Potentiation of the cytolytic activity of peripheral blood monocytes by lymphokines and interferon.

Human peripheral blood monocytes obtained by EDTA-reversible adherence to plastic surfaces precoated with autologous serum can rapidly lyse a variety of tumor cells. That the effector cells in this system are indeed monocytes has been demonstrated (1). Using a short-term (3 to 4 hr) 51Cr-release assay and the single cell conjugate cytotoxic assay, we studied the effects of lymphokine-rich supernatants containing gamma-interferon and partially purified fibroblast interferon on the monocyte cytolytic activity. Overnight incubation of the monocytes in fetal bovine serum-containing medium resulted in a relatively small decrease in cytotoxic activity compared to the one obtained with monocytes incubated in autologous serum. The addition of lymphokines or interferon under both incubation conditions resulted in augmented activity as measured in the 51Cr-release assay. However, the proportions of binding and cytotoxic monocytes, determined in the single cell conjugate assay, did not increase. These results suggest that augmented activity is not due to recruitment of inactive cells. Kinetics studies of tumor cell lysis indicate the increase in killing efficiency is probably due to both an increase in the rate of killing and in the recycling ability of the cytotoxic cells. Using the conjugate/agarose technique, we also demonstrated that excess tumor cells could impair the lytic machinery of freshly isolated monocytes, whereas monocytes treated with lymphokines or interferon partially lost their sensitivity to this inhibitory effect. The ability of tumor cells to impair the lytic machinery of monocytes could be one of the mechanisms by which tumors escape immunosurveillance.