Effect of lipopolysaccharide on intracellular killing of Leishmania enriettii and correlation with macrophage oxidative metabolism.

The effect of lipopolysaccharide (LPS) on the lymphokine (LK)-dependent activation of murine peritoneal macrophages for intracellular killing of Leishmania enriettii parasites was investigated. Exposure to LPS alone did not induce macrophages to kill the parasite. In the presence of LK or recombinant interferon-gamma, however, which by themselves rendered the macrophages only weakly cytotoxic, considerable stimulation of intracellular parasite killing was achieved already at a LPS concentration of 1 ng/ml. The response to LPS was of the same magnitude in macrophages tested for intracellular killing as in parallel assays of extracellular cytolysis of target cells. Acquisition of leishmanicidal activity by macrophages exposed to LK and LPS correlated with stimulation of the respiratory burst, as shown by increased hexose monophosphate shunt levels, and priming for elevated chemiluminescence and O2- and H2O2 production. Polymyxin B blocked both this LPS-dependent metabolic activity and intracellular parasite destruction. Intracellular killing was, however, not solely dependent on oxidative metabolism of macrophages since in the absence of LK, LPS stimulated respiratory burst activity, yet no intracellular killing was observed, and triggering of the respiratory burst by phorbol myristate acetate or zymosan did not affect intracellular parasite survival. These results suggest that, in this experimental model, efficient intracellular parasite killing depends both on increased production of oxygen metabolites and on the availability of so far unidentified factor(s), the synthesis of which requires exposure of macrophages to both LK and LPS.