Immunopathogenesis and immunotherapeutic approaches to type 1A diabetes

It is now clear that type 1A (immune-mediated) diabetes develops in genetically susceptible individuals where, prior to the onset of overt hyperglycaemia, there is usually a long prodrome characterised by the presence of autoimmunity directed at islet beta cells. It is the destruction of these insulin-producing cells that results in loss of metabolic regulation and the resultant hyperglycaemia and severe sequelae of type 1A diabetes. An extensive body of animal data and a developing body of human studies are now addressing therapies directed at this root immune cause of type 1A diabetes. Therapies can be considered in terms of the disease stage at which they are applied and in terms of their effects on the immune system (e.g., generalised immunosuppression, immunomodulation, antigen-specific therapies and tolerance-inducing therapies). As T cells are the primary mediators of islet beta cell destruction, it is likely that improved therapies and monitoring of T cell autoimmunity will be necessary to develop a safe and effective therapy for type 1A diabetes.     

T-cell assays to determine disease activity and clinical efficacy of immune therapy in type 1 diabetes

Type 1 (insulin-dependent) diabetes mellitus results from a T cell-mediated autoimmune destruction of the pancreatic beta cells in genetically predisposed individuals. Therapies directed against T cells have been demonstrated to halt the disease process and prevent recurrent beta-cell destruction after islet transplantation. Less is known about the nature and function of these T cells, the cause of the loss of tolerance to islet autoantigens, why the immune system apparently fails to suppress autoreactivity, and whether (or which) autoantigen(s) are critically involved in the initiation or progression of disease. Autoreactive T cells have proven to be valuable targets to study pathogenic or diabetes-related processes. Measuring T-cell autoreactivity also provided critical information to determine the fate of islet allografts transplanted to type 1 diabetic patients. Furthermore, these studies have provided proof of operational immunologic tolerance to islet allografts as well as valuable information to improve and customize immunosuppressive therapy. Currently, technologies to detect T-cell auto- and alloreactivity in type 1 diabetic recipients of islet allografts are applied to monitor islet allograft survival in relation with various immunosuppressive therapies and to guide tapering of these therapies after successful restoration of insulin production. Although it is generally appreciated that studies on cellular auto- and alloimmunity are hampered by the complex nature of such immune responses and the required technical and physical skills, it has been a worthwhile quest to unravel the role of T cells in the pathogenesis of type 1 diabetes and islet allograft destruction.     

Targeting T lymphocytes for immune monitoring and intervention in autoimmune diabetes

Recognition of a peptide-MHC complex by the T cell receptor (TCR) is a key interaction that initiates T lymphocyte activation or silencing during an immune response. Fluorochrome-labeled recombinant MHC class II-peptide reagents function as soluble mimetics of this interaction, bind to their specific TCR, and allow for detection of antigen-specific CD4+ T cells. These reagents are now under scrutiny for "immune staging" of patients at risk of type 1 diabetes, in an effort to diagnose islet autoimmunity early enough to block immune-mediated beta cell destruction. Several issues are currently being addressed to improve the performance of these T cell assays: enrichment steps for better sensitivity, multiplexing of several islet epitopes, simultaneous monitoring of CD4+ and CD8+ responses, detection of low avidity T cells, combination of quantitative (number of positive cells) and qualitative (cytokine secretion, naive/memory phenotype) readouts. CD4+ T cells are key effectors of autoimmunity, and these MHC class II peptide reagents, through their signaling properties, might also provide therapeutics to block the autoimmune process at its onset, analogous to the use of OKT3gammao1(AlaAla) anti-CD3 antibody but in an antigen-specific fashion. The aim of such therapeutics is to potentiate different physiological control mechanisms to restore immune tolerance. Mechanisms initiated by this pathway may be capable of triggering elimination of pathogenic T cells through antigen-specific apoptosis and anergy, combined with the induction of regulatory T cells with broad suppressive function.     

Local expression of B7-H1 promotes organ-specific autoimmunity and transplant rejection

A number of studies have suggested B7-H1, a B7 family member, inhibits T cell responses. Therefore, its expression on nonlymphoid tissues has been proposed to prevent T cell-mediated tissue destruction. To test this hypothesis, we generated transgenic mice that expressed B7-H1 on pancreatic islet beta cells. Surprisingly, we observed accelerated rejection of transplanted allogeneic B7-H1-expressing islet beta cells. Furthermore, transgenic B7-H1 expression broke immune tolerance, as some of the mice spontaneously developed T cell-dependent autoimmune diabetes. In addition, B7-H1 expression increased CD8+ T cell proliferation and promoted autoimmunity induction in a T cell adoptive transfer model of diabetes. Consistent with these findings, B7-H1.Ig fusion protein augmented naive T cell priming both in vitro and in vivo. Our results demonstrate that B7-H1 can provide positive costimulation for naive T cells to promote allograft rejection and autoimmune disease pathogenesis.     

Development of new strategies to prevent type 1 diabetes: the role of animal models

Type 1 diabetes is an immune‐mediated disease typically preceded by a long preclinical stage during which a growing number of islet‐cell‐specific autoantibodies appear in the serum. Although antigen‐specific T lymphocytes and cytokines rather than these autoantibodies are the likely executors of β‐cell‐destruction, these autoantibodies reflect the existence of autoimmunity that targets islet β‐cells. Abrogation of this autoimmunity during the preclinical stage would be the key to the prevention of type 1 diabetes. However, the quest of protecting islet‐cells from the immune attack requires detailed knowledge of mechanisms that control islet‐inflammation and β‐cell‐destruction, and of mechanisms that control immune tolerance to peripheral self‐antigens in general. This knowledge can only be obtained through further innovative research in experimental animal models. In this review, we will first examine how research in non‐obese diabetic mice has already led to promising new strategies of diabetes prevention now being tested in human clinical trials. Thereafter, we will discuss how recent advances in understanding the mechanisms that control immune response to peripheral self‐antigens such as β‐cell antigens may help to develop even more selective and effective strategies to prevent diabetes in the future.     

Immune intervention for type 1 diabetes mellitus

The major form of type 1 diabetes (T1D) is characterised by immune-mediated pancreatic islet β-cell destruction, and has also been called type 1A diabetes to distinguish it from idiopathic forms of islet β-cell loss. Since the first demonstration of islet cell antibodies in 1974, the concept has been that this form of diabetes is autoimmune in nature. The commonly accepted concept is that antibodies (representing the humoral arm of the immune system) do not mediate the β-cell destruction but rather serve as markers of that destruction, while the cellular arm of the immune system, specifically T-lymphocytes, mediate the β-cell destruction. Yet, the T-lymphocytes do not act alone. They receive help in initiating the response from antigen-presenting cells such as dendritic cells and macrophages, and appear to receive help also from B-lymphocytes. In addition, the initial immune response engenders secondary and tertiary responses - involving the whole immunological army - which collectively result in impairment of β-cell function, progressive destruction of β-cells, and consequent development of type 1A diabetes. The process is insidious and may evolve over many years, with the overt expression of clinical symptoms becoming apparent only when most β-cells have been destroyed. Yet, the process clearly evolves at different speeds - much more rapidly in young children, much more slowly in older individuals. And, although it has been thought that ultimately there is complete β-cell destruction, several studies have now demonstrated some degree of persistent β-cell function or existence (at autopsy) in long-standing T1D. A major focus of investigation in T1D is the preservation of β-cell function (and, it is hoped, of β-cells themselves), in the expectation that continuing endogenous insulin secretion will contribute towards better glycaemic control, reduce episodes of severe hypoglycaemia, and slow the development of complications such as retinopathy and nephropathy. Thus, there have been many studies designed to interdict the T1D disease process, mostly by altering the immune system, both during the stage of evolution of the disease and at the time of disease onset. This chapter of the Yearbook of Advanced Technology and Treatments in Diabetes reviews the key papers that have appeared in this field between July 2009 and June 2010. Articles selected were confined to studies in human beings. All immune intervention studies reported in this time frame were included. In addition, the author selected other relevant articles dealing with mechanisms, markers, triggers, and pathology of human type 1 diabetes.     

Recovery from diabetes in mice by beta cell regeneration

The mechanisms that regulate pancreatic beta cell mass are poorly understood. While autoimmune and pharmacological destruction of insulin-producing beta cells is often irreversible, adult beta cell mass does fluctuate in response to physiological cues including pregnancy and insulin resistance. This plasticity points to the possibility of harnessing the regenerative capacity of the beta cell to treat diabetes. We developed a transgenic mouse model to study the dynamics of beta cell regeneration from a diabetic state. Following doxycycline administration, transgenic mice expressed diphtheria toxin in beta cells, resulting in apoptosis of 70%-80% of beta cells, destruction of islet architecture, and diabetes. Withdrawal of doxycycline resulted in a spontaneous normalization of blood glucose levels and islet architecture and a significant regeneration of beta cell mass with no apparent toxicity of transient hyperglycemia. Lineage tracing analysis indicated that enhanced proliferation of surviving beta cells played the major role in regeneration. Surprisingly, treatment with Sirolimus and Tacrolimus, immunosuppressants used in the Edmonton protocol for human islet transplantation, inhibited beta cell regeneration and prevented the normalization of glucose homeostasis. These results suggest that regenerative therapy for type 1 diabetes may be achieved if autoimmunity is halted using regeneration-compatible drugs.     

MERTK on mononuclear phagocytes regulates T cell antigen recognition at autoimmune and tumor sites

Understanding mechanisms of immune regulation is key to developing immunotherapies for autoimmunity and cancer. We examined the role of mononuclear phagocytes during peripheral T cell regulation in type 1 diabetes and melanoma. MERTK expression and activity in mononuclear phagocytes in the pancreatic islets promoted islet T cell regulation, resulting in reduced sensitivity of T cell scanning for cognate antigen in prediabetic islets. MERTK-dependent regulation led to reduced T cell activation and effector function at the disease site in islets and prevented rapid progression of type 1 diabetes. In human islets, MERTK-expressing cells were increased in remaining insulin-containing islets of type 1 diabetic patients, suggesting that MERTK protects islets from autoimmune destruction. MERTK also regulated T cell arrest in melanoma tumors. These data indicate that MERTK signaling in mononuclear phagocytes drives T cell regulation at inflammatory disease sites in peripheral tissues through a mechanism that reduces the sensitivity of scanning for antigen leading to reduced responsiveness to antigen.     

Reversal of established autoimmune diabetes by restoration of endogenous beta cell function.

In NOD (nonobese diabetic) mice, a model of autoimmune diabetes, various immunomodulatory interventions prevent progression to diabetes. However, after hyperglycemia is established, such interventions rarely alter the course of disease or allow sustained engraftment of islet transplants. A proteasome defect in lymphoid cells of NOD mice impairs the presentation of self antigens and increases the susceptibility of these cells to TNF-alpha-induced apoptosis. Here, we examine the hypothesis that induction of TNF-alpha expression combined with reeducation of newly emerging T cells with self antigens can interrupt autoimmunity. Hyperglycemic NOD mice were treated with CFA to induce TNF-alpha expression and were exposed to functional complexes of MHC class I molecules and antigenic peptides either by repeated injection of MHC class I matched splenocytes or by transplantation of islets from nonautoimmune donors. Hyperglycemia was controlled in animals injected with splenocytes by administration of insulin or, more effectively, by implantation of encapsulated islets. These interventions reversed the established beta cell-directed autoimmunity and restored endogenous pancreatic islet function to such an extent that normoglycemia was maintained in up to 75% of animals after discontinuation of treatment and removal of islet transplants. A therapy aimed at the selective elimination of autoreactive cells and the reeducation of T cells, when combined with control of glycemia, is thus able to effect an apparent cure of established type 1 diabetes in the NOD mouse.     

Islet cell function: alpha and beta cells--partners towards normoglycaemia

Under normal conditions, insulin and glucagon are counter-regulatory hormones whose balanced action exhibits a relationship that ensures normoglycaemia. Elevated glucose levels following a meal stimulate pancreatic islet beta cells to secrete insulin and islet alpha cells to downregulate production of glucagon. With declining glucose and insulin levels, alpha-cell production of glucagon is increased to stimulate hepatic glucose production, preventing fasting hypoglycaemia. In type 2 diabetes mellitus (T2DM), beta-cell insulin response to glucose is blunted, including absence of early acute response, and alpha-cell response to glucose is impaired, resulting in absolute or relative hyperglucagonaemia and inappropriate hepatic glucose output that contributes to fasting hyperglycaemia. These changes are associated with structural and functional changes in pancreatic islets, including reduced beta-cell mass and reduced beta-cell:alpha-cell ratio. The role of the incretin hormone glucagon-like peptide-1 (GLP-1) in regulating glucose-dependent beta-cell insulin production and glucose-dependent alpha-cell glucagon production has been used to develop GLP-1-based therapies. These therapies may reduce the imbalances among insulin and glucagon that characterise T2DM, resulting in improved glycaemic control.     

Plasmid-based gene therapy of diabetes mellitus

Type I diabetes mellitus (T1D) is due to a loss of immune tolerance to islet antigen and thus, there is intense interest in developing therapies that can re-establish it. Tolerance is maintained by complex mechanisms that include inhibitory molecules and several types of regulatory T cells (Tr). A major historical question is whether gene therapy can be employed to generate Tr cells. This review shows that gene transfer of immunoregulatory molecules can prevent T1D and other autoimmune diseases. In our studies, non-viral gene transfer is enhanced by in vivo electroporation (EP). This technique can be used to perform DNA vaccination against islet cell antigens and when combined with appropriate immune ligands results in the generation of Tr cells and protection against T1D. In vivo EP can also be applied for non-immune therapy of diabetes. It can be used to deliver protein drugs such as glucagon-like peptide 1 (GLP-1), leptin or transforming growth factor beta (TGF-beta). These act in T1D or type II diabetes (T2D) by restoring glucose homeostasis, promoting islet cell survival and growth or improving wound healing and other complications. Furthermore, we show that in large animals EP can deliver peptide hormones, such as growth hormone releasing hormone (GHRH). We conclude that the non-viral gene therapy and EP represent a safe and efficacious approach with clinical potential.     

Gene therapy for type 1 diabetes: a novel approach for targeted treatment of autoimmunity

It has been difficult to develop therapies that target those T cells initiating and mediating the pathogenesis of autoimmune disease. Indeed, most current treatments indiscriminately affect both the autoreactive T cells and the "good" T cells, putting the patient at risk of compromised immune function. A new approach raises the possibility of targeted therapy for autoimmunity. Transplantation of hematopoietic stem cells modified to express a protective form of MHC class II corrects a defect in central tolerance. This method contrasts with other targeted therapies that attempt to modify peripheral tolerance, which is also defective in type 1 diabetes mellitus.     

Role of viruses in the pathogenesis of IDDM

Insulin-dependent diabetes mellitus (IDDM), also known as type I diabetes, results from the destruction of pancreatic beta cells. During the past few decades, genetic factors, autoimmunity and viral infections have been extensively studied as the possible cause of beta cell destruction. The evidence for virus-induced diabetes comes largely from experiments in animals, but several studies in humans also point to viruses as a trigger of this disease in some cases. There are at least two possible mechanisms for the involvement of viruses in the pathogenesis of IDDM: (a) cytolytic infection of beta cells may result in destruction of the cells without the induction of autoimmunity, or may be a final insult leading to the clinical onset of diabetes in individuals with an already decreased beta cell mass resulting from an autoimmune process; and (b) persistent viral infection (e.g. retrovirus, rubella virus, cytomegalovirus) may result in the triggering of autoimmune IDDM in certain circumstances. Regarding the latter possibility, viruses may insert, expose, or alter antigens in the plasma membrane of the beta cell, which may initiate autoimmunity leading to the destruction of the cells.     

Possible mechanisms in the pathogenesis of virus-induced diabetes mellitus

Insulin-dependent diabetes mellitus results from destruction of pancreatic beta cells. Viruses and autoimmunity have been implicated as possible causes of beta cell destruction in genetically predisposed individuals. The evidence for viruses comes largely from experiments in animals, but several studies in humans point to viruses as triggers in the pathogenesis of diabetes in some cases. In animal models, at least 4 different possible mechanisms for virus-induced diabetes have been proposed. The first mechanism is direct cytolytic infection of pancreatic beta cells. One group of viruses, including encephalomyocarditis virus, Mengovirus 2T, and Coxsackie B viruses, can directly infect and destroy pancreatic beta cells independent of autoimmune processes. The second mechanism is triggering of autoimmune responses. In contrast to the encephalomyocarditis virus-induced diabetes, reovirus type 1 and rubella virus seem to be somehow associated with autoimmunity in the genesis of a diabetes-like syndrome in a certain strain of suckling mice and hamsters, respectively. The third mechanism is cumulative environmental insults. The cumulative environmental insults with viruses and beta cell toxic chemicals can result in diabetes in genetically predisposed non-human primates and certain inbred strains of mice. The fourth mechanism is persistent infection. A certain virus, such as lymphocytic choriomeningitis virus, persistently infects murine pancreatic beta cells and produces hyperglycemia. The evidence that viruses cause diabetes in humans is more circumstantial.(ABSTRACT TRUNCATED AT 250 WORDS)     

Current concept of the pathogenesis of autoimmune type 1 diabetes mellitus

In type 1 diabetes, the beta cell is selectively destroyed by T lymphocytes in a genetically susceptible individual. Susceptible HLA-class II molecules which have low binding affinity with putative beta cell antigens may fail in induction of self-tolerance. beta cell-reactive T cells may expand and infiltrate around the islet (benign insulitis) by stimulation with environmental factors through molecular mimicry and/or beta cell antigens per se. Benign insulitis may develop to destructive insulitis under overexpression of Th1 cytokines, which may be affected by non-MHC genes. Immune interventions to shift a cytokine balance to Th2 dominance in the insulitis lesion may be most realistic approach to suppress development of type 1 diabetes after the induction of autoimmunity.     

CD25+ CD4+ T cells, expanded with dendritic cells presenting a single autoantigenic peptide, suppress autoimmune diabetes

In the nonobese diabetic (NOD) mouse model of type 1 diabetes, the immune system recognizes many autoantigens expressed in pancreatic islet beta cells. To silence autoimmunity, we used dendritic cells (DCs) from NOD mice to expand CD25+ CD4+ suppressor T cells from BDC2.5 mice, which are specific for a single islet autoantigen. The expanded T cells were more suppressive in vitro than their freshly isolated counterparts, indicating that DCs from autoimmune mice can increase the number and function of antigen-specific, CD25+ CD4+ regulatory T cells. Importantly, only 5,000 expanded CD25+ CD4+ BDC2.5 T cells could block autoimmunity caused by diabetogenic T cells in NOD mice, whereas 10(5) polyclonal, CD25+ CD4+ T cells from NOD mice were inactive. When islets were examined in treated mice, insulitis development was blocked at early (3 wk) but not later (11 wk) time points. The expanded CD25+ CD4+ BDC2.5 T cells were effective even if administered 14 d after the diabetogenic T cells. Our data indicate that DCs can generate CD25+ CD4+ T cells that suppress autoimmune disease in vivo. This might be harnessed as a new avenue for immunotherapy, especially because CD25+ CD4+ regulatory cells responsive to a single autoantigen can inhibit diabetes mediated by reactivity to multiple antigens.     

Role of Viruses in the Pathogenesis of IDDM

Insulin-dependent diabetes mellitus (IDDM), also known as type I diabetes, results from the destruction of pancreatic beta cells. During the past few decades, genetic factors, autoimmunity and viral infections have been extensively studied as the possible cause of beta cell destruction. The evidence for virus-induced diabetes comes largely from experiments in animals, but several studies in humans also point to viruses as a trigger of this disease in some cases. There are at least two possible mechanisms for the involvement of viruses in the pathogenesis of IDDM: (a) cytolytic infection of beta cells may result in destruction of the cells without the induction of autoimmunity, or may be a final insult leading to the clinical onset of diabetes in individuals with an already decreased beta cell mass resulting from an autoimmune process; and (b) persistent viral infection (e.g. retrovirus, rubella virus, cytomegalovirus) may result in the triggering of autoimmune IDDM in certain circumstances. Regarding the latter possibility, viruses may insert, expose, or alter antigens in the plasma membrane of the beta cell, which may initiate autoimmunity leading to the destruction of the cells.     

Type 1 diabetes mellitus: much progress,many opportunities

As part of the centennial celebration of insulin’s discovery, this review summarizes the current understanding of the genetics, pathogenesis, treatment, and outcomes in type 1 diabetes (T1D). T1D results from an autoimmune response that leads to destruction of the β cells in the pancreatic islet and requires lifelong insulin therapy. While much has been learned about T1D, it is now clear that there is considerable heterogeneity in T1D with regard to genetics, pathology, response to immune-based therapies, clinical course, and susceptibility to diabetes-related complications. This Review highlights knowledge gaps and opportunities to improve the understanding of T1D pathogenesis and outlines emerging therapies to treat or prevent T1D and reduce the burden of T1D.     

Adeno-associated virus vector mediated gene transfer to pancreatic beta cells

Insulin-dependent diabetes mellitus (IDDM) or type 1 diabetes is an autoimmune disease that results in destruction of the insulin-producing pancreatic islet beta cells. Several factors induce the invasion of immune cells into islets and trigger inflammation. Gene therapy approaches targeting the islet cells could be an effective treatment to prevent the onset or reverse type 1 diabetes. Allogeneic islet transplantation provides short-term treatment. However, genetically modified islets, which resist the host immune response, could provide long-term solutions. Adeno-associated virus (AAV) is emerging as a prominent vector system for delivering therapeutic genes for human gene therapy. AAV vector can transduce nondividing cells and provide long-term gene expression by integrating into host chromosome. Therefore, it is an appropriate vector system for islet cell gene therapy. To test the efficacy of AAV vector to transduce pancreatic endocrine cells, we constructed AAV vectors using plasmid pSub201. Wild-type AAV DNA analogue from plasmid psub201 was subcloned into a cloning plasmid pSP72 and AAV vectors were constructed by inserting the transgenes with heterologous promoter in place of AAV open reading frames (rep and cap). In this report we demonstrate the transduction of pancreatic islet cells with AAV vectors encoding bacterial -galactosidase enzyme or enhanced green fluorescent protein (EGFP) as reporter gene. Dispersed porcine and rat islet cells can be transduced by AAV vector, with an efficiency of 47% and 38%, respectively. In particular porcine islet insulin producing beta cells were transduced with an efficiency of 39%. Intact rat islet cells were transduced with an efficiency of 26% as estimated by FACS analysis following transduction with an AAV vector encoding EGFP. Transduction of intact rat islets with an AAV vector did not alter glucose-induced insulin secretion. AAV vector transduction was higher in transformed islet cell lines INS-1 and RIN m5F with an efficiency of 65% and 57%, respectively. These new results suggest that AAV vectors will provide an improved method of gene delivery to pancreatic islets and isolated pancreatic beta cells.     

Immunotherapeutic approaches to prevent, ameliorate, and cure type 1 diabetes

Type 1A diabetes (T1D) is caused by autoimmune islet beta cell destruction precipitated by environmental triggers in genetically predisposed individuals. Islet beta cells produce insulin and are the primary target of this autoimmune disorder. Insulin, glutamic acid decarboxylase, and insulinoma associated-2 autoantibodies (IAA, GAD65, and IA-2) are the autoantibodies that have been associated most clearly with the development of T1D. Despite our current ability to predict T1D using genetic markers and detecting islet autoantibodies, we have yet to find a safe way to prevent the disease. However, there are more than 100 different therapies that prevent T1D in the nonobese diabetic (NOD) mouse model or the BioBreeding (BB) rats. This paper reviews a few select therapeutic approaches that have been or are being evaluated as possibilities for the prevention, amelioration, or cure of T1D.