Immune system

Question 1

MALT

Answer 1

• The lymphoid tissue found in submucosal sites around the body
• It is populated by lymphocytes (B cells, T cells and macrophages)
• M cells are also present, which sample antigen from the lumen and deliver it to the lymphoid tissue

Question 2

GALT

Answer 2

• Gut associated lymphoid tissue

- Peyer’s patches are a component of GALT, found in the lining of the small intestine

Question 3

mannose receptor

Answer 3

• It is a pattern recognition receptor present on the surface of macrophages
• It recognises terminal mannose attached to glycans present on the surface of some microorganisms, that play a role in the innate and adaptive immune response
• A number of pathogenic organisms including C.albicans display glycans on their surface with terminal mannose residues that are recognised by the mannose receptors
• Upon recognition, the receptor internalises the bound pathogen and transports it to the lysozymes for degradation via the phagocytic pathway
• They mediate phagocytosis but do not initiate the inflammatory response

Question 4

Tuberculosis

Answer 4

• It is taken up by phagocytosis mediated by mannose receptors which internalise the pathogen once it is recognised
• This facilitates infection by mycobacterium tuberculosis
• The bacteria multiply inside the macrophage, preventing formation of the phagolysosome to avoid degradation
• It is latent and can manifest itself later

Question 5

Pattern recognition receptors

Answer 5

• They are key in innate immune response
• They are proteins expressed by cells of the immune system to identify 2 classes of molecules: pathogen associated molecular patterns (PAMPs), which are associated with microbial pathogens and damage associated molecular patterns (DAMPs), which are associated with components of host’s cells that are released during cell damage

Question 6

Types of PRRs

Answer 6

• Receptor kinases
• Toll-like receptors, they trigger the secretion of cytokines
• C-type lectin receptors, mannose receptors

Question 7

Toll like receptors

Answer 7

• A type of PRR
• Upon activation, TLRs recruit adaptor proteins in the cytosol of the immune cell
• The adaptor proteins initiate downstream cascade that results in the inflammatory response
• They span the membrane

Question 8

NLR

Answer 8

• A type of PRR
• They span the cytoplasm
• If a micorbe evades the phagocytes it will probably be picked up by the NLRs

Question 9

PAMPs

Answer 9

• Molecules associated with groups of pathogens that are recognised by cells of the innate immune system
• They are recognised by PRRs (including TLRs)
• Different molecules can serve as PAMPs including glycans
• PAMPs activate innate immune responses
• Bacterial LPS, flagellin, lipoteichoic acid peptidoglycan are considered to be PAMPs

Question 10

How to bacteria counteract recognition and killing?

Answer 10

• Escape from vacuole into cytoplasm by action of Listerolysin from Listeria monocytogenes
• It releases a cytolysin, it is cholesterol dependent, it does not attack its own membrane as it does not have cholesterol
• It lyses the cell membrane
• The cell membrane of eukaryotic cells has cholesterol

Question 11

Listeria

Answer 11

EXTRACELLULAR
- Whilst it is extracellular it can secrete listeriolysin (LLO), which can damage the membrane

INTRACELLULAR

• If listeria is taken up into a phagosome, it lowers its pH which produces more LLO
• The phagosome ruptures, which allows the cell to become intracellular
• It spreads
• It turns on other genes that recruit actin tails that push it along
• It can push through and infect another cell, it is now in a double membrane bound vacuole
• It produces another phospholipase that breaks down the double membrane

Question 12

Salmonella

Answer 12

• Salmonella use the TLRs to ensure they get a protective niche and then secrete proteins into the cell to prevent further damage
• Salmonella want to be taken up into vacuoles
• The TLRs recognise components on the surface of Salmonella
• It leads to signalling which lowers the pH of the vacuole
• Salmonella has engineered itself to hijack the TLRs to enable it to survive
• When it is in a Salmonella containing vacuole, it secretes effectors proteins (spi) which prevent fusion with the lysosome
• The effectors are only secreted at this pH
• In macrophages that do not have TLRs, the pH is not lowered, so no effector proteins are released and the vacuole fuses with a lysosyme

Question 13

Shigella

• induces cell death
• uses a type III secretion system to produce the effectors that help it escape from macrophages
• Intracellular spread to neighbouring enterocytes

Answer 13

• The shigella goes in through the M cells into a macrophage
• Shigella can escape the macrophage and travel to enterocytes in the gut through intracellular spread
• In order to escape the macrophage it must induce cell death in the macrophage by secreting T3SS effectors
• NLRs are going to recognise parts of the cell that are in the cytoplasm
• By putting these T3SS in the cytoplasm, the NLRs recognise them and tell the cell to rupture
• It allows shigella to escape and enter the neighbouring cells

Question 14

What is autophagy

Answer 14

• An intracellular degradation system that delivers cytoplasmic constituents to the lysosome
• It consists of several steps:

1. Sequestration
2. Transport to lysozymes
3. Degradation
4. Utilisation of degradation products

• Proteins ready to be degraded are tagged by ubiquitin
• It is benfeifical in clearning infectious diseases

Question 15

Autophagy process (a fall back mechanism to phagocytosis)

Answer 15

1. Autophagy induction
2. A cup shaped membrane sac called the isolation membrane starts to form
3. The isolation membrane elongates and closes to form a double membrane vesicle called the autophagosome
4. The outer membrane of the autophagosome fuses with a lysosome to form an autolysosome, leading to the degradation of the cytoplasmic contents inside the autophagosome

• The genes responsible for autophagy (ATG) genes
• The autophagosome can engulf bacteria if they are in the cytoplasm (xenophagy)
• Even before lysis of the vacuole membrane, the isolation membrane can wrap around it
• If the bacteria are able to escape the phagosome, the isolation membrane can wrap around it in a autophagosome and give it to a phagosome

Question 16

What starts the induction of autophagy?

Answer 16

1. Ubiquitin

2. LC3

Question 17

Ubiquitination

Answer 17

It is usually associated with marking proteins for degradation by proteasomes
but it can lead to autophagy

Question 18

Shigella destroyed by autophagy

Answer 18

• In escaping from macrophages they were leaving membrane debris around
• It was a signal for autophagy

Question 19

Adaptive immune system

The trimeric complex

Answer 19

CD8/ CD4 cell + peptide + MHC molecule

• MHC1 molecules present peptides to CD8+ T cells
• MHC11 molecules present peptides to CD4+ T cells
• Peptides from the cytosol are recognised by CD8+ T cells in the context of MHC1 molecules
• There isn’t just a trimeric complex. There are coreceptors and costimulation

Question 20

CD1d

Answer 20

• MHC- like complex

- It presents glycolipids to NK cells

Question 21

MHC1

Answer 21

• They have a B2 molecule attached to them, if the B2 molecule isn’t there, the MHC molecule does not present peptides to CD8 T cells
• CD8 T cells are directly important in controlling TB disease
• Usually MHC1 molecules present anitgens from the cytosol
• CD8+ T cells stimulate cytotoxic T cells
• If Memory T cells see the antigens again, they release perforins, granzymes that will destroy the cell

Question 22

Cytotoxic T cells

MHC1 - CD8+ - cytotoxic T cells

Answer 22

• Two major killing mechanisms: perforin and the CD95 pathway, granzymes through perforin or the apoptic pathway through CD95
• Perforin delivered from the cytoplasmic granules that fuse with the membrane of APC forming pores to allow the granzymes to enter the infected cell
  1. On degranulation, perforin binds to the target cell’s plasma membrane and forms pores on the target membrane
  2. The pore formed allows for the passive diffusion of granzymes into the target cell
• CD95 pathway
  It is a death receptor on the surface of cells that leads to programmed cell death
• Both pathways lead to cell apoptosis

Question 23

CD4+ T cells

Answer 23

• No B2 molecule
• They present antigens originating from the phagosome
• The peptides presented are longer than those by MHC1
• Do not to go through the phagolysosome fusion to generate the cellular debris for it to be presented
• They are T helper cells
• They produce different cytokines dependent on how they have been activated

Question 24

TB

Answer 24

INNATE GRANULOMA
TB is taken up by alveolar macrophages (innate)

IMMUNE GRANULOMA
The innate cells that have taken up the bacteria have produced cytokines that will have recruited more cells to the site of infection (T cells)

CHRONIC GRANULOMA
The bacteria wants to be walled off
Fibroblasts, macrophages, T cells and a middle of caseum
B cells are around the edge

How does TB spread?
- Breakdown of the caseus granuloma, liquefaction

Question 25

Antibodies

Answer 25

- They are produced by B cells | - They can recognise surface structures of the bacteria and facilitate opsonization of the bacteria into macrophages

Question 26

TB in infection

Answer 26

- It is causing alveolar macrophages to produce cytokines - They recruit more immune cells - TNFa is the prototypical proinflammatory cytokine produced by phagocytic cells

Question 27

Bioterrorism

Answer 27

- The deliberate dispersal of infective or toxic agents to kill or incapacitate man and crops - Used against civilian populations

Question 28

Biological warfare

Answer 28

- Used in war | - Usually involves nation states

Question 29

Biothreat agents

Answer 29

- Virus: smallpox, ebola, influenza + FMDV - Bacteria: anthrax + plague - Fungi: rice blast fungus - Toxin: ricin toxin

Question 30

Biological weapons | Smallpox

Answer 30

- Virus - Transmissable - Mortality 30-50% - ID = 100

Question 31

Biological weapons | Plague

Answer 31

- Caused by the bacterium Yersinia pestisis - Transmissable - ID= 1000

Question 32

Properties of biological weapons

Answer 32

- Cheap - Easy to produce - Potent - Difficult to attribute

Question 33

Produce biological weapons

Answer 33

- Many bacteria are simple to culture - Viruses are more difficult - Source and properties of the strain are a key issue