Do the peptide-binding properties of diabetogenic class II molecules explain autoreactivity?

Enhancing Antigen Presentation and Inducing Antigen-Specific Immune Tolerance with Amphiphilic Peptides

Ag-specific immunotherapy to restore immune tolerance to self-antigens, without global immune suppression, is a long-standing goal in the treatment of autoimmune disorders such as type 1 diabetes (T1D). However, vaccination with autoantigens such as insulin or glutamic acid decarboxylase have largely failed in human T1D trials. Induction and maintenance of peripheral tolerance by vaccination requires efficient autoantigen presentation by APCs. In this study, we show that a lipophilic modification at the N-terminal end of CD4⁺ epitopes (lipo-peptides) dramatically improves peptide Ag presentation. We designed amphiphilic lipo-peptides to efficiently target APCs in the lymph nodes by binding and trafficking with endogenous albumin. Additionally, we show that lipophilic modification anchors the peptide into the membranes of APCs, enabling a bivalent cell-surface Ag presentation. The s.c. injected lipo-peptide accumulates in the APCs in the lymph node, enhances the potency and duration of peptide Ag presentation by APCs, and induces Ag-specific immune tolerance that controls both T cell- and B cell-mediated immunity. Immunization with an amphiphilic insulin B chain 9-23 peptide, an immunodominant CD4⁺ T cell epitope in NOD mice, significantly suppresses the activation of T cells, increases inhibitory cytokine production, induces regulatory T cells, and delays the onset and lowers the incidence of T1D. Importantly, treatment with a lipophilic β-cell peptide mixture delays progression to end-stage diabetes in acutely diabetic NOD mice, whereas the same doses of standard soluble peptides were not effective. Amphiphilic modification effectively enhances Ag presentation for peptide-based immune regulation of autoimmune diseases.

The proinflammatory effects of macrophage-derived NADPH oxidase function in autoimmune diabetes

Type 1 diabetes (T1D) is an autoimmune disease culminating in the destruction of insulin-producing pancreatic β-cells. While ultimately a T cell-mediated disease, macrophages play an indispensable role in disease initiation and progression. Infiltrating macrophages generate an inflammatory environment by releasing NADPH oxidase-derived superoxide and proinflammatory cytokines. The synthesis of reactive oxygen species (ROS) is acknowledged as putative factors contributing to autoimmunity and β-cell damage in T1D. In addition to direct lysis, free radicals collectively participate in β-cell destruction by providing a redox-dependent third signal necessary for islet-reactive CD4 and CD8 T cell maturation and by inducing oxidative post-translational modifications of β-cell epitopes to further exacerbate autoimmune responses. This review will provide an overview of macrophage function and a synergistic cross-talk with redox biology that contributes to autoimmune dysregulation in T1D.

Human leukocyte Antigen-DM polymorphisms in autoimmune diseases

Classical MHC class II (MHCII) proteins present peptides for CD4⁺ T-cell surveillance and are by far the most prominent risk factor for a number of autoimmune disorders. To date, many studies have shown that this link between particular MHCII alleles and disease depends on the MHCII's particular ability to bind and present certain peptides in specific physiological contexts. However, less attention has been paid to the non-classical MHCII molecule human leucocyte antigen-DM, which catalyses peptide exchange on classical MHCII proteins acting as a peptide editor. DM function impacts the presentation of both antigenic peptides in the periphery and key self-peptides during T-cell development in the thymus. In this way, DM activity directly influences the response to pathogens, as well as mechanisms of self-tolerance acquisition. While decreased DM editing of particular MHCII proteins has been proposed to be related to autoimmune disorders, no experimental evidence for different DM catalytic properties had been reported until recently. Biochemical and structural investigations, together with new animal models of loss of DM activity, have provided an attractive foundation for identifying different catalytic efficiencies for DM allotypes. Here, we revisit the current knowledge of DM function and discuss how DM function may impart autoimmunity at the organism level.

A Perspective on the Role of Computational Models in Immunology

This is an exciting time for immunology because the future promises to be replete with exciting new discoveries that can be translated to improve health and treat disease in novel ways. Immunologists are attempting to answer increasingly complex questions concerning phenomena that range from the genetic, molecular, and cellular scales to that of organs, whole animals or humans, and populations of humans and pathogens. An important goal is to understand how the many different components involved interact with each other within and across these scales for immune responses to emerge, and how aberrant regulation of these processes causes disease. To aid this quest, large amounts of data can be collected using high-throughput instrumentation. The nonlinear, cooperative, and stochastic character of the interactions between components of the immune system as well as the overwhelming amounts of data can make it difficult to intuit patterns in the data or a mechanistic understanding of the phenomena being studied. Computational models are increasingly important in confronting and overcoming these challenges. I first describe an iterative paradigm of research that integrates laboratory experiments, clinical data, computational inference, and mechanistic computational models. I then illustrate this paradigm with a few examples from the recent literature that make vivid the power of bringing together diverse types of computational models with experimental and clinical studies to fruitfully interrogate the immune system.

Hydrophobic CDR3 residues promote the development of self-reactive T cells

Studies of individual T cell antigen receptors (TCRs) have shed some light on structural features that underlie self-reactivity. However, the general rules that can be used to predict whether TCRs are self-reactive have not been fully elucidated. Here we found that the interfacial hydrophobicity of amino acids at positions 6 and 7 of the complementarity-determining region CDR3β robustly promoted the development of self-reactive TCRs. This property was found irrespective of the member of the β-chain variable region (Vβ) family present in the TCR or the length of the CDR3β. An index based on these findings distinguished Vβ2(+), Vβ6(+) and Vβ8.2(+) regulatory T cells from conventional T cells and also distinguished CD4(+) T cells selected by the major histocompatibility complex (MHC) class II molecule I-A(g7) (associated with the development of type 1 diabetes in NOD mice) from those selected by a non-autoimmunity-promoting MHC class II molecule I-A(b). Our results provide a means for distinguishing normal T cell repertoires versus autoimmunity-prone T cell repertoires.

Beta cell antigens in type 1 diabetes: triggers in pathogenesis and therapeutic targets

Research focusing on type 1 diabetes (T1D) autoantigens aims to explore our understanding of these beta cell proteins in order to design assays for monitoring the pathogenic autoimmune response, as well as safe and efficient therapies preventing or stopping it. In this review, we will discuss progress made in the last 5 years with respect to mechanistic understanding, diagnostic monitoring, and therapeutic modulation of the autoantigen-specific cellular immune response in T1D. Some technical progress in monitoring tools has been made; however, the potential of recent technologies for highly multiplexed exploration of human cellular immune responses remains to be exploited in T1D research, as it may be the key to the identification of surrogate markers of disease progression that are still wanting. Detailed analysis of autoantigen recognition by T cells suggests an important role of non-conventional antigen presentation and processing in beta cell-directed autoimmunity, but the impact of this in human T1D has been little explored. Finally, therapeutic administration of autoantigens to T1D patients has produced disappointing results. The application of novel modes of autoantigen administration, careful translation of mechanistic understanding obtained in preclinical studies and in vitro with human cells, and combination therapies including CD3 antibodies may help to make autoantigen-based immunotherapy for T1D a success story in the future.

Endoplasmic reticulum targeting alters regulation of expression and antigen presentation of proinsulin

Peptide ligands presented by MHC class I (MHC-I) molecules are produced by degradation of cytosolic and nuclear, but also endoplasmic reticulum (ER)-resident, proteins by the proteasome. However, Ag processing of ER proteins remains little characterized. Studying processing and presentation of proinsulin, which plays a pivotal role in autoimmune diabetes, we found that targeting to the ER has profound effects not only on how proinsulin is degraded, but also on regulation of its cellular levels. While proteasome inhibition inhibited degradation and presentation of cytosolic proinsulin, as expected, it reduced the abundance of ER-targeted proinsulin. This targeting and protein modifications modifying protein half-life also had profound effects on MHC-I presentation and proteolytic processing of proinsulin. Thus, presentation of stable luminal forms was inefficient but enhanced by proteasome inhibition, whereas that of unstable luminal forms and of a cytosolic form were more efficient and compromised by proteasome inhibitors. Distinct stability of peptide MHC complexes produced from cytosolic and luminal proinsulin suggests that different proteolytic activities process the two Ag forms. Thus, both structural features and subcellular targeting of Ags can have strong effects on the processing pathways engaged by MHC-I-restricted Ags, and on the efficiency and regulation of their presentation.

Insufficient autoantigen presentation and failure of tolerance in a mouse model of rheumatoid arthritis

OBJECTIVE

In the K/BxN mouse model of rheumatoid arthritis, T cells reactive for the self antigen glucose-6-phosphate isomerase (GPI) escape negative selection even though GPI expression is ubiquitous. We sought to determine whether insufficient GPI presentation could account for the failure of negative selection and for the development of arthritis.

METHODS

To increase the antigen presentation of GPI, we generated transgenic mice expressing a membrane-bound form of GPI (mGPI) and crossed them with K/BxN mice. A monoclonal antibody specific for the α-chain of the KRN T cell receptor was generated to examine the fate of GPI-specific T cells.

RESULTS

The mGPI-transgenic mice presented GPI more efficiently and showed a dramatic increase in negative selection and an inhibition of arthritis. Interestingly, thymic negative selection remained incomplete in these mice, and the escaped autoreactive T cells were anergic in the peripheral lymphoid organs, suggesting that enhanced antigen presentation also induces peripheral tolerance. Despite this apparent tolerance induction toward GPI, these mice developed a chronic wasting disease, characterized by colonic inflammation with epithelial dysplasia, as well as a dramatic reduction in Treg cells.

CONCLUSION

These data indicate that insufficient autoantigen expression or presentation results in defects of both central and peripheral tolerance in the K/BxN mice. Our findings also support the idea that insufficient autoantigen levels may underlie the development of autoimmunity.

The Repertoires of Peptides Presented by MHC-II in the Thymus and in Peripheral Tissue: A Clue for Autoimmunity?

T-cell tolerance to self-antigens is established in the thymus through the recognition by developing thymocytes of self-peptide-MHC complexes and induced and maintained in the periphery. Efficient negative selection of auto-reactive T cells in the thymus is dependent on the in situ expression of both ubiquitous and tissue-restricted self-antigens and on the presentation of derived peptides. Weak or inadequate intrathymic expression of self-antigens increases the risk to generate an autoimmune-prone T-cell repertoire. Indeed, even small changes of self-antigen expression in the thymus affect negative selection and increase the predisposition to autoimmunity. Together with other mechanisms, tolerance is maintained in the peripheral lymphoid organs via the recognition by mature T cells of a similar set of self-peptides in homeostatic conditions. However, non-lymphoid peripheral tissue, where organ-specific autoimmunity takes place, often have differential functional processes that may lead to the generation of epitopes that are absent or non-presented in the thymus. These putative differences between peptides presented by MHC molecules in the thymus and in peripheral tissues might be a major key to the initiation and maintenance of autoimmune conditions.

Composition of the HLA-DR-associated human thymus peptidome

Major histocompatibility complex class II (MHC-II) molecules bind to and display antigenic peptides on the surface of antigen-presenting cells (APCs). In the absence of infection, MHC-II molecules on APCs present self-peptides and interact with CD4(+) T cells to maintain tolerance and homeostasis. In the thymus, self-peptides bind to MHC-II molecules expressed by defined populations of APCs specialised for the different steps of T-cell selection. Cortical epithelial cells present peptides for positive selection, whereas medullary epithelial cells and dendritic cells are responsible for peptide presentation for negative selection. However, few data are available on the peptides presented by MHC molecules in the thymus. Here, we apply mass spectrometry to analyse and identify MHC-II-associated peptides from five fresh human thymus samples. The data show a diverse self-peptide repertoire, mostly consisting of predicted MHC-II high binders. Despite technical limitations preventing single cell population analyses of peptides, these data constitute the first direct assessment of the HLA-II-bound peptidome and provide insight into how this peptidome is generated and how it drives T-cell repertoire formation.

Recognition of self and altered self by T cells in autoimmunity and allergy

T cell recognition of foreign peptide antigen and tolerance to self peptides is key to the proper function of the immune system. Usually, in the thymus T cells that recognize self MHC + self peptides are deleted and those with the potential to recognize self MHC + foreign peptides are selected to mature. However there are exceptions to these rules. Autoimmunity and allergy are two of the most common immune diseases that can be related to recognition of self. Many genes work together to lead to autoimmunity. Of those, particular MHC alleles are the most strongly associated, reflecting the key importance of MHC presentation of self peptides in autoimmunity. T cells specific for combinations of self MHC and self peptides may escape thymus deletion, and thus be able to drive autoimmunity, for several reasons: the relevant self peptide may be presented at low abundance in the thymus but at high level in particular peripheral tissues; the relevant self peptide may bind to MHC in an unusual register, not present in the thymus but apparent elsewhere; finally the relevant self peptide may be post translationally modified in a tissue specific fashion. In some types of allergy, the peptide + MHC combination may also be fully derived from self. However the combination in question may be modified by the presence of other ligands, such as small drug molecules or metal ions. Thus these types of allergies may act like the post translationally modified peptides involved some types of autoimmunity.

Do MHCII-presented neoantigens drive type 1 diabetes and other autoimmune diseases?

The strong association between particular MHCII alleles and type 1 diabetes is not fully understood. Two ideas that have been considered for many years are that autoimmunity is driven by (1) low-affinity CD4(+) T cells that escape thymic negative selection and respond to certain autoantigen peptides that are particularly well presented by particular MHCII molecules, or (2) CD4(+) T cells responding to neoantigens that are absent in the thymus, but uniquely created in the target tissue in the periphery and presented by particular MHCII alleles. Here we discuss the recent structural data in favor of the second idea. We review studies suggesting that peptide antigens recognized by autoimmune T cells are uniquely proteolytically processed and/or posttranslationally modified in the target tissue, thus allowing these T cells to escape deletion in the thymus during T-cell development. We postulate that an encounter with these tissue-specific neoantigenic peptides presented by the particular susceptible MHCII alleles in the peripheral tissues when accompanied by the appropriate inflammatory milieu activates these T-cell escapees leading to the onset of autoimmune disease.

Peptide linkage to the α-subunit of MHCII creates a stably inverted antigen presentation complex

Class II proteins of the major histocompatibility complex (MHCII) typically present exogenous antigenic peptides to cognate T cell receptors of CD4-T lymphocytes. The exact conformation of peptide-MHCII complexes (pMHCII) can vary depending on the length, register and orientation of the bound peptide. We have recently found the self-peptide CLIP (class-II-associated invariant chain-derived peptide) to adopt a dynamic bidirectional binding mode with regard to the human MHCII HLA-DR1 (HLA, human leukocyte antigen). We suggested that inversely bound peptides could activate specific T cell clones in the context of autoimmunity. As a first step to prove this hypothesis, pMHC complexes restricted to either the canonical or the inverted peptide orientation have to be constructed. Here, we show that genetically encoded linkage of CLIP and two other antigenic peptides to the HLA-DR1 α-chain results in stable complexes with inversely bound ligands. Two-dimensional NMR and biophysical analyses indicate that the CLIP-bound pMHC(inv) complex (pMHC(inv), inverted MHCII-peptide complex) displays high thermodynamic stability but still allows for the exchange against higher-affinity viral antigen. Complemented by comparable data on a corresponding β-chain-fused canonical HLA-DR1/CLIP complex, we further show that linkage of CLIP leads to a binding mode exactly the same as that of the corresponding unlinked constructs. We suggest that our approach constitutes a general strategy to create pMHC(inv) complexes. Such engineering is needed to create orientation-specific antibodies and raise T cells to study phenomena of autoimmunity caused by isomeric pMHCs.

On the perils of poor editing: regulation of peptide loading by HLA-DQ and H2-A molecules associated with celiac disease and type 1 diabetes

This review discusses mechanisms that link allelic variants of major histocompatibility complex (MHC) class II molecules (MHCII) to immune pathology. We focus on HLA (human leukocyte antigen)-DQ (DQ) alleles associated with celiac disease (CD) and type 1 diabetes (T1D) and the role of the murine DQ-like allele, H2-Ag7 (I-Ag7 or Ag7), in murine T1D. MHCII molecules bind peptides, and alleles vary in their peptide-binding specificity. Disease-associated alleles permit binding of disease-inducing peptides, such as gluten-derived, Glu-/Pro-rich gliadin peptides in CD and peptides from islet autoantigens, including insulin, in T1D. In addition, the CD-associated DQ2.5 and DQ8 alleles are unusual in their interactions with factors that regulate their peptide loading, invariant chain (Ii) and HLA-DM (DM). The same alleles, as well as other T1D DQ risk alleles (and Ag7), share nonpolar residues in place of Asp at β57 and prefer peptides that place acidic side chains in a pocket in the MHCII groove (P9). Antigen-presenting cells from T1D-susceptible mice and humans retain CLIP because of poor DM editing, although underlying mechanisms differ between species. We propose that these effects on peptide presentation make key contributions to CD and T1D pathogenesis.

Historical overview of immunological tolerance

A fundamental property of the immune system is its ability to mediate self-defense with a minimal amount of collateral damage to the host. The system uses several different mechanisms to achieve this goal, which is collectively referred to as the "process of immunological tolerance." This article provides an introductory historical overview to these various mechanisms, which are discussed in greater detail throughout this collection, and then briefly describes what happens when this process fails, a state referred to as "autoimmunity."

CD4 T cells and their antigens in the pathogenesis of autoimmune diabetes

Pathogenesis of type 1 diabetes (T1D) is mediated by effector T cells and CD4 Th1 and Th17T cells have important roles in this process. While effector function of Th1 cells is well established, because of their inherent plasticity Th17 cells have been more controversial. Th17 cells contribute to pathogenicity, but several studies indicate that Th17 cells transfer disease through conversion to Th1 cells in vivo. CD4T cells are attracted to islets by β-cell antigens which include insulin and the two new autoantigens, chromogranin A and islet amyloid polypeptide, all proteins of the secretory granule. Peptides of insulin and ChgA bind to the NOD class II molecule in an unconventional manner and since autoantigenic peptides may typically bind to MHC with low affinity, it is postulated that post-translational modifications of β-cell peptides could contribute to the interaction between peptides, MHC, and the autoreactive TCR.

Prevention of type 1 diabetes in mice by tolerogenic vaccination with a strong agonist insulin mimetope

Subimmunogenic vaccination with an agonist mimetope of insulin converts naive T cells into regulatory T cells and prevents type 1 diabetes in NOD mice.

Type 1 diabetes (T1D) results from the destruction of insulin-secreting pancreatic β cells by autoreactive T cells. Insulin is an essential target of the autoimmune attack. Insulin epitopes recognized by diabetogenic T cell clones bind poorly to the class II I-A^g7 molecules of nonobese diabetic (NOD) mice, which results in weak agonistic activity of the peptide MHC complex. Here, we describe a strongly agonistic insulin mimetope that effectively converts naive T cells into Foxp3⁺ regulatory T cells in vivo, thereby completely preventing T1D in NOD mice. In contrast, natural insulin epitopes are ineffective. Subimmunogenic vaccination with strongly agonistic insulin mimetopes might represent a novel strategy to prevent T1D in humans at risk for the disease.

Dysregulation of thymic clonal deletion and the escape of autoreactive T cells

Events ongoing in the thymus are critical for deleting developing thymocytes specific for tissue antigens, and establishing self-tolerance within the T cell compartment. Aberrant thymic negative selection, however, is believed to generate a repertoire with increased self-reactivity, which in turn can contribute to the development of T cell-mediated autoimmunity. In this review, mechanisms that regulate the efficacy of negative selection and influence the deletion of autoreactive thymocytes will be discussed.

I-Ag7 is subject to post-translational chaperoning by CLIP

Several MHC class II alleles linked with autoimmune diseases form unusually low-stability complexes with class II-associated invariant chain peptides (CLIP), leading us to hypothesize that this is an important feature contributing to autoimmune pathogenesis. We recently demonstrated a novel post-endoplasmic reticulum (ER) chaperoning role of the CLIP peptides for the murine class II allele I-E(d). In the current study, we tested the generality of this CLIP chaperone function using a series of invariant chain (Ii) mutants designed to have varying CLIP affinity for I-A(g7). In cells expressing these Ii CLIP mutants, I-A(g7) abundance, turnover and antigen presentation are all subject to regulation by CLIP affinity, similar to I-E(d). However, I-A(g7) undergoes much greater quantitative changes than observed for I-E(d). In addition, we find that Ii with a CLIP region optimized for I-A(g7) binding may be preferentially assembled with I-A(g7) even in the presence of higher levels of wild-type Ii. This finding indicates that, although other regions of Ii interact with class II, CLIP binding to the groove is likely to be a dominant event in assembly of nascent class II molecules with Ii in the ER.

Processing and presentation of (pro)-insulin in the MHC class II pathway: the generation of antigen-based immunomodulators in the context of type 1 diabetes mellitus

Both CD4(+) and CD8(+) T lymphocytes play a crucial role in the autoimmune process leading to T1D. Dendritic cells take up foreign antigens and autoantigens; within their endocytic compartments, proteases degrade exogenous antigens for subsequent presentation to CD4(+) T cells via MHC class II molecules. A detailed understanding of autoantigen processing and the identification of autoantigenic T cell epitopes are crucial for the development of antigen-based specific immunomodulators. APL are peptide analogues of auto-immunodominant T cell epitopes that bind to MHC class II molecules and can mediate T cell activation. However, APL can be rapidly degraded by proteases occurring in the extracellular space and inside cells, substantially weakening their efficiency. By contrast, protease-resistant APL function as specific immunomodulators and can be used at low doses to examine the functional plasticity of T cells and to potentially interfere with autoimmune responses. Here, we review the latest achievements in (pro)-insulin processing in the MHC class II pathway and the generation of APL to mitigate autoreactive T cells and to activate Treg cells.

Banting Lecture 2009: An unfinished journey: molecular pathogenesis to prevention of type 1A diabetes

The Banting Medal for Scientific Achievement Award is the American Diabetes Association's highest scientific award and honors an individual who has made significant, long-term contributions to the understanding of diabetes, its treatment, and/or prevention. The award is named after Nobel Prize winner Sir Frederick Banting, who codiscovered insulin treatment for diabetes.

Dr. Eisenbarth received the American Diabetes Association's Banting Medal for Scientific Achievement at the Association's 69th Scientific Sessions, June 5–9, 2009, in New Orleans, Louisiana. He presented the Banting Lecture, An Unfinished Journey—Type 1 Diabetes—Molecular Pathogenesis to Prevention , on Sunday, June 7, 2009.

The novel endosomal membrane protein Ema interacts with the class C Vps-HOPS complex to promote endosomal maturation

Endosomal maturation is critical for accurate and efficient cargo transport through endosomal compartments. Here we identify a mutation of the novel Drosophila gene, ema (endosomal maturation defective) in a screen for abnormal synaptic overgrowth and defective protein trafficking. Ema is an endosomal membrane protein required for trafficking of fluid-phase and receptor-mediated endocytic cargos. In the ema mutant, enlarged endosomal compartments accumulate as endosomal maturation fails, with early and late endosomes unable to progress into mature degradative late endosomes and lysosomes. Defective endosomal down-regulation of BMP signaling is responsible for the abnormal synaptic overgrowth. Ema binds to and genetically interacts with Vps16A, a component of the class C Vps-HOPS complex that promotes endosomal maturation. The human orthologue of ema, Clec16A, is a candidate susceptibility locus for autoimmune disorders, and its expression rescues the Drosophila mutant demonstrating conserved function. Characterizing this novel gene family identifies a new component of the endosomal pathway and provides insights into class C Vps-HOPS complex function.

Predominant occupation of the class I MHC molecule H-2Kwm7 with a single self-peptide suggests a mechanism for its diabetes-protective effect

Type 1 diabetes (T1D) is an autoimmune disease characterized by T cell-mediated destruction of insulin-producing pancreatic beta cells. In both humans and the non-obese diabetic (NOD) mouse model of T1D, class II MHC alleles are the primary determinant of disease susceptibility. However, class I MHC genes also influence risk. These findings are consistent with the requirement for both CD4(+) and CD8(+) T cells in the pathogenesis of T1D. Although a large body of work has permitted the identification of multiple mechanisms to explain the diabetes-protective effect of particular class II MHC alleles, studies examining the protective influence of class I alleles are lacking. Here, we explored this question by performing biochemical and structural analyses of the murine class I MHC molecule H-2K(wm7), which exerts a diabetes-protective effect in NOD mice. We have found that H-2K(wm7) molecules are predominantly occupied by the single self-peptide VNDIFERI, derived from the ubiquitous protein histone H2B. This unexpected finding suggests that the inability of H-2K(wm7) to support T1D development could be due, at least in part, to the failure of peptides from critical beta-cell antigens to adequately compete for binding and be presented to T cells. Predominant presentation of a single peptide would also be expected to influence T-cell selection, potentially leading to a reduced ability to select a diabetogenic CD8(+) T-cell repertoire. The report that one of the predominant peptides bound by T1D-protective HLA-A*31 is histone derived suggests the potential translation of our findings to human diabetes-protective class I MHC molecules.

Checkpoints in lymphocyte development and autoimmune disease

Antigen receptor-controlled checkpoints in B lymphocyte development are crucial for the prevention of autoimmune diseases such as systemic lupus erythematosus. Checkpoints at the stage of pre-B cell receptor (pre-BCR) and BCR expression can eliminate certain autoreactive BCRs either by deletion of or anergy induction in cells expressing autoreactive BCRs or by receptor editing. For T cells, the picture is more complex because there are regulatory T (T(reg)) cells that mediate dominant tolerance, which differs from the recessive tolerance mediated by deletion and anergy. Negative selection of thymocytes may be as essential as T(reg) cell generation in preventing autoimmune diseases such as type 1 diabetes, but supporting evidence is scarce. Here we discuss several scenarios in which failures at developmental checkpoints result in autoimmunity.

Analysis of T cell receptor beta chains that combine with dominant conserved TRAV5D-4*04 anti-insulin B:9-23 alpha chains

OBJECTIVE

The objective of this study was to define the spectrum of TCR beta chains permissive for T cells with alpha chains containing the conserved TRAV5D-4*04 sequence to target the insulin B:9-23 peptide, a major epitope for initiation of diabetes in the NOD mouse.

MATERIALS AND METHODS

We produced T cell hybridomas from mice with single T cell receptors (BDC12-4.1 TCR alpha(+)beta(+) double transgenic mice and BDC12-4.4 TCR alpha(+)beta(+) double retrogenic mice) or from mice with only the corresponding alpha chains transgene or retrogene and multiple endogenous TCR beta chains.

RESULTS

Hybridomas with the complete BDC12-4.1 and BDC12-4.4 T cell receptors, despite having markedly different TCR beta chains, responded to similar B:9-23 peptides. Approximately 1% of the hybridomas from mice with the fixed TRAV5D-4*04 alpha chains and multiple endogenous beta chains responded to B:9-23 peptides while the majority of hybridomas with different beta chains did not respond. There was no apparent conservation of TCR beta chain sequences in the responding hybridomas.

CONCLUSIONS

Approximately 1% of hybridomas utilizing different TCR beta chains paired with the conserved TRAV5D-4*04 containing alpha chains respond to insulin peptide B:9-23. Therefore, TCR beta chain sequences make an important contribution to insulin B:9-23 peptide recognition but multiple beta chain sequences are permissive for recognition.

Control of type 1 diabetes by CD4+Foxp3+ regulatory T cells: lessons from mouse models and implications for human disease

In recent years, there has been a revival of the concept of CD4(+) regulatory T (T(reg)) cells as being a central control point in various immune responses, including autoimmune responses and immunity to transplants, allergens, tumours and infectious microbes. The current literature suggests that T(reg) cells are diverse in their phenotype and mechanism(s) of action, and as such, may constitute a myriad of naturally occurring and induced T cell precursors with variable degrees of regulatory potential. In this review, we summarize research from various laboratories, including our own, showing that CD4(+)Foxp3(+) T(reg) cells are critical in the control of type 1 diabetes (T1D) in mouse models and humans. In this review, we also discuss cellular and molecular determinants that impact CD4(+)Foxp3(+) T(reg) cell development and function and consequential resistance to organ-specific autoimmune disease. Recent advances in the use of CD4(+)Foxp3(+) T(reg) cellular therapy to promote immunological tolerance in the absence of long-term generalized immunosuppression are also presented.

Dysrulation of T cell peripheral tolerance in type 1 diabetes

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease in which the insulin producing beta cells are destroyed. The breakdown of beta cell-specific self-tolerance by T cells involves a number of dysregulated events intrinsic and extrinsic to T cells. Herein, we review the key mechanisms that drive beta cell autoimmunity, with an emphasis on events that influence the expansion and differentiation of pathogenic T cells in the periphery.

A new model defines the minimal set of polymorphism in HLA-DQ and -DR that determines susceptibility and resistance to autoimmune diabetes

Background

The mechanism underlying autoimmune diabetes has been difficult to define. There is a strong genetic contribution and numerous studies associate the major histocompatibility complex, especially the class II region, with predisposition or resistance. However, how these molecules are implicated remains obscure.

Presentation of the hypothesis

We have supplemented structural analysis with computational biophysical and sequence analyses and propose an heuristic for distinguishing between human leukocyte antigen molecules that predispose to insulin dependent diabetes mellitus and those that are protective. Polar residues at both β37 and β9 suffice to distinguish accurately between class II alleles that predispose to type 1 diabetes and those that do not. The electrostatic potential within the peptide binding pocket exerts a strong influence on diabetogenic epitopes with basic residues. Diabetes susceptibility alleles are predicted to bind autoantigens strongly with tight affinity, prolonged association and altered cytokine expression profile. Protective alleles bind moderately, and neutral alleles poorly or not at all. Non-Asp β57 is a modifier that supplements disease risk but only in the presence of the polymorphic, polar pair at β9 and β37. The nature of β37 determines resistance on one hand, and susceptibility or dominant protection on the other.

Conclusion

The proposed ideas are illustrated with structural, functional and population studies from the literature. The hypothesis, in turn, rationalizes their results. A plausible mechanism of immune mediated diabetes based on binding affinity and peptide kinetics is discussed. The number of the polymorphic markers present correlates with onset of disease and severity. The molecular elucidation of disease susceptibility and resistance paves the way for risk prediction, treatment and prevention of disease based on analogue peptides.

Reviewers

This article was reviewed by Eugene V. Koonin, Michael Lenardo, Hossam Ashour, and Bhagirath Singh. For the full reviews, please go to the Reviewers' comments section.

Update in endocrine autoimmunity

CONTEXT

The endocrine system is a common target in pathogenic autoimmune responses, and there has been recent progress in our understanding, diagnosis, and treatment of autoimmune endocrine diseases.

SYNTHESIS

Rapid progress has recently been made in our understanding of the genetic factors involved in endocrine autoimmune diseases. Studies on monogenic autoimmune diseases that include endocrine phenotypes like autoimmune polyglandular syndrome type 1 and immune dysregulation, polyendocrinopathy, enteropathy, X-linked have helped reveal the role of key regulators in the maintenance of immune tolerance. Highly powered genetic studies have found and confirmed many new genes outside of the established role of the human leukocyte antigen locus with these diseases, and indicate an essential role of immune response pathways in these diseases. Progress has also been made in identifying new autoantigens and the development of new animal models for the study of endocrine autoimmunity. Finally, although hormone replacement therapy is still likely to be a mainstay of treatment in these disorders, there are new agents being tested for potentially treating and reversing the underlying autoimmune process.

CONCLUSION

Although autoimmune endocrine disorders are complex in etiology, these recent advances should help contribute to improved outcomes for patients with, or at risk for, these disorders.

Weak proinsulin peptide-major histocompatibility complexes are targeted in autoimmune diabetes in mice

OBJECTIVE

Weak major histocompatibility complex (MHC) binding of self-peptides has been proposed as a mechanism that may contribute to autoimmunity by allowing for escape of autoreactive T-cells from the thymus. We examined the relationship between the MHC-binding characteristics of a beta-cell antigen epitope and T-cell autoreactivity in a model of autoimmune diabetes.

RESEARCH DESIGN AND METHODS

The binding of a proinsulin epitope, proinsulin-1(47-64) (PI-1[47-64]), to the MHC class II molecules I-A(g7) and I-A(k) was measured using purified class II molecules. T-cell reactivity to the proinsulin epitope was examined in I-A(g7+) and I-A(k+) mice.

RESULTS

C-peptide epitopes bound very weakly to I-A(g7) molecules. However, C-peptide-reactive T-cells were induced after immunization in I-A(g7)-bearing mice (NOD and B6.g7) but not in I-A(k)-bearing mice (B10.BR and NOD.h4). T-cells reactive with the PI-1(47-64) peptide were found spontaneously in the peripancreatic lymph nodes of pre-diabetic NOD mice. These T-cells were activated by freshly isolated beta-cells in the presence of antigen-presenting cells and caused diabetes when transferred into NOD.scid mice.

CONCLUSIONS

These data demonstrate an inverse relationship between self-peptide-MHC binding and T-cell autoreactivity for the PI-1(47-64) epitope in autoimmune diabetes.