A single injection of 10(5) U/kg of recombinant rat IFN-gamma increases the amount of tissue dendritic cells up to sixfold, and concomitantly induces the (capillary) endothelial cells to express class II MHC antigens. Both responses peak on the third day after IFN-gamma injection, and the antigen expression returns to basic levels on day 7. Simultaneous administration of 1 mg/kg/d of methylprednisolone entirely abolishes both responses. These observations demonstrate, for the first time, that IFN-gamma and steroids have antagonistic effects on class II MHC antigen presentation in tissue, and suggest that one immunosuppressive mechanism of glucocorticosteroids in organ transplantation is downregulation of graft antigenicity.

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