MHC Flashcards
what does a T cell require to become activated?
- need to have an antigen presented to them by an APC
- the antigen is presented on MHC proteins
- which is recognised by the TCR
what is MHC class I?
- cell is infected with intracellular pathogen
- proteins broken into peptides and presented on the surface by MHC class I
what recognises MHC class I?
CD8 T cell
what is MHC class II?
- extracellular pathogen, responds and takes it up
- puts in an intracellular vesicle
- digests into peptides
- presents on MHC class II
what recognises MHC class II?
CD4 T helper cell
what is MHC?
- a gene cluster
- encodes MHC class I and II proteins
- main role is to present the antigens on the cell surface
- peptide-MHC complex is recognised by TCR
what is MHC class I expressed by?
nucleated cells
- any cell with a nucleus could be infected by a virus
what is MHC class II expressed by?
antigen presenting cells eg dendritic cells, macrophages, B cells
what is the structure of MHC class I?
- 2 subunits
- alpha chain with 3 domains
- B2 microglobulin (B2M)
- peptide binding groove is formed by alpha chain, a1 and a1 domains
- alpha chain has extensive polymorphism
what is the structure of MHC class II?
- has 2 subunits
- different domain structure
- alpha and beta chain are encoded in MHC gene cluster
- peptide binding groove formed by a1 and B1 domains
- extensive polymorphism shown in both a and B chains
how does TCR interact with MHC?
- TCR interacts simultaneously with both MHC and associated peptide
- TCR recognises peptide in the context of MHC
- T cell only activated if peptide is presented by MHC
how does MHC bind to peptides?
- peptide binding groove formed from 2 alpha helices, lie on top of a B sheet, forms the cleft
- for both MHC I and II
how does peptide bind in MHC class I?
- peptide is constrained at both ends by invariant amino acids
- bulky chains at the N terminus stop the peptide going further
- class I can only bind short peptides (8-10 aa)
how does peptide bind in MHC class II?
- open peptide binding groove
- peptide can protrude at either end
- can be 13+aa
how is MHC polygenic?
every person expresses several different variants
how is MHC polymorphic?
human population contains many different variants (ie alleles)
where is the human MHC?
gene cluster on chromosome 6
- Human MHC genes = HLA (human leukocyte antigen) genes
what gives MHC such great diversity?
- each of express 3 different types of MHC I alpha chain from each chromosome 6
- each person expresses 3x different MHC II a and b genes from each chromosome 6
- MHC genes are the most polymorphic gene known
how many MHC does a person have?
3x HLA per chromosome x2 chromosomes = 6 MHC per person
where does allelic variation localise?
the peptide binding groove
how do variations effect the peptide binding groove?
- polymorphisms alter shape of peptide binding groove
- altered shape means different variants bind different peptides
- peptides bind to particular MHCs through ‘anchor residues’
what do polymorphisms in MHC form?
distinct pockets that peptide anchor residue fits into
what is at the C terminus of the peptide?
- hydrophobic amino acids
- tyrosine residues are bulky side chains
- interact with the pockets known as anchor residues
- stick the peptide down into the MHC
whats difficult for pathogens as a result?
- difficult for pathogens to evolve to be non-presentable
- if we all shared one MHC it would be able to evolve
what are the 3 stages of presenting peptides from cystolic proteins on MHC class I?
- break down proteins into peptides
- transport of peptides into ER
- peptide coating onto MHC class I
how are proteins broken down into peptides during stage 1 of presenting peptides on MHC class I?
- proteins digested by proteasome
- all cells express constitutive proteasome
- IFNy induces immunoproteasome formation
what is the structure of the constitutive proteasome?
- 20s catalytic core = 28 subunits, protease activity
- 19s regulatory caps directs cystolic proteins to core
what is the role of the immunoproteasome?
- fires out better peptides and fires them out faster
- new catalytic core proteins = increased cleavage after hydrophobic residues
- new regulatory cap proteins = faster peptide release
how are peptides transported into the ER in stage 2 of antigen presentation on MHC class I?
- MHC I co-translationally translocated into the ER membrane
- active transport of peptides by ER membrane TAP1 and TAP2 proteins
- have a preference for peptides with hydrophobic residue at the C terminus
- constitutively expressed is IFN inducible, can be increase during infection
what is peptide coating onto MHC class I in stage 3?
- stable peptide is essential for cell surface MHC expression
- alpha chain folds and binds to B2M
- some ER peptides are too long
- form a peptide loading complex (PLC)
why is stable peptide binding essential for cell surface MHC I epxression?
- MHC proteins are unstable when not bound to peptides or weakly bound
- dont want the peptide to dissociate
what are long peptides trimmed by MHC I loading?
trimmed to 8-10 aa by ER amino peptidase
what is the peptide loading complex (PLC)?
- chaperone proteins, bind to MHC
- Tapasin, binds to MHC and TAP, pulls MHC close
- PLC performs peptide editing i.e remove low affinity peptides
- PLC ensures only high affinity peptides stably bind MHC
what are the 4 stages of MHC class I presentation?
- peptide generation in acidified endosome
- movement of MHC II to acidified endosomes
- invariant chain cleavage to CLIP
- HLA_DMs promotes stable peptide binding in MIIC
how is a peptide generated in acidified endosomes in MHC class II stage 1?
- convert the pathogens/proteins into peptides
- proteasomes not present in vesicle
- early endosome, fuse and acidify it
- acidic pH activates acid cysteine proteases eg cathepsins
how are MHC II moved to acidified endosomes during presentation stage 2?
- MHC II co-translationally translocated into the ER membrane
- dont want MHC II to bind peptides prematurely, prevent this with the invariant chain
how does the invariant chain prevent MHC II and peptides binding prematurely during stage 2 of presentation?
- block MHC II peptide binding groove and stops peptide binding in the ER
- directs MHC II from ER to low pH endosomes towards vesicular proteins
what is invariant chain cleavage to CLIP in stage 3 of MHC II presentation?
- MHC II associated with invariant chain = vesicle acidifies
- invariant chian degrades apart from the CLIP fragment
- acid vesicle is called the MIIC
- moving towards to acid endosome
how does HLA-DMs promote stable peptide binding in MIIC in stage of MHC II presentation?
- HLA_DM is a MHC II like molecule
HLA-DM promotes:
1. removal of CLIP fragment from MHC II binding groove
2. peptide editing
why do you need cross presentation?
- naive CD8 T cells needs to be activated by MHC I on a dendritic cell
- good if you have a virus that infects a DC
- problem if the virus doesnt affect a DC, need cross presentation from the MHCII pathway to the MHC I pathway
what cells can cross present?
cross presentation pathway is unique to the DC
how do cross presentation work?
- virus taken in by endocytosis into an endosome which becomes acidified and broken into peptides
- dendritic cells express endosomal channel protein Sec61
- shuttles peptides from the endosome into the cytosol
- enters the class I pathway
