Mycobacterium Pathogenesis Flashcards
What is the mechanism underlying granuloma formation?
resistance to microbicidal effects of a type 1 response results in chronic low level infection that requires an ongoing Th1 response to prevent pathogen proliferation and spread.
What is the characteristic feature of granulomas?
fusion of several macrophages to form multinucleated giant cells
Where are multinucleated giant cells found in the granuloma?
border of the central focus of macrophages and lympphcytes which surround them
Waht are type 1 cells?
NK cells ICL1 and Th1 cells
What are teh 3 general strategies for intracellular suvirval?
blockade of phagosome-lysosome fusion; escape from the phagosome into the cytosol and resistance for killing mechanisms within thep hagolysosome
What is tuberculin?
complex extract of peptides and carbohydrates derived from M.tuberculosis
Who gets a positive mantoux test?
those who have had BCG or have been infected by M.tb
What response causes a reaction with the mantoux test?
Th1 effector response- delayed type hypersensitivty reaction
What species make up the M.tb complex?
M.tb; M.africanuum and M.cannetti; M.bovis and M.microti
What does M. microti target?
small vertebrates
Which continent has all strains of M.tb and how does this relate to theories abotu the history of TB?
Africa- lines up with theory that TB evolved and spread with huams n
What is a problem with M.tb strains and research?
mainly made of Euro-American strains which does not reflect the global strain prevalence
What are teh 2 tests for determining latent TB?
TST and IGRA (immunoglobulin gamma release assay)
How is TB transmitted?
airborne
What is thought to vast amount of people who are connected with TB?
eliminate using their innate immune repsonse- will be completely in all testing as no T cell priming
What is multidrug resistant TB defined as?
resistance to at least isoniazid and rifampicin
What is XDR-TB defined as?
resistance to isoniazid and rifampicin; fluoroquinolone and any of the 3 injectable second line aminoglycosides
When is latent TB defined as being subclinical?
once there is positive culture or sputum smear- can transmit
What region of the genome is different between TB and BCG?
RD1
What does RD1 encode?
ESX-1 secretion system
what is the function of ESX-1?
once the bacteria have eben internalised in a phagosome by the host macropahges, ESX-1 mediates the delivery of bacterial productsi n to hte macropahge cytoplasm
What are the 2 broad outcomes of exposure to M.tb?
elimination or persistence of the pathogen
Why does a positive TST or IGRA not automatically mean a patient will benefit from LTBI treatment?
some individuals who sliminate the infection via the adaptive immune repsonse may have a positive test depending on whether they primed their T cells responses
What are the dominant cell type that M.tb infects?
alveolar macrophages
How do alveolar macropahges internalise M.tb?
receptor-mediated phagocytosis
What is the effect of surfactant D in M.tb phagocytosis?
can prevent it
What happens if the alveolar macrophages are no able to eliminate the bacteria?
bacteria invades the lung interstital tissue by direct infection fo the epithelium or the alveolar macropahges infected with M.tb migrating the lung parenchyma
What is one reason that M.tb uses the ESX-1 secretion system to inject bacterial products into the cytosol?
activation of cytosolic surveillance pathway resulting in a type I IFN response can help promote grwoth of intracellular bacterial pathogens
How is there georgraphical segreation in a grnuloma?
centre contains pro-inflammatory components whilst the surrounding tissue has anti-inflammatory ones
What suggests that other T-cell independet immune responses are involved in susceptibility to TB in HIV?
some studies indicate the risk of active TB disease is enhanced during hte early stage of HIV infection- when the number of CD4 cells is normal
Why is it unclear whether enhanced T cell repsonses give better protection?
in mice, increasing the total CD4 T cell responses lead to reduced protectio nand enahnced mortality
What suggests that M.tb might benefit from antigen-specfici CD4 T cell activation?
genomic studies of genes involved in the production of immunodominant CD4 T cell antigens do not vary across strains and lineages
How does M.tb develop drug ressitance?
through genetic mutations- not natural transformation
What are the 2 main mechanisms of M.tb drug resistance?
target modification; defective enzyme that converts a pro-drug into an active drug
What are the 2 tests available for identification of LTBI?
TST and the IGRA
What is the benefit of IGRA over TST?
IGRA can distinguish between BCG-induced and M.tb infection induced postiive TST
How is TST perfomed?
intradermal infection of 5 tuberculin units of purified protein derivative
What are the disadvantages of TST?
specificity is compromised by BCG vaccination and exposure to non-tuberculous mycobacteria; limited predictive value- most individuals with positive TST do not progress to active TB
How is IGRA carried out?
in vitro blood test of cell-mediated immune response- measure T cell release of IFNy following stimulation by RD1-encoded antigens
Why are RD1 antigens more specigic for M.tb than PPD antigens?
they are not encoded in the genome of any BCG vaccine strains or most species of non-tuberculous mycobacteria
What are the 4 main technologies for detection of active TB disease?
imaging-CXR and PET-CT; microscopy of sputum; culture based methodsl moelcualr tests
What is LAM rapid test?
detection of lipoarabinomannan antigen in urine
When should LAM rapid test be used?
HIB positive adults with signs of active TB with <100 CD4 or very ill HIV patients (specificity and sensitivity is highest in these patients)
What does the Xpert TB/Rif assay detect?
detect mycobacterium tb complex and ressitance to rifampicin
How does Xpert MTB/RIF assay work?
nucelic acid amplification system test using sputum
Why is sputum smear microscopy difficult in children?
young children are unable to produce sputum; disease may be extrapulmonary and low numbers of bacilli in children with TB
What are the methods of detection of drug resistance?
phenotypic; culture-based (ability of bacteria to grow in presence of anti-TB drugs); moeclular based (detection of genetic mutations)
What is the efficacy of BCG vaccine against pulmonary TB in adults?
0-80%
What is the effect of BCG vaccine in children?
infants and 5 years protects from severe, extrapulmonary forms of active TB disease– 50-80%
What is the most common drug to be resistant to in TB?
isoniazid
What is the lifetime risk of latent TB converting to active TB?
10%
What happens when bacteria are phagocytosed by alveolar macrophages?
pro-inflammatory response; recruit mononuclear cells; buidling blocks for granuloma
What is the caseum?
area rich in lipis and debris from necrotic macropahges
What is foudn between the macropahges and lymphocytes in the granuloma?
fibrotic region with ECM and fibroblasts
What happens when there is breakdown of the granuloma?
stimatulion of mycobacterial growth and then release of large numbers of bacteria
What is recruited to the phagosome to reduce the pH?
V-H ATPase
What are the features of live BCG phagosomes?
limited acidication-reduced V-ATPase; contain early endosome markers- Rab5 and 14 (blocking lysosome fusion)
What are the 3 strategies for mycobacterial arrest of phagosome maturation?
interactiosn with surface receptors; modification of phagosomal lumen; modification of phagosomal membrane
What is the effect of CR3 uptake of Mtb?
prevents respiratory burst; blocks phagosome maturation
What does uptake of Mtb by TLR2 stimualte?
IL-12 and iNOS
How does Mtb modify the phagosomal lumen?
secreted acid phosphatase; lipoarabinomannan; protein kinase G
What is the effect of secreted acid phosphatase?
cleaves PI 3-kinase and blocks phagosome-late endosome fusion (PIP3 is essential for phagolysosome biosynthesis)
What is the effect of lipoarabinomannan?
inactivates macrophages; scavenges oxidative radicals
What is the effect of protein kinase G
?
protein kinase G- blocks delivery of mycobacteria to lysosome
What happens when AX20017 is used to block protein kinase G?
M.tb are found in the lysosome
How does M.tb modify the phagosomal membrane?
mycobacterial lipids/proteins traffic out of immature phagosome
What proteins are invovled in trafficing mycobacterial lipids/proteins out of immature phagosome?
PDIM and PIM (pthiocerol dimycocerasat and phosphatidylinositol mannosides)
What do lipids released in to vacuole accumulate in?
multilamellar bodies which are released by exocytosis
What happens to the exosomes with lipids from M.tb?
internalised by neighbouring cells
Why is treatment difficult and prolonged?
cell wall; slow replication time; intra-cellular niche
Which TB drug is classed as bactericidal and target rapidly dividing bacilli?
isoniazid
Which TB drugs are considered sterilising drugs killing persistent, nonreplicating organisms?
rifampicin and pyrazinamide
What is antibiotic tolerance?
reduced susceptibility to killing by cell wall-active antibiotics- feature of persistent bacilli and LTBI- rate of bacterial killing is proportional to the rate of bacterial replication and metabolic activity
What is dormancy?
bacteria are viable but exhibit reduced metabolic activity
Waht is LTBI traditionally thought of?
population of nonreplicating, metabolically quiescent organisms that have entered a dormant state as an adaptive response to immune-based containment mechanisms
What suggests that the classical definition of LTBI of dormancy is incorrect?
isoniazid can prevent reactivation disease indicates that some portion of bacilli are sporadically multiplying
How is the population of LTBI considered?
heterogenous bacillary population with some more rapidly replicating and some which are metabolically quiescent
How do LTBI arise?
6-8 weeks after infection, granulomas undergo necrosis killing the majority of tubercle bacilli and destruction of host tissue with the small proportion of bacilli surviving in an altered state- LTBI
What suggests the importance of CD4 T cells in anti-TB immunity?
knock out mice have a much greater increase in bacillary numbers and death rate ; also prevent reactivation as depletion 6 months after infection results in rapid reactivation
What suggests that the role of CD4 cells in their protection is independent of IFNy and macropahge activation?
CD4 depletion did not result in diminshed expression of IFny or iNOS activity
What suggests that protein kinase G is important in Mycobacterial pathogenicitiyu?
it is the only soluble kinase maintained in all pathogenic mycobacteria including mycobacterium leprae- which is considered to have the minimal genes required for pathogenicity- without tey show attenuated phenotype, cannot resist lysosomal transfer and are rapidly degraded
How is protein kinase G thought to work?
phosphorylates a host molecule therby preventing the actiivty of the host factor in carrying out phagosome-lysosome fusion
Why is protein kinase G an attractive target for inhibition?
this wouldn’t be interfering with mycoabcterial physiology but allowing macrophage to carry out its innate antimicrobail activity; its secreted so drug does not need to get through teh impermeable mycobacterial cell wall
