CYTOKINE INTERACTIONS IN EXPERIMENTAL CUTANEOUS LEISHMANIASIS .2. ENDOGENOUS TUMOR-NECROSIS-FACTOR-ALPHA PRODUCTION BY MACROPHAGES IS INDUCED BY THE SYNERGISTIC ACTION OF INTERFERON (IFN)-GAMMA AND INTERLEUKIN (IL)-4 AND ACCOUNTS FOR THE ANTIPARASITIC EFFECT MEDIATED BY IFN-GAMMA AND IL-4

Tumor necrosis factor-alpha (TNF-alpha) strongly activates murine peritoneal macrophages (M-PHI) for killing of amastigotes from Leishmania major in the presence of low amounts of interferon-gamma (IFN-gamma). Recently, we found that IFN-gamma and interleukin 4 (IL 4) also synergistically enhance the antileishmanial potential of M-PHI. In this report, evidence is provided that the synergism of IFN-gamma and IL 4 is based on the ability of the lymphokines to induce the endogenous production of TNF-alpha. First, both IFN-gamma and IL 4 as single agents and in combination were potent inducers of TNF-alpha production by M-PHI infected with L. major amastigotes. Second, the synergistic effect of IFN-gamma and IL 4 on parasite killing by M-PHI strongly correlated with their synergistic effect on the release of TNF-alpha. Third, the IFN-gamma/IL 4-mediated parasite elimination was completely abrogated not only in the presence of antibodies to IFN-gamma and IL 4, but also with an antibody specific for TNF-alpha. Consistent with the conclusion that endogenously produced TNF-alpha accounts for the synergism of IL 4 with IFN-gamma is the finding that N-omega-monomethyl-L-arginine, an inhibitor of the L-arginine-dependent generation of microbicidal nitrogen intermediates, totally blocked the M-PHI activation induced by IFN-gamma combined with IL 4 as well as by IFN-gamma combined with TNF-alpha. These results underline the complex interplay of cytokines derived from lymphocytes and M-PHI and the role of TNF-alpha as pivotal factor for the induction of antileishmanial effector functions.