Lecture 14; Antigen Processing and presentation Flashcards
Describe MHC class 1 target and outcomes;
Endogenous pathway
Intracellular pathogens, activates CD8,
Expresses 8-10AA sequences
Predominantly viral
Describe MHC class 2 target and outcomes;
Exogenous pathway
Extracellular pathogens, any length but usually 8-30AA
Activates CD4
Bacterial, parasitic and toxins
What pathway is MHC1 restriction?
Endogenous pathway
What pathway is MHC2 restriction?
Exogenous pathway
Whats the difference in recognition between b and t cells?
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.
Describe what pathogen antigens are presented in class one MHC (endogenous pathway)
- Antigen peptides are generated from (viral) proteins produced by the presenting cell
Describe how antigens are presented in MHC class one;
- 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)
How are antigen peptide fragments processed in the endogenous pathway?
- Proteins are degraded by immunoproteasome
What does a regular proteasome do?
- 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
How is the immuno proteosome activated?
P28 causes the N terminal tails of the alpha sub units to flip upwards, thereby facilitating substrate entry and product exit.
What are some features of the immunoproteasome?
- Does not completely replace constitutive proteasome
- Considerably shorter half life
What does the immunoproteasome do?
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
What are trim peptidases?
- Proteases that trim the products of proteosomes that are too large for presentation
But major function is probably peptide degredation and AA recycling
What aids peptide loading onto MHC 1?
Tapasin
Describe tapasin;
Quality control regulator
- 48kDa protein
What is the function of tapasin?
- 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
Describe the exogenous pathway;
- 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
Give an overview of the exogenous pathway processing;
1) MHC assembly and transport to peptide loading compartment
2) Uptake and processing of exogenous antigen
3) Peptide loading (CLIP exchange)
What is required for MHC 2 assembly?
Invariant chain (seperate protein)
Describe the invariant chain structure;
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
What is the function of the invariant chain?
- 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
Describe the uptake of exogenous antigen;
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)
What are the types of endocytosis?
- Receptor-mediated endocytosis: mannose and lectin-like receptors
- Macropinocytosis: uptake of fluid-filled vesicles (mainly DC)
- Phagocytosis: uptake of complete cells
Describe the antigen processing that occurs in the exogenous pathway;
- Low pH of endosome degrades proteins (proton pump)
- Fusion of endosome with lysosome supplies proteases that are activated by low pH and degrade proteins
How is antigen loaded?
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
