Beta cell apoptosis in T cell-mediated autoimmune diabetes

Interferon-gamma impacts at multiple points during the progression of autoimmune diabetes

The role of interferon-gamma in autoimmune diabetes was assessed by breeding a null mutation of the interferon-gamma receptor alpha chain into the nonobese diabetic mouse strain, as well as into a simplified T cell receptor transgenic model of diabetes. In contrast to a previous report on abrogation of the interferon-gamma gene, mutation of the gene encoding its receptor led to drastic effects on disease in both mouse lines. Nonobese diabetic mice showed a marked inhibition of insulitis-both the kinetics and penetrance-and no signs of diabetes; the transgenic model exhibited near-normal insulitis, but this never evolved into diabetes, either spontaneously or after experimental provocation. This failure could not be explained by perturbations in the ratio of T helper cell phenotypes; rather, it reflected a defect in antigen-presenting cells or in the islet beta cell targets.

Irreversible inhibition of metabolic function and islet destruction after a 36-hour exposure to interleukin-1beta

The purpose of this study was to identify the duration of exposure of islets to interleukin 1beta (IL-1beta) that results in irreversible damage. Treatment of rat islets for 18 h with IL-1beta results in an inhibition of glucose-stimulated insulin secretion, mitochondrial aconitase activity, and total protein synthesis. The addition of N(G)-monomethyl-L-arginine (NMMA) or aminoguanidine to islets preincubated for 18 h with IL-1beta, followed by continued culture for 8 h (with both NMMA and IL-1beta), results in the recovery of islet secretory function, aconitase activity, and protein synthesis. However, islet metabolic function is irreversibly inhibited after a 36-h incubation with IL-1beta, as an additional 8-h incubation with NMMA or aminoguanidine does not stimulate the recovery of insulin secretion, aconitase activity, or protein synthesis. The irreversible inhibition of metabolic function correlates with the commitment of islets to destruction. Treatment of islets for 96 h with IL-1beta results in islet degeneration. NMMA, added to islets 24 h after the addition of IL-1beta, followed by continued culture for 72 h (with NMMA and IL-1beta), prevents islet degeneration. However, NMMA added to islets 36 h or 48 h after the addition of IL-1beta, followed by continued culture for a total of 96 h, does not prevent islet degeneration. New messenger RNA expression appears to be required for islet recovery from IL-1beta-induced damage as actinomycin D prevents the recovery of islet aconitase activity. Lastly, treatment of human islets with a combination of IL-1beta and interferon-gamma (IFNgamma) results in a potent inhibition of mitochondrial aconitase activity. NMMA, when cocultured with IL-1beta + IFNgamma, completely prevents cytokine-induced inhibition of human islet aconitase activity. NMMA, when added to human islets pretreated for 18 h with IL-1beta + IFNgamma, stimulates the recovery of mitochondrial aconitase activity after an additional 8 h incubation. These findings indicate that nitric oxide-induced islet damage is reversible; however, prolonged production of nitric oxide (after a 36-h exposure to IL-1beta) results in the irreversible inhibition of islet metabolic and secretory function.

Genetic control of diabetes progression

Autoimmune diabetes in both the human and the nonobese diabetic mouse has elaborate genetics; in the latter case, the disease is influenced by at least 15-20 loci. We anticipated that the genetics would be simpler in the BDC2.5 T cell receptor transgenic mouse model of diabetes, wherein many T cells express a particular diabetogenic specificity. Initiation of insulitis in this model was the same on the two genetic backgrounds analyzed, but the kinetics and penetrance of diabetes were strikingly different, permitting us to focus on genetic influences during a defined window of disease progression. The differences correlated with variations in five genomic intervals, certain ones of which have been previously implicated in susceptibility to autoimmune disease. This reductionist approach indeed simplified the analysis of diabetes susceptibility loci.

Nitric oxide primes pancreatic beta cells for Fas-mediated destruction in insulin-dependent diabetes mellitus

Fas is an apoptosis-inducing surface receptor involved in controlling tissue homeostasis and function at multiple sites. Here we show that beta cells from the pancreata of newly diagnosed insulin-dependent diabetes mellitus (IDDM) patients express Fas and show extensive apoptosis among those cells located in proximity to Fas ligand-expressing T lymphocytes infiltrating the IDDM islets. Normal human pancreatic beta cells that do not constitutively express Fas, become strongly Fas positive after interleuken (IL)-1beta exposure, and are then susceptible to Fas-mediated apoptosis. NG-monomethyl-L-arginine, an inhibitor of nitric oxide (NO) synthase, prevents IL-1beta-induced Fas expression, whereas the NO donors sodium nitroprusside and nitric oxide releasing compound (NOC)-18, induce functional Fas expression in normal pancreatic beta cells. These findings suggest that NO-mediated upregulation of Fas contributes to pancreatic beta cell damage in IDDM.

A mechanism for the major histocompatibility complex-linked resistance to autoimmunity

Certain major histocompatibility complex (MHC) class II haplotypes encode elements providing either susceptibility or dominant resistance to the development of spontaneous autoimmune diseases via mechanisms that remain undefined. Here we show that a pancreatic beta cell-reactive, I-Ag7-restricted, transgenic TCR that is highly diabetogenic in nonobese diabetic mice (H-2(g7)) undergoes thymocyte negative selection in diabetes-resistant H-2(g7/b), H-2(g7/k), H-2(g7/q), and H-2(g7/nb1) NOD mice by engaging antidiabetogenic MHC class II molecules on thymic bone marrow-derived cells, independently of endogenous superantigens. Thymocyte deletion is complete in the presence of I-Ab, I-Ak + I-Ek or I-Anb1 + I-Enb1 molecules, partial in the presence of I-Aq or I-Ak molecules alone, and absent in the presence of I-As molecules. Mice that delete the transgenic TCR develop variable degrees of insulitis that correlate with the extent of thymocyte deletion, but are invariably resistant to diabetes development. These results provide an explanation as to how protective MHC class II genes carried on one haplotype can override the genetic susceptibility to an autoimmune disease provided by allelic MHC class II genes carried on a second haplotype.

Mechanism underlying counterregulation of autoimmune diabetes by IL-4

Diabetes in nonobese diabetic (NOD) mice is an autoimmune disease characterized by the destruction of the beta cells in the pancreas. We have previously reported that transgenic expression of interleukin-4 (IL-4) counterregulates the disease process, completely protecting NOD mice from insulitis and diabetes. Here we demonstrate the presence of autoreactivity but lack of pathogenicity of the IL-4-regulated lymphocytes. The importance of T cell diversity for the protective effect of IL-4 is demonstrated through breeding with transgenic BDC2.5 mice, which have an almost exclusively monoclonal T cell repertoire. Limitation of T cell diversity abrogated the protection by IL-4. We suggest that "immune deviation" in NOD-IL-4 mice is mediated by the pancreatic tissue itself, which causes activation of distinct, nonpathogenic T cell specificities.

T helper 2 (Th2) T cells induce acute pancreatitis and diabetes in immune-compromised nonobese diabetic (NOD) mice

Autoimmune diabetes is caused by the CD4(+), T helper 1 (Th1) cell-mediated apoptosis of insulin-producing beta cells. We have previously shown that Th2 T cells bearing the same T cell receptor (TCR) as the diabetogenic Th1 T cells invade islets in neonatal nonobese diabetic (NOD) mice but fail to cause disease. Moreover, when mixed in excess and cotransferred with Th1 T cells, Th2 T cells could not protect NOD neonates from Th1-mediated diabetes. We have now found, to our great surprise, the same Th2 T cells that produced a harmless insulitis in neonatal NOD mice produced intense and generalized pancreatitis and insulitis associated with islet cell necrosis, abscess formation, and subsequent diabetes when transferred into immunocompromised NOD.scid mice. These lesions resembled allergic inflamation and contained a large eosinophilic infiltrate. Moreover, the Th2-mediated destruction of islet cells was mediated by local interleukin-10 (IL-10) production but not by IL-4. These findings indicate that under certain conditions Th2 T cells may not produce a benign or protective insulitis but rather acute pathology and disease. Additionally, these results lead us to question the feasibility of Th2-based therapy in type I diabetes, especially in immunosuppressed recipients of islet cell transplants.

Cytokines induce deoxyribonucleic acid strand breaks and apoptosis in human pancreatic islet cells

We have previously observed that a 6-day exposure of human pancreatic islets to a combination of cytokines (interleukin-1beta 50 U/ml + tumour necrosis factor-alpha 1000 U/ml + interferon-gamma 1000 U/ml) severely impairs beta-cell functions. In the present study, we examined whether this condition affects DNA integrity and viability of human islet cells. Cells were studied after 3, 6, and 9 days of cytokine treatment by both single cell gel electrophoresis (the "comet assay," a sensitive method for detection of DNA strand breaks) and by a cytotoxicity assay using the DNA binding dyes Hoechst 33342 and propidium iodide as indices for the number of viable, necrotic, and apoptotic cells. Cytokine treatment for 6 and 9 days resulted in a 50% increase in comet length (P < 0.01 vs. controls), indicating DNA strand breaks, as well as in a significant increase in the number of apoptotic cells (P < 0.02 vs. controls), but not in the number of necrotic cells. The arginine analogs N(G)-nitro-L-arginine and N(G)-monomethyl-L-arginine prevented nitric oxide formation by the cytokines but did not interfere with cytokine-induced DNA strand breaks and apoptosis. The present data suggest that prolonged (6-9 days) exposure of human pancreatic islets to a mixture of cytokines induces DNA strand breaks and cell death by apoptosis. These deleterious effects of cytokines appear to be independent of nitric oxide generation.