COLLAGEN-LIKE COMPLEMENT COMPONENT C1Q IS A MEMBRANE-PROTEIN OF HUMANMONOCYTE-DERIVED MACROPHAGES THAT MEDIATES ENDOCYTOSIS

The collagen-like Clq molecule, a subcomponent of the first component of complement, C1, is synthesized by macrophages (M phi). Previously, we have demonstrated that Clq is a membrane protein of guinea pig peri toneal macrophages (M phi). To extend this observation as a general bi ologic characteristic of M phi, we investigated human (hu) monocyte-de rived M phi. Interestingly, surface labeling with the biotin derivativ e idyl-6-(biotinamido)-hexanoate(biotinamido)-hexano of M phi, freshly isolated monocytes, lymphocytes, granulocytes, and myelomonocytic U93 7 cells revealed that Clq occurs only on the surface of M phi and not on the surface of the other cells types. Therefore, Clq appears to be a marker for differentiation into M phi. FITC-labeled, fixed Staphyloc occus aureus coupled to membrane C1q via a monoclonal alpha-hu-Clq Ab were used to demonstrate that membrane Clq is capable of mediating pha gocytosis. Various detergents (Nonidet P-40, digitonin, lubrol, and Tr iton X-114) were used to solubilize membrane Clq. Membrane Clq of hu M phi is tightly bound to or located in the intact membrane, since trea tment of cells with acidic buffers (''acid strip'') failed to remove C lq from the cell surface. However, repeated freezing and thawing of ce lls and washing of segregated membranes with buffer containing 1 M KCl and 3 M urea brought about a marked release of membrane C1q.