Monoclonal anti-human C4b antibodies: stabilization and inhibition of the classical-pathway C3 convertase.

Two IgG mouse monoclonal antibodies (MAbs), Abs 242 and 463, were prepared by fusion of spleen cells from mice immunized with human C4b with a myeloma cell line, P3/ X 63-Ag 8.653. They were assessed for their effect on the activation and stability of the cell-bound classical-pathway C3 convertase, EAC14b2a and on the binding of C2 and C4bp to EC4b. Ab 242 recognized a conformational neoantigen which appeared upon activation of C4 with C-1s and disappeared after chain separation of C4b, while Ab 463 recognized a linear epitope in the beta-chain of C4b. Ab 242 was found to be a C4bp-like MAb: it accelerates the decay-dissociation of C3 convertase and interferes with the binding of C2 to C4b. It also interfered with the binding of C4bp to C4b. These results suggest that Ab 242 recognizes an epitope which is closely related to the C2- and C4bp-binding sites in C4b. Ab 463, on the other hand, was found to be a nephritic factor like MAb: it prolongs the half-life of C3 convertase from 8 to 30 min at 37 degrees C.