Lec 3: Phagocytosis

Question 1

What are some clinical signs of SLE?

Answer 1

• synovitis, dermal reactions, oral erosions and ulcers, myositis, neuritis, meningitis, arteritis, myelopathy, pleuritis

Question 2

What are 2 examples of prion diseases?

Answer 2

BSE and CWD

Question 3

What are 3 different ways to group immune system responses?

Answer 3

innate vs adaptive
humoral vs cell mediated
local vs systemic

Question 4

How is phagocytosis a prototypical innate defence system?

Answer 4

• essential feature of immune system
• after physical barriers, come in direct contact with immune cells
• phagocytosis broadly used across evolution

Question 5

What are some phagocytes in tissue?

Answer 5

dendritic cells, microglia, macrophages, Kupffer cells

Question 6

What are some phagocytes in blood?

Answer 6

• neutrophils

- monocytes

Question 7

Who created the phagocytic theory?

Answer 7

Elie Metchnikoff

Question 8

How can phagocytosis be pro or anti inflammatory?

Answer 8

• phagocyte + pathogen = pro inflammatory

- phagocyte + apoptotic body = anti inflammatory

Question 9

What was phagocytosis originally developed for?

Answer 9

• control of normal cellular turnover

- dates back to biofilms

Question 10

Is phagocytosis highly conserved?

Answer 10

• yes, conserved over millions of years of evolution

- also highly integrated into innate and adaptive arms of immunity

Question 11

Why do mammals have the most phagocyte differentiation?

Answer 11

• the most evolutionarily advanced

Question 12

What does phagocytosis serve as a platform mechanism for?

Answer 12

• antimicrobial killing and degradation
• host cell turnover during inflammation
• immunomodulation
• antigen presentation

Question 13

How do different phagocytes use this mechanism differently?

Answer 13

• macrophages good at phagolysosome degradation
• dendritic cells good at antigen presentation
• neutrophils release cytotoxic molecules

Question 14

What are considered professional phagocytes?

Answer 14

• monocytes, macrophages, immature dendritic cells, neutrophils

Question 15

What are some examples of paraprofessional phagocytes?

Answer 15

• retinal epithelial cells (eat dying rod cells)
• thyroid and bladder epithelial cells (eat erythrocytes)
• fibroblasts, platelets

Question 16

What was one experiment that showed that most cells have the machinery to phagocytose?

Answer 16

• fibroblast cells were able to stably express a phagocytosis receptor

Question 17

What are the 7 steps in phagocytosis?

Answer 17

1. chemotaxis
2. adherence: receptor binding and recognition
3. receptor activation: cross linking and overcoming threshold of activation
4. induction of phagocytic signalling cascades
5. pseudopod formation and internalization
6. initiation of intracellular degradative mechanisms, killing responses (NO; RB) and release of pro inflammatory mediators
7. digestion and release of degradation products

Question 18

Give an example of an amoeba using phagocytosis for not immune purposes?

Answer 18

• particles (bacteria) from extracellular milieu serve as essential nutrient source

Question 19

What are 4 survival strategies used by intracellular pathogens?

Answer 19

1. prevent fusion of lysosomes to the phagosome
2. escape from phagosome before fusion of lysosomes (i.e. intracytoplasmic)
3. “tough it out” within phagolysosome
4. hijack other receptors and hang out in other place within cell without phagosome formation

Question 20

Define phagocytosis.

Answer 20

Internalization of particles that are bigger than 1 micron in diameter. Receptor mediated; clathrin-independent. e.g. bacteria apoptotic cells. Involved in immunity, tissue turnover and repair

Question 21

Define endocytosis.

Answer 21

Internalization of particles that are smaller than 1 micron in diameter. Receptor mediated; clathrin-dependent. e.g. viruses, small immune complexes. Involved in down regulation of surface receptors, nutrient uptake and synaptic vesicle recycling

Question 22

Define pinocytosis.

Answer 22

“Cell drinking” - non receptor mediated (non specific); clathrin dependent. Involved in uptake of fluid and soluble molecules from extracellular environment

Question 23

How does the innate immune system detect foreign invaders?

Answer 23

• germline encoded (not adaptive)
• limited number of receptors
• target motifs that are not found in eukaryotes and that have essential role for biology of pathogen
• these are “PRRs” that detect “PAMPs)

Question 24

What kind of receptors do phagocytes contain?

Answer 24

• a number are involved in innate immunity (TLRs)

- only a fraction of them are phagocytic receptors

Question 25

What are the 2 types of phagocytic receptors? Give examples.

Answer 25

- direct recognition (e.g. scavenger or mannose receptors) | - indirect recognition (e.g. antibodies, complement, focolins and collectins)

Question 26

Define affinity.

Answer 26

Strength of a non covalent binding interactions; the higher the affinity the higher the likelihood two partners will exist in a complex

Question 27

Define avidity.

Answer 27

Increased 'apparent' affinity of a molecule for its ligand due to the presence of multiple binding sites on both partners

Question 28

IgG is always present in serum. How is this relevant?

Answer 28

- only want it activated when there is lots of pathogen (threshold activation) - affinity and avidity allows you to define the threshold

Question 29

Describe how the phagolysosome is formed.

Answer 29

- once target is bound and activation signals have been provided, the phagosome must be taken into cell - requires actin rearrangements, myosin interaction with actin (stimulates entry) and dynamin (helps contract and pinch off the membrane) - phagosome fuses with primary, secondary granules and lysosomes forming destructive phagolysosome

Question 30

What does the phagolysosome of neutrophils contain?

Answer 30

- antimicrobial peptides - lysosomal enzymes (e.g. cathepsin G, elastase) - reactive oxygen and nitrogen compounds

Question 31

What is different during phagolysosome formation of macrophages and neutrophils?

Answer 31

- primary and secondary granules do not appear to be present

Question 32

Describe reactive oxygen intermediates.

Answer 32

- produced in the phagolysosome by NADPG oxidase - quite rapid - NADPH oxidase complex consists of membrane and cytosolic components which assemble in response to phagocytic signal

Question 33

Describe reactive nitrogen intermediates and give an example.

Answer 33

- e.g. nitric oxide - produced in the cytoplasm by the inducible nitric oxide synthase (iNOS) and then diffuses into the phagosome to react with internalized microbes - requires transcriptional induction of iNOS gene expression (e.g. in response to IFN-y and LPS in macrophages) - slower and long lasting

Question 34

How is apoptosis anti inflammatory?

Answer 34

- activation of specific proteases and nucleases leads to death characterized by chromatin condensation, protein and DNA degradation, loss of plasma membrane lipid asymmetry and disintegration of the cell into membrane bound fragments - cell death through apoptosis will contain its contents to prevent inflammation - phagocytosis results in release of anti inflammatory mediators including TGD-b, IL10 and prostaglandin E2

Question 35

Describe how Kupffer cells and splenic macrophages remove senescent erythrocytes from the liver and spleen.

Answer 35

- CD47 is receptor expressed on healthy erythrocytes - CD47 binds to SIRP-1a on macrophage surface - CD-47 binding increases the threshold of activation needed to induce phagocytosis - live erythrocytes are not removed prematurely

Question 36

How does autoimmune hemolytic anemia (AIHA) occur?

Answer 36

- reduction of CD47-SIRPa inhibitory signalling to splenic macrophages

Question 37

How does phagocytosis relate to SLE?

Answer 37

- SLE characterized by formation of autoantibodies to self antigens primarily against nucleic acids (may also target RBCs, platelets, lymphocytes, clotting factors, immunoglobulin) - immune complexes activates complement and attracts neutrophils - enzymes released from neutrophils cause damage to endothelial cells of basement membrane

Question 38

How does phagocytosis relate to prion diseases?

Answer 38

- high beta sheet formation of prion protein harder to digest as a phagocyte - translocation of infectious prion proteins across intestinal epithelium - protein will enter secondary lymphoid organs from gut and then go to brain