Identification of the serum factor required for liposome-primed activation of mouse peritoneal macrophages. Modified alpha 2-macroglobulin enhances Fc gamma receptor-mediated phagocytosis of opsonized sheep red blood cells.

Culture supernatant, prepared by incubating fetal calf serum and liposome-treated B cells, augments mouse peritoneal macrophage Fc gamma receptor-mediated phagocytosis of IgG-opsonized sheep red blood cells. The activation process was hypothesized to be as follows. B-cell membranous glycosidases stimulated by liposomes, convert serum factor to a macrophage-stimulating active serum factor. As the active serum factor loses its activation potential by the addition of mannose or by digestion with alpha-mannosidase, mannose residues at the terminal present in the active serum factor are hypothesized to contribute to macrophage activation. The active serum factor was purified on a concanavalin A-Sepharose 4B column, and identified as a modified form of alpha 2-macroglobulin by immunochemical analysises. On non-denaturing polyacrylamide gel electrophoresis, modified alpha 2-macroglobulin showed a slow form, while alpha 2-macroglobulin-trypsin and alpha 2-macroglobulin-methylamine complexes, which bind specifically to alpha 2-macroglobulin receptors, showed a fast form and did not activate macropahges. These findings demonstrate that alpha 2-macroglobulin is the essential serum factor in liposome-primed macrophage activation, and that modified alpha 2-macroglobulin with mannose residues at the terminal sugar chain binds to macrophage mannose receptors, but not alpha 2-macroglobulin receptors, and increases Fc gamma receptor-mediated phagocytosis of IgG-opsonized sheep red blood cells.