Effector molecules of the host defence mechanism against Mycobacterium avium complex: the evidence showing that reactive oxygen intermediates, reactive nitrogen intermediates, and free fatty acids each alone are not decisive in expression of macrophage antimicrobial activity against the parasites

Abstract

In this study, we evaluated the roles of reactive oxygen intermediates (ROI), reactive nitrogen intermediates (RNI), and free fatty acids (FFA) as effectors of the macrophage-mediated host defence mechanism against Mycobacterium avium complex (MAC). First, M. avium (three strains) and M. intracellulare (two strains) were treated with the H₂O₂-Fe²⁺-mediated halogenation system, acidified NaNO₂-derived RNI, or FFA (linolenic acid) in sodium acetate buffer pH 5.5, and then counted for the number of residual colony-forming units (CFU) of organisms. Although these effectors exerted strong bactericidal activity against the MAC, the susceptibility of test organisms markedly varied from strain to strain. There was no significant relationship between the degree of resistance of a given MAC strain to these effectors and its virulence in mice, indicating that ROI, RNI, and FFA each alone are not decisive as the effector components of the host defence mechanism against the MAC. Second, the increase in ROI-producing ability in murine peritoneal macrophages due to tumour necrosis factor-alpha (TNF-α) treatment was not accompanied by parallel potentiation of anti-MAC activity of the same macrophage population. This excludes the possibility that ROI play a central role in macrophage-mediated killing and inhibition of MAC organisms. Third, anti-MAC activity of BAM3 macrophage cell line was not significantly attenuated by N^G-monomethyl-l-arginine (NO synthase-inhibitor causing reduction of RNI production) or by quinacrine (phospholipase A₂-inhibitor causing reduction of FFA release), indicating that RNI and FFA each alone do not play crucial roles in the expression of macrophage antimicrobial activity against the MAC. The present findings suggest important roles of collaborating actions of various antimicrobial effectors and/or the participation of other kinds of effectors in macrophage-mediated killing and inhibition of MAC organisms.

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