Environmental Factors Contribute to β Cell Endoplasmic Reticulum Stress and Neo-Antigen Formation in Type 1 Diabetes

Immunopeptidome mining reveals a novel ERS-induced target in T1D

Autoreactive CD8+ T cells play a key role in type 1 diabetes (T1D), but the antigen spectrum that activates autoreactive CD8+ T cells remains unclear. Endoplasmic reticulum stress (ERS) has been implicated in β-cell autoantigen generation. Here, we analyzed the major histocompatibility complex class I (MHC-I)-associated immunopeptidome (MIP) of islet β-cells under steady and ERS conditions and found that ERS reshaped the MIP of β-cells and promoted the MHC-I presentation of a panel of conventional self-peptides. Among them, OTUB258-66 showed immunodominance, and the corresponding autoreactive CD8+ T cells were diabetogenic in nonobese diabetic (NOD) mice. High glucose intake upregulated pancreatic OTUB2 expression and amplified the OTUB258-66-specific CD8+ T-cell response in NOD mice. Repeated OTUB258-66 administration significantly reduced the incidence of T1D in NOD mice. Interestingly, peripheral blood mononuclear cells (PBMCs) from patients with T1D, but not from healthy controls, showed a positive IFN-γ response to human OTUB2 peptides. This study provides not only a new explanation for the role of ERS in promoting β-cell-targeted autoimmunity but also a potential target for the prevention and treatment of T1D. The data are available via ProteomeXchange with the identifier PXD041227.

PPARs at the crossroads of T cell differentiation and type 1 diabetes

T-cell-mediated autoimmune type 1 diabetes (T1D) is characterized by the immune-mediated destruction of pancreatic beta cells (β-cells). The increasing prevalence of T1D poses significant challenges to the healthcare system, particularly in countries with struggling economies. This review paper highlights the multifaceted roles of Peroxisome Proliferator-Activated Receptors (PPARs) in the context of T1D, shedding light on their potential as regulators of immune responses and β-cell biology. Recent research has elucidated the intricate interplay between CD4+ T cell subsets, such as Tregs and Th17, in developing autoimmune diseases like T1D. Th17 cells drive inflammation, while Tregs exert immunosuppressive functions, highlighting the delicate balance crucial for immune homeostasis. Immunotherapy has shown promise in reinstating self-tolerance and restricting the destruction of autoimmune responses, but further investigations are required to refine these therapeutic strategies. Intriguingly, PPARs, initially recognized for their role in lipid metabolism, have emerged as potent modulators of inflammation in autoimmune diseases, particularly in T1D. Although evidence suggests that PPARs affect the β-cell function, their influence on T-cell responses and their potential impact on T1D remains largely unexplored. It was noted that PPARα is involved in restricting the transcription of IL17A and enhancing the expression of Foxp3 by minimizing its proteasomal degradation. Thus, antagonizing PPARs may exert beneficial effects in regulating the differentiation of CD4+ T cells and preventing T1D. Therefore, this review advocates for comprehensive investigations to delineate the precise roles of PPARs in T1D pathogenesis, offering innovative therapeutic avenues that target both the immune system and pancreatic function. This review paper seeks to bridge the knowledge gap between PPARs, immune responses, and T1D, providing insights that may revolutionize the treatment landscape for this autoimmune disorder. Moreover, further studies involving PPAR agonists in non-obese diabetic (NOD) mice hold promise for developing novel T1D therapies.

Shedding new light on the role of ERAP1 in Type 1 diabetes: A perspective on disease management

Type 1 diabetes mellitus (T1D) is a multifactorial organ specific autoimmune disease which originates from the destruction of insulin-producing beta cells within the pancreatic islets by autoreactive CD8⁺ T lymphocytes. The autoimmune responses are raised against autoantigenic peptides presented in the context of the Major Histocompatibility Complex (MHC) class I molecules. Peptides are generated in the cytoplasm of the beta cell by degradation through the proteasome activity and other proteases. Proteolytic intermediate protein fragments are then vehicled into the endoplasmic reticulum (ER) by transporters associated with antigen processing TAP1 and TAP2. In the ER, Endoplasmic Reticulum Aminopeptidase 1 (ERAP1) and 2 (ERAP2) shape the intermediate proteins to produce the optimal peptide size for loading into the MHC class I molecules. Subsequently complexes are shuttled to the cell surface for antigen presentation. Genome Wide Association Studies (GWAS) have identified different SNPs of ERAP1 associated to several autoimmune diseases and in particular the T1D-related ERAP1 SNP rs30187 encoding for K528R ERAP1. An association between the ER stress and the increased exposure of beta cells to the immune system has been hypothesized to further contribute to the etiopathogenesis. In particular in a recent study by Thomaidou et al. 2020 (doi: https://doi.org/10.2337/db19-0984) the posttranscriptional regulation of ERAP1 is shown to shaping the recognition of the preproinsulin (PPI) signal peptide by cytotoxic T lymphocytes. In the light of foregoing ERAP1 inhibitors could potentially prevent the activation of epitope-specific autoimmune-promoting T cells and their cytokine production; further regulating ERAP1 expression at posttranscriptional level under stress conditions of the beta cells could help to reverse autoimmune process through limiting epitope-presentation to autoreactive T cells. In this article we provide a perspective on the role of ERAP1 as implicated in the pathogenesis of insulin-dependent diabetes mellitus by reviewing studies reported in literature and discussing our own experimental evidence.

HERV1-env Induces Unfolded Protein Response Activation in Autoimmune Liver Disease: A Potential Mechanism for Regulatory T Cell Dysfunction

Regulatory T cells (Tregs) are not terminally differentiated but can acquire effector properties. Here we report an increased expression of human endogenous retrovirus 1 (HERV1-env) proteins in Tregs of patients with de novo autoimmune hepatitis and autoimmune hepatitis, which induces endoplasmic reticulum (ER) stress. HERV1-env-triggered ER stress activates all three branches (IRE1, ATF6, and PERK) of the unfolded protein response (UPR). Our coimmunoprecipitation studies show an interaction between HERV1-env proteins and the ATF6 branch of the UPR. The activated form of ATF6α activates the expression of RORC and STAT3 by binding to promoter sequences and induces IL-17A production. Silencing of HERV1-env results in recovery of Treg suppressive function. These findings identify ER stress and UPR activation as key factors driving Treg plasticity (species: human).

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

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

Type 1 Diabetes Mellitus: A Review on Advances and Challenges in Creating Insulin Producing Devices

Type 1 diabetes mellitus (T1DM) is the most common autoimmune chronic disease in young patients. It is caused by the destruction of pancreatic endocrine β-cells that produce insulin in specific areas of the pancreas, known as islets of Langerhans. As a result, the body becomes insulin deficient and hyperglycemic. Complications associated with diabetes are life-threatening and the current standard of care for T1DM consists still of insulin injections. Lifesaving, exogenous insulin replacement is a chronic and costly burden of care for diabetic patients. Alternative therapeutic options have been the focus in these fields. Advances in molecular biology technologies and in microfabrication have enabled promising new therapeutic options. For example, islet transplantation has emerged as an effective treatment to restore the normal regulation of blood glucose in patients with T1DM. However, this technique has been hampered by obstacles, such as limited islet availability, extensive islet apoptosis, and poor islet vascular engraftment. Many of these unsolved issues need to be addressed before a potential cure for T1DM can be a possibility. New technologies like organ-on-a-chip platforms (OoC), multiplexed assessment tools and emergent stem cell approaches promise to enhance therapeutic outcomes. This review will introduce the disorder of type 1 diabetes mellitus, an overview of advances and challenges in the areas of microfluidic devices, monitoring tools, and prominent use of stem cells, and how they can be linked together to create a viable model for the T1DM treatment. Microfluidic devices like OoC platforms can establish a crucial platform for pathophysiological and pharmacological studies as they recreate the pancreatic environment. Stem cell use opens the possibility to hypothetically generate a limitless number of functional pancreatic cells. Additionally, the integration of stem cells into OoC models may allow personalized or patient-specific therapies.

Biomarkers of autoimmunity and beta cell metabolism in type 1 diabetes

Posttranslational protein modifications (PTMs) are an inherent response to physiological changes causing altered protein structure and potentially modulating important biological functions of the modified protein. Besides cellular metabolic pathways that may be dictated by PTMs, the subtle change of proteins also may provoke immune attack in numerous autoimmune diseases. Type 1 diabetes (T1D) is a chronic autoimmune disease destroying insulin-producing beta cells within the pancreatic islets, a result of tissue inflammation to specific autoantigens. This review summarizes how PTMs arise and the potential pathological consequence of PTMs, with particular focus on specific autoimmunity to pancreatic beta cells and cellular metabolic dysfunction in T1D. Moreover, we review PTM-associated biomarkers in the prediction, diagnosis and in monitoring disease activity in T1D. Finally, we will discuss potential preventive and therapeutic approaches of targeting PTMs in repairing or restoring normal metabolic pathways in pancreatic islets.

Posttranslational modifications in diabetes: Mechanisms and functions

As one of the most widespread chronic diseases, diabetes and its accompanying complications affect approximately one tenth of individuals worldwide and represent a growing cause of morbidity and mortality. Accumulating evidence has proven that the process of diabetes is complex and interactive, involving various cellular responses and signaling cascades by posttranslational modifications (PTMs). Therefore, understanding the mechanisms and functions of PTMs in regulatory networks has fundamental importance for understanding the prediction, onset, diagnosis, progression, and treatment of diabetes. In this review, we offer a holistic summary and illustration of the crosstalk between PTMs and diabetes, including both types 1 and 2. Meanwhile, we discuss the potential use of PTMs in diabetes treatment and provide a prospective direction for deeply understanding the metabolic diseases.

Lactiplantibacillus plantarum 299v supplementation modulates β-cell ER stress and antioxidative defense pathways and prevents type 1 diabetes in gluten-free BioBreeding rats

The increasing incidence of Type 1 diabetes has coincided with the emergence of the low-fiber, high-gluten Western diet and other environmental factors linked to dysbiosis. Since Lactiplantibacillus plantarum 299 v (Lp299v) supplementation improves gut barrier function and reduces systemic inflammation, we studied its effects in spontaneously diabetic DRlyp/lyp rats provided a normal cereal diet (ND) or a gluten-free hydrolyzed casein diet (HCD). All rats provided ND developed diabetes (62.5±7.7 days); combining ND with Lp299v did not improve survival. Diabetes was delayed by HCD (72.2±9.4 days, p = .01) and further delayed by HCD+Lp299v (84.9±14.3 days, p < .001). HCD+Lp299v pups exhibited increased plasma propionate and butyrate levels, which correlated with enriched fecal Bifidobacteriaceae and Clostridiales taxa. Islet transcriptomic and histologic analyses at 40-days of age revealed that rats fed HCD expressed an autophagy profile, while those provided HCD+Lp299v expressed ER-associated protein degradation (ERAD) and antioxidative defense pathways, including Nrf2. Exposing insulinoma cells to propionate and butyrate promoted the antioxidative defense response but did not recapitulate the HCD+Lp299v islet ERAD transcriptomic profile. Here, both diet and microbiota influenced diabetes susceptibility. Moreover, Lp299v supplement modulated antioxidative defense and ER stress responses in β-cells, potentially offering a new therapeutic direction to thwart diabetes progression and preserve insulin secretion.

Environmental Triggering of Type 1 Diabetes Autoimmunity

Type 1 diabetes (T1D) is a chronic autoimmune disease in which pancreatic islet β cells are destroyed by immune cells, ultimately leading to overt diabetes. The progressive increase in T1D incidence over the years points to the role of environmental factors in triggering or accelerating the disease process which develops on a highly multigenic susceptibility background. Evidence that environmental factors induce T1D has mostly been obtained in animal models. In the human, associations between viruses, dietary habits or changes in the microbiota and the development of islet cell autoantibodies or overt diabetes have been reported. So far, prediction of T1D development is mostly based on autoantibody detection. Future work should focus on identifying a causality between the different environmental risk factors and T1D development to improve prediction scores. This should allow developing preventive strategies to limit the T1D burden in the future.

SARS-CoV-2 induced post-translational protein modifications: A trigger for developing autoimmune diabetes?

Emerging evidence indicates a bi-directional relationship between SARS-CoV-2 and diabetes. The possibility exists that SARS-CoV-2 could induce diabetes, but it is not yet clear whether this might be a fulminant-type diabetes, autoimmune diabetes, or a new-onset transient hyperglycaemia. This viewpoint discusses mechanisms by which SARS-CoV-2 might trigger type 1 diabetes mellitus (T1DM). Specifically, we looked at the role of post-translational protein modifications (PTMs) and the generation of neoepitopes as a potential mechanism in the induction of islet autoimmunity, and the pathways via which coronavirus infections might exacerbate the formation of PTMs and, in so doing, provoke the onset of T1DM.

An accomplice more than a mere victim: The impact of β-cell ER stress on type 1 diabetes pathogenesis

BACKGROUND

Pancreatic β-cells are the insulin factory of an organism with a mission to regulate glucose homeostasis in the body. Due to their high secretory activity, β-cells rely on a functional and intact endoplasmic reticulum (ER). Perturbations to ER homeostasis and unmitigated stress lead to β-cell dysfunction and death. Type 1 diabetes (T1D) is a chronic inflammatory disease caused by the autoimmune-mediated destruction of β-cells. Although autoimmunity is an essential component of T1D pathogenesis, accumulating evidence suggests an important role of β-cell ER stress and aberrant unfolded protein response (UPR) in disease initiation and progression.

SCOPE OF REVIEW

In this article, we introduce ER stress and the UPR, review β-cell ER stress in various mouse models, evaluate its involvement in inflammation, and discuss the effects of ER stress on β-cell plasticity and demise, and islet autoimmunity in T1D. We also highlight the relationship of ER stress with other stress response pathways and provide insight into ongoing clinical studies targeting ER stress and the UPR for the prevention or treatment of T1D.

MAJOR CONCLUSIONS

Evidence from ex vivo studies, in vivo mouse models, and tissue samples from patients suggest that β-cell ER stress and a defective UPR contribute to T1D pathogenesis. Thus, restoration of β-cell ER homeostasis at various stages of disease presents a plausible therapeutic strategy for T1D. Identifying the specific functions and regulation of each UPR sensor in β-cells and uncovering the crosstalk between stressed β-cells and immune cells during T1D progression would provide a better understanding of the molecular mechanisms of disease process, and may reveal novel targets for development of effective therapies for T1D.

High-fat diet prevents the development of autoimmune diabetes in NOD mice

AIMS

Type 1 diabetes (T1D) has a strong genetic predisposition and requires an environmental trigger to initiate the beta-cell autoimmune destruction. The rate of childhood obesity has risen in parallel to the proportion of T1D, suggesting high-fat diet (HFD)/obesity as potential environmental triggers for autoimmune diabetes. To explore this, non-obese diabetic (NOD) mice were subjected to HFD and monitored for the development of diabetes, insulitis and beta-cell stress.

MATERIALS AND METHODS

Four-week-old female NOD mice were placed on HFD (HFD-NOD) or standard chow-diet. Blood glucose was monitored weekly up to 40 weeks of age, and glucose- and insulin-tolerance tests performed at 4, 10 and 15 weeks. Pancreata and islets were analysed for insulin secretion, beta-cell mass, inflammation, insulitis and endoplasmic reticulum stress markers. Immune cell levels were measured in islets and spleens. Stool microbiome was analysed at age 4, 8 and 25 weeks.

RESULTS

At early ages, HFD-NOD mice showed a significant increase in body weight, glucose intolerance and insulin resistance; but paradoxically, they were protected from developing diabetes. This was accompanied by increased insulin secretion and beta-cell mass, decreased insulitis, increased splenic T-regulatory cells and altered stool microbiome.

CONCLUSIONS

This study shows that HFD protects NOD mice from autoimmune diabetes and preserves beta-cell mass and function through alterations in gut microbiome, increased T-regulatory cells and decreased insulitis. Further studies into the exact mechanism of HFD-mediated prevention of diabetes in NOD mice could potentially lead to interventions to prevent or delay T1D development in humans.

Targeting β-cell dedifferentiation and transdifferentiation: opportunities and challenges

The most distinctive pathological characteristics of diabetes mellitus induced by various stressors or immune-mediated injuries are reductions of pancreatic islet β-cell populations and activity. Existing treatment strategies cannot slow disease progression; consequently, research to genetically engineer β-cell mimetics through bi-directional plasticity is ongoing. The current consensus implicates β-cell dedifferentiation as the primary etiology of reduced β-cell mass and activity. This review aims to summarize the etiology and proposed mechanisms of β-cell dedifferentiation and to explore the possibility that there might be a time interval from the onset of β-cell dysfunction caused by dedifferentiation to the development of diabetes, which may offer a therapeutic window to reduce β-cell injury and to stabilize functionality. In addition, to investigate β-cell plasticity, we review strategies for β-cell regeneration utilizing genetic programming, small molecules, cytokines, and bioengineering to transdifferentiate other cell types into β-cells; the development of biomimetic acellular constructs to generate fully functional β-cell-mimetics. However, the maturation of regenerated β-cells is currently limited. Further studies are needed to develop simple and efficient reprogramming methods for assembling perfectly functional β-cells. Future investigations are necessary to transform diabetes into a potentially curable disease.

Glycolysis Inhibition Induces Functional and Metabolic Exhaustion of CD4+ T Cells in Type 1 Diabetes

In Type 1 Diabetes (T1D), CD4+ T cells initiate autoimmune attack of pancreatic islet β cells. Importantly, bioenergetic programs dictate T cell function, with specific pathways required for progression through the T cell lifecycle. During activation, CD4+ T cells undergo metabolic reprogramming to the less efficient aerobic glycolysis, similarly to highly proliferative cancer cells. In an effort to limit tumor growth in cancer, use of glycolytic inhibitors have been successfully employed in preclinical and clinical studies. This strategy has also been utilized to suppress T cell responses in autoimmune diseases like Systemic Lupus Erythematosus (SLE), Multiple Sclerosis (MS), and Rheumatoid Arthritis (RA). However, modulating T cell metabolism in the context of T1D has remained an understudied therapeutic opportunity. In this study, we utilized the small molecule PFK15, a competitive inhibitor of the rate limiting glycolysis enzyme 6-phosphofructo-2-kinase/fructose-2,6- biphosphatase 3 (PFKFB3). Our results confirmed PFK15 inhibited glycolysis utilization by diabetogenic CD4+ T cells and reduced T cell responses to β cell antigen in vitro. In an adoptive transfer model of T1D, PFK15 treatment delayed diabetes onset, with 57% of animals remaining euglycemic at the end of the study period. Protection was due to induction of a hyporesponsive T cell phenotype, characterized by increased and sustained expression of the checkpoint molecules PD-1 and LAG-3 and downstream functional and metabolic exhaustion. Glycolysis inhibition terminally exhausted diabetogenic CD4+ T cells, which was irreversible through restimulation or checkpoint blockade in vitro and in vivo. In sum, our results demonstrate a novel therapeutic strategy to control aberrant T cell responses by exploiting the metabolic reprogramming of these cells during T1D. Moreover, the data presented here highlight a key role for nutrient availability in fueling T cell function and has implications in our understanding of T cell biology in chronic infection, cancer, and autoimmunity.

Citrullination and PAD Enzyme Biology in Type 1 Diabetes - Regulators of Inflammation, Autoimmunity, and Pathology

The generation of post-translational modifications (PTMs) in human proteins is a physiological process leading to structural and immunologic variety in proteins, with potentially altered biological functions. PTMs often arise through normal responses to cellular stress, including general oxidative changes in the tissue microenvironment and intracellular stress to the endoplasmic reticulum or immune-mediated inflammatory stresses. Many studies have now illustrated the presence of 'neoepitopes' consisting of PTM self-proteins that induce robust autoimmune responses. These pathways of inflammatory neoepitope generation are commonly observed in many autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes (T1D), among others. This review will focus on one specific PTM to self-proteins known as citrullination. Citrullination is mediated by calcium-dependent peptidylarginine deiminase (PAD) enzymes, which catalyze deimination, the conversion of arginine into the non-classical amino acid citrulline. PADs and citrullinated peptides have been associated with different autoimmune diseases, notably with a prominent role in the diagnosis and pathology of rheumatoid arthritis. More recently, an important role for PADs and citrullinated self-proteins has emerged in T1D. In this review we will provide a comprehensive overview on the pathogenic role for PADs and citrullination in inflammation and autoimmunity, with specific focus on evidence for their role in T1D. The general role of PADs in epigenetic and transcriptional processes, as well as their crucial role in histone citrullination, neutrophil biology and neutrophil extracellular trap (NET) formation will be discussed. The latter is important in view of increasing evidence for a role of neutrophils and NETosis in the pathogenesis of T1D. Further, we will discuss the underlying processes leading to citrullination, the genetic susceptibility factors for increased recognition of citrullinated epitopes by T1D HLA-susceptibility types and provide an overview of reported autoreactive responses against citrullinated epitopes, both of T cells and autoantibodies in T1D patients. Finally, we will discuss recent observations obtained in NOD mice, pointing to prevention of diabetes development through PAD inhibition, and the potential role of PAD inhibitors as novel therapeutic strategy in autoimmunity and in T1D in particular.

Endoplasmic Reticulum-Mitochondria Crosstalk and Beta-Cell Destruction in Type 1 Diabetes

Beta-cell destruction in type 1 diabetes (T1D) results from the combined effect of inflammation and recurrent autoimmunity. In response to inflammatory signals, beta-cells engage adaptive mechanisms where the endoplasmic reticulum (ER) and mitochondria act in concert to restore cellular homeostasis. In the recent years it has become clear that this adaptive phase may trigger the development of autoimmunity by the generation of autoantigens recognized by autoreactive CD8 T cells. The participation of the ER stress and the unfolded protein response to the increased visibility of beta-cells to the immune system has been largely described. However, the role of the other cellular organelles, and in particular the mitochondria that are central mediator for beta-cell survival and function, remains poorly investigated. In this review we will dissect the crosstalk between the ER and mitochondria in the context of T1D, highlighting the key role played by this interaction in beta-cell dysfunctions and immune activation, especially through regulation of calcium homeostasis, oxidative stress and generation of mitochondrial-derived factors.

Type I interferons as key players in pancreatic β-cell dysfunction in type 1 diabetes

Type 1 diabetes (T1D) is a chronic autoimmune disease characterized by pancreatic islet inflammation (insulitis) and specific pancreatic β-cell destruction by an immune attack. Although the precise underlying mechanisms leading to the autoimmune assault remain poorly understood, it is well accepted that insulitis takes place in the context of a conflicting dialogue between pancreatic β-cells and the immune cells. Moreover, both host genetic background (i.e., candidate genes) and environmental factors (e.g., viral infections) contribute to this inadequate dialogue. Accumulating evidence indicates that type I interferons (IFNs), cytokines that are crucial for both innate and adaptive immune responses, act as key links between environmental and genetic risk factors in the development of T1D. This chapter summarizes some relevant pathways involved in β-cell dysfunction and death, and briefly reviews how enteroviral infections and genetic susceptibility can impact insulitis. Moreover, we present the current evidence showing that, in β-cells, type I IFN signaling pathway activation leads to several outcomes, such as long-lasting major histocompatibility complex (MHC) class I hyperexpression, endoplasmic reticulum (ER) stress, epigenetic changes, and induction of posttranscriptional as well as posttranslational modifications. MHC class I overexpression, when combined with ER stress and posttranscriptional/posttranslational modifications, might lead to sustained neoantigen presentation to immune system and β-cell apoptosis. This knowledge supports the concept that type I IFNs are implicated in the early stages of T1D pathogenesis. Finally, we highlight the promising therapeutic avenues for T1D treatment directed at type I IFN signaling pathway.

Neuroplastin Modulates Anti-inflammatory Effects of MANF

Endoplasmic reticulum (ER) stress is known to induce pro-inflammatory response and ultimately leads to cell death. Mesencephalic astrocyte-derived neurotrophic factor (MANF) is an ER-localized protein whose expression and secretion is induced by ER stress and a crucial survival factor. However, the underlying mechanism of how MANF exerts its cytoprotective activity remains unclear due to the lack of knowledge of its receptor. Here we show that Neuroplastin (NPTN) is such a receptor for MANF. Biochemical analysis shows the physiological interaction between MANF and NPTN on the cell surface. Binding of MANF to NPTN mitigates the inflammatory response and apoptosis via suppression of NF-kβ signaling. Our results demonstrate that NPTN is a cell surface receptor for MANF, which modulates inflammatory responses and cell death, and that the MANF-NPTN survival signaling described here provides potential therapeutic targets for the treatment of ER stress-related disorders, including diabetes mellitus, neurodegeneration, retinal degeneration, and Wolfram syndrome.

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Neuroplastin (NPTN) is a plasma membrane receptor for MANF

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NPTN regulates MANF-mediated suppression of inflammation

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NPTN regulates cell survival mediated by MANF under ER stress

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MANF-NPTN survival pathway provides potential therapeutic targets for ER stress-related disorders

β-Cell Stress Shapes CTL Immune Recognition of Preproinsulin Signal Peptide by Posttranscriptional Regulation of Endoplasmic Reticulum Aminopeptidase 1

The signal peptide of preproinsulin is a major source for HLA class I autoantigen epitopes implicated in CD8 T cell (CTL)-mediated β-cell destruction in type 1 diabetes (T1D). Among them, the 10-mer epitope located at the C-terminal end of the signal peptide was found to be the most prevalent in patients with recent-onset T1D. While the combined action of signal peptide peptidase and endoplasmic reticulum (ER) aminopeptidase 1 (ERAP1) is required for processing of the signal peptide, the mechanisms controlling signal peptide trimming and the contribution of the T1D inflammatory milieu on these mechanisms are unknown. Here, we show in human β-cells that ER stress regulates ERAP1 gene expression at posttranscriptional level via the IRE1α/miR-17-5p axis and demonstrate that inhibition of the IRE1α activity impairs processing of preproinsulin signal peptide antigen and its recognition by specific autoreactive CTLs during inflammation. These results underscore the impact of ER stress in the increased visibility of β-cells to the immune system and position the IRE1α/miR-17 pathway as a central component in β-cell destruction processes and as a potential target for the treatment of autoimmune T1D.

The Role of β Cell Stress and Neo-Epitopes in the Immunopathology of Type 1 Diabetes

Due to their secretory function, β cells are predisposed to higher levels of endoplasmic reticulum (ER) stress and greater sensitivity to inflammation than other cell types. These stresses elicit changes in β cells that alter their function and immunogenicity, including defective ribosomal initiation, post-translational modifications (PTMs) of endogenous β cell proteins, and alternative splicing. Multiple published reports confirm the presence of not only CD8+ T cells, but also autoreactive CD4+ T cells within pancreatic islets. Although the specificities of T cells that infiltrate human islets are incompletely characterized, they have been confirmed to include neo-epitopes that are formed through stress-related enzymatic modifications of β cell proteins. This article summarizes emerging knowledge about stress-induced changes in β cells and data supporting a role for neo-antigen formation and cross-talk between immune cells and β cells that provokes autoimmune attack - leading to a breakdown in tissue-specific tolerance in subjects who develop type 1 diabetes.

What Is the Sweetest UPR Flavor for the β-cell? That Is the Question

Unfolded protein response (UPR) is a process conserved from yeasts to mammals and, based on the generally accepted dogma, helps the secretory performance of a cell, by improving its capacity to cope with a burden in the endoplasmic reticulum (ER). The ER of β-cells, "professional secretory cells", has to manage tremendous amounts of insulin, which elicits a strong pressure on the ER intrinsic folding capacity. Thus, the constant demand for insulin production results in misfolded proinsulin, triggering a physiological upregulation of UPR to restore homeostasis. Most diabetic disorders are characterized by the loss of functional β-cells, and the pathological side of UPR plays an instrumental role. The transition from a homeostatic to a pathological UPR that ultimately leads to insulin-producing β-cell decay entails complex cellular processes and molecular mechanisms which remain poorly described so far. Here, we summarize important processes that are coupled with or driven by UPR in β-cells, such as proliferation, inflammation and dedifferentiation. We conclude that the UPR comes in different "flavors" and each of them is correlated with a specific outcome for the cell, for survival, differentiation, proliferation as well as cell death. All these greatly depend on the way UPR is triggered, however what exactly is the switch that favors the activation of one UPR as opposed to others is largely unknown. Substantial work needs to be done to progress the knowledge in this important emerging field as this will help in the development of novel and more efficient therapies for diabetes.

Setting the Stage for Islet Autoimmunity in Type 2 Diabetes: Obesity-Associated Chronic Systemic Inflammation and Endoplasmic Reticulum (ER) Stress

Islet autoimmunity has been identified as a component of both type 1 (T1D) and type 2 (T2D) diabetes, but the pathway through which islet autoimmunity develops in T1D and T2D may be different. Acknowledging the presence of islet autoimmunity in the pathophysiology of T2D, a historically nonautoimmune metabolic disease, would pave the way for important changes in classifications of and therapeutic options for T2D. In order to fully appreciate the importance of islet autoimmunity in T2D, the underlying mechanisms for immune system activation need to be explored. In this review, we focus on the potential origin of immune system activation (innate and adaptive) leading to the development of islet autoimmunity in T2D.

Molecular Insights Into the Relationship Between Autoimmune Thyroid Diseases and Breast Cancer: A Critical Perspective on Autoimmunity and ER Stress

The etiopathologies behind autoimmune thyroid diseases (AITDs) unravel misbehavior of immune components leading to the corruption of immune homeostasis where thyroid autoantigens turn foe to the self. In AITDs lymphocytic infiltration in the thyroid shows up a deranged immune system charging the follicular cells of the thyroid gland (thyrocytes) leading to the condition of either hyperthyroidism or hypothyroidism. The inflammation in AITDs consistently associate with ER function due to which disturbances in the ER protein homeostasis leads to unfolded protein response (UPR) that promotes pathogenesis of autoimmunity. The roles of ER stress in the instantaneous downregulation of MHC class I molecules on thyrocytes and the relevance of IFN γ in the pathogenesis of AITD has been well-documented. Thyroglobulin being the major target of autoantibodies in most of the AITDs is because of its unusual processing in the ER. Autoimmune disorders display a conglomeration of ER stress-induced UPR activated molecules. Several epidemiological data highlight the preponderance of AITDs in women as well as its concurrence with breast cancer. Both being an active glandular system displaying endocrine activity, thyroid as well as breast tissue show various commonalities in the expression pattern of heterogenous molecules that not only participate in the normal functioning but at the same time share the blame during disease establishment. Studies on the development and progression of breast carcinoma display a deranged and uncontrolled immune response, which is meticulously exploited during tumor metastasis. The molecular crosstalks between AITDs and breast tumor microenvironment rely on active participation of immune cells. The induction of ER stress by Tunicamycin advocates to provide a model for cancer therapy by intervening glycosylation. Therefore, this review attempts to showcase the molecules that are involved in feeding up the relationship between breast carcinoma and AITDs.

Bioluminescent reporter assay for monitoring ER stress in human beta cells

During type 1 diabetes development, cells in the islets of Langerhans engage adaptive mechanisms in response to inflammatory signals to cope with stress, to restore cellular homeostasis, and to preserve cell function. Disruption of these mechanisms may induce the formation of a repertoire of stress-induced neoantigens, which are critical in the loss of tolerance to beta cells and the development of autoimmunity. While multiple lines of evidence argue for a critical role of the endoplasmic reticulum in these processes, the lack of tools to specifically monitor beta cell stress hampers the development of therapeutic interventions focusing on maintaining endoplasmic reticulum homeostasis. Here we designed and evaluated a stress-induced reporter in which induction of stress correlates with increased light emission. This Gaussia luciferase-based reporter system employs the unconventional cytoplasmic splicing of XBP1 to report ER stress in cells exposed to known ER-stress inducers. Linking this reporter to a human beta cell-specific promotor allows tracing ER-stress in isolated human beta cells as well as in the EndoC-βH1 cell line. This reporter system represents a valuable tool to assess ER stress in human beta cells and may aid the identification of novel therapeutics that can prevent beta cell stress in human pancreatic islets.

Antigen recognition in autoimmune diabetes: a novel pathway underlying disease initiation

Development of human autoimmune disorders results from complex interplay among genetic, environmental, and immunological risk factors. Despite much heterogeneity in environmental triggers, the leading genes that give the propensity for tissue-specific autoimmune diseases, such as type 1 diabetes, are those associated with particular class II major histocompatibility complex alleles. Such genetic predisposition precipitates presentation of tissue antigens to MHC-II-restricted CD4 T cells. When properly activated, these self-reactive CD4 T cells migrate to the target tissue and trigger the initial immune attack. Using the non-obese diabetic mouse model of spontaneous autoimmune diabetes, much insight has been gained in understanding how presentation of physiological levels of self-antigens translates into pathological outcomes. In this review, we summarize recent advances illustrating the features of the antigen presenting cells, the sites of the antigen recognition, and the nature of the consequent T cell responses. We emphasize emerging evidence that highlights the importance of systemic presentation of catabolized tissue antigens in mobilization of pathogenic T cells. The implication of these studies in therapeutic perspectives is also discussed.

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.

Autocatalytic cycle in the pathogenesis of diabetes mellitus: biochemical and pathophysiological aspects of metabolic therapy with natural amino acids on the example of glycine

In this work systematization (classification) of biochemical and physiological processes that cause disorders in the human body during the development of diabetes mellitus is carried out. The development of the disease is considered as the interaction and mutual reinforcement of two groups of parallel processes. The first group has a molecular nature and it is associated with impairment of ROS-regulation system which includes NADPH oxidases, RAGE receptors, mitochondria, cellular peroxireductase system and the immune system. The second group has a pathophysiological nature and it is associated with impairment of microcirculation and liver metabolism. The analysis of diabetes biochemistry based on different published references yields a creation of a block diagram evaluating the disease development over time. Two types of autocatalytic processes were identified: autocatalysis in the cascade of biochemical reactions and "cross-section" catalysis, in which biochemical and pathophysiological processes reinforce each other. The developed model has shown the possibility of using pharmacologically active natural metabolite glycine as a medicine inhibiting the development of diabetes. Despite the fact that glycine is a substitute amino acid the drop in the glycine blood concentration occurs even in the early stages of diabetes development and can aggravate the disease. It is shown that glycine is a potential blocker of key autocatalytic cycles, including biochemical and pathophysiological processes. The analysis of the glycine action based on the developed model is in complete agreement with the results of clinical trials in which glycine has improved blood biochemistry of diabetic patients and thereby it prevents the development of diabetic complications.

Comparison of Acute Gene Expression Profiles of Islet Cells Obtained via Laser Capture Microdissection between Alloxan- and Streptozotocin-treated Rats

To identify the molecular profiles of islets from alloxan (ALX)- and streptozotocin (STZ)-treated rats, a microarray-based global gene expression analysis was performed on frozen islets isolated via laser capture microdissection. At 6 weeks old, rats were injected with ALX (40 mg/kg) or STZ (50 or 100 mg/kg) and then euthanized 24 hr later. Histopathological analysis showed β-cell necrosis, macrophage infiltration, and islet atrophy. The extent of these changes was more notable in the STZ groups than in the ALX group. Transcriptome analysis demonstrated a significant up- or downregulation of cell cycle arrest-related genes in the p53 signaling pathway. Cyclin D2 and cyclin-dependent kinase inhibitor 1A, mediators of G1 arrest, were remarkably altered in STZ-treated rats. In contrast, cyclin-B1 and cyclin-dependent kinase 1, mediators of G2 arrest, were remarkably changed in ALX-treated rats. Genes involved in the intrinsic mitochondria-mediated apoptotic pathway were upregulated in the ALX and STZ groups. Moreover, heat-shock 70 kDA protein 1A ( Hspa1a), Hsp90ab1, and Hsph1 were upregulated in ALX-treated rats, suggesting that ALX treatment injures β cells via endoplasmic reticulum stress. These results contribute to a better understanding of gene expression in the pathogenesis of islet toxicity.

IRE1α Implications in Endoplasmic Reticulum Stress-Mediated Development and Pathogenesis of Autoimmune Diseases

Inositol-requiring transmembrane kinase/endoribonuclease 1α (IRE1α) is the most prominent and evolutionarily conserved endoplasmic reticulum (ER) membrane protein. This transduces the signal of misfolded protein accumulation in the ER, named as ER stress, to the nucleus as “unfolded protein response (UPR).” The ER stress-mediated IRE1α signaling pathway arbitrates the yin and yang of cell life. IRE1α has been implicated in several physiological as well as pathological conditions, including immune disorders. Autoimmune diseases are caused by abnormal immune responses that develop due to genetic mutations and several environmental factors, including infections and chemicals. These factors dysregulate the cell immune reactions, such as cytokine secretion, antigen presentation, and autoantigen generation. However, the mechanisms involved, in which these factors induce the onset of autoimmune diseases, are remaining unknown. Considering that these environmental factors also induce the UPR, which is expected to have significant role in secretory cells and immune cells. The role of the major UPR molecule, IRE1α, in causing immune responses is well identified, but its role in inducing autoimmunity and the pathogenesis of autoimmune diseases has not been clearly elucidated. Hence, a better understanding of the role of IRE1α and its regulatory mechanisms in causing autoimmune diseases could help to identify and develop the appropriate therapeutic strategies. In this review, we mainly center the discussion on the molecular mechanisms of IRE1α in the pathophysiology of autoimmune diseases.

Immune Recognition of β-Cells: Neoepitopes as Key Players in the Loss of Tolerance

Prior to the onset of type 1 diabetes, there is progressive loss of immune self-tolerance, evidenced by the accumulation of islet autoantibodies and emergence of autoreactive T cells. Continued autoimmune activity leads to the destruction of pancreatic β-cells and loss of insulin secretion. Studies of samples from patients with type 1 diabetes and of murine disease models have generated important insights about genetic and environmental factors that contribute to susceptibility and immune pathways that are important for pathogenesis. However, important unanswered questions remain regarding the events that surround the initial loss of tolerance and subsequent failure of regulatory mechanisms to arrest autoimmunity and preserve functional β-cells. In this Perspective, we discuss various processes that lead to the generation of neoepitopes in pancreatic β-cells, their recognition by autoreactive T cells and antibodies, and potential roles for such responses in the pathology of disease. Emerging evidence supports the relevance of neoepitopes generated through processes that are mechanistically linked with β-cell stress. Together, these observations support a paradigm in which neoepitope generation leads to the activation of pathogenic immune cells that initiate a feed-forward loop that can amplify the antigenic repertoire toward pancreatic β-cell proteins.