Iron Acquisition by Mycobacterium tuberculosis Within Phagocytes

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DESCRIPTION (provided by applicant): The pathophysiology of tuberculosis (TB) is linked to the ability of the causative organism, Mycobacterium tuberculosis (M.tb), to grow within human macrophages. Lung myeloid dendritic cells are a newly recognized reservoir of M.tb during lung infection. M.tb metabolism in macrophages and dendritic cells may differ. Iron (Fe) acquisition is critical for M.tb growth. In vivo, most extracellular Fe is chelated to transferrin (TF) and lactoferrin (LF), decreasing microbial access to Fe. We find that M.tb in human macrophages can acquire Fe from TF and LF, as well as the macrophage cytoplasm. Fe uptake from extracellular TF and LF may involve Fe moving into the cytoplasm. Some (e.g. hereditary hemochromatosis) conditions that lower intracellular Fe in macrophages, decrease M.tb Fe uptake. Whether other factors that regulate macrophage Fe metabolism (e.g. ferroportin, heme oxygenase 1 (HO-1), hepcidin) impact M.tb Fe acquisition is unknown. Similarly unknown is whether Fe acquisition by M.tb growing in dendritic cells is different from that occurring in macrophages. We hypothesize that: 1) M.tb acquires Fe from an intracellular Fe pool(s) in macrophages and dendritic cells; 2) acquisition of extracellular Fe by M.tb involves initial Fe movement into this pool; 3) the kinetics/route of M.tb Fe uptake varies with the extracellular Fe chelate and host cell type (macrophage vs. dendritic cell) involved; 4) genetic or physiological factors that modulate macrophage and/or dendritic cell cytoplasmic Fe alter Fe available for use by M.tb; and 4) administration of Ga, which disrupts M.tb Fe-dependent metabolism could prove to be a novel therapy for TB. In order to test these hypotheses we will pursue the following specific aims: 1. Delineate and compare the ability of M.tb residing within human macrophages and myeloid dendritic cells to: A) acquire Fe from physiologic extracellular chelates (e.g. TF and LF) and from cytoplasmic sites in the macrophage and dendritic cell; and B) to what extent Fe acquisition from extracellular chelates involves initial Fe movement into these same cytoplasmic sites. 2. Determine: A) if M.tb modulates macrophage Fe metabolism, which in turn alters Fe acquisition and growth by the bacteria; and B) the role of key components of macrophage Fe metabolism (e.g. ferroportin, HFE, HO-1, hepcidin) on M.tb Fe acquisition and growth. 3. Determine the efficacy of Ga as a therapeutic agent against M.tb using a murine model of M.tb infection. If positive results are seen, further ascertain the mechanism(s) of Ga's anti-tuberculous activity. The proposed studies use human macrophages and dendritic cells along with fully virulent M.tb strains to accomplish these aims. Experimental methods employed include: measuring uptake of 59Fe, siRNA, confocal microscopy, immunoblot analysis, RT-PCR, and ELISA and use of a murine model of pulmonary TB. Insight into M.tb Fe metabolism could lead to novel treatments that would benefit veterans. PUBLIC HEALTH RELEVANCE: Project Narrative - Relevance to VA Mission: Veterans are at high risk for pulmonary TB. There is a growing problem in treating tuberculosis in that the causative organism, Myocobacterium tuberculosis, is becoming increasingly resistant to available antibiotics. The ability to acquire iron (Fe) from the infected host is required for M. tuberculosis to be able to grow and therefore cause infection. Insight into M. tuberculosis Fe metabolism could lead to development of novel approaches to interfere with the organism's ability to acquire Fe treatments that could lead to the development of new kinds of antibiotics. This application will explore one potential mechanism of doing this using gallium. Thus, increased understanding of how M. tuberculosis acquires and uses Fe during infection could lead to the development of new antibiotics to be used in treating TB, something that would be of great benefit to veterans.