MHC-linked protection from diabetes dissociated from clonal deletion of T cells

Jin-Gui-Shen-Qi Wan alleviates fibrosis in mouse diabetic nephropathy via MHC class II

ETHNOPHARMACOLOGICAL RELEVANCE

Jin-Gui-Shen-Qi Wan (JGSQW) is a traditional Chinese medicine formula that has been traditionally used to alleviate urinary system ailments such as frequent urination and polyuria. Clinical studies have indicated that when combined with hypoglycaemic drugs, JGSQW exhibits a synergistic effect and can improve diabetic nephropathy (DN), yet its underlying mechanism and targets remain unclear.

AIM OF THE STUDY

This study aims to investigate the therapeutic efficacy of JGSQW and its underlying mechanisms using a DN db/db mouse model.

MATERIALS AND METHODS

Ultrahigh-performance liquid chromatography coupled with mass spectrometry was utilized to analyse the primary active compounds, blood levels, and pharmacokinetics of JGSQW. Additionally, the therapeutic effects of JGSQW and metformin on blood glucose levels, lipid levels, renal function, and renal pathology in diabetic nephropathy mice were investigated using a db/db mouse model. Proteomic analysis was carried out to identify the primary target of JGSQW in treating DN. The mechanism of action was verified by western blotting, immunohistochemistry, and immunofluorescence. Then, molecular docking and molecular dynamics, transfection, drug affinity responsive target stability (DARTS) assay and cell thermal migration assay (CETSA) further validated the targeted binding effect.

RESULTS

JGSQW combined with metformin significantly improved the blood glucose levels, blood lipids, renal function, and renal pathology of DN mice. JGSQW mainly exerted its therapeutic effect on DN by targeting major histocompatibility complex class II (MHC class II) molecules. Immunohistochemistry results showed that JGSQW inhibited the expression of collagen I, fibronectin, and alpha smooth muscle actin (α-SMA) expression. Immunofluorescence and Western blot results showed that JGSQW inhibited the expression of H2-Ab1 and H2-Aa, which are MHC class II molecules, thereby suppressing CD4+ T-cell infiltration and improving diabetic kidney fibrosis. The binding ability of paeoniflorin to H2-Aa was predicted and verified by molecular, DARTS, and CETSA assays. Treatment with 80 μM paeoniflorin effectively alleviated high glucose-induced injury in the MPC-5 injury model. H2-Aa was overexpressed at this model concentration, and Western blotting further confirmed that paeoniflorin reduced glomerular podocyte fibrosis by regulating H2-Aa.

CONCLUSIONS

JGSQW combined with metformin may have a synergistic effect to alleviates renal fibrosis in diabetic nephropathy by downregulating immune complex MHC class II molecules and attenuating the antigen presentation effect of MHC class II on CD4.

Gut Microbiota, Leaky Gut, and Autoimmune Diseases

With the rising prevalence of autoimmune diseases, the role of the environment, specifically the gut microbiota, in disease development has grown to be a major area of study. Recent advances show a relationship and possible cause and effect between the gut microbiota and the initiation or exacerbation of autoimmune diseases. Furthermore, microbial dysbiosis and leaky gut are frequent phenomena in both human autoimmune diseases and the murine autoimmunity models. This review will focus on literature in recent years concerning the gut microbiota and leaky gut in relation to the autoimmune diseases, including systemic lupus erythematosus, type 1 diabetes, and multiple sclerosis.

Clostridial Butyrate Biosynthesis Enzymes Are Significantly Depleted in the Gut Microbiota of Nonobese Diabetic Mice

Increasing evidence suggests that the intestinal microbiota is involved in the pathogenesis of type 1 diabetes (T1D). Here we sought to determine which gut microbial taxa and functions vary between nonobese diabetic (NOD) mice and genetically modified NOD mice protected from T1D (Eα16/NOD) at 10 weeks of age in the time window between insulitis development and T1D onset. The gut microbiota of NOD mice were investigated by analyzing stool samples with a metaproteogenomic approach, comprising both 16S rRNA gene sequencing and microbial proteome profiling through high-resolution mass spectrometry. A depletion of Firmicutes (particularly, several members of Lachnospiraceae) in the NOD gut microbiota was observed compared to the level in the Eα16/NOD mice microbiota. Moreover, the analysis of proteins actively produced by the gut microbiota revealed different profiles between NOD and Eα16/NOD mice, with the production of butyrate biosynthesis enzymes being significantly reduced in diabetic mice. Our results support a model for gut microbiota influence on T1D development involving bacterium-produced metabolites as butyrate.IMPORTANCE Alterations of the gut microbiota early in age have been hypothesized to impact T1D autoimmune pathogenesis. In the NOD mouse model, protection from T1D has been found to operate via modulation of the composition of the intestinal microbiota during a critical early window of ontogeny, although little is known about microbiota functions related to T1D development. Here, we show which gut microbial functions are specifically associated with protection from T1D in the time window between insulitis development and T1D onset. In particular, we describe that production of butyrate biosynthesis enzymes is significantly reduced in NOD mice, supporting the hypothesis that modulating the gut microbiota butyrate production may influence T1D development.

Protective major histocompatibility complex allele prevents type 1 diabetes by shaping the intestinal microbiota early in ontogeny

Certain MHC-II or HLA-D alleles dominantly protect from particular autoimmune diseases. For example, expression of the MHC-II Eα:Eβ complex potently protects nonobese diabetic (NOD) mice, which normally lack this isotype, from spontaneous development of type 1 diabetes. However, the underlying mechanisms remain debated. We investigated MHC-II-mediated protection from type 1 diabetes using a previously reported NOD mouse line expressing an Eα transgene and, thereby, the Eα:Eβ complex. Eα16/NOD females vertically protected their NOD offspring from diabetes and insulitis, an effect that was dependent on the intestinal microbiota; moreover, they developed autoimmunity when treated with certain antibiotics or raised in a germ-free environment. Genomic and proteomic analyses revealed NOD and Eα16/NOD mice to host mild but significant differences in the intestinal microbiotas during a critical early window of ontogeny, and transfer of cecal contents from the latter to the former suppressed insulitis. Thus, protection from autoimmunity afforded by particular MHC/HLA alleles can operate via intestinal microbes, highlighting potentially important societal implications of treating infants, or even just their pregnant mothers, with antibiotics.

Restoring Regulatory T Cells in Type 1 Diabetes

Genetic and cellular studies of type 1 diabetes in patients and in the nonobese diabetic mouse model of type 1 diabetes point to an imbalance between effector T cells and regulatory T cells (Tregs) as a driver of the disease. The imbalance may arise as a consequence of genetically encoded defects in thymic deletion of islet antigen-specific T cells, induction of islet antigen-specific thymic Tregs, unfavorable tissue environment for peripheral Treg induction, and failure of islet antigen-specific Tregs to survive in the inflamed islets secondary to insufficient IL-2 signals. These understandings are the foundation for rationalized design of new therapeutic interventions to restore the balance by selectively targeting effector T cells and boosting Tregs.

Autoimmune Diabetes: An Overview of Experimental Models and Novel Therapeutics

Type 1 diabetes (T1D) results from a chronic and selective destruction of insulin-secreting β-cells within the islets of Langerhans of the pancreas by autoreactive CD4(+) and CD8(+) T lymphocytes. The use of animal models of T1D was instrumental for deciphering the steps of the autoimmune process leading to T1D. The non-obese diabetic (NOD) mouse and the bio-breeding (BB) rat spontaneously develop the disease similar to the human pathology in terms of the immune responses triggering autoimmune diabetes and of the genetic and environmental factors influencing disease susceptibility. The generation of genetically modified models allowed refining our understanding of the etiology and the pathogenesis of the disease. In the present review, we provide an overview of the experimental models generated and used to gain knowledge on the molecular and cellular mechanisms underlying the breakdown of self-tolerance in T1D and the progression of the autoimmune response. Immunotherapeutic interventions designed in these animal models and translated into the clinical arena in T1D patients will also be discussed.

Non-classical MHC I-E negatively regulates macrophage activation and Th17 cell development in NOD mice

Transgenic expression of I-E molecules prevents diabetes in NOD mice. So far, the precise role of these non-classical MHC II molecules remains elusive. Here, we showed that transgenic expression of I-E^k alpha 16 molecule in NOD mice selectively reduced Th17 cells in the thymus and pancreatic draining lymph nodes. The reduction in Th17 cells was associated with both attenuated IL-6 production and decreased activation of macrophages. Mechanistically, transgenic expression of the I-E molecule diminished expression of intracellular classical MHC II molecule and led to impaired TLR4-mediated signaling. In contrast to classical MHC II molecule, this non-classical MHC II molecule negatively regulates the inflammatory responses of macrophages. Altogether, our study reveals a novel regulatory role of I-E molecules in modulating inflammatory immune responses.

Population dynamics of islet-infiltrating cells in autoimmune diabetes

Type-1 diabetes in the nonobese diabetic (NOD) mouse starts with an insulitis stage, wherein a mixed population of leukocytes invades the pancreas, followed by overt diabetes once enough insulin-producing β-cells are destroyed by invading immunocytes. Little is known of the dynamics of lymphocyte movement into the pancreas during disease progression. We used the Kaede transgenic mouse, whose photoconvertible fluorescent reporter permits noninvasive labeling and subsequent tracking of immunocytes, to investigate pancreatic infiltrate dynamics and the requirement for antigen specificity during progression of autoimmune diabetes in the unmanipulated NOD mouse. Our results indicate that the insulitic lesion is very open with constant cell influx and active turnover, predominantly of B and T lymphocytes, but also CD11b(+)c(+) myeloid cells. Both naïve- and memory-phenotype lymphocytes trafficked to the insulitis, but Foxp3(+) regulatory T cells circulated less than their conventional CD4(+) counterparts. Receptor specificity for pancreatic antigens seemed irrelevant for this homing, because similar kinetics were observed in polyclonal and antigen-specific transgenic contexts. This "open" configuration was also observed after reversal of overt diabetes by anti-CD3 treatment. These results portray insulitis as a dynamic lesion at all stages of disease, continuously fed by a mixed influx of immunocytes, and thus susceptible to evolve over time in response to immunologic or environmental influences.

T cell-B cell thymic cross-talk: maintenance and function of thymic B cells requires cognate CD40-CD40 ligand interaction

Thymic development requires bidirectional interaction or cross-talk between developing T cells and thymic stromal cells, a relationship that has been best characterized for the interaction between thymocytes and thymic epithelial cells. We have characterized in this article the requirement for similar cross-talk in the maintenance and function of thymic B cells, another population that plays a role in selection of developing thymic T cells. We found that maintenance of thymic B cells is strongly dependent on the presence of mature single-positive thymocytes and on the interactions of these T cells with specific Ag ligand. Maintenance of thymic B cell number is strongly dependent on B cell-autonomous expression of CD40, but not MHC class II, indicating that direct engagement of CD40 on thymic B cells is necessary to support their maintenance and proliferation. Thymic B cells can mediate negative selection of superantigen-specific, self-reactive, single-positive thymocytes, and we show that CD40 expression on B cells is critical for this negative selection. Cross-talk with thymic T cells is thus required to support the thymic B cell population through a pathway that requires cell-autonomous expression of CD40, and that reciprocally functions in negative selection of autoreactive T cells.

MHC Class II Polymorphisms, Autoreactive T-Cells, and Autoimmunity

Major histocompatibility complex (MHC) genes, also known as human leukocyte antigen genes (HLA) in humans, are the prevailing contributors of genetic susceptibility to autoimmune diseases such as Type 1 Diabetes (T1D), multiple sclerosis, and rheumatoid arthritis, among others (1–3). Although the pathways through which MHC molecules afford autoimmune risk or resistance remain to be fully mapped out, it is generally accepted that they do so by shaping the central and peripheral T-cell repertoires of the host toward autoimmune proclivity or resistance, respectively. Disease-predisposing MHC alleles would both spare autoreactive thymocytes from central tolerance and bias their development toward a pathogenic phenotype. Protective MHC alleles, on the other hand, would promote central deletion of autoreactive thymocytes and skew their development toward non-pathogenic phenotypes. This interpretation of the data is at odds with two other observations: that in MHC-heterozygous individuals, resistance is dominant over susceptibility; and that it is difficult to understand how deletion of one or a few clonal autoreactive T-cell types would suffice to curb autoimmune responses driven by hundreds if not thousands of autoreactive T-cell specificities. This review provides an update on current advances in our understanding of the mechanisms underlying MHC class II-associated autoimmune disease susceptibility and/or resistance and attempts to reconcile these seemingly opposing concepts.

Antidiabetogenic MHC class II promotes the differentiation of MHC-promiscuous autoreactive T cells into FOXP3+ regulatory T cells

Polymorphisms in MHC class II molecules, in particular around β-chain position-57 (β57), afford susceptibility/resistance to multiple autoimmune diseases, including type 1 diabetes, through obscure mechanisms. Here, we show that the antidiabetogenic MHC class II molecule I-A(b) affords diabetes resistance by promoting the differentiation of MHC-promiscuous autoreactive CD4(+) T cells into disease-suppressing natural regulatory T cells, in a β56-67-regulated manner. We compared the tolerogenic and antidiabetogenic properties of CD11c promoter-driven transgenes encoding I-A(b) or a form of I-A(b) carrying residues 56-67 of I-Aβ(g7) (I-A(b-g7)) in wild-type nonobese diabetic (NOD) mice, as well as NOD mice coexpressing a diabetogenic and I-A(g7)-restricted, but MHC-promiscuous T-cell receptor (4.1). Both I-A transgenes protected NOD and 4.1-NOD mice from diabetes. However, whereas I-A(b) induced 4.1-CD4(+) thymocyte deletion and 4.1-CD4(+)Foxp3(+) regulatory T-cell development, I-A(b-g7) promoted 4.1-CD4(+)Foxp3(+) Treg development without inducing clonal deletion. Furthermore, non-T-cell receptor transgenic NOD.CD11cP-I-A(b) and NOD.CD11cP-IA(b-g7) mice both exported regulatory T cells with superior antidiabetogenic properties than wild-type NOD mice. We propose that I-A(b), and possibly other protective MHC class II molecules, afford disease resistance by engaging a naturally occurring constellation of MHC-promiscuous autoreactive T-cell clonotypes, promoting their deviation into autoregulatory T cells.

Early window of diabetes determinism in NOD mice, dependent on the complement receptor CRIg, identified by noninvasive imaging

All juvenile NOD mice exhibit insulitis, but there is substantial variation in their progression to diabetes. We demonstrate that a patient-validated magnetic-resonance-imaging (MRI) strategy to non-invasively visualize local effects of pancreatic-islet inflammation can predict diabetes onset in NOD mice. MRI signals acquired during a narrow early time-window allowed pre-sorting into disease-progressors and -nonprogressors and an estimate of time-to-diabetes. We exploited this capability to identify novel elements correlated with disease protection, including CRIg (complement receptor of the immunoglobulin superfamily), which marked a subset of macrophages associated with diabetes resistance. Administration of CRIg-Fc depressed MRI signals and diabetes incidence. In addition to identifying regulators of disease progression, this study shows that diabetes is set at an early age in NOD mice.

Commercially available outbred mice for genome-wide association studies

Genome-wide association studies using commercially available outbred mice can detect genes involved in phenotypes of biomedical interest. Useful populations need high-frequency alleles to ensure high power to detect quantitative trait loci (QTLs), low linkage disequilibrium between markers to obtain accurate mapping resolution, and an absence of population structure to prevent false positive associations. We surveyed 66 colonies for inbreeding, genetic diversity, and linkage disequilibrium, and we demonstrate that some have haplotype blocks of less than 100 Kb, enabling gene-level mapping resolution. The same alleles contribute to variation in different colonies, so that when mapping progress stalls in one, another can be used in its stead. Colonies are genetically diverse: 45% of the total genetic variation is attributable to differences between colonies. However, quantitative differences in allele frequencies, rather than the existence of private alleles, are responsible for these population differences. The colonies derive from a limited pool of ancestral haplotypes resembling those found in inbred strains: over 95% of sequence variants segregating in outbred populations are found in inbred strains. Consequently it is possible to impute the sequence of any mouse from a dense SNP map combined with inbred strain sequence data, which opens up the possibility of cataloguing and testing all variants for association, a situation that has so far eluded studies in completely outbred populations. We demonstrate the colonies' potential by identifying a deletion in the promoter of H2-Ea as the molecular change that strongly contributes to setting the ratio of CD4+ and CD8+ lymphocytes.

The defect in T-cell regulation in NOD mice is an effect on the T-cell effectors

FoxP3(+) regulatory T cells (Tregs) protect against autoimmunity, type 1 diabetes (T1D) in particular, prompting the hypothesis that a deficiency in Tregs is a critical determinant of diabetes susceptibility in NOD mice. However, tests of this hypothesis have yielded contradictory results. We confirmed that NOD mice, compared with reference strains, do not have a primary deficit in Treg numbers in the lymphoid organs, whether in prediabetic mice of any age or in animals with recent-onset diabetes. NOD Tregs did show a defect in standard in vitro T cell suppression assays, particularly at low suppressor/effector ratios. Gene expression profiling revealed the vast majority of transcripts constituting the "Treg signature" to be normally distributed in NOD Tregs versus CD4(+) T conventional (Tconv) cells, although there were a few differences affecting one or the other population. According to results from criss-cross experiments, the functional inefficacy was not rooted in NOD Tregs, which suppressed as well as their C57BL/6 (B6) counterparts, but rather in NOD Tconv, which were less prone to suppression than were B6 Tconv cells. They also responded more effectively to anti-CD3/28 monoclonal antibody (mAb) stimulation in vitro or to a natural pancreatic antigen in vivo. This difference was independent of autoimmune inflammation, did not map to the idd3 region, and was not due to the overproduction of interleukin-21 in NOD mice. That the immune dysregulation in this T1D model is rooted in the ability of effector T cells to be regulated, rather than in Tregs themselves, has implications for proposed therapeutic interventions.

PD-1 deficiency reveals various tissue-specific autoimmunity by H-2b and dose-dependent requirement of H-2g7 for diabetes in NOD mice

Although many autoimmune diseases are associated with particular HLA/H-2 haplotypes, the mechanisms through which specific HLA/H-2 haplotypes afford autoimmune susceptibility remain enigmatic. Here, we analyzed the effects of the diabetes-associated (H-2(g7)) and an antidiabetogenic (H-2(b)) H-2 haplotypes in NOD mice deficient for programmed cell death-1 (PD-1, Pdcd1), a unique model of type 1 diabetes that confers complete penetrance and rapid onset of the disease. NOD-H2(b/b)Pdcd1(-/-) mice were completely protected from diabetes, confirming that H-2(g7) is indispensable for diabetes even in the absence of PD-1. However, NOD-H2(b/b)Pdcd1(-/-) mice developed autoimmune inflammation in multiple tissues including peripheral nerves, stomachs, and exocrine tissues, demonstrating that autoreactive T cells are generated in the context of H-2(b). These autoreactive T cells damaged target tissues only in the absence of PD-1, confirming that PD-1 deficiency unravels the hidden autoimmune susceptibility of the strain by reducing the threshold of T cell activation. Transfer experiments revealed that CD4 T cells are the effector cells of neuritis, and nerve-infiltrating CD4 T cells are strongly deviated toward Th1. Interestingly, neuritogenic T cells were also generated in the context of H-2(g7), in sharp contrast to the strict requirement of H-2(g7) for diabetes. In addition, 60% of the NOD-H2(b/g7)Pdcd1(-/-) mice developed diabetes, suggesting that H-2(b) does not dominantly suppress diabetes and that H-2(g7) induces diabetes in a dose-dependent fashion.

Enhanced thymic selection of FoxP3+ regulatory T cells in the NOD mouse model of autoimmune diabetes

FoxP3(+)CD4(+) regulatory T cells (Tregs) play a key role in the maintenance of peripheral self-tolerance, and it has been suggested that diabetes-susceptible nonobese diabetic (NOD) mice are defective in the generation and numbers of Tregs. We found thymic selection of Tregs to be under genetic control. Fetal thymic organ cultures on the NOD background required 3- to 10-fold more antigen than corresponding cultures on the B6 background for optimal induction of Tregs, but once the threshold for induction was reached the NOD background yielded close to 10-fold more Tregs. This increased selection of Tregs was also found in nontransgenic NOD mice in fetal through adult stages. This trait did not map to the MHC, idd3, or the chromosome 3 (Chr3) regions that control clonal deletion, but mainly to two regions on Chr1 and Chr11. Thus, NOD mice do not have a global defect in the generation or maintenance of Tregs; if anything, they show the opposite.

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.

Significance of MHC class II haplotypes and IgG Fc receptors in SLE

Systemic lupus erythematosus (SLE) is a systemic antibody-mediated autoimmune disease that develops under the control of multiple susceptibility genes. Genetic studies in murine and human SLE have identified several chromosomal intervals that contain candidate susceptibility genes. However, the ultimate identification of the genes and their roles in disease process need much further investigation. Spontaneous murine SLE models provide useful tools in this respect. In this chapter, we show this line of investigation, particularly focusing on the roles of major histocompatibility complex (MHC) class II and immunoglobulin G Fc receptors (FcgammaRs). The existence of high-affinity autoantibodies is evidence that autoimmunity in SLE is antigen-driven. Thereby, MHC class II haplotypes have been implicated in SLE susceptibility; however, because of the linkage disequilibrium that exists among the class I, II and III genes within the MHC complex, it has been difficult to discriminate the relative contributions of individual loci. On the other hand, the extent of antibody synthesis upon antigen stimulation and associated inflammatory cascades are controlled in several ways by the balance of stimulatory and inhibitory signaling molecules on immune cells. Stimulatory/inhibitory FcgammaRs mediate one such mechanism, and there are reports indicating the association between polymorphic FcgammaRs and SLE. However, as stimulatory and inhibitory FcgammaRs cluster on the telomeric chromosome 1, the absolute contribution of individual genes has been difficult to dissect. In studies of genetic dissection using interval-congenic and intragenic recombinant mouse strains of SLE models, we show evidence and discuss how and to what extent MHC class II molecules and stimulatory/inhibitory FcgammaRs are involved in SLE susceptibility.

Breakpoints in immunoregulation required for Th1 cells to induce diabetes

We describe a novel TCR-transgenic mouse line, TCR7, where MHC class II-restricted, CD4+ T cells are specific for the subdominant H-2b epitope (HEL74-88) of hen egg lysozyme (HEL), and displayed an increased frequency in the thymus and in peripheral lymphoid compartments over that seen in non-transgenic littermate controls. CD4+ T cells responded vigorously to HEL or HEL74-88 epitope presented on APC and could develop into Th1 or Th2 cells under appropriate conditions. Adoptive transfer of TCR7 Ly5.1 T cells into Ly5.2 rat insulin promoter (RIP)-HEL transgenic recipient hosts did not lead to expansion of these cells or result in islet infiltration, although these TCR7 cells could expand upon transfer into mice expressing high levels of HEL in the serum. Islet cell infiltration only occurred when the TCR7 cells had been polarized to either a Th1 or Th2 phenotype prior to transfer, which led to insulitis. Progression from insulitis to autoimmune diabetes only occurred in these recipients when Th1 but not Th2 TCR7 cells were transferred and CTLA-4 signaling was simultaneously blocked. These findings show that regulatory pathways such as CTLA-4 can hold in check already differentiated autoreactive effector Th1 cells, to inhibit the transition from tolerance to autoimmune diabetes.

The multigenic structure of the MHC locus contributes to positive selection efficiency: a role for MHC class II gene-specific restriction

The study of T cell positive selection in the thymus has long been focused on the specificity of the MHC-TCR interactions, making use of genetically manipulated mice that display TCR specificities or selecting peptides of limited diversity. However, little is known on the role of the MHC molecules irrespective of the peptide specificity and the implications of MHC multigenic structure in thymic positive selection have not been addressed. Here, we investigated the effect of MHC class II genetic configuration on the positive selection efficiency of naturally generated pre-selection repertoires in the mouse thymus. Analysis of positively selected thymocyte populations in MHC-congenic and -transgenic mice revealed that expression of I-E molecule in the thymic cortex increases positive selection efficiency of CD4 cells by approximately 50%. We show that increments in positive selection attributable to either the I-A and I-E genes are not due to increased MHC class II expression in the thymic cortex and are not affected by the number of MHC alleles. Collectively, our findings imply that MHC class II gene-restricted TCR specificities significantly contribute to positive selection efficiency, introducing the notion that multigenic structure of the MHC locus serves to increase selection of non-overlapping TCR repertoires.

Effect of X- and Y-box deletions on the development of diabetes in H-2Ealpha-chain transgenic nonobese diabetic mice

The development of type 1 diabetes in nonobese diabetic (NOD) mice is influenced by major histocompatibility complex (MHC) class II genes. The NOD-E transgenic mouse, which expresses H2-E as a result of the introduction of an Ead gene, is protected from development of type 1 diabetes. While the mechanism of protection remains unclear, the effect has been regarded as a model system for MHC protection from autoimmunity. We have investigated the effect of deletions of the Ea promoter region, which, in turn, affect H2-E expression patterns in transgenic NOD mice. We have constructed transgenic NOD mice where the X (DeltaX) and Y (DeltaY) boxes of the Ead gene have, respectively, been functionally deleted. Previous reports, using X- or Y-box-deleted H2-E transgenic mice, made by crossing the appropriate transgenes onto the NOD background from C57BL/6 transgenic mice, indicated that promoter mutation abrogated the H2-E-mediated protection seen in NOD-E. The NOD DeltaX and NOD DeltaY transgenic mice generated in the present study differ in susceptibility to diabetes from wild-type NOD mice. NOD DeltaY1 animals are protected from diabetes development, while DeltaX mice remain susceptible, albeit to a lesser extent than the parental NOD strain.

Facilitating physiologic self-regeneration: a step beyond islet cell replacement

Type 1 diabetes (T1D) is an autoimmune disease, the clinical onset of which most frequently presents in children and adolescents who are genetically predisposed. T1D is characterized by specific insulin-producing beta cell destruction. The well-differentiated and specialized islet beta cells seem to physiologically retain the ability to compensate for the cells lost by reproducing themselves, whereas undifferentiated cell sources may help in generating new ones, even while the autoimmune process takes place. Diabetes clinical onset, i.e., establishment of a detectable, chronic hyperglycemia, occurs at a critical stage when autoimmunity, having acted for a while, supersedes the regenerative effort and reduces the number of beta cells below the physiologic threshold at which the produced insulin becomes insufficient for the body's needs. Clinical solutions aimed at avoiding cumbersome daily insulin administrations by the reestablishment of physiologic insulin production, like whole pancreas or pancreatic islet allotransplantation, are limited by the scarcity of pancreas donors and by the toxic effects of the immunosuppressive drugs administered to prevent rejection. However, new accumulating evidence suggests that, once autoimmunity is abrogated, the endocrine pancreas properties may be sufficient to allow the physiological regenerative process to restore endogenous insulin production, even after the disease has become clinically manifest. Knowledge of these properties of the endocrine pancreas suggests the testing of reliable and clinically translatable protocols for obliterating autoimmunity, thus allowing the regeneration of the patient's own endocrine cells. The safe induction of an autoimmunity-free status might become a new promising therapy for T1D.

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.

Noninvasive imaging of pancreatic inflammation and its reversal in type 1 diabetes

A major stumbling block for research on and treatment of type 1 diabetes is the inability to directly, but noninvasively, visualize the lymphocytic/inflammatory lesions in the pancreatic islets. One potential approach to surmounting this impediment is to exploit MRI of magnetic nanoparticles (MNP) to visualize changes in the microvasculature that invariably accompany inflammation. MNP-MRI did indeed detect vascular leakage in association with insulitis in murine models of type 1 diabetes, permitting noninvasive visualization of the inflammatory lesions in vivo in real time. We demonstrate, in proof-of-principle experiments, that this strategy allows one to predict, within 3 days of completing treatment with an anti-CD3 monoclonal antibody, which NOD mice with recent-onset diabetes are responding to therapy and may eventually be cured. Importantly, an essentially identical MNP-MRI strategy has previously been used with great success to image lymph node metastases in prostate cancer patients. This success strongly argues for rapid translation of these preclinical observations to prediction and/or stratification of type 1 diabetes and treatment of individuals with the disease; this would provide a crucially needed early predictor of response to therapy.

NOD B-cells are insufficient to incite T-cell-mediated anti-islet autoimmunity

Although it is well established that B-cells are required for the development of diabetes in the nonobese diabetic (NOD) mouse, the nature of their role remains unknown. Herein, we investigate the hypothesis that B-cells in this autoimmune background actively disrupt the tolerant state of those T-cells with which they interact. We demonstrate that NOD B-cells express elevated levels of crucial molecules involved in antigen presentation (including CD21/35, major histocompatibility complex class II, and CD40), alterations that invite the possibility of inappropriate T-cell activation. However, when chimeric animals are generated in which all B-cells are NOD-derived, a tolerant state is maintained. These data demonstrate that although B-cells are required for the development of autoimmunity, they are not sufficient to disrupt established tolerance. Moreover, non-B-cell antigen-presenting cells may be the critical actors in the establishment of the tolerant state; this function may be absent in NOD mice as they are characterized by deficient professional antigen-presenting cell function.

The NOD mouse: a model of immune dysregulation

Autoimmunity is a complex process that likely results from the summation of multiple defective tolerance mechanisms. The NOD mouse strain is an excellent model of autoimmune disease and an important tool for dissecting tolerance mechanisms. The strength of this mouse strain is that it develops spontaneous autoimmune diabetes, which shares many similarities to autoimmune or type 1a diabetes (T1D) in human subjects, including the presence of pancreas-specific autoantibodies, autoreactive CD4+ and CD8+ T cells, and genetic linkage to disease syntenic to that found in humans. During the past ten years, investigators have used a wide variety of tools to study these mice, including immunological reagents and transgenic and knockout strains; these tools have tremendously enhanced the study of the fundamental disease mechanisms. In addition, investigators have recently developed a number of therapeutic interventions in this animal model that have now been translated into human therapies. In this review, we summarize many of the important features of disease development and progression in the NOD strain, emphasizing the role of central and peripheral tolerance mechanisms that affect diabetes in these mice. The information gained from this highly relevant model of human disease will lead to potential therapies that may alter the development of the disease and its progression in patients with T1D.

Dissection of the role of MHC class II A and E genes in autoimmune susceptibility in murine lupus models with intragenic recombination

Systemic lupus erythematosus (SLE) is a multigenic autoimmune disease, and the major histocompatibility complex (MHC) class II polymorphism serves as a key genetic element. In SLE-prone (NZB x NZW)F(1) mice, the MHC H-2(d/z) heterozygosity (H-2(d) of NZB and H-2(z) of NZW) has a strong impact on disease; thus, congenic H-2(d/d) homozygous F(1) mice do not develop severe disease. In this study, we used Ea-deficient intra-H-2 recombination to establish A(d/d)-congenic (NZB x NZW)F(1) mice, with or without E molecule expression, and dissected the role of class II A and E molecules. Here we found that A(d/d) homozygous F(1) mice lacking E molecules developed severe SLE similar to that seen in wild-type F1 mice, including lupus nephritis, autoantibody production, and spontaneously occurring T cell activation. Additional evidence revealed that E molecules prevent the disease in a dose-dependent manner; however, the effect is greatly influenced by the haplotype of A molecules, because wild-type H-2(d/z) F(1) mice develop SLE, despite E molecule expression. Studies on the potential of dendritic cells to present a self-antigen chromatin indicated that dendritic cells from wild-type F(1) mice induced a greater response of chromatin-specific T cells than did those from A(d/d) F(1) mice, irrespective of the presence or absence of E molecules, suggesting that the self-antigen presentation is mediated by A, but not by E, molecules. Our mouse models are useful for analyzing the molecular mechanisms by which MHC class II regions regulate the process of autoimmune responses.

Imaging inflammation of the pancreatic islets in type 1 diabetes

Type 1 diabetes is the clinical manifestation of aberrant leukocytic infiltration of the pancreatic islets; it is usually diagnosed only very late in disease progression, after the critical autoimmune phenomena have mostly played out. A noninvasive means of directly monitoring the evolution of islet infiltrates would have important research and clinical applications. We have exploited fluorescence and MRI of long-circulating magnetofluorescent nanoparticles to visualize micro-vascular leakage, as an indicator of inflammation, in pancreata of mouse models of type 1 diabetes ex vivo or in vivo. We could detect the onset and evolution of insulitis in vivo and in real time, permitting us to study the natural history of diabetes in individual animals.

Ex vivo analysis of thymic CD4 T cells in nonobese diabetic mice with tetramers generated from I-A(g7)/class II-associated invariant chain peptide precursors

The MHC determines susceptibility and resistance to type 1 diabetes in humans and nonobese diabetic (NOD) mice. To investigate how a disease-associated MHC molecule shapes the T cell repertoire in NOD mice, we generated a series of tetramers from I-A(g7)/class II-associated invariant chain peptide precursors by peptide exchange. No CD4 T cell populations could be identified for two glutamic acid decarboxylase 65 peptides, but tetramers with a peptide mimetic recognized by the BDC-2.5 and other islet-specific T cell clones labeled a distinct population in the thymus of young NOD mice. Tetramer-positive cells were identified in the immature CD4(+)CD8(low) population that arises during positive selection, and in larger numbers in the more mature CD4(+)CD8(-) population. Tetramer labeling was specific based on the use of multiple control tetramers, including one with a single amino acid analog peptide in which a critical TCR contact residue was substituted. The T cell population was already present in the thymus of 2-wk-old NOD mice before the typical onset of insulitis and was detected in B10 mice congenic for the NOD MHC locus, but not B10 control mice. These results demonstrate that a T cell population can expand in the thymus of NOD mice to levels that are at least two to three orders of magnitude higher than estimated for a given specificity in the naive T cell pool. Based on these data, we propose a model in which I-A(g7) confers susceptibility to type 1 diabetes by biasing positive selection in the thymus and later presenting peptides from islet autoantigens to such T cells in the periphery.

In a Transgenic Model of Spontaneous Autoimmune Diabetes, Expression of a Protective Class II MHC Molecule Results in Thymic Deletion of Diabetogenic CD8+ T Cells

H-2(d) mice expressing both the influenza virus hemagglutinin (HA) as a transgene-encoded protein on pancreatic islet beta cells (InsHA), as well as the Clone 4 TCR specific for the dominant H-2K(d)-restricted HA epitope, can be protected from the development of spontaneous autoimmune diabetes by expression of the H-2(b) haplotype. Protection occurs due to the deletion of K(d)HA-specific CD8+ T cells. This was unexpected as neither the presence of the InsHA transgene nor H-2(b), individually, resulted in thymic deletion. Further analyses revealed that thymic deletion required both a hybrid MHC class II molecule, Ebeta(b) Ealpha(d), and the K(d) molecule presenting the HA epitope, which together synergize to effect deletion of CD4+CD8+ thymocytes. This surprising example of protection from autoimmunity that maps to a class II MHC molecule, yet effects an alteration in the CD8+ T cell repertoire, suggests that selective events in the thymus represent the integrated strength of signal delivered to each cell through recognition of a variety of different MHC-peptide ligands.

Overexpression of bcl-2 in T cells affects insulitis in the nonobese diabetic mouse

The nonobese diabetic (NOD) mouse is a useful model for human autoimmune diabetes. The gene for the anti-apoptotic protein Bcl-2 has previously been suggested as a probable susceptibility candidate for the NOD mouse disease. In this study, we investigated how overexpression of Bcl-2 in lymphocytes might affect insulitis in NOD mice. A bcl-2 transgene expressed constitutively under the SV40-promoter and the 5'Igh enhancer, Emu, was bred onto NOD background. Two bcl-2 transgenic NOD strains were produced and analysed, one with overexpression of Bcl-2 on only B cells and the other with overexpression of Bcl-2 on both B and T cells. Subsequent to verification of expression pattern and functionality of the transgene, insulitis intensity was investigated in different backcross generations of the two transgenic strains. Overexpression of Bcl-2 on both B and T cells leads to a statistically significant protection of the mice from insulitis compared with normal littermates. Overexpression of Bcl-2 on only B cells, on the other hand, does not have any statistically significant effect on insulitis. Possible mechanisms for the effect of Bcl-2 on insulitis in NOD mice are discussed.

Cross-reactive mycobacterial and self hsp60 epitope recognition in I-A(g7) expressing NOD, NOD-asp and Biozzi AB/H mice

The highly conserved 60 kD endogenous heat shock protein (hsp60) has been suggested to be a target for T cell recognition in autoimmune diseases such as type I diabetes. We previously reported cross-recognition of both mycobacterial hsp60 (Mt60) and self hsp60 (m60) by Mt60 immunized NOD mice. To identify the epitopes involved, we generated T cell lines against m60 or its mycobacterial counterpart and tested these lines for recognition of complete sets of overlapping peptides spanning either hsp60 sequence. T cell lines responded to identical regions in the hsp60 proteins, regardless of their degree of conservation or I-A(g7) binding-affinity. Additionally, we determined whether a protective genetic background would affect the presence of hsp60 cross-reactive T cells in the peripheral repertoire by comparing epitope recognition in I-A(g7) expressing NOD, NOD-asp and Biozzi AB/H mice. Two out of five immunodominant murine peptides were able to induce proliferation in NOD and NOD-asp Mt60 T cell lines, but not in Biozzi AB/H T cell lines. Our results point out that Mt60 immunization not necessarily leads to proliferative T cells responding to endogenous hsp60 peptides in the context of diabetes-predisposing I-A(g7). Moreover, the capacity of T cells to respond to self hsp60 is not influenced by the presence of protective I-A(g7asp). Yet, proliferation of hsp60 autoreactive T cells is solely measured in combination with insulitis and as such serves as a surrogate marker for islet inflammation.

T-cell tolerance by dendritic cells and macrophages as a mechanism for the major histocompatibility complex-linked resistance to autoimmune diabetes

For poorly understood reasons, the development of autoimmune diabetes in humans and mice is dominantly inhibited by major histocompatibility complex (MHC) class II molecules with diverse antigen-binding sites. We have previously shown that thymocytes expressing a highly diabetogenic I-A(g7)-restricted T-cell receptor (TCR) (4.1-TCR) undergo negative selection in mice carrying one copy of the antidiabetogenic H-2(b) haplotype in an I-A(b)-dependent but superantigen-independent manner. Here, we show that 4.1-TCR-transgenic thymocytes undergo different forms of tolerance in NOD mice expressing antidiabetogenic I-A(d), I-A(g7PD), or I-Ealpha(k) transgenes. The ability of protective MHC class II molecules to induce thymocyte tolerance in 4.1-TCR-transgenic NOD mice correlates with their ability to prevent diabetes in non-TCR-transgenic mice and is associated with polymorphisms within positions 56-67 of their beta1 domains. The 4.1-thymocyte tolerogenic activity of these MHC class II molecules is mediated by dendritic cells and macrophages but not by B-cells or thymic epithelial cells and is a peptide-dependent process. Antidiabetogenic MHC class II molecules may thus afford diabetes resistance by presenting, on dendritic cells and macrophages, tolerogenic peptides to a subset of highly diabetogenic and MHC-promiscuous CD4(+) T-cells that play a critical role in the initiation of diabetes.

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.

Neonatal pattern of V(H) gene utilization in nonobese diabetic mice does not correlate with development of insulitis

The nonobese diabetic (NOD) mouse model is a model of human autoimmune insulin dependent diabetes, IDDM. The effector cells of the disease have been shown to be T cells, but also B cells seem to contribute. Adult NOD mice have been shown to display a bias in their utilization of immunoglobulin (Ig) variable heavy (V(H)) genes. In this study the analysis of VH gene utilization in NOD mice protected from insulitis by transgenic insertion of a major histocompatibility complex (MHC) class II E(alpha) gene, point out that the bias in V(H) gene expression is not correlated to disease development. The aberrant V(H) gene utilization pattern in mice with the NOD genetic background is instead suggested to be a consequence of a deregulation of the apoptosis inhibiting gene bcl-2. We also investigated if prolonged in vitro survival of NOD lymphocytes is correlated to disease development. The E(alpha) transgenic NOD mice were shown to display a prolonged in vitro survival of spleen T cells, similar to normal NOD mice. These results indicate that defective death mechanisms of T cells may not be primarily involved in the development of autoimmune disease in these mice. However, in contrast to results from other groups, no difference in in vitro survival could be detected for B cells from mice with NOD genetic background compared to C57BL/6 mice.

IL-12 administration reveals diabetogenic T cells in genetically resistant I-Ealpha-transgenic nonobese diabetic mice: resistance to autoimmune diabetes is associated with binding of Ealpha-derived peptides to the I-A(g7) molecule

Nonobese diabetic (NOD) and NOD-DRalpha transgenic (tg) mice, expressing Aalpha(d):Abeta(g7) and Aalpha(d):Abeta(g7) plus DRalpha:Ebeta(g7) class II molecules, respectively, both develop insulin-dependent diabetes mellitus (IDDM), whereas NOD-Ealpha tg mice expressing Aalpha(d):Abeta(g7) plus Ealpha:Ebeta(g7) are protected. We show that IL-12 administration induces rapid IDDM onset in NOD-DRalpha but fails to provoke insulitis and diabetes in NOD-Ealpha tg mice. Nevertheless, T cells from IL-12-treated NOD-Ealpha tg mice secrete IFN-gamma and transfer IDDM to NOD-SCID and NOD-Ealpha-SCID recipients, demonstrating the presence of peripheral diabetogenic Th1 cells in the protected mice. Surprisingly, regulatory cells were undetectable. Moreover, Ealpha:Ebeta(g7) could substitute for DRalpha:Ebeta(g7) in Ag presentation, arguing against mechanisms of protection involving capture of diabetogenic I-A(g7)-restricted epitopes by Ealpha:Ebeta(g7)molecules. Interestingly, the expression of naturally processed epitopes derived from DRalpha- and Ealpha-chains bound to I-A(g7) is different in the two strains of tg mice, and the difference is enhanced by IL-12 administration. I-A(g7) molecules from both NOD-DRalpha and NOD-Ealpha tg mice present the conserved DRalpha/Ealpha 52-68 sequence, at high and low levels, respectively. In addition, only IDDM-resistant NOD-Ealpha tg mice possess APCs bearing Ealpha65-77/I-A(g7) complexes, which tolerize the specific T cells. This is associated with the selective inhibition of the response to insulinoma-associated protein 2 (IA-2), an autoantigen in IDDM. Our results support protective mechanisms based on I-A(g7) blockade by peptides unique to the Ealpha-chain, such as Ealpha65-77 and/or tolerance of diabetogenic T cells cross-reactive with Ealpha-peptide/I-A(g7) complexes.

The regulatory role of DR4 in a spontaneous diabetes DQ8 transgenic model

MHC class II molecules are critical determinants of genetic susceptibility to human type 1 diabetes. In patients, the most common haplotype contains the DRA1*0101-DRB1*0401 (DR4) and DQA1*0301-DQB1*0302 (DQ8) loci. To assess directly the relative roles of HLA-DQ8 and DR4 for diabetes development in vivo, we generated C57BL/6 transgenic mice that lack endogenous mouse MHC class II molecules but express HLA-DQ8 and/or DR4. Neither HLA-DQ nor HLA-DR transgenic mice developed insulitis or spontaneous diabetes. However, when they were crossed to transgenic mice (C57BL/6) expressing the B7.1 costimulatory molecules on pancreatic beta cells that do not normally develop diabetes, T cells from these double transgenic mice were no longer tolerant to islet autoantigens. The majority of DQ8/RIP-B7 mice developed spontaneous diabetes, whereas only 25% of DR4/RIP-B7 mice did so. Interestingly, when DQ8 and DR4 were coexpressed (DQ8DR4/RIP-B7), only 23% of these mice developed diabetes, an incidence indistinguishable from the DR4/RIP-B7 mice. T cells from both DR4/RIP-B7 and DQ8DR4/RIP-B7 mice, unlike those from DQ8/RIP-B7 mice, exhibited a Th2-like phenotype. Thus, the expression of DR4 appeared to downregulate DQ8-restricted autoreactive T cells in DQ8DR4/RIP-B7 mice. Our data suggest that although both DQ8 and DR4 can promote spontaneous diabetes in mice with a non-autoimmune-prone genetic background, the diabetogenic effect of the DQ8 allele is much greater, whereas DR4 expression downregulates the diabetogenic effect of DQ8, perhaps by enhancing Th2-like immune responses.

Expression of a tolerizing tumor antigen in peripheral tissue does not preclude recovery of high-affinity CD8+ T cells or CTL immunotherapy of tumors expressing the antigen

Transgenic (TG) mice were generated selectively expressing the gag protein of Friend murine leukemia virus (FMuLV) in the liver. FMuLV(gag) is also expressed by the FBL leukemia, and is the immunodominant tumor Ag of the CD8(+) T cell response in C57BL/6 mice. gag-TG mice expressing FMuLV(gag) in the liver were tolerant to the protein and failed to generate a CTL response to either FBL or FMuLV(gag). This tolerance reflected anergy rather than deletion, as CTL responsiveness could be recovered after four cycles of in vitro stimulation. Adoptively transferred gag-specific T cells were not anergized in gag-TG recipients, as revealed by antitumor activity in vivo. Also, such T cells did not induce detectable autoimmune injury in gag-TG liver cells. These results suggest that the requirements for a tissue Ag to provide a tolerizing stimulus are distinct from those for being the target of a T cell-mediated autoimmune response and that the requirements for induction and maintenance of peripheral tolerance are distinct for naive and primed T cells. That anergic T cells reactive with tumor-associated Ags can be recovered by repetitive in vitro stimulation and can mediate tumor therapy suggests strategies that use such Ags to generate CTL for adoptive immunotherapy should be further developed.

Pinpointing when T cell costimulatory receptor CTLA-4 must be engaged to dampen diabetogenic T cells

Engagement of the T cell costimulatory receptor CTLA-4 can potently down-regulate an immune response. For example, in a T cell receptor transgenic mouse model of autoimmune diabetes, CTLA-4 interactions keep pancreatic islet-reactive T cells in check, evidenced by the finding that mAb blockade of CTLA-4 rapidly provokes diabetes in animals that would not normally succumb until many months later. Interestingly, this effect is only observed early in the course of disease, before insulitis is stably entrenched. Here, we have exploited a highly synchronous and easily manipulable transfer system to determine precisely when CTLA-4 must be engaged to check the diabetogenicity of islet-reactive T cells. Our results indicate that CTLA-4 interactions during initial priming of the T cells in the pancreatic lymph nodes are not determinant. Rather, the critical interactions occur when the T cells secondarily reencounter their antigen in the target organ, the pancreatic islets. In addition, we made use of CTLA-4-deficient mice to bolster our interpretation that CTLA-4 engagement has a dampening rather than an enhancing influence on diabetes progression.

MHC structure and autoimmune T cell repertoire development

Recent work has continued to clarify the relationship between MHC structure and thymic selection that leads to peripheral T cell repertoire development in the pathogenesis of autoimmune diseases. Particular attention has been focused on the nonobese diabetic model of autoimmune diabetes, in which a unique MHC class II molecule (I-Ag7) plays a central role. In the past year, reports on the biochemistry of I-Ag7-combined with analysis of the role of I-Ag7 in T cell repertoire selection--support a model of defective thymic selection as the basis of the association between particular MHC molecules and autoimmune diseases. Analogous work has been done on the structure of the human MHC disease-susceptible and -resistant alleles, DQA1*0301 DQB1*0302 and DQA1*0102 DQB1*0602, and their effect on autoimmune repertoire selection. Comparison of these results (in naturally occurring, spontaneous autoimmune human and murine diabetes), with results in a variety of transgenic and knockout models, has produced an integrated view of how avidity considerations in repertoire selection in the thymus could affect predisposition towards autoimmunity.

E expression is needed on both bone marrow derived cells and thymic epithelium to increase IL-4 production and achieve protection in NOD bone marrow chimeras

The NOD mouse is an animal model for insulin-dependent diabetes with many similarities to the human disease. NOD mice which are transgenic for the Ea gene, allowing expression of the E molecule, are protected from diabetes and rarely develop insulitis. We have constructed bone marrow chimeras between transgenic and non-transgenic NOD mice to study the correlation of E expression on bone marrow derived cells and thymic epithelium vs the production of IL-4 and IFN-gamma. We show that NOD-E-->NOD-E and NOD-E-->NOD chimeras have elevated levels of IL-4 compared to NOD-->NOD and NOD-->NOD-E chimeras in the thymus. However, in the periphery the protected NOD-E-->NOD-E show much higher IL-4 levels than any of the other chimeras. This drop in peripheral IL-4 production seen in NOD-E-->NOD, NOD-->NOD-E and NOD-->NOD chimeras correlates with the increased insulitis seen in these mice compared to NOD-E-->NOD-E. In contrast, there were no differences in IFN-gamma production between the chimeras. We suggest that the precommitted, regulatory T cells, selected in an E-expressing thymic environment, need continuous interaction with E-expressing primary antigen presenting cells in the periphery for optimal IL-4 production. Decrease in IL-4 production correlates with increased insulitis.

Minute defects in the expression of MHC E molecules lead to impaired protection from autoimmunity in NOD mice

The E complex of the major histocompatibility complex (MHC) can prevent the spontaneous development of diabetes in nonobese diabetic (NOD) mice transgenic for the Ea gene. None of three promoter-mutated Ea constructs with Ea expression directed to different subsets of immunocompetent cells exerts full protection in NOD mice. The promoter-mutated constructs are all capable of mediating intrathymic elimination of I-E-restricted T cells. Thus, thymic negative selection is not responsible for the protective effect but a more complex effect is likely. Here we show that combinations of two or three different mutated Ea constructs do not protect against intra-islet insulitis either. We also show that spleen cells from protected animals are sufficient to protect NOD mice in adoptive transfer experiments. The only detectable expression defects in splenic cells or cells influencing the repertoire of splenic cells are in the B-cell compartment. Furthermore, in three construct combinations, the differences to wild-type expression are extremely small. Thus, we conclude that even minute disturbances of the E expression pattern might reduce the protection of NOD mice from insulitis.

Protection from Radiation-Induced Colitis Requires MHC Class II Antigen Expression by Cells of Hemopoietic Origin

Ulcerative colitis, an inflammatory bowel disease, is believed to result from a breakdown of dominant tolerance mechanisms that normally control intestinal immunity. Although CD4+ T lymphocyte subpopulations and expression of MHC class II molecules have been shown to play a role in the pathogenesis of the disease, the nature of the responsible mechanisms remains unclear. In this paper we describe a novel mouse model for inflammatory bowel disease, radiation-induced colitis, that occurs with complete penetrance 6–8 wk postinduction. A combination of high dose gamma-irradiation and lack of MHC class II expression on cells of hemopoietic origin results in development of colitis in C57BL/6 mice. Because of its versatility (due to susceptibility of mice of the widely genetically manipulated C57BL/6 background), high reproducibility, and 100% penetrance, radiation-induced colitis will be a useful mouse model for colitis and a significant tool to study dominant immunological tolerance mechanisms. Moreover, our data imply that tolerization to enteric Ags requires MHC class II mediated presentation by APC of hemopoietic origin.

A Peptide Binding Motif for I-Eg7, the MHC Class II Molecule That Protects Eα-Transgenic Nonobese Diabetic Mice from Autoimmune Diabetes

The nonobese diabetic (NOD) mouse, a model of spontaneous insulin-dependent diabetes mellitus (IDDM), fails to express surface MHC class II I-Eg7 molecules due a deletion in the Eα gene promoter. Eα-transgenic NOD mice express the EαEβg7 dimer and fail to develop either insulitis or IDDM. A number of hypotheses have been proposed to explain the mechanisms of protection, most of which require peptide binding to I-Eg7. To define the requirements for peptide binding to I-Eg7, we first identified an I-Eg7-restricted T cell epitope corresponding to the sequence 4–13 of Mycobacterium tuberculosis 65-kDa heat shock protein (hsp). Single amino acid substitutions at individual positions revealed a motif for peptide binding to I-Eg7 characterized by two primary anchors at relative position (p) 1 and 4, and two secondary anchors at p6 and p9. This motif is present in eight of nine hsp peptides that bind to I-Eg7 with high affinity. The I-Eg7 binding motif displays a unique p4 anchor compared with the other known I-E motifs, and major differences are found between I-Eg7 and I-Ag7 binding motifs. Analysis of peptide binding to I-Eg7 and I-Ag7 molecules as well as proliferative responses of draining lymph node cells from hsp-primed NOD and Eα-transgenic NOD mice to overlapping hsp peptides revealed that the two MHC molecules bind different peptides. Of 80 hsp peptides tested, none bind with high affinity to both MHC molecules, arguing against some of the mechanisms hypothesized to explain protection from IDDM in Eα-transgenic NOD mice.

Murine endothelia do not express MHC class II I-Ealpha subunit and differentially regulate I-Aalpha expression along the vascular tree

Cellular elements of the vascular wall, such as endothelium (En) and smooth muscle cells/pericytes (SM/P) possess important immunologic properties. We have previously reported that murine brain microvessel En cells and SM/P express Major Histocompatibility (MHC) class II molecules and activate syngeneic CD4+ T cells in a class II dependent way. Herein we compare MHC class II expression on brain microvessel En to aorta large vessel En cells in order to explore the mechanisms of immune responses in brain tissue versus other peripheral tissues. Interestingly, we demonstrate that En cells from brain microvessel and large aortic vessel express the I-A but not the I-E subunit of MHC class II molecules. The expression of I-A class II molecules can be upregulated on brain microvessel and aortic En cells by interferon-gamma (IFN-gamma). Similarly, the expression of I-A, but not I-E, MHC class II molecules on brain microvessel endothelial cells was upregulated in the presence of activated T cells. Interleukin-10 (IL-10) was found to inhibit IFN-gamma-mediated upregulation of I-A class II molecule expression on aortic but not on microvessel En cells. Our data may indicate that some differences in organ-specific immune responses, are defined by local parameters, such as MHC distribution and regulation.