Vaccinia virus complement control protein is capable of protecting xenoendothelial cells from antibody binding and killing by human complement and cytotoxic cells

Background.Vaccinia virus complement control protein (VCP) was the first secretory microbial protein shown to have structural similarity to the family of complement control proteins, VCP can block both the classical and alternate complement pathways. Recently, VCP has been shown to bind to heparin, and this property contributes to separate functions, making the molecule a multifunctional protein. Methods. VCP prepared from a natural infection of RK-13 cells with vaccinia virus was purified to homogeneity, Cultured pig aortic endothelial cells (PAECs) were mixed with human serum, anti-Gal alpha1,3 Gal antibody, neutrophils, or natural killer (NK) cells in the presence or absence of VCP and either direct binding of FITC-labeled antibody or billing by cytotoxic cells was estimated, Results. Our cell culture studies demonstrate that VCP blocks complement-mediated killing of PAECs by human serum in a dose-dependent manner. We also demonstrate that VCP is capable of blocking Gal alpha1,3 Gal binding sites on PAECS, Surprisingly, VCP effectively blocked interactions between PAECs and cytotoxic cells such as human naive neutrophils and NR cells. Conclusion. VCP is a novel protein amongst the complement control protein family and can, not only block xenorejection by inhibiting complement but also by blocking killing by cytotoxic cells.