Antibody-independent interactions between Escherichia coli J5 and human complement components.

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Abstract
With the characterization of an increasing number of molecules that are capable of activating the 1st component of the classical pathway of complement (C), the possibility that some Gram-negative bacteria may activate C1 independent of naturally occurring antibody has been reexamined. We have confirmed a previous report that purified C1 (the activated form of C1) can bind to certain strains of bacteria and it retains its enzymatic activity when thus bound. The availability of purified C1 in its precursor form has allowed us to extend these observations to the native C1 molecule. Using a semirough mutant of Escherichia coli, the galactose epimerase-deficient strain E. coli J5, we have examined the binding and activation of radiolabeled C1. J5 bound radiolabeled C1 in a dose-dependent manner and essentially all of the bound C1 was activated as judged by SDS-PAGE. The bacteria-C1 complex consumed purified C4 and C2 and the consumption of C2 was proportional to the C4 concentration. Subsequent addition of terminal C components C3-9 supplied as serum-EDTA caused a highly significant decrease in bacterial viability. These results demonstrate that C1 may bind to the bacterial membrane in such a manner as to initiate a bactericidal reaction. Therefore, antibody-independent binding and activation of C1 must be considered in the assessment of serum sensitivity of Gram-negative bacteria.