Tuberculosis

Question 1

What are the features of TB disease?

Answer 1

• Common as chronic pneumonia
• Caused by Mycobacterium tuberculosis
• Can affect sites other than lungs
• Deceptive onset
• Persistent productive cough
• Fever, night sweats, weight loss
• Lungs most commonly infected (pulmonary TB)

Question 2

What’s the difference between TB infection and TB disease?

Answer 2

• Infection/Latent TB

o Immune system controls disease

• Disease/Symptoms of TB

o Bacteria escapes immune response

Question 3

What is the significance of TB?

Answer 3

• 2nd most common single infectious cause of death (first= HIV)
• 1/3 world infected
• 1.4 million deaths 2010
• Multidrug resistance (1st line drugs: INH, RIF and additional drugs)and completely drug resistant strains

Question 4

What issues arise for AIDs patients when it comes to TB?

Answer 4

• Co-infections HIV and M. tuberculosis big issue (developing countries)

o Reactivate latent infection, rapid progression of new infection

• AIDs patients likely to get disseminated disease (lack of CD4 T cells/cell mediated immunity)
• AIDs patients susceptible to opportunistic infection (MAC)

Question 5

What is MAC?

Answer 5

• MAC = mycobacterium avium complex
• From the environment (water, soil), colonises GIT and invades deeper tissue

Question 6

What are the characteristic of mycobacteria?

Answer 6

• Often environmental, harmless species
• Pathogens = M. tuberculosis, MAC, M. Bovis
• Cell wall prevents gram staining
• Cell wall acid fast

Question 7

What are the features of M. tuberculosis as bacteria?

Answer 7

• Obligate aerobe
• Acid fast
• Resistant to drying
• But sensitive to heat
• Grows very slow (use diagnostics other than culture, long time to determine antimicrobial susceptibility and treat)
• Survive in host macrophages

Question 8

What is special about mycobacteria cell walls?

Answer 8

• Cell wall acid fast
• Other molecules outside peptidoglycan layer
• Lipo-arabinomannam: superficial lipids, mycolic acids, arabinogalactam
• Extra cell wall components can help modulate host immune response
• Retain basic dye

Question 9

What does acid fast mean? What staining procedures are used for these organisms?

Answer 9

• Retain basic dye
• Ziel Neilsen stain
• 1. Use strong basic dye and apply with heat (10mins)-everything pink
• 2. Decolourise smear with acid alcohol- goes colourless but mycobacteria bacilli retain pink dye
• 3. Counterstain with blue dye to stain everything else on slide

Question 10

How can TB be transmitted?

Answer 10

• Inhale nuclei droplets from aerosols

Question 11

What sizes of droplet nuclei are most efficient for spreading TB? Why?

Answer 11

• Ideal to be <5microns because stay suspended longer and penetrate into alveoli
• If larger than 5 microns, settle rapidly or removed by cilia and mucus
• Evade mucociliary elevator of large airways, enter alveoli, enter macrophages

Question 12

What usually happens when LRT pathogens interact with the innate immune system?

Answer 12

• Microbes access lungs, alveolar macrophages
• Bind multiple receptors, activate bactericidal processes
• Cytokines, chemokines
• Microbe removed, adaptive system may help

Question 13

What happens when M. tuberculosis interacts with the innate immune system?

Answer 13

• Inhale nuclei droplet
• Go to alveolar macrophages
• Binding to receptors = poor activation of phagosome maturation and bactericidal processes
• Lysosome fusion inhibited by lipids on mycobacteria
• Phagosome pH maintained, bacteria can replicate inside
• Limited degradation, but enough to activate CD4 T cells
• Macrophage release (symptoms)

o IL-1

o IL-8

o IL 12 – TH1 cells induced

o TNF α

• Mycobacterium survive and replicates

Question 14

What occurs in the lymphoid tissue due to M. tuberculosis?

Answer 14

• APCS display M. tuberculosis peptides, induce TH1 (IL-12) cells to secret IFNγ
• Macrophages are activated and results in inflammation and tissue damage
• TH17 induced too for neutrophil recruitment (but neutrophils can’t kill)
• Mycobacterium resistant to macrophage killing so TH1 and TH17 cells continuously made

Question 15

What is the cycle that occurs following inhalation of TB?

Answer 15

• 1. Inhale TB laden droplet nuclei
• 2. Multiplication in alveolar macrophages
• 3. Cytokine and chemokine production, monocyte influx, limited degradation
• 4. Macrophage and DC migrate to hilar LN, present mycobacteria antigens to T cells
• 5. T cell activation
• 6. Activated CD8, TH1, CD4, other IFNγ secreting cells migrate to infection site and further activate infected macrophages, recruit neutrophils (back to step 3)
• 7. Mycobacteria persist – prolonged infection

o Delayed type hypersensitivity and granuloma formation

Question 16

What occurs due to IFNγ activation of macrophages?

Answer 16

• IFNγ activate macrophage, produce IL-1, IL-8, TNFα (activate endothelium, phagocytes and lymphocytes migrate from circulation)
• More IL-1 (fever)
• More TNFα (weight loss, granuloma formation, death of some infected macrophages)
• GRANULOMA

Question 17

What makes up a granuloma?

Answer 17

• Accumulation of multinucleate giant cell (macrophages fuse), surrounded by epitheliod cells, surrounded by T lymphocytes
• Contains organism in 90% infections

Question 18

What are the features of latent TB?

Answer 18

• 90% of infected have controlled infection due to delayed type hypersensitivity response
• No symptoms and non-infectious

Question 19

What are primary and secondary TB? What increases the risk of getting these?

Answer 19

• Primary TB

o 5% patients don’t control initial infection

o Worsening pneumonia

o Possible dissemination to other organs

• Secondary TB

o Infection controlled by 5-10% have reactivation of latent disease

• Compromised immune system more susceptible to primary or secondary TB

Question 20

How does secondary TB arise?

Answer 20

• Reactivation commonly in lung (bacteria replicate there)

o Caseous necrosis

o Cavity formation

• Granulomas enlarge, increased immune response, excess cytokines, tissue damage, TB symptoms
• Lots of sputum with acid fast bacilli
• Contagious patient

Question 21

How can we screen for M. tuberculosis? What are the features of the processes?

Answer 21

• Doesn’t indicate immunity or disease
• Tuberculin Skin Test/Mantoux Test

o Inject purified protein derivative (PPD) (tuberculin)

o Memory T cells migrate to injection site, induce inflammation, show delayed hypersensitivity reaction

• Interferon production in response to in vitro stimulation with specific TB antigens (IGRA)

o Collect blood and add to culture medium o Add antigens

o Incubate

o Test for IFNγ secreted into medium by activated memory T cells activated by the PPD

Question 22

How can active TB disease be diagnosed in the lab?

Answer 22

• X ray
• Acid fast bacilli
• Culture on enriched medium

o Solid 4-8 weeks

o Specialised liquid 7-14 days

• Unreliable antibody detection
• New nucleic acid tests

o Cartridge based automated

o PCR and identifies m. tuberculosis DNA (presence) and resistance to Rifampicin

Question 23

What is short course therapy? What kind of TB disease is it used for?

Answer 23

• For active disease
• 6 months
• First Line:

o Rifampicin

o Isoniazid (INH)

o Pyrazinamide (PZA)

o Ethambutol

o RIPE

• All four for 2 months then R and I for 4months

Question 24

What is special about INH and PZA?

Answer 24

• INH and PZA = pro-drugs
• Activated by mycobacterial enzymes (KatG, Pyrazinamidase)

Question 25

How can latent TB be treated?

Answer 25

• If positive and no evidence of active diseases: recently infected or latent disease
• Use INH to reduce reactivation
• Chemoprophylaxis of latent TB reduced incidence of progression to active disease

Question 26

What are the features of the TB vaccine?

Answer 26

• Long term M. bovis culture
• 100% conversion to tuberculin positive in vaccinees
• Variable protection
• Live attenuated dangerous if immunocompromised
• Current recommendation- use BCG where TB incidence high or in young