Esat-6 protein of mycobacterium tuberculosis increases holotransferrin-mediated iron uptake in macrophages by downregulating surface hemochromatosis protein hfe

Iron is an essential element for Mycobacterium tuberculosis; it has at least 40 enzymes that require iron as a cofactor. Accessibility of iron at the phagosomal surface inside macrophage is crucial for survival and virulence of M. tuberculosis. ESAT-6, a 6-kDasecreted protein of region of difference 1, is known to play a crucial role in virulence and pathogenesis of M. tuberculosis. In our earlier study, we demonstrated that ESAT-6 protein interacts with b-2-microglobulin (b2M) and affects class I Ag presentation through sequestration of b2M inside endoplasmic reticulum, which contributes toward inhibition of MHC class I:b2M:peptide complex formation. The 6 aa at C-terminal region of ESAT-6 are essential for ESAT6:b2M interaction. b2M is essential for proper folding of HFE, CD1, and MHC class I and their surface expression. It is known that M. tuberculosis recruit holotransferrin at the surface of the phagosome. But the upstream mechanism by which it modulates holotransferrin-mediated iron uptake at the surface of macrophage is not well understood. In the current study, we report that interaction of the ESAT-6 protein with b2M causes downregulation of surface HFE, a protein regulating iron homeostasis via interacting with transferrin receptor 1 (TFR1). We found that ESAT-6:b2M interaction leads to sequestration of HFE in endoplasmic reticulum, causing poorer surface expression of HFE and HFE:TFR1 complex (nonfunctional TFR1) in peritoneal macrophages from C57BL/6 mice, resulting in increased holotransferrin-mediated iron uptake in these macrophages. These studies suggest that M. tuberculosis probably targets the ESAT-6 protein to increase iron uptake.