Receptors for complement on echinoid phagocytes. II. Purified human complement mediates echinoid phagocytosis

The ingestion of sheep erythrocytes (E) coated with specific IgM antibody (EA) and various purified human complement components (EAC 1, EAC 14, EAC 142, EAC 1423b, EAC 43b) by echinoid phagocytes was examined. E, EA, EAC 1, EAC 14 and EAC 142 were ingested at a similar and very low level, while a strong opsonic effect was found by coating red cells with purified C 3 (EAC 1423b and EAC 43b). When erythrocyte-bound C 3b was converted to the hemolytically inactive C 3bi, the opsonic effect was markedly increased. Conversion of C 3bi to C 3d resulted in a total loss of opsonic activity. A similar response pattern was found when binding of erythrocytes to mouse macrophages was recorded. Since it cannot be ruled out that phagocytes convert C 3b to C 3bi by endogeneous C 3 inactivator, it is concluded that echinoid phagocytes display CR3 (C 3bi) and possibly also CR1 (C 3b) receptors, while CR2 (C 3d) receptors are lacking.