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In vitro effect of bacterial lipopolysaccharide on the cytotoxicity of human natural killer cells.

Preincubation with a number of mediators of infection, such as Gram negative bacteria (S. typhi), bacterial lipopolysaccharide (LPS), tumor necrotic factor-alpha (TNF-alpha), and interleukin-2 (IL-2), significantly increases natural killer (NK) cell activity in samples of human peripheral blood mononuclear cells (PBMC), without changing the levels of either the phenotypic CD16/56 or stimulatory CD25 marker. We now report similar results after preincubation of highly purified NK cell preparations (CD16 + 56 > 95%; the rest corresponding to CD3+ T-cells) with either S. typhi, TNF-alpha or IL-2. However, in similar experiments, LPS inhibits, in a dose-dependent manner (final conc. 2.5, 5.0 or 10.0 micrograms/mL), NK cell cytotoxicity against K-562 tumor cells. Preincubation of purified NK cells with LPS (25 micrograms/mL; 10 and 30 min) produced significant alterations in the tyrosine phosphorylation/dephosphorylation pattern of several intracellular proteins, including a significant increase (10 min) in the phosphorylation of the 120; 100; 72 and 59 kDa proteins, followed (30 min) by the essentially complete desphosphorylation of the p59 protein. Qualitatively similar results were obtained at lower LPS concentrations e.g., range 2.5 to 20 micrograms/mL. The absence of phosphoproteins in the 40-44 kDa range, known to be present after incubation of monocytes with LPS, raises the possibility that these "class" of proteins may be critical in explaining the LPS inhibitory effect on NK lytic function. Our finding may contribute to a better understanding of the mechanisms involved in the complex in vivo interaction between LPS, monocytes and NK cells.