IFN-gamma/TNF-alpha synergism as the final effector in autoimmune diabetes: A key role for STAT1/IFN regulatory factor-1 pathway in pancreatic beta cell death

Fas ligand (FasL), perforin, TNF-alpha, IL-1, and NO have been considered a s effector molecule(s) leading to beta cell death in autoimmune diabetes. H owever, the real culprit(s) in beta cell destruction have long been elusive , despite intense investigation. We and others have demonstrated that FasL is not a major effector molecule in autoimmune diabetes, and previous inabi lity to transfer diabetes to Fas-deficient nonobese diabetic (NOD)-lpr mice was due to constitutive FasL expression on lymphocytes from these mice. He re, we identified IFN-gamma /TNF-alpha synergism as the final effector mole cules in autoimmune diabetes of NOD mice. A combination of IFN-gamma and TN F-alpha, but neither cytokine alone, induced classical caspase-dependent ap optosis in insulinoma and pancreatic islet cells. IFN-gamma treatment confe rred susceptibility to TNF-alpha -induced apoptosis on otherwise resistant insulinoma cells by STAT1 activation followed by IFN regulatory factor (IRF )-1 induction. IRF-1 played a central role in IFN-gamma /TNF-alpha -induced cytotoxicity because inhibition of IRF-1 induction by antisense oligonucle otides blocked IFN-gamma /TNF-alpha -induced cytotoxicity, and transfection of IRF-1 rendered insulinoma cells susceptible to TNF-alpha -induced cytot oxicity. STAT1 and IRF-1 were expressed in pancreatic islets of diabetic NO D mice and colocalized with apoptotic cells. Moreover, anti-TNF-alpha Ab in hibited the development of diabetes after adoptive transfer. Taken together , our results indicate that IFN-gamma /TNF-alpha synergism is responsible f or autoimmune diabetes in vivo as well as beta cell apoptosis in vitro and suggest a novel signal transduction in IFN-gamma /TNF-alpha synergism that may have relevance in other autoimmune diseases and synergistic anti-tumor effects of the two cytokines.