1642 Retarded Inactivation of C

Cleavage of C3 by the physiologic C3 convertases or by trypsin results in the generation of a metastable form of C3b bearing a reactive internal thioester (1). This thioester may undergo hydrolysis by solvent water molecules or may form ester or amide linkages with suitable acceptors. Immunoglobulin G (IgG) is reported to display a measurable affinity for uncleaved C3 (2) and has been shown to be an excellent acceptor of nascent C3b (3). In accord with these observations, it has been shown that complement activation by soluble, IgGbearing immune complexes or IgG-sensitized bacteria results in substantial deposition of C3b onto antibody molecules (4-6). More recently, "innocent bystander" IgG in serum has been shown to bind C3b when complement is activated by exogenous immune complexes (7). Several lines of evidence suggest that binding to IgG may alter the characteristics of C3b. It is well known that IgG enhances the rate and/or extent of alternative pathway activation in an Fc fragment-independent manner on a variety of activating surfaces (8-10). Further, IgG can confer alternative pathway-activating potential on nonactivating surfaces (1 1, 12). Sensitization with IgG dramatically augments complement-mediated killing of some serum-resistant bacteria. Recent studies by Joiner et al. (13) have shown that this effect is obtained only if IgG is present on the bacterial surface at the time of initial C3b deposition; addition of IgG subsequent to this step has little influence (13). This effect, which persists even when overall C3b uptake on the subject organisms is equalized, suggests an intimate interaction between C3b and IgG on the bacterial surface that is not replaced by random juxtaposition of the molecules. Using a fluid phase system, Medoff et al. (14) have shown that C3b which is incorporated into soluble, IgG-bearing immune complexes is largely insusceptible to the action of factors H (/31H) and I (C3b inactivator) at concentrations capable of quickly cleaving free C3b or C3b bound to "nonactivating" particles such as sheep erythrocytes. In view of these data, we have undertaken studies of the influence of binding to IgG on the subsequent behavior of C3b. We report here that small, heterodimeric complexes of C3b and IgG are readily formed in vitro by trypsin cleavage Reprint requests should be addressed to L. Fries at the Laboratory of Clinical Investigation, NIAID, NIH, Building 10, Room 11N-214, Bethesda, MD 20205.