MOLECULAR-CLONING OF A PIG HOMOLOG OF MEMBRANE COFACTOR PROTEIN (CD46)

Organs of transgenic pigs that express human complement regulatory pro teins are under assessment as an alternative to transplantation. A maj or barrier to the transplantation of pig organs is the hyperacute reje ction caused by pre-existing antibodies and complement. Pig cells are very susceptible to human complement, presumably because pig cell-surf ace complement regulatory proteins are inefficient against it. Express ion of human complement regulatory proteins, such as decay-acceleratin g factor and membrane cofactor proteins (MCP or CD46), by means of tra nsgenes would confer resistance to human complement upon pig cells, th ereby 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 protein s would be useful. To obtain their cDNAs, we transfected human cells w ith a pig cDNA library, selected cells by incubation with pig compleme nt 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 wi th 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 transmem brane and cytoplasmic domains had factor I cofactor activity in C3b cl eavage, indicating that it is functionally, as well as structurally ho mologous to MCP. FAGS analysis with anti-pMCP mAb demonstrated that pM CP is expressed on all blood leukocytes, erythrocytes, and on endothel ial and epithelial cell lines.