The battle for iron. Macrophages use multiple pathways to restrict the essential growth factor iron for intracellular mycobacteria. First, cytokines such as IFN-γ inhibit the transcriptional expression of transferrin receptor (TfR). TfR is a major source of iron for mycobacteria, because the bacteria can utilize its ligand, transferrin iron, following its endosomal transfer. Macrophages produce lipocalin-2 (Lcn2), which binds and neutralizes siderophores produced by M. tuberculosis to scavenge and re-utilize cytoplasmic iron. Furthermore, macrophage-derived cytokines such as TNF-α induce the formation of the iron-binding protein ferritin, which incorporates iron into its core rendering it unavailable for intracellular bacteria as well as the iron binding protein lactoferrin (Lf), which also scavenges this metal. Activated macrophages express the phagolysomal protein Nramp1, which among other effects pumps iron out of macrophages, thereby reducing the availability of the metal in the phagosome for mycobacteria. Finally, upon formation of nitric oxide, the transcription factor Nrf2 is activated, stimulating the expression of the major iron export protein ferroportin (FP1) pumping iron out of the phagolyosome and of the cytoplasma of macrophages. By mechanisms that remain elusive, the stimulation of hepcidin expression in the liver, which is a major mechanism for iron restriction to extracellular pathogens, is circumvented. All these events result in reduction in intracellular iron levels and a limited availability of iron for intra-macrophage bacteria. Based on the negative regulatory effects of iron on IFN-γ activity, the reduction in this metal's availability results in strengthened innate anti-microbial immune responses. Importantly, some of the pathways shown in this figure have been investigated for other intracellular bacteria such as S. typhimurium, and their importance for M. tuberculosis remains to be shown.