Microbicidal activity of eosinophils is associated with activation of the arginine-NO pathway

In order to investigate the ability of rat peritoneal eosinophils to produce nitric oxide (NO) induced by cytokines in vitro, these cells were activated with several cytokines (IL-5, IL-8, Rantes, TNF-alpha, IFN-gamma) in association or not with LPS. Under these conditions, we were able to detect nitrite in the incubation medium when the eosinophils were stimulated with IFN-gamma or IL-8 in the presence of LPS. LPS alone also induced nitrite production. Significant levels of nitrite in the medium were already present after 12 h of stimulation and increased steadily within the next 48 h. Regarding NO synthase, its highest activity was achieved at 12 h after IFN-gamma/LPS stimulation. After this peak, the enzymatic activity reduced gradually to control levels 48 h after the stimulation. The simultaneous addition of the NO synthase inhibitor L-NIO (100 microM) to the eosinophil suspension blocked nitrite production and NO synthase activity. On the other hand, neither IL-5, Rantes nor TNF-alpha were able to induce the release of nitrite in the presence or absence of LPS. To evaluate the microbicidal effect of these cells against the Leishmania parasite, eosinophils were infected with Leishmania major. It was observed that these cells were able to produce nitrite and to kill the parasite after activation with LPS/IFN-gamma. Moreover, L-NIO blocked this leishmanicidal activity and the nitrite production. Our results suggest that activated eosinophils release NO which is involved in their microbicidal activity against Leishmania major.