Autoimmune diabetes: how many steps for one disease?

Attenuation of Th1 response through galectin-9 and T-cell Ig mucin 3 interaction inhibits autoimmune diabetes in NOD mice

Galectin-9 (gal-9), widely expressed in many tissues, regulates Th1 cells and induces their apoptosis through its receptor, T-cell Ig mucin 3, which is mainly expressed on terminally differentiated Th1 cells. Type 1 diabetes is a Th1-dominant autoimmune disease that specifically destroys insulin-producing beta cells. To suppress the Th1 immune response in the development of autoimmune diabetes, we overexpressed gal-9 in NOD mice by injection of a plasmid encoding gal-9. Mice treated with gal-9 plasmid were significantly protected from diabetes and showed less severe insulitis compared with controls. Flow cytometric analyses in NOD-T1/2 double transgenic mice showed that Th1-cell population in spleen, pancreatic lymph node and pancreas was markedly decreased in gal-9 plasmid-treated mice, indicating a negative regulatory role of gal-9 in the development of pathogenic Th1 cells. Splenocytes from gal-9 plasmid-treated mice were less responsive to mitogenic stimulation than splenocytes from the control group. However, adoptive transfer of splenocytes from gal-9-treated or control mice caused diabetes in NOD/SCID recipients with similar kinetics, suggesting that gal-9 treatment does not induce active tolerance in NOD mice. We conclude that gal-9 may downregulate Th1 immune response in NOD mice and could be used as a therapeutic target in autoimmune diabetes.

The autoimmune contrivance: genetics in the mouse model

Autoimmunity and inheritance of complex characters behold an explosive interest in biology over the last 15 years. Research in the genetics of autoimmunity has been impelled by the isolation of genetic markers allowing tracing of heredity. The annotation and sequencing of the human and mouse genomes provide with the potential for further advancements, through the development of new technologies. This review aims to summarize advances made in the autoimmunity field, centered in type 1 diabetes in the NOD mouse model. It also aims to demonstrate that animal models, albeit some phenotypic and genetic dissimilarities with the human diseases, still remain the best way to move towards an understanding of the molecular mechanisms involved in autoimmunity. Assessing the current state of research in this field together with the increasing potential of novel biotechnology advancements, new insights to disease pathogenesis and discovery of molecular targets for intervention strategies are anticipated in the coming years.

Identification and expansion of pancreatic stem/progenitor cells

Pancreatic islet transplantation represents an attractive approach for the treatment of diabetes. However, the limited availability of donor islets has largely hampered this approach. In this respect, the use of alternative sources of islets such as the ex vivo expansion and differentiation of functional endocrine cells for treating diabetes has become the major focus of diabetes research. Adult pancreatic stem cells /progenitor cells have yet to be recognized because limited markers exist for their identification. While the pancreas has the capacity to regenerate under certain circumstances, questions where adult pancreatic stem/progenitor cells are localized, how they are regulated, and even if the pancreas harbors a stem cell population need to be resolved. In this article, we review the recent achievements both in the identification as well as in the expansion of pancreatic stem/progenitor cells.

Caspase-3-dependent beta-cell apoptosis in the initiation of autoimmune diabetes mellitus

beta-Cell apoptosis is a key event contributing to the pathogenesis of type 1 diabetes mellitus. In addition to apoptosis being the main mechanism by which beta cells are destroyed, beta-cell apoptosis has been implicated in the initiation of type 1 diabetes mellitus through antigen cross-presentation mechanisms that lead to beta-cell-specific T-cell activation. Caspase-3 is the major effector caspase involved in apoptotic pathways. Despite evidence supporting the importance of beta-cell apoptosis in the pathogenesis of type 1 diabetes, the specific role of caspase-3 in this process is unknown. Here, we show that Caspase-3 knockout (Casp3(-/-) mice were protected from developing diabetes in a multiple-low-dose streptozotocin autoimmune diabetes model. Lymphocyte infiltration of the pancreatic islets was completely absent in Casp3(-/-) mice. To determine the role of caspase-3-dependent apoptosis in disease initiation, a defined antigen-T-cell receptor transgenic system, RIP-GP/P14 double-transgenic mice with Casp3 null mutation, was examined. beta-cell antigen-specific T-cell activation and proliferation were observed only in the pancreatic draining lymph node of RIP-GP/P14/Casp3(+/-) mice, but not in mice lacking caspase-3. Together, our findings demonstrate that caspase-3-mediated beta-cell apoptosis is a requisite step for T-cell priming, a key initiating event in type 1 diabetes.

A T-cell functional phenotype common among autoimmune-prone rodent strains

The genetic basis and familial clustering of autoimmunity suggest that common phenotypic traits predispose individuals to disease. We found a hyporesponsive T-cell phenotype that was shared by all autoimmune-prone mouse and rat strains tested, including MRL, nonobese diabetic (NOD), NZB, NZW, NZB/W F1, SJL and SWR mice, as well as DA and BB rats, but was not evident in nonautoimmune-prone rodents. This T-cell intrinsic, age-independent hyporesponsiveness is measured as an increased activation threshold for upregulation of activation markers upon T-cell receptor (TCR) cross-linking both in vitro and in vivo. Inefficient deletion of CD4 and CD8 single-positive, heat stable antigen (HSA)hi medullary thymocytes was also observed in hyporesponsive donors. We interpret these data to suggest that increased TCR-mediated signalling thresholds in autoimmune-prone individuals may contribute to the escape of autoreactive thymocytes from negative selection.

Long-term islet graft survival in streptozotocin- and autoimmune-induced diabetes models by immunosuppressive and potential insulinotropic agent FTY720

BACKGROUND

Toxicity of current immunosuppressive agents to islet grafts is one of the major obstacles to clinical islet transplantation (Tx). This study was designed to assess the efficacy of FTY720, a novel immunomodulator with a unique mechanism of action, on islet graft survival and function in streptozotocin (STZ)- and autoimmune-induced diabetic recipients.

METHODS

Islet allograft from BALB/C mice or islet isografts were transplanted into STZ-induced diabetic CBA mice and autoimmune nonobese diabetic (NOD) mice. FTY720 was administered orally at 0.5 mg/kg per day in STZ diabetic recipients or 3 mg/kg per day in NOD recipients after Tx. Functional status of the islet graft was monitored by measuring blood glucose daily. Insulin secretion from mouse islets was measured with an insulin scintillation proximity assay.

RESULTS

Under the treatment of FTY720, long-term normoglycemia (>100 days) was achieved in 100% of STZ diabetic recipients and 50% of diabetic NOD recipients compared with a respective 11 and 7 days in untreated animals after allogeneic islet Tx. Normoglycemia persisted only temporarily (<4 weeks) in untreated NOD recipients of NOD islets, but was maintained for >100days with FTY720 treatment. Histologically, leukocyte infiltration observed in untreated animals was largely inhibited in FTY720-treated ones. Additionally, FTY720 stimulated insulin secretion from isolated islets by approximately twofold under both normoglycemic and hyperglycemic conditions.

CONCLUSIONS

FTY720 is highly effective in protecting allo- and autoimmune response-mediated islet graft destruction without direct toxicity to the islets. The effect is likely attributable to its action in preventing effector lymphocyte infiltration into the grafted tissue.

Specific deficiency of p56lck expression in T lymphocytes from type 1 diabetic patients

Peripheral T lymphocyte activation in response to TCR/CD3 stimulation is reduced in type 1 diabetic patients. To explore the basis of this deficiency, a comprehensive analysis of the signal transduction pathway downstream of the TCR/CD3 complex was performed for a cohort of patients (n = 38). The main result of the study shows that T cell hyporesponsiveness is positively correlated with a reduced amount of p56(lck) in resting T lymphocytes. Upon CD3-mediated activation, this defect leads to a hypophosphorylation of the CD3zeta-chain and few other polypeptides without affecting the recruitment of ZAP70. Other downstream effectors of the TCR/CD3 transduction machinery, such as phosphatidylinositol 3-kinase p85alpha, p59(fyn), linker for activation of T cells (LAT), and phospholipase C-gamma1, are not affected. In some patients, the severity of this phenotypic deficit could be linked to low levels of p56(lck) mRNA and resulted in the failure to efficiently induce the expression of the CD69 early activation marker. We propose that a primary deficiency in human type 1 diabetes is a defect in TCR/CD3-mediated T cell activation due to the abnormal expression of the p56(lck) tyrosine kinase.

Regulation of TCR-induced IFN-gamma release from islet-reactive non-obese diabetic CD8(+) T cells by prostaglandin E(2) receptor signaling

Prostaglandins (PG) are released during tissue injury and inflammation, and inhibit immune responses at many points. PG may be one of several factors that protect not only against injury-induced, but also spontaneous, organ-specific autoimmune disease. Here we show that the production of PGE(2), normally produced at a very low rate in islets of Langerhans, is significantly increased in inflamed islets of non-obese diabetic (NOD) mice. We investigated a possible role of PGE(2) in controlling TCR-dependent release of IFN-gamma from islet-reactive NOD CD8(+) T cells. PGE(2) inhibited anti-TCR antibody-triggered release of IFN-gamma from CD8(+) T cell clone 8D8 and from polyclonal cytotoxic T lymphocytes (CTL). Using receptor subtype selective agonists, we present evidence that the effect of PGE(2) is mediated by EP(2) and EP(4) receptors, both of which are coupled to an increase in intracellular cAMP production. The cAMP analogs 8-Br-cAMP and Sp-cAMPS mimic the effect of EP(2)/EP(4) receptor agonists, inhibiting TCR-triggered IFN-gamma release from NOD CD8(+) T cells in a dose-dependent manner. The inhibitory effect of PGE(2) was largely reversed by IL-2 added at the time of culture initiation and decreased with increasing strength of stimulation through the TCR. Resting CTL were more sensitive to PGE(2) than recently expanded CTL and NOD CD8(+) T cells remained insensitive to PGE(2) for a longer time than BALB/c cells. Our study suggests that PGE(2) may be part of a regulatory network that controls local activation of T cells and may play a role in the balance between the development of islet autoimmunity or maintenance of tolerance.

Tumor necrosis factor-alpha and the progression of diabetes in non-obese diabetic mice

In the past decade, a wealth of information has accumulated through studies in non-obese diabetic (NOD) mice regarding the molecular and cellular events that participate in the progression to diabetes in insulin-dependent diabetes mellitus (IDDM). One molecule that has received considerable attention is the inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). TNF-alpha has been demonstrated to have a positive or negative effect on the progression to diabetes in NOD mice, although the mechanism by which TNF-alpha exerts these differential outcomes is unknown. Here we describe a new NOD model for analyzing the role of TNF-alpha in IDDM, TNF-alpha-NOD mice. TNF-alpha-NOD mice express TNF-alpha solely in their islets from neonatal life onwards, and develop accelerated progression to diabetes. This rapid progression to diabetes is related to earlier and more aggressive infiltration of the islets with immune cells and an enhancement in the presentation of islet antigen in situ in the islets by islet-infiltrating antigen-presenting cells to T cells. Although adoptive transfer studies demonstrated that TNF-alpha can enhance presentation of islet antigen to both effector CD4+ and CD8+ T cells, further investigations in TNF-alpha-NOD mice deficient in either CD4+ or CD8+ T cells demonstrated that diabetes progression is dependent on CD8+ T cells, with CD4+ T cells playing a lesser role. The data accumulating from TNF-alpha-NOD mice, described in this review, indicates novel pathways by which inflammatory stimuli can precipitate autoimmunity, and suggests newer approaches in the design of therapeutic treatments that prevent beta-cell destruction in IDDM.