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

Abstract

Aims/hypothesis. Interferon-γ (IFNγ) and TNFα synergistically induce pancreatic beta-cell apoptosis. Apart from their direct effect, we studied the possible indirect immunological role of IFNγ/TNFα 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 IFNγ. The effects of nicotinamide on islet-cell apoptosis and diabetes mellitus were examined using MTT assay and adoptive transfer model. Results. IFNγ alone induced MHC class II expression on a small number of insulinoma cells. TNFα alone did not induce MHC class II expression, but enhanced IFNγ-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 IFNγ and TNFα in combination or protect against the development of diabetes mellitus after adoptive transfer of diabetogenic lymphocytes. Conclusion/interpretation. IFNγ and TNFα 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.