Lecture 6 - MHC class II presenting Flashcards
MHC-II is a dimeric glycoprotein composed of two transmembrane, two domain, glycoprotein chains
Alpha chain (34kDa)
Beta chain (29kDa)
Peptide binding cleft is open at each end and is generated between the β1 and α1 subunits,
Polymorphisms are found primarily in the β1 and α1 subunits, specifically the areas forming the cleft
Alpha and beta chains from the different chromosomes can pair up
i.e. Up to 4X DP, 4X DQ, 4X DR
(N/B DR alpha chain is monomorphic-so 4 not 8 DRs)
Open-ended MHC-II molecules bind longer peptides of between 10 and 30 amino acids.
The ends of the peptide are not bound in the groove-different than MHC-I.
Peptide lies in an extended conformation and is held by hydrogen bonds distributed along the length of the peptide
Peptide is held by interactions with both polymorphic A.A’s (within pockets in the groove) and conserved side chains of invariant amino acids that line the peptide-binding groove.
Since peptide is not bound at the ends and protrudes out of the MHC molecule any length of peptide is theoretically possible.
Long peptides appear to be trimmed by peptidases, to make peptides of 13-17 A.A
Peptides binding to HLA-DR3 display varying lengths but core structures have similar properties, if not sequences.
Anchor residues of peptide bind to polymorphic residues in MHC, giving specificity of binding
Same principle as MHC class I
MHC binding: process in a nut shell
MHC interacts with peptide backbone and amino acid side chains that insert into the pockets
Difference between MHC class I and II molecules: peptide binding domain, nature of peptide-binding cleft, general size of bound peptides, peptide motifs involved in MHC binding, and the nature of bound peptide?
MHC class I:
* α1/α2
* closed at both ends
* anchor residues at both ends of peptide - typically hydrophobic carboxyl-terminal anchors
* extended structure - both ends interact with MHC cleft but middle arches up away from MHC molecule
MHC class II:
* α1/β1
* open at both ends
* anchor residues distributed equally along the peptides
* extended structure - held at a constant elevation above MHC cleft
MHC class II cells: what do they do?
The job of MHC class II is to instruct CD4+ T cells to help other immune cells to mediate their functions (i.e. macrophages, B cells)
CD4+ T cells are (normally) required to deal with extracellular pathogens
Thus, MHC class II typically displays peptides originating from extracellular proteins
How does the cell process and direct intracellular and extracellular antigens for display by the appropriate MHC class I and class II molecules, respectively?
As MHC class II molecules are assembled in the lumen of the endoplasmic reticulum, how does the cell prevent peptides destined for display by MHC class I being presented by MHC class II molecules
Remember, TAP transports peptides into the lumen of the ER
Peptides presented by MHC class II are generated in endocytic vesicles
These proteins NOT in cytosol so NOT degraded by proteasome
Extracellular proteins (and bacteria etc) internalised into endocytic vesicles (endosomes or phagosomes) *
Internalised proteins/pathogens are degraded by acid proteases as the endosome/phagosome acidifies, ultimately fusing with lysosome.
macrophage/DC activation makes endosomal acidification and lysosomal fusion more efficient
Consequences for cross-presentation
MHC class II variation
Genes encoding for MHC pair up with their respective counterpart from the maternal/paternal side but may also move around and pair with …
Cathepsins: what are they, what do they do, and what are their effects in MHC?
Cysteine proteases
Cut in the middle (endoproteases?)
S and L cathepsin deficient mice display deficiencies in antigen processing
The importance of GILT
Gamma-in
Disulfide bonds may need to be reduced before proteins are digested in endosomes
Performed by an IFN-γ-induced thiol reductase (gamma interferon-induced lysosomal thiol reductase (GILT)
GILT clearly localises in endosomal (Lamp2+) compartments
The CD4+ T cell response to proteins containing disulphide bonds is reduced in GILT-/- mice (open circles) compared with intact mice (closed circles)
The response to proteins not containing disulphide bonds (i.e. casein) is intact in GILT-/- mice
The invariant chain (li)
MHC class II synthesised in ER, but it must be blocked from binding peptides (destined for MHC-I) until it is in the correct endosomal compartment
MHC class II –associated invariant chain (li) inhibits peptide binding by sitting in the peptide binding groove
Calnexin stabilises the assembly of the li-MHC complex
li chain (via signals from transmembrane segment) also targets the MHC complex for delivery to low-pH endosomal compartments
In the MIIC (MHC class II compartment) the li is cleaved by proteases (including cathepsin S in APC, L in thymic epithelial cells) to form the class II-associated invariant chain peptide (CLIP)
HLA-DM (or H-2M) molecule
DM molecule -Closely resembles MHC-II, with α and β chains
It catalyses the release of CLIP in MIIC, enabling peptides to bind to MHC class II molecules
It stabilises empty MHC class II molecules in MIIC before peptides bind
It does not bind peptides as the binding groove is closed
It performs “peptide editing” removing unstably bound peptides from the MHC-II complex. This enables stable long lasting expression of MHC II-peptide complexes on cell surface.
It is inhibited by HLA-DO, which prevents peptide loading
MARCH-1 controls MHC-II expression in immature DCs
In immature DCs MARCH-1 promotes degradation of MHC-II.
Ubiquitylated proteins are targeted to proteasome
In activated DCs MARCH-1 expression is suppressed, leading to improved expression of MHC-II-peptide complexes
Ensures DCs can optimally interact with CD4+ T cells during an immune response
Summary: Overview of Classical exogenous (endosomal) antigen processing: MHC class II
Do cells also need to present endogenous peptides on MHC class II?
Cross presentation allows MHC class I presentation of exogenous (extracellular) peptides
- Essential for priming of CD8+ T cell responses in some infections (when the pathogen infects non-APCs)
Is there a mechanism that allows the opposite – i.e. MHC class II presentation of endogenous (intracellular) peptides?
- in what context would this be important for the development of a CD4+ T cell immune response?
