Phagocytosis by human monocytes of unopsonized particulate activators of the human alternative complement pathway: Induction by cytokine

Monocytes isolated from human blood by centrifugal elutriation exhibited little ability to ingest rabit erythrocytes (E(R)), zymosan particles, or desialated sheep erythrocytes. In contrast, 85 to 95% of these cells rosetted with C3b- or C3bi-bearing sheep erythrocytes (E(S)) or ingested IgG-coated E(S). Preincubation of the monocytes with human lymphocytes increased their ability to ingest E(R). The E(R) phagocytosis-inducing activity was contained in the 105,000 x G supernatant of lymphocyte lysates. These supernatants increased the percentage of ingesting monocytes from 5 to 15% to 80% within 60 min. The soluble factor was found to be relatively heat stable, inactivated by trypsin, and distinct from IFN-γ. Its m.w. is less than 13,000. It was present in B and T lymphocytes and also in U937 cells. These results suggest that the ability of human monocytes to ingest nonopsonized particulate activators of the alternative complement pathway is a cytokine-inducible property and that the effect of the cytokine on complement receptor- or Fc receptor-dependent adherence or ingestion of opsonized particles is minor.