BACKGROUND: Factors that contribute to xenograft (Xg) rejection were investigated in complement C6-deficient (C-) PVG rats. METHODS: First and second hamster hearts were transplanted in C6-deficient and C6-sufficient PVG rats. Xenoantibody (XAb) formation, hemolytic C (CH50) activity and immunohistochemistry were studied. RESULTS: PVG C6-deficient rats rejected Xgs 3 days later than PVG C6-sufficient rats. Surprisingly, C activation participated in the rejection in PVG C- rats, as shown by partially recovered serum CH50 levels and deposition of C factors in the Xgs. As we found that cultured endothelial cells produced C6 in vitro, we hypothesized that Xg endothelial cells corrected the C6 defect in PVG C- rats. This was probably induced by IgM XAbs as: (1) it did not occur in immunosuppressed PVG C- rats in which XAb formation was prevented, and (2) transfer of IgM XAbs to naive, xenotransplanted PVG C- rats accelerated the recovery of CH50 and concomitantly Xg rejection. Thirty days after rejection of a first Xg, when no IgM XAbs or CH50 activity but high levels of IgG XAbs were detected in PVG C- rats, second Xgs underwent a hyperacute rejection. This time, complement was not involved, as no serum CH50 nor C deposition was found in the Xg. Instead, IgG antibody-dependent cellular cytotoxicity was involved as: (1) IgG XAbs were deposited in the Xg and (2) hyperacute rejection was induced in naive PVG C- rats by transfer of IgG XAbs, and (3) this rejection was delayed to 5+/-3 days if the adoptive hosts were first irradiated. CONCLUSIONS: In the face of a defect of host C factors, IgM XAb may induce cells of the Xg to secrete C factors which may correct the C defect of the host. Even if activation of lytic C can be prevented, IgG XAb may still provoke an acute Xg rejection by antibody-dependent cellular cytotoxicity.