Differentiated U937 cells and human monocytes exhibit a differential production of extracellular oxygen species: O2.- excretion versus H2O2 diffusion.

The nature and the localization of the oxidative response triggered by different stimuli in either differentiated U937 cells and peripheral blood-derived human monocytes was investigated using luminometric and cytofluorometric techniques. Differentiated U937 cells essentially produced extracellular superoxide anion (O2.-), whatever the stimulus used. Monocytes, however, responded to Salmonella typhimurium, phorbol esters, and opsonized zymosan by an intracellular, an extracellular, and both an intra- and extracellular production of oxygen species, respectively. Furthermore, H2O2 but not O2.- was detected in the extracellular oxidative response of monocytes. Using differentiated U937 cells, luminol was found to be as efficient as lucigenin in the detection of extracellular O2.-, providing sufficient concentrations of extracellular horseradish peroxidase were present. However, both azide and histidine inhibited the lucigenin-enhanced chemiluminescence, suggesting an initial and transient production of singlet oxygen by differentiated U937 cells. Taken together these results strongly suggest that, when stimulated, differentiated U937 cells directly excrete O2.- in the extracellular medium while, within monocytes, O2.- is rapidly dismutated in H2O2 which can eventually diffuse outside the cell. Such differences in the oxidative response between the two cell types could be explained by the lack of total closure of the phagosome, only observed in differentiated U937 cells.