What is the shape of M. tuberculosis?
Unicellular rods
Is M. tuberculosis gram positive or gram negative?
Gram positive -> doesn’t stain very well
What gives M. tuberculosis the characteristic acid fastness?
Mycolic acid
Where is lipoarabinomannan found?
In the outer leaflet
What are the clinical manifestations of TB?
- Fever
- Weight loss
- Weakness
- Consumption
- Cachexia -> extreme weight loss and muscle wasting
Where can M. tuberculosis spread?
- CNS
- Lymphatic and genitourinary systems
- Bones and joints
What percentage of individuals will have a primary active disease withing the first 2 years of infection?
5%
Describe the pathogenesis of M. tuberculosis
- Aerosol travels to alveoli of the lungs
- M. tuberculosis is engulfed by alveolar macrophages
- If activated due to an acquire immune response (e.g. healthy adult), host may clear bacteria or at least contain the infection
- If unactivated (e.g. infant or naive adult) bacteria survive and replicate in macrophages
- This attracts more cells, damages tissue and forms a granulomatous tubercule which becomes increasingly fibrotic as time goes on
- The immune response and granuloma are protective
- Cytokines and chemokines drive granuloma formation
Describe a caseous granuloma
- Has a necrotic caseous centre to the lesion
- Surrounded by inflammatory cells
- Centre is full of mycobacteria
- Necessary for transmission of TB
Describe the Ghon complex
- Lesion in the lung and in draining lymph node
- Largely a contained infection
Describe cavitary TB
- Massive destruction of the lung
- Large area of cavitation
Describe the immune response in TB
- Clinical syndrome requires intact immune response
- Inflammatory response correlates with degree of cavitation
- Reduced infectivity of pulmonary TB in late HIV
- Steroids increase the rate of sputum clearance
- The immune response and granuloma are required for transmission
What receptors do phagocytes use in TB?
- Complement receptor 3
- Mannose receptor
- TLR2
What is the mannose receptor used for?
Often used for detection of microbes and inducing phagocytosis
What is the TLR2 used for?
Often used to ligate lipoproteins on microbes
How do Mycobacteria resist antimicrobial stress?
- Enter via receptors that avoid stimulation of the oxidative burst -> preferentially bind CR3 as the mannose receptor activates the oxidative burst
- Detoxifies ROIs by production of superoxide dismutase and catalase
- Induces an efficient stress response to resist effects of damaged proteins -> induces heat shock proteins Hsp70 and Hsp60 to refold damaged proteins and stops them aggregating -.> uses proteasome to degrade damaged proteins -> has DNA repair systems
Describe normal phagosome mutation
- Uptake
- The small GTPase Rab5 is recruited to the phagosome membrane
- Rab5 directs stimulation of phosphatidylinositol 3-kinase (PI3K) which is the enzyme that generates phosphatidylinositol 3-phosphate (PI3P)
- PI3P is essential for phagosome maturation
- Early endosomal antigen 1 (EEA1) accumulates, binding to Rab5 and PI3P
- Rab5 is involved in recruiting Rab7
- Rab5 and EEA1 are lost
- Rab7 is important for fusion of this late endosome to lysosomes forming the phagolysosome
- This maturation process is accompanied by gradual acquisition of a vacuolar ATPase proton pump which acidifies the phagosome
At what stage does M. tuberculosis inhibit phagosome maturation?
Inhibits maturation so the phagosome remains as an early endosomal compartment. This means the pH is kept at around 6.3.
How does M. tuberculosis cause phagosome arrest?
Lipoarabinomannan (LAM) has some ability to arrest phagosome maturation -> found on the outer leaflet of the Mycobacteria cell wall
How does M. tuberculosis modulate adaptive immunity?
- Inhibits antigen presentation to T cells
- Activated macrophages are able to overcome the M. tuberculosis counter measures -> macrophages are activated by IFNgamma produced by T cells
Describe the process of MHC class II being presented
- MHC II and invariant chain (Ii) made in the ER are transported through the Golgi to a vacuole that fuses with late endosomes
- This forms the MHC class II enriched compartment
- Ii is in the peptide binding groove
- Antigen enters via receptor-mediated endocytosis via early endosome until it forms the MIIC late endosome
- This changes the pH and degrades the protein
- Ii degrades leaving CLIP, then HLA-DM facilitates the replacement of CLIP with a peptide 13-18 amino acids in length
- Finally the complex is transported to the cell surface
How does M. tuberculosis inhibit presentation of MHC class II?
- LAM inhibits MHC II by chronic stimulation of TLR2 on the antigen presenting cell
- Normally MHC II is induced by stimulation of the antigen presenting cell and signal transduction down the JAK STAT pathway to produce the class II transactivator molecule (CIITA) which activates transcription of MHC II genes
- LAM activation of TLR2 leads to the production of an inhibitor molecule C/EBP which inhibits transcription of CIITA and therefore expression of MHC II
- Other glycolipids are also able to inhibit expression of MHC II
How does M. tuberculosis escape the phagosome?
- Disrupts the phagosome membrane
- Uses a type 7 secretion system, ESX-1
- EsxA and EsxB are secreted as a heterodimer using the T7SS
What is the function of EsxA?
Involved in the disruption of the phagosome membrane
