Acquired spontaneous cytotoxicity of K562 in a subpopulation of monocytes

Human peripheral blood monocytes were placed on a discontinuous density gradient of bovine serum albumin and fractionated into five subpopulations. Cells from each subpopulation were assayed for spontaneous cytotoxic activity against K562 tumor cells. Immediately following fractionation, monocytes were not cytotoxic. Following incubation for at least 48 hr, monocytes from two layers of the gradients clearly exhibited greater spontaneous cytotoxic activity than all others. The degree of cytotoxicity expressed by cells of these layers was enhanced by the addition of indomethacin and inhibited by prostaglandin E2 (PGE2). Monocytes acquiring spontaneous cytotoxicity did not secrete measurable levels of PGE2 and had increased levels of purine nucleoside phosphorylase after 72 hs of culture in vitro. Surface markers HNK-1 and Mac-1 normally associated with cytotoxic function, were detected on these cells by indirect immunofluorescence at isolation and after culture. The fraction with the greatest cytotoxic activity showed an increase in the proportion of cells displaying reactivity to HNK-1 after culture compared to initial isolation.