" alloreactive cells while retaining a normal immune repertoire with retention of memory to previously encountered pathogens. This would overcome the problems occurring as a result of therapies that use massive depletion of T cells to allow acceptance of organ transplants or bone marrow grafts. We therefore used a skin graft model of CD4-mediated T cell rejection across a major H-2 mismatch (C57BL/6 (H-2(b)) to BALB/c (H-2(d)) mice) to evaluate whether nondividing CD4(+) T cells derived from a mixed lymphocyte culture would exhibit tolerance to a skin graft from the initial stimulator strain. We demonstrate that selective removal of dividing alloreactive CD4(+) T cells resulted in marked specific prolongation of allogeneic skin graft survival, and that the nondividing CD4(+) T cells retained a broad TCR repertoire and the ability to maintain memory. This novel way of depleting alloreactive T cells may serve as a useful strategy in combination with other mechanisms to achieve transplant tolerance.     

CD8-independent tumor cell recognition is a property of the T cell receptor and not the T cell

The CD8 coreceptor enhances T cell function by stabilizing the TCR/peptide/MHC complex and/or increasing T cell avidity via interactions with the intracellular kinases Lck and LAT. We previously reported a CD4(+) T cell (TIL 1383I), which recognizes the tumor-associated Ag tyrosinase in the context of HLA-A2. To determine whether CD8 independent tumor cell recognition is a property of the TCR, we used retroviral transduction to express the TIL 1383I TCR in the CD8(-) murine lymphoma, 58 alpha(-)/beta(-). Immunofluorescent staining of TCR-transduced cells with human TCR V beta subfamily-specific and mouse CD3-specific Abs confirmed surface expression of the transferred TCR and coexpression of mouse CD3. Transduced effector cells secreted significant amounts of IL-2 following Ag presentation by tyrosinase peptide-pulsed T2 cells as well as stimulation with HLA-A2(+) melanoma lines compared with T2 cells alone or HLA-A2(-) melanoma cells. Further analysis of TCR-transduced clones demonstrated a correlation between T cell avidity and cell surface expression of the TCR. Therefore, the TIL 1383I TCR has sufficient affinity to mediate recognition of the physiologic levels of Ag expressed by tumor cells in the absence of CD8 expression.     

Shaping of the autoreactive regulatory T cell repertoire by thymic cortical positive selection

The main function of regulatory T lymphocytes is to keep autoimmune responses at bay. Accordingly, it has been firmly established that the repertoire of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) is enriched in autospecific cells. Differences in thymic-positive and/or -negative selection may account for selection of the qualitatively distinct regulatory and conventional T cell (Tconv) repertoires. It has previously been shown that precursors for Tregs are less sensitive to negative selection than Tconv precursors. Studies with TCR/ligand doubly transgenic mice suggested that an agonist ligand might induce positive selection of Treg (but not Tconv) cells. However, massive deletion of Tconv (but not Treg) cell precursors observed in these mice renders interpretation of such data problematic and a potential role for positive selection in generation of the autospecific Treg repertoire has remained therefore incompletely understood. To study this important unresolved issue and circumvent use of TCR/ligand-transgenic mice, we have developed transgenic mice expressing a single MHC class II/peptide ligand on positively selecting thymic cortical epithelial cells. We found that functional Treg (but not Tconv) cells specific for the single ligand were preferentially selected from the naturally diverse repertoire of immature precursors. Our data therefore demonstrate that thymic cortical positive selection of regulatory and Tconv precursors is governed by distinct rules and that it plays an important role in shaping the autoreactive Treg repertoire.     

Peripheral "CD8 tuning" dynamically modulates the size and responsiveness of an antigen-specific T cell pool in vivo

In this study, we suggest that CD8 levels on T cells are not static, but can change and, as a result, modulate CD8(+) T cell responses. We describe three models of CD8 modulation using novel weak-agonist (K1A) and super-agonist (C2A) altered peptide ligands of the HY smcy peptide. First, we used peripheral nonresponsive CD8(low) T cells produced after peripheral HY-D(b) MHC class I tetramer stimulation of female HY TCR transgenic and wild-type mice. Second, we used genetically lowered CD8(int) T cells from heterozygote CD8(+/0) mice. Finally, we used pre-existing nonresponsive CD8(low) T cells from male HY TCR transgenic mice. In CD8(low) and CD8(high) mice, presence of a lower level of CD8 greatly decreased the avidity of the peptide-MHC for HY TCR as reflected by avidity (K(D)) and dissociation constant (T(1/2)) measurements. All three models demonstrated that lowering CD8 levels resulted in the requirement for a higher avidity peptide-MHC interaction with the TCR to respond equivalently to unmanipulated CD8(high) T cells of the same specificity. Additionally, direct injections of wild-type HY-D(b) and C2A-D(b) tetramers into female HY TCR or female B6 mice induced a high frequency of peripheral nonresponsive CD8(low) T cells, yet C2A-D(b) was superior in inducing a primed CD8(+)CD44(+) memory population. The ability to dynamically modulate the size and responsiveness of an Ag-specific T cell pool by "CD8 tuning" of the T cell during the early phases of an immune response has important implications for the balance of responsiveness, memory, and tolerance.     

T cells primed by Leishmania major infection cross-react with alloantigens and alter the course of allograft rejection

Alloreactive T lymphocytes can be primed through direct presentation of donor MHC:peptide complexes on graft cells and through indirect presentation of donor-derived determinants expressed by recipient APCs. The large numbers of determinants on an allograft and the high frequency of the alloreactive repertoire has further led to speculation that exposure to environmental Ags may prime T cells that cross-react with alloantigens. We sought to develop a model in which to test this hypothesis. We found that CD4(+) T cells obtained from C57BL/6 (B6) mice that clinically resolved Leishmania major infection exhibited statistically significant cross-reactivity toward P/J (H-2(p)) Ags compared with the response to other haplotypes. B6 animals that were previously infected with L. major specifically rejected P/J skin grafts with second set kinetics compared with naive animals. Although donor-specific transfusion combined with costimulatory blockade (anti-CD40 ligand Ab) induced prolonged graft survival in naive animals, the same treatment was ineffective in mice previously infected with L. major. The studies demonstrate that cross-reactive priming of alloreactive T cells can occur and provide direct evidence that such T cells can have a significant impact on the outcome of an allograft. The results have important implications for human transplant recipients whose immune repertoires may contain cross-reactively primed allospecific T cells.     

Alloantigen affinity and CD4 help determine severity of graft-versus-host disease mediated by CD8 donor T cells

TCR affinity dictates T cell selection in the thymus and also has a high impact on the fate of peripheral T cells. Graft-vs-host disease (GVHD) is a pathological process initiated by activation of donor T cells after adoptive transfer into an allogeneic recipient. How TCR affinity affects the potential of alloreactive T cells to induce GVHD is unclear. Using alloreactive CD4+ and CD8+ TCR transgenic (Tg) T cells, GVHD models are presented that allow for the visualization of how CD8+ alloreactive T cells behave in response to alloantigens with different TCR affinity in the absence or presence of CD4 help. In a nonmyeloablative transplant model where GVHD lethality is due to marrow aplasia, alloreactive CD8+ TCR Tg T cells induced significantly more severe GVHD in the recipients that express an intermediate-affinity alloantigen than in the recipients that express a high-affinity alloantigen. In a myeloablative transplant model where GVHD lethality is due to epithelium injury, CD8+ TCR Tg cells were also more pathogenic in the recipients with an intermediate-affinity alloantigen than in those with a high-affinity alloantigen. The presence of alloreactive CD4+ TCR Tg cells enhanced the potential of CD8+ TCR Tg cells to cause GVHD in recipients with an intermediate-, but not with a high-, affinity alloantigen. These findings underscore that alloantigen affinity and CD4 help control the fate and pathogenicity of alloreactive CD8+ T cells in vivo.     

Competitive inhibition in vivo and skewing of the T cell repertoire of antigen-specific CTL priming by an anti-peptide-MHC monoclonal antibody.

We have recently described a mAb, KP15, directed against the MHC-I/peptide molecular complex consisting of H-2D(d) and a decamer peptide corresponding to residues 311-320 of the HIV IIIB envelope glycoprotein gp160. When administered at the time of primary immunization with a vaccinia virus vector encoding gp160, the mAb blocks the subsequent appearance of CD8(+) CTL with specificity for the immunodominant Ag, P18-I10, presented by H-2D(d). This inhibition is specific for this particular peptide Ag; another H-2D(d)-restricted gp160 encoded epitope from a different HIV strain is not affected, and an H-2L(d)-restricted epitope encoded by the viral vector is also not affected. Using functional assays and specific immunofluorescent staining with multivalent, labeled H-2D(d)/P18-I10 complexes (tetramers), we have enumerated the effects of blocking of priming on the subsequent appearance, avidity, and TCR Vbeta usage of Ag-specific CTL. Ab blocking skews the proportion of high avidity cells emerging from immunization. Surprisingly, Vbeta7-bearing Ag-specific TCR are predominantly inhibited, while TCR of several other families studied are not affected. The ability of a specific MHC/peptide mAb to inhibit and divert the CD8(+) T cell response holds implications for vaccine design and approaches to modulate the immune response in autoimmunity.     

Participation of L3T4 in T cell activation in the absence of class II major histocompatibility complex antigens. Inhibition by anti-L3T4 antibodies is a function both of epitope density and mode of presentation of anti-receptor antibody

The recognition of many class II major histocompatibility complex (MHC)-associated antigens by T cells requires the participation of the L3T4 molecule. It has been proposed that this molecule acts to stabilize low affinity binding to antigen in association with MHC and thereby increases the avidity of T cell/antigen interactions. By using antibodies against the T cell antigen receptor (TCR) to activate T cells, thereby circumventing the requirement for antigen presenting cells and MHC-associated antigen, we have been able to study the function of L3T4 in the absence of class II MHC. We have used two monoclonal antibodies, KJ16-133.18 and F23.1, that recognize a determinant encoded by the T cell receptor V beta 8 gene family. These antibodies were used to select two clones of T cells with surface phenotype Thy-1.2+, L3T4+, Lyt-2-, KJ16-133.18+, F23.1+, IA-, IE-. One of these clones (E9.D4) was hapten-specific (anti-ABA + Iak), the other (4.35F2) was alloreactive (anti-Iak). Activation of these clones by antigen, concanavalin A (Con A) or by the F23.1 antibody was studied by assaying the production of interleukin 3 (IL 3). Both soluble and solid phase-coupled F23.1 induced T cell activation in the complete absence of class II MHC, immobilized antibody (either Sepharose-coupled or plastic-adsorbed) being more effective. The induction of IL 3 production by suboptimal doses of either Con A or plastic-adsorbed F23.1 was inhibited by the anti-L3T4 antibody GK1.5, as was the response to F23.1 coupled to Sepharose-4B beads. However, the responses to optimal or superoptimal doses of these stimuli were not inhibited. In contrast, weak responses to non-TCR cross-linking stimuli such as phorbol myristate acetate (PMA) or low concentrations of soluble F23.1 were not inhibited by GK1.5 (the latter response was usually slightly enhanced). These results show that anti-L3T4 antibodies are not inherently inhibitory, but require both ligation and cross-linking of the TCR for their effect. We propose a model whereby L3T4 interacts with the TCR during T cell activation. Anti-L3T4 antibodies sterically hinder the formation of TCR complexes and so prevent activation. However, by increasing the epitope density of the activating ligand, the avidity of the T cell/ligand interaction can be increased sufficiently to prevent this disruption.(ABSTRACT TRUNCATED AT 400 WORDS)     

TCR beta chain that forms peptide-independent alloreactive TCR transfers reduced reactivity with irrelevant peptide/MHC complex

A major feature of the TCR repertoire is strong alloreactivity. Peptides presented by allogeneic MHC are irrelevant for recognition by a subset of alloreactive T cells. To characterize peptide-independent TCRs at the molecular level, we forced the expression of a TCRbeta chain isolated from a peptide-independent alloreactive CD8+ T cell line. The alloreactive TCR repertoire in the transgenic mouse was peptide dependent. However, analysis of essential TCR contacts formed during the recognition of self-MHC-restricted Ag showed that fewer contacts with peptide were established by the transgenic TCRbeta chain, and that this was compensated by additional contacts formed by endogenous TCRalpha chains. Thus, reduced interaction with the peptide appears to be a transferable feature of the peptide-independent TCRbeta chain. In addition, these findings demonstrate that reactivity to peptides is preferred over the reactivity to MHC during the formation of the TCR repertoire.     

The TCR repertoire of an immunodominant CD8+ T lymphocyte population

The TCR repertoire of an epitope-specific CD8(+) T cell population remains poorly characterized. To determine the breadth of the TCR repertoire of a CD8(+) T cell population that recognizes a dominant epitope of the AIDS virus, the CD8(+) T cells recognizing the tetrameric Mamu-A*01/p11C(,CM) complex were isolated from simian immunodeficiency virus (SIV)-infected Mamu-A*01(+) rhesus monkeys. This CD8(+) T cell population exhibited selected usage of TCR V beta families and complementarity-determining region 3 (CDR3) segments. Although the epitope-specific CD8(+) T cell response was clearly polyclonal, a dominance of selected V beta(+) cell subpopulations and clones was seen in the TCR repertoire. Interestingly, some of the selected V beta(+) cell subpopulations and clones maintained their dominance in the TCR repertoire over time after infection with SIV of macaques. Other V beta(+) cell subpopulations declined over time in their relative representation and were replaced by newly evolving clones that became dominant. The present study provides molecular evidence indicating that the TCR repertoire shaped by a single viral epitope is dominated at any point in time by selected V beta(+) cell subpopulations and clones and suggests that dominant V beta(+) cell subpopulations and clones can either be stable or evolve during a chronic infection.     

Distinct footprints of TCR engagement with highly homologous ligands

T cell receptor engagement promotes proliferation, differentiation, survival, or death of T lymphocytes. The affinity/avidity of the TCR ligand and the maturational stage of the T cell are thought to be principal determinants of the outcome of TCR engagement. We demonstrate in this study that the same mouse TCR preferentially uses distinct residues of homologous peptides presented by the MHC molecules to promote specific cellular responses. The preference for distinct TCR contacts depends on neither the affinity/avidity of TCR engagement (except in the most extreme ranges), nor the maturity of engaged T cells. Thus, different portions of the TCR ligand appear capable of biasing T cells toward specific biological responses. These findings explain differences in functional versatility of TCR ligands, as well as anomalies in the relationship between affinity/avidity of the TCR for the peptide/MHC and cellular responses of T cells.     

EBV-specific CD4+ T cell clones exhibit vigorous allogeneic responses

Alloreactive T cells play a key role in mediating graft-vs-host disease and allograft rejection, and recent data suggest that most T cell alloreactivity resides within the CD4 T cell subset. Particularly, T cell responses to herpesvirus can shape the alloreactive repertoire and influence transplantation outcomes. In this study, we describe six distinct EBV-specific CD4(+) T cell clones that cross-reacted with EBV-transformed lymphoblastoid cell lines (LCLs), dendritic cells, and endothelial cells expressing MHC class II alleles commonly found in the population. Allorecognition showed exquisite MHC specificity. These CD4(+) T cell clones efficiently killed dendritic cells or LCLs expressing the cross-reactive allogeneic MHC class II molecules, whereas they did not kill autologous LCLs. Endothelial cells expressing the proper allogeneic MHC molecules were poorly killed, but they induced high-level TNF-alpha production by the EBV-specific CD4(+) T cell clones. As already proposed, the strong alloreactivity toward LCLs suggest that these cells could be used for selective depletion of alloreactive T cells.     

Selection and long-term persistence of reactive CTL clones during an EBV chronic response are determined by avidity, CD8 variable contribution compensating for differences in TCR affinities.

Recent studies have suggested that the diversity of TCR repertoire after primary immunization is conserved in memory T cells and that a progressive narrowing of this repertoire may take place during recall infections. It now remains to be investigated which parameters determine the repertoire of the memory response and possibly restrict its diversity after subsequent antigenic challenges. To address this question, we took advantage of a panel of CD8+ T cell clones from the joint of a rheumatoid arthritis patient and selected for their reactivity against a single MHC/peptide complex. Characterization of both TCR chains documented a great diversity among those clones and the persistence of clonotypes over a 2-yr period. Strikingly, despite the observed repertoire heterogeneity, all clones displayed a narrow range of MHC/peptide density requirements in cytotoxicity assays (ED50 between 9 and 36 nM). TCR affinities were then indirectly estimated by blocking CD8 interaction with an anti-CD8 mAb. We found a wide range of TCR affinities among the different clonotypes that segregated with Vbeta usage. We thus propose that during an in vivo chronic response, a narrow range of avidity of the TCR-CD8 complex conditions long-term clonotype persistence, and that the level of CD8 contribution is adjusted to keep clonotypes with variable TCR affinities within this avidity window.     

An MHC class Ib-restricted TCR that cross-reacts with an MHC class Ia molecule

TCR transgenic 6C5 T cells recognize an insulin B chain epitope presented by the nonclassical class I MHC molecule, Qa-1(b). Positive selection of these T cells was shown previously to require Qa-1(b). Despite dedicated specificity for Qa-1(b), evidence presented in the current study indicates that 6C5 T cells can cross-recognize a classical class I molecule. Clonal deletion was observed unexpectedly in 6C5.H-2(bxq) mice, which do not express I-E MHC class II molecules and thus should not be subject to superantigen-mediated negative selection. 6C5 T cells were observed to respond in vivo and in vitro to spleen cells from allogeneic H-2(q) mice, and specificity was mapped to D(q). Evidence was obtained for direct recognition of D(q), rather than indirect presentation of a D(q)-derived peptide presented by Qa-1(b). Polyclonal CD8(+) T cells from class Ia-deficient K(b)D(b-/-) mice reacted in vitro to allogeneic spleen cells with an apparent frequency comparable to conventional class Ia-restricted T cells. Our results provide a clear example of a Qa-1-specific TCR that can cross-react with a class Ia molecule and evidence supporting the idea that this may be a common property of T cells selected by class Ib molecules.