Functional discrimination by human monocytes between their C3b receptors and their recognition units for particulate activators of the alternative complement pathway.

A trypsin-sensitive recognition unit for ingestion of particulate activators of the human alternative complement pathway is present on human monocytes and persists during 48 hr of culture in synthetic medium. Further, after inactivation by trypsin, the recognition unit for activators of the alternative pathway is fully regenerated in terms of function during the 48 hr of culture. In contrast, the C3b receptor on human monocytes is relatively trypsin resistant and is lost during 48 hr of culture, as assessed by its capacity to mediate C3b-dependent immune adherence or enhanced phagocytosis. The prior immune adherence of the monocytes to C3b-bearing particles does not alter the capacity of the monocytes to ingest a particulate activator, and the ingestion of particulate activators does not induce phagocytosis of the rosetted C3b-bearing particles. Thus, the functions linked to the recognition of C3b and of particulate activators of the alternative pathway are expressed independently by monocytes when the determinants reside on separate particles. When the determinants are expressed by the same particle, such as an activator bearing C3b, the monocytes exhibit a synergistic response through their distinct recognition units for C3b and particulate activators of the alternative pathway.