Cytolytic activity of circulating human monocytes on transformed and untransformed human fibroblasts

Monocytes from the peripheral blood of 10 normal adult human donors were tested for their cytolytic activity on untransformed or SV40-transformed, early passage, human fibroblasts. Two pairs of human fibroblast lines (2303 and SV40-2303, 2931 and SV40-2931) were used and lysis was assessed in terms of release of [3H]thymidine from prelabelled target cells over a 72-h incubation period. High levels of cytolytic activity were consistently observed on transformed fibroblasts at attacker to target cell (A:T) ratios ranging from 5:1 to 40:1. Low, but significant, lysis of untransformed cells was observed only with two donors at the highest (40:1) A:T ratio tested. With all donors, the cytolytic activity on transformed lines was significantly greater than on untransformed fibroblasts. Thus natural human monocyte-mediated cytotoxicity is preferentially expressed on transformed, as compared to untransformed, cells although sparing of the latter is not absolute.