Macrophage activation: Synergism between hybridoma MAF and poly(I). Poly(C) delivered by liposomes

High concentrations of a murine T cell hybridoma culture supernatant containing macrophage-activating factor (MAF) rendered resident mouse peritoneal macrophages cytotoxic for P815 mastocytoma cells. The capacity of the hybridoma-derived MAF [MAF(H)] to induce tumoricidal activity increased 10³ to 10⁴-fold when the lymphokine was excapsulated into liposomes. Combinations of MAF(H) and poly(I) · poly(C) acted synergistically to render macrophages potency cytotoxic. Subthreshold (nonactivating) concentrations of free or liposome-encapsulated MAF(H) increased the potency of free poly(I) · poly(C) and liposome encapsulated poly(I) · poly(C). Either as free agent or encapsulated in liposomes, single-stranded poly(I) or poly(C) did not activate macrophages in the presence or absence of MAF(H). Double-stranded poly(I) · poly(C) was thus required for macrophage activation and synergism with MAF(H).