827 In concert with others we have demonstrated that combined anti-CD40L and CTLA-4Ig (costimulation blockade) treatment produce long-term cardiac allograft survival in mice and that cyclosporine (CsA) totally blocked the costimulation blockade induced allograft tolerance. In contrast, rapamycin (RPM) plus costimulation blockade enables tolerance induction. The mechanism by which CsA and RPM exert differential effects on costimulation blockade induced tolerance is unknown. In the present study, we examined intragraft gene expression of CTL attacking molecules FasL and Granzyme B in recipient mice treated with CTLA-4Ig and anti-CD40L, CsA, RPM, as well as combined treatment with CTLA-4Ig and anti-CD40L plus CsA or RPM. We found that: 1) costimulation blockade markedly inhibited granzyme B and FasL gene expression as compared to untreated controls; 2) costimulation blockade plus RPM completely abolished intragraft expression of FasL and granzyme B; 3) costimulation blockade plus CsA failed to block FasL and granzyme B gene expression. We conclude that the biological effect of CsA and RPM on costimulation blockade induced tolerance is strikingly different and CsA and RPM have different effects on expression of CTL markers.