Vaccinia virus complement control protein ameliorates hyperacute xenorejection by inhibiting xenoantibody binding

Abstract

DEPENDING ON THE SPECIES involved, either the classical or alternative complement activation pathways mediate hyperacute rejection in cardiac xenotransplantation. Cardiac transplants between mice and sensitized rats or between pigs and primates are hyperacutely rejected by the classical pathway. In these species, where the classical pathway predominates, hyperacute rejection can be diverted by either removal of xenoantibodies or by inhibition of complement activation. Vaccinia virus complement control protein (VCP) has been shown to block the classical complement pathway of activation. Therefore we hypothesized that VCP can reduce cardiac hyperacute rejection. VCP was first identified in 1988. It is a soluble protein encoded by vaccinia virus and is secreted from vaccinia virus-infected cells. VCP is structurally most similar to human C4b binding protein (C4b-BP) but is also significantly similar to members of the family of complement control proteins. VCP consists of four complement control protein modules; each module contains 60 to 65 amino acids. It is functionally similar to the family of human complement control proteins such as decay accelerating factor (DAF), membrane cofactor protein (MCP), and soluble complement receptor 1 (CR1). Purified bioactive VCP binds to C4b, blocks the formation of the classical pathway C3 convertase, binds C3b, causes the accelerated decay of the classical pathway C3 convertase, and blocks the conversion of C3 to C3b in both the classical and alternative pathways by acting as a cofactor for factor I cleavage of C3b. VCP also has been shown to have heparin binding capabilities. VCP’s heparin binding ability provides it with many additional functions that most other complement regulatory proteins lack, with the possible exception of C4b-BP and factor H (fH), which have been shown to bind heparin and complement. Through this additional heparin binding activity, VCP blocks MIP-1 activity, thereby inhibiting monocyte attachment and egression through endothelial cells in vitro. It is postulated that VCP binds to heparan sulfate proteoglycans on the surface of endothelial cells, directly blocking MIP-1 binding, thus inhibiting formation of a chemokine gradient and subsequent chemotaxis. This property also enables VCP to inhibit specific antibody binding to MHC class I molecules on human endothelial cells, suggesting that VCP can interfere with molecular interactions with infected cells and possibly prevent antibody-dependent cell-mediated cytotoxicity (ADCC) as well as other cytotoxic cell interactions with target cells. Amazingly, VCP has recently been shown to inhibit human anti-Gal 1-3 Gal antibody attachment onto cultured porcine endothelial cells, and to reduce human neutrophil and NK killing of pig aortic endothelial cells (PAEC) through its ability to bind heparan sulfate. VCP has also been shown to inhibit complement activation while remaining bound to heparan sulfate, allowing the protein to perform several functions simultaneously. Here we demonstrate, in an in vivo heterotopic cervical heart transplant model, that VCP is able to ameliorate xenorejection by blocking complement fixation and activation as well as binding to the endothelial surface and preventing xenoantibody attachment.