A NOVEL, COMPLEMENT FACTOR H-RELATED REGULATORY PROTEIN EXPRESSED ON THE SURFACE OF HUMAN B-CELL LINES

Complement regulatory proteins present on the surface of various mamma lian cells play an important role in controlling homologous lysis, by interacting with C3 (and usually C4). These proteins have a similar st ructural motif (''short consensus repeat'') (Reid, K.B.M., Bentley, R. D., Campbell, R.D., Chung, L.P., Sim, R.B., Kristensen, T. and Tack, B .F., Immunol. Today 1986.7: 230), and the genes encoding them are memb ers of the family of regulators of complement activation. Here we desc ribe a hitherto unknown member of this family, a molecule expressed by B lymphoblastoid cells. This protein is recognized by polyclonal anti bodies fo factor H and by MAH4, a monoclonal antibody reacting with th e N-terminal portion of factor H. The cell surface protein is built up of two disulfide-linked chains of approximately 68 and 75 kDa. Biosyn thetic labeling studies confirmed that it is synthesized by B cells on ly but not by the investigated lines of other origin. When tested for its functional activity, this molecule was shown to act as cofactor fo r factor I-mediated cleavage of fluid-phase C3b to C3bi. The protein a ppears to be encoded by a 3.5-kb mRNA, hybridizing with a cDNA probe c oding for the N-terminal portion of factor H. Due to its cross-reactiv ity with anti-H antibodies, cofactor activity for factor I and hybridi zation with factor H cDNA, despite its two-chain composition, it is co nsidered a factor H-like protein.