Cytokine interactions that determine the outcome of Mycobacterial infection of macrophages

Macrophages are the target cells for mycobacterial infections. They are also largely responsible for intracellular killing of mycobacteria, which is dependent on the cytokine environment. Interferon-gamma (IFN-gamma) is chiefly responsible for macrophage activation and bactericidal capacity while Th2 cytokines have a contrasting effect. However, cytokines rarely act in isolation during an infection. Instead, multiple cytokines, both activating and inhibitory, are present and their concentration levels and mutual interactions are likely to determine the ultimate outcome of an infection. Here, we used an in vitro infection model of mouse macrophages to study the effect of cytokine interactions on the infection with Mycobacterium bovis strain BCG. We measured nitric oxide (NO) production and bacterial survival in cells following stimulation with various combinations of cytokines. The surprising finding was that high concentrations of IL-10 (i.e., above 16.5 ng/ml), which is generally considered to be a macrophage-suppressive cytokine, enhanced IFN-gamma-induced NO production. Furthermore, the simultaneous addition of either of the two Th2 cytokines IL-4 or IL-13, strongly inhibited IFN-gamma-mediated NO production and bacterial killing even at a low concentration of 0.62 ng/ml, but could not reverse the synergistic action of IFN-gamma and TNF-alpha, even when the Th2 cytokines were present at high concentrations (i.e., 50 ng/ml). Therefore, macrophage activity is heavily dependent on the cytokine micro-environment where the final outcome is determined in equal measures by the nature of cytokines present, the timing of their accumulation and their concentration levels.