DISCRIMINATION BETWEEN ALTERNATIVELY SPLICED STP-A AND STP-B ISOFORMSOF CD46

CD46 (membrane cofactor protein; MCP) is ubiquitously expressed on nuc leated human cells; it has a protective function, binding C3b and C4b, which are then cleaved by serum factor I. CD46 molecules (55 000-65 0 00 MW) have four short consensus repeats (SCR): the function of SCR-1 and -2 is unknown; SCR-3 and -4 bind C3b and C4b. These are succeeded by the STP region, which can contain three separate regions (STP-A, -B , -C) rich in serine, threonine and proline and which are heavily glyc osylated, succeeded by transmembrane and cytoplasmic tail regions (of which there are several). Multiple isoforms exist due to the different splicing of exons: STP-A and -B can thus be present or absent. So far these products can only be detected separately by polymerase chain re action (PCR) and RNA studies; we now describe their detection by anti- peptide antibodies. Peptides whose sequences corresponded with those o f STP-A and STP-B were synthesized and used for the immunization of mi ce; although they differ in only seven of 21 amino acids, monoclonal a ntibodies (mAb) that reacted specifically with STP-A but not with STP- B, and mAb that reacted specifically with STP-B but not with STP-A, we re produced; these reacted specifically with native CD46 on human tiss ues and cell lines. STP-A mAb reacted with tissues in which STP-A RNA had been found, some leukaemias and cell lines; in normal tissue expre ssion was mainly found in the intestine (large and small) and salivary gland. Anti-STP-B reacted with most tissues and cell lines. The antib odies should be of use in defining the expression and function of CD46 in different tissues.