Mycobacterium tuberculosis

• Describe M. Tb epidemiology

  • Spread via aerosols:
  • Infectious disease estimated around 1 bacterium
  • Infected individuals have a 5-10% risk of developing TB unless they have a predisposing factor
  • Susceptible individuals include immunocompromised and those with genetic predispositions
• Describe the symptoms of TB

  • Primary TB is often asymptomatic
  • This is followed by fever, coughing, weight loss and lung damage
  • M. Tb spreads via lymphatics and the bloodstream to create systemic disease which can affect bones, liver, brain. etc.
  • Systemic form is almost always fatal
• Describe M. tb

  • Mycobacteriaceae
  • Gram negative, rod shaped, non-spore forming obligate aerobe
  • Contains complex cell wall with lipoarabinomannan and mycolic acid
  • Mycolic acids are strong hydrophobic molecules that form a lipid shell
• Describe the main virulence factors for M. Tb

  • Reprogram macrophages after primary infection to prevent destruction
  • Initiate granuloma formation to create confined environment
  • Can enter latency phase, which renders itself extremely resistant to host defences and drug treatment
• Describe the stages of TB pathogenesis

  1. Inhalation of bacteria
  2. Bacteria is ingested by alveolar macrophages
  3. Bacteria replicate within macrophages and induce localised inflammatory response
  4. Granuloma forms and matures as lymphocytes surround infected macrophages
  5. Granuloma necrosis with extracellular M. Tb replication.
  6. Continuous stimulation of T cells and macrophages causes lung damage and bacteria disseminate to cause systemic infection
• Describe the structure of granulomas

  • M. Tb are at the centre of the granuloma, within alveolar macrophages
  • Caesum: consists of dead phagocytes
  • Monocytes and dendritic cells migrate to the lymph nodes to primer T cells, which try to kill bacteria
  • Fibroblasts form a border around the edge of the granuloma
  • Core of epitheliod cells, macrophages and some giant cells usually surrounded by lymphocytes.
• What are epitheliod cells?

  These are derived from activated macrophages due to chronic stimulation with cytokines. Epitheloid cells fuse to form multinucleate giant cells that are important mediators of tissue remodelling and sequestration of intracellular bacteria.
• What are foamy macrophages?

  These are macrophages loaded with lipid droplets and are often observed within the tissues with proinflammatory stimulus. They play an active role in necrosis formation and the accumulation of caseous debris at the heart of the granuloma.
• Describe the importance of IFN-gamma in M. Tb killing

  CD4+ T cells activate natural killer cells to kill infected macrophages, which results in release of intracellular M. Tb. IFN-gamma activates macrophages to mop up the M. Tb cells.
• Describe phagosome maturation

  • Respiratory burst oxidase forms at the phagosome membrane to produce ROS
  • Coronin 1 needs to dissociate to allow the phagosome to mature
  • Early endosome fuses with the phagosome with Rab5 and Rab5 effectors, PI3K and EEA1, are essential for maturation
  • PIP3 is produced by PI3K and signals for late endosome fusion with Rab7, which controls lysosome fusion
  • Ca2+ rises and activates calmodulin, which recruits lysosomal hydrolases and results in acidification
• Describe the virulence factor, LAM, in M. Tb

  • Complex glycolipid that contains arabinose mannose disaccharide units
  • Prevents acidification of phagosomal vacuole by inhibiting Ca2+ entry and blocks recruitment of PI3K
  • Scavenges ROS and blocks MHCII expression on macrophages.
• Describe the virulence factor, mycolic acid, in M. Tb

  Mycolic acid inactivates the respiratory burst and prevents toxic effects of cationic proteins and lysozymes.
• Describe the virulence factor, PI phosphatases, in M. Tb

  PI phosphatase are secreted to maintain low levels of PIP3.

  • SapM: hydrolyses PIP3
  • MptpA: de phosphorylated and inactivates VPS33B, which is involved in membrane fusion
• Describe the role of eukaryotic type protein kinases in M. Tb virulence

  These eukaryotic type protein kinases are secreted into the macrophage cytosol and phosphorylate host proteins involved in trafficking pathways.
• How does M. Tb adapt to nutrient stress?

  • Urease: counter effects in drop in pH and inhibits phagosome maturation
  • Superoxide dismutants: catalase, reduce ROS
  • Two compoenent regulatory systems allows M. Tb to response to environmental stress
• Describe M. Tb secretion systems

  • 5 specific Type VII secretory systems that export effector proteins:
  • ESX-1: exports substrates involved in phagosomal rupture, which allows the phagosome contents to mix with the cytosol
  • ESX-3: iron acquisition
  • ESX-5: secretes PE and PPE proteins, maintain intracellular peristence.
• Describe the importance of ESX-1 system for M. Tb virulence

  This is responsible for secreting proteins involved in phagosomal rupture, which allows the contents of the phagosome to mix with the cytosol. This trigger STING TBK-1 signalling, which triggers IFN-alpha synthesis and promoters infection. It also trigger ubiquitylation of phagosomes which tags them to fuse with lysosomes.