Effect of in vivo inhibition of nitric oxide production in murine leishmaniasis.

In vitro experiments suggest that cytokine induced nitric oxide (NO) synthesis from L-arginine is a major effector mechanism against prokaryotic and eukaryotic intracellular pathogens. N omega monomethyl L-arginine (MLA), an active site inhibitor of the cytokine induced NO synthase, inhibits cytokine induced resistance of mammalian cells to intracellular microbes in vitro. In our experiments, we show that Leishmania infection markedly increases NO synthesis in the genetically resistant C3H/HeN mouse strain. In addition, administration of 50 mM MLA in the drinking water inhibits endogenous NO synthesis as well as natural resistance to footpad Leishmania infections in both susceptible BALB/c and resistant C3H/HeN mice. Leishmania parasites continued to proliferate in MLA-treated C3H/HeN mice after footpad inoculation. Similarly C3H/HeN mice treated for 3 wk with MLA had an increased parasite load and sloughing of the footpad. Footpad size of C3H/HeN mice not treated with MLA returned to base-line diameter and the regional nodes contained few amastigotes. These in vivo effects were paralleled by endogenous NO synthesis (high when Leishmania was controlled and low when Leishmania was not controlled). In addition, inhibition of NO synthesis in Leishmania-infected mice leads to a cachectic state not caused by infection alone or inhibition of NO synthesis alone. The cachexia appeared to be due to decreased food intake that occurs when NO synthesis is inhibited in Leishmania-infected mice. Our results show that cytokine induced NO has a major effector role in resistance of murine hosts to Leishmania infection.