Suppression of the activity of natural killer-like cells by peritoneal macrophages obtained from Lewis lung tumor-bearing, Propionibacterium granulosum KP-45-treated, or normal, untreated mice.

Peritoneal adherent cells (PAC) obtained from Propionibacterium granulosum KP-45-treated or Lewis lung carcinoma-bearing BDF1 mice suppressed in vitro the NK-like cytotoxic activity of murine splenocytes against YAC-1 tumor target cells. Maximum inhibition occurred when suppressor and effector cells were preincubated together for 18 h, but the effect was demonstrable also when the two groups of cells were mixed only at the onset of the 4-h cytotoxic assay (i.e. without previous contact). Inhibitory cells appeared to be mostly macrophages, as judged by adherence to plastic and morphologic features, and as little as 5 to 20% of PAC, relative to the total number of co-incubated cells, were required for the clear demonstration of the effect. In addition to activated also normal, resident PAC obtained from untreated animals inhibited the NK cell-mediated cytotoxicity, but the effect was significantly pronounced only when 20% of suppressor cells were incubated overnight with effector splenocytes. The results favor the hypothesis that both functionally activated as well as resting macrophages operate as important regulators of the activity of NK cells in vivo.