A role for clonal inactivation in T cell tolerance to Mls-1a

Prenatal Administration of Betamethasone Causes Changes in the T Cell Receptor Repertoire Influencing Development of Autoimmunity

Prenatal glucocorticoids are routinely administered to pregnant women at risk of preterm delivery in order to improve survival of the newborn. However, in half of the cases, birth occurs outside the beneficial period for lung development. Glucocorticoids are potent immune modulators and cause apoptotic death of immature T cells, and we have previously shown that prenatal betamethasone treatment at doses eliciting lung maturation induce profound thymocyte apoptosis in the offspring. Here, we asked if there are long-term consequences on the offspring’s immunity after this treatment. In the non-obese diabetic mouse model, prenatal betamethasone clearly decreased the frequency of pathogenic T cells and the incidence of type 1 diabetes (T1D). In contrast, in the lupus-prone MRL/lpr strain, prenatal glucocorticoids induced changes in the T cell repertoire that resulted in more autoreactive cells. Even though glucocorticoids transiently enhanced regulatory T cell (Treg) development, these cells did not have a protective effect in a model for multiple sclerosis which relies on a limited repertoire of pathogenic T cells for disease induction that were not affected by prenatal betamethasone. We conclude that prenatal steroid treatment, by inducing changes in the T cell receptor repertoire, has unforeseeable consequences on development of autoimmune disease. Our data should encourage further research to fully understand the consequences of this widely used treatment.

MMTV superantigens coerce an unconventional topology between the TCR and MHC class II

Mouse mammary tumor virus superantigens (vSAGs) are notorious for defying structural characterization, and a consensus has yet to be reached regarding their ability to bridge the TCR to MHC class II (MHCII). In this study, we determined the topology of the T cell signaling complex by examining the respective relation of vSAG7 with the MHCII molecule, MHCII-associated peptide, and TCR. We used covalently linked peptide/MHCII complexes to demonstrate that vSAG presentation is tolerant to variation in the protruding side chains of the peptide, but can be sensitive to the nature of the protruding N-terminal extension. An original approach in which vSAG was covalently linked to either MHCII chain confirmed that vSAG binds outside the peptide binding groove. Also, whereas the C-terminal vSAG segment binds to the MHCII α-chain in a conformation-sensitive manner, the membrane-proximal N-terminal domain binds the β-chain. Because both moieties of the mature vSAG remain noncovalently associated after processing, our results suggest that vSAG crosslinks MHCII molecules. Comparing different T cell hybridomas, we identified key residues on the MHCII α-chain that are differentially recognized by the CDR3β when engaged by vSAG. Finally, we show that the highly conserved tyrosine residue found in the vSAg TGXY motif is required for T cell activation. Our results reveal a novel SAG/MHCII/TCR architecture in which vSAGs coerce a near-canonical docking between MHCII and TCR that allows eschewing of traditional CDR3 binding with the associated peptide in favor of MHCII α-chain binding. Our findings highlight the plasticity of the TCR CDRs.

Challenging the Tritope Model of T cell receptor structure-function relationships with classical data on 'super' and 'allo-MHC' antigens

The response of the immune system to allo-MHC-encoded antigens and Mls 'superantigens' has been experimentally analysed in detail, but the data have not been coupled to a theoretical framework. It should therefore be instructive to see how well the newly proposed Tritope Model of TCR structure-function relationships deals with the signalling interactions between the TCR and the above antigens. We will pay heed to William Bateson's admonition, 'treasure the exceptions', by showing how a meaningful theory interrogates the data with the same validity that the data interrogate the theory. The concordances, as well as the contradictions, with the Tritope Model are a test of its heuristic value.

Prevalent de novo somatic mutations in superantigen genes of mouse mammary tumor viruses in the genome of C57BL/6J mice and its potential implication in the immune system

Background

Superantigens (SAgs) of mouse mammary tumor viruses (MMTVs) play a crucial role in T cell selection in the thymus in a T cell receptor (TCR) Vβ-specific manner and SAgs presented by B cells activate T cells in the periphery. The peripheral T cell repertoire is dynamically shaped by the steady induction of T cell tolerance against self antigens throughout the lifespan. We hypothesize that de novo somatic mutation of endogenous MMTV SAgs contributes to the modulation of the peripheral T cell repertoire.

Results

SAg coding sequences were cloned from the genomic DNAs and/or cDNAs of various tissues of female C57BL/6J mice. A total of 68 unique SAg sequences (54 translated sequences) were identified from the genomic DNAs of liver, lungs, and bone marrow, which are presumed to harbor only three endogenous MMTV loci (Mtv-8, Mtv-9, and Mtv-17). Similarly, 69 unique SAg sequences (58 translated sequences) were cloned from the cDNAs of 18 different tissues. Examination of putative TCR Vβ specificity suggested that some of the SAg isoforms identified in this study have Vβ specificities different from the reference SAgs of Mtv-8, Mtv-9, or Mtv-17.

Conclusion

The pool of diverse SAg isoforms, generated by de novo somatic mutation, may play a role in the shaping of the peripheral T cell repertoire including the autoimmune T cell population.

Predominant clonal accumulation of CD8+ T cells with moderate avidity in the central nervous systems of Theiler's virus-infected C57BL/6 mice

Induction of antigen-specific CD8(+) T cells bearing a high-avidity T-cell receptor (TCR) is thought to be an important factor in antiviral and antitumor immune responses. However, the relationship between TCR diversity and functional avidity of epitope-specific CD8(+) T cells accumulating in the central nervous system (CNS) during viral infection is unknown. Hence, analysis of T-cell diversity at the clonal level is important to understand the fate and function of virus-specific CD8(+) T cells. In this study, we examined the Vbeta diversity and avidity of CD8(+) T cells specific to the predominant epitope (VP2(121-130)) of Theiler's murine encephalomyelitis virus. We found that Vbeta6(+) CD8(+) T cells, associated with epitope specificity, predominantly expanded in the CNS during viral infection. Further investigations of antigen-specific Vbeta6(+) CD8(+) T cells by CDR3 spectratyping and sequencing indicated that distinct T-cell clonotypes are preferentially increased in the CNS compared to the periphery. Among the epitope-specific Vbeta6(+) CD8(+) T cells, MGX-Jbeta1.1 motif-bearing cells, which could be found at a high precursor frequency in naïve mice, were expanded in the CNS and tightly associated with gamma interferon production. These T cells displayed moderate avidity for the cognate epitope rather than the high avidity normally observed in memory/effector T cells. Therefore, our findings provide new insights into the CD8(+) T-cell repertoire during immune responses to viral infection in the CNS.

Junctional diversity prevents negative selection of an antigen-specific T cell repertoire

Endogenous mouse mammary tumor proviruses (MMTV; Mtv loci) deletes Vbeta6 expressing T cells in the thymus of Mtv-7(+) DBA/2 (H2(d)) mice through negative selection. We found that in Mtv-7(-) BALB/c (H2(d)) mice, Vbeta6 is a dominant V gene used in T cell responses to an 18 amino acid long peptide antigen: EYKEYAEYAEYAEYAEYA [abbreviated as K5 or EYK(EYA)(5)]. It was therefore surprising to find that despite the deletion of Vbeta6+ T cells, vigorous K5 specific T cell responses that use Vbeta6 can be raised in DBA/2 mice. Sequence analysis of Vbeta6 junctional diversity in K5 specific T cell lines revealed that the DBA/2 K5 repertoire compensates for the loss of most Vbeta6 T cells by overusing and amplifying Vbeta6+ T cells escaping central deletion and peripheral tolerization. In order to address the inability of some Vbeta6 T cells to recognize Mtv-7(+) we analyzed a panel of BALB/c Vbeta6 expressing T cell hybridomas. This data supported the argument that certain Vbeta6 junctional sequences preclude Mtv recognition and allows their escape from central deletion in DBA/2 mice. These cells are not anergic and can be activated with cognate peptide antigen in periphery. We suggest that junctional diversity at the V region of some of the T cell receptors does not allow these cells to recognize self-superantigens with high enough affinity and thus they escape negative selection in the thymus. These results for the first time provide a molecular explanation of how the immune system compensates for "hole in the repertoire" caused by deletion of the majority of T cells carrying certain V region segments.

How do regulatory T cells work?

CD4⁺ T cells are commonly divided into regulatory T (Treg) cells and conventional T helper (Th) cells. Th cells control adaptive immunity against pathogens and cancer by activating other effector immune cells. Treg cells are defined as CD4⁺ T cells in charge of suppressing potentially deleterious activities of Th cells. This review briefly summarizes the current knowledge in the Treg field and defines some key questions that remain to be answered. Suggested functions for Treg cells include: prevention of autoimmune diseases by maintaining self-tolerance; suppression of allergy, asthma and pathogen-induced immunopathology; feto-maternal tolerance; and oral tolerance. Identification of Treg cells remains problematic, because accumulating evidence suggests that all the presently-used Treg markers (CD25, CTLA-4, GITR, LAG-3, CD127 and Foxp3) represent general T-cell activation markers, rather than being truly Treg-specific. Treg-cell activation is antigen-specific, which implies that suppressive activities of Treg cells are antigen-dependent. It has been proposed that Treg cells would be self-reactive, but extensive TCR repertoire analysis suggests that self-reactivity may be the exception rather than the rule. The classification of Treg cells as a separate lineage remains controversial because the ability to suppress is not an exclusive Treg property. Suppressive activities attributed to Treg cells may in reality, at least in some experimental settings, be exerted by conventional Th cell subsets, such as Th1, Th2, Th17 and T follicular (Tfh) cells. Recent reports have also demonstrated that Foxp3⁺ Treg cells may differentiate in vivo into conventional effector Th cells, with or without concomitant downregulation of Foxp3.

Antigen-specific tolerance of human alpha1-antitrypsin induced by helper-dependent adenovirus

As efficient and less toxic virus-derived gene therapy vectors are developed, a pressing problem is to avoid immune response to the therapeutic gene product. Secreted therapeutic proteins potentially represent a special problem, as they are readily available to professional antigen-presenting cells throughout the body. Some studies suggest that immunity to serum proteins can be avoided in some mouse strains by using tissue-specific promoters. Here we show that expression of human alpha1-antitrypsin (AAT) was nonimmunogenic in the immune-responsive strain C3H/HeJ, when expressed from helper-dependent (HD) vectors using ubiquitous as well as tissue-specific promoters. Coadministration of less immunogenic HD vectors with an immunogenic first-generation vector failed to immunize, suggesting immune suppression rather than immune stealth. Indeed, mice primed with HD vectors were tolerant to immune challenge with hAAT emulsified in complete Freund's adjuvant. Such animals developed high-titer antibodies to coemulsified human serum albumin, showing that tolerance was antigen specific. AAT-specific T cell responses were depressed in tolerized animals, suggesting that tolerance affects both T and B cells. These results are consistent with models of high-dose tolerance of B cells and certain other suppressive mechanisms, and suggest that a high level of expression from HD vectors can be sufficient to induce specific immune tolerance to serum proteins.

Negative thymocyte selection to HERV-K18 superantigens in humans

An experimental system to explore central tolerance in humans is unavailable. However, the human endogenous retrovirus K-18 (HERV-K18) region on chromosome 1 provides an excellent model: HERV-K18 encodes a superantigen (SAg) stimulating Vβ7CD4 T cells that is implicated in type 1 diabetes and Epstein-Barr virus persistence. In this study, we have addressed thymic HERV-K18 SAg expression, the capacity of SAg to induce negative selection, and the consequences of this for peripheral tolerance compared with SAg reactivity. We demonstrate that thymic HERV-K18 SAg expression is constitutive and is restricted in time and space such that it can induce negative selection. We developed an in vitro assay capable of detecting negative human thymocyte selection by bacterial SAgs presented on extrathymic antigen-presenting cells (APCs). Using this assay, the HERV-K18 SAg is necessary and sufficient for negative selection of immature or semimature Vβ7CD4 thymocytes. Decreases of SAg reactive Vβ7CD4 T cells generated in the thymus predict low or absent SAg reactivity. Therefore, these results indicate that negative thymic selection to HERV-K18 SAgs constitutes a first checkpoint controlling peripheral tolerance compared with SAg reactivity. This study now offers a framework to dissect negative selection and its interplay with viral persistence and autoimmunity in humans.

T-cell tolerance and autoimmunity to systemic and tissue-restricted self-antigens

We have used transgenic mouse models to examine the mechanisms of tolerance in CD4(+) T lymphocytes to soluble, systemic and cell-associated, tissue-restricted self-antigens. Anergy to an islet antigen, as a model of a tissue antigen, is dependent on the inhibitory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4), and tissue-restricted autoimmunity is inhibited by regulatory T lymphocytes. Anergy to a circulating systemic antigen can occur independently of CTLA-4 signals, and it is induced primarily by a block in proximal receptor-initiated signals. CD4(+)CD25(+) regulatory T cells are generated in response to both forms of self-antigens, but the induction is much more efficient with the tissue antigen. Receptor desensitization can be induced by the systemic antigen even in the absence of regulatory T cells, but tolerance can be broken by immunization much more easily if these cells are absent. Deletion of mature T cells is striking with the systemic antigen; there is little evidence to support peripheral deletion as a mechanism of tolerance to the tissue antigen. Thus, both distinct and overlapping mechanisms account for unresponsiveness to different forms of self-antigens. These results establish a foundation for searching for genetic influences and pathogenic mechanisms in organ-specific and systemic autoimmune diseases.

A comparative study between T regulatory type 1 and CD4+CD25+ T cells in the control of inflammation

There is now compelling evidence that CD4(+)CD25(+) T cells play a major role in the maintenance of tolerance. Besides CD4(+)CD25(+) T cells, different populations of regulatory CD4(+) T cells secreting high amounts of IL-10 (T regulatory type 1 (Tr1)) or TGF-beta (Th3) have also been described in in vivo models. In the lymphocyte transfer model of inflammatory bowel disease, we show here that the control of inflammation during the first weeks is not due to a complete inhibition of differentiation of aggressive proinflammatory T cells, but is the result of a balance between proinflammatory and Tr cells. We also show that in the first weeks continuous IL-10 secretion was required to actively control inflammation. Indeed, treatment with anti-IL-10R Abs 3 wk after the start of the experiment completely reversed the protective effect of Tr cells. IL-10 secretion and control of inflammation could be provided by late injection of Tr1 cells that efficiently cure ongoing inflammatory responses in two different models of inflammation. In contrast, inflammation was not controlled when high numbers of CD4(+)CD45RB(low) or CD4(+)CD25(+) T cells were injected as early as 1 wk after the start of the experiment. These results confirm in vitro studies showing that CD4(+)CD45RB(low) do not contain high IL-10-producing cells and suggest that CD4(+)CD45RB(low) Tr cells maintain tolerance in vivo, in part indirectly, through the differentiation of IL-10-secreting Tr1 cells.

T-helper cell tolerance to ubiquitous nuclear antigens

Systemic autoimmune diseases are characterized by the development of antinuclear autoantibodies. In order to understand the immunologic events leading to the development of such antibodies, knowledge of mechanisms of immune tolerance to nuclear antigens is required. By utilizing adoptive T-cell transfer strategies with transgenic mouse models expressing nuclear neo-self antigens, T-cell tolerance to the lupus-related nuclear antigens human La and nRNP A has been demonstrated. These findings also indicate the existence in normal animals of autoreactive B cells continuously presenting nuclear antigen, suggesting that nuclear antigens are not sequestered from the immune system. Investigations of CD4+ T-cell tolerance to non-nuclear antigens have revealed a number of mechanisms that protect the host from autoreactivity, including autoreactive T-cell deletion, regulatory T-cell development and anergy induction. Recent studies using T-cell receptor and neo-self nuclear antigen transgenic mice are revealing the importance of such mechanisms in maintaining tolerance to nuclear antigens. Mechanisms of tolerogenic antigen presentation, identification of tolerogenic antigen source(s) and the pathways leading to loss of tolerance to nuclear antigens in systemic autoimmune disease states are currently being sought.

Clonal deletion of simian virus 40 large T antigen-specific T cells in the transgenic adenocarcinoma of mouse prostate mice: an important role for clonal deletion in shaping the repertoire of T cells specific for antigens overexpressed in solid tumors

In addition to their overexpression in cancer cells, most of the tumor-associated Ags are expressed at low but detectable levels in normal tissues. It is not clear whether the repertoire of T cells specific for unmutated tumor Ags is shaped by negative selection during T cell development. The transgenic adenocarcinoma of mouse prostate (TRAMP) model is transgenic for the SV40 large T Ag (Tag) under the control of the rat probasin regulatory elements. Although it has been established that T lymphocytes from TRAMP mice are tolerant to SV40 Tag, the mechanism of the tolerance is largely unknown. To examine whether the T cell clonal deletion is responsible for the tolerance, we crossed the TRAMP mice with mice transgenic for a rearranged TCR specific for SV40 Tag presented by the H-2K(k). Double transgenic TRAMP/TCR mice showed profound thymic deletion of SV40 Tag-reactive T cells, including a 6- to 10-fold reduction in the total thymocyte numbers and a >50-fold reduction in phenotypically mature T cells. Consistent with this finding, we observed that the SV40 Tag and endogenous mouse probasin genes are expressed at low levels in the thymus. These results demonstrate that clonal deletion is a major mechanism for tolerance to Ags previously regarded as prostate-specific, and provide direct evidence that the T cell repertoire specific for an unmutated tumor Ag can be shaped by clonal deletion in the thymus.

Genetic control of T and B lymphocyte activation in nonobese diabetic mice

Type 1 diabetes in nonobese diabetic (NOD) mice is characterized by the infiltration of T and B cells into pancreatic islets. T cells bearing the TCR Vbeta3 chain are disproportionately represented in the earliest stages of islet infiltration (insulitis) despite clonal deletion of most Vbeta3(+) immature thymocytes by the mammary tumor virus-3 (Mtv-3) superantigen (SAg). In this report we showed that a high frequency of NOD Vbeta3(+) T cells that escape deletion are activated in vivo and that this phenotype is linked to the Mtv-3 locus. One potential mechanism of SAg presentation to peripheral T cells is by activated B cells. Consistent with this idea, we found that NOD mice harbor a significantly higher frequency of activated B cells than nondiabetes-prone strains. These activated NOD B cells expressed cell surface molecules consistent with APC function. At the molecular level, the IgH repertoire of activated B cells in NOD mice was equivalent to resting B cells, suggesting a polyclonal response in vivo. Genetic analysis of the activated B cell phenotype showed linkage to Idd1, the NOD MHC haplotype (H-2(g7)). Finally, Vbeta3(+) thymocyte deletion and peripheral T cell activation did not require B cells, suggesting that other APC populations are sufficient to generate both Mtv-3-linked phenotypes. These data provide insight into the genetic regulation of NOD autoreactive lymphocyte activation that may contribute to failure of peripheral tolerance and the pathogenesis of type I diabetes.

Human autoreactive (Th0) CD4(+) T-cell clones with cytolytic activity recognizing autologous activated T cells as the target

In attempt to obtain a clue to understanding possible physiological roles played by autoreactive T cells, autoreactive T-cell clones originally derived from an allogeneic mixed lymphocyte culture have been analyzed for their target spectrum, lytic function and cytokine profiles. Five CD4(+) T-cell clones established from allogeneic MLR, in which the stimulator cells shared certain class II MHC antigens with the responder, turned out to be reactive to autologous PBL. Among these, three clones were cytolytic against autologous B-cell line. These three cytolytic autoreactive clones were shown to be capable of specifically lysing autologous activated T cells expressing class II MHC molecules, raising possibility that such autoreactive clones might play a role in negatively regulating T cell responses. Cytolysis by an autoreactive clone 21C5 was inhibited completely by concanamycin A (CMA) known as a specific inhibitor of perforin, suggesting an involvement of the perforin/granzyme system. T-cell clones derived from the same MLC showed distinct correlation between their specificity and lymphokine profiles. Thus, the three cytolytic autoreactive clones belonged to Th0, whereas the two noncytolytic autoreactive clones belonged to Th2 and three alloreactive CD4(+) clones derived from the same culture were of Th1 type.

Anergic T lymphocytes selectively express an integrin regulatory protein of the cytohesin family

It has been proposed that the maintenance of T cell anergy depends on the induction of negative regulatory factors. Differential display of reverse transcribed RNA was used to identify novel genes that might mediate this function in anergic Th1 clones. We report that anergic Th1 clones do indeed express a genetic program different from that of responsive T cells. Moreover, one gene, the general receptor of phosphoinositides 1 (GRP1), was selectively induced in anergic T cells. The GRP1, located in the plasma membrane, regulated integrin-mediated adhesion and was invariably associated with unresponsiveness in multiple models of anergy. T cells expressing retrovirally transduced GRP1 exhibited normal proliferation and cytokine production. However, GRP1-transduced T cells were not stable and rapidly lost GRP1 expression. Thus, although GRP1 may not directly mediate T cell anergy, it regulates cell expansion and survival, perhaps through its integrin-associated activities.

Identification of a New Type of Invariant Vα14+ T Cells and Responsiveness to a Superantigen, Yersinia pseudotuberculosis- Derived Mitogen

We examined the expression of the H4 T cell activation marker in thymic T cell subpopulations and found that TCR-αβ+ CD4+ thymic T cells are segregated into three subpopulations based upon H4 levels. Thymic T cells with either no or low H4 expression differentiate via the mainstream differentiation pathway in the thymus. H4int thymic T cells, which express a skewed Vβ repertoire of Vβ2, -7, and -8 in their TCRs, show the phenotype of NKT cells: CD44high, Ly6Chigh, NK1.1+, and TCR-αβlow. H4high thymic T cells also show a skewed Vβ repertoire, Vβ2, -7, and -8, and predominantly express an invariant Vα14-Jα281+ α-chain in their TCRs but constitute a distinct population in that they are CD44int, Ly6C−, NK1.1−, and TCR-αβhigh. Thus, invariant Vα14+ thymic T cells consist of ordinary NKT cells and a new type of T cell population. Vβ7+ and Vβ8.1+ invariant Vα14+ thymic T cells are present in DBA/2 mice, which carry mammary tumor virus-7-encoded superantigens, in comparable levels to those in BALB/c mice. Furthermore, Vβ7+ invariant Vα14+ thymic T cells in DBA/2 mice are in the immunologically responsive state, and Yersinia pseudotuberculosis-derived mitogen-induced Vβ7+ invariant Vα14+ thymic T cell blasts from DBA/2 and BALB/c mice exhibited equally enhanced responses upon restimulation with Y. pseudotuberculosis-derived mitogen. Thus, invariant Vα14+ thymic T cells that escape negative selection in DBA/2 mice contain T cells as functionally mature as those in BALB/c mice.

Treatment of an autoimmune disease with "classical" T cell veto: a proposal

Immune responses protect against infectious diseases and cancers. In normal circumstances, the immune system is tolerant to self. However, under certain conditions this tolerance is broken. The immune system attacks otherwise normal tissue. An autoimmune disease ensues. Strategies are now being sought that remove the pathogenic T cells without affecting other immune functions. "Classical" veto has been described as an immune suppressive mechanism able to remove T cells in a highly specific and effective manner. The present article briefly reviews the current knowledge on the development of autoreactive T cells and their regulation in the periphery. It describes "classical" veto, its mechanisms, and its novel applications. Finally, it argues that "classical" veto can be adapted to treat an autoimmune disease, such as type I diabetes mellitus.

Anergic CD8+ T Cells Can Persist and Function In Vivo

Using a mouse model system, we demonstrate that anergic CD8+ T cells can persist and retain some functional capabilities in vivo, even after the induction of tolerance. In TCR Vβ5 transgenic mice, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction. CD8low cells are characterized by an activated phenotype, are functionally compromised in vitro, and are slated for deletion in vivo. We now demonstrate that CD8low cells derive from a proliferative compartment, but do not divide in vivo. CD8low cells persist in vivo with a t1/2 of 3–5 days, in contrast to their in vitro t1/2 of 0.5–1 day. During this unexpectedly long in vivo life span, CD8low cells are capable of producing IFN-γ in vivo despite their inability to proliferate or to kill target cells in vitro. CD8low cells also accumulate at sites of inflammation, where they produce IFN-γ. Therefore, rather than withdrawing from the pool of functional CD8+ T cells, anergic CD8low cells retain a potential regulatory role despite losing their capacity to proliferate. The ability of anergic cells to persist and function in vivo adds another level of complexity to the process of tolerance induction in the lymphoid periphery.

Role of the T cell receptor alpha chain in stabilizing TCR-superantigen-MHC class II complexes

Superantigens (SAGs) activate T cells by simultaneously binding the Vbeta domain of the TCR and MHC class II molecules on antigen-presenting cells. The preferential expression of certain Valpha regions among SAG-reactive T cells has suggested that the TCR alpha chain may modulate the level of activation through an interaction with MHC. We demonstrate that the TCR alpha chain is required for maximum stabilization of the TCR-SAG-MHC complex and that the alpha chain increases the half-life of the complex to match those of TCR-peptide/MHC complexes. The site on the TCR alpha chain responsible for these effects is CDR2. Thus, the overall stability of the TCR-SAG-MHC complex is determined by the combination of three distinct interactions: TCR-SAG, SAG-MHC, and MHC-TCR.

Chronic Modulation of the TCR Repertoire in the Lymphoid Periphery

Using TCR Vβ5 transgenic mice as a model system, we demonstrate that the induction of peripheral tolerance can mold the TCR repertoire throughout adult life. In these mice, three distinct populations of peripheral T cells are affected by chronic selective events in the lymphoid periphery. First, CD4+Vβ5+ T cells are deleted in the lymphoid periphery by superantigens encoded by mouse mammary tumor viruses-8 and -9 in an MHC class II-dependent manner. Second, mature CD8+Vβ5+ T cells transit through a CD8lowVβ5low deletional intermediate during tolerance induction by a process that depends upon neither mouse mammary tumor virus-encoded superantigens nor MHC class II expression. Third, a population of CD4−CD8−Vβ5+ T cells arises in the lymphoid periphery in an age-dependent manner. We analyzed the TCR Vα repertoire of each of these cellular compartments in both Vβ5 transgenic and nontransgenic C57BL/6 mice as a function of age. This analysis revealed age-related changes in the expression of Vα families among different cellular compartments, highlighting the dynamic state of the peripheral immune repertoire. Our work indicates that the chronic processes maintaining peripheral T cell tolerance can dramatically shape the available TCR repertoire.

The avidity spectrum of T cell receptor interactions accounts for T cell anergy in a double transgenic model

The mechanism of self-tolerance in the CD4(+) T cell compartment was examined in a double transgenic (Tg) model in which T cell receptor (TCR)-alpha/beta Tg mice with specificity for the COOH-terminal peptide of moth cytochrome c in association with I-Ek were crossed with antigen Tg mice. Partial deletion of cytochrome-reactive T cells in the thymus allowed some self-specific CD4(+) T cells to be selected into the peripheral T cell pool. Upon restimulation with peptide in vitro, these cells upregulated interleukin (IL)-2 receptor but showed substantially lower cytokine production and proliferation than cells from TCR Tg controls. Proliferation and cytokine production were restored to control levels by addition of saturating concentrations of IL-2, consistent with the original in vitro definition of T cell anergy. However, the response of double Tg cells to superantigen stimulation in the absence of exogenous IL-2 was indistinguishable from that of TCR Tg controls, indicating that these self-reactive cells were not intrinsically hyporesponsive. Measurement of surface expression of Tg-encoded TCR alpha and beta chains revealed that cells from double Tg mice expressed the same amount of TCR-beta as cells from TCR Tg controls, but only 50% of TCR-alpha, implying expression of more than one alpha chain. Naive CD4(+) T cells expressing both Tg-encoded and endogenous alpha chains also manifested an anergic phenotype upon primary stimulation with cytochrome c in vitro, suggesting that low avidity for antigen can produce an anergic phenotype in naive cells. The carboxyfluorescein diacetate succinimidyl ester cell division profiles in response to titered peptide +/- IL-2 indicated that expression of IL-2 receptor correlated with peptide concentration but not TCR level, whereas IL-2 production was profoundly affected by the twofold decrease in specific TCR expression. Addition of exogenous IL-2 recruited double Tg cells into division, resulting in a pattern of cell division indistinguishable from that of controls. Thus, in this experimental model, cells expressing more than one alpha chain escaped negative selection to a soluble self-protein in the thymus and had an anergic phenotype indistinguishable from that of low avidity naive cells. The data are consistent with the notion that avidity-mediated selection for self-reactivity in the thymus may lead to the appearance of anergy within the peripheral, self-reactive T cell repertoire, without invoking the induction of hyporesponsiveness to TCR-mediated signals.

Tolerant CD8 T Cells Induced by Multiple Injections of Peptide Antigen Show Impaired TCR Signaling and Altered Proliferative Responses In Vitro and In Vivo

The mechanisms responsible for peripheral CD8 T cell tolerance to foreign Ags remain poorly understood. In this study we have characterized the state of CD8 T cell tolerance induced in F5 TCR transgenic mice by multiple peptide injections in vivo. The tolerant state of CD8 T cells is characterized by impaired proliferative responses, increased sensitivity to cell death, and failure to acquire cytotoxic effector function after in vitro antigenic challenge. In vivo monitoring of CD8 T cell proliferation using 5-carboxyfluorescein diacetate succinimidyl ester showed that a large subset of the tolerant T cell population failed to divide in response to peptide. TCR down-regulation could not account for this loss of responsiveness to Ag since recombination-activating gene-1 (RAG-1)−/−F5 CD8 T cell responses were similar to those of RAG-1−/−F5 × RAG-1−/− F1 T lymphocytes, which express lower levels of the transgenic TCR. Analysis of early signal transduction in tolerant CD8 T cells revealed high basal levels of cytoplasmic calcium as well as impaired calcium mobilization and tyrosine phosphorylation after cross-linking of CD3ε and CD8α. Together these data indicate that repeated exposure to soluble antigenic peptide in vivo can induce a state of functional tolerance characterized by defective TCR signaling, impaired proliferation, and increased sensitivity to cell death.

Pristane-induced arthritis in mice. V. Susceptibility to pristane-induced arthritis is determined by the genetic regulation of the T cell repertoire

OBJECTIVE

Pristane-induced arthritis (PIA) is an experimental seropositive arthritis that is characterized by serologic and cellular immune abnormalities and is dependent on the presence of a competent CD4+ T cell population. We examined the regulation of PIA by genes of the major histocompatibility complex (MHC) and the Mls-1 loci to determine whether the selection of the T cells that infiltrate arthritic joints is a critical factor in disease susceptibility.

METHODS

Genetic regulation of PIA was investigated using F1 hybrid and congenic strain analysis to determine the influence of MHC and Mls-1 genes. The T cell receptor Vbeta phenotypes of lymph node cells and T cells infiltrating arthritic joints were examined with 2-color flow cytometry and reverse transcription-polymerase chain reaction techniques.

RESULTS

F1 hybrid offspring from 2 major PIA-susceptible strains (DBA/1 x BALB/c) were resistant to the induction of arthritis because of the interaction between genes of the MHC and the Mls-1 loci, which modified the T cell repertoire. This conclusion was supported by the observed resistance to PIA in BALB/ c-Mls-1a mice, where T cells expressing the Vbeta8.1 and Vbeta6 phenotypes were absent. The receptor phenotype of T cells infiltrating arthritic joints in DBA/1 mice was markedly skewed toward Vbeta8.1 and Vbeta6 compared with the population observed in lymph nodes from either PIA or normal control DBA/1 mice.

CONCLUSION

The data support the hypothesis that PIA is a T cell-mediated disease. While pristane causes a polyclonal T cell expansion that gives rise to lymphadenopathy, the development of arthritis in susceptible strains of mice occurs due to the preservation of specific T cell subsets with the capacity to infiltrate synovial joints.

Positive versus negative signaling by lymphocyte antigen receptors

Antigen receptors on lymphocytes play a central role in immune regulation by transmitting signals that positively or negatively regulate lymphocyte survival, migration, growth, and differentiation. This review focuses on how opposing positive or negative cellular responses are brought about by antigen receptor signaling. Four types of extracellular inputs shape the response to antigen: (a) the concentration of antigen; (b) the avidity with which antigen is bound; (c) the timing and duration of antigen encounter; and (d) the association of antigen with costimuli from pathogens, the innate immune system, or other lymphocytes. Intracellular signaling by antigen receptors is not an all-or-none event, and these external variables alter both the quantity and quality of signaling. Recent findings in B lymphocytes have clearly illustrated that these external inputs affect the magnitude and duration of the intracellular calcium response, which in turn contributes to differential triggering of the transcriptional regulators NF kappa B, JNK, NFAT, and ERK. The regulation of calcium responses involves a network of tyrosine kinases (e.g. lyn, syk), tyrosine or lipid phosphatases (CD45, SHP-1, SHIP), and accessory molecules (CD21/CD19, CD22, FcR gamma 2b). Understanding the biochemistry and logic behind these integrative processes will allow development of more selective and efficient pharmaceuticals that suppress, modify, or augment immune responses in autoimmunity, transplantation, allergy, vaccines, and cancer.

Unique molecular surface features of in vivo tolerized T cells

Differential expression of surface markers can frequently be used to distinguish functional subsets of T cells, yet a surface phenotype unique to T cells induced into an anergic state has not been described. Here, we report that CD4 T cells rendered anergic in vivo by superantigen can be identified by loss of the 6C10 T cell marker. Inoculation of Vbeta8.1 T cell antigen receptor (TCR) transgenic mice with a Vbeta8.1-reactive minor lymphocyte-stimulating superantigen (Mls-1(a)) induces tolerance to Mls-1(a) by clonal anergy. CD4 lymph node T cells from Mls-1(a) inoculated transgenic mice enriched for the 6C10(-) phenotype neither proliferate nor produce interleukin-2 upon TCR engagement, whereas 6C10(+) CD4 T cells retain responsiveness. Analysis of T cell memory markers demonstrate that 6C10(-) T cells remain 3G11(hi) but express heterogeneous levels of CD45RB, CD62L, CD44, and the CD69 early activation marker, suggesting that T cells at various degrees of activation can be functionally anergic. These studies demonstrate that anergic T cells can be purified based on 6C10 expression permitting examination of issues concerning biochemical and biological features specific to T cell anergy.

Cutting Edge: Multiple Mechanisms of Peripheral T Cell Tolerance to the Fetal “Allograft”

The fetus represents a foreign entity to the maternal immune system, yet this “natural” allograft is not normally rejected. This unique situation provides a physiologic system to evaluate peripheral tolerance in which the maternal immune system is challenged with relatively rare Ags not previously encountered in the thymus. Using H-Y-specific TCR transgenic mice, we demonstrate that T cells specific for fetal Ags decrease in an Ag-specific manner during pregnancy and remain low postpartum, the result of an encounter with fetal cells expressing the appropriate MHC/peptide complexes. The finding that placental trophoblasts can induce Fas-mediated death of T cells is consistent with peripheral clonal deletion as one mechanism of tolerance. The remaining clonotypic T cells are unresponsive to antigenic stimulation, although neither TCR nor coreceptor is down-regulated. Our study demonstrates that specific recognition of fetal allogeneic Ags by maternal T cells results in tolerance induction of reactive T cells via multiple mechanisms.

Biochemical features of anergic T cells

T cell anergy is a functionally defined state of hyporesponsiveness in which T cells neither proliferate nor produce IL2 following subsequent TCR ligation. Recent biochemical data from in vitro studies suggest that anergic cells do not utilize all of the signaling pathways normally initiated by TCR triggering. These findings appear to hold true for T cells rendered anergic in vivo, as well; however, biochemical studies on clonal anergy in vivo have been limited by the inability to recover a homogeneous population of anergic T cells. Here we review progress on TCR mediated signaling pathways as well as the description of surface marker phenotypes specific to T cell anergy.

Suppression of Immune Responses by CD8 Cells. I. Superantigen-Activated CD8 Cells Induce Unidirectional Fas-Mediated Apoptosis of Antigen-Activated CD4 Cells

Stimulation of mature CD4 cells through the TCR induces cellular activation and expansion that are often followed by clonal elimination by a form of apoptosis3 termed activation-induced cell death. This process of CD4 cell apoptosis is generally thought to reflect clonal suicide and to be independent of other cell types. Here we show that during the response to the superantigen Staphylococcal enterotoxin A, activated CD8 cells, but not activated CD4 cells, suppress the CD4 proliferative response. Suppression by CD8 cells reflects their ability to induce CD4 cell apoptosis via ligation of Fas. Moreover, although activated CD8 cells that express Fas ligand and Fas eliminate CD4 cells through a Fas-dependent mechanism, they are themselves resistant to Fas-dependent apoptosis. These findings indicate a fundamental difference between the two major T cell subsets with regard to sensitivity to Fas-dependent apoptosis, expression of Fas ligand, and mediation of suppressive activity following immunization with superantigen.

Effects of oral tolerance induction by myelin basic protein on Vbeta8+ Lewis rat T cells

Encephalitogenic T cells from Lewis rats use a restricted T cell receptor (TCR) gene combination, Vbeta8.2 and Valpha2. The oral administration of myelin basic protein (MBP) to Lewis rats prior to encephalitogenic challenge results in a marked inhibition of clinical neurologic signs of encephalitis, reduced central nervous system pathology, suppressed T cell reactivity to MBP, and decreased serum anti-MBP antibody responses. The present study determined the TCR Vbeta8 gene usage in rats rendered orally tolerant to MBP as compared with vehicle-fed or unfed controls. Total RNA was extracted from lymph node cells (LNC), Northern blots run, and hybridizations performed using a rat beta chain V region probe positive for Vbeta8.2. The results indicate that feeding MBP results in a decrease in Vbeta8+ TCR RNA expression in lymph nodes draining the site of encephalitogenic challenge. T cell proliferation was reduced in LNC of tolerized rats relative to control rats. No change in the Vbeta8+ TCR RNA expression or MBP reactivity was observed in the mesenteric lymph nodes (MLN) of vehicle-fed or MBP-fed rats, although an increase in cell number was found in the MLN of both groups. These results suggest that the mechanisms of orally induced tolerance involve local clonal deletion or migration of Vbeta8+ T cells, of which MBP-specific T cells are a part.

Coxsackieviruses and diabetes

Insulin-dependent diabetes mellitus (IDDM) is an autoimmune disease whose etiology is complex. Both genetic susceptibility, which is polygenic, and environmental factors, including virus infections, appear to be involved in the development of IDDM. In this review, we have tried to balance the discussion of diabetes by examining both immunological and virological perspectives. Several mouse models, including viral and non-viral models, have been used to study diabetes. For this review, we include lessons gleaned from the non-obese diabetic (NOD) mouse and from mouse models of coxsackievirus- and encephalomyocarditis-virus-induced diabetes. Finally, we present a multi-stage model in which several viral infections, including the coxsackieviruses, are postulated to play a role in the autoimmune destruction of pancreatic beta cells.

A range of CD4 T cell tolerance: partial inactivation to organ-specific antigen allows nondestructive thyroiditis or insulitis

T cell receptor (TCR) transgenic mice specific for hen egg lysozyme (HEL) were crossed with mice expressing HEL on the thyroid epithelium, on pancreatic islet beta cells, or systemically. Depending on the pattern of HEL expression, deletion of double-positive thymocytes ranged from minimal to complete, and peripheral CD4 cells exhibited graded reduction in TCR expression, in vitro responsiveness, and in vivo helper ability. CD4 cells were least tolerant in TCR/thyroid-HEL and TCR/islet-HEL mice, which developed an extensive lymphocytic thyroiditis or insulitis that nevertheless did not eliminate HEL-expressing endocrine cells. Autoreactive CD4 clones thus escape the thymus under a range of circumstances, retain sufficient function to initiate subclinical autoimmune inflammation when self-antigens are concentrated in the thyroid or pancreas, and may regulate progression of subclinical inflammation to destructive autoimmune disease.

Peptide antigen treatment of naive and virus-immune mice: antigen-specific tolerance versus immunopathology

Peptide-specific down-regulation of T cell responses may represent a powerful tool to intervene in autoimmune diseases or graft rejections. It is therefore important to know whether peptide treatment tolerizes both naive and antigen-experienced memory T lymphocytes. Here we show that a major histocompatibility complex class I binding peptide, derived from the glycoprotein (GP33 peptide) of lymphocytic choriomeningitis virus (LCMV), specifically tolerized naive cytotoxic T lymphocytes (CTL) when administered three times intraperitoneally in incomplete Freund's adjuvants. However, in the presence of GP33-specific memory CTL in LCMV-primed mice, the same treatment had a general immunosuppressive effect on unrelated third-party antigen-specific T cell responses and caused severe immunopathological damage to the spleen.

Autoimmunity caused by disruption of central T cell tolerance. A murine model of drug-induced lupus

A side effect of therapy with procainamide and numerous other medications is a lupus-like syndrome characterized by autoantibodies directed against denatured DNA and the (H2A-H2B)-DNA subunit of chromatin. We tested the possibility that an effect of lupus-inducing drugs on central T cell tolerance underlies these phenomena. Two intrathymic injections of procainamide-hydroxylamine (PAHA), a reactive metabolite of procainamide, resulted in prompt production of IgM antidenatured DNA antibodies in C57BL/6xDBA/2 F1 mice. Subsequently, IgG antichromatin antibodies began to appear in the serum 3 wk after the second injection and were sustained for several months. Specificity, inhibition and blocking studies demonstrated that the PAHA-induced antibodies showed remarkable specificity to the (H2A-H2B)-DNA complex. No evidence for polyclonal B cell activation could be detected based on enumeration of Ig-secreting B cells and serum Ig levels, suggesting that a clonally restricted autoimmune response was induced by intrathymic PAHA. The IgG isotype of the antichromatin antibodies indicated involvement of T cell help, and proliferative responses of splenocytes to oligonucleosomes increased up to 100-fold. As little as 5 microM PAHA led to a 10-fold T cell proliferative response to chromatin in short term organ culture of neonatal thymi. We suggest that PAHA interferes with self-tolerance mechanisms accompanying T cell maturation in the thymus, resulting in the emergence of chromatin-reactive T cells followed by humoral autoimmunity.

Long-term survival of solid organ allografts by brief anti-lymphocyte function-associated antigen-1 monoclonal antibody monotherapy

Strategies targeting lymphocyte function-associated antigen-1 (LFA-1, CD11a/CD18) and intercellular adhesion molecule-1 (ICAM-1) have previously been shown to produce long-term survival of solid organ allografts in animals only when both CD11a and ICAM-1 are targeted for a brief (6-7 days) time or when extended (14 weeks) treatment with anti-CD11a monoclonal antibody (mAb) is administered. We show that recipient pretreatment followed by a brief (13 days) treatment course with high-dose anti-CD11a mAb alone produces long-term survival of cardiac allografts in the rigorous, nonprimarily vascularized heart allograft model in mice. This treatment regimen induces specific unresponsiveness in our model. In recipients bearing long-term beating cardiac grafts after treatment with anti-CD11a mAb, there still exists a high frequency of potentially antigen-reactive T cells in isolated peripheral blood lymphocyte fractions. Therefore, clonal deletion does not appear to explain the induction of specific unresponsiveness by treatment with anti-CD11a mAb in this model. These findings support the further investigation of the use of high-dose anti-LFA-1 mAb monotherapy in the pre- and early postoperative period to promote solid organ allograft survival.

Liver allograft rejection in rats depleted of CD8+ cells

The mechanism(s) of rejection or tolerance induction is a competitive, complex process that presumably involves interactions between multiple subpopulations of T lymphocytes. We investigated the roles of CD8+ cytolytic and CD4+ helper T cells in rat strains that tolerate liver allografts and that differ at both the major histocompatibility complex (MHC) (RT1) and minor histocompatibility genes. Orthotopic liver transplantation (OLT) with arterial reconstruction was performed with Brown Norway (BN) (RT1n) donors and Lewis (RT1(1)) recipients, some of which were untreated, others treated with anti-CD8 antibody, and still others treated with anti-CD4 antibody. Liver graft rejection was monitored for 28 days on the basis of two criteria: (1) serum levels of AST enzyme at 3-day intervals and (2) liver biopsies at weekly intervals and at the time of sacrifice at the end of the study period. In the untreated control group, an elevation of AST was found to peak at day 6 after grafting, and it remained elevated until day 28 (AST 542 +/- 72 U/l). Histologically, signs of severe rejection were first observed on day 9; these changed to moderate rejection about day 21 and to mild rejection by day 28, when the animals were sacrificed. Recipients pre-treated with anti-CD8 demonstrated a significant elevation of AST within 6 days that, unlike in the control recipients, continued to rise sharply through the observation period (AST 1127 +/- 181 U/l, P = 0.009 vs control group). Liver biopsies showed mild rejection at day 9 and moderate rejection at days 21 through 28. Recipients pretreated with anti-CD4 showed a time course of enzyme elevation and severity of rejection that was not significantly different from that observed in the control group. However, anti-CD4 treatment resulted in only 75% depletion of CD4+ cells in peripheral blood as compared to complete elimination of CD8+ cells following anti-CD8 treatment. Functional studies of spleen and liver-infiltrating lymphocytes obtained after 28 days showed low proliferative response in mixed lymphocyte culture with both BN and PVG stimulator spleen and lymph node cells. These results suggest that in this donor/recipient combination, removal of CD8+ cells increases the severity of rejection as demonstrated by a progressive rise in AST and histology. Moreover, OLT in this combination results in a profound, nonspecific inhibition of proliferative T-cell responses to MHC alloantigens.

Role of the T cell receptor alpha-chain in superantigen recognition

Superantigens bind to antigen-presenting cells on the outside of the major histocompatibility complex (MHC) class II molecule and to T cells via the external face of the T cell receptor (TCR) V beta element. As a consequence, superantigens stimulate populations of T cells in a V beta-specific, non-MHC-restricted manner. However, accumulating evidence has shown an additional contribution of the TCR alpha-chain and polymorphic residues of the MHC molecule to superantigen recognition by some T cells. These data suggest that the TCR and MHC come into contact during superantigen engagement and indirectly modulate the superantigen reactivity. Thus, additional interactions between non-V beta elements of the TCR and MHC play a role in the overall stability of the superantigen/MHC/TCR complex, explaining the influence of the TCR alpha-chain. It is likely that this additional interaction is of greater consequence for weakly reactive T cells. This modulation of superantigen reactivity in individual T cells may have physiological consequences, for example, in the induction of autoimmunity.

Anergic T cells are defective in both jun NH2-terminal kinase and mitogen-activated protein kinase signaling pathways

T helper type 1 cells (Th1) become anergic when stimulated through the antigen receptor in the absence of costimulation. They do not produce IL-2 or proliferate in response to subsequent stimulation. Previous studies have indicated that anergic T cells are defective in the trnsactivational activity of the transcription factor, AP-1, which is required for optimal IL-2 transcription. Using two murine Th1 cell clones, we demonstrate that anergic Th1 cells have defects in both jun NH2-terminal kinase (JNK) and extracellular signal-regulated kinase (ERK) activities. These kinases have been shown to be important for the upregulation of AP-1 activity. Furthermore, our data show that ERK and JNK activities are restored when anergy is induced in the presence of the protein synthesis inhibitor cycloheximide, or when anergic T cells are allowed to proliferate in response to exogenous IL-2. These treatments have previously been shown to prevent or reverse the anergic state. Our results suggest that defects in both JNK and ERK may result in the decreased AP-1 activity and the reduced IL-2 transcription observed in anergic T cells.

Balancing immunity and tolerance: deleting and tuning lymphocyte repertoires

Immunological self-tolerance is ensured by eliminating or inhibiting self-reactive lymphocyte clones, creating physical or functional holes in the B- and T-lymphocyte antigen receptor repertoires. The nature and size of these gaps in our immune defenses must be balanced against the necessity of mounting rapid immune responses to an everchanging array of foreign pathogens. To achieve this balance, only a fraction of particularly hazardous self-reactive clones appears to be physically eliminated from the repertoire in a manner that fully prevents their recruitment into an antimicrobial immune response. Many self-reactive cells are retained with a variety of conditional and potentially flexible restraints: (i) their ability to be triggered by antigen is diminished by mechanisms that tune down signaling by their antigen receptors, (ii) their ability to carry out inflammatory effector functions can be inhibited, and (iii) their capacity to migrate and persist is constrained. This balance between tolerance and immunity can be shifted, altering susceptibility to autoimmune disease and to infection by genetic or environmental differences either in the way antigens are presented, in the tuning molecules that adjust triggering set points for lymphocyte responses to antigen, or in the effector molecules that eliminate, retain, or expand particular clones.

Mature T cell reactivity altered by peptide agonist that induces positive selection

Recent studies have investigated how defined peptides influence T cell development. Using a T cell receptor-transgenic beta2-microglobulin-deficient model, we have examined T cell maturation in fetal thymic organ cultures in the presence of various peptides containing single-alanine substitutions of the strong peptide agonist, p33. Cocultivation with the peptide A4Y, which contains an altered T cell contact residue, resulted in efficient positive selection. Several in vitro assays demonstrated that A4Y was a moderate agonist relative to p33. Although A4Y promoted positive selection over a wide concentration range, high doses of this peptide could not induce clonal deletion. Thymocytes maturing in the presence of A4Y were no longer able to respond to A4Y, but could proliferate against p33. These studies demonstrate that (a) peptides that induce efficient positive selection at high concentrations are not exclusively antagonists; (b) some agonists do not promote clonal deletion; (c) positive selection requires a unique T cell receptor-peptide-major histocompatibility complex interaction; and (d) interactions with selecting peptides during T cell ontogeny may define the functional reactivity of mature T cells.

Pancreatic islet allograft immunity and tolerance: the two-signal hypothesis revisited

The principle assumption of this discussion is that costimulation (CoS) forms the primary stimulus that compels T cells to mount a response to their specific antigen. However, this response can be either positive or negative, depending on the developmental stage of the T cell and the microenvironment in which the antigen and CoS are received. Thus, both immunity and tolerance may represent different outcomes of a two-signal process. We would emphasize that CoS is a functional term and not a strict molecular definition. While many molecular interactions have been described as providing CoS activity, notably those involving the B-7 family of cell surface molecules, it is not yet clear what combination(s) of non-antigen-specific signals may fulfil this function. This point is important because many studies have achieved tolerance through strategies designed to inhibit specific CoS molecules. However, it may be that differential signaling through distinct CoS molecules, rather than a global inhibition of CoS per se, plays a role in the generation of active tolerance in such studies (Bluestone 1995). A corollary of this notion is that antigen (signal 1) delivery to T cells is a null event and so is not an inherently paralysing signal. Of course, if signal 1 is not itself a tolerogenic signal, then other mechanisms are necessary to explain many empirical observations of tolerance to allogeneic or self antigens. This is best illustrated by those forms of functional tolerance to either alloantigens or self antigens that do not appear to be the result of clonal deletion/inactivation. It would be relatively simple to invoke a model of tolerance whereby the relevant tissue-destructive cell is eliminated or inactivated; such a model would preclude the necessity to suggest active regulatory mechanisms of tolerance. However, in several model systems, including our own observations concerning tolerance induction to APC-depleted islet allografts, tissue-destructive T cells can persist in recipients tolerant to allogeneic or self antigens. Furthermore, there are key examples in which tolerance demonstrates a dominant phenotype; that is, tolerant cells can regulate the activity of naive, non-tolerant cells. This latter observation points to the function of an active, regulatory form of tolerance. As such, we would emphasize that tolerance should not be defined as unresponsiveness since the tolerant state is the consequence of very active immune reactions.

Age-related occurrence of inhibitory antibodies to streptococcal pyrogenic superantigens

Several bacteria, such as staphylococci and streptococci, can produce superantigens (SA) that induce the activation of T cells in humans. Although these organisms are the major causes of infection in children, the evidence that T cells are vigorously activated by SA produced by such organisms has not been reported except for toxic shock syndrome. In a previous paper, we demonstrated that inhibitory IgG antibodies (Ab) to SA in humans may protect against SA stimulation. In the present study, we investigated the occurrence of these inhibitory Ab to SA in 94 healthy children by the enzyme linked immunosorbent assay technique and the suppressive effect on T cell stimulation by SA. The positivity of Ab to streptococcal pyrogenic exotoxin (SPE)-A, SPE-C and staphylococcal enterotoxin B (SEB) increased with age. The age at which more than 50% of children exhibited Ab to SA was 1 year for SEB, 6 years for SPE-C and 11 years for SPE-A. Sera from these children were inhibitory to T cell proliferation elicited by SA in proportion to the concentration of IgG Ab to each SA. Sera supplemented with IgG Ab to SA by gamma-globulin therapy became inhibitory to T cell proliferation by SA. We conclude that, as children grow, they can develop Ab to SA that may play a role in protecting them against vigorous T cell activation by SA.