MODIFICATION OF HUMORAL RESPONSES BY THE COMBINATION OF LEFLUNOMIDE AND CYCLOSPORINE IN LEWIS RATS TRANSPLANTED WITH HAMSTER HEARTS

Background. Vigorous antibody-mediated responses prevent the successfu l engraftment of hamster hearts transplanted into Lewis rats. Early an tibody responses mediating acute rejection of the xenograft are T cell -independent and resistant to the T-cell immunosuppressant, cyclospori ne (CsA). Immunosuppression with the combination of leflunomide plus C sA completely prevents xenograft rejection, but when such immunosuppre ssion is stopped the hamster heart is rejected by a process that we te rm late xenograft rejection. me report here on some of the immunologic al features of late xenograft rejection. Methods. Lewis rats transplan ted with hamster hearts were treated with leflunomide (5 mg/kg/day by gavage) for 14-21 days and CsA (20 mg/kg/day by gavage) continuously f rom the day of transplant. Serum was harvested and the functional acti vities of the xenoreactive antibodies were quantitated by in vivo pass ive transfer of sera, flow cytometry, in vitro C3 deposition assays, a nd Western blotting, Results. CsA alone prevented late xenograft rejec tion and the accompanying production of xenoreactive antibodies. The x enoreactive antibodies accompanying acute or late xenograft rejection were predominantly IgM, but only serum from rats undergoing acute xeno graft rejection was able to induce hyperacute rejection, The ability o f serum to induce hyperacute rejection correlated with its ability to induce C3 deposition on hamster lymphocytes in vitro, The repertoire o f hamster antigens recognized by IgM in the serum of rats undergoing l ate xenograft rejection is more restricted than that of IgM in the ser um of rats undergoing acute xenograft rejection. We additionally demon strate that long-term graft survival is not dependent on graft accommo dation. Conclusions. These studies demonstrate that a brief treatment with the combination of leflunomide and CsA profoundly modifies the hu moral xenoreactivity in the recipient, converting it from a T-independ ent into a T cell-dependent response. Differences in functional activi ty of sera from acute or late xenograft rejection suggest that antigen ic specificity defines the ability of IgM to induce complement activat ion and hyperacute rejection.