Dose-dependent inhibition of complement in baboons by vaccinia virus complement control protein: implications in xenotransplantation

Vaccinia virus complement control protein (VCP) is a potent inhibitor of both the alternative and the classical complement pathways through its binding to activated third and fourth components. In addition to its complement inhibiting abilities, VCP can bind heparan sulfate on cell surfaces, resulting in further functional activities. Altogether, the multiple functions of VCP have been shown to reduce the inflammatory response of the host, helping the vaccinia virus to evade immune destruction. Recently, we reported that VCP is able to block hyperacute xenograft rejection, significantly prolonging graft survival in two separate in vivo heterotopic cervical cardiac xenograft models. Histopathological examination of the transplanted hearts receiving VCP revealed marked VCP deposition on the endothelium, a significant reduction in cardiac tissue damage, and significantly less C3, IgG and IgM deposition in the tissue. It is concluded that VCP may inhibit hyperacute xenorejection by binding to the endothelial surface, blocking complement fixation activation, thereby preventing xenoantibody attachment. In the current study, the level of serum complement inhibition was evaluated following different bolus dosages of VCP in baboons. The results indicated that to achieve a satisfactory level of complement inhibition higher doses of VCP are needed in baboons, than previously observed in rats. The current observations are critical for future assessment of the role of VCP to suppress hyperacute rejection following pig-to-baboon xenotransplantation.