Spontaneous cytotoxicity by monocyte-enriched subpopulations of human peripheral blood mononuclear cells against human or mouse anchorage-dependent tumour cell lines. Contribution of NK-like cells.

Human peripheral blood mononuclear cells (PBMNC) were found to be cytotoxic for mouse or human anchorage-dependent target cell lines in a 48-72 h [125I]iododeoxyuridine (IUDR) release assay. Unfractionated, adherent or nonadherent cells had significant levels of cytotoxicity, as did cells fractionated according to size into 'lymphocytes' or 'monocytes' by elutriation. Intermediate size cells, not enriched for monocytes, had high levels of cytotoxicity. In all fractions tested, including adherent populations, some cells with the morphology of large granular cells were observed. Treatment of all fractions with interferon (IFLrA, a purified, recombinant alpha-IFN) boosted cytotoxicity against four target cells lines. Treatment with lymphokines containing putative 'macrophage-activating factor' (MAF) also enhanced cytotoxicity in fractions depleted of monocytes. Culture in fetal bovine serum enhanced cytotoxicity mainly in unfractionated and nonadherent PBMNC. These experiments indicated that NK-like cells can be appreciable contaminants in elutriator-purified monocyte-enriched or adherent cell populations and thereby contribute to observed cytotoxicity, particularly after pretreatment with IFN or other stimulatory factors.