Phagocytosis of target particles bearing C3b-IgG covalent complexes by human monocytes and polymorphonuclear leucocytes.

Immunoglobulin G (IgG) provides an efficient acceptor site for nascent C3b, and complement activation on the surface of IgG-coated bacteria has been shown to generate significant numbers of C3b-IgG complexes. We have studied the relative efficiency of IgG alone, C3b-IgG complexes, and similar densities of IgG and C3b residues deposited independently, in mediating ingestion of sheep erythrocyte (E) targets by human phagocytes. Human ¹²⁵I-C3b covalently bound to rabbit anti-Forssman IgG was generated as described elsewhere (Fries et al., 1985). E, EIgMC4b, or EIgMC4b3b (prepared with IgM antibody and purified complement components) were sensitized with radiolabelled anti-Forssman IgG or C3b-IgG heterodimers to generate targets beating IgG alone, C3b-IgG covalent complexes, or C3b and IgG in equivalent numbers but not bound to each other. Phagocytosis by monocytes and polymorphonuclear leucocytes (PMN) of targets bearing C3b-IgG was markedly enhanced relative to those bearing IgG alone, especially at levels of < 2000 opsonin residues/target cell. Uptake of C3b-IgG-hearing targets was also significantly more resistant to competitive inhibition by ambient monomeric IgG. Phagocytosis of EIgMCAb+C3b-IgG by monocytes was superior to the uptake of either EAC4b + IgG or EAC4b3b + IgG bearing equivalent amounts of C3b and IgG not in covalent complex (P < 0·05, n = 10). Similar results were obtained with PMN Thus, generation of C3b-IgG complexes in vivo may not only promote complement activation and enhance C3b deposition, but also produce a compound opsonic residue which is a more potent promoter of phagocytosis than an equal number of C3b and IgG residues randomly distributed relative to each other. [ABSTRACT FROM AUTHOR]