Origin and selection of peripheral CD4-CD8- T cells bearing alpha/beta T cell antigen receptors in autoimmune gld mice

Characterization of MHC class-I restricted TCRalphabeta+ CD4- CD8- double negative T cells recognizing the gp100 antigen from a melanoma patient after gp100 vaccination

The immune attack against malignant tumors require the concerted action of CD8+ cytotoxic T lymphocytes (CTL) as well as CD4+ T helper cells. The contribution of T cell receptor (TCR) alphabeta+ CD4- CD8- double-negative (DN) T cells to anti-tumor immune responses is widely unknown. In previous studies, we have demonstrated that DN T cells with a broad TCR repertoire are present in humans in the peripheral blood and the lymph nodes of healthy individuals. Here, we characterize a human DN T cell clone (T4H2) recognizing an HLA-A2-restricted melanoma-associated antigenic gp100-peptide isolated from the peripheral blood of a melanoma patient. Antigen recognition by the T4H2 DN clone resulted in specific secretion of IFN-gamma and TNF. Although lacking the CD8 molecule the gp100-specific DN T cell clone was able to confer antigen-specific cytotoxicity against gp100-loaded target cells as well as HLA-A2+ gp100 expressing melanoma cells. The cytotoxic capacity was found to be perforin/granzymeB-dependent. Together, these data indicate that functionally active antigen-specific DN T cells recognizing MHC class I-restricted tumor-associated antigen (TAA) may contribute to anti-tumor immunity in vivo.

CD95 ligand mediates T-cell receptor-induced apoptosis of a CD4+ CD8+ double positive thymic lymphoma

Tumors in the thymus can be of different cellular origin. Among the most common tumors are thymoma and lymphoma, which are derived from transformed thymic epithelial cells and transformed lymphocytes, respectively. Thymic lymphoma and their response to apoptotic stimuli are poorly characterized. Here, we analyse apoptosis events in the thymic lymphoma cell line Thy278, which expresses cell surface antigens characteristic of immature double positive thymocytes. Upon T-cell receptor (TCR)/CD3 stimulation, Thy278 cells die by apoptosis, similar as primary thymocytes during negative selection. Caspases are crucial for deletion of both Thy278 cells and normal thymocytes. Moreover, we show that deletion of primary thymocytes and Thy278 cells upon CD3 stimulation is considerably impaired by neutralizing CD95L antibody. Thus, our results not only demonstrate that TCR-induced apoptosis is still functional in transformed thymocytes, but also suggest that Thy278 cells are a helpful model for the molecular analysis of negative selection.

Death receptor signaling and autoimmunity

In recent years, it has become clear that self-nonself discrimination by the immune system is driven not so much by the specificities of the antigen receptors themselves, but by ligand-receptor systems that sense the presence of foreign pathogens (toll-like receptors) and those that regulate the balance between cellular proliferation and programmed cell death (tumor necrosis factor [TNF] family ligands and receptors). Interestingly, these two receptor families share a number of common signaling pathways, mediated by the cytoplasmic proteins containing death domains and TRAF domains, which trigger the complementary processes of programmed cell death and inflammation. Both humans and mice with genetic defects in the TNF-receptor family member Fas accumulate abnormal lymphocytes and develop systemic autoimmunity. These findings highlighted the importance of this TNF-receptor family member in the homeostasis of the immune system. In particular, the Fas receptor has been shown to be important in immunoreceptor-mediated apoptosis of activated T and B lymphocytes. Six members of the TNF-receptor superfamily share a common signaling domain with Fas, termed the death domain, that directly links these receptors to the apoptotic machinery of the cell, and, collectively, these receptors have been designated as "death receptors."We are currently investigating a number of important unresolved issues in this field, including: (1). how susceptibility to apoptosis through death receptors is regulated, (2). how Fas and related death receptors function in the maintenance of self-tolerance and homeostasis in the major cell types of the immune system, and (3). recently described nonapoptotic lymphocyte activation signals that use components of death receptor signaling.

B220+ double-negative T cells suppress polyclonal T cell activation by a Fas-independent mechanism that involves inhibition of IL-2 production

Fas-mediated apoptosis is a key mechanism for elimination of autoreactive T cells, yet loss of function mutations in the Fas signaling pathway does not result in overt T cell-mediated autoimmunity. Furthermore, mice and humans with homozygous Fas(lpr) or Fas ligand(gld) mutations develop significant numbers of B220+ CD4- CD8- double-negative (DN) alphabeta T cells (hereafter referred to as B220+ DN T cells) of poorly understood function. In this study, we show that B220+ DN T cells, whether generated in vitro or isolated from mutant mice, can suppress the ability of activated T cells to proliferate or produce IL-2, IL-10, and IFN-gamma. B220+ DN T cells that were isolated from either lpr or gld mice were able to suppress proliferation of autologous and syngeneic CD4 T cells, showing that suppression is Fas independent. Furthermore, restoration of Fas/Fas ligand interaction did not enhance suppression. The mechanism of suppression involves inhibition of IL-2 production and its high affinity IL-2R alpha-chain (CD25). Suppression also requires cell/cell contact and TCR activation of B220+ DN T cells, but not soluble cytokines. These findings suggest that B220+ DN T cells may be involved in controlling autoreactive T cells in the absence of Fas-mediated peripheral tolerance.

FAS gene mutation in a case of autoimmune lymphoproliferative syndrome type IA with accumulation of gammadelta+ T cells

A 6-month-old girl presented to the hospital with cervical lymphadenopathy and hepatosplenomegaly. She was known to have an enlarged spleen, anemia, and thrombocytopenia since the age of 1 month. A lymph node biopsy showed a diffuse proliferation of blasts with few remnants of follicles. The blasts were CD3+CD57+CD4-CD8-, consistent with the usual autoimmune lymphoproliferative syndrome phenotype. However, these double negative T cells stained positive for gammadelta T-cell receptors, whereas double negative T cells in patients with autoimmune lymphoproliferative syndrome usually bear alphabeta T-cell receptor. Mutation analysis of the FAS gene revealed a mutation in the death domain of the FAS gene, which is a frequent finding in patients with autoimmune lymphoproliferative syndrome. Based on these results, the diagnosis of autoimmune lymphoproliferative syndrome was established. RT-PCR analysis of the affected lymph node tissue revealed a strong upregulation of interleukin 10 and a moderate upregulation of interferon-gamma expression compared with normal tissue. Our findings indicate that autoimmune lymphoproliferative syndrome can result in a prominent proliferation of gammadelta+ double negative T cells. It is important to distinguish this benign polyclonal proliferation from neoplastic gammadelta+ T-cell proliferations, such as hepatosplenic gammadelta T-cell lymphomas. Factors contributing to the accumulation of these gammadelta+ double negative T cells may be an unidentified infection in combination with the young age of onset in this patient.

The role of Fas and related death receptors in autoimmune and other disease states

The Fas receptor, also known as APO-1 or CD95, has emerged as a key initiator of apoptotic programmed cell death in a variety of cell types. CD4(+) T cells are unique in their ability to commit "suicide" by stimulating their own Fas receptors with secreted or membrane-bound Fas ligand. This takes place in the setting of repeated stimulation with T-cell antigens and is thought to be a mechanism for controlling the expansion of T cells during viral infections and autoimmune disease states. T cells can also trigger apoptosis in B cells, macrophages, and other cell types through Fas ligand. These interactions negatively regulate the immune system but can also contribute to immunopathology, as occurs in Fas-mediated damage of target tissues in hepatitis and other organ-specific autoimmune diseases. The dual role of Fas in the immune response complicates the understanding of its role in disease states and may limit its potential as a therapeutic target. Despite the many roles of Fas in immunoregulation, findings in experimental mouse strains and human patients with genetic deficiencies in the Fas pathway have shown that the main result of disrupting this pathway in vivo is systemic autoimmunity and a predisposition toward lymphoid malignancies. The role of Fas in various cell types and the lessons we have learned from Fas-deficient patients with the autoimmune lymphoproliferative syndrome will be discussed.

Induction of experimental autoimmune neuritis in CD4-8-C57BL/6J mice

The C57BL/6J mice strain is known to be reputedly resistant to induction of experimental autoimmune neuritis (EAN), an animal model of Guillain-Barré syndrome in humans. Here we describe the induction of EAN in mice of the C57BL/6J background by transfer into naive syngeneic recipients bovine peripheral nerve myelin (BPM)-primed donor lymph node cells that had been stimulated in vitro with the bovine peripheral nervous system (PNS) myelin P2 protein peptide 57-81 followed by challenge with BPM, Freund's complete adjuvant and pertussis toxin. EAN was more severe, both clinically and histologically, and accompanied by extensive infiltration of inflammatory cells and demyelination in peripheral nerves when examined on day 30 after transfer of primed T cells from CD4- 8- mice into identical naive hosts than after transfer of cells from primed wild type, CD4-/- or CD8-/- mice to corresponding recipient animals. EAN in CD4-8- mice was also associated with elevated numbers of P2 peptide-reactive interferon-y (TFN-gamma) secreting cells and alphabeta T cells were present in lymph nodes and spleens. The data suggest that PNS myelin activated T cells from an EAN-resistant mice strain are capable of homing to the PNS. The expanded CD4-8- alphabeta T cells may have helper and effector functions, related to initiation of EAN in the CD4-8- mice. Lack of CD4+ and CD8+ expressing cells does not prevent the initiation of an autoimmune disease.

Fas/Fas Ligand Signaling During Gestational T Cell Development

Most thymocytes express high levels of Fas Ag (Apo-1/CD95); however, the role of Fas/Fas ligand-mediated apoptosis in thymocyte development remains unclear. During gestational development of thymocytes in C57BL/6(B6) +/+ mice, the highest levels of Fas ligand mRNA and Fas ligand protein expression were detected at gestational day (GD) 15, and there was a ninefold decrease in Fas ligand mRNA expression between GD 15 and 17 accompanied by a sixfold increase in Fas mRNA. Apoptotic thymocytes were first detected in the medulla at GD 15, and increasing numbers of cortical clusters and scattered, single apoptotic cells were present on GD 16 and 17. Thus, early apoptosis correlated with high expression of Fas ligand. High levels of Fas ligand mRNA were maintained throughout gestational development in thymocytes of Fas-deficient B6-lpr/lpr mice, but cortical clusters and scattered apoptotic cells were decreased relative to B6 +/+ mice before GD 17. Kinetic analysis of fetal thymic organ cultures treated with anti-Fas Ab demonstrated that thymocytes become sensitive to Fas-mediated apoptosis during the transition from the CD4−CD8− to the CD4+CD8+ phenotype. More mature CD4+CD8+ thymocytes and CD4+ and CD8+ thymocytes became resistant to Fas-mediated apoptosis after GD 17, despite high expression of Fas. However, low avidity engagement of the TCR on Fas-sensitive CD4+CD8+ thymocytes before GD 17 induced resistance to Fas-mediated apoptosis. The present results indicate that Fas plays a critical role in mediating apoptosis during early gestational thymocyte development and that thymocytes that receive a survival signal through TCR/CD3 become resistant to Fas-mediated apoptosis.

A role for FADD in T cell activation and development

FADD is a cytoplasmic adapter molecule that links the family of death receptors to the activation of caspases during apoptosis. We have produced transgenic mice expressing a dominantly interfering mutant of FADD, lacking the caspase-dimerizing death effector domain, as well as mice overexpressing the poxvirus serpin, CrmA, an inhibitor of caspases downstream of FADD. While thymocytes from either line of mice were completely protected from CD95-dependent cytotoxicity, neither transgene afforded protection from apoptosis induced during thymocyte selection and neither led to the lymphoproliferative disorders associated with deficiencies in CD95. However, in FADD dominant negative (FADDdd) mice, early thymocyte development was retarded and peripheral lymphocyte pools were devoid of normal populations of T cells. We show that thymocytes and peripheral T cells from FADDdd display signaling anomalies, implying that FADD plays a previously uncharacterized role in T cell development and activation.

Fas- and perforin-independent mechanism of cytotoxic T lymphocyte

Cytotoxic T lymphocytes (CTLs) play an important role in elimination of virus-infected cells (1). Recent studies revealed at least two distinct mechanisms that CTLs utilize to destroy their target cells. Both mechanisms induce target cell apoptosis specifically and directionally, but these processes are totally different. One is pore formation on target cell membrane by perforin secreted from CTLs (perforin-granzyme pathway), and the other is ligation of Fas, which is expressed on the surface of target cells and Fas ligand, on the surface of CTLs (Fas-FasL pathway) (2). Here we review our work and describe CTL clones that have novel lytic mechanisms derived from CD4-CD8- lymph node cells of gld mice.

Kinetics of Fas-induced apoptosis in thymic organ culture

Although most thymocytes express high levels of Fas antigen (CD95), the role of Fas in apoptosis signaling during thymocyte maturation has not been defined. Fas apoptosis occurs primarily in the CD4+CD8+ subpopulations of thymocytes. Fas expression and apoptosis function were investigated in the CD4-8-, CD4+8+, and CD4+ and CD8 single positive thymocyte subpopulations by in vivo injection of anti-Fas and in vitro incubation of Fas with thymic organ cultures. Fas was first expressed on CD4-8- thymocytes coincident with expression of IL-2R and CD44. In Fas mutant lpr/lpr mice, defective Fas expression correlated with overproduction of late-stage CD4-8(-)-thymocytes. Fas was highly expressed on CD3dull and CD3bright thymocytes. CD4+8+CD3dull thymocytes were sensitive to Fas apoptosis, whereas more mature CD4+8+CD3bright thymocytes were resistant to Fas apoptosis. Anti-Fas incubation with established thymic organ culture for 24 hr resulted in apoptosis of approximately 25% of thymocytes. Continued incubation of thymic organ culture with anti-Fas resulted in an apoptosis rate of 25% of CD4+CD8+ thymocytes per day for the first 3 days of culture. Continued culture for further time points up to 6 days did not result in further apoptosis of the CD4+CD8+ thymocytes. These results suggest that CD4-CD8-CD44+ IL-2R+ thymocytes express Fas and there is overpopulation of the subsequent developmental stage of thymocytes in Fas mutant lpr mice. Also, early-stage CD4+8+ thymocytes are susceptible to Fas apoptosis, whereas Fas apoptosis resistance is required after 3 days of thymic organ culture. We conclude that these two populations of thymocytes are susceptible to Fas ligand-mediated apoptosis during T cell development in the thymus.

Murine lupus in MRL/lpr mice lacking CD4 or CD8 T cells

MRL/lpr mice develop a systemic autoimmune disease similar to systemic lupus erythematosus in humans. The mice show progressive lymphadenopathy due to the accumulation of an unusual population of CD4-8-(DN) B220+ alpha beta+ T cells. We bred MRL/lpr mice with mice lacking CD4+ or CD8+ T cells by gene targeting via homologous recombination in embryonal stem cells to determine the roles of these cells in the autoimmune disease. No difference in survival or autoantibody levels was noted between CD8-/-lpr and littermate controls. Interestingly, these CD8-/- lpr mice have a reduced level of B220+ DN T cells despite the fact that the degree of lymphadenopathy was unaltered. CD4-/- lpr mice had a diminished autoimmune disease with a reduction in autoantibody production and skin vasculitits, and increased survival compared to littermate controls. However, CD4-/- lpr mice had an enhanced splenomegaly that developed massively by 16-20 weeks of age (5 to 8 greater than lpr control mice) due to the accumulation of DN B220+ T cells. In addition, there were no differences in peripheral lymph node enlargement, although the proportion of DN B220+ T cells was about twofold higher in the CD4-/- lpr mice. These cells were phenotypically identical to the DN population in control lpr mice, indicating that the accumulating DN T cells can be dissociated from the autoimmune disease in these mice. Collectively, our results reveal that the autoimmune disease is dependent on CD4+, but not CD8+ T cells, and that many of the B220+ DN T cells traverse a CD8 developmental pathway.

The pattern of T lymphocyte differentiation is altered during thymic involution

Thymic involution is likely to be a significant factor in the alteration of peripheral T lymphocyte function with age. The process of thymic involution involves the progressive loss of normal organ architecture and cellular composition, and a significant reduction in the output of mature T lymphocytes. The present study assesses the impact of thymic involution on the T cell differentiation process by quantitating the number and percent representation of various phenotypically distinguishable T cell developmental intermediates in C57BL/6 mice of various ages. The results suggest that several distinct sites in the developmental sequence are impacted by aging. By middle-age (14-17 months), significant perturbations in the frequencies of several CD4-CD8- (DN) subpopulations have occurred. These include a shift towards an increased percentage of Pgp-1+ IL-2R- DN cells, the earliest thymic progenitors, and a decreased percentage and total number of Pgp-1- IL-2R+ DN cells. Furthermore there is a threefold increase in the percentage of DN cells which express CD3 (from 16.6% to 45.5%) which occurs between 4 and 14 months of age. By 24-27 months of age, the percentage of the total DN population increases two- to threefold over that of young (2-3 months) animals, while the fraction of CD4+CD8+ (DP) is significantly reduced. These alterations are consistent with the possibility that thymic involution results in one or more 'developmental' blocks which limit key differentiative transitions within the DN population, and furthermore, the marked increase in the frequency of DN cells displaying CD3 argues that an alternative T cell differentiation pathway plays an increasingly significant role with advancing age.

Reduced development of CD4-8-B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules

The lpr gene has recently been shown to encode a functional mutation in the Fas receptor, a molecule involved in transducing apoptotic signals. Mice homozygous for the lpr gene develop an autoimmune syndrome accompanied by massive accumulation of double-negative (DN) CD4-8-B220+ T cell receptor-alpha/beta+ cells. In order to investigate the origin of these DN T cells, we derived lpr/lpr mice lacking major histocompatibility complex (MHC) class I molecules by intercrossing them with beta 2-microglobulin (beta 2m)-deficient mice. Interestingly, these lpr beta 2m-/- mice develop 13-fold fewer DNT cells in lymph nodes as compared to lpr/lpr wild-type (lprWT) mice. Analysis of anti-DNA antibodies and rheumatoid factor in serum demonstrates that lpr beta 2m-/- mice produce comparable levels of autoantibodies to lprWT mice. Collectively our data indicate that MHC class I molecules control the development of DN T cells but not autoantibody production in lpr/lpr mice and support the hypothesis that the majority of DN T cells may be derived from cells of the CD8 lineage.

Autoimmunity in mice bearing lprcg: a novel mutant gene

A novel mutation at the lpr (lymphoproliferation)(Fas) locus, lprcg, that can complement gld (generalized lymphoproliferative disease) in induction of lymphadenopathy was discovered in CBA/K1Jms mice. The lpr and lprcg mutations are a defective allele of the Fas locus that encodes an apoptosis-mediating receptor. The former does not express the receptor and the latter expresses the point-mutated nonfunctional receptor. The gld locus is hypothesized to encode a ligand for the receptor and the gld mutation to have a defect that leads to incompetent expression of the ligand. The absence and non-functioning of the receptor in lpr/lpr and lprcg/lprcg mice, respectively, and the lack of the ligand in gld/gld mice may arrest apoptosis of lymphoid cells in the thymus, resulting in the same type of lymphadenopathy characterized by expansion of unusual CD4-CD8- (DN) T cells. Less severe lymphadenopathy induced by complementarity between lprcg and gld may be explained by less efficient apoptosis resulting from competition for the ligand between the functional and nonfunctional receptors. Phenotypically, lpr and lprcg are different from gld in the function at bone marrow (BM) and lymph node (LN) levels: lpr/lpr and lprcg/lprcg BM cause atrophy but gld/gld BM hyperplasia of wild-type (+/+) LNs, and lpr/lpr and lprcg/lprcg LNs but not gld/gld LNs allow the homing of lpr- and lprcg-induced DN T cells. Lymphadenopathy is equally prominent in CBA-lprcg/lprcg and MRL-lprcg/lprcg mice. Hypergammaglobulinemia, autoantibodies and circulating immune complexes are detectable at significant levels in both lprcg/lprcg mice but at higher levels on the MRL background. Pathological signs like glomerulonephritis and vasculitis are clinically unimportant in CBA-lprcg/lprcg but strikingly severe in MRL-lprcg/lprcg mice. Noticeably, clinically significant glomerulonephritis and vasculitis also develop with slight but significant serological aberrations in MRL-lprcg/+ heterozygotes. Graft-vs.-host disease-like syndrome in the lprcg/lprcg BM-->+/+ chimera is minimal on the CBA but as severe as life-threatening on the MRL background as in the MRL-lpr/lpr BM-->MRL(-)+/+ chimera. Thus, autoimmune diseases induced by the lpr, lprcg and gld genes are actually indistinguishable in the clinical, serological and pathological aspects on the same strain background and the disease caused by the interaction between lprcg and gld is less severe in all the aspects, consistent with the receptor-ligand theory. The lprcg/lprcg mice with different strain backgrounds together with lpr/lpr and gld/gld mice will serve as a powerful tool for elucidation of the mechanism of development of single-gene autoimmune diseases at a molecular biological level.

Gld and lpr mice: single gene mutant models for failed self tolerance

Mice homozygous for the gld or lpr mutations develop autoimmunity, and a lymphoproliferative disorder involving accumulation of huge numbers of unusual CD4-CD8-TCR alpha beta lo T cells. Here we review our past work with gld mice, and attempt to explain lymphoproliferation in terms of current models of T cell maturation and self-tolerance induction. The availability of molecular probes to the gene products of lpr and gld should shortly lead to a better understanding of the acquisition of self tolerance during T cell maturation and of autoimmunity.

Dissociation of autoaggression and self-superantigen reactivity

Self-superantigens have been described as products of endogenous retroviruses of the mouse ('minor lymphocyte stimulating loci') that are capable of interacting without prior processing with conserved domains of TCR V beta chains, causing the activation and deletion of most T cells expressing products of determined V beta gene families [1-4]. The fact that superantigens activate a far higher percentage of T cells (1-20%) than conventional, peptidic antigens (< 0.1%) provides the methodological advantage that the degree of clonal deletion may be measured by the analysis of the TCR repertoire using appropriate anti-V beta antibodies. Although much information on the spatio-temporal organization of repertoire-purging has been gathered by virtue of self-superantigens, serious doubts exist as to the possibility that such structures serve as pathogenetically relevant autoantigens. Thus, certain inbred mice spontaneously develop autoimmune diseases, although they bear T-cell repertoires that appear to be purged from self-superantigen-reactive V beta products. In addition, therapeutic interventions targeted to V beta gene products that are not specific for self-superantigens are successful in preventing disease development. The lack of correlation between superantigen-related V beta deletions and autoimmune disease development is substantiated in further models of murine autoimmunity. Based on these observations, we formulate the hypothesis that self-superantigen-reactive T cells are not involved in the development of autoimmune diseases.

Functional analyses of lpr gene in MRL/Mp-lpr/lpr mice. Role of lymph node stromal cells in lpr-lymphadenopathy

To clarify the mechanism by which the lpr gene causes lymphadenopathy, we established an experimental system to induce lymph node (LN) swelling in unaffected mice. In MRL-(+)/+ mice that had been 5 Gy-irradiated and grafted with bone marrow cells (BMCs) plus LN from MRL-lpr/lpr mice, a remarkable enlargement of the LN grafts was seen. The enlarged grafts lacked normal LN structure and were indistinguishable from LNs of MRL-lpr/lpr mice. The induction of LN swelling by this method was achieved not only in [MRL-lpr/lpr-->MRL-(+)/+] but also in [MRL-lpr/lpr-->BALB/c], [MRL-lpr/lpr-->C3H], [B6-lpr/lpr-->B10.Thy1.1], and [B6-lpr/lpr-->BALB/c] combinations. Furthermore, the lpr/lpr LN grafts developed lymph node swelling even without the transplantation of BMCs. Most cells in the grafted LNs disappeared within a few days, and large clear fibroblast-like cells then became dominant for 1 to 4 weeks. Thereafter, lymphoid cells increased and had filled the graft by the 8th week. The LN grafts obtained from MRL-lpr/lpr (but not MRL-(+)/+) mice showed the ability to transfer LN node swelling into the secondary MRL-(+)/+ hosts two weeks after the primary transplantation. These results strongly suggest that the fibroblast-like LN stromal cells play a crucial role in lpr-associated lymphadenopathy.

Adoptive transfer of the entire gld (generalized lymphoproliferative disease) syndrome in nude beige mice by a single gld thymocyte graft

C57BL/6 nude beige mice (B6 nubq; no T cell, no NK activity) were used as recipients for the adoptive transfer of thymocytes from B6 gld mice (generalized lymphoproliferative disease) which are a model of systemic lupus erythematous. The [gld----nubg] chimeras showed several similarities with gld control mice including the T cell disorders (lymphoproliferation and Con A-response deficiency of splenocytes) and B cell disorders (hyperglobulinemia and elevated anti-single-stranded DNA antibody titers). This suggests that the gld lymphoproliferative disorder has a thymic origin (and does not result from an abnormally extrathymic T cell development) and that the gld T cells have an essential role for the emergence of the disorders of both the T and B cells.

Mechanisms of autoimmunity in the context of T-cell tolerance: insights from natural and transgenic animal model systems

There are a number of mechanisms which cooperate to produce and maintain T-cell tolerance. First, and perhaps most important, is the clonal deletion in the thymus of T cells with high affinity for self antigens. However, to ensure that a wide repertoire of T cells is available in the periphery to combat foreign antigens, the threshold of clonal deletion may be set low enough so that T cells whose TCR's have sub-threshold affinity for self antigens mature and migrate to the periphery. T cells which recognize self antigen-derived peptides not expressed or presented in the thymus will also fail to be deleted. For those self-reactive T cells which are not deleted in the thymus, other mechanisms may produce tolerance, including an undefined alteration of signalling pathways which produces clonal anergy, and lowering the avidity of the TCR for its ligand by downregulating coreceptor and accessory molecules. Active suppression of T-cell responses in another well-described phenomenon whose mechanism is undefined. From our observations with the model systems discussed here, we have observed three distinct mechanisms by which T-cell tolerance can be circumvented, allowing autoimmune phenomena to occur. These mechanisms may have relevance for different types of autoimmune diseases seen in humans. In gld mice, the autoimmune disease seems to be related to a global defect in T-cell differentiation and function, which allows for the expansion of autoimmune B cells. While we showed that clonal deletion of V beta-bearing T cells is appropriate in certain cases, aberrant lymphokine secretion by the abnormal T cells or disruption of immune system regulation are most probably responsible for allowing autoantibody production. While human lupus erythematosis shares much of the pathology of lpr and gld mice, there is no expansion of T cells with a similar phenotype in human lupus. There are environmental factors which must play a role in the development of human lupus, since the incidence of the disease does not follow an absolute genetic pattern. The escape from clonal deletion and subsequent reactivation of autoimmune T cells which we observed in V beta 8.1 TCR-transgenic mice can be a model for human autoimmune diseases such as multiple sclerosis and type I diabetes, in which T cells are directed against a specific autoantigen. According to this model, susceptibility loci for autoimmune disease such as the MHC would function by producing different repertoires of T cells which in some cases could gain autoreactivity following activation.(ABSTRACT TRUNCATED AT 400 WORDS)

T-cell receptor and autoimmune disease

Since the genes encoding the TCR have been cloned, their structure, organization, pattern of rearrangement, diversification and expression in ontogeny have been classified. However, there are still many important questions to be addressed, such as the nature of thymic education, tolerance, the mechanism of MHC-restricted antigen recognition and the relation between TCR repertoire and autoimmunity. In the future, new approaches to study these issues, such as transgenic mice, X-ray crystallography, and severe combined immune deficiency mice reconstituted with human hematopoietic cells will lead to a more profound understanding of these questions. This will hopefully allow us to manipulate the immune response in different and more effective ways than are currently available.

Self-reactive T cells can escape clonal deletion in T-cell receptor V beta 8.1 transgenic mice

To study the mechanisms of tolerance in detail, we have constructed transgenic mice expressing a V beta 8.1-D beta 2-J beta 2.3-C beta 2 T-cell receptor (TCR) gene. Since expression of V beta 8.1 is known to correlate with reactivity of CD4+CD8- T cells to minor lymphocyte-stimulating locus 1a (Mls-1a), we expected to induce tolerance in most CD4+CD8- T cells in V beta 8.1 transgenic mice of the Mls-1a allele. In one line of Mls-1b V beta 8.1 transgenic mice, the V beta 8.1 TCR was expressed on greater than 98% of mature T cells and their response to Mls-1a was highly enriched. In Mls-1a V beta 8.1 transgenic mice, CD4+CD8- T cells in these mice were severely reduced among both peripheral T cells and thymocytes. However, the deletion of these cells was not complete, and most of the residual CD4+CD8- mature T cells still expressed normal densities of V beta 8.1 TCR. The residual CD4+CD8- T cells did not respond to Mls-1a but were still able to proliferate in response to other stimuli via the TCR. Interestingly, CD4+CD8- V beta 8.1+ T-cell clones isolated from Mls-1a V beta 8.1 transgenic mice could respond to Mls-1a. We suggest that these types of T cells escape clonal deletion in the thymus.