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

Organs of transgenic pigs that express human complement regulatory proteins are under assessment as an alternative to transplantation. A major barrier to the transplantation of pig organs is the hyperacute rejection caused by pre-existing antibodies and complement. Pig cells are very susceptible to human complement, presumably because pig cell-surface complement regulatory proteins are inefficient against it. Expression of human complement regulatory proteins, such as decay-accelerating factor and membrane cofactor proteins (MCP or CD46), by means of transgenes would confer resistance to human complement upon pig cells, thereby preventing hyperacute rejection. To express sufficient levels of human complement regulatory proteins at appropriate sites, regulatory elements of genes of pig membrane-bound complement regulatory proteins would be useful. To obtain 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 a predicted protein of 363 amino acids with 42% amino acid identity with human MCP. The pMCP consisted of four short consensus repeats, a Ser/Thr/Pro-rich domain, and transmembrane and cytoplasmic domains. Recombinant soluble pMCP that lacked transmembrane and cytoplasmic domains had factor I cofactor activity in C3b cleavage, indicating that it is functionally, as well as structurally homologous to MCP. FACS analysis with anti-pMCP mAb demonstrated that pMCP is expressed on all blood leukocytes, erythrocytes, and on endothelial and epithelial cell lines.