Characterization of C1q by monoclonal antibodies

The effect of a purified monoclonal anti-C1q antibody (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 within 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. One out of seven monoclonal antibodies generated against mouse macrophages (M phi was found) to recognize isolated heterologous C1q. This antibody was shown to be cytotoxic and to react in a strain independent way with mouse M phi derived from bone marrow cells as well as with M phi from the peritoneal activity. However, it did not react with mouse granulocytes, thymocytes, T- and B-lymphocytes. The hemolytic activity of fluid phase C1q was inhibited to 50% at a 2 X 10(-4) dilution of hybridoma supernatant, whereas a 100-fold higher concentration was required to inhibit C1q bound to immune complexes (EAC1q) to the same extent. It was demonstrated that this antibody recognizes the isolated globular, Fc-binding portions of the C1q molecule and react with the A and B chains. Since M phi have been shown to synthesize C1q, the Fc-recognizing subcomponent of the first component of complement, evidence was provided that endogenous C1q can serve as an Fc receptor on M phi during secretion. This was demonstrated by a dose-dependent inhibition of Fc receptor activity for EIgG by the F (ab')2 fragment of this monoclonal antibody. In a fluorescence activated cell sorter (FACS) analysis Ab 146 F (ab')2 recognizes up to 75% of unstimulated NMRI peritoneal exudate cells (PEC), 60% and 53% of cells stimulated by thioglycollate and ConA, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)