Molecular cloning of a pig homologue of membrane cofactor protein (CD46)

Organs of transgenic pigs that express human complement regulatory proteinsare under assessment as an alternative to transplantation. A major barrierto the transplantation of pig organs is the hyperacute rejection caused bypre-existing antibodies and complement. Pig cells are very susceptible tohuman complement, presumably because pig cell- surface complementregulatory proteins are inefficient against it. Expression of humancomplement regulatory proteins, such as decay- accelerating factor andmembrane cofactor proteins (MCP or CD46), by means of transgenes wouldconfer resistance to human complement upon pig cells, thereby preventinghyperacute rejection. To express sufficient levels of human complementregulatory proteins at appropriate sites, regulatory elements of genes ofpig membrane-bound complement regulatory proteins would be useful. Toobtain their cDNAs, we transfected human cells with a pig cDNA library,selected cells by incubation with pig complement and rescued the plasmids.We cloned a cDNA for the pig homologue of MCP, pMCP. The cDNA encoded apredicted protein of 363 amino acids with 42% amino acid identity withhuman MCP. The pMCP consisted of four short consensus repeats, aSer/Thr/Pro-rich domain, and transmembrane and cytoplasmic domains.Recombinant soluble pMCP that lacked transmembrane and cytoplasmic domainshad factor I cofactor activity in C3b cleavage, indicating that it isfunctionally, as well as structurally homologous to MCP. FACS analysis withanti-pMCP mAb demonstrated that pMCP is expressed on all blood leukocytes,erythrocytes, and on endothelial and epithelial cell lines.