Complement factor H-binding protein of Raji cells and tonsil B lymphocytes.

Several reports have indicated that Factor H has specific effects on certain cell populations, suggesting that Factor H receptors may exist. Lambris & Ross [(1982) J. Exp. Med. 155, 1400-1411] purified a protein from Raji B-lymphoblastoid cell culture supernatants, using Factor H-Sepharose affinity chromatography. This species appeared to consist of two disulphide-linked components each of Mr 50,000, with an additional 50,000-Mr chain attached non-covalently. The existence of cell-surface Factor H-binding proteins has now been re-investigated with 125I surface-labelled Raji and tonsil B cells. Non-ionic-detergent extracts of the cells, in 0.1% Nonidet P40/10 mM-sodium phosphate buffer, pH 7.4, were incubated with Factor H-Sepharose in the presence of proteinase inhibitors. After the beads had been washed, bound components were eluted with 50 mM-NaCl. A single radioactive species was eluted from the resin, which migrates identically with Factor H (apparent Mr 170,000) in SDS/polyacrylamide-gel electrophoresis under reducing and non-reducing conditions. Biosynthetic radiolabelling studies confirmed that this species was synthesized by Raji cells. Examination of culture supernatants from biosynthetically radiolabelled Raji cells showed again the presence of a single soluble species that bound to Factor H-Sepharose, but this species was of lower Mr (approx. 105,000) than the membrane-derived protein. The soluble form may be produced by proteolysis of the membrane form, or may be of separate origin. The similarity in size of the cell-surface protein to Factor H was initially confusing, but it is distinct from cell-surface Factor H on the basis of three criteria: (1) it is not recognized by anti-(Factor H) monoclonal antibodies MRC OX23 and MRC OX24, nor by polyclonal F(ab')2 anti-(Factor H); (2) it does not bind to Zn2+-chelate resin, whereas Factor H does; (3) cell-surface Factor H present on U937 cells does not bind to Factor H-Sepharose.