Role for CD47-SIRPalpha signaling in xenograft rejection by macrophages.

We have previously proven that human macrophages can phagocytose porcine cells even in the absence of Ab or complement opsonization, indicating that macrophages present a pivotal immunological obstacle to xenotransplantation. A recent report indicates that the signal regulatory protein (SIRP)alpha is a critical immune inhibitory receptor on macrophages, and its interaction with CD47, a ligand for SIRPalpha, prevents autologous phagocytosis. Considering the limited compatibility (73%) in amino acid sequences between pig and human CD47, we hypothesized that the interspecies incompatibility of CD47 may contribute to the rejection of xenogeneic cells by macrophages. In the present study, we have demonstrated that porcine CD47 does not induce SIRPalpha tyrosine phosphorylation in human macrophage-like cell line, and soluble human CD47-Fc fusion protein inhibits the phagocytic activity of human macrophages toward porcine cells. In addition, we have verified that manipulation of porcine cells for expression of human CD47 radically reduces the susceptibility of the cells to phagocytosis by human macrophages. These results indicate that the interspecies incompatibility of CD47 significantly contributes to the rejection of xenogeneic cells by macrophages. Genetic induction of human CD47 on porcine cells could provide inhibitory signaling to SIRPalpha on human macrophages, providing a novel approach to preventing macrophage-mediated xenograft rejection.