Interferon-γ Regulates the Death of M. tuberculosis-Infected Macrophages

We previously described a caspase-independent death induced in macrophages by a high intracellular burden of Mycobacterium tuberculosis (Mtb). This death, with features of apoptosis and necrosis, releases viable bacilli for spreading infection. Interferon (IFN)-γ promotes survival of macrophages with a low intracellular Mtb load by inhibiting bacterial replication. Macrophages in naïve hosts are unable to restrict Mtb replication following aerosol transmission, but IFN-γ is increasingly present when adaptive immunity is expressed in the lungs ~2 weeks post-infection. We therefore investigated the effects of IFN-γ on macrophages challenged with Mtb at high multiplicity of infection (MOI). In contrast to the response at low MOI, IFN-γ accelerated the death of heavily infected macrophages and altered the characteristics of the dying cells. IFN-γ increased caspase-dependent DNA cleavage and apoptotic vesicle formation, but it also increased mitochondrial injury and release of LDH and HMGB1 in a caspase-independent manner. Adaptive immunity in tuberculosis (TB), mediated primarily by IFN-γ, has differential effects on Mtb-induced macrophage cell death depending on the intracellular bacillary load. While IFN-γ generally promotes host defense, our data suggest that its effects on heavily infected macrophages could also accelerate necrosis and spreading infection in TB disease.