Augmentation of natural killer cell activity by lipopolysaccharide through separable effects on the binding of nonadherent lymphocytes to tumor targets and tumor killing.

A single-cell assay was utilized to study the augmentation by Escherichia coli lipopolysaccharide (LPS) of the cytotoxicity of human lymphocytes for the human myeloid tumor K562. Preincubation with LPS at 20 micrograms/ml for 30 min at 37 degrees increased the binding of all nonadherent (NA) lymphocyte populations to K562 tumors [unseparated NA lymphocytes from 13.1 to 25.1%, immunoglobulin G Fc receptor-enriched lymphocytes from 27.6 to 42.9%, and immunoglobulin G Fc receptor-depleted lymphocytes from 14.0 to 23.7%, at p less than 0.001]. In contrast, interferon (IFN) at 10 units/ml had no effect on the overall binding of lymphocytes to K562 tumors. When lymphocyte-tumor conjugates were dispersed in agarose, cytotoxic activity of unseparated NA lymphocytes at 1 to 3 hr was markedly increased by preincubation with LPS (p less than 0.001). However, LPS did not enhance cytotoxicity if conjugates were formed in its absence. IFN, likewise, increased cytotoxic activity in unseparated NA lymphocytes at 1 to 3 hr (p less than 0.001). No synergistic cytotoxicity was seen with concurrent exposure to LPS and IFN. LPS increased cytotoxicity in the Fc receptor-enriched:tumor conjugates at 1 to 3 hr (p less than 0.001) and appeared to promote more efficient killing in individual conjugates over time. Cytotoxicity in the Fc receptor-depleted:tumor conjugates was not enhanced by LPS. Thus, LPS may enhance natural killer cell-like activity by increasing the binding of human lymphocytes to K562 tumors and by rearranging the population of binding cells to include more efficient killer cells. While the effects of LPS on binding appear independent of IFN, selective recruitment of more efficient killer cells could be through an IFN mechanism.