Receptors for complement on echinoid phagocytes. I. The opsonic effect of vertebrae sera on echinoid phagocytosis

The ingestion of sheep erythrocytes (SRBC) by echinoid phagocytes was greatly increased after treatment of SRBC with sera from mouse, human and fish. The opsonic principle in the sera was heat labile (56 degrees C 1/2 hr). Opsonization with mammalian sera was studied more extensively. It was dependent on pre-sensitization of SRBC with specific antibody (IgM), required Ca2+, and was inhibited by low temperature (4 degrees C). The finding that serum depleted of C 3 only opsonized very weakly strongly indicates that the opsonic principles is the complement cascade C 1-4-2-3 activated via the classical pathway, and that the opsonic effect largely coincides with the coating of SRBC with C 3b. A role of C 5 or later components was excluded, since the mouse AKR serum was C 5 deficient, and the human serum was normally treated with zymosan. When variations known to impair the opsonic function of C 3 was introduced in the opsonization procedure, a parallel inhibition was recorded on attachment of SRBC to mouse peritoneal macrophages and on ingestion of SRBC by echinoid cells. Thus C 3 receptors are probably present on echinoid phagocytes.