Murine splenic macrophage tumoricidal activation by cytokines

Interleukin-2 (IL-2), IL-1 beta, interferon-gamma (IFN-gamma), and tumor necrosis factor-alpha (TNF-alpha), each alone and in all possible combinations, were studied for their capacity to activate murine resident splenic macrophages to a tumoricidal state. Two approaches were used in these studies. The first approach was to add cytokine directly to the adherent macrophages that had been washed free of nonadherent spleen cells. The only agent effective alone was IL-2, inducing significant tumoricidal activity in macrophages obtained after culturing whole spleen cell suspensions for 4, but not 1 to 3, days. Nonadherent splenic populations were required during this 4-day macrophage "culture conditioning." Only combinations of cytokines containing IL-2 were effective, but none more than IL-2 alone. The second approach was to add cytokine to the whole spleen cell suspensions for an activation period before isolation of adherent macrophages. Again, the only agent effective alone was IL-2. Macrophage tumoricidal activity was highest when IL-2 was added to the whole spleen cell suspensions at the initiation of the 4-day activation culture. In addition, TNF-alpha, but none of the other cytokines, significantly augmented the IL-2-induced effect. The tumoricidal activity was not a consequence of lipopolysaccharide contamination or of lymphokine-activated killer cells. Based on the utilization of neutralizing antibodies, IL-1 alpha, IL-1 beta, and IFN-gamma were not involved as soluble mediators during the activation of tumoricidal splenic macrophages by IL-2 with or without TNF-alpha.