The effect of specific antibody on antibody-independent interactions between E. coli J5 and human complement

We previously reported that Escherichia coli J5, the galactose epimerase-deficient mutant of E. coli O111:B4, can bind and activate purified human C1. The effects of hyperimmune rabbit anti-J5 IgG or IgM on E. coli J5 interactions with human C have been examined. Specific IgG or IgM increased the binding of 125I-C1 by J5. However, the rate of C1 activation, as determined by SDS-PAGE of eluted 125I-C1s, was decreased if bacteria were preincubated with immune IgG. Complexes formed between J5 preincubated with immune Ig and C1, under conditions in which all of the C1 was allowed to activate, consumed more C4 than J5 alone plus C1. However, the amount of C4 consumed per C1 molecule was identical for all bacteria preparations. Concentrations of specific IgG or IgM that significantly increased C1 binding did not appear to enhance C3b deposition upon incubation of E. coli J5 in NHS. Thus, although specific antibody may enhance C1 binding by E. coli J5, the ability of these additional C1 molecules to alter later events in the C cascade may depend on the control of C1 activation and its subsequent activity when bound to different membrane components.