Murine monoclonal anti-Ba antibody that enhances haemolytic activity of factor B.

A murine monoclonal antibody (mAb 20-ET) (IgG1, kappa) was selected from a panel of stable hybridomas produced by fusion of P3-X-63-Ag8-U1 (P3UI) myeloma cells with spleen cells from a BALB/c mouse immunized with human factor B. This antibody was shown by the immune blotting method to be directed against the Ba domain of factor B. The haemolytic activity of factor B was enhanced dose-dependently by mAb 20-ET when it was incubated with factor B and EAC4b, 3b cells (sensitized erythrocytes bearing complement fragments C4b and C3b). However, when the antibody was added after factor B had been bound to EAC4b,3b cells and the cells had been washed, it caused little enhancement of the haemolytic activity. The enhancing effect of this antibody was not due to its stabilization of the C3b-B complex, because EAC4b,3b dissociated from mAb 20-ET-bound factor B complexes rather more readily than from uncomplexed factor B. The presence of mAb 20-ET in the reaction mixture caused and maintained a much higher steady-state level of binding of factor B with EAC4b,3b cells than that in its absence. Factor P caused delayed dissociation of mAb-bound factor B from EAC4b,3b cells, thus enhancing the haemolytic activity of factor B bound with the mAb.