Immunoglobulin-independent utilization of the classical complement pathway in opsonophagocytosis of Escherichia coli by human peripheral leukocytes.

Participation of immunoglobulin (Ig) G in normal human serum in formation of the classical complement (C) pathway C3 convertase on the surfaces of two clinical isolates of Escherichia coli, (P) and (A), was investigated. Bacteria were treated with purified normal human IgG or buffer and then sequentially interacted with functionally purified human C1, C4, and C2 lacking detectable Ig. The bacteria were incubated with purified C3 trace-labeled with 125I, and surface deposition of C3b was quantitated. C3b deposition on E. coli (P) was demonstrated after interaction with C1, C4, C2, and C3 with or without pretreatment of the bacterium with IgG. The number of surface-bound C3b molecules and the kinetics of C3b deposition after assembly of the C3 convertase with and without IgG were equivalent. Interaction of E. coli (A) with C1, C4, C2, and C3 without pretreatment of the bacterium with IgG resulted in minimal deposition of C3b. IgG-dependent convertase formation with resultant C3b deposition on E. coli (A) was comparable to that on E. coli (P). Uptake and intracellular killing by human peripheral leukocytes of 3H-labeled bacteria treated with combinations of IgG, C1, C4, C2, and nonlabeled C3 were also determined. The level and kinetics of uptake and kitting of E. coli (P) treated with IgG and/or C1, C4, C2, and C3 were similar. Uptake and killing of E. coli (A) treated with C1, C4, C2, and C3 were demonstrated only when the bacterium was pretreated with IgG. Uptake and killing of both bacterial strains treated with IgG or combinations of proteins lacking C3 were negligible. Ingestion of E. coli (P) treated with C1, C4, C2, and C3 by polymorphonuclear neutrophils was confirmed by electron microscopy. Binding of purified human C1 q to the bacterial surfaces was assessed indirectly by the C1q deviation test. E. coli (P) bound C1q without pretreatment with IgG, whereas pretreatment of E. coli (A) with IgG was required for binding of C1q. Pretreatment of E. coli (P) with IgG augmented binding of C1q. Experiments that measured binding of 125I-labeled IgG to the bacterial surfaces demonstrated directly that IgG bound to both E. coli (P) and (A). The data presented in this investigation document the role of the classical C pathway in facilitating opsonophagocytosis and intracellular killing of E. coli by human peripheral leukocytes and suggest that utilization of the classical pathway in these events can occur in the absence of Ig.