IFNγ/TNFα synergism in MHC class II induction:: effect of nicotinamide on MHC class II expression but not on islet-cell apoptosis

Aims/hypothesis. Interferon-gamma (IFNgamma) and TNFalpha synergistically induce pancreatic beta-cell apoptosis. Apart from their direct effect, we studied the possible indirect immunological role of IFNgamma/TNFalpha synergism on pancreatic beta-cell death by investigating MHC class II induction by cytokines. The effect of nicotinamide on the cytokine-induced MHC class II expression and pancreatic beta-cell death was also studied. Methods. Immunocytochemistry, flow cytometry and RNase protection assay were used to study MHC class II expression. Immunoblotting was done to study downstream signals of IFNgamma. The effects of nicotinamide on islet-cell apoptosis and diabetes mellitus were examined using MTT assay and adoptive transfer model. Results. IFNgamma alone induced MHC class II expression on a small number of insulinoma cells. TNFalpha alone did not induce MHC class II expression, but enhanced IFNgamma-induced MHC class II expression. MHC class II expression by cytokine(s) was due to the induction of class II transactivator (CIITA). Nicotinamide reduced MHC class II expression by cytokine(s) but did not protect insulinoma-cell apoptosis by IFNgamma and TNFalpha in combination or protect against the development of diabetes mellitus after adoptive transfer of diabetogenic lymphocytes. Conclusion/interpretation. IFNgamma and TNFalpha synergistically induced MHC class II expression on insulinoma cells through the induction of CIITA; nicotinamide reduced the expression of cytokine-induced MHC class II expression on insulinoma cells through its effect on CIITA expression; and the preventive effect of nicotimamide on Type I (insulin-dependent) diabetes mellitus is probably due to its effect of MHC class II expression rather than that on islet cell apoptosis.