OVEREXPRESSION OF CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX ACCOMPANIES INSULITIS IN THE NONOBESE DIABETIC MOUSE AND IS PREVENTED BY ANTI-INTERFERON-GAMMA ANTIBODY

Overexpression of class I major histocompatibility complex (MHC) proteins on pancreatic islet cells is a characteristic of autoimmune Type 1 (insulin-dependent) diabetes mellitus in humans and in animal models. Studies of post-mortem pancreases from humans with Type 1 diabetes suggest that overexpression of class I MHC proteins may precede mononuclear cell infiltration of the islets (insulitis). Pancreatic histology from the earliest stages of human Type 1 diabetes is rarely available. We have used the non-obese diabetic mouse, given cyclophosphamide to accelerate Beta-cell destruction, to investigate the temporal relationship between the overexpression of class I MHC protein and mRNA and other pathological changes associated with Beta-cell destruction. Prior to cyclophosphamide, immunoperoxidase staining showed that expression of class I MHC proteins was greater on islet cells and infiltrating inflammatory cells of the non-obese diabetic mouse than on islet cells of other mouse strains, whereas staining on exocrine cells was similar. On day three after cyclophosphamide administration, when insulitis had regressed, islet class I MHC protein expression had diminished. A dramatic increase in class I MHC protein expression occurred between days seven and nine, concomitant with reinfiltration of the islets by mononuclear cells; overexpression was seen both on islet cells and on surrounding exocrine cells, but only in the presence of mononuclear cell infiltration. By day 21, class I MHC protein overexpression was again confined to the islets, the exocrine pancreas being free of infiltration. Class I mRNA also increased dramatically by day eight but had virtually returned to normal by day 12. Overexpression of class I MHC protein following cyclophosphamide was prevented by administration of anti-interferon-gamma-antibody. Expression of class II MHC proteins was not detected on pancreatic cells following cyclophosphamide but was present on infiltrating mononuclear cells. These findings demonstrate a close association between class I MHC protein and mRNA overexpression and insulitis in non-obese diabetic mice given cyclophosphamide. They are consistent with the view that class I MHC overexpression is effected by cytokines secreted by activated immuno-inflammatory cells. Class I MHC overexpression should enhance targeting of cytotoxic T cells to Beta cells bearing autoantigen.