Pre-ligation of CR1 enhances IgG-dependent phagocytosis by cultured human monocytes

Opsonization of the C3b receptor (CR1) on phagocytic cells with C3b enhances both attachment of targets to the cells and subsequent IgG-dependent ingestion of these targets. To explore mechanisms involved in this increased phagocytosis, we adhered cultured human monocytes to surfaces pre-coated with CR1 ligand or control proteins and quantitated ingestion of sheep E opsonized with IgG alone. Three ligands for CR1 resulted in markedly enhanced phagocytosis of targets when compared individually to a panel of non-ligands, as determined by both the proportion of monocytes ingesting targets (percent phagocytosis) and by the number of targets ingested per 100 monocytes (phagocytic index). The ligands included purified C3b, iC3, and Fab fragments of 1B4, a monoclonal anti-CR1, which resulted in a percent phagocytosis of 56.3 (p less than 0.01), 59.0 (p less than 0.01), and 54.4 (p less than 0.02) and a phagocytic index of 281.2 (p less than 0.01), 281.1 (p less than 0.01), and 247.1 (p less than 0.02), respectively. Control proteins including human serum albumin, hemoglobin, Fab fragments of anti-fibronectin, anti-beta 2 microglobulin, and MOPC 21, and Fc fragments of 1B4 and MOPC 21 produced no significant stimulation of phagocytosis, nor did F(ab')2 fragments of monoclonal anti-CR3, M1/70. CR1-specific augmentation of target ingestion was apparent with monocytes cultured in serum-free medium for 1 to 7 days, but was not seen with freshly elutriated cells. Phagocytosis of unopsonized or IgM-coated targets was minimal. These results suggest that the adherent monocytes are primed by CR1 cross-linking for enhanced FcR-mediated phagocytosis even when the CR1 ligand is not present on the targets. This contrasts with the behavior of CR3, and demonstrated functional divergence between these C3 fragment receptors in the phagocytic process.