Molecular Basis of the Disease State

How Tuberculosis is Spread

Mycobacterium tuberculosis is most often spread through coughing (Li et al. 2002). The bacteria travels through the air in droplets from the patient’s infected lungs to the recipient’s respiratory tract (Li et al. 2002). When the bacteria reach the alveolar cells, they secrete virulence determinants that are affected by host environmental conditions (Li et al. 2002). Furthermore, host environment determines the efficiency of the bacteria entering macrophages (Li et al. 2002).

The Granuloma Model

Granulomas are organized clusters of macrophages and epithelial cells surrounded by T cells (Guirado et al. 2015). The role of granulomas are to contain Mycobacterium tuberculosis  in a specific area (Guirado et al. 2015). Since the bacteria possess defenses for its survival, it cannot be completely killed within granulomas, which is why latent tuberculosis can persist within the host (Guirado et al. 2015).

 Metabolic Pathways

When developing drugs in order to treat tuberculosis, it is important to target metabolic pathways that support the bacteria’s defense against host immune response. The biosynthesis of cysteine is crucial to bacterium survival in granulomas (Ågren et al. 2008). It had previously been thought that latent tuberculosis can survive in granulomas due to its ability to fight oxidative stress and reactive nitrogen species by producing mycothiol (Schnell and Schneider 2010). Mycothiol contains important constituents of cysteine, which are used to synthesize it (Schnell and Schneider 2010). When oxidative stress and reactive nitrogen intermediates target cysteine residues, Mycobacterium tuberculosis can still survive due to mycothiol’s cysteine-constituent reserve (Ågren et al. 2008 and Schnell and Schneider 2010). One of these constituents, O-acetyl-L-serine (OAS), was thought to be the sulfur acceptor of CysM during the biosynthesis of cysteine in Mycobacterium tuberculosis (Ågren et al. 2008). It was thought that by targeting OAS, Mycobacterium tuberculosis would not be able to stabilize the amount of cysteine necessary for survival (Ågren et al. 2008). However, it was discovered that CysM uses another sulfur acceptor, O-phosphoserine (OPS), which utilizes a different pathway for synthesizing cysteine (Ågren et al. 2008).

Between the two substrates of CysM, OAS and OPS, it was determined that OPS allows for the rapid production of an aminoacrylate intermediate, which is part of the first half-reaction in the biosynthesis of cysteine (Ågren et al. 2008). It was also determined that the second order rate constant for this half-reaction was 600x greater when CysM donated sulfur to OPS when compared to CysM donating sulfur to OAS (Ågren et al. 2008).

Once a crystal structure was determined for CysM, the active site provided evidence for CysM’s preferential interaction with OPS (Ågren et al. 2008). An arginine residue within the active site was shown to specifically recognize OPS as a substrate (Ågren et al. 2008). Before the discovery of an OPS-dependent pathway, it was thought that Mycobacterium tuberculosis used OAS for cysteine biosynthesis (Ågren et al. 2008). However, with this additional pathway, drug therapies used to target the OAS-pathway do not completely inhibit bacterium survival (Ågren et al. 2008). Thus, the additional targeting the OPS-pathway for cysteine biosynthesis will provide further survival inhibition, making this a landmark experiment (Ågren et al. 2008).

Cysteine Biosynthesis Pathways Using Either OAS or OPS. Source: http://www.jbc.org/content/283/46/31567.long
Cysteine Biosynthesis Pathways Using Either OAS or OPS. Source: http://www.jbc.org/content/283/46/31567.long

Defense Against Immune Response

What makes tuberculosis such a deadly infection is that Mycobacterium tuberculosis can suppress host immune response (Wu et al. 2015). There are various components of the immune response that are affected by Mycobacterium tuberculosis, thus providing multiple defenses. One of the major ways in which this bacterium fights against immune response is by decreasing type 1 helper T cell (Th1) response and reducing interferon-gamma (Gong et al. 1996). This is accomplished through the production of interleukin-10 (IL-10) by phagocytes, which is induced by the bacterium (Gong et al. 1996). IL-10 inhibits the production of interleukin-12 (IL-12), which elicits the production of interferon-gamma and increases CTLA-4 expression (Gong et al. 1996). By decreasing the production of interferon-gamma, macrophage activity will greatly decrease (Flynn et al. 1993). Therefore, Th1 response will be suppressed, thus enabling Mycobacterium tuberculosis to persist in its host (Gong et al. 1996).

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Overview of Suppressed Immune Response

Recently, it has been shown that patients of tuberculosis have heightened levels of CD25+, CD4+, and FOXP3+ regulatory T cells (Wu et al. 2015). Antigens ESAT-6 and Ag85B secreted by Mycobacterium tuberculosis cause an increase in these regulatory T cells, thereby suppressing effector T cell activity (Wu et al. 2015). Furthermore, it has been shown that T cells undergo higher levels of apoptosis in patients with tuberculosis compared to those not infected (Elliot et al. 2015). Therefore, the bacterium can cause a decrease in host immune response (Wu et al. 2015).

6 Replies to “Molecular Basis of the Disease State”

  1. Hi – Very thorough work. I found your explanation of the metabolic pathways to be especially clear and thorough. My question is about the “How tuberculosis is spread” part. You mention secretion of virulence determinants, and I was wondering if you could expand on that idea. What kind of molecules are you talking about (e.g. proteins? small molecules?), and how are they connected with the disease? It would be bad enough if a foreign bacterium entered someone’s lungs, but the term “virulence determinant” (and your explanations on this page) make me thing that these molecules can add to the picture of how nasty tuberculosis can be. This makes me want to know more!
    Thank you- Zach Zimmerman

    1. Thank you for your feedback Zach! The majority of virulence determinant molecules are proteins. Although many the function of many of these proteins have yet to be determined in regards to the role they play in virulence, it has been shown that knockout of the genes encoding for these proteins have led to a decrease in pathogenicity of Mycobacterium tuberculosis. However, the function of some of these proteins have been determined.

      Some cell wall proteins help maintain the integrity of the cell envelope, therefore providing defense against host immune response. Other secreted or surface-bound cell wall proteins allow the bacterium to enter host cells and survive within macrophages.

      There are so many of these proteins that are virulence determinants that it wouldn’t be possible for me to list them all and include their function. However, the majority allow the bacterium to enter host cell and survive when facing host immune response. I hope this answered your question!

  2. I’m really intrigued by the concept of targeting cysteine metabolism to treat TB, especially since none of the conventionally used agents have this mechanism of action. With such rapidly growing resistance against isoniazid and rifampin, more agents to add into combination therapies would be so helpful! What’s the homology like between our Cys biosynthesis pathway and the pathway Mycobacterium uses? Do you think it would be likely that there might be crossover between drugs targeting Mycobacterial Cys synthesis ezymes and our Cys enzymes? Also, were you able to see in the literature if Mycobacterium is capable of scavenging Cys from the Cys we make/consume? That would definitely complicate things…

    Nice job on this project!

    1. Thank you so much for your feedback Rebecca! I really appreciate it! As far as I could tell from my research, targeting Cys biosynthesis in the bacteria may interfere with our Cys biosynthesis. Between CysM and our cystathionine-beta-synthetase, there is about a 50% similarity, however, we lack the enzymes necessary for the de novo cysteine biosynthesis pathway of Mycobacterium tuberculosis. Therefore, by targeting other enzymes within the pathway, there shouldn’t be any interference. Also, I couldn’t find anything about Mycobacterium tuberculosis using host Cys, but that’s not to say that they don’t. Again, I really appreciate your feedback, and I hope I sufficiently answered your question!

  3. Hi Nikki,

    While I was researching granulomata in my project I constantly came across resources involving granuloma formation in tuberculosis. However, I have some questions as to the bacterial defense mechanism. Does the bacteria induce IL-10 production after it is phagocytosed by the macrophage? Is there a biochemical mechanism for how M. tuberculosis induces this cellular response?

  4. Hi Elliott,
    The bacteria do in fact induce IL-10 production after phagocytosis by macrophages. From what I could find from my research, the macrophages that contain M. tuberculosis has receptors that interact with apoptotic cells, and these receptors that interact with apoptotic cells are the ones that recognize the pathogens. When recognizing apoptotic cells, IL-10 is released, triggering an anti-inflammatory response. I hope this sufficiently answers your questions!

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