The expression of monomorphic determinants of class I and class II histocompatibility antigens in human heart before and after cardiac transplantation was examined using monoclonal antibodies and an immunoperoxidase technique. Thirty-five biopsy specimens were examined. Seven were from nontransplanted hearts and 28 were from patients who had received orthotopic cardiac transplantation 5-270 days previously. In normal hearts no class I was found on the surface of the myocardium, but intercalated discs were occasionally stained and interstitial structures (blood vessels and discrete mononuclear cells) strongly expressed the antigen. In contrast, of the 28 specimens from transplanted patients, 24 expressed class I antigen strongly on the outer surface and intercalated discs of the myocardium as well as on the interstitial structures. Class II antigen was not found on the myocardium of normal heart. It was present on small capillaries, the endothelia of some large capillaries, and discrete mononuclear cells. In 20 of 25 samples from transplant patients there was increased expression of class II antigen on interstitial structures but not on myocardium. The increased expression of class I antigen on the myocardium following transplantation may make it a potential target for cytotoxic T cells. The relevance of increased expression of class I and class II antigens in response to transplantation is discussed.
