Antibody-independent C1 activation by E. coli

Antibody-independent interactions of C1 with several E. coli strains were examined. Purified C1 was directly activated by the semi-rough mutant E. coli J-5, its parental wild-type strain, E. coli 0111:B4, and two clinical isolates, E. coli (P) and E. coli (A), in the absence of C1 inhibitor. E. coli J-5 activated C1 about 10-fold more rapidly and bound approximately threefold more C1 than the other strains. E. coli J-5, but not the other strains, also bound C1s2, provided that the subcomponent was offered to the bacteria in the presence of C1q and calcium; such binding was thus independent of the presence or absence of C1r2. After C1 activation in the absence of C1 inhibitor, activated C1s spontaneously dissociated from E. coli 0111:B4, (P), and (A), but remained associated with E. coli J-5. The regulatory protein C1 inhibitor prevented C1 activation by the weaker activators, E. coli strains 0111:B4, (P), and (A), but had no effect on C1 activation by E. coli J-5. Although C1 inhibitor thus failed to modulate C1 activation by E. coli J-5, it did block the enzymatic activity of activated C1 bound to this strain. Analyses of the molecular processes involved revealed differences with other systems. In the presence of C1 inhibitor, the C1s subunit of C1 activated by E. coli J-5 underwent further cleavage with the release into the supernatant of C1s fragments and complexes of C1 inhibitor with light chain fragments. Such fragments were not disulfide-linked to the remainder of the C1s molecule. The bulk of the heavy chain remained adherent to the surface of E. coli J-5. This finding documents the presence of a binding site for activated C1s on the surface of E. coli J-5 and localizes this site to the heavy chain. These studies thus indicate that several E. coli strains are direct C1 activators. Furthermore, E. coli J-5 provides another example of a direct C1 activator having binding sites not only for C1q but also for dimeric C1s. The studies also show that there are multiple properties of particles which determine the ability to activate C1, the rate of activation, the possibility of regulation of the activation process by C1 inhibitor, and the fate of activated C1.