Activated Macrophages Destroy Intracellular Leishmania Major Amastigotes by an l-Arginine-Dependent Killing Mechanism

Macrophages infected with amastigotes of Leishmania major and treated with IFNg in vitro develop potent antimicrobial activities that eliminate the intracellular parasite. This antileishmanial activity was suppressed in a dose dependent fashion by NG-monomethyl-L-arginine (NGMMLA), a competitive inhibitor of nitrite, nitric oxide and L-citrulline synthesis from L-arginine. Excess L- arginine added to infected macrophage cultures reversed the inhibitory effects of NGMMLA. Addition of arginase to culture media inhibited intracellular killing by IFNg-treated cells. Similar effects were seen with macrophages obtained from BCG-infected C3H/HeN mice. Increased levels of nitrite, an oxidative product of the L-arginine dependent effector mechanism, was measured in cultures of infected IFNg-treated macrophages as well as infected BCG-activated macrophages. Nitrite production correlated with development of antileishmanial activity. Nitrite production and microbicidal activity both decreased when in vivo or in vitro-activated macrophages were cultured in the presence of either arginase or NGMMLA. Nitric oxide synthesized from a terminal guanidino nitrogen atom of L- arginine and a precursor of the nitrite measured, may disrupt Fe-dependent enzymatic pathways vital to the survival of amastigotes within macrophages. Reprints.