Opsonization of clinical isolates of B. fragilis and B. thetaiotaomicron with the six isolated proteins of the alternative complement pathway under physiological conditions resulted in considerable C3 deposition on the bacterial surfaces. The time course of C3 deposition was similar to that observed in EGTA-serum; however, the magnitude of C3 deposition was twofold greater in EGTA-serum. Opsonization of the bacteria with the isolated alternative pathway proteins failed to promote adherence, uptake, or killing by polymorphonuclear leukocytes, whereas opsonization of the bacteria with EGTA-serum facilitated these events. The difference in opsonic capacity of isolated proteins and EGTA-serum was not related to the quantitative difference in C3 deposition, because repeated opsonization of the bacteria with isolated proteins resulting in C3 deposition comparable to that observed in EGTA-serum only minimally increased adherence of the bacteria to polymorphonuclear leukocytes. SDS-PAGE and autoradiographic analysis of C3 extracted from bacteria opsonized with isolated proteins or EGTA-serum using methylamine and SDS demonstrated that the predominant form of C3 bound by ester bonds under both sets of conditions was iC3b. A low molecular weight C3 cleavage fragment was detected in extracts from bacteria opsonized with isolated proteins, but it accounted for only a minor fraction of the bound C3. The results of our study demonstrate that the early phase of opsonization involving activation of the alternative pathway by B. fragilis and B. thetaiotaomicron and resultant C3 deposition on the bacterial surfaces does not require auxiliary serum factors, but the effector phase of opsonization of these bacteria involving recognition of bacteria-bound C3 by polymorphonuclear leukocytes and the induction of phagocytosis and intracellular killing is dependent on such factors. Natural IgM antibodies serve as auxiliary factors is opsonization of B. thetaiotaomicron by the alternative pathway, whereas additional serum factors are required for alternative pathway-mediated opsonization of B. fragilis.

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