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Cultured human monocytes have been shown to be susceptible to lysis by autologous lymphokine-activated killer (LAK) cells. To determine factors that might modulate the sensitivity of monocytes to lysis, we cultured adherent peripheral blood leukocytes (PBL) in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) or interleukin-3 (IL-3) since these cytokines have been reported to affect both functional and physical characteristics of monocytes. Both recombinant human GM-CSF and IL-3 were found to significantly enhance the susceptibility of monocytes to lysis by LAK cells in a dose- dependent manner, with GM-CSF being slightly more effective. In a kinetics study, the lysability of monocytes increased after two days of incubation with either cytokine, with maximal susceptibility occurring after four to six days of culture. The effects of GM-CSF and IL-3 appeared to be specific for monocytes since culture of either nonadherent cells or granulocytes, which are normally resistant to LAK- mediated lysis, did not induce sensitivity. While the effects of GM-CSF and IL-3 have been shown to be synergistic in some cases, they did not act synergistically to induce monocyte susceptibility to LAK lysis. In cold target experiments cytokine-treated monocytes reciprocally blocked lysis, suggesting that similar target structures were modulated with either factor. FACS analysis and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated comparable modulation of surface antigens with either GM-CSF or IL-3. Thus, these cytokines can serve to augment susceptibility of monocytes to LAK cells, emphasizing the complex interactions that occur in the immune system.