Cell interactions in the initial contact between cultured melanoma cells and syngeneic macrophages

Thioglycolate-induced peritoneal macrophages from melanoma-bearing mice (immune macrophages) or from control mice (control macrophages) were cultured with syngeneic melanoma cells (P51) to determine the surface characteristics of the effector cells during interaction and destruction of the target cells. After a short culture period (3 hours), immune macrophages had extensive connections via filopodia and ruffled membranes to the surfaces of the melanoma cells; control macrophages did not exhibit the same behavior. A dense region in the cytoplasm immediately beneath the macrophage plasmalemma was observed at the point of contact with the target tumor cell. With longer periods of culture (24 hours), effector cells began phagocytosis of the target cells; immune macrophages, however, had more fine filopodial connections and were more cytostatic than were controls. These observations indicate that one of the initial mechanisms of tumor cell destruction was contact-induced lysis, with phagocytosis playing a minor part.