Activation of the first component of complement, C1, by a monoclonal antibody recognizing the C chain of C1q

The effect of a purified monoclonal anti-C1q anti-body (Ab 242 G3) on the function of C1q, a subcomponent of the first component of complement C1, was studied. No inhibition of purified activated C1 was observed, whereas binding of the Ab to fluid phase C1q, to C1q bound to immune complexes (EAC1q), or to serum C1 in fluid phase resulted in a dose-dependent inhibition of the hemolytic activity of C1. In contrast, when the effect of the Ab on serum C1 bound to immune complexes (EAC1) was measured, no inhibition, but a dose-dependent enhancement, of the hemolytic activity was obtained. The dose-response curve of the Ab-treated cell-bound serum C1 was indistinguishable from that of activated C1. Isolated Fab fragments of this Ab did not cause an increase in C1 activity. After separation of the A, B, and C chains of C1q by SDS-PAGE, Ab 242 G3 reacted in immunoblotting selectively with the C chain. These data indicate that cross-linking of C1q via the C chain of C1q might lead to an internal activation of C1.