L20 intracellular pathogens and TB

Question 1

Who gets Tuberculosis? What causes Tuberculosis?

Answer 1

indiscriminate of age, sex, race or social class
mycobacterium tuberculosis

Question 2

How many drugs in treatment and for how long?

Answer 2

2-4 drugs for 6-12 months.

Question 3

Where does TB live?

Answer 3

It’s an intracellular pathogen that lives in vacuoles.

Question 4

At what speed does it grow?

Answer 4

It’s slow growing; 12-24 hours replication.

Question 5

What 3 key innate killing mechanisms used by phagocytes does TB inhibit?

Answer 5

• ROS radical generation
• Reactive NO radical generation
• Phagosome fusion with lysosomes

Question 6

What are TB clinical symptoms?

Answer 6

Breathlessness, chest pains, loss of appetite and weight, persistent cough, night sweats, tiredness

Question 7

What do macrophages need to enhance their activation?

Answer 7

Th1 cells enhance macrophage function through their activation. Phagolysosome fusion is more efficient after activation. But we are not very good at killing TB as 30% of global pop is latently infected.

Question 8

What % of the global population is latently infected?

Answer 8

30%

Question 9

How does the immune system quarantine TB in the lungs?

Answer 9

Lung bacteria are restricted to granulomas. Sometimes granulomas collapse and become caseous - mass of dead macrophages with semi solid pus. A granuloma consists of macrophages that fuse to form multi-nucleated giant cells with engulfed bacteria, surrounded by T cells all around.

Question 10

What happens when granulomas leak?

Answer 10

This enables bacteria to spread. Granulomas grow until they erode into an air pocket, blood vessels or another granuloma. This leads to the release of millions of bacteria.

Question 11

What are the conditions like in granulomas?

Answer 11

anaerobic, lack of blood supply, cytokines include IFNγ, TNFα, macrophages and mycobacteria. T cell on outside surround it. Infectious granulomas have lymphocytes within and around macrophage mass. CD4 try and activate and CD8 try and kill. There is a presence of cell necrosis in central areas. If the infection can be contained, fibroblasts and collagen layer form around the granuloma isolating it from the rest of the body. But this result in loss of organ function. This can get calcified seen in X-rays.

Question 12

What do we have to prevent TB?

Answer 12

BCG vaccine. 70-80% effective against most severe forms of TB e.g. TB meningitidis. Less good at preventing respiratory disease. Its a live vaccine so cant give to immunodeficient patients or immunosuppressed.

Question 13

What influences susceptibility to TB?

Answer 13

HIV, malnutrition, close living, stress, diabetes, alcohol/drugs and immunosuppression.
Genetic predisposition: CD4 T cells defects SCID, TH1 cytokine receptors defects in IFNγ, IL-12/23 defects

Question 14

How does TB spread?

Answer 14

Sputum - coughing.
Within the individual it can spread from one are area of lungs to other by coughing.
Tight living conditions

Question 15

What happens after exposure to TB?

Answer 15

70% no infection
30% infected. Out of those infected: >90% get adequate host immune response and disease containment. 5-10% get primary disease.
Out of the ones that contain it 10% get reactivation. 90% get chronic latent infection.

Question 16

What kind of bacterium is mycobacterium TB?

Answer 16

Bacilli, gram positive

Question 17

What’s up with TB drug resistance?

Answer 17

It’s increasing

Question 18

Who tends to get TB?

Answer 18

Higher rates of TB with HIV coinfection.

Question 19

Where do most cases occur?

Answer 19

In resource limited countries

Question 20

How many cases and deaths per year?

Answer 20

8 million cases per year

1.4 million deaths per year

Question 21

Where do intracellular pathogens live in cells?

Answer 21

Vacuoles or cytoplasm.

Question 22

What are the advantages and disadvantages of bacteria surviving in phagocytes?

Answer 22

+ intracellular environment provides continuous supply of nutrients to the pathogen, its shielded from extracellular immune system e.g. complement, abs, drugs

-need to overcome innate antimicrobial killing mechanisms of the phagocyte.

Question 23

What can be done about a pathogen that can survive inside a phagocyte?

Answer 23

CD4 - TH1 for vacuole restricted bacteria

CD8 T cells and NK cells for cytoplasmic localised bacteria

Question 24

How do bacteria evade intracellular killing by phagocytes?

Answer 24

Macrophages:

E.g. mycobacterium tuberculosis.
In the pathway from engulfing it to presenting via MHC2, pathogen will replicate in early phagosome and block fusion of early phagosomes with cell lysosomes.

Or bacteria can e.g. listeria monocytogenes
Leave the early endosome after being engulfed and escape into the cytosol

Question 25

What are some innate mechanisms for intracellular microbial pathogen killing?

Answer 25

acidification, toxic oxygen derived products e.g. ROS, hydrogen peroxide, NO radicals, antimicrobial peptides, enzymes like lysozyme and competitors e.g. lactoferrin (binds Fe)

Question 26

What T cell type is important in intracellular pathogen killing?

Answer 26

Th1-cell mediated immunity

Question 27

How are CD4 Th1 cells helping macrophages kill intracellular pathogens?

Answer 27

IFNγ and CD40 ligand–>activates macrophage to destroy engulfed bacteria.
Fas ligand–> kills chronically infected cells released bacteria to be destroyed by fresh macrophages
IL-2 –>induces T cell proliferation, increasing numbers of effector cells
TNFα - activates endotheliuum to induce macrophage binding and exit from blood vessel to site of infection

Question 28

What happens if you don’t have T cells in terms of bacterial infections?

Answer 28

No Th1 cells = no bacterial clearance.

Question 29

Describe the journey of a CD4 Th1 cell to the infected macrophage.

Answer 29

CD4 Th1 cells primed by dendritic cells in lymph node/spleen through APCs that have a lot of MHC2. They recognise that a cell is infected through MHC2 presented as processed peptides from the intracellular pathogen. This leads to proliferation and differentiation of T cell to acquire effector function. Primed Th1 seek out infected cells, recognise complex of bacterial peptide with MHC2 and activates macrophage. Their activation enhances macrophage killing.

Question 30

How do macrophages respond to Th1 activation?

Answer 30

They increase expression of MHC1 and MHC2, CD40 and CD80,86 amplification.
Fas expression (apoptosis)
Increase in TNF and TNFRs promotes NO production and apoptosis.
Lysosome numbers increase, ROS and factors that promote phagolysosome maturation.
macrophage doubles in size

Question 31

What happens to bacteria in cytosol?

Answer 31

Antigen presented through MHC1, CD8 will kill this cell; independent of where infecting bacterium resides in the cell; vacuole or cytosol.

Question 32

Which cytokines are important for T cell activation and differentiation into effectors?

Answer 32

IL-12

IL-18

Question 33

What does IFNγ do in this context?

Answer 33

IFNγ release by cytotoxic T cells and CD4 cells. Which promotes macrophage activation, increasing killing and APC capacity. This response is enhanced by CD40/CD40L stimulation.

Question 34

How do B and T cells work together at site of intracellular pathogen infection?

Answer 34

CD8 secrete IFNγ helping activate macrophages and CD4 secrete IL2 which promote local CD8 proliferation and survival.

Question 35

What are granulomas?

Answer 35

result from chronic inflammatory reaction as final attempt of immune system to try and isolate and wall off chronic infection.