L04: Cellular mediators of innate immunity 1

Question 1

Clearance of extracellular pathogens

Answer 1

1. Neutrophil, eosinophil, basophil, mast cell
2. Granulocytes/polymorphonuclear leukocytes: neutrophil, basophil, eosinophil
3. Eosinophils, basophils and mast cells destroy large extracelular pathogens, activated by cross-linking of high affinity IgE receptors and release toxic proteins, proteases and inflammatory mediators

Question 2

Clearance of intracellular pathogens

Answer 2

1. Macrophage: long lived phagocytic cells with range of anti-microbial pathways, key mediators of inflammatory response, release cytokines and chemokines for inflammation
2. NK cell: clearance of cytoplasmic pathogens and tumour cells, destroy infected cells through detection of infected cells with altered or absent MHC1, activated by cytokines
3. Most cells: mount an anti-viral response by producing type 1 interferon which induce gene expression that enables viral RNA degradation, inhibition of protein translation and thus viral replication

Question 3

Phagocytosis

Answer 3

Specific form on endocytosis in which specialised cells take up particles into membrane-bound compartments called phagosomes which fuse with lysosomes to form phagolysosomes where particles are destroyed

Question 4

Opsonins

Answer 4

Agents which coat particles to enhance the process of phagocytosis (e.g. antibodies and complement compounds). Macrophages have specific opsonic recceptors

Question 5

Macrophages can phagocytose microbes through (2)

Answer 5

1. Non-opsonic phagocytosis: direct recognition
2. Opsonic phagocytosis: indirect recognition
   Macrophages can acidfy their environment to kill pathogens, directly lyse them, produce ROS through the NADPH oxidase complex, use degradative enzymes, use direct lysing peptides, use nutrient starvation strategy to limit growth and phagolysosomes

Question 6

Immediate vs delayed macrophage anti-microbial pathways

Answer 6

Immediate: ROS, proteases, acdification of phagosome
Delayed: NO production, anti-microbial peptide production, nutrient depletion and macrophage cell death

Question 7

ROS/ROI and RNS/RNI

Answer 7

Destroy microbes via:

1. Inactivating key metabolitc enzymes required for microbial survival
2. Destroying protein structure (cysteine oxidation)
3. Cytotoxicity via lipid and DNA oxidation
4. Contributing to solubilisation of cationic proteases delivered to phagosome (indirect effect)

Question 8

Pathogens that survive within macrophages use 1/3 approaches:

Answer 8

1. Subvert trafficking and reside in vesicles
2. Escape the phagosome to survive in cytoplasm
3. Tough it out in the phagolysosome

Question 9

Salmonella

Answer 9

Avoids attack through pathogenicity islands, SPI-1/2 to enable bacterial invasion via epithelial cells and facilitate intramacrophage survival respectively. SPI-1 injected into epithelial cells to trick the epithelium to stimulate micropinocytosis to take up bacterium. SPI-2 causes mislocalisation of antimicrobial functions (iNOS & NADPH oxidase) and so effect mechanisms are diverted

Question 10

Anti-viral response

Answer 10

Virus infected host cells - IFNa/IFNb - induce resistance to viral replication in all cells, increase MHC class 1 expression and antigen presentation in all cells, activate NK cells to kill  virus-infected cells.
Main components: production of type 1 IFNs and activation of NK cells

Question 11

NK cell activating and inhibiting receptors

Answer 11

Activating receptor: associates with adaptor molecules DAP10/DAP12. DAPs contain ITAMs (immunoreceptor tyrosine-based activation motifs); phosporylation of ITAMs recruits kinases and enables cellular response
Inhibitory receptors: contains ITIMs which recruit tyrosine phosphatase (SHP-1) and prevents cellular activation

Question 12

NK cells mediating ADCC

Answer 12

1. Ab binds surface of target cell
2. FcyR3 recognises Ab and NK cell binds
3. Crosslinking of FcyR3 induces activation of NK cells
4. Perforins and granzymes released triggering cell death
5. ADCC