Nucleotide sequence of mutant I-A beta bm12 gene is evidence for genetic exchange between mouse immune response genes

Pathophysiology of Childhood-Onset Myasthenia: Abnormalities of Neuromuscular Junction and Autoimmunity and Its Background

The pathophysiology of myasthenia gravis (MG) has been largely elucidated over the past half century, and treatment methods have advanced. However, the number of cases of childhood-onset MG is smaller than that of adult MG, and the treatment of childhood-onset MG has continued to be based on research in the adult field. Research on pathophysiology and treatment methods that account for the unique growth and development of children is now desired. According to an epidemiological survey conducted by the Ministry of Health, Labour and Welfare of Japan, the number of patients with MG by age of onset in Japan is high in early childhood. In recent years, MG has been reported from many countries around the world, but the pattern of the number of patients by age of onset differs between East Asia and Western Europe, confirming that the Japanese pattern is common in East Asia. Furthermore, there are racial differences in autoimmune MG and congenital myasthenic syndromes according to immunogenetic background, and their pathophysiology and relationships are gradually becoming clear. In addition, treatment options are also recognized in different regions of the world. In this review article, I will present recent findings focusing on the differences in pathophysiology.

Lymph-Node-Targeted Drug Delivery for Effective Immunomodulation to Prolong the Long-Term Survival After Heart Transplantation

The chronic rejection responses and side effects of the systematic administration of immunosuppressants are the main obstacles to heart allograft and patient survival. The development of xenotransplantation also urgently requires more efficient immune regulation strategies. Herein, it is demonstrated that lymph-node (LN)-targeted drug delivery can realize LN-specific immunomodulation with attenuated immune suppression on distant peripheral immune organs to effectively prolong long-term survival after heart transplantation in a chronic murine heart transplantation model. A chemokine C-C motif ligand 21 (CCL21) specific aptamer for LN targeting is decorated onto the surface of the hybrid nanoparticular delivery vector mainly composed of CaCO₃ /CaP/heparin. The targeting delivery system can dramatically enhance accumulation of the loaded immunosuppressant, fingolimod hydrochloride (FTY720), in draining lymph nodes (dLNs) for inducing powerful immune suppression. By promoting the generation of endogenous regulatory T cells (T_regs ) and decreasing the proportion of effector T cells (T_effs ) in dLNs after heart transplantation, the LN-targeting strategy can effectively regulate local immune responses instead of systemic immunity, which reduces the incidence of long-term complications. This study provides an efficient strategy to improve the survival rate after organ transplantation by precise and localized immunoregulation with minimized side effects of immunosuppression.

Mouse Models of Antigen Presentation in Hematopoietic Stem Cell Transplantation

Allogeneic stem cell transplantation (alloSCT) is a curative therapy for hematopoietic malignancies. The therapeutic effect relies on donor T cells and NK cells to recognize and eliminate malignant cells, known as the graft-versus-leukemia (GVL) effect. However, off target immune pathology, known as graft-versus-host disease (GVHD) remains a major complication of alloSCT that limits the broad application of this therapy. The presentation of recipient-origin alloantigen to donor T cells is the primary process initiating GVHD and GVL. Therefore, the understanding of spatial and temporal characteristics of alloantigen presentation is pivotal to attempts to separate beneficial GVL effects from detrimental GVHD. In this review, we discuss mouse models and the tools therein, that permit the quantification of alloantigen presentation after alloSCT.

Graft IL-33 regulates infiltrating macrophages to protect against chronic rejection

Alarmins, sequestered self-molecules containing damage-associated molecular patterns, are released during tissue injury to drive innate immune cell proinflammatory responses. Whether endogenous negative regulators controlling early immune responses are also released at the site of injury is poorly understood. Herein, we establish that the stromal cell-derived alarmin interleukin 33 (IL-33) is a local factor that directly restricts the proinflammatory capacity of graft-infiltrating macrophages early after transplantation. By assessing heart transplant recipient samples and using a mouse heart transplant model, we establish that IL-33 is upregulated in allografts to limit chronic rejection. Mouse cardiac transplants lacking IL-33 displayed dramatically accelerated vascular occlusion and subsequent fibrosis, which was not due to altered systemic immune responses. Instead, a lack of graft IL-33 caused local augmentation of proinflammatory iNOS+ macrophages that accelerated graft loss. IL-33 facilitated a metabolic program in macrophages associated with reparative and regulatory functions, and local delivery of IL-33 prevented the chronic rejection of IL-33-deficient cardiac transplants. Therefore, IL-33 represents what we believe is a novel regulatory alarmin in transplantation that limits chronic rejection by restraining the local activation of proinflammatory macrophages. The local delivery of IL-33 in extracellular matrix-based materials may be a promising biologic for chronic rejection prophylaxis.

Optimizing PLG nanoparticle-peptide delivery platforms for transplantation tolerance using an allogeneic skin transplant model

A robust regimen for inducing allogeneic transplantation tolerance involves pre-emptive recipient treatment with donor splenocytes (SP) rendered apoptotic by 1-ethyl-3-(3'-dimethylaminopropyl)-carbodiimide(ECDI) treatment. However, such a regimen is limited by availability of donor cells, cost of cell procurement, and regulatory hurdles associated with cell-based therapies. Nanoparticles (NP) delivering donor antigens are a promising alternative for promoting transplantation tolerance. Here, we used a B6.C-H-2^bm12(bm12) to C57BL/6(B6) skin transplant model involving a defined major histocompatibility antigen mismatch to investigate design parameters of poly(lactide-co-glycolide) (PLG) NPs delivering peptides containing the donor antigen for optimizing skin allograft survival. We showed that an epitope-containing short peptide (P1) was more effective than a longer peptide (P2) at providing graft protection. Importantly, the NP and P1 complex (NP-ECDI-P1) resulted in a significant expansion of graft-infiltrating Tregs. Interestingly, in comparison to donor ECDI-SP that provided indefinite graft protection, NP-ECDI-P1 targeted different splenic phagocytes and skin allografts in these recipients harbored significantly more graft-infiltrating CD8⁺IFN-γ⁺ cells. Collectively, the current study provides initial engineering parameters for a cell-free and biocompatible NP-peptide platform for transplant immunoregulation. Moreover, it also provides guidance to future NP engineering endeavors to recapitulate the effects of donor ECDI-SP as a goal for maximizing tolerance efficacy of NP formulations.

T-bet+CD11c+ B cells are critical for antichromatin immunoglobulin G production in the development of lupus

Background

A hallmark of systemic lupus erythematosus is high titers of circulating autoantibodies. Recently, a novel CD11c⁺ B-cell subset has been identified that is critical for the development of autoimmunity. However, the role of CD11c⁺ B cells in the development of lupus is unclear. Chronic graft-versus-host disease (cGVHD) is a lupus-like syndrome with high autoantibody production. The purpose of this study was to explore the role of CD11c⁺ B cells in the pathogenesis of lupus in cGVHD mice.

Methods

cGVHD was induced by an intraperitoneal injection of 5 × 10⁷ Bm12 splenocytes into B6 mice. Flow cytometry was used to analyze mice splenocytes and human samples. Magnetic beads were used to isolate mice B cells. Gene expression was determined by real-time quantitative polymerase chain reaction (RT-qPCR). Enzyme-linked immunosorbent assay (ELISA) was used to detect antibodies in serum and supernatants.

Results

The percentage and absolute number of CD11c⁺ B cells was increased in cGVHD-induced lupus, with elevated levels of antichromatin immunoglobulin (Ig)G and IgG2a in sera. CD11c⁺ plasma cells from cGVHD mice produced large amounts of antichromatin IgG2a upon stimulation. Depletion of CD11c⁺ B cells reduced antichromatin IgG and IgG2a production. T-bet was upregulated in CD11c⁺ B cells. Knockout of T-bet in B cells alleviated cGVHD-induced lupus. Importantly, the percentage of T-bet⁺CD11c⁺ B cells increased in lupus patients and positively correlated with serum antichromatin levels.

Conclusion

T-bet⁺CD11c⁺ B cells promoted high antichromatin IgG production in the lupus-like disease model cGVHD. In lupus patients, the percentage of T-bet⁺CD11c⁺ B cells was elevated and positively correlated with antichromatin antibodies. The findings provide potential therapeutic insight into lupus disease treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1438-2) contains supplementary material, which is available to authorized users.

Regulatory T cell expressed MyD88 is critical for prolongation of allograft survival

MyD88 signaling directly promotes T-cell survival and is required for optimal T-cell responses to pathogens. To examine the role of T-cell-intrinsic MyD88 signals in transplantation, we studied mice with targeted T-cell-specific MyD88 deletion. Contrary to expectations, we found that these mice were relatively resistant to prolongation of graft survival with anti-CD154 plus rapamycin in a class II-mismatched system. To specifically examine the role of MyD88 in Tregs, we created a Treg-specific MyD88-deficient mouse. Transplant studies in these animals replicated the findings observed with a global T-cell MyD88 knockout. Surprisingly, given the role of MyD88 in conventional T-cell survival, we found no defect in the survival of MyD88-deficient Tregs in vitro or in the transplant recipients and also observed intact cell homing and expression of Treg effector molecules. MyD88-deficient Tregs also fail to protect allogeneic bone marrow transplant recipients from chronic graft-versus-host disease, confirming the observations of defective regulation seen in a solid organ transplant system. Together, our data define MyD88 as having a divergent requirement for cell survival in non-Tregs and Tregs, and a yet-to-be defined survival-independent requirement for Treg function during the response to alloantigen.

Is allograft rejection a clue to the mechanism promoting MHC polymorphism?

Extreme polymorphism at MHC loci is generally thought to be maintained by natural selection at a population level. Because allelic MHC molecules differ in their influence on the immune response, polymorphism ensures that the response repertoire is broad and that the group survives microbial challenge. Here Leif Anderson and his colleagues propose a different view, based on the observation that certain pathogens passively acquire MHC antigens from their primary host and carry these antigens into subsequent hosts. An immune response to the pathogen may depend on recognition of the non-self MHC antigens.

Transplantation of tail skin to study allogeneic CD4 T cell responses in mice

The study of T cell responses and their consequences during allo-antigen recognition requires a model that enables one to distinguish between donor and host T cells, to easily monitor the graft, and to adapt the system in order to answer different immunological questions. Medawar and colleagues established allogeneic tail-skin transplantation in mice in 1955. Since then, the skin transplantation model has been continuously modified and adapted to answer specific questions. The use of tail-skin renders this model easy to score for graft rejection, requires neither extensive preparation nor deep anesthesia, is applicable to animals of all genetic background, discourages ischemic necrosis, and permits chemical and biological intervention. In general, both CD4(+) and CD8(+) allogeneic T cells are responsible for the rejection of allografts since they recognize mismatched major histocompatibility antigens from different mouse strains. Several models have been described for activating allogeneic T cells in skin-transplanted mice. The identification of major histocompatibility complex (MHC) class I and II molecules in different mouse strains including C57BL/6 mice was an important step toward understanding and studying T cell-mediated alloresponses. In the tail-skin transplantation model described here, a three-point mutation (I-A(bm12)) in the antigen-presenting groove of the MHC-class II (I-A(b)) molecule is sufficient to induce strong allogeneic CD4(+) T cell activation in C57BL/6 mice. Skin grafts from I-A(bm12) mice on C57BL/6 mice are rejected within 12-15 days, while syngeneic grafts are accepted for up to 100 days. The absence of T cells (CD3(-/-) and Rag2(-/-) mice) allows skin graft acceptance up to 100 days, which can be overcome by transferring 2 x 10(4) wild type or transgenic T cells. Adoptively transferred T cells proliferate and produce IFN-γ in I-A(bm12)-transplanted Rag2(-/-) mice.

Profound depletion of host conventional dendritic cells, plasmacytoid dendritic cells, and B cells does not prevent graft-versus-host disease induction

The efficacy of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD). Host hematopoietic APCs are important initiators of GVHD, making them logical targets for GVHD prevention. Conventional dendritic cells (DCs) are key APCs for T cell responses in other models of T cell immunity, and they are sufficient for GVHD induction. However, we report in this article that in two polyclonal GVHD models in which host hematopoietic APCs are essential, GVHD was not decreased when recipient conventional DCs were inducibly or constitutively deleted. Additional profound depletion of plasmacytoid DCs and B cells, with or without partial depletion of CD11b(+) cells, also did not ameliorate GVHD. These data indicate that, in contrast with pathogen models, there is a surprising redundancy as to which host cells can initiate GVHD. Alternatively, very low numbers of targeted APCs were sufficient. We hypothesize the difference in APC requirements in pathogen and GVHD models relates to the availability of target Ags. In antipathogen responses, specialized APCs are uniquely equipped to acquire and present exogenous Ags, whereas in GVHD, all host cells directly present alloantigens. These studies make it unlikely that reagent-based host APC depletion will prevent GVHD in the clinic.

Profound depletion of host conventional dendritic cells, plasmacytoid dendritic cells, and B cells does not prevent graft-versus-host disease induction

The efficacy of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD). Host hematopoietic APCs are important initiators of GVHD, making them logical targets for GVHD prevention. Conventional dendritic cells (DCs) are key APCs for T cell responses in other models of T cell immunity, and they are sufficient for GVHD induction. However, we report in this article that in two polyclonal GVHD models in which host hematopoietic APCs are essential, GVHD was not decreased when recipient conventional DCs were inducibly or constitutively deleted. Additional profound depletion of plasmacytoid DCs and B cells, with or without partial depletion of CD11b(+) cells, also did not ameliorate GVHD. These data indicate that, in contrast with pathogen models, there is a surprising redundancy as to which host cells can initiate GVHD. Alternatively, very low numbers of targeted APCs were sufficient. We hypothesize the difference in APC requirements in pathogen and GVHD models relates to the availability of target Ags. In antipathogen responses, specialized APCs are uniquely equipped to acquire and present exogenous Ags, whereas in GVHD, all host cells directly present alloantigens. These studies make it unlikely that reagent-based host APC depletion will prevent GVHD in the clinic.

Allogeneic T regulatory cell-mediated transplantation tolerance in adoptive therapy depends on dominant peripheral suppression and central tolerance

T regulatory cells (Tregs) represent agents to mediate tolerance to allografts so that the use of immunosuppressive drugs is avoided. In this regard, we previously demonstrated that the adoptive transfer of allogeneic Tregs into IL-2Rbeta(-/-) mice prevented autoimmunity and led to allograft tolerance. Here, we investigated the requirements and mechanisms that favor this long-lasting tolerance. The most potent tolerance required exact matching of all alloantigens between the adoptively transferred allogeneic Tregs and allogeneic skin grafts, but tolerance to such allografts that lacked expression of major histocompatibility complex class I or II molecules also occurred. Thus, Tregs are not required to directly recognize major histocompatibility complex class II alloantigens to suppress skin transplant rejection. Depletion of allogeneic Tregs substantially, but not completely, abrogated this form of tolerance. However, thymocytes from allogeneic Treg adoptively transferred IL-2Rbeta(-/-) mice did not reject the corresponding allogeneic skin graft in secondary Scid recipients. Consistent with a requirement for a deletional mechanism in this IL-2Rbeta(-/-) model, a small number of wild-type T cells readily abrogated the immune tolerant state. Collectively, these findings indicate that full tolerance induction is largely dependent on substantial Treg-mediated suppression and thymic deletion of alloreactive T cells and may represent general conditions for Treg-mediated transplantation tolerance.

Chlamydia muridarum-specific CD4 T-cell clones recognize infected reproductive tract epithelial cells in an interferon-dependent fashion

During natural infections Chlamydia trachomatis urogenital serovars replicate predominantly in the epithelial cells lining the reproductive tract. This tissue tropism poses a unique challenge to host cellar immunity and future vaccine development. In the experimental mouse model, CD4 T cells are necessary and sufficient to clear Chlamydia muridarum genital tract infections. This implies that resolution of genital tract infection depends on CD4 T-cell interactions with infected epithelial cells. However, no laboratory has shown that Chlamydia-specific CD4 T cells can recognize Chlamydia antigens presented by major histocompatibility complex class II (MHC-I) molecules on epithelial cells. In this report we show that MHC-II-restricted Chlamydia-specific CD4 T-cell clones recognize infected upper reproductive tract epithelial cells as early as 12 h postinfection. The timing of recognition and degree of T-cell activation are dependent on the interferon (IFN) milieu. Beta IFN (IFN-beta) and IFN-gamma have different effects on T-cell activation, with IFN-beta blunting IFN-gamma-induced upregulation of epithelial cell surface MHC-II and T-cell activation. Individual CD4 T-cell clones differed in their degrees of dependence on IFN-gamma-regulated MHC-II for controlling Chlamydia replication in epithelial cells in vitro. We discuss our data as they relate to published studies with IFN knockout mice, proposing a straightforward interpretation of the existing literature based on CD4 T-cell interactions with the infected reproductive tract epithelium.

OX40 (CD134) engagement drives differentiationof CD4+ T cells to effector cells

Naive, CD4+ T cells proliferate extensively but fail to differentiate when they are transferred into unirradiated recipients that express alloantigen or transgenic antigen on all MHC class II+ cells. Addition of an agonist antibody to OX40 (CD134), a costimulatory TNF receptor family member expressed on activated CD4+ T cells, enables the proliferating T cells to accumulate as differentiated effector cells and kill the host animals. The donor T cells from anti-OX40-treated animals express high levels of IL-2R alpha (CD25) and acquire the ability to secrete IFN-gamma when stimulated with IL-12 and IL-18. OX40 promotes differentiation by 48 h in T cell priming, before changes in Bcl-2 expression or cell recovery become apparent. We found that a Bcl-2 transgene or deficiency in Fas or TNFR1 failed to influence accumulation of differentiated donor cells, and found larger changes in expression of cytokine and cytokine receptor genes than in survival genes. Accumulation of differentiated CD4+ effector T cells is initiated directly through OX40, but some OX40-deficient donor cells can gain effector function as bystanders to OX40+/+ cells. Taken together, these data suggest that CD4+ T cell differentiation to effector function is an important effect of OX40 engagement under conditions of ubiquitous antigen presentation.

Specificity of CD4+CD25+ regulatory T cell function in alloimmunity

CD4+CD25+ regulatory T cells (TRegs) are critical for the acquisition of peripheral allograft tolerance. However, it is unclear whether TRegs are capable of mediating alloantigen-specific suppressive effects and, hence, contributing to the specificity of the tolerant state. In the current report we have used the ABM TCR transgenic (Tg) system, a C57BL/6-derived strain in which CD4+ T cells directly recognize the allogeneic MHC-II molecule I-A(bm12), to assess the capacity of TRegs to mediate allospecific effects. In these mice, 5-6% of Tg CD4+ T cells exhibit conventional markers of the TReg phenotype. ABM TRegs are more effective than wild-type polyclonal TRegs at suppressing effector immune responses directed against I-A(bm12) alloantigen both in vitro and in vivo. In contrast, they are incapable of suppressing responses directed against third-party alloantigens unless these are expressed in the same allograft as I-A(bm12). Taken together, our results indicate that in transplantation, TReg function is dependent on TCR stimulation, providing definitive evidence for their specificity in the regulation of alloimmune responses.

Vaccination with a MHC class II peptide in Alum and inactive pertussis strongly ameliorates clinical MG in C57BL/6 mice

We have investigated the efficacy of immunization against peptides from predisposing MHC class II molecules in human-compatible adjuvants for ameliorating experimental autoimmune myasthenia gravis (EAMG). C57BL/6 mice were immunized three times with the peptide I-Abetab62-76 in Alum+killed pertussis organisms (PT) prior to two injections with tAChR. The treatment greatly reduced the occurrence and severity of clinical MG relative to controls that received saline/Alum+PT or none. It also reduced antibody and T-cell responses against tAChR. The results have important implications for the possible immunotherapy of MG by targeting disease-associated MHC.

Allograft rejection in a new allospecific CD4+ TCR transgenic mouse

The application of TCR transgenic mice to transplantation immunology is hampered by the limited lines available. Recently, we reported CD4+ T cell receptor (TCR) transgenic mice specific for I-Abm12 expressed on B6.C.H-2bm12 mice. Here, we characterized rejection of skin and vascularized cardiac allografts in these mice, which we term ABM (for anti-bm12). In vivo proliferative experiments reveal that all CD4 T cells in ABM mice react to bm12 antigens. Surprisingly, while ABM mice have accelerated (compared to B6 recipients) rejection of bm12 skin allografts, they, like B6 recipients, fail to acutely reject bm12 cardiac allografts. This is not due to lack of immunogenicity of bm12 hearts, as these grafts are acutely rejected by primed ABM recipients, although not by primed B6 recipients. Lastly, long-term surviving bm12 grafts in ABM recipients are relatively free from chronic rejection (compared with B6 recipients), which may be due to skewing of the CD4 repertoire towards direct alloreactivity, and consequent lack of CD4 mediated indirect allorecognition as evidenced by the lack of IgG deposition in those grafts. The results indicate that a complex interplay between responder frequency, priming and repertoire dictates the occurrence, or lack thereof, of acute and chronic rejection.

Suppressed clinical experimental autoimmune myasthenia gravis in bm12 mice is linked to reduced intracellular calcium mobilization and IL-10 and IFN-gamma release by acetylcholine receptor-specific T cells

Class II MHC mutant bm12 mice have an increased resistance to experimental autoimmune myasthenia gravis (EAMG) compared to C57BL/6 mice. In vitro, this relative resistance was mainly associated with a reduced cytokine response to acetylcholine receptor (AChR) and its dominant pathogenic peptide alpha 146-162, whereas the response to the epitope alpha 111-126 remained intact. Calcium mobilization after stimulation of AChR-immune T cells with AChR or alpha 146-162 peptide, but not alpha 111-126 peptide, was decreased in bm12 compared to C57BL/6. Thus, the reduced incidence of clinical EAMG in bm12 is linked to lower IFN-gamma and IL-10 release, and intracellular calcium mobilization by alpha 146-162-specific T cells.

Dendritic cells prime in vivo alloreactive CD4 T lymphocytes toward type 2 cytokine- and TGF-beta-producing cells in the absence of CD8 T cell activation

The mechanisms that influence the polarization of CD4 T cells specific for allogeneic MHC class II molecules in vivo are still poorly understood. We have examined the pathway of alloreactive CD4 T cell differentiation in a situation in which only CD4 T cells could be activated in vivo. In this report we show that priming of adult mice with allogeneic APC, in the absence of MHC class I-T cell interactions, induces a strong expansion of type 2 cytokine-producing allohelper T cells. These alloantigen-specific CD4 T cells directly recognize native allogeneic MHC class II molecules on APC and secrete, in addition to the prototypic Th2 cytokines IL-4, IL-5, and IL-10, large amounts of TGF-beta. The default Th2-phenotype acquisition is not genetically controlled and occurred both in BALB/c and C57BL/6 mice. CD8 T cells are the principal cell type that controls CD4 T cell differentiation in vivo. Furthermore, we demonstrate that strong Th2 priming can be induced not only with allogeneic splenocytes but also with a low number of bone marrow-derived dendritic cells. Finally, using a passive transfer system, we provide direct evidence that CD8 T cell expansion in situ promotes alloreactive Th1 cell development principally by preventing their default development to the Th2 pathway in a mechanism that is largely IFN-gamma independent. Therefore, this work demonstrates that type 2 cytokine production represents a dominant pathway of alloreactive CD4 T cell differentiation in adult mice, a phenomenon that was initially thought to occur only during the neonatal period.

Thymic selection generates T cells expressing self-reactive TCRs in the absence of CD45

The CD45 protein tyrosine phosphatase regulates Ag receptor signaling in T and B cells. In the absence of CD45, TCR coupling to downstream signaling cascades is profoundly reduced. Moreover, in CD45-null mice, the maturation of CD4+CD8+ thymocytes into CD4+CD8- or CD4-CD8+ thymocytes is severely impaired. These findings suggest that thymic selection may not proceed normally in CD45-null mice, and may be biased in favor of thymocytes expressing TCRs with strong reactivity toward self-MHC-peptide ligands to compensate for debilitated TCR signaling. To test this possibility, we purified peripheral T cells from CD45-null mice and fused them with the BWalpha-beta- thymoma to generate hybridomas expressing normal levels of TCR and CD45. The reactivity of these hybridomas to self or foreign MHC-peptide complexes was assessed by measuring the amount of IL-2 secreted upon stimulation with syngeneic or allogeneic splenocytes. A very high proportion (55%) of the hybridomas tested reacted against syngeneic APCs, indicating that the majority of T cells in CD45-null mice express TCRs with high avidity for self-MHC-peptide ligands, and are thus potentially autoreactive. Furthermore, a large proportion of TCRs selected in CD45-null mice (H-2b) were also shown to display reactivity toward closely related MHC-peptide complexes, such as H-2bm12. These results support the notion that modulating the strength of TCR-mediated signals can alter the outcome of thymic selection, and demonstrate that CD45, by molding the window of affinity/avidity for positive and negative selection, directly participates in the shaping of the T cell repertoire.

Suppression of experimental myasthenia gravis by monoclonal antibodies against MHC peptide region involved in presentation of a pathogenic T-cell epitope

We have prepared monoclonal antibodies (mAbs) against an antigen-binding region of I-A, region 62-76 of I-Abeta(b), which is involved in the T-cell participation in the pathogenesis of EAMG. The mAbs reacted with its parent molecules and inhibited the proliferation of disease-related T-cells. Passive transfer of these mAbs suppressed the occurrence of clinical EAMG, which was accompanied by decreased T-cell and Ab responses to tAChR. The results indicated that blocking the function of disease-related MHC by targeting a disease-associated region on MHC molecules could be an effective, straightforward and feasible strategy for immunointervention in MG.

Effect of the bm12 class II mutation on proliferative and cytokine responses of encephalitogenic T cells in myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis

The bm12 mutation in the class II I-A(b)molecule can profoundly influence experimental autoimmune disease, enhancing the development of systemic lupus erythematosus-like syndromes in NZB.H-2(bm12)mice or, conversely, abolishing the susceptibility of C57BL/6J (H-2(b)) mice to the induction of experimental autoimmune myasthenia gravis. We have studied the effect of this mutation on experimental autoimmune encephalomyelitis (EAE), induced in H-2(b)mice by myelin oligodendrocyte glycoprotein (MOG), and recently showed that MOG 35-55 peptide (pMOG 35-55), which represents the immunodominant encephalitogenic region for H-2(b)mice, is also a strong encephalitogen for H-2(bm12)mice. Nevertheless, although the differences in fine epitope specificity and TCR-Vbeta gene usage between encephalitogenic pMOG 35-55-specific T cells from H-2(b)and H-2(bm12)mice were subtle, H-2(bm12)and H-2(b)antigen presenting cells failed to effectively cross-present pMOG 35-55 non-syngeneically to I-A(b)/pMOG 33-55- and I-A(bm12)/pMOG 35-55-specific T cells, respectively. In the present study, we show that the abrogation of the response to pMOG 35-55 by the Th1 encephalitogenic pMOG 35-55-specific T cells upon non-syngeneic cross-presentation is neither due to a cytokine shift to a Th2 pattern, nor a result of anergy induction. Therefore, we suggest that presentation of pMOG 35-55 to I-A(b)/pMOG 35-55-specific T cells via the bm12 class II MHC molecule resulted in ineffective stimulation, similar to a weak agonistic effect.

Experimental autoimmune encephalomyelitis induced in B6.C-H-2bm12 mice by myelin oligodendrocyte glycoprotein: effect of MHC class II mutation on immunodominant epitope selection and fine epitope specificity of encephalitogenic T cells

The effect of the bm12 mutation on susceptibility to MOG-induced EAE, TCR repertoire and fine epitope specificity of the encephalitogenic T-cells, was assessed. prMOG35-55 was encephalitogenic for H-2bm12 and H-2b mice. Despite only minor differences in TCRVbeta expression and fine epitope specificity, H-2bm12/ and H-2b/prMOG35-55-specific T-cells failed to recognize Ab/prMOG35-55 and Abm12/prMOG35-55, respectively. rhMOG-induced EAE was milder in H-2bm12 mice, possibly as a result of co-dominant responses to prMOG35-55 and to the non-encephalitogenic pMOG94-116, rather than a single dominant response to prMOG35-55 in H-2b mice.

Gene conversion can create new MHC alleles

It is possible to measure gene conversion of MHC genes with the help of a semi-nested PCR assay. Several considerations are of utmost importance when such an assay is set up. Using this assay, we have found that gene conversion occurs in MHC class II genes in mouse sperm, but not in somatic cells tested. Although this gene conversion occurs in germline cells, it is already completed in spermatogonia, and consequently is mitotic event unlinked to meiosis. The frequency of gene conversion events in MHC class II genes varies strongly from one allele to another, with the highest detected frequencies as high as 1/40,000 for an individual heterozygous for both donor and acceptor sequences. Deletions or insertions in one gene relative to the other seem to lower the efficiency of gene conversion considerably. Stretches within MHC genes amenable to gene conversion are located in CpG clusters, whereas MHC genes not involved in gene conversion have background CpG levels. DNA damage, either chemical or radiation induced, increases the frequency of gene conversion of MHC class II genes in cultured cells of the fibroblastoid lineage. The effect of chemical DNA damage seems roughly dose dependent, whereas irradiation has a maximal effect at low doses.

T cell responses in EAMG-susceptible and non-susceptible mouse strains after immunization with overlapping peptides encompassing the extracellular part of Torpedo californica acetylcholine receptor alpha chain. Implication to role in myasthenia gravis of autoimmune T-cell responses against receptor degradation products

To study the role in myasthenia gravis (MG) of peptides resulting from acetylcholine receptor (AChR) degradation, we examined the ability of AChR peptides to induce T cell responses that are capable of cross-reacting with intact AChR. The studies were carried out in an experimental autoimmune MG (EAMG)-susceptible mouse strain [C57BL/6 (B6)] as well as in two non-susceptible strains [B6.C-H-2bm12 (bm12) and C3H/He]. A set of overlapping peptides encompassing the extracellular part (residues 1-210) of the alpha-chain of Torpedo californica (t) AChR were used, individually or in equimolar mixtures, as immunogens. In B6, immunization with peptides alpha45-60, alpha111-126, alpha146-162 and alpha182-198 gave T cells that responded in vitro to the correlate immunizing peptide. Only the T cells against the latter three peptides cross-reacted with tAChR. Peptide alpha146-162 exhibited the highest in vitro reaction with the immunizing peptide and cross-reaction with tAChR. T cells obtained by immunization of B6 with an equimolar mixture of the peptides responded in vitro to peptides alpha111-126, alpha146-162 and alpha182-198 and cross-reacted very strongly with tAChR. In bm12 and C3H/He, a number of peptides evoked, when used individually as immunogens, strong or moderate T cell responses that recognized in vitro the correlate immunizing peptide but cross-reacted poorly with tAChR. Immunization with the mixture of the peptides gave T cells that recognized several peptides in each strain butdid not cross-react with alpha146-162 or tAChR. The results indicate that the ability to recognize alpha146-162 or AChR by T cells against peptides resulting from receptor degradation can account for the susceptibility to, and aggravation of, MG in B6.

Factors that determine the severity of experimental myasthenia gravis

Based on our current information, the robust differences in responses of B6 and bm12 mice after immunization with AChR are as follows: (1) The AChR-specific T cell repertoires are strikingly different. The epitope specificities, as well as the rearranged TCR alpha and beta chains and their CDR3 domains, are virtually nonoverlapping in the two strains of mice. (2) The AChR antibody responses are quantitatively different, both to Torpedo AChR and to the autoantigen--mouse AChR. (3) The isotype distribution of AChR antibodies favors IgG2b in B6 mice, but not in bm12 mice. (4) The clinical manifestations of EAMG are qualitatively and quantitatively different in the two strains. These considerations have led to the following scheme, illustrated diagrammatically in FIGURE 2, to explain the differences in EAMG in B6 and bm12 mice: (1) The MHC Class II of B6 mice binds the alpha 146-162 peptide of Torpedo AChR with high affinity, while the genetically altered MHC Class II of bm12 mice does not, as previously suggested (see FIGURE 2). (2) The alpha 146-162/MHC Class II complex occurs only in B6 mice and interacts with T cells having appropriate TCRs, resulting in their stimulation and expansion. Although T cells of appropriate specificity are also available in the bm12 strain, the relevant peptide/MHC Class II complex is not present. Therefore, very few T cells with specificity for alpha 146-162 are stimulated, and those that are stimulated have different TCRs. T cells with specificity for other AChR peptides are also present and expanded in both strains of mice, but they have less influence on the outcome of the immune response. (3) The alpha 146-162-specific T cells of B6 mice, in turn, interact strongly with AChR-specific B cells of B6 mice. These B cells present the same epitope/MHC Class II complex as the APCs and therefore interact well with the alpha 146-162-specific T cells (FIGURE 2). Thus, T cells of this specificity appear to provide more efficient help for AChR antibody production than T cells with specificity for other Torpedo AChR epitopes. This results in production of greater amounts of AChR antibodies, including a critical subset that cross-reacts with autologous mouse AChR. The higher autoantibody levels contribute to the greater susceptibility to EAMG and to the greater severity of manifestations in the B6 strain compared with the bm12 strain. (4) There is a bias in B6 mice toward the production of AChR antibodies of IgG2b isotype. We suggest that T cells specific for alpha 146-162 may contribute to this isotype bias. The IgG2b antibodies appear to have particularly potent "myasthenogenic" effects in rats and mice. (5) Finally, it should be emphasized that these differences in immunological and clinical aspects of EAMG in B6 and bm12 mice are relative rather than absolute. T cells that respond to AChR epitopes other than alpha 146-162 can also provide help for AChR antibody production, albeit less potent. In a sense, this model represents a special case of molecular mimicry. In this case, the source of the foreign antigenic molecule is injection rather than the more usual route of infection. The antigen (Torpedo AChR) is one that these mice would never naturally encounter, and the critical amino acid (lysine 155) of the key epitope (alpha 146-162) is present only in the AChR of electric organs of electric fish and not in the AChR of mice, chickens, cows, or humans. The important point is that a detail of the structure of the foreign antigen--that is, a particular peptide of Torpedo AChR--can determine the severity of an antibody-mediated autoimmune disease, depending on how it interacts with a detail of the structure of the MHC Class II molecule and, in turn, on how the peptide/MHC Class II complex interacts with the available T cell repertoire. (ABSTRACT TRUNCATED)

Immunobiological studies on experimental visceral Leishmaniasis. V. The I-A(Bm12) mutation specifies resistance to infection

The I-A(Bm12) mutation of the I-A beta subunit converted Leishmania donovani-susceptible C57BL/6 (B6) mice into the relatively resistant B6C-H-2 Bm12 (Bm12) strain. The relative resistance was reflected not only in the reduced splenic and hepatic parasite burden in Bm12 (compared with B6) but also by the ability of Bm12 mice to mount a T-cell proliferative response to parasite antigens. Assay of antileishmanial antibody (immunoglobulin G (IgG)2a and IgG1) in the sera of infected mice showed that in Bm12 mice the predominant isotype was IgG2a, rather than IgG1, whereas a similar level of both isotypes were found in B6 mice. From the serum immunoglobulin isotype titre it appeared that the antileishmanial T-cell response was biased towards a T helper (Th) 1 response in Bm12 mice whereas it was a mixed Th1 and Th2 response in B6 mice. These observations provide credence to the notion that polymorphism in class II major histocompatibility complex (MHC) molecules is responsible for the difference in the disease phenotype.

Peptide dependence of major histocompatibility complex class II specific alloreactive responses

Splenic cells from transgenic mice, in which a single peptide is complexed to all major histocompatibility complex (MHC) class II molecules, are found to be incapable of triggering primary allogeneic mixed lymphocyte/leucocyte reactions (MLR) when co-cultured with lymphocytes from MHC class II congenic mouse strains. In addition, a single HLA-DR-blocking peptide can completely abrogate the capacity of splenocytes from chimeric HLA-DR/H2-E transgenic mice to stimulate primary MLR of T cells from wild-type mice. These results indicate that the primary alloreactive response is directed against a multitude of peptides presented by allogeneic MHC molecules.

Timing and effects of template number for gene conversion of major histocompatibility complex genes in the mouse

Pachytene spermatocytes and haploid spermatocytes from adult mice, as well as testis cells from prepubescent mice were analysed for the presence of gene conversion products resulting from the transfer between the MHC class II genes Ebd and Abk in a PCR assay. Gene conversion products were detected in testis cells as early as in 8 days old mice, where the only existing spermatogenic cells are spermatogonia. The frequency of gene conversion products remains the same as the cells reach meiosis in 18 days old mice or in pachytene cells, and is unchanged after meiosis is completed in haploid spermatocytes, thus indicating that gene conversion is largely completed already in the mitotically dividing spermatogonia. We have furthermore analysed the frequency of gene conversion between the Eb and Ab loci within the same MHC haplotype, the k haplotype. The frequency of such events is the same in animals with one and two copies of the same haplotype, and consequently seems to be independent of template number.

How subtle differences in MHC class II affect the severity of experimental myasthenia gravis

Myasthenia gravis is an autoimmune disorder characterized by muscle weakness, due to an antibody-mediated deficit of acetylcholine receptors (AChRs) at neuromuscular junctions. We analyzed the factors that determine the severity of experimental myasthenia gravis (EAMG) induced by immunization with Torpedo AChR, in two congenic strains of mice--B6 mice, which are highly susceptible to EAMG; and bm12 mice, which are relatively resistant, and differ only in a change of three amino acids in MHC Class II. We prepared large numbers of AChR-specific T cell hybridomas from each strain and characterized their epitope specificities and T cell receptor (TCR) gene usage: Half the B6 hybridomas responded to a single AChR peptide (alpha 146-162), and their TCR genes encoded restricted V alpha and V beta chains and CDR3 motifs. bm12 hybridomas had different epitope specificities and different, less restricted TCR genes. APCs were able to present AChR or AChR-derived peptides virtually exclusively to hybridomas of their own strain. Levels of antibodies to Torpedo and autoantibodies to mouse AChR were higher in B6 mice, and were biased toward the IgG2b isotype. We conclude that the "better fit" of MHC II, peptide, and TCR in the B6 mice enhanced cognate interactions of APCs with T cells, and T cells with B cells, resulting in a more abundant and pathogenic AChR antibody response, and thus more severe EAMG.

Gene conversion of major histocompatibility complex genes in the mouse spermatogenesis is a premeiotic event

The molecular genetic mechanism of gene conversion in higher eukaryotes remains unknown. We find it of considerable interest to determine when during spermatogenesis gene conversion occurs. We have therefore purified pachytene spermatocytes and haploid spermatocytes from adult mice and analyzed these fractions for the presence of gene conversion products resulting from the transfer between the major histocompatibility complex class II genes Ebd and Abk in a polymerase chain reaction assay. We have further isolated spermatogenic cells from prepubescent mice and analyzed them for the presence of the same gene conversion products. We can detect gene conversion products in testis cells as early as in 8-d-old mice where the only existing spermatogenic cells are spermatogonia. The frequency of gene conversion products remains the same as the cells reach meiosis in 18-d-old mice, and is unchanged after meiosis is completed in haploid spermatocytes. Gene conversion of this specific fragment therefore appears to be a premeiotic event and, consequently, relies on genetic mechanisms other than normal meiotic recombination.

Abundance of H-2 promiscuous T cells specific for mycobacterial determinants in H-2b/d F1 hybrid mice

A majority of immunodominant epitopes of mycobacterial antigens are known to be recognized by murine T cells in the context of several H-2 haplotypes. In this study, we established the frequency of T cells able to recognize these peptides promiscuously, i.e. in the context of allogeneic antigen-presenting cells, using hybridomas from peptide-immunized H-2 homologous and heterologous mice. The degree of promiscuity in homozygous mice varied between 4-27% between different specificities and genetic backgrounds. In particular, the results showed that promiscuity between Ab and Ad in respect to a peptide from the Mycobacterium tuberculosis 38-kDa protein (residues 350-369) was displayed by 22% of BALB/c and 4% of C57BL/10-derived hybrids, but by 42% of [BALB/c x C57BL/10] F1-derived clones. This represents a significant increase (p < 0.001) of T cell promiscuity compared to the parental haplotypes. It is noteworthy that considerably lower peptide concentrations were able to stimulate the promiscuous hybridomas compared to the H-2-restricted hybrids. This finding suggests a functional advantage of promiscuous T cells which enables them to expand preferentially in the initial stages of infections with M. tuberculosis and thus enables the host to mount a rapid protective immune response.

New HLA-DPB1 alleles generated by interallelic gene conversion detected by analysis of sperm

The rate at which allelic diversity at the HLA loci evolves has been the subject of considerable controversy. The patchwork pattern of sequence polymorphism within the second exon of the HLA class II loci, particularly in the DPB1 locus, may have been generated by segmental exchange (gene conversion). We have analysed the frequency of variant DPB1 sequences that have been created by interallelic gene conversion in the germline by screening pools of sperm using PCR amplification and oligonucleotide probe typing. Our results indicate that about 1/10,000 sperm represents a new DPB1 sequence generated by short tracts of segmental exchange (gene conversion) within the second exon, suggesting that interallelic gene conversion may have an important role in generating the extensive allelic diversity at the HLA loci.

Analysis of mutational changes at the HLA locus in single human sperm

Using a simple and efficient single sperm PCR and direct sequencing method, we screened for HLA-DPB1 gene mutations that may give rise to new alleles at this highly polymorphic locus. More than 800 single sperm were studied from a heterozygous individual whose two alleles carried 16 nucleotide sequence differences clustered in six polymorphic regions. A potential microgene conversion event was detected. Unrepaired heteroduplex DNA similar to that which gives rise to postmeiotic segregation events in yeast was observed in three cases. Control experiments also revealed unusual sperm from DPB1 homozygous individuals. The data may help explain allelic diversity in the MHC and suggest that a possible source of human mosaicism may be incomplete DNA mismatch repair during gametogenesis.

Different HLA-DQ are positively and negatively associated in Swedish patients with myasthenia gravis

The aim of this study was to determine the association between HLA-DQ and myasthenia gravis (MG) in 79 Swedish patients and 155 unrelated population based controls. HLA genotyping was done using polymerase chain reaction combined with sequence specific oligonucleotide probes. The DQB allele, DQB1*0201 was positively associated with MG, 39/79 (49%) patients and 43/152 (28%) controls (OR 2.47, Pc = 0.037). DQB1*0201 was observed more frequently in patients with an early onset of disease, below 30 years (Pc = 0.033). A negative association was found for DQA1*0103, 7/78 (9%) patients and 38/154 (25%) controls (OR 0.30, Pc = 0.037). DQA1*0501-DQB1*0201 and DQA1*0201-DQB1*0201 together was significantly increased in patients when compared to controls (OR 2.68; Pc = 0.019). In conclusion, two different DQ2 haplotypes (DQA1*0501-DQB1*0201 and DQA1*0201-DQB1*0201) were positively and the DQA1*0103 allele was negatively associated with MG. Susceptibility and resistance to MG in Swedish patients is mediated by HLA-DQ.

A determination of the frequency of gene conversion in unmanipulated mouse sperm

Gene conversion, sometimes also called micro gene conversion or gene conversion-like events, has been proposed to act on a number of genes in higher eukaryotes, such as gamma-globin, beta-tubulin, major urinary protein, and amyloid A genes. In the immune system, immunoglobulin genes and major histocompatibility complex class I and class II genes have been implicated. The notion that integral segments of DNA have been transferred from one gene to another in these cases has, however, met considerable resistance. We have devised a PCR assay detecting only the molecule that results if the E beta d-derived fragment analogous to that introduced in the bm12 mutation is transferred to the A beta k gene. We have proceeded to analyze sperm from the F1 cross C3H/HeJ (haplotype k) x BALB/c (haplotype d). In our assay, we find that the frequency for conversion of this particular DNA segment is 2 x 10(-6). This frequency is relevant only in the germ line; when liver cells were tested as an example of somatic cells, no events were observed, implying a frequency of < 2 x 10(-8) in liver. Fragments > 100 bp seem to be possible to transfer in this conversion.

Promiscuous T cell recognition of an H-2 IA-presented mycobacterial epitope

Genetically permissive T cell epitopes are an important prerequisite for the development of peptide-based vaccines or immunodiagnostic reagents. We have investigated the structural requirements of permissive T cell recognition of peptide p350-369 from the 38-kDa antigen of Mycobacterium tuberculosis. This peptide was found to be immunogenic in mice of the H-2b, bm12, d, s and k, but not of the H-2f genotype. T cell responses were restricted by I-A class II molecules. The same epitope core was recognized in the H-2b, d and k genotypes. T cell hybrids from BALB/c and C57BL/10 mice were used to determine: (i) the critical residues using substituted peptide derivatives and (ii) the degree of T cell promiscuity. Two out of five BALB/c (H-2d)-derived hybridomas tested displayed promiscuous peptide recognition in the context of H-2b and H-2bm12 antigen-presenting cells. The recognition of critical residues was found to be uniform for all five hybridomas when tested with syngeneic antigen-presenting cells; additional critical residues were identified when the peptide was recognized in the context of allogeneic antigen-presenting cells. Only one of the four tested C57BL/10 (H-2b) hybridomas showed promiscuity in the context of H-2bm12. Each of these C57BL/10-derived clones had a distinct response profile toward the critical residues. We propose that the demonstrated T cell promiscuity involves peptide interaction with polymorphic H-2 I-A residues.

Allogeneic recognition of class I molecules: anti-H-2Ld repertoire of H-2b mice includes T cells recognizing mutant class II H-2b (Abm12) molecules

Two major histocompatibility complex (MHC) class I-reactive T cell clones derived from H-2b mice, generated against the allogeneic Ld molecule, were found to recognize the H-2b class II mutant Abm12 molecule as well. In addition, these clones also recognize the class II A(s) molecule, and display a class II-dependent reactivity to staphylococcal enterotoxin B. Neither the class I nor the class II alloreactivities of the clones were found to be dependent on other MHC molecules. Both clones express CD4+CD8- phenotypes. The CD4 molecule appears to be involved in their class II reactivity, while little or no role for CD4 could be detected in the class I reactivity. This is the first report of a class I/class II cross-reactivity being mediated by CD4+ T cells. The structural basis for this cross-reactivity is discussed.

A highly polymorphic microsatellite in the class II Eb gene allows tracing of major histocompatibility complex evolution in mouse

A hallmark of major histocompatibility complex (MHC) genes is their extraordinarily high level of polymorphism. Polymorphic residues on MHC molecules determine which peptide ligands they bind and present to effector T lymphocytes. Although the genetic mechanisms responsible for MHC polymorphism have been delineated, the timetable and the pathway of their diversification remain unclear. To trace MHC evolution, we have characterized a highly polymorphic microsatellite containing tandem repeats (TRs) of two tetranucleotide units, TGGA and GGCA, located at the 3' end of the second intron in the class II Eb gene of mouse. On the basis of length as well as sequence variations, 11 TR alleles were defined in 55 inbred mouse strains, which included MHC recombinant haplotypes and haplotypes derived from different subspecies of mouse. In this extensive sampling, a striking concordance was observed between the serologically identified class II proteins and the associated TR alleles. Examination of several strains carrying the same MHC haplotypes as well as strains carrying recombinant MHC haplotypes indicates that TR alleles are extremely stable. These observations suggest that TR polymorphism predates the separation of various subspecies of mouse. On the basis of sequence divergence, a genealogical tree has been constructed to depict evolution of the different TR alleles. Finally, evidence is presented that suggests this microsatellite polymorphism is generated by slipped-strand mispairing during DNA replication.

Possible role of Abb gene in mouse resistance to EL4 metastases

"S" (survivor) mutants were produced in mice for genetic analysis of host resistance to metastatic cancers. S-mutants S-27 and S-31 resist transplantation of lymphoma EL4 of parental C57BL/6J (B6) mice while they accept parental skin grafts. Mutant S-27 also resists formation of spontaneous metastases from intradermally growing EL4 tumor into lymph nodes; mutant S-31 is highly susceptible to EL4 metastases. Another mutant, H-2bm26 (bm26), resists EL4 and rejects B6 skin grafts. Major histocompatibility complex (MHC) class I and class II gene expression was compared in these mutants and normal B6 mice. All three mutants tested, S-27, S-31, and bm26, expressed a low amount of Kb mRNA in organ-specific fashion. Mutants bm26 and S-31 expressed a low amount of Abb mRNA and of Ab antigen on their spleen cells. Some oligonucleotide probes designed to hybridize to the second exon of the class II MHC gene Abb did not hybridize with DNA from all three mutants. These findings suggest extensive sequence alterations in the Abb gene in mutants S-27, S-31, and bm26; they also suggest a major role of MHC in the control of host resistance to spontaneous metastases of the EL4 tumor.

Structure-function relationship among T-cell receptors specific for lysozyme peptides bound to Ab or Abm-12 molecules

The alpha beta T-cell receptor (TCR) recognizes antigenic peptides bound to major histocompatibility complex (MHC) molecules. In contrast to the antibody combining site, for which the antigen contact or complementarity-determining residues (CDRs) have been precisely defined, the location and function of the corresponding CDR regions of the alpha and beta TCR chains are not known. To develop a model system for systematic analysis of the CDRs of the alpha beta TCR, we isolated a panel of murine T-cell clones that recognize a lysozyme peptide containing residues 74-88 bound to either Ab or Abm-12 MHC class II molecules. Although these two MHC molecules differ by only three amino acid residues within the A beta chain, each of the T-cell clones was specific for peptide bound to the self-MHC molecule and did not recognize the same peptide bound to the other MHC molecule. The structural basis for this exquisite ligand specificity of the TCRs was analyzed by isolation and characterization of alpha and beta chain genes from five closely related T-cell clones. Comparison of predicted amino acid sequences mapped the ligand specificity differences to residues present within the alpha chain variable region segment and the alpha and beta chain variable-joining region junction regions. Thus with current models of TCR-ligand interactions, the results suggest that residues 26-30 of the alpha chain variable region may constitute one of the CDR regions of the TCR.