MHC structure and autoimmune T cell repertoire development

Type 1 diabetes pathogenesis and the role of inhibitory receptors in islet tolerance

Type 1 diabetes (T1D) affects over a million Americans, and disease incidence is on the rise. Despite decades of research, there is still no cure for this disease. Exciting beta cell replacement strategies are being developed, but in order for such approaches to work, targeted immunotherapies must be designed. To selectively halt the autoimmune response, researchers must first understand how this response is regulated and which tolerance checkpoints fail during T1D development. Herein, we discuss the current understanding of T1D pathogenesis in humans, genetic and environmental risk factors, presumed roles of CD4+ and CD8+ T cells as well as B cells, and implicated autoantigens. We also highlight studies in non-obese diabetic mice that have demonstrated the requirement for CD4+ and CD8+ T cells and B cells in driving T1D pathology. We present an overview of central and peripheral tolerance mechanisms and comment on existing controversies in the field regarding central tolerance. Finally, we discuss T cell- and B cell-intrinsic tolerance mechanisms, with an emphasis on the roles of inhibitory receptors in maintaining islet tolerance in humans and in diabetes-prone mice, and strategies employed to date to harness inhibitory receptor signaling to prevent or reverse T1D.

Prediction and characterization of helper T-cell epitopes from pneumococcal surface adhesin A

Pneumococcal surface adhesin A (PsaA) is a multifunctional lipoprotein known to bind nasopharyngeal epithelial cells, and is significantly involved in bacterial adherence and virulence. Identification of PsaA peptides that optimally bind human leucocyte antigen (HLA) and elicit a potent immune response would be of great importance to vaccine development. However, this is hindered by the multitude of HLA polymorphisms in humans. To identify the conserved immunodominant epitopes, we used an experimental dataset of 28 PsaA synthetic peptides and in silico methods to predict specific peptide-binding to HLA and murine MHC class II molecules. We also characterized spleen and cervical lymph node (CLN) -derived T helper (Th) lymphocyte cytokine responses to these peptides after Streptococcus pneumoniae strain EF3030 challenge in mice. Individual, yet overlapping, peptides 15 amino acids in length revealed residues of PsaA that consistently caused the highest interferon-γ, interleukin-2 (IL-2), IL-5 and IL-17 responses and proliferation as well as moderate IL-10 and IL-4 responses by ex vivo re-stimulated splenic and CLN CD4⁺ T cells isolated from S. pneumoniae strain EF3030-challenged F1 (B6 × BALB/c) mice. In silico analysis revealed that peptides from PsaA may interact with a broad range of HLA-DP, -DQ and -DR alleles, due in part to regions lacking β-turns and asparagine endopeptidase sites. These data suggest that Th cell peptides (7, 19, 20, 22, 23 and 24) screened for secondary structures and MHC class II peptide-binding affinities can elicit T helper cytokine and proliferative responses to PsaA peptides.

The potential role of dendritic cells in the therapy of Type 1 diabetes

Type 1 diabetes (T1D) is the result of T-cell mediated autoimmune destruction of pancreatic islet β-cells. The two current treatments for T1D are based on insulin or islet-cell replacement rather than the pathogenesis of T1D and remain problematic. Islet/pancreas transplantation does not cater for the majority of sufferers due to the lack of supply of organs and the need for continuous immunosuppression regimens. The mainstay treatment is insulin replacement, but this is disruptive to lifestyle and does not protect against severe long-term complications. An early vaccination and long-term restoration of immune tolerance to self-antigens in T1D patients (reversing the immunopathogenesis of the disease) would be preferable. Dendritic cells (DCs) are potent APCs and play an important role in inducing and maintaining immune tolerance. Targeting DCs through different DC surface molecules shows effective modulation of immune responses. Their feasibility for immunotherapy to prolong transplant survival and cancer immunotherapy has been demonstrated. Therefore, DCs could potentially be used in the treatment of autoimmune diseases. This review summarizes new insights into DCs as a potential therapeutic target for the treatment of T1D.

HLA-DQ association and allele competition in Chinese narcolepsy

In Japanese, Koreans and Caucasians, narcolepsy/hypocretin deficiency is tightly associated with the DRB1*15:01-DQA1*01:02-DQB1*06:02 haplotype. Studies in African-Americans suggest a primary effect of DQB1*06:02, but this observation has been difficult to confirm in other populations because of high linkage disequilibrium between DRB1*15:01/3 and DQB1*06:02 in most populations. In this study, we studied human leucocyte antigen (HLA) class II in 202 Chinese narcolepsy patients (11% from South China) and found all patients to be DQB1*06:02 positive. Comparing cases with 103 unselected controls, and 110 and 79 controls selected for the presence of DQB1*06:02 and DRB1*15:01, we found that the presence of DQB1*06:02 and not DRB1*15:01 was associated with narcolepsy. In particular, Southern Chinese haplotypes such as the DRB1*15:01-DQA1*01:02-DQB1*06:01 and DRB1*15:01-DQA1*01:02-DQB1*05 were not associated with narcolepsy. As reported in Japanese, Koreans, African-Americans and Caucasians, additional protective effects of DQA1*01 (non-DQA1*01:02) and susceptibility effects of DQB1*03:01 were observed. These results illustrate the extraordinary conservation of HLA class II effects in narcolepsy across populations and show that DRB1*15:01 has no effect on narcolepsy susceptibility in the absence of DQB1*06:02. The results are also in line with a previously proposed 'HLA-DQ allelic competition model' that involves competition between non-DQA1*01:02, non-DQB1*06:02 'competent' (able to dimerize together) DQ1 alleles and the major DQα*01:02/ DQβ*06:02 narcolepsy heterodimer to reduce susceptibility.

Recent insights into CD4+ T-cell specificity and function in type 1 diabetes

Type 1 diabetes (T1D) is caused by T-cell-mediated destruction of the insulin-producing beta-cells in the pancreas. Genetic and immunological evidence from humans and mouse models indicates that CD4(+) T cells play a crucial role in the development and prevention of T1D. The dichotomy between CD4(+) T regulatory and effector T cells has encouraged research into the role of these cell subsets in T1D. New antigens and epitopes recognized by CD4(+) T cells in affected individuals have been identified. Growing knowledge of T-cell specificity and function is helping to develop new assays for analyzing islet antigen-specific CD4(+) T cells from human blood. Here we discuss, with particular reference to human studies, advances in our understanding of CD4(+) T-cell responses in T1D.

Helper T cell epitope-mapping reveals MHC-peptide binding affinities that correlate with T helper cell responses to pneumococcal surface protein A

Understanding the requirements for protection against pneumococcal carriage and pneumonia will greatly benefit efforts in controlling these diseases. Several proteins and polysaccharide capsule have recently been implicated in the virulence of and protective immunity against Streptococcus pneumonia. Pneumococcal surface protein A (PspA) is highly conserved among S. pneumonia strains, inhibits complement activation, binds lactoferrin, elicits protective systemic immunity against pneumococcal infection, and is necessary for full pneumococcal virulence. Identification of PspA peptides that optimally bind human leukocyte antigen (HLA) would greatly contribute to global vaccine efforts, but this is hindered by the multitude of HLA polymorphisms. Here, we have used an experimental data set of 54 PspA peptides and in silico methods to predict peptide binding to HLA and murine major histocompatibility complex (MHC) class II. We also characterized spleen- and cervical lymph node (CLN)-derived helper T lymphocyte (HTL) cytokine responses to these peptides after S. pneumonia strain EF3030-challenge in mice. Individual, yet overlapping peptides, 15 amino acids in length revealed residues 199 to 246 of PspA (PspA_199–246) consistently caused the greatest IFN-γ, IL-2, IL-5 and proliferation as well as moderate IL-10 and IL-4 responses by ex vivo stimulated splenic and CLN CD4⁺ T cells isolated from S. pneumonia strain EF3030-challeged F₁ (B6×BALB/c) mice. IEDB, RANKPEP, SVMHC, MHCPred, and SYFPEITHI in silico analysis tools revealed peptides in PspA_199–246 also interact with a broad range of HLA-DR, -DQ, and -DP allelles. These data suggest that predicted MHC class II-peptide binding affinities do not always correlate with T helper (Th) cytokine or proliferative responses to PspA peptides, but when used together with in vivo validation can be a useful tool to choose candidate pneumococcal HTL epitopes.

HLA-DR alleles determine responsiveness to Borrelia burgdorferi antigens in a mouse model of self-perpetuating arthritis

OBJECTIVE

Arthritis is a prominent manifestation of Lyme disease, which is caused by infection with Borrelia burgdorferi (Bb). Chronic Lyme arthritis persisting even after antibiotic treatment is linked to HLA-DRB1*0401 (DR4) and related alleles. In contrast, patients whose Lyme arthritis resolves within 3 months postinfection show an increased frequency of HLA-DRB1*1101 (DR11). The aim of this study was to analyze the underlying mechanism by which HLA-DR alleles confer genetic susceptibility or resistance to antibiotic-refractory Lyme arthritis.

METHODS

We generated DR11-transgenic (DR11-Tg) mice on a murine MHCII-/- background and compared their immune response to Bb antigens with the response of DR4-Tg mice after immunization with Bb outer surface protein A (OspA) or infection with live Bb.

RESULTS

T cells from OspA-immunized and Bb-infected DR11-Tg mice had defective production of interferon-gamma as compared with those from DR4-Tg mice. In contrast, DR11-Tg mice developed higher titers of anti-OspA and anti-Bb antibodies, respectively, than did DR4-Tg mice. Consistent with this observation, we found that the Bb-infected DR11-Tg mice had a decreased spirochetal burden as compared with the DR4-Tg mice, as measured by quantitative polymerase chain reaction.

CONCLUSION

This study provides direct evidence that in the presence of HLA-DR11, the immune response against Bb antigens is directed toward a protective antibody response. In contrast, an inflammatory Th1 response is induced in the presence of DR4. These observations offer an explanation for the differential genetic susceptibility of DR4+ and DR11+ individuals to the development of chronic Lyme arthritis and, eventually, the progression to antibiotic-refractory Lyme arthritis.

Persistent arthritis in Borrelia burgdorferi-infected HLA-DR4-positive CD28-negative mice post-antibiotic treatment

OBJECTIVE

The immunologic events that lead to persistent joint inflammation in certain patients with Lyme arthritis post-antibiotic treatment have been elusive so far. The prevalence of this condition is highest in individuals with rheumatoid arthritis-associated HLA-DR alleles. This study was undertaken to generate a murine model with persistent arthritis post-antibiotic treatment.

METHODS

We have previously shown that CD28(-/-) mice develop intermittent monarticular Lyme arthritis that is responsive to antibiotics. Since there seems to be a link in humans between persistent arthritic manifestations post-antibiotic treatment and the HLA-DR4 allele, we generated DR4+/+CD28(-/-)MHCII(-/-) mice, infected them with Borrelia burgdorferi, and subsequently treated them with antibiotics.

RESULTS

Thirty-eight percent of the B burgdorferi-infected DR4+/+CD28(-/-)MHCII(-/-) mice, but none of the B burgdorferi-infected CD28(-/-)MHCII(-/-) mice, remained arthritic post-antibiotic treatment. A significant fraction (36%) of these mice, but none of the mice in which arthritis resolved, had serum antibodies to outer surface protein A of B burgdorferi. After abrogation of active B burgdorferi infection, the inflammatory reaction in mice with persistent joint inflammation was restricted to the joints, since their draining lymph nodes were no longer enlarged. Increased CD20 and interferon-gamma messenger RNA expression in the inflamed joints of these mice suggested a possible role of B cells and inflammatory cytokines in the pathogenesis of persistent arthritis post-antibiotic treatment.

CONCLUSION

The establishment of this murine model allows, for the first time, the elucidation of the immunologic events that lead to persistent Lyme arthritis post-antibiotic therapy in genetically susceptible individuals.

The NOD allele of the Idd5 locus on chromosome 1 mediates a non-cell-autonomous defect in negative selection of T cells

Recent data have suggested that non-obese diabetic (NOD) mice display a defect in negative thymic selection. Using mixed bone marrow chimeras, we demonstrate that the NOD allele of the diabetes susceptibility region 5 (Idd5) locus on chromosome 1, confers defective negative selection in response to endogenous superantigens (SAg) Mtv8 and Mtv9. We generated mixed bone marrow (BM) chimeras in which the donor cells of NOD and C3H or NOD.Idd5(b10) and C3H coexist and are similarly exposed to the Mtv8 and Mtv9 SAg. Under these conditions, SAg-mediated deletion of Vbeta11+ T cells is less efficient in chimeric mice reconstituted with NOD+C3H BM, compared with chimeras reconstituted with NOD.Idd5(b10)+C3H BM. Interestingly, the observed discrepancy was not T cell autonomous but was found to be mediated by a BM derived cellular subset, and under control of a gene(s) in the Idd5 region.

Animal models of spontaneous autoimmune disease: type 1 diabetes in the nonobese diabetic mouse

The nonobese diabetic (NOD) mouse represents probably the best spontaneous model for a human autoimmune disease. It has provided not only essential information on type 1 diabetes (T1D) pathogenesis, but also valuable insights into mechanisms of immunoregulation and tolerance. Importantly, it allows testing of immunointervention strategies potentially applicable to man. The fact that T1D incidence in the NOD mouse is sensitive to environmental conditions, and responds, sometimes dramatically, to immunomanipulation, does not represent a limit of the model, but is likely to render it even more similar to its human counterpart. In both cases, macrophages, dendritic cells, CD4+, CD8+, and B cells are present in the diseased islets. T1D is a polygenic disease, but, both in human and in NOD mouse T1D, the primary susceptibility gene is located within the MHC. On the other hand, T1D incidence is significantly higher in NOD females, although insulitis is similar in both sexes, whereas in humans, T1D occurs with about equal frequency in males and females. In addition, NOD mice have a more widespread autoimmune disorder, which is not the case in the majority of human T1D cases. Despite these differences, the NOD mouse remains the most representative model of human T1D, with similarities also in the putative target autoantigens, including glutamic acid decarboxylase IA-2, and insulin.

Encephalitogenic and tolerogenic potential of altered peptide ligands of MOG and PLP in Biozzi ABH mice

Altered peptide ligands (APL) are highly effective in inhibiting experimental autoimmune encephalomyelitis (EAE) in rodents although clinical trials in multiple sclerosis reveal severe limitations probably due to the diverse and differential effects of APL in vivo compared to in vitro. Myelin oligodendrocyte glycoprotein (MOG 8-21) induces relapsing EAE in ABH (A(g7)) mice associated with broadening of the autoimmune repertoire thus providing a dynamic system to examine the efficacy of peptide analogues. Subtle changes in MOG 8-21 dramatically influenced disease susceptibility and T cell responses in vitro. Non-encephalitogenic APL that induce production of the 'regulatory' cytokines IL-10 and/or TGFbeta and concomitant low levels of the 'proinflammatory' cytokines IFNgamma and TNFalpha modulated relapsing EAE but were far less effective than the 'proinflammatory' wild-type MOG 8-21 peptide. These data reveal that APL differ greatly in their ability to activate encephalitogenic T cells. The extensive heterogeneity of responses of APL in vitro suggests that selection of APL on this criteria is highly unpredictable and probably less effective for therapy than selecting the dominant wild-type epitope and delivering it using a tolerogenic route.

Class II major histocompatibility complex-peptide tetramer staining in relation to functional avidity and T cell receptor diversity in the mouse CD4(+) T cell response to a rheumatoid arthritis-associated antigen

OBJECTIVE

Although studies have suggested that human cartilage (HC) gp-39 may be an antigen recognized by autoreactive CD4(+) T cells in rheumatoid arthritis, we previously failed to identify specific CD4(+) T cells in patients' synovial fluid or blood using a class II major histocompatibility complex-peptide tetramer composed of the immunodominant HC gp-39(263-275) epitope covalently linked to DR4. We undertook this study to better understand the parameters for specific binding of this tetramer.

METHODS

DR4-transgenic mice were immunized with the HC gp-39 peptide, and a set of peptide-responsive hybridomas was derived. Hybridomas were stained with the DR4-gp-39 tetramer and cultured with increasing amounts of peptide in the presence of DR4-expressing antigen-presenting cells to determine functional avidity.

RESULTS

Great variability was apparent in the ability of the tetramer to stain the hybridomas, and there was a strong correlation between the intensity of tetramer staining and functional avidity. Importantly, nearly 30% of the hybridomas did not stain with tetramer, and these cells exhibited relatively low functional avidity. Although the addition of an anti-T cell receptor (anti-TCR) monoclonal antibody during the staining procedure enhanced binding of the tetramer to a number of the hybridomas, a significant percentage remained unstainable. Analysis of TCR expression showed that >90% of the hybridomas expressed the same TCR beta-chain variable region (V(beta)10), and sequencing of the TCR junctional regions showed diversity in the third complementarity-determining region.

CONCLUSION

These results suggest that immune responses dominated by relatively low-affinity TCR interactions, such as those that may occur in autoimmune disease, will be difficult to detect using standard tetramer techniques.

Restricted MHC-peptide repertoire predisposes to autoimmunity

MHC molecules associated with autoimmunity possess known structural features that limit the repertoire of peptides that they can present. Such limitation gives a selective advantage to TCRs that rely on interaction with the MHC itself, rather than with the peptide residues. At the same time, negative selection is impaired because of the lack of negatively selecting peptide ligands. The combination of these factors may predispose to autoimmunity. We found that mice with an MHC class II–peptide repertoire reduced to a single complex demonstrated various autoimmune reactions. Transgenic mice bearing a TCR (MM14.4) cloned from such a mouse developed severe autoimmune dermatitis. Although MM14.4 originated from a CD4⁺ T cell, dermatitis was mediated by CD8⁺ T cells. It was established that MM14.4⁺ is a highly promiscuous TCR with dual MHC class I/MHC class II restriction. Furthermore, mice with a limited MHC–peptide repertoire selected elevated numbers of TCRs with dual MHC class I/MHC class II restriction, a likely source of autoreactivity. Our findings may help to explain the link between MHC class I responses that are involved in major autoimmune diseases and the well-established genetic linkage of these diseases with MHC class II.

Autoimmune cholangitis in a patient with thymoma

Autoimmune cholangitis is characterized biochemically by chronic cholestasis and histopathologically by chronic non-suppurative destructive cholangitis. It is associated with positive antinuclear antibody test and negative antimitochondrial antibody test results. Recently, we experienced a case of a 35-year-old woman with autoimmune cholangitis associated with thymoma who presented with pruritus, jaundice, chronic fatigue and anterior chest discomfort. Her laboratory examinations revealed marked increases in levels of serum alkaline phosphatase and gamma-glutamyl transpeptidase. In serological tests, antinuclear antibody was found, but antimitochondrial antibody was not. Liver biopsy findings were compatible with chronic non-suppurative destructive cholangitis. On computed tomography (CT) of the chest, a large anterior mediastinal mass was found. The mass was totally resected and the patient was treated with ursodeoxy cholic acid. Thereafter, her clinical symptoms improved and liver functions completely returned to the normal range. We describe here an uncommon association of autoimmune cholangitis with thymoma, which has not been reported previously in the English-written literature.

The T cells specific for the carboxyl-terminal determinants of self (rat) heat-shock protein 65 escape tolerance induction and are involved in regulation of autoimmune arthritis

Immunization of Lewis rats with heat-killed Mycobacterium tuberculosis H37Ra leads to development of polyarthritis (adjuvant-induced arthritis; AA) that shares several features with human rheumatoid arthritis (RA). Immune response to the 65-kDa mycobacterial heat-shock protein (Bhsp65) is believed to be involved in induction of AA as well as in experimental modulation of this disease. However, the understanding of several critical aspects of the pathogenesis of AA in the Lewis rat has severely been hampered by the lack of information both regarding the level as well as epitope specificity of tolerance to the mammalian self (rat) homologue of Bhsp65, 65-kDa rat heat-shock protein (Rhsp65), and about the functional attributes of the T cell repertoire specific for this self protein. In this study, we established that tolerance to Rhsp65 in the Lewis rat is incomplete, and that the residual T cells primed upon challenge with this self hsp65 are disease regulating in nature. We also have defined the T cell epitopes in the C-terminal region within Rhsp65 that contribute predominantly to the immune reactivity as well as the AA-protective effect of this self protein. Furthermore, the T cells primed by peptides comprising these C-terminal determinants can be efficiently restimulated by the naturally generated epitopes from endogenous Rhsp65, suggesting that self hsp65 might also be involved in natural remission from acute AA. These novel first experimental insights into the self hsp65-directed regulatory T cell repertoire in AA would help develop better immunotherapeutic approaches for autoimmune arthritis.

Expansion and Functional Relevance of High-Avidity Myelin-Specific CD4+T Cells in Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease in which myelin-specific T cells are believed to play a crucial pathogenic role. Nevertheless, so far it has been extremely difficult to demonstrate differences in T cell reactivity to myelin Ag between MS patients and controls. We believe that by using unphysiologically high Ag concentrations previous studies have missed a highly relevant aspect of autoimmune responses, i.e., T cells recognizing Ag with high functional avidity. Therefore, we focused on the characterization of high-avidity myelin-specific CD4+ T cells in a large cohort of MS patients and controls that was matched demographically and with respect to expression of MHC class II alleles. We demonstrated that their frequency is significantly higher in MS patients while the numbers of control T cells specific for influenza hemagglutinin are virtually identical between the two cohorts; that high-avidity T cells are enriched for previously in vivo-activated cells and are significantly skewed toward a proinflammatory phenotype. Moreover, the immunodominant epitopes that were most discriminatory between MS patients and controls differed from those described previously and were clearly biased toward epitopes with lower predicted binding affinities to HLA-DR molecules, pointing at the importance of thymic selection for the generation of the autoimmune T cell repertoire. Correlations between selected immunological parameters and magnetic resonance imaging markers indicate that the specificity and function of these cells influences phenotypic disease expression. These data have important implications for autoimmunity research and should be considered in the development of Ag-specific therapies in MS.

T-cell compartments of prediabetic NOD mice

Given the importance of the NOD mouse as a model of type 1 diabetes, there is a surprising lack of published information on the overall composition of the thymic and peripheral T-cell compartments. In this study, we revisited some earlier reports of T-cell abnormalities in this strain and examined a number of additional parameters to provide a global view of T-cells in prediabetic NOD mice. In some cases, we concur with past conclusions, but in other important areas, we find that NOD mice closely resemble nonautoimmune strains. Specifically, and contrary to published reports, the thymocyte subset distribution, the rate and composition of thymic export, and the composition of the peripheral T-cell pool, including the proportion of CD25(+)CD4(+) T-cells, are essentially normal in prediabetic NOD mice. These factors are therefore unlikely to be involved in the loss of tolerance that leads to autoimmunity within this strain.

Directed evolution of a single-chain class II MHC product by yeast display

Many autoimmune diseases have been linked to the class II region of the major histocompatibility complex (MHC). The linkage is thought to be a result of autoreactive T cells that recognize self-peptides bound to a product of this locus. For example, T cells from non-obese diabetic mice recognize specific 'diabetogenic' peptides bound to a class II MHC allele called I-A(g7). The I-A(g7) molecule is noted for being unstable and difficult to work with, especially in soluble form. In this work, yeast surface display combined with fluorescence-activated cell sorting was used as a means of directed evolution to engineer stabilized variants of a single-chain form of I-A(g7). A library containing mutations at two residues (positions 56 and 57 of the I-A(g7) beta-chain) that are important in the class II disease associations yielded stabilized mutants with preferences for a glutamic acid at residue 56 and a leucine at residue 57. Random mutation of I-A(g7) followed by selection with an anti-I-A(g7) antibody also yielded stabilized variants with mutations in other residues. The methods described here allow the discovery of novel MHC complexes that could facilitate structural studies and provide new opportunities in the development of diagnostics or antagonists of class II MHC-associated diseases.

The GTP-binding domain of class II transactivator regulates its nuclear export

The transcriptional coactivator class II transactivator (CIITA), although predominantly localized in the nucleus, is also present in the cytoplasm. The subcellular distribution of CIITA is actively regulated by the opposing actions of nuclear export and import. In this study, we show that nuclear export is negatively regulated by the GTP-binding domain (GBD; aa 421-561) of CIITA: mutation or deletion of the GBD markedly increased export of CIITA from the nucleus. Remarkably, a CIITA GBD mutant binds CRM1/exportin significantly better than does wild-type CIITA, leading to the conclusion that GTP is a negative regulator of CIITA nuclear export. We also report that, in addition to the previously characterized N- and C-terminal nuclear localization signal elements, there is an additional N-terminal nuclear localization activity, present between aa 209 and 222, which overlaps the proline/serine/threonine-rich domain of CIITA. Thus, fine-tuning of the nucleocytoplasmic distribution of coactivator proteins involved in transcription is an active and dynamic process that defines a novel mechanism for controlling gene regulation.

The thymus and negative selection

Maintenance of tolerance to self antigens is presumed to reflect a combination of central and peripheral tolerance. For T cells, central tolerance occurs during early T cell development in the thymus and causes cells with strong reactivity to self antigens to be destroyed in situ (negative selection). Here, we summarize evidence that negative selection can occur in the thymic medulla and affects a population of semimature HSA+ T cells. The influence of costimulatory molecules, Fas and cytokines on negative selection is discussed.

T-cell signalling and autoimmunity: molecular mechanisms of disease

The genetic manipulation of mice has led to insights into the molecular mechanisms of autoimmune disease. Recent studies have begun to identify ways in which signalling cascades can be disrupted that preclude the development of autoimmunity. This review outlines a new model for the induction of T-cell-mediated autoimmune diseases. I highlight recent data that illustrate the ways in which the altered survival of T cells and defects in the inhibitory signalling pathways of T cells can contribute to autoimmunity.

Understanding autoimmune diabetes: insights from mouse models

Type 1 or insulin-dependent diabetes is an autoimmune disease that causes the selective destruction of insulin-secreting beta cells in the pancreatic islets. Although this is a polygenic disease, with at least 20 genes implicated, the dominant susceptibility locus maps to the major histocompatibility complex (MHC), both in humans and in rodent models. However, in spite of progress on several fronts, the molecular pathology of autoimmune diabetes remains incompletely defined. Major areas of research include environmental trigger factors, the identification and role of beta-cell antigens in inducing and maintaining the autoimmune response, and the nature of the pathogenic and protective lymphocytes involved. In this review, we will focus on these areas to highlight recent advances in understanding the pathogenesis of autoimmune diabetes, drawing extensively on insights gained by studying the non-obese diabetic (NOD) mouse.

A defect in central tolerance in NOD mice

The predisposition of nonobese diabetic (NOD) mice to develop autoimmune disease is usually attributed to defects in peripheral tolerance mechanisms. Here, evidence is presented that NOD mice display a defect in central tolerance (negative selection) of thymocytes. Impaired central tolerance in NOD mice was most prominent in a population of semi-mature thymocytes found in the medulla. The defect was apparent in vivo as well as in vitro, was independent of IAbetag7 expression and affected both Fas-dependent and Fas-independent pathways of apoptosis; for Fas-dependent apoptosis, the defective tolerance of NOD thymocytes correlated with the strong T cell receptor-mediated up-regulation of caspase 8-homologous FLICE (Fas-associated death-domain-like interleukin 1beta-converting enzyme)-inhibitory protein. In light of these findings, disease onset in NOD mice may reflect defects in central as well as peripheral tolerance.

Increased entry into the IFN-gamma effector pathway by CD4+ T cells selected by I-Ag7 on a nonobese diabetic versus C57BL/6 genetic background

IFN-gamma-mediated Th1 effects play a major role in the pathogenesis of autoimmune diabetes in nonobese diabetic (NOD) mice. We analyzed functional responses of CD4(+) T cells from NOD and B6.G7 MHC congenic mice, which share the H2(g7) MHC region but differ in their non-MHC genetic background. T cells from each strain proliferated equally to panstimulation with T cell lectins as well as to stimulation with glutamic acid decarboxylase 524-543 (self) and hen egg lysozyme 11-23 (foreign) I-A(g7)-binding peptide epitopes. Despite comparable proliferative responses, NOD CD4(+) T cells had significantly increased IFN-gamma intracellular/extracellular protein and mRNA responses compared with B6.G7 T cells as measured by intracellular cytokine analysis, time resolved fluorometry, and RNase protection assays. The increased IFN-gamma production was not due to an increase in the amount of IFN-gamma produced per cell but to an increase in the number of NOD CD4(+) T cells entering the IFN-gamma-producing pathway. The increased IFN-gamma response in NOD mice was not due to increased numbers of activated precursors as measured by activation/memory markers. B6.G7 lymphoid cells demonstrated an absolute decrease in IFN-gamma mRNA, an increase in IL-4 mRNA production, and a significantly decreased IFN-gamma:IL-4 mRNA transcript ratio compared with NOD cells. CD4(+) T cells from C57BL6 mice also showed significantly decreased IFN-gamma production compared with CD4(+) T cells from NOD.H2(b) MHC-congenic mice (which have an H2(b) MHC region introgressed onto an NOD non-MHC background). Therefore, the NOD non-MHC background predisposes to a quantitatively increased IFN-gamma response, independent of MHC class II-mediated T cell repertoire selection, even when compared with a prototypical Th1 strain.

Pathways for self-tolerance and the treatment of autoimmune diseases

Antigen delivers both immunogenic and tolerogenic signals to lymphocytes. The outcome of antigen exposure represents a complex integration of the timing of antigen binding with signals from many other immunogenic and tolerogenic costimulatory pathways. A road map of these signalling pathways is only beginning to be charted, revealing the mechansim of action and limitations of current immunotherapeutic agents and the points of attack for new agents. Ciclosporin and tacrolimus interfere with tolerogenic signals from antigen in addition to blocking immunogenic signals, thus preventing active establishment of tolerance. Corticosteroids inhibit a key immunogenic pathway, NFkappaB, and more specific inhibitors of this pathway may allow tolerance to be actively established while immune responses are blocked. New experimental therapies aim to mimic tolerogenic antigen signals by chronically stimulating antigen receptors with antigen or antibodies to the receptor, or aim to block costimulatory pathways involving CD40 ligand, B7, or interleukin 2. Obtaining the desired response with these strategies is unpredictable because many of these signals have both tolerogenic and immunogenic roles. The cause of autoimune diseases has been determined for several rare monogenic disorders, revealing inherited deficiencies in tolerogenic costimulatory pathways such as FAS. Common autoimmune disorders may have a biochemically related pathogenesis.

[Thymoma and disseminated lupus erythematosus. Two new cases and review of the literature]

INTRODUCTION

Thymoma is a tumour originating in the epithelial cells of the thymus, associated with several immunologic disorders. The association of thymoma with systemic lupus erythematosus has rarely been described. We report two cases of this association.

EXEGESIS

Description of two cases and a review of the literature. Mr T. was 41 years old when the diagnosis of thymoma and lupus was made. The thymectomy did not influence the evolution of his lupus. Mrs G. had been treated because of a lupus for 8 years prior to developing a thymoma. One year later she presented with erythroblastopenia, which was only sensitive to cyclosporin.

CONCLUSION

The association between lupus and thymoma has been reported in 36 cases in the literature. Thymoma is benign in 59% of the cases. The clinical presentation of lupus is nonspecific except for age, median 48 years, and sex ratio, 4:3. The clinical outcome of the lupus is not influenced by the thymectomy. Thymoma may precede lupus with a delay of several years or it may be diagnosed concurrently or several years later. This association is not accidental, though the pathogenic link between these conditions remains unknown. One could suppose that the decrease of the thymic function in the course of thymoma could enhance the expression of autoreactive T lymphocytes as well as the activation of B cells. Patients should be followed after thymectomy because autoimmune diseases, particularly lupus, may develop belatedly. On the other hand, thymoma may be suspected mainly when lupus occurs in patients around 50 years of age.

Nonobese diabetic mice display elevated levels of class II-associated invariant chain peptide associated with I-Ag7 on the cell surface

Peptide presentation by MHC class II molecules plays a pivotal role in determining the peripheral T cell repertoire as a result of both positive and negative selection in the thymus. Homozygous I-A(g7) expression imparts susceptibility to autoimmune diabetes in the nonobese diabetic mouse, and recently, it has been proposed that this arises from ineffectual peptide binding. Following biosynthesis, class II molecules are complexed with class II-associated invariant chain peptides (CLIP), which remain associated until displaced by Ag-derived peptides. If I-A(g7) is a poor peptide binder, then this may result in continued occupation by CLIP to the point of translocation to the cell surface. To test this hypothesis we generated affinity-purified polyclonal antisera that recognized murine CLIP bound to class II molecules in an allele-independent fashion. We have found abnormally high natural levels of cell surface class II occupancy by CLIP on nonobese diabetic splenic B cells. Experiments using I-A-transfected M12.C3 cells showed that I-A(g7) alone was associated with elevated levels of CLIP, suggesting that this was determined solely by the amino acid sequence of the class II molecule. These results indicated that an intrinsic property of I-A(g7) would affect both the quantity and the repertoire of self-peptides presented during thymic selection.

Animal models of autoimmunity and their relevance to human diseases

T cells mediate various autoimmune diseases. Pathologic autoimmunity can be induced by manipulating thymic or peripheral control of self-reactive T cells. There is, for example, accumulating evidence that elimination or dysfunction of regulatory T cells can elicit T cell mediated, destructive autoimmune disease in otherwise normal animals and enhance autoimmunity in spontaneous models.

Present difficulties and future promise of MHC multimers in autoimmune exploration

Class I tetramers have been used to track cytotoxic T cells during bacterial and viral infections. During the past year, the use of such molecules has revealed important information about the role of CD8(+) T cells in autoimmune diabetes. Furthermore, class II multimers have been produced and successfully used to stain autoreactive CD4(+) T cells from patients with rheumatoid arthritis or Borrelia-burgdorferi-induced Lyme arthritis.

Thymoma

Thymomas are fascinating tumors because of their multifaceted clinical presentation, including an unrivaled frequency of associated paraneoplastic autoimmune diseases and an astounding histologic heterogeneity. Thus, thymomas challenge the clinician, pathologist, and immunologist alike with complex diagnostic and therapeutic problems in almost all fields of medicine, radiology, nuclear medicine, surgery, radiooncology, and pathology. This review highlights recent progress in these fields, including the new World Health Organization's classification of thymomas, and reports on the beginning discovery of the genetic basis of thymoma oncogenesis and the cellular and molecular immunopathology of thymoma-associated paraneoplastic autoimmunity.

A structural framework for deciphering the link between I-Ag7 and autoimmune diabetes

Susceptibility to murine and human insulin-dependent diabetes mellitus correlates strongly with major histocompatibility complex (MHC) class II I-A or HLA-DQ alleles that lack an aspartic acid at position beta57. I-Ag7 lacks this aspartate and is the only class II allele expressed by the nonobese diabetic mouse. The crystal structure of I-Ag7 was determined at 2.6 angstrom resolution as a complex with a high-affinity peptide from the autoantigen glutamic acid decarboxylase (GAD) 65. I-Ag7 has a substantially wider peptide-binding groove around beta57, which accounts for distinct peptide preferences compared with other MHC class II alleles. Loss of Asp(beta57) leads to an oxyanion hole in I-Ag7 that can be filled by peptide carboxyl residues or, perhaps, through interaction with the T cell receptor.