Enhanced Pathogenicity of Diabetogenic T Cells Escaping a Non-MHC Gene-Controlled Near Death Experience

Advancing Animal Models of Human Type 1 Diabetes

Multiple rodent models have been developed to study the basis of type 1 diabetes (T1D). However, nonobese diabetic (NOD) mice and derivative strains still provide the gold standard for dissecting the basis of the autoimmune responses underlying T1D. Here, we review the developmental origins of NOD mice, and how they and derivative strains have been used over the past several decades to dissect the genetic and immunopathogenic basis of T1D. Also discussed are ways in which the immunopathogenic basis of T1D in NOD mice and humans are similar or differ. Additionally reviewed are efforts to "humanize" NOD mice and derivative strains to provide improved models to study autoimmune responses contributing to T1D in human patients.

Proteomic identification of MHC class I-associated peptidome derived from non-obese diabetic mouse thymus and pancreas

The peptides repertoire presented to CD8⁺ T cells by major histocompatibility complex (MHC) class I molecules is referred to as the MHC I-associated peptidome (MIP), which regulates thymus development, peripheral survival and function during lifetime of CD8⁺ T cells. Type 1 diabetes (T1D) is an organ-specific autoimmune disease caused by pancreatic β cells destruction mediated primarily by autoreactive CD8⁺ T cells. Non-obese diabetic (NOD) mouse is an important animal model of T1D. Here, we deeply analyzed the MIP derived from NOD mice thymus and pancreas, and demonstrated that the thymus MIP source proteins partially shared with the MIP source proteins derived from NOD mice pancreas and β cell line. One H-2Kd restricted peptide SLC35B1_26-34 which was shared by MIP derived from both NOD mice pancreatic tissues and islet β-cell line, but absent in MIP from NOD thymus tissues, showed ability to stimulate IFN-γ secretion and proliferation of NOD mice splenic CD8⁺ T cells. The global view of the MHC I-associated self-peptides repertoire in the thymus and pancreas of NOD mice may serve as a biological reference to identify potential autoantigens targeted by autoreactive CD8⁺ T cells in T1D. Data are available via ProteomeXchange with identifier PXD031966. SIGNIFICANCE: The peptides repertoire presented to CD8⁺ T cells by major histocompatibility complex (MHC) class I molecules is referred to as the MHC I-associated peptidome (MIP). The MIP presented by thymic antigen presenting cells (APCs) is crucial for shaping CD8⁺ T cell repertoire and self-tolerance, while the MIP presented by peripheral tissues and organs is not only involved in maintaining periphery CD8⁺ T cell survival and homeostasis, but also mediates immune surveillance and autoimmune responses of CD8⁺ T cells under pathological conditions. Type 1 diabetes (T1D) is an organ-specific autoimmune disease caused by the destruction of pancreatic β cells, mediated primarily by autoreactive CD8⁺ T cells. Non-obese diabetic (NOD) mouse is one of important animal models of spontaneous autoimmune diabetes that shares several key features with human T1D. The global view of the MHC I-associated self-peptides repertoire in the thymus and pancreas of NOD mice may serve as a good biological reference to identify potential autoantigens targeted by autoreactive CD8⁺ T cells in T1D. It has great significance for further clarifying the immune recognition and effect mechanism of autoreactive CD8⁺ T cells in the pathogenesis of T1D, and then developing antigen-specific immune intervention strategies.

CD5 levels reveal distinct basal T-cell receptor signals in T cells from non-obese diabetic mice

Type 1 diabetes in non-obese diabetic (NOD) mice occurs when autoreactive T cells eliminate insulin producing pancreatic β cells. While extensively studied in T-cell receptor (TCR) transgenic mice, the contribution of alterations in thymic selection to the polyclonal T-cell pool in NOD mice is not yet resolved. The magnitude of signals downstream of TCR engagement with self-peptide directs the development of a functional T-cell pool, in part by ensuring tolerance to self. TCR interactions with self-peptide are also necessary for T-cell homeostasis in the peripheral lymphoid organs. To identify differences in TCR signal strength that accompany thymic selection and peripheral T-cell maintenance, we compared CD5 levels, a marker of basal TCR signal strength, on immature and mature T cells from autoimmune diabetes-prone NOD and -resistant B6 mice. The data suggest that there is no preferential selection of NOD thymocytes that perceive stronger TCR signals from self-peptide engagement. Instead, NOD mice have an MHC-dependent increase in CD4⁺ thymocytes and mature T cells that express lower levels of CD5. In contrast, T cell-intrinsic mechanisms lead to higher levels of CD5 on peripheral CD8⁺ T cells from NOD relative to B6 mice, suggesting that peripheral CD8⁺ T cells with higher basal TCR signals may have survival advantages in NOD mice. These differences in the T-cell pool in NOD mice may contribute to the development or progression of autoimmune diabetes.

CD70 Inversely Regulates Regulatory T Cells and Invariant NKT Cells and Modulates Type 1 Diabetes in NOD Mice

The CD27-CD70 costimulatory pathway is essential for the full activation of T cells, but some studies show that blocking this pathway exacerbates certain autoimmune disorders. In this study, we report on the impact of CD27-CD70 signaling on disease progression in the NOD mouse model of type 1 diabetes (T1D). Specifically, our data demonstrate that CD70 ablation alters thymocyte selection and increases circulating T cell levels. CD27 signaling was particularly important for the thymic development and peripheral homeostasis of Foxp3⁺Helios⁺ regulatory T cells, which likely accounts for our finding that CD70-deficient NOD mice develop more-aggressive T1D onset. Interestingly, we found that CD27 signaling suppresses the thymic development and effector functions of T1D-protective invariant NKT cells. Thus, rather than providing costimulatory signals, the CD27-CD70 axis may represent a coinhibitory pathway for this immunoregulatory T cell population. Moreover, we showed that a CD27 agonist Ab reversed the effects of CD70 ablation, indicating that the phenotypes observed in CD70-deficient mice were likely due to a lack of CD27 signaling. Collectively, our results demonstrate that the CD27-CD70 costimulatory pathway regulates the differentiation program of multiple T cell subsets involved in T1D development and may be subject to therapeutic targeting.

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.

A Hypermorphic Nfkbid Allele Contributes to Impaired Thymic Deletion of Autoreactive Diabetogenic CD8+ T Cells in NOD Mice

In both NOD mice and humans, the development of type 1 diabetes (T1D) is dependent in part on autoreactive CD8+ T cells recognizing pancreatic β cell peptides presented by often quite common MHC class I variants. Studies in NOD mice previously revealed that the common H2-Kd and/or H2-Db class I molecules expressed by this strain aberrantly lose the ability to mediate the thymic deletion of pathogenic CD8+ T cell responses through interactions with T1D susceptibility genes outside the MHC. A gene(s) mapping to proximal chromosome 7 was previously shown to be an important contributor to the failure of the common class I molecules expressed by NOD mice to mediate the normal thymic negative selection of diabetogenic CD8+ T cells. Using an inducible model of thymic negative selection and mRNA transcript analyses, we initially identified an elevated Nfkbid expression variant as a likely NOD-proximal chromosome 7 region gene contributing to impaired thymic deletion of diabetogenic CD8+ T cells. CRISPR/Cas9-mediated genetic attenuation of Nfkbid expression in NOD mice resulted in improved negative selection of autoreactive diabetogenic AI4 and NY8.3 CD8+ T cells. These results indicated that allelic variants of Nfkbid contribute to the efficiency of intrathymic deletion of diabetogenic CD8+ T cells. However, although enhancing thymic deletion of pathogenic CD8+ T cells, ablating Nfkbid expression surprisingly accelerated T1D onset that was associated with numeric decreases in both regulatory T and B lymphocytes in NOD mice.

BIM Deficiency Protects NOD Mice From Diabetes by Diverting Thymocytes to Regulatory T Cells

Because regulatory T-cell (Treg) development can be induced by the same agonist self-antigens that induce negative selection, perturbation of apoptosis will affect both negative selection and Treg development. But how the processes of thymocyte deletion versus Treg differentiation bifurcate and their relative importance for tolerance have not been studied in spontaneous organ-specific autoimmune disease. We addressed these questions by removing a critical mediator of thymocyte deletion, BIM, in the NOD mouse model of autoimmune diabetes. Despite substantial defects in the deletion of autoreactive thymocytes, BIM-deficient NOD (NODBim(-/-)) mice developed less insulitis and were protected from diabetes. BIM deficiency did not impair effector T-cell function; however, NODBim(-/-) mice had increased numbers of Tregs, including those specific for proinsulin, in the thymus and peripheral lymphoid tissues. Increased levels of Nur77, CD5, GITR, and phosphorylated IκB-α in thymocytes from NODBim(-/-) mice suggest that autoreactive cells receiving strong T-cell receptor signals that would normally delete them escape apoptosis and are diverted into the Treg pathway. Paradoxically, in the NOD model, reduced thymic deletion ameliorates autoimmune diabetes by increasing Tregs. Thus, modulating apoptosis may be one of the ways to increase antigen-specific Tregs and prevent autoimmune disease.

MHC-mismatched mixed chimerism mediates thymic deletion of cross-reactive autoreactive T cells and prevents insulitis in nonobese diabetic mice

Type 1 diabetic NOD mice have defects in both thymic negative selection and peripheral regulation of autoreactive T cells, and induction of mixed chimerism can effectively reverse these defects. Our recent studies suggest that MHC-mismatched mixed chimerism mediates negative selection of autoreactive thymocytes in wild-type NOD and TCR-transgenic NOD.Rag1(+/+).BDC2.5 mice. However, it remains unknown how mismatched I-A(b) MHC class II can mediate deletion of autoreactive T cells positively selected by I-A(g7). In the present study, we directly tested the hypothesis that mismatched MHC class II in mixed chimeras mediates deletion of cross-reactive autoreactive thymocytes. We first identify that transgenic BDC2.5 T cells from NOD.Rag1(+/+).BDC2.5 but not NOD.Rag1(-/-).BDC2.5 mice possess cross-reactive TCRs with endogenous TCRα-chains; MHC-mismatched H-2(b) but not matched H-2(g7) mixed chimerism mediates thymic deletion of the cross-reactive transgenic T cells in NOD.Rag1(+/+).BDC2.5 mice. Second, by transplanting T cell-depleted (TCD) bone marrow (BM) cells from NOD.Rag1(+/+).BDC2.5 or NOD.Rag1(-/-).BDC2.5 mice into lethally irradiated MHC-mismatched H-2(b) C57BL/6 or MHC-matched congenic B6.H-2(g7) recipients, we demonstrate that NOD.Rag1(+/+).BDC2.5 BM-derived cross-reactive transgenic T cells, but not NOD.Rag1(-/-).BDC2.5 BM-derived non-cross-reactive transgenic T cells, can be positively selected in MHC-mismatched H-2(b) thymus. Third, by cotransplanting NOD.Rag1(+/+).BDC2.5 TCD BM cells with BM cells from MHC-mismatched T cell-deficient C57BL/6 mice into lethally irradiated MHC-matched B6.H-2(g7) recipients, we demonstrate that thymic deletion of the cross-reactive transgenic T cells is dependent on MHC-mismatched donor BM-derived APCs but not on donor BM-derived T cells. Taken together, our studies indicate that MHC-mismatched mixed chimerism can mediate thymic deletion of cross-reactive autoreactive T cells that express more than one TCR.

Single-cell mass cytometry of TCR signaling: amplification of small initial differences results in low ERK activation in NOD mice

Signaling from the T-cell receptor (TCR) conditions T-cell differentiation and activation, requiring exquisite sensitivity and discrimination. Using mass cytometry, a high-dimensional technique that can probe multiple signaling nodes at the single-cell level, we interrogate TCR signaling dynamics in control C57BL/6 and autoimmunity-prone nonobese diabetic (NOD) mice, which show ineffective ERK activation after TCR triggering. By quantitating signals at multiple steps along the signaling cascade and parsing the phosphorylation level of each node as a function of its predecessors, we show that a small impairment in initial pCD3ζ activation resonates farther down the signaling cascade and results in larger defects in activation of the ERK1/2-S6 and IκBα modules. This nonlinear property of TCR signaling networks, which magnifies small initial differences during signal propagation, also applies in cells from B6 mice activated at different levels of intensity. Impairment in pCD3ζ and pSLP76 is not a feedback consequence of a primary deficiency in ERK activation because no proximal signaling defect was observed in Erk2 KO T cells. These defects, which were manifest at all stages of T-cell differentiation from early thymic pre-T cells to memory T cells, may condition the imbalanced immunoregulation and tolerance in NOD T cells. More generally, this amplification of small initial differences in signal intensity may explain how T cells discriminate between closely related ligands and adopt strongly delineated cell fates.

Advances in our understanding of the pathophysiology of Type 1 diabetes: lessons from the NOD mouse

T1D (Type 1 diabetes) is an autoimmune disease caused by the immune-mediated destruction of pancreatic β-cells. Studies in T1D patients have been limited by the availability of pancreatic samples, a protracted pre-diabetic phase and limitations in markers that reflect β-cell mass and function. The NOD (non-obese diabetic) mouse is currently the best available animal model of T1D, since it develops disease spontaneously and shares many genetic and immunopathogenic features with human T1D. Consequently, the NOD mouse has been extensively studied and has made a tremendous contribution to our understanding of human T1D. The present review summarizes the key lessons from NOD mouse studies concerning the genetic susceptibility, aetiology and immunopathogenic mechanisms that contribute to autoimmune destruction of β-cells. Finally, we summarize the potential and limitations of immunotherapeutic strategies, successful in NOD mice, now being trialled in T1D patients and individuals at risk of developing T1D.

B7x in the periphery abrogates pancreas-specific damage mediated by self-reactive CD8 T cells

B7x (B7-H4 or B7S1) is the seventh member of the B7 family, and its in vivo function remains largely unknown. Despite new genetic data linking the B7x gene with autoimmune diseases, how exactly it contributes to peripheral tolerance and autoimmunity is unclear. In this study, we showed that B7x protein was not detected on APCs or T cells in both human and mice, which is unique in the B7 family. Because B7x protein is expressed in some peripheral cells such as pancreatic β cells, we used a CD8 T cell-mediated diabetes model (AI4αβ) in which CD8 T cells recognize an endogenous self-Ag, and found that mice lacking B7x developed more severe diabetes than control AI4αβ mice. Conversely, mice overexpressing B7x in the β cells (Rip-B7xAI4αβ) were diabetes free. Furthermore, adoptive transfer of effector AI4αβ CD8 T cells induced diabetes in control mice, but not in Rip-B7xAI4αβ mice. Mechanistic studies revealed that pathogenic effector CD8 T cells were capable of migrating to the pancreas but failed to robustly destroy tissue when encountering local B7x in Rip-B7xAI4αβ mice. Although AI4αβ CD8 T cells in Rip-B7xAI4αβ and AI4αβ mice showed similar cytotoxic function, cell death, and global gene expression profiles, these cells had greater proliferation in AI4αβ mice than in RIP-B7xAI4αβ mice. These results suggest that B7x in nonlymphoid organs prevents peripheral autoimmunity partially through inhibiting proliferation of tissue-specific CD8 T cells, and that local overexpression of B7x on pancreatic β cells is sufficient to abolish CD8 T cell-induced diabetes.

Thymic negative selection is functional in NOD mice

Based on analyses of multiple TCR transgenic (tg) models, the emergence of pathogenic T cells in diabetes-prone NOD mice has been ascribed to a failure to censure autoreactive clones in the thymus. In contrast, using isolated and preselected thymocytes, we show that nonobese diabetic (NOD) genetic variation impairs neither clonal deletion nor downstream transcriptional programs. However, we find that NOD genetic variation influences αβ/γδ-lineage decisions promoted by early expression of tg αβ-TCRs at the double-negative (DN) stage. In B6 and other genetic backgrounds, tg αβ-TCRs behave like γδ-TCRs and commit a large fraction of DNs toward the γδ-lineage, thereby decreasing the size of the double-positive (DP) pool, which is efficiently positively and negatively selected. In NOD DNs, αβ-TCR signalosomes instead behave like pre-TCRs, resulting in high numbers of DPs competing for limited selection niches, and poor positive and negative selection. Once niche effects are neutralized in mixed bone marrow chimeras, positive and negative selection are equally efficient on B6 and NOD backgrounds. Biochemical analysis revealed a selective defect in the activation of Erk1/2 downstream of NOD αβ-TCR signalosomes. Therefore, NOD genetic variation influences αβ/γδ-lineage decisions when the αβ-TCR heterodimer is prematurely expressed, but not the process of negative selection.

Unmasking genes in a type 1 diabetes-resistant mouse strain that enhances pathogenic CD8 T-cell responses

OBJECTIVE

Nominally resistant mouse strains such as C57BL/6 (B6) harbor latent type 1 diabetes susceptibility genes uncovered in outcross to disease-susceptible NOD mice. However, identification of possible recessively acting B6-derived susceptibility genes is limited because very few F2 progeny derived from outcrossing this strain with NOD develop spontaneous autoimmune diabetes. Thus, we assessed whether a transgenic T-cell receptor (TCR) disease transfer model allowed the mapping of recessively acting B6 genetic loci that in the proper context contribute to diabetes.

RESEARCH DESIGN AND METHODS

CD8 T-cells transgenically expressing the diabetogenic AI4 TCR were transferred into 91 (NODxB6.H2(g7))F1xB6.H2(g7) first-backcross (BC1) females. A genome-wide scan was performed for loci affecting clinical diabetes and insulitis severity.

RESULTS

A major locus on chromosome 11 in tight linkage with the marker D11Mit48 (logarithm of odds score = 13.2) strongly determined whether BC1 progeny were susceptible to AI4 T-cell-mediated diabetes. Mice homozygous versus heterozygous for B6 markers of this chromosome 11 genetic locus were, respectively, highly susceptible or resistant to AI4-induced insulitis and diabetes. The genetic effect is manifest by host CD4 T-cells. Microarray analyses of mRNA transcript expression identified a limited number of candidate genes.

CONCLUSIONS

The distal region of chromosome 11 in B6 mice harbors a previously unrecognized recessively acting gene(s) that can promote autoreactive diabetogenic CD8 T-cell responses. Future identification of this gene(s) may further aid the screening of heterogeneous humans at future risk for diabetes, and might also provide a target for possible disease interventions.

Visualizing the role of Cbl-b in control of islet-reactive CD4 T cells and susceptibility to type 1 diabetes

The E3 ubiquitin ligase Cbl-b regulates T cell activation thresholds and has been associated with protecting against type 1 diabetes, but its in vivo role in the process of self-tolerance has not been examined at the level of potentially autoaggressive CD4(+) T cells. In this study, we visualize the consequences of Cbl-b deficiency on self-tolerance to lysozyme Ag expressed in transgenic mice under control of the insulin promoter (insHEL). By tracing the fate of pancreatic islet-reactive CD4(+) T cells in prediabetic 3A9-TCR × insHEL double-transgenic mice, we find that Cbl-b deficiency contrasts with AIRE or IL-2 deficiency, because it does not affect thymic negative selection of islet-reactive CD4(+) cells or the numbers of islet-specific CD4(+) or CD4(+)Foxp3(+) T cells in the periphery, although it decreased differentiation of inducible regulatory T cells from TGF-β-treated 3A9-TCR cells in vitro. When removed from regulatory T cells and placed in culture, Cblb-deficient islet-reactive CD4(+) cells reveal a capacity to proliferate to HEL Ag that is repressed in wild-type cells. This latent failure of T cell anergy is, nevertheless, controlled in vivo in prediabetic mice so that islet-reactive CD4(+) cells in the spleen and the pancreatic lymph node of Cblb-deficient mice show no evidence of increased activation or proliferation in situ. Cblb deficiency subsequently precipitated diabetes in most TCR:insHEL animals by 15 wk of age. These results reveal a role for peripheral T cell anergy in organ-specific self-tolerance and illuminate the interplay between Cblb-dependent anergy and other mechanisms for preventing organ-specific autoimmunity.

Invariant natural killer T-cell control of type 1 diabetes: a dendritic cell genetic decision of a silver bullet or Russian roulette

OBJECTIVE

In part, activation of invariant natural killer T (iNKT)-cells with the superagonist alpha-galactosylceramide (alpha-GalCer) inhibits the development of T-cell-mediated autoimmune type 1 diabetes in NOD mice by inducing the downstream differentiation of antigen-presenting dendritic cells (DCs) to an immunotolerogenic state. However, in other systems iNKT-cell activation has an adjuvant-like effect that enhances rather than suppresses various immunological responses. Thus, we tested whether in some circumstances genetic variation would enable activated iNKT-cells to support rather than inhibit type 1 diabetes development.

RESEARCH DESIGN AND METHODS

We tested whether iNKT-conditioned DCs in NOD mice and a major histocompatibility complex-matched C57BL/6 (B6) background congenic stock differed in capacity to inhibit type 1 diabetes induced by the adoptive transfer of pathogenic AI4 CD8 T-cells.

RESULTS

Unlike those of NOD origin, iNKT-conditioned DCs in the B6 background stock matured to a state that actually supported rather than inhibited AI4 T-cell-induced type 1 diabetes. The induction of a differing activity pattern of T-cell costimulatory molecules varying in capacity to override programmed death-ligand-1 inhibitory effects contributes to the respective ability of iNKT-conditioned DCs in NOD and B6 background mice to inhibit or support type 1 diabetes development. Genetic differences inherent to both iNKT-cells and DCs contribute to their varying interactions in NOD and B6.H2(g7) mice.

CONCLUSIONS

This great variability in the interactions between iNKT-cells and DCs in two inbred mouse strains should raise a cautionary note about considering manipulation of this axis as a potential type 1 diabetes prevention therapy in genetically heterogeneous humans.

Through regulation of TCR expression levels, an Idd7 region gene(s) interactively contributes to the impaired thymic deletion of autoreactive diabetogenic CD8+ T cells in nonobese diabetic mice

When expressed in NOD, but not C57BL/6 (B6) genetic background mice, the common class I variants encoded by the H2g7 MHC haplotype aberrantly lose the ability to mediate the thymic deletion of autoreactive CD8+ T cells contributing to type 1 diabetes (T1D). This indicated some subset of the T1D susceptibility (Idd) genes located outside the MHC of NOD mice interactively impair the negative selection of diabetogenic CD8+ T cells. In this study, using both linkage and congenic strain analyses, we demonstrate contributions from a polymorphic gene(s) in the previously described Idd7 locus on the proximal portion of Chromosome 7 predominantly, but not exclusively, determines the extent to which H2g7 class I molecules can mediate the thymic deletion of diabetogenic CD8+ T cells as illustrated using the AI4 TCR transgenic system. The polymorphic Idd7 region gene(s) appears to control events that respectively result in high vs low expression of the AI4 clonotypic TCR alpha-chain on developing thymocytes in B6.H2g7 and NOD background mice. This expression difference likely lowers levels of the clonotypic AI4 TCR in NOD, but not B6.H2g7 thymocytes, below the threshold presumably necessary to induce a signaling response sufficient to trigger negative selection upon Ag engagement. These findings provide further insight to how susceptibility genes, both within and outside the MHC, may interact to elicit autoreactive T cell responses mediating T1D development in both NOD mice and human patients.

Selective delivery of beta cell antigen to dendritic cells in vivo leads to deletion and tolerance of autoreactive CD8+ T cells in NOD mice

Type 1 diabetes (T1D) is an autoimmune disease resulting from defects in central and peripheral tolerance and characterized by T cell-mediated destruction of islet beta cells. Cytotoxic CD8(+) T cells, reactive to beta cell antigens, are required for T1D development in the NOD mouse model of the disease, and CD8(+) T cells specific for beta cell antigens can be detected in the peripheral blood of T1D patients. It has been evident that in nonautoimmune-prone mice, dendritic cells (DCs) present model antigens in a tolerogenic manner in the steady state, e.g., in the absence of infection, and cause T cells to proliferate initially but then to be deleted or rendered unresponsive. However, this fundamental concept has not been evaluated in the setting of a spontaneous autoimmune disease. To do so, we delivered a mimotope peptide, recognized by the diabetogenic CD8(+) T cell clone AI4, to DCs in NOD mice via the endocytic receptor DEC-205. Proliferation of transferred antigen-specific T cells was initially observed, but this was followed by deletion. Tolerance was achieved because rechallenge of mice with the mimotope peptide in adjuvant did not induce an immune response. Thus, targeting of DCs with beta cell antigens leads to deletion of autoreactive CD8(+) T cells even in the context of ongoing autoimmunity in NOD mice with known tolerance defects. Our results provide support for the development of DC targeting of self antigens for treatment of chronic T cell-mediated autoimmune diseases.

Impairment of organ-specific T cell negative selection by diabetes susceptibility genes: genomic analysis by mRNA profiling

BACKGROUND

T cells in the thymus undergo opposing positive and negative selection processes so that the only T cells entering circulation are those bearing a T cell receptor (TCR) with a low affinity for self. The mechanism differentiating negative from positive selection is poorly understood, despite the fact that inherited defects in negative selection underlie organ-specific autoimmune disease in AIRE-deficient people and the non-obese diabetic (NOD) mouse strain

RESULTS

Here we use homogeneous populations of T cells undergoing either positive or negative selection in vivo together with genome-wide transcription profiling on microarrays to identify the gene expression differences underlying negative selection to an Aire-dependent organ-specific antigen, including the upregulation of a genomic cluster in the cytogenetic band 2F. Analysis of defective negative selection in the autoimmune-prone NOD strain demonstrates a global impairment in the induction of the negative selection response gene set, but little difference in positive selection response genes. Combining expression differences with genetic linkage data, we identify differentially expressed candidate genes, including Bim, Bnip3, Smox, Pdrg1, Id1, Pdcd1, Ly6c, Pdia3, Trim30 and Trim12.

CONCLUSION

The data provide a molecular map of the negative selection response in vivo and, by analysis of deviations from this pathway in the autoimmune susceptible NOD strain, suggest that susceptibility arises from small expression differences in genes acting at multiple points in the pathway between the TCR and cell death.

"Humanized" HLA transgenic NOD mice to identify pancreatic beta cell autoantigens of potential clinical relevance to type 1 diabetes

The mechanistic basis by which the H2(g7) major histocompatibility complex (MHC) provides the primary risk factor for the development of T cell-mediated autoimmune type 1 diabetes (T1D) in non-obese diabetic (NOD) mice involves contributions not only from the unusual A(g7) class II molecule, but also from the more common K(d) and/or D(b) class I variants it encodes. Similarly, transgenic studies in NOD mice have confirmed the possibility first suggested in association studies that in the proper genetic context the common human HLA-A2.1 class I variant can mediate diabetogenic CD8 T cell responses. T1D continues to develop in a further refined NOD stock that expresses human HLA-A2.1, but no murine class I molecules (designated NOD.beta2m-.HHD). Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is an important antigenic target of diabetogenic CD8 cells in standard NOD mice. Three IGRP-derived peptides have also been identified that are presented by human HLA-A2.1 molecules to diabetogenic CD8 T cells in NOD.beta2m-.HHD mice. At least one of these IGRP peptides (265-273) can also be the target of autoreactive CD8 T cells in HLA-A2.1-expressing human T1D patients. Studies are currently under way to determine whether HLA-A2.1-restricted IGRP peptides can be used in a tolerance-inducing protocol that inhibits T1D development in NOD. beta2m-.HHD mice. If so, this knowledge could ultimately lead to the development of a similar T1D prevention protocol in humans.

Partial versus Full Allogeneic Hemopoietic Chimerization Is a Preferential Means to Inhibit Type 1 Diabetes as the Latter Induces Generalized Immunosuppression

In both humans and NOD mice, particular combinations of MHC genes provide the primary risk factor for development of the autoreactive T cell responses causing type 1 diabetes (T1D). Conversely, other MHC variants can confer dominant T1D resistance, and previous studies in NOD mice have shown their expression on hemopoietically derived APC is sufficient to induce disease protection. Although allogeneic hemopoietic chimerization can clearly provide a means for blocking T1D development, its clinical use for this purpose has been obviated by a requirement to precondition the host with what would be a lethal irradiation dose if bone marrow engraftment is not successful. There have been reports in which T1D-protective allogeneic hemopoietic chimerization was established in NOD mice that were preconditioned by protocols not including a lethal dose of irradiation. In most of these studies, virtually all the hemopoietic cells in the NOD recipients eventually converted to donor type. We now report that a concern about such full allogeneic chimeras is that they are severely immunocompromised potentially because their T cells are positively selected in the thymus by MHC molecules differing from those expressed by the APC available in the periphery to activate T cell effector functions. However, this undesirable side effect of generalized immunosuppression is obviated by a new protocol that establishes without a lethal preconditioning component, a stable state of mixed allogeneic hemopoietic chimerism sufficient to inhibit T1D development and also induce donor-specific tolerance in NOD recipients.

CD38 Is Required for the Peripheral Survival of Immunotolerogenic CD4+Invariant NK T Cells in Nonobese Diabetic Mice

T cell-mediated autoimmune type-1 diabetes (T1D) in NOD mice partly results from this strain's numerical and functional defects in invariant NK T (iNKT) cells. T1D is inhibited in NOD mice treated with the iNKT cell superagonist alpha-galactosylceramide through a process involving enhanced accumulation of immunotolerogenic dendritic cells in pancreatic lymph nodes. Conversely, T1D is accelerated in NOD mice lacking CD38 molecules that play a role in dendritic cell migration to inflamed tissues. Unlike in standard NOD mice, alpha-galactosylceramide pretreatment did not protect the CD38-deficient stock from T1D induced by an adoptively transferred pancreatic beta cell-autoreactive CD8 T cell clone (AI4). We found that in the absence of CD38, ADP-ribosyltransferase 2 preferentially activates apoptotic deletion of peripheral iNKT cells, especially the CD4+ subset. Therefore, this study documents a previously unrecognized role for CD38 in maintaining survival of an iNKT cell subset that preferentially contributes to the maintenance of immunological tolerance.

Lessons on autoimmune diabetes from animal models

T1DM (Type I diabetes mellitus) results from selective destruction of the insulin-producing beta-cells of the pancreas by the immune system, and is characterized by hyperglycaemia and vascular complications arising from suboptimal control of blood glucose levels. The discovery of animal models of T1DM in the late 1970s and early 1980s, particularly the NOD (non-obese diabetic) mouse and the BB (BioBreeding) diabetes-prone rat, had a fundamental impact on our ability to understand the genetics, aetiology and pathogenesis of this disease. NOD and BB diabetes-prone rats spontaneously develop a form of diabetes that closely resembles the human counterpart. Early studies of these animals quickly led to the realization that T1DM is caused by autoreactive T-lymphocytes and revealed that the development of T1DM is controlled by numerous polymorphic genetic elements that are scattered throughout the genome. The development of transgenic and gene-targeting technologies during the 1980s allowed the generation of models of T1DM of reduced genetic and pathogenic complexity, and a more detailed understanding of the immunogenetics of T1DM. In this review, we summarize the contribution of studies in animal models of T1DM to our current understanding of four fundamental aspects of T1DM: (i) the nature of genetic elements affording T1DM susceptibility or resistance; (ii) the mechanisms underlying the development and recruitment of pathogenic autoreactive T-cells; (iii) the identity of islet antigens that contribute to the initiation and/or progression of islet inflammation and beta-cell destruction; and (iv) the design of avenues for therapeutic intervention that are rooted in the knowledge gained from studies of animal models. Development of new animal models will ensure continued progress in these four areas.

HLA-A*0201-restricted T cells from humanized NOD mice recognize autoantigens of potential clinical relevance to type 1 diabetes

In both humans and NOD mice, particular MHC genes are primary contributors to development of the autoreactive CD4+ and CD8+ T cell responses against pancreatic beta cells that cause type 1 diabetes (T1D). Association studies have suggested, but not proved, that the HLA-A*0201 MHC class I variant is an important contributor to T1D in humans. In this study, we show that transgenic expression in NOD mice of HLA-A*0201, in the absence of murine class I MHC molecules, is sufficient to mediate autoreactive CD8+ T cell responses contributing to T1D development. CD8+ T cells from the transgenic mice are cytotoxic to murine and human HLA-A*0201-positive islet cells. Hence, the murine and human islets must present one or more peptides in common. Islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP) is one of several important T1D autoantigens in standard NOD mice. Three IGRP-derived peptides were identified as targets of diabetogenic HLA-A*0201-restricted T cells in our NOD transgenic stock. Collectively, these results indicate the utility of humanized HLA-A*0201-expressing NOD mice in the identification of T cells and autoantigens of potential relevance to human T1D. In particular, the identified antigenic peptides represent promising tools to explore the potential importance of IGRP in the development of human T1D.

The why and how of thymocyte negative selection

The generation of T cell receptor (TCR) sequence diversity is the strength of adaptive immunity, yet is also the Achilles' heel. To purge highly self-reactive T cells from the immune system, generation of diversity has coevolved with a mechanism of negative selection. Recent studies have revealed new insights addressing the why and how of negative selection by examining situations in which negative selection has failed in human and animals models of autoimmunity. Both thymocyte extrinsic and intrinsic mechanisms are required to restrict the TCR repertoire to a non-autoreactive set. Negative selection also ensures that T cells emerge with receptors that are focussed on the peptide moiety of MHC-peptide complexes.

How defects in central tolerance impinge on a deficiency in regulatory T cells

Both central (thymic) and peripheral (nonthymic) mechanisms are important for the induction and maintenance of T cell tolerance. Mice with a defect in Foxp3, required for the generation and activity of CD4(+)CD25(+) regulatory T cells, exhibit massive lymphoproliferation and severe inflammatory infiltration of multiple organs, in particular the lungs, liver, and skin. We have explored how this phenotype is influenced by an additional defect in central tolerance induction, generated by either crossing in a null mutation of the Aire gene or substituting the nonobese diabetic (NOD) genetic background. The double-deficient mice had fulminant autoimmunity in very early life and a gravely shortened lifespan vis-à-vis single-deficient littermates. They showed massive lymphoproliferation and exacerbated inflammatory damage, particularly in the lungs and liver. Yet, the range of affected sites was not noticeably extended, and, surprisingly, many organs, or regions of organs, remained untouched, suggesting additional important mechanisms to enforce immunological self-tolerance.

Diabetes Resistance/Susceptibility in T Cells of Nonobese Diabetic Mice Conferred by MHC and MHC-Linked Genes

Polymorphism of MHC and MHC-linked genes is tightly associated with susceptibility to type 1 diabetes (T1D) in human and animal models. Despite the extensive studies, however, the role of MHC and MHC-linked genes expressed by T cells on T1D susceptibility remains unclear. Because T cells develop from TCR(-) thymic precursor (pre-T) cells that undergo MHC restriction mediated by thymic stroma cells, we reconstituted the T cell compartment of NOD.scid-RIP-B7.1 mice using pre-T cells isolated from NOD, NOR, AKR, and C57BL/6 (B6) mice. T1D developed rapidly in the mice reconstituted with pre-T cells derived from NOD or NOR donors. In contrast, most of the NOD.scid-RIP-B7.1 mice reconstituted with pre-T cells from AKR or B6 donors were free of T1D. Further analysis revealed that genes within MHC locus of AKR or B6 origin reduced incidence of T1D in the reconstituted NOD.scid-RIP-B7.1 mice. The expression of MHC class I genes of k, but not b haplotype, in T cells conferred T1D resistance. Replacement of an interval near the distal end of the D region in T cells of B6 origin with an identical allele of 129.S6 origin resulted in T1D development in the reconstituted mice. These results provide evidence that the expression of MHC class I and MHC-linked genes in T cells of NOD mice indeed contributes to T1D susceptibility, while expression of specific resistance alleles of MHC or MHC-linked genes in T cells alone would effectively reduce or even prevent T1D.

The autoimmune contrivance: genetics in the mouse model

Autoimmunity and inheritance of complex characters behold an explosive interest in biology over the last 15 years. Research in the genetics of autoimmunity has been impelled by the isolation of genetic markers allowing tracing of heredity. The annotation and sequencing of the human and mouse genomes provide with the potential for further advancements, through the development of new technologies. This review aims to summarize advances made in the autoimmunity field, centered in type 1 diabetes in the NOD mouse model. It also aims to demonstrate that animal models, albeit some phenotypic and genetic dissimilarities with the human diseases, still remain the best way to move towards an understanding of the molecular mechanisms involved in autoimmunity. Assessing the current state of research in this field together with the increasing potential of novel biotechnology advancements, new insights to disease pathogenesis and discovery of molecular targets for intervention strategies are anticipated in the coming years.

Modifier loci condition autoimmunity provoked by Aire deficiency

Loss of function mutations in the autoimmune regulator (Aire) gene in autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy patients and mutant mice lead to autoimmune manifestations that segregate as a monogenic trait, but with wide variation in the spectrum of organs targeted. To investigate the cause of this variability, the Aire knockout mutation was backcrossed to mice of diverse genetic backgrounds. The background loci strongly influenced the pattern of organs that were targeted (stomach, eye, pancreas, liver, ovary, thyroid, and salivary gland) and the severity of the targeting (particularly strong on the nonobese diabetic background, but very mild on the C57BL/6 background). Autoantibodies mimicked the disease pattern, with oligoclonal reactivity to a few antigens that varied between Aire-deficient strains. Congenic analysis and a whole genome scan showed that autoimmunity to each organ had a distinctive pattern of genetic control and identified several regions that controlled the pattern of targeting, including the major histocompatibility complex and regions of Chr1 and Chr3 previously identified in controlling type 1 diabetes.

Of mice and men: use of animal models to identify possible interventions for the prevention of autoimmune type 1 diabetes in humans

Animal model and clinical studies indicate that type 1 diabetes (T1D) results from T cell-mediated autoimmune destruction of insulin-producing pancreatic beta-cells. This review discusses the knowledge gained from animal models about the nature of the autoreactive T cells that cause T1D and the possible basis for their development. Based on this information, the possible positive and negative aspects of various antigen-specific and non-specific immunotherapies, which could potentially prevent the onset of T1D in at risk individuals, are discussed.

Novel leptin receptor mutation in NOD/LtJ mice suppresses type 1 diabetes progression: I. Pathophysiological analysis

A spontaneous single-base mutation in the leptin receptor of type 1 diabetes-prone NOD/LtJ mice (designated as Lepr(db-5J)) produced a glycine640valine transversion in the extracellular domain. All mutant mice became obese and hyperinsulinemic at weaning, with 70-80% developing early-onset hyperglycemia. However, these obese diabetic mice continued to gain weight without insulin therapy. Spontaneous diabetes remission was observed in all obese females and a subset of obese males. Insulitis was largely limited to islet perimeters, with intraislet insulitis infrequently observed. In 17 obese males (age 39 weeks), we observed phenotypic heterogeneity, including full remission from hyperglycemia (24%), intermediate hyperglycemia with elevated body weight (41%), and severe hyperglycemia and weight loss (35%). The remitting normoglycemic and intermediate hyperglycemic phenotypes were associated with extensive beta-cell hyperplasia. Unlike the extensive intraislet insulitis present in diabetic lean NOD/Lt mice, the severe obese diabetic phenotype was associated with islet atrophy without extensive intraislet insulitis. These results indicated that the manipulation of the leptin/leptin receptor axis may provide a novel means of downregulating autoimmunity in type 1 diabetes and confirmed a role for leptin as a mediator in the development of this disease in NOD mice.

The good turned ugly: immunopathogenic basis for diabetogenic CD8+ T cells in NOD mice

Type 1 diabetes (T1D) in both humans and nonobese diabetic (NOD) mice is a T-cell-mediated autoimmune disease in which the insulin-producing pancreatic islet beta-cells are selectively eliminated. As a result, glucose metabolism cannot be regulated unless exogenous insulin is administered. Both the CD4(+) and the CD8(+) T-cell subsets are required for T1D development. Approximately 20 years ago, an association between certain class II major histocompatibility complex (MHC) alleles and susceptibility to T1D was reported. This finding led to enormous interest in the CD4(+) T cells participating in the development of T1D, while the CD8(+) subset was relatively ignored. However, the isolation of beta-cell-autoreactive CD8(+) T-cell clones from the islets of NOD mice helped to generate interest in the pathogenic role of this subset, as has accumulating evidence that certain class I MHC alleles are additional risk factors for T1D development in humans. Three distinct diabetogenic CD8(+) T-cell populations have now been characterized in NOD mice. Here, we review recent investigations exploring their selection, activation, trafficking, and antigenic specificities. As CD8(+) T cells are suspected contributors to beta-cell demise in humans, continued exploration of these critical areas could very possibly lead to tangible benefits for T1D patients and at-risk individuals.

Cellular and genetic mechanisms of self tolerance and autoimmunity

The mammalian immune system has an extraordinary potential for making receptors that sense and neutralize any chemical entity entering the body. Inevitably, some of these receptors recognize components of our own body, and so cellular mechanisms have evolved to control the activity of these 'forbidden' receptors and achieve immunological self tolerance. Many of the genes and proteins involved are conserved between humans and other mammals. This provides the bridge between clinical studies and mechanisms defined in experimental animals to understand how sets of gene products coordinate self-tolerance mechanisms and how defects in these controls lead to autoimmune disease.

Single cell analysis shows decreasing FoxP3 and TGFbeta1 coexpressing CD4+CD25+ regulatory T cells during autoimmune diabetes

Natural CD4⁺CD25⁺ regulatory T (CD4⁺CD25⁺ T reg) cells play a key role in the immunoregulation of autoimmunity. However, little is known about the interactions between CD4⁺CD25⁺ T reg cells and autoreactive T cells. This is due, in part, to the difficulty of using cell surface markers to identify CD4⁺CD25⁺ T reg cells accurately. Using a novel real-time PCR assay, mRNA copy number of FoxP3, TGFβ1, and interleukin (IL)-10 was measured in single cells to characterize and quantify CD4⁺CD25⁺ T reg cells in the nonobese diabetic (NOD) mouse, a murine model for type 1 diabetes (T1D). The suppressor function of CD4⁺CD25⁺CD62L^hi T cells, mediated by TGFβ, declined in an age-dependent manner. This loss of function coincided with a temporal decrease in the percentage of FoxP3 and TGFβ1 coexpressing T cells within pancreatic lymph node and islet infiltrating CD4⁺CD25⁺CD62L^hi T cells, and was detected in female NOD mice but not in NOD male mice, or NOR or C57BL/6 female mice. These results demonstrate that the majority of FoxP3-positive CD4⁺CD25⁺ T reg cells in NOD mice express TGFβ1 but not IL-10, and that a defect in the maintenance and/or expansion of this pool of immunoregulatory effectors is associated with the progression of T1D.

Genetic lesions in T-cell tolerance and thresholds for autoimmunity

The cause of common organ-specific autoimmune diseases is poorly understood because of genetic and cellular complexity in humans and animals. Recent advances in the understanding of the mechanisms of the defects underlying autoimmune disease in autoimmune polyendocrinopathy syndrome type 1 and non-obese diabetic mice suggest that failures in central tolerance play a key role in predisposition towards organ-specific autoimmunity. The lessons from such rare monogenic autoimmune disorders and well-characterized polygenic traits demonstrate how subtle quantitative trait loci can result in large changes in the susceptibility to autoimmunity. These data allow us to propose a model relating efficiency of thymic deletion to T-cell tolerance and susceptibility to autoimmunity.

Nonobese diabetic mice and the genetics of diabetes susceptibility

The nonobese diabetic mouse spontaneously develops an autoimmune, T-cell-mediated type 1 diabetes (T1D). Common and rare alleles both within a diabetogenic major histocompatibility complex (MHC) and multiple non-MHC genes combine to impair normal communication between the innate and acquired immune system, leading to loss of immune tolerance. An understanding of how variable collections of genes interact with each other and with environmental cues offers important insights as to the complexities of T1D inheritance in humans.

Generalized resistance to thymic deletion in the NOD mouse; a polygenic trait characterized by defective induction of Bim

The cause of common polygenic autoimmune diseases is not understood because of genetic and cellular complexity. Here, we pinpoint the action of a subset of autoimmune susceptibility loci in the NOD mouse strain linked to D1mit181, D2mit490, D7mit101, and D15mit229, which cause a generalized resistance to thymic deletion in vivo that applies equally to Aire-induced organ-specific gene products in the thymic medulla and to systemic antigens expressed at high levels throughout the thymus and affects CD4(+), CD4(+)8(+), and CD4(+)25(+) thymocytes. Resistance to thymic deletion does not reflect a general deficit in TCR signaling to calcineurin- or ERK-induced genes, imbalance in constitutive regulators of apoptosis, nor excessive signaling to prosurvival genes but is distinguished by failure to induce the proapoptotic gene and protein, Bim, during in vivo encounter with high-avidity autoantigen. These findings establish defects in thymic deletion and Bim induction as a key mechanism in the pathogenesis of autoimmunity.