The interaction of Mycobacterium tuberculosis with antigen presenting cells
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Mycobacterium tuberculosis (Mtb), the etiologic agent of tuberculosis, infects nearly one third of all humans, making it the world's most successful bacterial pathogen. Despite the availability of antibiotic therapies, 1.5 million individuals succumb to tuberculosis related disease annually. A hallmark of the tuberculosis infection is its ability to subvert host immunity, thereby facilitating intracellular survival to establish a latent infection that can reactivate to cause disease at a later time.;Interactions of Mtb with host antigen presenting cells, such as macrophages and dendritic cells (DC), define the progression of tuberculosis infection. Understanding these interactions would likely shed light on Mtb pathogenesis. The first part of the thesis work focuses on the host protein TACO (t&barbelow;ryptophane a&barbelow;spartate-containing coat protein, also known as Coronin). TACO has recently been shown to be actively recruited and retained on bacilli-containing phagosomes after infection of macrophages by BCG, the avirulent vaccine mycobacterial strain. By the retention of TACO, mycobacteria are able to interrupt maturation of bacilli-containing phagosomes, thereby preventing phagolysosomal (PL) fusion. This interruption allows evasion of the anti-microbial environment in the lysosomes, thus promoting bacterial persistence. However, the role of TACO in the interaction of primary human macrophages with virulent Mtb has not been evaluated. An in vitro experimental system has been established to address this issue by confocal microscopy. Preliminary results showed that within primary human macrophages, TACO is retained on the surface of Mtb-containing phagosomes that co-localized with Rab5. This in vitro model is being used to carry out detailed kinetic and quantitative analysis of the trafficking of TACO, Rab5, and LAMP-1 on Mtb-containing phagosomes in primary human macrophages and DC.;In parallel to the TACO work, studies have been initiated to test the hypothesis that Mtb can inhibit PL-fusion in DC, thereby promoting persistence and attenuate the antigen-presenting capacity of this antigen presenting cells. Using primary human dendritic cells and green fluorescence protein-expressing M. tuberculosis, confocal microscopic studies have revealed that live M. tuberculosis, compared with killed bacteria, when internalized into phagosomes, were able to avoid delivery to the lysosomal compartment. In addition, we determined that activation of the dendritic cells with interferon (IFN)-gamma abrogated the phagosome maturation blockade caused by live M. tuberculosis, allowing maturation to phagolysosome. When taken together, these findings demonstrate a mechanism for M. tuberculosis evasion of the host immune response that can be potentially exploited for designing more effective anti-tuberculosis therapy.;In summary, results generated by the studies of the thesis work have revealed novel mechanisms by which Mtb manipulates host macrophages and DC, and as a result, promotes persistence. Characterization of these putative mechanisms at the molecular and biochemical level will likely guide the design of novel anti-tuberculosis therapies.
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