Correction of the Iron Overload Defect in β-2-Microglobulin Knockout Mice by Lactoferrin Abolishes Their Increased Susceptibility to Tuberculosis

As a resident of early endosomal phagosomes, Mycobacterium tuberculosis is connected to the iron uptake system of the host macrophage. β-2-microglobulin (β2m) knockout (KO) mice are more susceptible to tuberculosis than wild-type mice, which is generally taken as a proof for the role of major histocompatibility complex class I (MHC-I)–restricted CD8 T cells in protection against M. tuberculosis. However, β2m associates with a number of MHC-I–like proteins, including HFE. This protein regulates transferrin receptor mediated iron uptake and mutations in its gene cause hereditary iron overload (hemochromatosis). Accordingly, β2m-deficient mice suffer from tissue iron overload. Here, we show that modulating the extracellular iron pool in β2m–KO mice by lactoferrin treatment significantly reduces the burden of M. tuberculosis to numbers comparable to those observed in MHC class I–KO mice. In parallel, the generation of nitric oxide impaired in β2m–KO mice was rescued. Conversely, iron overload in the immunocompetent host exacerbated disease. Consistent with this, iron deprivation in infected resting macrophages was detrimental for intracellular mycobacteria. Our data establish: (a) defective iron metabolism explains the increased susceptibility of β2m-KO mice over MHC-I–KO mice, and (b) iron overload represents an exacerbating cofactor for tuberculosis.