Lectures 4-6

Question 1

What are the mechanisms of endocytosis in professional APCs?

Answer 1

macropinocytosis, phagocytosis, receptor-mediated endocytosis, pinocytosis and autophagocytosis

Question 2

Which methods of endocytosis are mediated by actin and which are mediated by clathrin?

Answer 2

actin: macropinocytosis and phagocytosis
clathrin: receptor-mediated endocytosis and pinocytosis

Question 3

Which methods of endocytosis are triggered by a surface receptor?

Answer 3

phagocytosis and receptor-mediated endocytosis

Question 4

What are the most important mechanisms of endocytosis for MHC II presentation?

Answer 4

macropinocytosis, phagocytosis and receptor-mediated endocytosis

Question 5

What are examples of receptors which trigger phagocytosis?

Answer 5

PRRs, FcRs, CRs and apoptotic corpse receptors

Question 6

What do PRRs recognise?

Answer 6

structures found only on the surface of microbes

Question 7

What do apoptotic corpse receptors recognise?

Answer 7

“eat me” ligands

Question 8

How are dying cells removed from the body?

Answer 8

release of “find me” signals by dying cells + shifting the balance of “eat me” and “don’t eat me” signals (efferocytosis)

Question 9

What is an example of an “eat me” signal?

Answer 9

increased exposure of phosphatidyl serine on the extracellular side (molecule which is highly exposed on the cytosolic side)
this occurs when flippases are downregulated during apoptosis resulting in increased PS exposure to the extracellular environment

Question 10

What is exposed PS recognised by?

Answer 10

phagocytes

Question 11

What is defective efferocytosis associated with?

Answer 11

sustained inflammation (because damaged tissue is not cleared properly), autoimmunity and cancer

Question 12

What does autophagy consist of?

Answer 12

the engulfment of cytosolic material by a membrane, forming an autophagosome -> accesses the endocytic route

Question 13

Why is autophagy mostly used?

Answer 13

to allow digestion of the autophagocytosed material by the hydrolytic system of lysosomes

Question 14

What are the steps in autophagy?

Answer 14

a double-membrane structure, the phagophore, grows around cytosolic material -> a double-membrane vesicle forms, the autophagosome -> endosomes and lysosomes fuse with the autophagosome to generate autolysosomes -> autolysosome content is degraded and degradation products transported to the cytosol for recycling

Question 15

What material do B lymphocytes present?

Answer 15

only present material captured by the surface receptor

Question 16

What happens once B lymphocytes have presented material captured by the surface receptor?

Answer 16

a T lymphocyte will recognise the material as foreign and become activated -> activation of the B lymphocyte to differentiate into an antibody producing cell

Question 17

If it is good for B lymphocytes to present antigen, why don’t all B lymphocytes present as dendritic cells do (anything from the extracellular environment)?

Answer 17

it will become activated and the T lymphocyte will activate the B lymphocyte leading to the production of antibodies that are useless (very inefficient)
this is why B cells have restricted the antigen presenting capacity to material that they can recognise with their surface receptor

Question 18

How are endocytosed proteins denatured in endosomal compartments?

Answer 18

acidification (H+-pump ATPases) and reduction of S-S bonds (GILT)

Question 19

Why does MHC II associate with li?

Answer 19

to prevent binding of ER components

Question 20

What is the role of the “molecular code” carried by li?

Answer 20

“tags” MHC II-li for transport to the endocytic route; the complexes would otherwise traffic directly to the PM (like MHC I molecules)

Question 21

What is CLIP known as and why?

Answer 21

a promiscuous peptide because it fits in the peptide binding site of all MHC II allotypes

Question 22

What are the steps which lead to substitution of antigenic peptides for CLIP?

Answer 22

a yet unknown protease cleaves the “zipper” region of li to generate lip10, releasing three aB dimers, each associated with one li-p10 fragment -> cathepsin S cleaves li-p10 at the edge of the peptide binding site, generating aB-CLIP -> the chaperone HLA-DM interacts with aB-CLIP inducing an “open” conformation in the aB dimer -> CLIP is substituted by an antigenic peptide

Question 23

What is the role of HLA-DM?

Answer 23

subjects the MHC II-peptide complexes to editing so that only complexes carrying peptides that confer a minimum stability leave the endosomes

Question 24

Why do we have dendritic cells?

Answer 24

because T lymphocytes cannot scan the whole body searching for pathogens

Question 25

What are dendritic cells that have not seen pathogen dedicated to?

Answer 25

sample their environment by endocytosis and detect pathogens or tissue damage (immature DC)

Question 26

What are dendritic cells that have seen pathogen dedicated to?

Answer 26

present antigens captured in their immune state and inform about type and location of pathogen (activated DC)

Question 27

Why don't dendritic cells present antigen after they have undergone maturation?

Answer 27

there is a decrease in the rate of synthesis of MHC class II molecules and a decrease in the ubiquitination of molecules that go to the surface (MARCH 1 is downregulated) -> provides a frozen snapshot of the material that was present when the dendritic cell was activated

Question 28

What would happen if a dendritic cell first encounters the adjuvant of the vaccine and then there was activation and then it encounters the antigen of the vaccine?

Answer 28

this activated dendritic cell has downregulated MHC class II and downregulated endocytosis so now it would not be able to present the antigen

Question 29

What happens if you link the antigen and the adjuvant together in a vaccine?

Answer 29

you ensure that every dendritic cell captures both at the same time so that you will get efficient presentation with every dendritic cell that encounters the vaccine

Question 30

What happens during malaria as an example of having too much activation of dendritic cells?

Answer 30

the parasite multiplies and induces systemic inflammation which leads to all of the dendritic cells in the body becoming activated (therefore unable to inform the immune system because they will not present the antigen from the infection that happens after undergoing systematic activation)

Question 31

Why don’t we want all cells to cross-present?

Answer 31

the T cells that you activate to kill the virally infected cells will kill not only the cells the cells that are infected with the virus, but every cell that is cross-presenting (needs to be restricted to certain cells)