The effects of a purified bacterial lipopolysaccharide endotoxin on homogenous populations of rabbit polymorphonuclear leucocytes have been studied in vitro under defined conditions. Employing a 500-fold range of concentration (0.1 to 50.0 µg./ml.), it was shown that endotoxin enhanced the rate at which staphylococci were killed by leucocytes. The mechanism underlying the increased killing was found to be a direct stimulation of the phagocytic activity of the leucocyte and not mediated by the release of bactericidins or opsonins from the treated cells. In the presence of 10 per cent serum all concentrations of endotoxin enhanced phagocytosis, whereas at lower serum concentrations, the higher doses of lipopolysaccharide inhibited the phagocytic activity of the cells.

Similar concentrations of endotoxin were capable of increasing the utilization of glucose and the production of lactic acid. Endotoxin treated leucocytes exhibited no change in oxygen consumption, and only a slight depression in glycogen synthesis.

It appeared that endotoxin could interact and alter the functional and metabolic properties of leucocytes in the absence of serum.

The demonstration of enhanced phagocytic activity of endotoxin-treated cells was dependent upon the particular opsonic requirements for the organism under study.

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