The human complement factor B-C3b complex: Investigation of the interaction using C3b bound to thiol-Sepharose

Abstract Factor B, a serine protease proenzyme and part of the complement system, binds to other complement proteins such as C3b and properdin. While it is known to be activated by factor D, other interactions that interfere with the binding between Factor B and C3b are less well-understood. We attached C3b to a thiol Sepharose via its free SH group and conducted a competition assay employing 125 I-labelled factor B to study competitive binding. Two anti-C3d monoclonal antibodies (F49b and 4C2) were found to partially inhibit the binding of Factor B to C3b. The inhibitory effects of these monoclonal antibodies were found to be dose-dependent. 4C2 was found to have a maximum inhibition of 50%, while the maximum inhibition by F49b was ∼40%. In contrast, two anti-C3c monoclonal antibodies (F39b and F20b) were able to enhance the formation of the factor B-C3b complex in a dose-dependent manner. Competition binding studies with isolated C3 fragments further supported the involvement of the C3d region in factor B binding, as C3d inhibited the binding of factor B to C3b completely. Furthermore, additional competition binding studies conducted with each of the three domains of factor B (Ba, vWF and SP) demonstrated that each domain independently inhibited the binding of intact factor B to C3b by ∼50%.