Intracellular activation of human and rodent macrophages by human lymphokines encapsulated in liposomes

Cell-free culture supernatant fluids rich in macrophage-activating factor (MAF) activity were obtained from mitogen-stimulated human peripheral blood mononuclear leukocytes (MNL). The MAF preparations were incubated with human peripheral blood monocytes, rat alveolar macrophages (AM), and mouse peritoneal exudate macrophages (PEM). Only human monocytes were rendered tumorilytic against the human A375 melanoma cells, whereas rat AM or mouse PEM were not activated to lyse their respective syngeneic tumor targets. In contrast, once entrapped in multilamellar liposomes, the human MAF activated the human and rodent macrophages to become tumoricidal. The MAF activity was not due to contamination with endotoxins nor to the presence of gamma interferon. These data suggest that in contrast to macrophage surface receptors, which are species specific, the intracellular target sites for human MAF may cross species barriers.