In this study, we have extended a cyclophosphamide (CP)-induced tolerance system to kidney transplantation in rats to examine whether or not we can overcome fully allogeneic (major histocompatibility complex plus minor histocompatibility) antigen barriers in organ transplantation. In the recipient Lewis (LEW, RT11) rats that were primed intravenously with 4 X 108 spleen cells plus 2 X 108 bone marrow cells from Brown-Norway (BN, RT1n) rats and treated intraperitoneally with 100 mg./kg. of cyclophosphamide (CP) 2 days later, the survival of kidney allografts, but not skin allografts, from BN was prolonged as compared with that in the untreated LEW rats. Some of the kidney allografts survived for more than 100 days without further immunosuppressants. The tolerant state induced was tolerogen specific, and the suppression of tissue damage of the grafted kidney in such tolerant rats was also confirmed by the histopathological findings of the grafted kidney. These results indicate that considerable levels of tolerance can be induced, at least in organ transplantation, across fully allogeneic antigen barriers in rats by a CP-induced tolerance system. We believe that the present study is the first step in applying our CP-induced tolerance system using skin grafting in the murine model to clinical organ transplantation.
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