Minimal impact of a de novo-expressed beta-cell autoantigen on spontaneous diabetes development in NOD mice

Immunological and virological triggers of type 1 diabetes: insights and implications

Type 1 diabetes (T1D) is caused by an autoimmune process which culminates in the destruction of insulin-producing beta cells in the pancreas. It is widely believed that a complex and multifactorial interplay between genetic and environmental factors, such as viruses, play a crucial role in the development of the disease. Research over the past few decades has shown that there is not one single viral culprit, nor one single genetic pathway, causing the disease. Rather, viral infections, most notably enteroviruses (EV), appear to accelerate the autoimmune process leading to T1D and are often seen as a precipitator of clinical diagnosis. In support of this hypothesis, the use of anti-viral drugs has recently shown efficacy in preserving beta cell function after onset of diabetes. In this review, we will discuss the various pathways that viral infections utilize to accelerate the development of T1D. There are three key mechanisms linking viral infections to beta-cell death: One is modulated by the direct infection of islets by viruses, resulting in their impaired function, another occurs in a more indirect fashion, by modulating the immune system, and the third is caused by heightened stress on the beta-cell by interferon-mediated increase of insulin resistance. The first two aspects are surprisingly difficult to study, in the case of the former, because there are still many questions about how viruses might persist for longer time periods. In the latter, indirect/immune case, viruses might impact immunity as a hit-and-run scenario, meaning that many or all direct viral footprints quickly vanish, while changes imprinted upon the immune system and the anti-islet autoimmune response persist. Given the fact that viruses are often associated with the precipitation of clinical autoimmunity, there are concerns regarding the impact of the recent global coronavirus-2019 (COVID-19) pandemic on the development of autoimmune disease. The long-term effects of COVID-19 infection on T1D will therefore be discussed, including the increased development of new cases of T1D. Understanding the interplay between viral infections and autoimmunity is crucial for advancing our knowledge in this field and developing targeted therapeutic interventions. In this review we will examine the intricate relationship between viral infections and autoimmunity and discuss potential considerations for prevention and treatment strategies.

Infections: a double-edge sword in autoimmunity

PURPOSE OF REVIEW

Infections play a role in the pathogenesis of autoimmune diseases (AID). Several bacterial and viral pathogens play a double role, as both inducers and inhibitors of AID. In this review, we will present current evidence and discuss different aspects of this notion.

RECENT FINDINGS

Infectors that both inhibit and induce AID include Helicobacter pylori, Klebsiella pneumoniae, hepatitis B virus, group B Coxsackieviruses, Epstein-Barr virus and Lymphocytic choriomeningitis virus. Numerous AID are affected by infections, including polyarteritis nodosa, inflammatory bowel disease, and type 1 diabetes. Some pathogens, such as group B Coxsackieviruses, may induce and inhibit the development of the same AID. This reveals a complex role of infections in autoimmunity pathogenesis.

SUMMARY

Elucidating the exact role of each pathogen on each specific AID is important, as this will enable evaluating the manipulation of these infections in the treatment of AID.

Mechanisms of diabetic autoimmunity: I--the inductive interface between islets and the immune system at onset of inflammation

The mechanisms of autoimmune reactivity onset in type 1 diabetes (T1D) remain elusive despite extensive experimentation and discussion. We reconsider several key aspects of the early stages of autoimmunity at four levels: islets, pancreatic lymph nodes, thymic function and peripheral immune homeostasis. Antigen presentation is the islets and has the capacity to provoke immune sensitization, either in the process of physiological neonatal β cell apoptosis or as a consequence of cytolytic activity of self-reactive thymocytes that escaped negative regulation. Diabetogenic effectors are efficiently expanded in both the islets and the lymph nodes under conditions of empty lymphoid niches during a period of time coinciding with a synchronized wave of β cell apoptosis surrounding weaning. A major drive of effector cell activation and expansion is inherent peripheral lymphopenia characteristic of neonates, though it remains unclear when is autoimmunity triggered in subjects displaying hyperglycemia in late adolescence. Our analysis suggests that T1D evolves through coordinated activity of multiple physiological mechanisms of stimulation within specific characteristics of the neonate immune system.

Autoimmune Diabetes: An Overview of Experimental Models and Novel Therapeutics

Type 1 diabetes (T1D) results from a chronic and selective destruction of insulin-secreting β-cells within the islets of Langerhans of the pancreas by autoreactive CD4(+) and CD8(+) T lymphocytes. The use of animal models of T1D was instrumental for deciphering the steps of the autoimmune process leading to T1D. The non-obese diabetic (NOD) mouse and the bio-breeding (BB) rat spontaneously develop the disease similar to the human pathology in terms of the immune responses triggering autoimmune diabetes and of the genetic and environmental factors influencing disease susceptibility. The generation of genetically modified models allowed refining our understanding of the etiology and the pathogenesis of the disease. In the present review, we provide an overview of the experimental models generated and used to gain knowledge on the molecular and cellular mechanisms underlying the breakdown of self-tolerance in T1D and the progression of the autoimmune response. Immunotherapeutic interventions designed in these animal models and translated into the clinical arena in T1D patients will also be discussed.

Beta-cell specific production of IL6 in conjunction with a mainly intracellular but not mainly surface viral protein causes diabetes

Inflammatory mechanisms play a key role in the pathogenesis of type 1 and type 2 diabetes. IL6, a pleiotropic cytokine with impact on immune and non-immune cell types, has been proposed to be involved in the events causing both forms of diabetes and to play a key role in experimental insulin-dependent diabetes development. The aim of this study was to investigate how beta-cell specific overexpression of IL-6 influences diabetes development. We developed two lines of rat insulin promoter (RIP)-lymphocytic choriomeningitis virus (LCMV) mice that also co-express IL6 in their beta-cells. Expression of the viral nucleoprotein (NP), which has a predominantly intracellular localization, together with IL6 led to hyperglycemia, which was associated with a loss of GLUT-2 expression in the pancreatic beta-cells and infiltration of CD11b(+) cells, but not T cells, in the pancreas. In contrast, overexpression of the LCMV glycoprotein (GP), which can localize to the surface, with IL-6 did not lead to spontaneous diabetes, but accelerated virus-induced diabetes by increasing autoantigen-specific CD8(+) T cell responses and reducing the regulatory T cell fraction, leading to increased pancreatic infiltration by CD4(+) and CD8(+) T cells as well as CD11b(+) and CD11c(+) cells. The production of IL-6 in beta-cells acts prodiabetic, underscoring the potential benefit of targeting IL6 in diabetes.

Nonviral delivery of small interfering RNA into pancreas-associated immune cells prevents autoimmune diabetes

The development of small interfering RNA (siRNA) for the treatment of human disorders has been often hampered by their low transfection efficiency in vivo. In order to overcome this major drawback, various in vivo siRNA transfection methods have been developed. However, their capacity to transfect immune or insulin-producing β-cells within the pancreas for the treatment of autoimmune diabetes remains undetermined. We found that lipid- or polyethylenimine-based delivery agents were efficient to address siRNA molecules within pancreas-associated antigen-presenting cells (APCs) (but not β-cells) and particularly a CD11b(+) cell population comprising both CD11b(+)CD11c(neg) macrophages and CD11b(+)CD11c(+) dendritic cells. However, the route of administration and the carrier composition greatly affected the transfection efficacy. Therapeutically, we showed that early (starting at 6-week-old) short-course treatment with lipid/Alox15-specific siRNA complex promoted long-term protection from type 1 diabetes (T1D) in wild-type (WT) nonobese diabetic (NOD) mice. Alox15 downregulation in pancreas-associated CD11b(+) cells significantly upregulated a variety of costimulatory molecules and particularly the programmed death 1 ligand 1 (PD-L1) pathway involved in tolerance induction. Concomitantly, we found that regulatory T cells were increased in the pancreas of lipid/Alox15 siRNA-treated NOD mice. Collectively, our data provide new insights into the development of siRNA-based therapeutics for T1D.

Interleukin-21 receptor-mediated signals control autoreactive T cell infiltration in pancreatic islets

It remains unclear how interleukin-21 receptor (IL-21R) contributes to type 1 diabetes. Here we have shown that dendritic cells (DCs) in the pancreas required IL-21R not for antigen uptake, but to acquire the chemokine receptor CCR7 and migrate into the draining lymph node. Consequently, less antigen, major histocompatibility complex (MHC) class II, and CD86 was provided to autoreactive effector cells in Il21r(-/-) mice, impairing CD4(+) T cell activation, CD40:CD40L interactions, and pancreatic infiltration by autoreactive T cells. CD40 crosslinking restored defective CD4(+) cell expansion and CD4 independently expanded autoreactive CD8(+) cells, but CD8(+) cells still required CD4(+) cells to reach the pancreas and induce diabetes. Diabetes induction by transferred T cells required IL-21R-sufficient host antigen-presenting cells. Transferring IL-21R-sufficient DCs broke diabetes resistance in Il21r(-/-) mice. We conclude that IL-21R controls both antigen transport by DCs and the crucial beacon function of CD4(+) cells for autoreactive CD8(+) cells to reach the islets.

Viruses and cytotoxic T lymphocytes in type 1 diabetes

Histopathological studies on pancreas tissues from individuals with recent-onset type 1 diabetes (T1D) consistently find that CD8 T cells substantially contribute to the formation of islet lesions. CD8 T cells reactive against islet-associated antigens can also be found in blood samples from T1D patients. Mechanistic studies on the pathogenic role of this T cell subset have mostly focused on two animal models, i.e., the non-obese diabetic mouse and the virally induced rat insulin promoter-lymphocytic choriomeningitis virus model. Data were obtained in support of a role for viral infection in expanding a population of diabetogenic cytotoxic T lymphocytes. In view of the theorized association of viral infection with initiation of islet autoimmunity and progression to clinically overt disease, CD8 T cells thus represent an attractive target for immunotherapy. We will review here arguments in favor of a pivotal role for CD8 T cells in driving T1D development and speculate on etiologic agents that may provoke their aberrant activation.

Viral infection prevents diabetes by inducing regulatory T cells through NKT cell-plasmacytoid dendritic cell interplay

Type 1 diabetes (T1D) is an autoimmune disease resulting from T cell-mediated destruction of insulin-producing β cells, and viral infections can prevent the onset of disease. Invariant natural killer T cells (iNKT cells) exert a regulatory role in T1D by inhibiting autoimmune T cell responses. As iNKT cell-plasmacytoid dendritic cell (pDC) cooperation controls viral replication in the pancreatic islets, we investigated whether this cellular cross talk could interfere with T1D development during viral infection. Using both virus-induced and spontaneous mouse models of T1D, we show that upon viral infection, iNKT cells induce TGF-β-producing pDCs in the pancreatic lymph nodes (LNs). These tolerogenic pDCs convert naive anti-islet T cells into Foxp3(+) CD4(+) regulatory T cells (T reg cells) in pancreatic LNs. T reg cells are then recruited into the pancreatic islets where they produce TGF-β, which dampens the activity of viral- and islet-specific CD8(+) T cells, thereby preventing T1D development in both T1D models. These findings reveal a crucial cooperation between iNKT cells, pDCs, and T reg cells for prevention of T1D by viral infection.

Expression level of a pancreatic neo-antigen in beta cells determines degree of diabetes pathogenesis

It is not fully understood how the expression level of autoantigens in beta cells impacts autoimmune diabetes (T1D) development. Earlier studies using ovalbumin and also insulin had shown that secreted antigens could enhance diabetes development through facilitated presentation by antigen presenting cells. Here we sought to determine how the expression level of a membrane bound, non-secreted or cross-presented neo-antigen, the glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV), would influence T1D. We found that an RIP-LCMV transgenic mouse line exhibiting higher levels of beta cell GP expression developed more severe diabetes after LCMV infection or transfer of high numbers of activated autoreactive T cells. Importantly, all beta cells were lost and a significant increase in morbidity and mortality from T1D was noted. Insulitis and accumulation of autoaggressive CD8 cells was more profound in the RIP-LCMV-GP high-expressor line. Interestingly, the additional introduction of neo-antigen-specific CD4(+) helper or regulatory T cells was able to influence diabetogenesis positively or negatively. We conclude that a higher degree of autoantigen expression results in increased diabetes susceptibility. Therefore, autoantigens such as insulin that are expressed at higher levels in beta cells might have a more profound impact on diabetes pathogenesis.

Virus infections: lessons from pancreas histology

Type 1 diabetes mellitus is a chronic autoimmune disease resulting from the progressive immune-mediated destruction of pancreatic beta cells in genetically susceptible individuals, with the likely contribution of environmental factors, among which viruses have been extensively studied. The pathologic hallmark of the disease is insulitis-a process characterized by islet infiltration of immunocompetent cells that has been well characterized in animal models of islet autoimmunity, and to a lesser extent, in humans. Insulitis characterization has provided valuable information to gain insights into the disease pathogenesis. We review the recent literature on the viral contribution to beta-cell destruction and dysfunction in type 1 diabetes, with particular reference to the pathology of the pancreatic islet in humans and in animal models of the disease.

99th Dahlem conference on infection, inflammation and chronic inflammatory disorders: viruses, autoimmunity and immunoregulation

Based on studies in animal models, viral infections, in particular by enteroviruses, can accelerate or halt type 1 diabetes (T1D) development. Among factors that determine the outcome are the degree of viral replication in the target organ (viral titres), the tropism of the virus for beta cells, and the precise time-point of infection in relation to the diabetogenic process. Mechanisms underlying these phenomena have been assessed in mouse studies and should now be verified for human T1D. For enhancement of diabetes development, up-regulation of interferon pathways, expression of class-I major histocompatibility complexes and Toll-like receptor-dependent immunity appear important. In contrast, prevention of T1D involves pathways that the immune system usually invokes to shut down anti-viral responses to limit immunopathology, and which can 'clean out' autoreactive memory effector T cells as a bystander phenomenon: up-regulation of inhibitory molecules and invigoration of regulatory T cell (T(reg)) function. Importantly, these immunoregulatory processes also appear to foster and sustain persistent viral infections. Induction of immunoregulatory mechanisms, and in particular the phenotype and function of T(regs), is of interest therapeutically and will be discussed.

Genetic-induced variations in the GAD65 T-cell repertoire governs efficacy of anti-CD3/GAD65 combination therapy in new-onset type 1 diabetes

To enhance efficacy of forthcoming type 1 diabetes (T1D) clinical trials, combination therapies (CTs) are envisaged. In this study, we showed that efficacy of a CT, using anti-CD3 antibody and glutamic acid decarboxylase of 65 kd (GAD65)-expressing plasmid, to reverse new-onset T1D was dependent upon the genetic background. Synergism between both treatments was only observed in the RIP-LCMV-GP but not in the nonobese diabetic (NOD) or RIP-LCMV-NOD models. Efficacy was associated with an expansion of bystander suppressor regulatory T cells (Tregs) recognizing the C-terminal region of GAD65 and secreting interleukin-10 (IL-10), transforming growth factor-beta (TGF-beta), and interferon-gamma (IFN-gamma). In addition, we found that frequency and epitope specificity of GAD65-reactive CD4(+) T cells during antigen priming at diabetes onset and Tregs detected after CT correlated. Consequently, NOD mice harbored significantly lower levels of GAD65-reactive CD4(+) T cells than RIP-LCMV-GP before and after treatment. Our results demonstrate that antigen-specific T cells available at treatment may differ between various major histocompatibility complex (MHC) and genetic backgrounds. These cells play a major role in shaping T-cell responses following antigen-specific immune intervention and determine whether a beneficial Tregs response is generated. Our findings hold important implications to understand and predict the success of antigen-based clinical trials, where responsiveness to immunotherapy might vary from patient to patient.

Immunoregulatory mechanisms triggered by viral infections protect from type 1 diabetes in mice

Type 1 diabetes (T1D) is an autoimmune disease that is caused by the destruction of insulin-producing beta cells. Viral infections induce immune responses that can damage beta cells and promote T1D or on the other hand prevent the development of the disease. However, the opposing roles of viral infections in T1D are not understood mechanistically. We report here that viruses that do not inflict damage on beta cells provided protection from T1D by triggering immunoregulatory mechanisms. Infection of prediabetic NOD mice with Coxsackie virus B3 or lymphocytic choriomeningitis virus (LCMV) delayed diabetes onset and reduced disease incidence. Delayed T1D onset was due to transient upregulation of programmed cell death-1 ligand 1 (PD-L1) on lymphoid cells, which prevented the expansion of diabetogenic CD8+ T cells expressing programmed cell death-1 (PD-1). Reduced T1D incidence was caused by increased numbers of invigorated CD4+CD25+ Tregs, which produced TGF-beta and maintained long-term tolerance. Full protection from T1D resulted from synergy between PD-L1 and CD4+CD25+ Tregs. Our results provide what we believe to be novel mechanistic insight into the role of viruses in T1D and should be valuable for prospective studies in humans.

NKT cell-plasmacytoid dendritic cell cooperation via OX40 controls viral infection in a tissue-specific manner

Invariant natural killer T (iNKT) cells promote immune responses to various pathogens, but exactly how iNKT cells control antiviral responses is unclear. Here, we showed that iNKT cells induced tissue-specific antiviral effects in mice infected by lymphocytic choriomeningitis virus (LCMV). Indeed, iNKT cells inhibited viral replication in the pancreas and liver but not in the spleen. In the pancreas, iNKT cells expressed the OX40 molecule and promoted type I interferon (IFN) production by plasmacytoid dendritic cells (pDCs) through OX40-OX40 ligand interaction. Subsequently, this iNKT cell-pDC cooperation attenuated the antiviral adaptive immune response in the pancreas but not in the spleen. The dampening of pancreatic anti-LCMV CD8(+) T cell response prevented tissue damage in transgenic mice expressing LCMV protein in islet beta cells. Thus, this study identifies pDCs as an essential partner of iNKT cells for mounting an efficient, nondeleterious antiviral response in peripheral tissue.