Lecture 14; Antigen Processing and presentation

Question 1

Describe MHC class 1 target and outcomes;

Answer 1

Endogenous pathway

Intracellular pathogens, activates CD8,

Expresses 8-10AA sequences

Predominantly viral

Question 2

Describe MHC class 2 target and outcomes;

Answer 2

Exogenous pathway

Extracellular pathogens, any length but usually 8-30AA

Activates CD4

Bacterial, parasitic and toxins

Question 3

What pathway is MHC1 restriction?

Answer 3

Endogenous pathway

Question 4

What pathway is MHC2 restriction?

Answer 4

Exogenous pathway

Question 5

Whats the difference in recognition between b and t cells?

Answer 5

B cells recognise epitopes i.e conformational or linear

T cells recognise linear epitopes only, therefore antigen must be presented on MHC molecules. TcR and CD co-detection.

Question 6

Describe what pathogen antigens are presented in class one MHC (endogenous pathway)

Answer 6

• Antigen peptides are generated from (viral) proteins produced by the presenting cell

Question 7

Describe how antigens are presented in MHC class one;

Answer 7

• Generated antigen peptides from pathogen
• Peptides processed in cytosol
• Transported to ER and loaded on MHC 1
• MHC1+peptide is transported through golgi complex to the cell surface via the secretory pathway

(CD8 activation and cell death)

Question 8

How are antigen peptide fragments processed in the endogenous pathway?

Answer 8

• Proteins are degraded by immunoproteasome

Question 9

What does a regular proteasome do?

Answer 9

• Difunctional ribosome products
• Non-functional and potentially toxic proteins
• Proteins synthesised in excess
• Regulatory proteins

About 1% of the peptide pool can bind to MHC class 1

Question 10

How is the immuno proteosome activated?

Answer 10

P28 causes the N terminal tails of the alpha sub units to flip upwards, thereby facilitating substrate entry and product exit.

Question 11

What are some features of the immunoproteasome?

Answer 11

• Does not completely replace constitutive proteasome

- Considerably shorter half life

Question 12

What does the immunoproteasome do?

Answer 12

It has an altered cleavage site preference with a strong preference to cleave behind residues that represent correct C terminus anchors for MHC 1

PA28 simply enhances the Hz of usage of the minor cleavage sites to provide more peptides suitable for MHC1 presentation

Question 13

What are trim peptidases?

Answer 13

• Proteases that trim the products of proteosomes that are too large for presentation

But major function is probably peptide degredation and AA recycling

Question 14

What aids peptide loading onto MHC 1?

Answer 14

Tapasin

Question 15

Describe tapasin;

Answer 15

Quality control regulator

• 48kDa protein

Question 16

What is the function of tapasin?

Answer 16

• Stabilises TAP1/TAP2, enhancing peptide transport (transports peptides from cytosol into ER so it can be loaded onto MHC before it is transported to membrane)
• Bridges MHC class 1 to TAP (structural component)
• Facilitates peptide loading
• Stabilises empty peptide-receptive MHC

= Optimises peptide repertoire

= Ensures quality peptides with strong affinity

Question 17

Describe the exogenous pathway;

Answer 17

• Antigen peptides are generated from proteins engulfed by professional antigen APC
• Peptides are processed in the endosomal compartment
• MHC class 2 presents to CD4
• Range of responses

Question 18

Give an overview of the exogenous pathway processing;

Answer 18

1) MHC assembly and transport to peptide loading compartment
2) Uptake and processing of exogenous antigen
3) Peptide loading (CLIP exchange)

Question 19

What is required for MHC 2 assembly?

Answer 19

Invariant chain (seperate protein)

Question 20

Describe the invariant chain structure;

Answer 20

Seperate domains of chain;

• short N-terminal cytosolic domain (sorting motif)
• single TM domain
• class II-associated invariant chain peptide (CLIP)
• C-terminal trimerisation motif

Question 21

What is the function of the invariant chain?

Answer 21

• Scaffold to ensure proper folding + assembly of MHC class 2
• Blocks premature class 2 peptide association
• Direct trafficking of MHC 2 invariant chain to endosomal pathway

Question 22

Describe the uptake of exogenous antigen;

Answer 22

Endocytosis: Uptake of material into the cell by the formation of a membrane-bound vesicle.

Endosome: endocytotic vesicle derived from the plasma membrane. (where the endocytotic vessel ends up)

Question 23

What are the types of endocytosis?

Answer 23

1. Receptor-mediated endocytosis: mannose and lectin-like receptors
2. Macropinocytosis: uptake of fluid-filled vesicles (mainly DC)
3. Phagocytosis: uptake of complete cells

Question 24

Describe the antigen processing that occurs in the exogenous pathway;

Answer 24

• Low pH of endosome degrades proteins (proton pump)

- Fusion of endosome with lysosome supplies proteases that are activated by low pH and degrade proteins

Question 25

How is antigen loaded?

Answer 25

In the endosome,

CLIP protein is excised by HLA-DM which stablises MHC2

Proteases then degrade CLIP and Invariant chain

HLA-DM helps peptide binding and dissociated as MHC2 is expressed.

HLA-DM has a similar function as tapasin in the endogenous pathway