An insulin-IAPP hybrid peptide is an endogenous antigen for CD4 T cells in the non-obese diabetic mouse

The immunology of type 1 diabetes

Following the seminal discovery of insulin a century ago, treatment of individuals with type 1 diabetes (T1D) has been largely restricted to efforts to monitor and treat metabolic glucose dysregulation. The recent regulatory approval of the first immunotherapy that targets T cells as a means to delay the autoimmune destruction of pancreatic β-cells highlights the critical role of the immune system in disease pathogenesis and tends to pave the way for other immune-targeted interventions for T1D. Improving the efficacy of such interventions across the natural history of the disease will probably require a more detailed understanding of the immunobiology of T1D, as well as technologies to monitor residual β-cell mass and function. Here we provide an overview of the immune mechanisms that underpin the pathogenesis of T1D, with a particular emphasis on T cells.

The insulin secretory granule is a hotspot for autoantigen formation in type 1 diabetes

In type 1 diabetes, the insulin-producing beta cells of the pancreas are destroyed through the activity of autoreactive T cells. In addition to strong and well-documented HLA class II risk haplotypes, type 1 diabetes is associated with noncoding polymorphisms within the insulin gene locus. Furthermore, autoantibody prevalence data and murine studies implicate insulin as a crucial autoantigen for the disease. Studies identify secretory granules, where proinsulin is processed into mature insulin, stored and released in response to glucose stimulation, as a source of antigenic epitopes and neoepitopes. In this review, we integrate established concepts, including the role that susceptible HLA and thymic selection of the T cell repertoire play in setting the stage for autoimmunity, with emerging insights about beta cell and insulin secretory granule biology. In particular, the acidic, peptide-rich environment of secretory granules combined with its array of enzymes generates a distinct proteome that is unique to functional beta cells. These factors converge to generate non-templated peptide sequences that are recognised by autoreactive T cells. Although unanswered questions remain, formation and presentation of these epitopes and the resulting immune responses appear to be key aspects of disease initiation. In addition, these pathways may represent important opportunities for therapeutic intervention.

An Insulin-Chromogranin A Hybrid Peptide Activates DR11-Restricted T Cells in Human Type 1 Diabetes

Hybrid insulin peptides (HIPs) formed through covalent cross-linking of proinsulin fragments to secretory granule peptides are detectable within murine and human islets. The 2.5HIP (C-peptide-chromogranin A [CgA] HIP), recognized by the diabetogenic BDC-2.5 clone, is a major autoantigen in the nonobese diabetic mouse. However, the relevance of this epitope in human disease is currently unclear. A recent study probed T-cell reactivity toward HIPs in patients with type 1 diabetes, documenting responses in one-third of the patients and isolating several HIP-reactive T-cell clones. In this study, we isolated a novel T-cell clone and showed that it responds vigorously to the human equivalent of the 2.5HIP (designated HIP9). Although the responding patient carried the risk-associated DRB1*04:01/DQ8 haplotype, the response was restricted by DRB1*11:03 (DR11). HLA class II tetramer staining revealed higher frequencies of HIP9-reactive T cells in individuals with diabetes than in control participants. Furthermore, in DR11+ participants carrying the DRB4 allele, HIP9-reactive T-cell frequencies were higher than observed frequencies for the immunodominant proinsulin 9-28 epitope. Finally, there was a negative correlation between HIP9-reactive T-cell frequency and age at diagnosis. These results provide direct evidence that this C-peptide-CgA HIP is relevant in human type 1 diabetes and suggest a mechanism by which nonrisk HLA haplotypes may contribute to the development of β-cell autoimmunity.

ARTICLE HIGHLIGHTS

Mapping of a hybrid insulin peptide in the inflamed islet β-cells from NOD mice

There is accumulating evidence that pathogenic T cells in T1D recognize epitopes formed by post-translational modifications of β-cell antigens, including hybrid insulin peptides (HIPs). The ligands for several CD4 T-cell clones derived from the NOD mouse are HIPs composed of a fragment of proinsulin joined to peptides from endogenous β-cell granule proteins. The diabetogenic T-cell clone BDC-6.9 reacts to a fragment of C-peptide fused to a cleavage product of pro-islet amyloid polypeptide (6.9HIP). In this study, we used a monoclonal antibody (MAb) to the 6.9HIP to determine when and where HIP antigens are present in NOD islets during disease progression and with which immune cells they associate. Immunogold labeling of the 6.9HIP MAb and organelle-specific markers for electron microscopy were employed to map the subcellular compartment(s) in which the HIP is localized within β-cells. While the insulin B9-23 peptide was present in nearly all islets, the 6.9HIP MAb stained infiltrated islets only in NOD mice at advanced stages of T1D development. Islets co-stained with the 6.9HIP MAb and antibodies to mark insulin, macrophages, and dendritic cells indicate that 6.9HIP co-localizes within insulin-positive β-cells as well as intra-islet antigen-presenting cells (APCs). In electron micrographs, the 6.9HIP co-localized with granule structures containing insulin alone or both insulin and LAMP1 within β-cells. Exposing NOD islets to the endoplasmic reticulum (ER) stress inducer tunicamycin significantly increased levels of 6.9HIP in subcellular fractions containing crinosomes and dense-core granules (DCGs). This work demonstrates that the 6.9HIP can be visualized in the infiltrated islets and suggests that intra-islet APCs may acquire and present HIP antigens within islets.

Non-mutational neoantigens in disease

The ability of mammals to mount adaptive immune responses culminating with the establishment of immunological memory is predicated on the ability of the mature T cell repertoire to recognize antigenic peptides presented by syngeneic MHC class I and II molecules. Although it is widely believed that mature T cells are highly skewed towards the recognition of antigenic peptides originating from genetically diverse (for example, foreign or mutated) protein-coding regions, preclinical and clinical data rather demonstrate that novel antigenic determinants efficiently recognized by mature T cells can emerge from a variety of non-mutational mechanisms. In this Review, we describe various mechanisms that underlie the formation of bona fide non-mutational neoantigens, such as epitope mimicry, upregulation of cryptic epitopes, usage of non-canonical initiation codons, alternative RNA splicing, and defective ribosomal RNA processing, as well as both enzymatic and non-enzymatic post-translational protein modifications. Moreover, we discuss the implications of the immune recognition of non-mutational neoantigens for human disease.

The beta cell-immune cell interface in type 1 diabetes (T1D)

BACKGROUND

T1D is an autoimmune disease in which pancreatic islets of Langerhans are infiltrated by immune cells resulting in the specific destruction of insulin-producing islet beta cells. Our understanding of the factors leading to islet infiltration and the interplay of the immune cells with target beta cells is incomplete, especially in human disease. While murine models of T1D have provided crucial information for both beta cell and autoimmune cell function, the translation of successful therapies in the murine model to human disease has been a challenge.

SCOPE OF REVIEW

Here, we discuss current state of the art and consider knowledge gaps concerning the interface of the islet beta cell with immune infiltrates, with a focus on T cells. We discuss pancreatic and immune cell phenotypes and their impact on cell function in health and disease, which we deem important to investigate further to attain a more comprehensive understanding of human T1D disease etiology.

MAJOR CONCLUSIONS

The last years have seen accelerated development of approaches that allow comprehensive study of human T1D. Critically, recent studies have contributed to our revised understanding that the pancreatic beta cell assumes an active role, rather than a passive position, during autoimmune disease progression. The T cell-beta cell interface is a critical axis that dictates beta cell fate and shapes autoimmune responses. This includes the state of the beta cell after processing internal and external cues (e.g., stress, inflammation, genetic risk) that that contributes to the breaking of tolerance by hyperexpression of human leukocyte antigen (HLA) class I with presentation of native and neoepitopes and secretion of chemotactic factors to attract immune cells. We anticipate that emerging insights about the molecular and cellular aspects of disease initiation and progression processes will catalyze the development of novel and innovative intervention points to provide additional therapies to individuals affected by T1D.

Hybrid insulin peptide isomers spontaneously form in pancreatic beta-cells from an aspartic anhydride intermediate

Hybrid insulin peptides (HIPs) form in beta-cells when insulin fragments link to other peptides through a peptide bond. HIPs contain nongenomic amino acid sequences and have been identified as targets for autoreactive T cells in type 1 diabetes. A subgroup of HIPs, in which N-terminal amine groups of various peptides are linked to aspartic acid residues of insulin C-peptide, was detected through mass spectrometry in pancreatic islets. Here, we investigate a novel mechanism that leads to the formation of these HIPs in human and murine islets. Our research herein shows that these HIPs form spontaneously in beta-cells through a mechanism involving an aspartic anhydride intermediate. This mechanism leads to the formation of a regular HIP containing a standard peptide bond as well as a HIP-isomer containing an isopeptide bond by linkage to the carboxylic acid side chain of the aspartic acid residue. We used mass spectrometric analyses to confirm the presence of both HIP isomers in islets, thereby validating the occurrence of this novel reaction mechanism in beta-cells. The spontaneous formation of new peptide bonds within cells may lead to the development of neoepitopes that contribute to the pathogenesis of type 1 diabetes as well as other autoimmune diseases.

The biogenesis of the immunopeptidome

The immunopeptidome is the repertoire of peptides bound and presented by the MHC class I, class II, and non-classical molecules. The peptides are produced by the degradation of most cellular proteins, and in some cases, peptides are produced from extracellular proteins taken up by the cells. This review attempts to first describe some of its known and well-accepted concepts, and next, raise some questions about a few of the established dogmas in this field: The production of novel peptides by splicing is questioned, suggesting here that spliced peptides are extremely rare, if existent at all. The degree of the contribution to the immunopeptidome by degradation of cellular protein by the proteasome is doubted, therefore this review attempts to explain why it is likely that this contribution to the immunopeptidome is possibly overstated. The contribution of defective ribosome products (DRiPs) and non-canonical peptides to the immunopeptidome is noted and methods are suggested to quantify them. In addition, the common misconception that the MHC class II peptidome is mostly derived from extracellular proteins is noted, and corrected. It is stressed that the confirmation of sequence assignments of non-canonical and spliced peptides should rely on targeted mass spectrometry using spiking-in of heavy isotope-labeled peptides. Finally, the new methodologies and modern instrumentation currently available for high throughput kinetics and quantitative immunopeptidomics are described. These advanced methods open up new possibilities for utilizing the big data generated and taking a fresh look at the established dogmas and reevaluating them critically.

Cathepsin D Drives the Formation of Hybrid Insulin Peptides Relevant to the Pathogenesis of Type 1 Diabetes

Hybrid insulin peptides (HIPs) form in pancreatic β-cells through the formation of peptide bonds between proinsulin fragments and other peptides. HIPs have been identified in pancreatic islets by mass spectrometry and are targeted by CD4 T cells in patients with type 1 diabetes (T1D) as well as by pathogenic CD4 T-cell clones in nonobese diabetic (NOD) mice. The mechanism of HIP formation is currently poorly understood; however, it is well established that proteases can drive the formation of new peptide bonds in a side reaction during peptide bond hydrolysis. Here, we used a proteomic strategy on enriched insulin granules and identified cathepsin D (CatD) as the primary protease driving the specific formation of HIPs targeted by disease-relevant CD4 T cells in T1D. We also established that NOD islets deficient in cathepsin L (CatL), another protease implicated in the formation of disease-relevant HIPs, contain elevated levels of HIPs, indicating a role for CatL in the proteolytic degradation of HIPs. In summary, our data suggest that CatD may be a therapeutic target in efforts to prevent or slow the autoimmune destruction of β-cells mediated by HIP-reactive CD4 T cells in T1D.

Calcitonin gene-related peptide is a potential autoantigen for CD4 T cells in type 1 diabetes

The calcitonin gene-related peptide (CGRP) is a 37-amino acid neuropeptide with critical roles in the development of peripheral sensitization and pain. One of the CGRP family peptides, islet amyloid polypeptide (IAPP), is an important autoantigen in type 1 diabetes. Due to the high structural and chemical similarity between CGRP and IAPP, we expected that the CGRP peptide could be recognized by IAPP-specific CD4 T cells. However, there was no cross-reactivity between the CGRP peptide and the diabetogenic IAPP-reactive T cells. A set of CGRP-specific CD4 T cells was isolated from non-obese diabetic (NOD) mice. The T-cell receptor (TCR) variable regions of both α and β chains were highly skewed towards TRAV13 and TRBV13, respectively. The clonal expansion of T cells suggested that the presence of activated T cells responded to CGRP stimulation. None of the CGRP-specific CD4 T cells were able to be activated by the IAPP peptide. This established that CGRP-reactive CD4 T cells are a unique type of autoantigen-specific T cells in NOD mice. Using IAg7-CGRP tetramers, we found that CGRP-specific T cells were present in the pancreas of both prediabetic and diabetic NOD mice. The percentages of CGRP-reactive T cells in the pancreas of NOD mice were correlated to the diabetic progression. We showed that the human CGRP peptide presented by IAg7 elicited strong CGRP-specific T-cell responses. These findings suggested that CGRP is a potential autoantigen for CD4 T cells in NOD mice and probably in humans. The CGRP-specific CD4 T cells could be a unique marker for type 1 diabetes. Given the ubiquity of CGRP in nervous systems, it could potentially play an important role in diabetic neuropathy.

A Novel Tolerogenic Antibody Targeting Disulfide-Modified Autoantigen Effectively Prevents Type 1 Diabetes in NOD Mice

Increasing evidence suggested that the islet amyloid polypeptide (IAPP) is an essential autoantigen in the pathogenesis of type 1 diabetes (T1D) in humans and non-obese diabetic (NOD) mice. A unique disulfide containing IAPP-derived peptide KS20 is one of the highly diabetogenic peptides in NOD mice. The KS20-reactive T cells, including prototypic pathogenic BDC5.2.9, accumulate in the pancreas of prediabetic and diabetic mice and contribute to disease development. We generated a monoclonal antibody (LD96.24) that interacts with IA^g7-KS20 complexes with high affinity and specificity. LD96.24 recognized the IA^g7-KS20 disulfide loop and blocked the interaction between IA^g7-KS20 tetramers and cognate T cells but not other autoantigen-reactive T cells. The in vivo LD96.24 studies, at either early or late stages, drastically induced tolerance and delayed the onset of T1D disease in NOD mice by reducing the infiltration of not only IAPP-specific T cells but also chromogranin A and insulin-specific T cells in the pancreas, together with B cells and dendritic cells. LD96.24 can also significantly increase the ratio of Foxp3⁺ regulatory T cells with Interferon-gamma-secreting effector T cells. Our data suggested the important role of disulfide-modified peptides in the development of T1D. Targeting the complexes of Major histocompatibility complex (MHC)/disulfide modified antigens would influence the thiol redox balance and could be a novel immunotherapy for T1D.

Insulin B-chain hybrid peptides are agonists for T cells reactive to insulin B:9-23 in autoimmune diabetes

Insulin is considered to be a key antigenic target of T cells in Type 1 Diabetes (T1D) and autoimmune diabetes in the NOD mouse with particular focus on the B-chain amino acid sequence B:9-23 as the primary epitope. Our lab previously discovered that hybrid insulin peptides (HIPs), comprised of insulin C-peptide fragments fused to other β-cell granule peptides, are ligands for several pathogenic CD4 T cell clones derived from NOD mice and for autoreactive CD4 T cells from T1D patients. A subset of CD4 T cell clones from our panel react to insulin and B:9-23 but only at high concentrations of antigen. We hypothesized that HIPs might also be formed from insulin B-chain sequences covalently bound to other endogenously cleaved ß-cell proteins. We report here on the identification of a B-chain HIP, termed the 6.3HIP, containing a fragment of B:9-23 joined to an endogenously processed peptide of ProSAAS, as a strong neo-epitope for the insulin-reactive CD4 T cell clone BDC-6.3. Using an I-Ag7 tetramer loaded with the 6.3HIP, we demonstrate that T cells reactive to this B-chain HIP can be readily detected in NOD mouse islet infiltrates. This work suggests that some portion of autoreactive T cells stimulated by insulin B:9-23 may be responding to B-chain HIPs as peptide ligands.

Editing T cell repertoire by thymic epithelial cell-directed gene transfer abrogates risk of type 1 diabetes development

Insulin is the primary autoantigen (Ag) targeted by T cells in type 1 diabetes (T1D). Although biomarkers precisely identifying subjects at high risk of T1D are available, successful prophylaxis is still an unmet need. Leaky central tolerance to insulin may be partially ascribed to the instability of the MHC-InsB_9-23 complex, which lowers TCR avidity, thus resulting in defective negative selection of autoreactive clones and inadequate insulin-specific T regulatory cell (Treg) induction. We developed a lentiviral vector (LV)-based strategy to engineer thymic epithelial cells (TECs) to correct diabetogenic T cell repertoire. Intrathymic (it) LV injection established stable transgene expression in EpCAM⁺ TECs, by virtue of transduction of TEC precursors. it-LV-driven presentation of the immunodominant portion of ovalbumin allowed persistent and complete negative selection of responsive T cells in OT-II chimeric mice. We successfully applied this strategy to correct the diabetogenic repertoire of young non-obese diabetic mice, imposing the presentation by TECs of the stronger agonist InsulinB_9-23R22E and partially depleting the existing T cell compartment. We further circumscribed LV-driven presentation of InsulinB_9-23R22E by micro-RNA regulation to CD45⁻ TECs without loss of efficacy in protection from diabetes, associated with expanded insulin-specific Tregs. Overall, our gene transfer-based prophylaxis fine-tuned the central tolerance processes of negative selection and Treg induction, correcting an autoimmune prone T cell repertoire.

Modulation of autoimmune diabetes by N-ethyl-N-nitrosourea- induced mutations in non-obese diabetic mice

Genetic association studies of type 1 diabetes (T1D) in humans, and in congenic non-obese diabetic (NOD) mice harboring DNA segments from T1D-resistant mice, face the challenge of assigning causation to specific gene variants among many within loci that affect disease risk. Here, we created random germline mutations in NOD/NckH mice and used automated meiotic mapping to identify mutations modifying T1D incidence and age of onset. In contrast with association studies in humans or congenic NOD mice, we analyzed a relatively small number of genetic changes in each pedigree, permitting implication of specific mutations as causative. Among 844 mice from 14 pedigrees bearing 594 coding/splicing changes, we identified seven mutations that accelerated T1D development, and five that delayed or suppressed T1D. Eleven mutations affected genes not previously known to influence T1D (Xpnpep1, Herc1, Srrm2, Rapgef1, Ppl, Zfp583, Aldh1l1, Col6a1, Ccdc13, Cd200r1, Atrnl1). A suppressor mutation in Coro1a validated the screen. Mutagenesis coupled with automated meiotic mapping can detect genes in which allelic variation influences T1D susceptibility in NOD mice. Variation of some of the orthologous/paralogous genes may influence T1D susceptibility in humans.

Activation pathways that drive CD4+ T cells to break tolerance in autoimmune diseases. Immunol Rev 2022;307:161-190. [PMID: 35142369 PMCID: PMC9255211 DOI: 10.1111/imr.13071]

Autoimmune diseases are characterized by dysfunctional immune systems that misrecognize self as non-self and cause tissue destruction. Several cell types have been implicated in triggering and sustaining disease. Due to a strong association of major histocompatibility complex II (MHC-II) proteins with various autoimmune diseases, CD4+ T lymphocytes have been thoroughly investigated for their roles in dictating disease course. CD4+ T cell activation is a coordinated process that requires three distinct signals: Signal 1, which is mediated by antigen recognition on MHC-II molecules; Signal 2, which boosts signal 1 in a costimulatory manner; and Signal 3, which helps to differentiate the activated cells into functionally relevant subsets. These signals are disrupted during autoimmunity and prompt CD4+ T cells to break tolerance. Herein, we review our current understanding of how each of the three signals plays a role in three different autoimmune diseases and highlight the genetic polymorphisms that predispose individuals to autoimmunity. We also discuss the drawbacks of existing therapies and how they can be addressed to achieve lasting tolerance in patients.

Designing Personalized Antigen-Specific Immunotherapies for Autoimmune Diseases-The Case for Using Ignored Target Cell Antigen Determinants

We have proposed that antigen-specific immunotherapies (ASIs) for autoimmune diseases could be enhanced by administering target cell antigen epitopes (determinants) that are immunogenic but ignored by autoreactive T cells because these determinants may have large pools of naïve cognate T cells available for priming towards regulatory responses. Here, we identified an immunogenic preproinsulin determinant (PPI_L4-20) that was ignored by autoimmune responses in type 1 diabetes (T1D)-prone NOD mice. The size of the PPI_L4-20-specific splenic naive T cell pool gradually increased from 2–12 weeks in age and remained stable thereafter, while that of the major target determinant insulin B-chain_9-23 decreased greatly after 12 weeks in age, presumably due to recruitment into the autoimmune response. In 15–16 week old mice, insulin B-chain_9-23/alum immunization induced modest-low level of splenic T cell IL-10 and IL-4 responses, little or no spreading of these responses, and boosted IFNγ responses to itself and other autoantigens. In contrast, PPI_L4-20/alum treatment induced robust IL-10 and IL-4 responses, which spread to other autoantigens and increased the frequency of splenic IL-10-secreting Treg and Tr-1-like cells, without boosting IFNγ responses to ß-cell autoantigens. In newly diabetic NOD mice, PPI_L4-20, but not insulin B-chain_9-23 administered intraperitoneally (with alum) or intradermally (as soluble antigen) supplemented with oral GABA induced long-term disease remission. We discuss the potential of personalized ASIs that are based on an individual’s naïve autoantigen-reactive T cell pools and the use of HLA-appropriate ignored autoantigen determinants to safely enhance the efficacy of ASIs.

Tolerogenic Delivery of a Hybrid Insulin Peptide Markedly Prolongs Islet Graft Survival in the NOD Mouse

The induction of antigen (Ag)-specific tolerance and replacement of islet β-cells are major ongoing goals for the treatment of type 1 diabetes (T1D). Our group previously showed that a hybrid insulin peptide (2.5HIP) is a critical autoantigen for diabetogenic CD4+ T cells in the NOD mouse model. In this study, we investigated whether induction of Ag-specific tolerance using 2.5HIP-coupled tolerogenic nanoparticles (NPs) could protect diabetic NOD mice from disease recurrence upon syngeneic islet transplantation. Islet graft survival was significantly prolonged in mice treated with 2.5HIP NPs, but not NPs containing the insulin B chain peptide 9-23. Protection in 2.5HIP NP-treated mice was attributed both to the simultaneous induction of anergy in 2.5HIP-specific effector T cells and the expansion of Foxp3+ regulatory T cells specific for the same Ag. Notably, our results indicate that effector function of graft-infiltrating CD4+ and CD8+ T cells specific for other β-cell epitopes was significantly impaired, suggesting a novel mechanism of therapeutically induced linked suppression. This work establishes that tolerance induction with an HIP can delay recurrent autoimmunity in NOD mice, which could inform the development of an Ag-specific therapy for T1D.

Lysosomal cathepsin creates chimeric epitopes for diabetogenic CD4 T cells via transpeptidation

The identification of the peptide epitopes presented by major histocompatibility complex class II (MHCII) molecules that drive the CD4 T cell component of autoimmune diseases has presented a formidable challenge over several decades. In type 1 diabetes (T1D), recent insight into this problem has come from the realization that several of the important epitopes are not directly processed from a protein source, but rather pieced together by fusion of different peptide fragments of secretory granule proteins to create new chimeric epitopes. We have proposed that this fusion is performed by a reverse proteolysis reaction called transpeptidation, occurring during the catabolic turnover of pancreatic proteins when secretory granules fuse with lysosomes (crinophagy). Here, we demonstrate several highly antigenic chimeric epitopes for diabetogenic CD4 T cells that are produced by digestion of the appropriate inactive fragments of the granule proteins with the lysosomal protease cathepsin L (Cat-L). This pathway has implications for how self-tolerance can be broken peripherally in T1D and other autoimmune diseases.

Recognition of Multiple Hybrid Insulin Peptides by a Single Highly Diabetogenic T-Cell Receptor

The mechanisms underlying the major histocompatibility complex class II (MHCII) type 1 diabetes (T1D) association remain incompletely understood. We have previously shown that thymocytes expressing the highly diabetogenic, I-Ag7-restricted 4.1-T-cell receptor (TCR) are MHCII-promiscuous, and that, in MHCII-heterozygous mice, they sequentially undergo positive and negative selection/Treg deviation by recognizing pro- and anti-diabetogenic MHCII molecules on cortical thymic epithelial cells and medullary hematopoietic antigen-presenting cells (APCs), respectively. Here, we use a novel autoantigen discovery approach to define the antigenic specificity of this TCR in the context of I-Ag7. This was done by screening the ability of random epitope-GS linker-I- A β g 7 chain fusion pools to form agonistic peptide-MHCII complexes on the surface of I- A α d chain-transgenic artificial APCs. Pool deconvolution, I-Ag7-binding register-fixing, TCR contact residue mapping, and alanine scanning mutagenesis resulted in the identification of a 4.1-TCR recognition motif XL(G/A)XEXE(D/E)X that was shared by seven agonistic hybrid insulin peptides (HIPs) resulting from the fusion of several different chromogranin A and/or insulin C fragments, including post-translationally modified variants. These data validate a novel, highly sensitive MHCII-restricted epitope discovery approach for orphan TCRs and suggest thymic selection of autoantigen-promiscuous TCRs as a mechanism for the murine T1D-I-Ag7-association.

T cell receptor recognition of hybrid insulin peptides bound to HLA-DQ8

HLA-DQ8, a genetic risk factor in type I diabetes (T1D), presents hybrid insulin peptides (HIPs) to autoreactive CD4+ T cells. The abundance of spliced peptides binding to HLA-DQ8 and how they are subsequently recognised by the autoreactive T cell repertoire is unknown. Here we report, the HIP (GQVELGGGNAVEVLK), derived from splicing of insulin and islet amyloid polypeptides, generates a preferred peptide-binding motif for HLA-DQ8. HLA-DQ8-HIP tetramer⁺ T cells from the peripheral blood of a T1D patient are characterised by repeated TRBV5 usage, which matches the TCR bias of CD4+ T cells reactive to the HIP peptide isolated from the pancreatic islets of a patient with T1D. The crystal structure of three TRBV5+ TCR-HLA-DQ8-HIP complexes shows that the TRBV5-encoded TCR β-chain forms a common landing pad on the HLA-DQ8 molecule. The N- and C-termini of the HIP is recognised predominantly by the TCR α-chain and TCR β-chain, respectively, in all three TCR ternary complexes. Accordingly, TRBV5 + TCR recognition of HIP peptides might occur via a 'polarised' mechanism, whereby each chain within the αβTCR heterodimer recognises distinct origins of the spliced peptide presented by HLA-DQ8.

Noncontiguous T cell epitopes in autoimmune diabetes: From mice to men and back again

Type 1 diabetes (T1D) is a T cell-mediated autoimmune disease that affects the insulin-producing beta cells of the pancreatic islets. The nonobese diabetic mouse is a widely studied spontaneous model of the disease that has contributed greatly to our understanding of T1D pathogenesis. This is especially true in the case of antigen discovery. Upon review of existing knowledge concerning the antigens and peptide epitopes that are recognized by T cells in this model, good concordance is observed between mouse and human antigens. A fascinating recent illustration of the contribution of the nonobese diabetic mouse in the area of epitope identification is the discovery of noncontiguous CD4+ T cell epitopes. This novel epitope class is characterized by the linkage of an insulin-derived peptide to, most commonly, a fragment of a natural cleavage product of another beta cell secretory granule constituent. These so-called hybrid insulin peptides are also recognized by T cells in patients with T1D, although the precise mechanism for their generation has yet to be defined and is the subject of active investigation. Although evidence from the tumor immunology arena documented the existence of noncontiguous CD8+ T cell epitopes, generated by proteasome-mediated peptide splicing involving transpeptidation, such CD8+ T cell epitopes were thought to be a rare immunological curiosity. However, recent advances in bioinformatics and mass spectrometry have challenged this view. These developments, coupled with the discovery of hybrid insulin peptides, have spurred a search for noncontiguous CD8+ T cell epitopes in T1D, an exciting frontier area still in its infancy.

T-cell responses to hybrid insulin peptides prior to type 1 diabetes development

T-cell responses to posttranslationally modified self-antigens are associated with many autoimmune disorders. In type 1 diabetes, hybrid insulin peptides (HIPs) are implicated in the T-cell-mediated destruction of insulin-producing β-cells within pancreatic islets. The natural history of the disease is such that it allows for the study of T-cell reactivity prior to the onset of clinical symptoms. We hypothesized that CD4 T-cell responses to posttranslationally modified islet peptides precedes diabetes onset. In a cohort of genetically at-risk individuals, we measured longitudinal T-cell responses to native insulin and hybrid insulin peptides. Both proinflammatory (interferon-γ) and antiinflammatory (interluekin-10) cytokine responses to HIPs were more robust than those to native peptides, and the ratio of such responses oscillated between pro- and antiinflammatory over time. However, individuals who developed islet autoantibodies or progressed to clinical type 1 diabetes had predominantly inflammatory T-cell responses to HIPs. Additionally, several HIP T-cell responses correlated to worsening measurements of blood glucose, highlighting the relevance of T-cell responses to posttranslationally modified peptides prior to autoimmune disease development.

The Role of Programmed Death-1 in Type 1 Diabetes

PURPOSE OF REVIEW

Programmed death-1 (PD-1) is an inhibitory receptor that controls T and B cell proliferation and function through interacting with its ligand PD-L1 or PD-L2. PD-1/PD-L1 blockade reboots anti-tumor immunity and is currently used to treat > 15 different types of cancer. However, the response rate is not at 100% and some patients relapse. Importantly, up to 37% of patients treated with PD-1/PD-L1 blocking antibodies develop immune-related adverse events, including overt autoimmunity, such as type 1 diabetes (T1D). Herein, we discuss the role of PD-1, PD-L1, and PD-L2 signaling in pre-clinical models of T1D, including recent work from our laboratory.

RECENT FINDINGS

We highlight ongoing efforts to harness PD-1/PD-L1 signaling and treat autoimmunity. We also evaluate studies aimed at defining biomarkers that could reliably predict the development of immune-related adverse events after clinical PD-1/PD-L1 blockade. With increasing use of PD-1 blockade in the clinic, onset of autoimmunity is a growing health concern. In this review, we discuss what is known about the role of PD-1 pathway signaling in T1D and comment on ongoing efforts to identify patients at risk of T1D development after PD-1 pathway blockade.

Spliced HLA-bound peptides: a Black Swan event in immunology

Peptides that bind to and are presented on the cell surface by human leucocyte antigen (HLA) molecules play a critical role in adaptive immunity. For a long time it was believed that all the HLA-bound peptides were generated through simple proteolysis of linear sequences of cellular proteins, and therefore are templated in the genome and proteome. However, evidence for untemplated peptide ligands of HLA molecules has accumulated during the last two decades, with a recent global analysis of HLA-bound peptides suggesting that a considerable proportion of HLA-bound peptides are potentially generated through splicing/fusion of discontinuous peptide segments from one or two distinct proteins. In this review, we will evaluate recent discoveries and debates on the contribution of spliced peptides to the HLA class I immunopeptidome, consider biochemical rules for splicing and the potential role of these spliced peptides in immune recognition.

Hidden in Plain View: Discovery of Chimeric Diabetogenic CD4 T Cell Neo-Epitopes

The T cell antigens driving autoimmune Type 1 Diabetes (T1D) have been pursued for more than three decades. When diabetogenic CD4 T cell clones and their relevant MHCII antigen presenting alleles were first identified in rodents and humans, the path to discovering the peptide epitopes within pancreatic beta cell proteins seemed straightforward. However, as experimental results accumulated, definitive data were often absent or controversial. Work within the last decade has helped to clear up some of the controversy by demonstrating that a number of the important MHCII presented epitopes are not encoded in the natural beta cell proteins, but in fact are fusions between peptide fragments derived from the same or different proteins. Recently, the mechanism for generating these MHCII diabetogenic chimeric epitopes has been attributed to a form of reverse proteolysis, called transpeptidation, a process that has been well-documented in the production of MHCI presented epitopes. In this mini-review we summarize these data and their implications for T1D and other autoimmune responses.

Neoepitopes in Type 1 Diabetes: Etiological Insights, Biomarkers and Therapeutic Targets

The mechanisms underlying type 1 diabetes (T1D) pathogenesis remain largely unknown. While autoantibodies to pancreatic beta-cell antigens are often the first biological response and thereby a useful biomarker for identifying individuals in early stages of T1D, their role in T1D pathogenesis is not well understood. Recognition of these antigenic targets by autoreactive T-cells plays a pathological role in T1D development. Recently, several beta-cell neoantigens have been described, indicating that both neoantigens and known T1D antigens escape central or peripheral tolerance. Several questions regarding the mechanisms by which tolerance is broken in T1D remain unanswered. Further delineating the timing and nature of antigenic responses could allow their use as biomarkers to improve staging, as targets for therapeutic intervention, and lead to a better understanding of the mechanisms leading to loss of tolerance. Multiple factors that contribute to cellular stress may result in the generation of beta-cell derived neoepitopes and contribute to autoimmunity. Understanding the cellular mechanisms that induce beta-cells to produce neoantigens has direct implications on development of therapies to intercept T1D disease progression. In this perspective, we will discuss evidence for the role of neoantigens in the pathogenesis of T1D, including antigenic responses and cellular mechanisms. We will additionally discuss the pathways leading to neoepitope formation and the cross talk between the immune system and the beta-cells in this regard. Ultimately, delineating the timing of neoepitope generation in T1D pathogenesis will determine their role as biomarkers as well as therapeutic targets.

Identification of spliced peptides in pancreatic islets uncovers errors leading to false assignments

Proteasomal spliced peptides (PSPs) have been identified in the class I major histocompatibility complex (MHC) peptidomes of several tumors and have emerged as novel neoantigens that can stimulate highly specific T cells. Much debate has surrounded the percentage of PSPs in the immunopeptidome; reported numbers have ranged from <1-5% to 12-45%. Recently, our laboratory demonstrated in nonobese diabetic (NOD) mice that hybrid insulin peptides (HIPs), a special class of spliced peptides, are formed during insulin granule degradation in crinosomes of the pancreatic β cells and that modified peptides comprised a significant source of false positive HIP assignments. Herein, this study is extended to crinosomes isolated from other mouse strains and to two recent MHC class I studies, to see if modified peptides explained discrepancies in reported percentages of PSPs. This analysis revealed that both MHC-I peptidomes contained many spectra erroneously assigned as PSPs. While many false positive PSPs did arise from modified peptides, others arose from probable data processing errors. Thus, the reported numbers of PSPs in the literature are likely elevated due to errors associated with data processing and analysis.

Peptide-Spectrum Match Validation with Internal Standards (P-VIS): Internally-Controlled Validation of Mass Spectrometry-Based Peptide Identifications

Liquid chromatography-tandem mass spectrometry is an increasingly powerful tool for studying proteins in the context of disease. As technological advances in instrumentation and data analysis have enabled deeper profiling of proteomes and peptidomes, the need for a rigorous, standardized approach to validate individual peptide-spectrum matches (PSMs) has emerged. To address this need, we developed a novel and broadly applicable workflow: PSM validation with internal standards (P-VIS). In this approach, the fragmentation spectrum and chromatographic retention time of a peptide within a biological sample are compared with those of a synthetic version of the putative peptide sequence match. Similarity measurements obtained for a panel of internal standard peptides are then used to calculate a prediction interval for valid matches. If the observed degree of similarity between the biological and the synthetic peptide falls within this prediction interval, then the match is considered valid. P-VIS enables systematic and objective assessment of the validity of individual PSMs, providing a measurable degree of confidence when identifying peptides by mass spectrometry.

Hybrid Insulin Peptides Are Recognized by Human T Cells in the Context of DRB1*04:01

T cells isolated from the pancreatic infiltrates of nonobese diabetic mice have been shown to recognize epitopes formed by the covalent cross-linking of proinsulin and secretory granule peptides. Formation of such hybrid insulin peptides (HIPs) was confirmed through mass spectrometry, and responses to HIPs were observed among the islet-infiltrating T cells of pancreatic organ donors and in the peripheral blood of individuals with type 1 diabetes (T1D). However, questions remain about the prevalence of HIP-specific T cells in humans, the sequences they recognize, and their role in disease. We identified six novel HIPs that are recognized in the context of DRB1*04:01, discovered by using a library of theoretical HIP sequences derived from insulin fragments covalently linked to one another or to fragments of secretory granule proteins or other islet-derived proteins. We demonstrate that T cells that recognize these HIPs are detectable in the peripheral blood of subjects with T1D and exhibit an effector memory phenotype. HIP-reactive T-cell clones produced Th1-associated cytokines and proliferated in response to human islet preparations. These results support the relevance of HIPs in human disease, further establishing a novel posttranslational modification that may contribute to the loss of peripheral tolerance in T1D.

Vitiligo: Mechanisms of Pathogenesis and Treatment

Vitiligo is an autoimmune disease of the skin that targets pigment-producing melanocytes and results in patches of depigmentation that are visible as white spots. Recent research studies have yielded a strong mechanistic understanding of this disease. Autoreactive cytotoxic CD8+ T cells engage melanocytes and promote disease progression through the local production of IFN-γ, and IFN-γ-induced chemokines are then secreted from surrounding keratinocytes to further recruit T cells to the skin through a positive-feedback loop. Both topical and systemic treatments that block IFN-γ signaling can effectively reverse vitiligo in humans; however, disease relapse is common after stopping treatments. Autoreactive resident memory T cells are responsible for relapse, and new treatment strategies focus on eliminating these cells to promote long-lasting benefit. Here, we discuss basic, translational, and clinical research studies that provide insight into the pathogenesis of vitiligo, and how this insight has been utilized to create new targeted treatment strategies.

The MHC-II peptidome of pancreatic islets identifies key features of autoimmune peptides

The nature of autoantigens that trigger autoimmune diseases has been much discussed, but direct biochemical identification is lacking for most. Addressing this question demands unbiased examination of the self-peptides displayed by a defined autoimmune major histocompatibility complex class II (MHCII) molecule. Here we examined the immunopeptidome of the pancreatic islets in non-obese diabetic (NOD) mice, which spontaneously develop autoimmune diabetes based on the I-A^g7 variant of MHCII. The relevant peptides that induced pathogenic CD4⁺ T cells at the initiation of diabetes derived from proinsulin. These peptides were also found in the MHCII peptidome of the pancreatic lymph nodes and spleen. The proinsulin-derived peptides followed a trajectory from their generation and exocytosis in β cells, to uptake and presentation in islets and peripheral sites. Such a pathway generated conventional epitopes but also resulted in the presentation of post-translationally modified peptides, including deamidated sequences. These analyses reveal the key features of a restricted component in the self-MHCII peptidome that caused autoreactivity.

Diabetes-induced Proteome Changes Throughout Development

BACKGROUND

Diabetes Mellitus (DM) is a multisystemic disease involving the homeostasis of insulin secretion by the pancreatic islet beta cells (β-cells). It is associated with hypertension, renal disease, and arterial and arteriolar vascular diseases.

DISCUSSION

The classification of diabetes is identified as type 1 (gene linked β-cell destruction in childhood) and type 2 (late onset associated with β-cell overload and insulin resistance in peripheral tissues. Type 1 diabetes is characterized by insulin deficiency, type 2 diabetes by both insulin deficiency and insulin resistance. The former is a genetically programmed loss of insulin secretion whereas the latter constitutes a disruption of the homeostatic relationship between the opposing activity of β- cell insulin and alpha cell (α-cell) glucagon of the Islets of Langerhans. The condition could also occur in pregnancy, as a prenatal occurring event, possibly triggered by the hormonal changes of pregnancy combined with β-cell overload. This review discusses the molecular basis of the biomolecular changes that occur with respect to glucose homeostasis and related diseases in DM. The underlying link between pancreatic, renal, and microvascular diseases in DM is based on oxidative stress and the Unfolded Protein Response (UPR).

CONCLUSION

Studying proteome changes in diabetes can deepen our understanding of the biomolecular basis of disease and help us acquire more efficient therapies.

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.

Hybrid Insulin Peptides Are Autoantigens in Type 1 Diabetes

We recently established that hybrid insulin peptides (HIPs) are present in human islets and that T cells reactive to HIPs are found in the residual islets of organ donors with type 1 diabetes (T1D). Here, we investigate whether HIP-reactive T cells are indicative of ongoing autoimmunity in patients with T1D. We used interferon-γ enzyme-linked immune absorbent spot analyses on peripheral blood mononuclear cells (PBMCs) to determine whether patients with new-onset T1D or control subjects displayed T-cell reactivity to a panel of 16 HIPs. We observed that nearly one-half of the patients responded to one or more HIPs. Responses to four HIPs were significantly elevated in patients with T1D but not in control subjects. To characterize the T cells reactive to HIPs, we used a carboxyfluorescein succinimidyl ester-based assay to clone T cells from PBMCs. We isolated six nonredundant, antigen-specific T-cell clones, most of which reacting to their target HIPs in the low nanomolar range. One T-cell clone was isolated from the same patient on two different blood draws, indicating persistence of this T-cell clone in the peripheral blood. This work suggests that HIPs are important target antigens in human subjects with T1D and may play a critical role in disease.

Immune cell trafficking to the islets during type 1 diabetes

Inhibition of immune cell trafficking to the pancreatic islets during type 1 diabetes (T1D) has therapeutic potential, since targeting of T cell and B cell trafficking has been clinically effective in other autoimmune diseases. Trafficking to the islets is characterized by redundancy in adhesion molecule and chemokine usage, which has not enabled effective targeting to date. Additionally, cognate antigen is not consistently required for T cell entry into the islets throughout the progression of disease. However, myeloid cells are required to enable T cell and B cell entry into the islets, and may serve as a convergence point in the pathways controlling this process. In this review we describe current knowledge of the factors that mediate immune cell trafficking to pancreatic islets during T1D progression.

Alterations in Beta Cell Identity in Type 1 and Type 2 Diabetes

PURPOSE OF REVIEW

To discuss the current understanding of "β cell identity" and factors underlying altered identity of pancreatic β cells in diabetes, especially in humans.

RECENT FINDINGS

Altered identity of β cells due to dedifferentiation and/or transdifferentiation has been proposed as a mechanism of loss of β cells in diabetes. In dedifferentiation, β cells do not undergo apoptosis; rather, they lose their identity and function. Dedifferentiation is well characterized by the decrease in expression of key β cell markers such as genes encoding major transcription factors, e.g., MafA, NeuroD1, Nkx6.1, and Foxo1, and an increase in atypical or "disallowed" genes for β cells such as lactate dehydrogenase, monocarboxylate transporter MCT1, or progenitor cell genes (Neurog3, Pax4, or Sox9). Moreover, altered identity of mature β cells in diabetes also involves transdifferentiation of β cells into other islet hormone producing cells. For example, overexpression of α cell specific transcription factor Arx or ablation of Pdx1 resulted in an increase of α cell numbers and a decrease in β cell numbers in rodents. The frequency of α-β double-positive cells was also prominent in human subjects with T2D. These altered identities of β cells likely serve as a compensatory response to enhance function/expand cell numbers and may also camouflage/protect cells from ongoing stress. However, it is equally likely that this may be a reflection of new cell formation as a frank regenerative response to ongoing tissue injury. Physiologically, all these responses are complementary. In diabetes, (1) endocrine identity recapitulates the less mature/less-differentiated fetal/neonatal cell type, possibly representing an adaptive mechanism; (2) residual β cells may be altered in their subtype proportions or other molecular features; (3) in humans, "altered identity" is a preferable term to dedifferentiation as their cellular fate (differentiated cells losing identity or progenitors becoming more differentiated) is unclear as yet.

The Immunoreactive Platform of the Pancreatic Islets Influences the Development of Autoreactivity

Tissue homeostasis is maintained through a finely tuned balance between the immune system and the organ-resident cells. Disruption of this process not only results in organ dysfunction but also may trigger detrimental autoimmune responses. The islet of Langerhans consists of the insulin-producing β-cells essential for proper control of body metabolism, but less appreciated is that these cells naturally interact with the immune system, forming a platform by which the β-cell products are sensed, processed, and responded to by the local immune cells, particularly the islet-resident macrophages. Although its physiological outcomes are not completely understood, this immunoreactive platform is crucial for precipitating islet autoreactivity in individuals carrying genetic risks, leading to the development of type 1 diabetes. In this Perspective, we summarize recent studies that examine the cross talk between the β-cells and various immune components, with a primary focus on discussing how antigenic information generated during normal β-cell catabolism can be delivered to the resident macrophage and further recognized by the adaptive CD4 T-cell system, a critical step to initiate autoimmune diabetes. The core nature of the islet immune platform can be extrapolated to other endocrine tissues and may represent a common mechanism underlying the development of autoimmune syndromes influencing multiple endocrine organs.

Hybrid insulin peptides are neo-epitopes for CD4 T cells in autoimmune diabetes

PURPOSE OF REVIEW

The current review covers recent advances in our knowledge of the newest autoantigen neo-epitopes in type 1 diabetes (T1D): hybrid insulin peptides or HIPs. These ligands for autoreactive T cells are formed by peptide fusion, a novel posttranslational modification process that we first reported in 2016.

RECENT FINDINGS

Two major HIPs in the nonobese diabetic mouse model, ligands for diabetogenic CD4 T-cell clones, have been incorporated into tetramers and used to track HIP-reactive T cells during progression of disease. HIPs have also been used in strategies for induction of antigen-specific tolerance and show promise for delaying or reversing disease in the nonobese diabetic mouse. Importantly, CD4 T cells reactive to various HIPs have been detected in the islets and peripheral blood mononuclear cell of T1D patients and newly developed human T-cell clones are being employed to gather more data on the phenotype and function of HIP-reactive T cells in patients.

SUMMARY

These new hybrid insulin peptide epitopes may provide the basis for establishing autoreactive T cells as biomarkers of disease and as potential tolerogens for treatment of T1D.

HIPs and HIP-reactive T cells

Mounting evidence implicates hybrid insulin peptides (HIPs) as important autoantigens in the development of type 1 diabetes (T1D). These fusion peptides formed between insulin and other pancreatic beta cell-derived peptides contain non-genomically encoded amino acid sequences, making them plausible targets for autoreactive T cells in T1D. HIPs are detectable by mass spectrometry in human and murine islets and are targeted by diabetes-inducing T cells in non-obese diabetic mice as well as by T cells isolated from the residual pancreatic islets of human organ donors with T1D. The discovery of HIPs comes with numerous new challenges, as well as opportunities to study the pathogenesis of T1D. Here we review the original discovery of HIPs and describe recent studies investigating the role of HIP-reactive T cells in the development of diabetes. We also discuss potential mechanisms that may be responsible for the generation of HIPs in beta cells and describe challenges that need to be addressed in the field of mass spectrometry to enable the discovery of new HIPs. The identification of these potentially disease-driving antigens in T1D is of key interest to the field as it may provide new tools to predict, prevent and potentially reverse the disease.

Biomarker discovery in pre-Type 1 Diabetes; Th40 cells as a predictive risk factor

CONTEXT

The incidence of Type 1 Diabetes (T1D) is increasing worldwide. The quest to understand T1D etiology as well as how to predict diabetes is ongoing and, in many ways, those goals intertwine. While genetic components associate with T1D, not all T1D individuals have those components and not all subjects with those components develop disease.

OBJECTIVE

More robust methods for prediction of T1D are needed. Can high CD4+CD40+ T cell (Th40) levels be used as a biomarker in addition to other markers?

METHODS

Th40 levels were assessed along with other parameters in blood collected from prediabetic TrialNet subjects.

RESULTS

Pre-diabetic subjects, stratified according to their Th40 cell levels, demonstrate patterns that parallel those seen between control and T1D subjects. Cytokine patterns are significantly different between Th40-high and -low subjects and a CD4/CD8 double-positive population is more represented in Th40-high groups. Subjects experiencing impaired glucose tolerance present a significantly higher Th40 level than control subjects do. HLA DR4/DR4 and DQ8/DQ8, HLAs associated with T1D, are more likely found among Th40-high subjects. Interestingly, HLA DR4/DR4 subjects were significantly older compared with all other subjects, suggesting that this haplotype together with a high Th40 level may represent someone who will onset after age 30, which is reported for 42% of T1D cases.

CONCLUSION

Considering the differences found in relation to prediabetic Th40 cell level, it may be possible to devise methods that more accurately predicts who will proceed toward diabetes and, possibly, at what stage of prediabetes a subject is.

How C-terminal additions to insulin B-chain fragments create superagonists for T cells in mouse and human type 1 diabetes

In type 1 diabetes (T1D), proinsulin is a major autoantigen and the insulin B:9-23 peptide contains epitopes for CD4⁺ T cells in both mice and humans. This peptide requires carboxyl-terminal mutations for uniform binding in the proper position within the mouse IA^g7 or human DQ8 major histocompatibility complex (MHC) class II (MHCII) peptide grooves and for strong CD4⁺ T cell stimulation. Here, we present crystal structures showing how these mutations control CD4⁺ T cell receptor (TCR) binding to these MHCII-peptide complexes. Our data reveal stricking similarities between mouse and human CD4⁺ TCRs in their interactions with these ligands. We also show how fusions between fragments of B:9-23 and of proinsulin C-peptide create chimeric peptides with activities as strong or stronger than the mutated insulin peptides. We propose transpeptidation in the lysosome as a mechanism that could accomplish these fusions in vivo, similar to the creation of fused peptide epitopes for MHCI presentation shown to occur by transpeptidation in the proteasome. Were this mechanism limited to the pancreas and absent in the thymus, it could provide an explanation for how diabetogenic T cells escape negative selection during development but find their modified target antigens in the pancreas to cause T1D.

CD11c+ Cells Are Gatekeepers for Lymphocyte Trafficking to Infiltrated Islets During Type 1 Diabetes

Type 1 diabetes (T1D) is a T cell mediated autoimmune disease that affects more than 19 million people with incidence increasing rapidly worldwide. For T cells to effectively drive T1D, they must first traffic to the islets and extravasate through the islet vasculature. Understanding the cues that lead to T cell entry into inflamed islets is important because diagnosed T1D patients already have established immune infiltration of their islets. Here we show that CD11c⁺ cells are a key mediator of T cell trafficking to infiltrated islets in non-obese diabetic (NOD) mice. Using intravital 2-photon islet imaging we show that T cell extravasation into the islets is an extended process, with T cells arresting in the islet vasculature in close proximity to perivascular CD11c⁺ cells. Antigen is not required for T cell trafficking to infiltrated islets, but T cell chemokine receptor signaling is necessary. Using RNAseq, we show that islet CD11c⁺ cells express over 20 different chemokines that bind chemokine receptors expressed on islet T cells. One highly expressed chemokine-receptor pair is CXCL16-CXCR6. However, NOD. CXCR6^-/- mice progressed normally to T1D and CXCR6 deficient T cells trafficked normally to the islets. Even with CXCR3 and CXCR6 dual deficiency, T cells trafficked to infiltrated islets. These data reinforce that chemokine receptor signaling is highly redundant for T cell trafficking to inflamed islets. Importantly, depletion of CD11c⁺ cells strongly inhibited T cell trafficking to infiltrated islets of NOD mice. We suggest that targeted depletion of CD11c⁺ cells associated with the islet vasculature may yield a therapeutic target to inhibit T cell trafficking to inflamed islets to prevent progression of T1D.

Human amylin induces CD4+Foxp3+ regulatory T cells in the protection from autoimmune diabetes

Autoimmune diabetes is a disorder of immune homeostasis that leads to targeted insulin-secreting islet β cell destruction characterized by insulitis. Human amylin (hA) is an important neuroendocrine hormone co-secreted with insulin by pancreatic β cells. Here, we report hA immune-modulatory action through inducing regulatory T cells. We ex vivo-treated human peripheral blood mononuclear cells (hPBMCs) with hA for 24 h and counted CD4+Foxp3+ regulatory T cells (Treg) using flow cytometry. Diabetic status was monitored and splenic Treg were measured in non-obese diabetic (NOD) male mice. NOD mice were intraperitoneally injected once daily with hA (n = 25) or solvent for control (n = 25) for 7 months continuously. Spleen tissues were collected at the end of intervention and processed for flow cytometry and Western blot. We found a 2.9-fold (p < 0.05) increase of CD4+Foxp3+ Treg in hPBMCs treated with 10 nmol/L hA compared with negative control. Incidence of diabetes in hA-treated NOD mice decreased 44% (p = 0.045) in the 6th month and 57% (p = 0.0002) in the 7th month. Meanwhile, the hA treatment induced a 1.5-fold increase of CD4+Foxp3+ Treg from mouse splenocytes (p = 0.0013). Expression of transforming growth factor-β (TGF-β) and toll-like receptor-4 (TLR-4) were upregulated in hA-treated mice. Human amylin might protect against autoimmune diabetes via the induction of CD4+Foxp3+ Treg, which suggests a novel approach to improve autoimmune conditions.

Identification of Hybrid Insulin Peptides (HIPs) in Mouse and Human Islets by Mass Spectrometry

We recently discovered hybrid insulin peptides (HIPs) as a novel class of post-translationally modified peptides in murine-derived beta cell tumors, and we demonstrated that these molecules are autoantigens in type 1 diabetes (T1D). A HIP consists of an insulin fragment linked to another secretory granule peptide via a peptide bond. We verified that autoreactive CD4 T cells in both mouse and human autoimmune diabetes recognize these modified peptides. Here, we use mass spectrometric analyses to confirm the presence of HIPs in both mouse and human pancreatic islets. We also present criteria for the confident identification of these peptides. This work supports the hypothesis that HIPs are autoantigens in human T1D and provides a foundation for future efforts to interrogate this previously unknown component of the beta cell proteome.

Inflammation-Induced Citrullinated Glucose-Regulated Protein 78 Elicits Immune Responses in Human Type 1 Diabetes

The β-cell has become recognized as a central player in the pathogenesis of type 1 diabetes with the generation of neoantigens as potential triggers for breaking immune tolerance. We report that posttranslationally modified glucose-regulated protein 78 (GRP78) is a novel autoantigen in human type 1 diabetes. When human islets were exposed to inflammatory stress induced by interleukin-1β, tumor necrosis factor-α, and interferon-γ, arginine residue R510 within GRP78 was converted into citrulline, as evidenced by liquid chromatography-tandem mass spectrometry. This conversion, known as citrullination, led to the generation of neoepitopes, which effectively could be presented by HLA-DRB1*04:01 molecules. With the use of HLA-DRB1*04:01 tetramers and ELISA techniques, we demonstrate enhanced antigenicity of citrullinated GRP78 with significantly increased CD4⁺ T-cell responses and autoantibody titers in patients with type 1 diabetes compared with healthy control subjects. Of note, patients with type 1 diabetes had a predominantly higher percentage of central memory cells and a lower percentage of effector memory cells directed against citrullinated GRP78 compared with the native epitope. These results strongly suggest that citrullination of β-cell proteins, exemplified here by the citrullination of GRP78, contributes to loss of self-tolerance toward β-cells in human type 1 diabetes, indicating that β-cells actively participate in their own demise.

Global Identification of Post-Translationally Spliced Peptides with Neo-Fusion

Post-translationally spliced peptides have recently garnered significant interest as potential targets for cancer immunotherapy and as contributors to autoimmune diseases such as type 1 diabetes, yet feasible identification methods for spliced peptides have yet to be developed. Here we present Neo-Fusion, a search program for discovering spliced peptides in tandem mass spectrometry data. Neo-Fusion utilizes two separated ion database searches to identify the two halves of each spliced peptide, and then it infers the full spliced sequence. This strategy allows for the identification of spliced peptides without peptide length constraints, providing a broadly applicable tool suitable for identification of spliced peptides in a variety of systems, such as the HLA-I and HLA-II immunopeptidomes and in vitro digested protein samples obtained from organelles, cells, or tissues of interest. Using simulated spliced peptides to benchmark Neo-Fusion, 25% of all simulated spliced peptides were identified at a measured false-discovery rate of 5% for HLA-I. Neo-Fusion provides the research community with a powerful new tool to aid in the study of the prevalence and biological significance of post-translationally spliced peptides.

The use of proteomics to understand antiviral immunity

Viruses are intracellular pathogens that cause a vast array of diseases, which are often severe and typified by high morbidity and mortality rates. Viral infections continue to be a global health burden and effective vaccines and therapeutics are constantly sought to prevent and treat these infections. The development of such treatments generally relies on understanding the mechanisms that underpin efficient host antiviral immune responses. This review summarises recent developments in our understanding of antiviral adaptive immunity and in particular, highlights the use of mass spectrometry to elucidate viral antigens and their processing and presentation to T cells and other immune effectors. These processed peptides serve as potential vaccine candidates or may facilitate clinical monitoring, diagnosis and immunotherapy of infectious diseases.