Effector mechanisms involved in cytokine-mediated bacteriostasis of Mycobacterium avium infections in murine macrophages

Abstract

In this study we found that addition of a range of doses of interferon-gamma (IFN-gamma), tumour necrosis factor-alpha (TNF-alpha), or granulocyte-macrophage colony stimulating factor (GM-CSF) to cultures of bone marrow-derived murine macrophages infected with the 25291 strain of Mycobacterium avium gave rise to varying degrees of bacteriostasis. In contrast, similar treatment with interleukin-4 (IL-4) or IL-6 had no effect. However, when similar experiments with the former set of cytokines were performed using a panel of M. avium isolates, substantial isolate-to-isolate variation was observed. In cultures containing IFN-gamma, synthesis of substantial levels of reactive nitrogen intermediates was observed; however, neither these materials, nor reactive oxygen intermediates, were found to be responsible for observed bacteriostasis. In further experiments, in which the culture medium was supplemented with various concentrations of a weak acid or a weak base in order to influence the pH of macrophage intracellular compartments, it was found that the presence of the weak acid augmented the activity of IFN-gamma, whilst the weak base counteracted this effect. These data support the hypothesis, therefore, that the bacteriostatic effect of IFN-gamma against the growth of M. avium, rather than depending on reactive radical production, is mediated through acidification of the infected phagosome, perhaps through activation of proton pumps in the phagosomal membrane.