Lecture 16

Question 1

What is the difference in the source of peptides between each MHC class?

Answer 1

Class I presents peptides from inside the cell while class II presents peptides from outside the cell

Question 2

What is the difference in the size of peptides between each MHC class?

Answer 2

Class I presents peptides 8-10 amino acids while class II can present peptides of any length but typically limits itself to 8-30 amino acids

Question 3

What is the difference in the T cell response to each MHC class?

Answer 3

Class I is responded to by Cytotoxic CD8 T cells which lead to killing of the infected cell
Class II is responded to by helper CD4 T cells which will lead to activation of T cells, B cells and macrophages as well as triggering Ig isotype switching occurs

Question 4

What is the difference in the type of infection resolved by each MHC class?

Answer 4

Class I deals with typically viral infection while Class II deals with bacterial, parasitic and toxins

Question 5

What are the two different pathways for antigen presentation?

Answer 5

The endogenous pathway seen in Class I MHC and the exogenous pathway seen in class II MHC with the key difference being the source of antigen

Question 6

How much of the peptide pool is available to the immune system?

Answer 6

About 1/3 of intracellular proteolysis in mammalian cells is directed to nascent proteins such as defective ribosomal products, non-functional and potentially toxic proteins, proteins synthesized in excess and regulatory proteins resulting in only about 1% of the peptide pool being available to the immune system

Question 7

What is the immunoproteosome?

Answer 7

This is a variant on the traditional proteasome which does not completely replace the original and has a considerably shorter half life along with an altered cleavage site preferences, with a strong preferences to cleave behind residues that represent correct C-terminal anchors for MHC I presentation
PA28 does not confer new cleavage site specificities but enhances the frequency of the usage of minor cleavage to provide more peptides for MHC presentation

Question 8

What is the endogenous/MHC I pathway of antigen presentation?

Answer 8

Antigen peptides are generated from proteins produced by the presenting cell, they are processed in the cytosol and then transported into the ER lumen where they are loaded onto an MHC I complex which will then be transported through the golgi complex to the surface via the secretory pathway

Question 9

What is the exogenous/MHC II pathway of antigen presentation?

Answer 9

Antigens processed are from proteins engulfed by professional antigen presenting cells
These peptides are processed in the endosomal compartment

Question 10

What are the differences between the proteasome and immunoproteasome?

Answer 10

The immunoprotesome contains beta1i (low molecular weight protein), beta2i multicatalytic endopeptidase complex like 1 MECL 1), beta5i (low molecular weight protein 7), POMP (proteasome maturation protein) and PA28 (proteasome activator)

Question 11

What is the role of Trim-peptidases in peptide processing in the endogenous pathway?

Answer 11

Peptides produced by proteasomes are often too large for presentation (8-11aa) or for TAP (8-16aa) so several cytosolic and ER proteases are involved in trimming (though their, main function is probably peptide degradation for aa recycling)

Question 12

How is MHC I molecule and peptide loading complex assembled?

Answer 12

The HC of MHC complexes with CNX to stabilize itself until the beta2-microglobulin permanently associates with the HC chain
The TAP transporter (through which peptide will pass) is associated with tapasin which is itself associated with CRT through the ERp57 which cover the molecule until peptide arrives

Question 13

How does peptide loading occur in MHC class I?

Answer 13

Peptides pass into the ER through the TAP transporter protein where it can associate with MHC class I

Question 14

What is the role of tapasin in peptide loading?

Answer 14

This glycoprotein stabilizes the TAP1/TAP2 complex to enhance peptide transport
It acts as a structural component bridging MHC class I to TAP
It facilitates peptide loading
Stabilizes empty peptide receptive MHC complexes and acts as a peptide editor to stabalize the peptide repertoire

Question 15

How is the MHC class II molecule assembled?

Answer 15

These molecules are made from two chains, a generic alpha chain and a highly polymorphic beta-chain
These molecules associate together in the lumen along with a CLIP molecule to form a trimerization domain, these domains will associate with 2 other domains to from a nonameric complex

Question 16

What are the domains and functions of the invariant chain Ii in MHC molecule assembly?

Answer 16

This has a short N-terminal cytosolic domain (sorting motif)
A single transmembrane domain
Class II associated invariant chain peptide
C-terminal trimerisation motif
This molecule functions as a scaffold to facilitate proper folding and assembly of MHC II, blocking premature class II peptide association, direct trafficking of MHC invariant chain to endosomal pathway

Question 17

What is the process of uptake of exogenous antigen and peptide loading into MHC II?

Answer 17

Material from the external environment is taken up by endocytosis which will form a vesicle which will decrease in pH leading to unfolding of proteins along with the actions of proteases leading to the production of peptides
MHC is also translocated to this vesicle where AEP cleaves off most of the CLIP chain leaving just a small fragment covering the peptide binding region
HLA-DM then functions as tapasin regulating the peptides which bind to MHC class II to ensure that they are stable