M. Tuberculosis is an important human pathogen of serious and escalating morbidity and mortality. Our ability to develop new approaches to preventing and treating tuberculosis is hampered by our lack of knowledge of the intracellular biology of M. tuberculosis, including the composition of its phagosome and the interaction of the phagosome with other organelles of the host cell - the human mononuclear phagocyte. This proposal seeks to use cell biological, molecular biological, biochemical, and immunological approaches to characterize the composition of the phagosome - both in terms of host cell proteins and in terms of mycobacterial antigens - and to characterize the interaction of the phagosome with the host cell. In preliminary studies, we have used the cryosection immunogold technique to determine which known markers of the endosomal-lysosomal pathway are present on the M. tuberculosis phagosome. Based upon the results of these studies, we have hypothesized the M. tuberculosis arrest the maturation of its phagosome at an early endosome- like stage. This proposal will further explore this hypothesis, among other goals. Specific goals and aims of this proposal are: Goal I) Define the specific intracellular compartment in which M. tuberculosis resides in host mononuclear phagocytes.
Aim a) Characterize the contribution of host cell molecules to the composition of the M. tuberculosis phagosome at various times after infection.
Aim b) Characterize the interaction of the M. tuberculosis phagosome with endocytic and exocytic pathways.
Aim c) Determine the effect of various cytokines and calcitriol on the composition of the mycobacterial phagosome and its interaction with other host cell organelles. Goal II) Characterize the contribution of M. tuberculosis molecules to the composition of its phagosome and determine if M. tuberculosis antigens are transported to extraphagosomal compartments.
Aim d) Determine which M. tuberculosis proteins are induced or repressed during intracellular growth in human mononuclear phagocytes, and determine whether these responses mirror the mycobacterial response to environmental stresses (heat shock, osmotic shock) when growing extracellularly.
Aim e) Determine which M. tuberculosis antigens are present on and in the phagosome at various times after infection.
Aim f) Determine if M. tuberculosis antigens enter extraphagosomal host cell compartments. Data obtained from these studies will shed considerable light on the intracellular biology of M. tuberculosis, including mechanisms of virulence of M. tuberculosis, and on the host cell response to M. tuberculosis infection. By providing new insights into the biology of M. tuberculosis, these studies will hopefully lead to new strategies for the prevention and treatment of tuberculosis.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
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Special Emphasis Panel (SRC (36))
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University of California Los Angeles
Internal Medicine/Medicine
Schools of Medicine
Los Angeles
United States
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Horwitz, Lawrence D; Horwitz, Marcus A (2014) The exochelins of pathogenic mycobacteria: unique, highly potent, lipid- and water-soluble hexadentate iron chelators with multiple potential therapeutic uses. Antioxid Redox Signal 21:2246-61
Clemens, Daniel L; Lee, Bai-Yu; Horwitz, Marcus A (2002) The Mycobacterium tuberculosis phagosome in human macrophages is isolated from the host cell cytoplasm. Infect Immun 70:5800-7
Land, K M; Clemens, D L; Johnson, P J (2001) Loss of multiple hydrogenosomal proteins associated with organelle metabolism and high-level drug resistance in trichomonads. Exp Parasitol 97:102-10
Pahl, P M; Horwitz, M A; Horwitz, K B et al. (2001) Desferri-exochelin induces death by apoptosis in human breast cancer cells but does not kill normal breast cells. Breast Cancer Res Treat 69:69-79
Byrd, T F; Horwitz, M A (2000) Aberrantly low transferrin receptor expression on human monocytes is associated with nonpermissiveness for Legionella pneumophila growth. J Infect Dis 181:1394-400
Clemens, D L; Johnson, P J (2000) Failure to detect DNA in hydrogenosomes of Trichomonas vaginalis by nick translation and immunomicroscopy. Mol Biochem Parasitol 106:307-13
Clemens, D L; Lee, B Y; Horwitz, M A (2000) Deviant expression of Rab5 on phagosomes containing the intracellular pathogens Mycobacterium tuberculosis and Legionella pneumophila is associated with altered phagosomal fate. Infect Immun 68:2671-84
Clemens, D L; Lee, B Y; Horwitz, M A (2000) Mycobacterium tuberculosis and Legionella pneumophila phagosomes exhibit arrested maturation despite acquisition of Rab7. Infect Immun 68:5154-66
Gobin, J; Wong, D K; Gibson, B W et al. (1999) Characterization of exochelins of the Mycobacterium bovis type strain and BCG substrains. Infect Immun 67:2035-9
Calder, K M; Horwitz, M A (1998) Identification of iron-regulated proteins of Mycobacterium tuberculosis and cloning of tandem genes encoding a low iron-induced protein and a metal transporting ATPase with similarities to two-component metal transport systems. Microb Pathog 24:133-43

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