In previous studies of the role of IgG and C3b in opsonization, C3b has not been present on the target particle in the complete absence of other serum proteins. We have developed a system in which C3b alone is present on the particle, using trypsin cleavage of purified C3. Sheep erythrocytes (E) coated with trypsin-generated C3b (ETC3b) efficiently bound to but were not ingested by human neutrophils. Specificity of binding was demonstrated by inhibition of rosette formation with fluid phase C3 or C3b, and by a decrease in rosettes after treatment of ETC3b with C3b inactivator (C3bINA). Addition of IgG anti-E but not IgM anti-E to ETC3b markedly enhanced neutrophil binding and stimulated ingestion of these erythrocytes. There were minimal binding and ingestion of E coated with IgG alone.

Binding of ETC3b to neutrophils was also insufficient to stimulate the release of superoxide anion (O2-), β-glucuronidase, and lysozyme. EIgG, prepared with increasing concentrations of IgG, promoted the release of O2- and granule enzymes in a concentration-dependent fashion, and ingestion was not required. Addition to ETC3b of a small quantity of IgG synergistically enhanced O2- release and degranulation.

These results suggest that the Fc and C3b receptors on neutrophils have separate but complementary functions. C3b serves to promote efficient particle-cell attachment, thereby mediating contact between particle-bound IgG and the neutrophil's Fc receptor. This latter contact triggers ingestion and the intracellular events involved in microbial killing.

1

This work was supported by United States Public Health Service Grants AI 14148 and CA 16673. S. L. N. was supported by National Institutes of Health Training Grant GM 07164.

2

This paper was presented in part at the annual meeting of the American Association of Immunologists, Atlanta, Georgia, June 5, 1978.

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