Lymphokine-stimulated macrophage phagocytosis of fluorescent microspheres: a rapid new assay

A highly sensitive and rapid in vitro macrophage phagocytosis assay is described for screening of lymphokine preparations with macrophage activating properties. Non-induced mouse peritoneal macrophages were incubated on glass slides for 60 min in the presence of fluorescent 2 micron latex beads and partially purified lymphokine fractions or media control. Lymphokine samples were prepared from culture supernatants of the B lymphoblastoid cell line RPMI 1788 by high performance liquid chromatography of a soluble trichloroacetic acid extract of concentrated culture supernatant. Phagocytosis was measured by direct counts of the percent phagocytic cells and the number of intracellular beads per 100 cells as seen by fluorescence microscopy. Phagocytic uptake in the presence of as little as 0.1 ng of active lymphokine could readily be determined qualitatively by correlation plots and quantitatively by appropriate statistical programs for data processing by computer. This technique provides a rapid, reproducible, screening assay for biological activity of macrophage activating lymphokines and other compounds affecting macrophage phagocytosis.