Bingye Han

Identification of the β cell antigen targeted by a prevalent population of pathogenic CD8+ T cells in autoimmune diabetes

Type 1 diabetes is an autoimmune disease in which autoreactive T cells attack and destroy the insulin-producing pancreatic β cells. CD8+ T cells are essential for this β cell destruction, yet their specific antigenic targets are largely unknown. Here, we reveal that the autoantigen targeted by a prevalent population of pathogenic CD8+ T cells in nonobese diabetic mice is islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP). Through tetramer technology, IGRP-reactive T cells are readily detected in islets and peripheral blood directly ex vivo. The human IGRP gene maps to a diabetes susceptibility locus, suggesting that IGRP also may be an antigen for pathogenic T cells in human type 1 diabetes and, thus, a new, potential target for diagnostic and therapeutic approaches.

CD40 Ligation Releases Immature Dendritic Cells from the Control of Regulatory CD4+CD25+ T Cells

We report that disruption of CD154 in nonobese diabetic (NOD) mice abrogates the helper function of CD4+CD25- T cells without impairing the regulatory activity of CD4+CD25+ T cells. Whereas CD4+ T cells from NOD mice enhanced a diabetogenic CD8+ T cell response in monoclonal TCR-transgenic NOD mice, CD4+ T cells from NOD.CD154(-/-) mice actively suppressed it. Suppression was mediated by regulatory CD4+CD25+ T cells capable of inhibiting CD8+ T cell responses induced by peptide-pulsed dendritic cells (DCs), but not peptide/MHC monomers. It involved inhibition of DC maturation, did not occur in the presence of CD154+ T-helper cells, and could be inhibited by activation of DCs with LPS, CpG DNA, or an agonistic anti-CD40 mAb. Thus, in at least some genetic backgrounds, CD154-CD40 interactions and innate stimuli release immature DCs from suppression by CD4+CD25+ T cells.

Prevention of diabetes by manipulation of anti-IGRP autoimmunity: high efficiency of a low-affinity peptide

Antigen therapy may hold great promise for the prevention of autoimmunity; however, most clinical trials have failed, suggesting that the principles guiding the choice of treatment remain ill defined. Here, we examine the antidiabetogenic properties of altered peptide ligands of CD8+ T cells recognizing an epitope of islet-specific glucose-6-phosphatase catalytic subunit–related protein (IGRP206–214), a prevalent population of autoreactive T cells in autoimmune diabetes. We show that islet-associated CD8+ T cells in nonobese diabetic mice recognize numerous IGRP epitopes, and that these cells have a role in the outcome of protocols designed to induce IGRP206–214-specific tolerance. Ligands targeting IGRP206–214-reactive T cells prevented disease, but only at doses that spared low-avidity clonotypes. Notably, near complete depletion of the IGRP206–214-reactive T-cell pool enhanced the recruitment of subdominant specificities and did not blunt diabetogenesis. Thus, peptide therapy in autoimmunity is most effective under conditions that foster occupation of the target organ lymphocyte niche by nonpathogenic, low-avidity clonotypes.

Individual Nonobese Diabetic Mice Exhibit Unique Patterns of CD8+ T Cell Reactivity to Three Islet Antigens, Including the Newly Identified Widely Expressed Dystrophia Myotonica Kinase

Spontaneous autoimmune diabetes development in NOD mice requires both CD8+ and CD4+ T cells. Three pathogenic CD8+ T cell populations (represented by the G9C8, 8.3, and AI4 clones) have been described. Although the Ags for G9C8 and 8.3 are known to be insulin and islet-specific glucose-6-phosphatase catalytic subunit-related protein, respectively, only mimotope peptides had previously been identified for AI4. In this study, we used peptide/MHC tetramers to detect and quantify these three pathogenic populations among β cell-reactive T cells cultured from islets of individual NOD mice. Even within age-matched groups, each individual mouse exhibited a unique distribution of β cell-reactive CD8+ T cells, both in terms of the number of tetramer-staining populations and the relative proportion of each population in the islet infiltrate. Thus, the inflammatory process in each individual follows its own distinctive course. Screening of a combinatorial peptide library in positional scanning format led to the identification of a peptide derived from dystrophia myotonica kinase (DMK) that is recognized by AI4-like T cells. Importantly, the antigenic peptide is naturally processed and presented by DMK-transfected cells. DMK is a widely expressed protein that is nonetheless the target of a β cell-specific autoimmune response.

Developmental control of CD8+ T cell–avidity maturation in autoimmune diabetes

The progression of immune responses is generally associated with an increase in the overall avidity of antigen-specific T cell populations for peptide-MHC. This is thought to result from preferential expansion of high-avidity clonotypes at the expense of their low-avidity counterparts. Since T cell antigen-receptor genes do not mutate, it is puzzling that high-avidity clonotypes do not predominate from the outset. Here we provide a developmental basis for this phenomenon in the context of autoimmunity. We have carried out comprehensive studies of the diabetogenic CD8 T cell population that targets residues 206-214 of the beta cell antigen islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP(206-214)) and undergoes avidity maturation as disease progresses. We find that the succession of IGRP(206-214)-specific clonotypes with increasing avidities during the progression of islet inflammation to overt diabetes in nonobese diabetic mice is fueled by autoimmune inflammation but opposed by systemic tolerance. As expected, naive high-avidity IGRP(206-214)-specific T cells respond more efficiently to antigen and are significantly more diabetogenic than their intermediate- or low-avidity counterparts. However, central and peripheral tolerance selectively limit the contribution of these high-avidity T cells to the earliest stages of disease without abrogating their ability to progressively accumulate in inflamed islets and kill beta cells. These results illustrate the way in which incomplete deletion of autoreactive T cell populations of relatively high avidity can contribute to the development of pathogenic autoimmunity in the periphery.

Cross-Priming of Diabetogenic T Cells Dissociated from CTL-Induced Shedding of β Cell Autoantigens

Cross-presentation of self Ags by APCs is key to the initiation of organ-specific autoimmunity. As MHC class I molecules are essential for the initiation of diabetes in nonobese diabetic (NOD) mice, we sought to determine whether the initial insult that allows cross-presentation of β cell autoantigens in diabetes is caused by cognate interactions between naive CD8+ T cells and β cells. Naive splenic CD8+ T cells from transgenic NOD mice expressing a diabetogenic TCR killed peptide-pulsed targets in the absence of APCs. To ascertain the role of CD8+ T cell-induced β cell lysis in the initiation of diabetes, we expressed a rat insulin promoter (RIP)-driven adenovirus E19 transgene in NOD mice. RIP-E19 expression inhibited MHC class I transport exclusively in β cells and rendered these cells resistant to lysis by CD8+ (but not CD4+) T cells, both in vitro and in vivo. Surprisingly, RIP-E19 expression impaired the accumulation of CD8+ T cells in islets and delayed the onset of islet inflammation, without affecting the timing or magnitude of T cell cross-priming in the pancreatic lymph nodes, which is the earliest known event in diabetogenesis. These results suggest that access of β cell autoantigens to the cross-presentation pathway in diabetes is T cell independent, and reveal a previously unrecognized function of MHC class I molecules on target cells in autoimmunity: local retention of disease-initiating clonotypes.

RAG-dependent peripheral T cell receptor diversification in CD8+ T lymphocytes

Rearrangement of T cell receptor (TCR) genes is driven by transient expression of V(D)J recombination-activating genes (RAGs) during lymphocyte development. Immunological dogma holds that T cells irreversibly terminate RAG expression before exiting the thymus, and that all of the progeny arising from mature T cells express the parental TCRs. When single pancreatic islet-derived, NRP-A7 peptide-reactive CD8+ T cells from nonobese diabetic (NOD) mice were repeatedly stimulated with peptide-pulsed dendritic cells, daughter T cells reexpressed RAGs, lost their ability to bind to NRP-A7/Kd tetramers, ceased to transcribe tetramer-specific TCR genes, and, instead, expressed a vast array of other TCR rearrangements. Pancreatic lymph node (PLN) CD8+ T cells from animals expressing a transgenic NRP-A7-reactive TCR transcribed and translated RAGs in vivo and displayed endogenous TCRs on their surface. RAG reexpression also occurred in the PLN CD8+ T cells of wild-type NOD mice and could be induced in the peripheral CD8+ T cells of nondiabetes-prone TCR-transgenic B10.H2g7 mice by stimulation with peptide-pulsed dendritic cells. In contrast, reexpression of RAGs could not be induced in the CD8+ T cells of B6 mice expressing an ovalbumin-specific, Kb-restricted TCR, or in the CD8+ T cells of NOD mice expressing a lymphocytic choriomeningitis virus-specific, Db-restricted TCR. Extra-thymic reexpression of the V(D)J recombination machinery in certain CD8+ T cell subpopulations, therefore, enables further diversification of the peripheral T cell repertoire.

CD154-Dependent Priming of Diabetogenic CD4+ T Cells Dissociated from Activation of Antigen-Presenting Cells

We followed the fate of K(d)- or I-A(g7)-restricted beta cell-autoreactive T cells in monoclonal TCR-transgenic NOD mice expressing or lacking CD154. 8.3-NOD.RAG-2(-/-)/CD154(-/-) mice, which bear autoreactive CD8(+) T cells, developed diabetes with the same incidence and tempo as 8.3-NOD.RAG-2(-/-)/CD154(+) mice. Recruitment of CD154(-/-) 8.3-CD8(+) CTL was accelerated by CD154(+)CD4(+) T cells, by expression of a B7.1 transgene in beta cells or by treatment of the mice with CpG-DNA or an agonistic anti-CD40 antibody. In contrast, the autoreactive CD4(+) T cells maturing in 4.1-NOD.RAG-2(-/-) mice lost their diabetogenic potential if they lacked CD154, even in the presence of CD154(+)CD4(+) T cells, B7.1 molecules on beta cells, CpG-DNA treatment, or systemic CD40 ligation. These results demonstrate the existence of a novel, CD154-dependent pathway of CD4(+) T cell activation that is independent of CD40-mediated activation of APCs.

Requirement for Both H-2Db and H-2Kd for the Induction of Diabetes by the Promiscuous CD8+ T Cell Clonotype AI4

The NOD mouse is a model for autoimmune type 1 diabetes in humans. CD8+ T cells are essential for the destruction of the insulin-producing pancreatic β cells characterizing this disease. AI4 is a pathogenic CD8+ T cell clone, isolated from the islets of a 5-wk-old female NOD mouse, which is capable of mediating overt diabetes in the absence of CD4+ T cell help. Recent studies using MHC-congenic NOD mice revealed marked promiscuity of the AI4 TCR, as the selection of this clonotype can be influenced by multiple MHC molecules, including some class II variants. The present work was designed, in part, to determine whether similar promiscuity also characterizes the effector function of mature AI4 CTL. Using splenocyte and bone marrow disease transfer models and in vitro islet-killing assays, we report that efficient recognition and destruction of β cells by AI4 requires the β cells to simultaneously express both H-2Db and H-2Kd class I MHC molecules. The ability of the AI4 TCR to interact with both H-2Db and H-2Kd was confirmed using recombinant peptide libraries. This approach also allowed us to define a mimotope peptide recognized by AI4 in an H-2Db-restricted manner. Using ELISPOT and mimotope/H-2Db tetramer analyses, we demonstrate for the first time that AI4 represents a readily detectable T cell population in the islet infiltrates of prediabetic NOD mice. Our identification of a ligand for AI4-like T cells will facilitate further characterization and manipulation of this pathogenic and promiscuous T cell population.

Autoantigen recognition is required for recruitment of IGRP(206-214)-autoreactive CD8+ T cells but is dispensable for tolerance.

The progression of autoimmune responses is associated with an avidity maturation process driven by preferential expansion of high avidity clonotypes at the expense of their low avidity counterparts. Central and peripheral tolerance hinder the contribution of high-avidity clonotypes targeting residues 206–214 of islet-specific glucose-6-phosphatase catalytic subunit-related protein (IGRP206–214) during the earliest stages of autoimmune diabetes. In this study, we probe the molecular determinants and biochemical consequences of IGRP206–214/Kd recognition by high-, intermediate-, and low-avidity autoreactive CD8+ T cells, and we investigate the effects of genetic IGRP206–214 silencing on their developmental biology. We find that differences in avidity for IGRP206–214/Kd map to CDR1α and are associated with quantitative differences in CD3ε proline-rich sequence exposure and Nck recruitment. Unexpectedly, we find that tolerance of high-avidity CD8+ T cells, unlike their activation and recruitment into the pancreas, is dissociated from recognition of IGRP206–214, particularly in adult mice. This finding challenges the view that tolerance of pathogenic autoreactive T cells is invariably triggered by recognition of the peptide–MHC complex that drives their activation in the periphery, indicating the existence of mechanisms of tolerance that are capable of sensing the avidity, hence pathogenicity of autoreactive T cells without the need to rely on local autoantigen availability.