530 Cyclosporine A (CsA) has been so far required in the immunosuppression regimen of recipient of vascularized xenograft beyond the phase of hypercute rejection. Effects of CsA on non lymphoid immune cells such as endothelial cells (EC) are not well established and sometimes contradictory since both protective and adverse effects have been reported. In the present study, we have investigated whether CsA could contribute to alter the antigenicity of porcine aortic endothelial cells (PAEC) by reducing class I and class II MHC antigen expression. The ability of CsA to prevent MHC antigens expression during human TNFα- or lymphocytes-mediated EC activation has been evaluated by Facs and semiquantitative RT-PCR and correlated to the ability of EC to promote human lymphocyte proliferation. First, Facs analysis showed that overexpression of class II MHC antigens on PAEC, mediated by human TNFα, was strongly inhibited in the presence of CsA. The expression level of class I MHC antigens was reduced by CsA to less than 50 % of the value for EC treated with TNFα alone whereas the presence of CsA completely prevented the induction of class II MHC molecules. CsA-mediated inhibition was dose-dependent within drug concentrations ranging between 2.5 and 20 μg/ml and was consistently observed at all time points (24h to 72h) of the activation period. The presence of CsA during activation of EC with TNFα dramatically reduce the ability of EC to further promote human PBL proliferation. Indeed, when PAEC were pretreated with both TNFα and CsA, lymphocyte proliferation was below that induced by resting EC for all time points (proliferation index: 0.6, 0.2 and 0.23 for day 3, 5 and 7, respectively). Similar level of inhibition was achieved by using an anti-porcine class II MHC blocking mAb. We also observed that expression of E-selectin, as well as class I and class II porcine MHC Ag, was induced following coculture of porcine EC with human PBL. Pretreatment of porcine EC with CsA for 4h, before coculture experiments, efficiently prevented the induction of E-selectin and class II MHC antigens and inhibits overexpression of class I antigens. Semiquantitative RT-PCR experiments showed that the presence of CsA markedly reduced the steady-state level of porcine class II (SLADRA and SLADQA) mRNA at 16h as compared with PAEC stimulated with TNFα alone. This reduced level of mRNA for class II MHC was associated with the lack of class II transcriptional coactivator, CIITA, expresssion at this time point. The ability of CsA to promote the rapid decay of CIITA mRNA is analysed. Therefore, our data indicate that CsA could contribute to prevent direct presentation of porcine MHC antigens by xenogeneic EC to human T lymphocytes.