Monoclonal antibody defines a macrophage intracellular Ca ²⁺-binding protein which is phosphorylated by phagocytosis

The macrophage is a highly phagocytic cell which has been widely used in attempts to dissect the mechanisms of endocytosis¹. In immune phagocytosis specific interactions occur between ligands on the presenting particle and Fc or C3b specific receptors expressed on the plasma membrane ^2,3, but macrophages also avidly ingest particles such as polystyrene latex by an immunologically nonspecific mechanism. This process is guided by sequential ligand-receptor interactions^4,5. Biochemical studies indicate that actin, myosin and several regulatory components distinct from the troponin system of skeletal muscle, take part in phagocytosis by macrophages and polymorphonuclear leukocytes (PMN)⁶ and indirect immunofluorescence confirms the presence of these components at sites of phagocytic activity ⁷. However, comparatively little is known about coupling between receptor binding and assembly of the contractile system, although electrical changes⁸ and the flux of divalent cations⁹ have been implicated. Here we report a monoclonal antibody that detects an intracellular protein from macrophages which binds Ca²⁺ in non-phagocytosing cells, and becomes phosphorylated as a result of phagocytosis. It might therefore take part in signal transduction during this process.