Reduced MHC class II antigen expression on rat heart and kidney cells after in vitro exposure to longwave ultraviolet irradiation.

The pretreatment of both kidney and heart donor with the photosensitizer 8-methoxy-psoralen followed by in vitro longwave ultraviolet graft irradiation (PUVA therapy) significantly prolonged graft survival times in rats even across a strong major histocompatibility complex (MHC) barrier. Immunohistological studies using monoclonal antibodies (Mab) to rat MHC class I and II antigens showed a significant reduction of class II antigen expression after PUVA therapy in both Sprague-Dawley rat kidneys and hearts. Using MHC class I Mab there were no differences in distribution pattern in untreated as well as in PUVA treated organs. Thus, our results represent the first in vivo evidence that photochemotherapy-induced graft survival time prolongation is closely connected with its ability to reduce MHC class II antigen expression.