Antigen Processing and the MHC

Question 1

MHC general properties

Answer 1

• Member of the immunoglobulin superfamily
• 2 classes *Class 1: HLA-A ,B, C, D, E, F, G

*Class 2: HLA-DP, DQ, DR, DM, DO

Question 2

HLA polymorphism

Answer 2

• MHC class 1 *HLA locus A, B - MHC class 2 *HLA locus DRB1 - HLA-A, B and HLA-DR are used to “match” donor and recipient for transplantation

Question 3

Chromosome from which MHC is encoded

Answer 3

• Chromosome 6

Question 4

Haplotype

Answer 4

• In respect to a linked cluster of polymorphic genes, the set of alleles carried on a single chromosome - Every person inherits two haplotypes, one from each parent

Question 5

MHC Inheritance

Answer 5

• Co-dominant expression; protein products of both alleles at a given locus are expressed

Question 6

Tissue/Cells that express MHC class 1

Answer 6

• All nucleated cells (excludes erythrocytes)

Question 7

Tissue/Cells that express MHC class 2

Answer 7

• T-cells
• B-cells
• Macrophages
• Dendritic cells
• Thymic Epithelium

Question 8

MHC Class 1 structure

Answer 8

Question 9

MHC Class 2 structure

Answer 9

Question 10

MHC Class 1 and 2 structural similarities

Answer 10

• Alpha helix “walls”
• Beta pleated sheet “floor”

Question 11

MHC Class 1 and 2 structural differences

Answer 11

• Difference: peptide binding cleft of Class 2 MHC is not closed at both ends like Class 1
• Peptides are less uniform in size for Class 2

Question 12

MHC Restriction

Answer 12

• T-cells recognize peptides ONLY in the context of a particular MHC isoform

Question 13

Which MHC class associates with shorter peptides?

Answer 13

• Class 1 appears to associate w/ peptides of 8-10 aa b/c they are anchored

Question 14

Exogenous antigen processing for presentation on Class 2 MHC

Answer 14

• Most peptides that are destined for presentation on MHC class 2 molecules (to CD4+ cells) are captured and internalized into endosomes by antigen presenting cells (dendritic cells, mononuclear phagocytes and B cells)
• After internalization, proteins are localalized into endosomes
• Class 2 molecules are synthesized in the ER, where they couple w/ a protein called the invariant chain, which blocks peptide binding within the peptide binding cleft of MHC Class 2

*binding of the invariant chain prevents binding of peptides to Class 2 MHC in the ER

• MHC Class 2 are exported from the ER into exocytic vesicles, where they are targeted to late endosomes and lysosomes; fusion of the exocytic vesicles w/ the compartments brings together the MHC Class 2 proteins w/ processed peptides derived from extracellular proteins

*These fused structures, called MHC Class 2 compartments contain proteolytic enzymes, Class 2 MHC, the invariant chain and HLA-DM

• The invariant chain is acted upon by proteolytic enzymes leaving only a small remnant called the Class 2-associated invariant chain peptide (CLIP)

*CLIP sits in the peptide binding groove in the same way as a peptide would. The groove becomes vacant through the action of HLA-DM, which appears to catalyse the removal of CLIP

*Once vacant, it appears the the peptide binding groove is filled w/ peptide and that HLA-DM may facilitate this

• Class 2 MHC are stabalized by the bound peptide and these stable complexes are delivered to the surface of the cell

Question 15

Endogenous antigen for presentation on Class 1 MHC

Answer 15

• This pathway is used for processing of cytosolic proteins for presentation at the cell surface on Class 1 MHC. These proteins would include viral gene products, proteins from phagocytosed microbes that have either leaked out of or been transported from vesicles into the cytoplasm (more about this later) or mutated/altered host cell genes
• A portion of all proteins synthesized within a cell and are present in the cytoplasm are targeted for destruction

*proteins are complexed w/ ubiquitin

*targets these proteins for transport to a proteosome

*proteosome: 28 subunit cylindrical protease complex > degrades protein into peptide > peptides transported to endoplasmic reticulum by TAP transporter

• Meanwhile, newly synthesized MHC Class 1 and Beta 2 microglobulin are transported to the ER where they form a loose assoc. w/ TAP and other chaperone molecules
• Tapasin forms a bridge b/w Class 1 and TAP transporter and allows transfer of a peptide into the peptide binding cleft
• Class 1 MHC: Beta 2 microglobulin complex then becomes properly folded and is exported to the cell surface

Question 16

Cross Presentation

Answer 16

• Dendritic cells: major antigen presenting cell invoved in activation of naive T cell (priming)
• Dendritic cells can present internalized (exogenous) antigen on either MHC Class 1 or Class 2
• Invovles transfer of peptide from the endocytic pathway to the cytosol
• Can then be picked up by a proteosome, processed and transported to the ER for presentation on Class 1 MHC

Question 17

MHC peptide binding characteristics

Answer 17

• Selective, not antigen specific like antigen receptors on B and T cells
• Broad specificity; many different peptides can bind to the same MHC molecule

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Question 18

Macrophages can bind to antigen through

Answer 18

• C3b or Fc receptors

Question 19

Exogenous antigen processing

Answer 19

• Class II molecules are synthesized in the ER, where they couple w/ a protein called the invariant chain, which blocks peptide binding within the peptide binding cleft of MHC class II; prevents binding of peptides to class II MHC in the ER
• MHC class II are exported from the ER into exocytic vesicles, where they are targeted to late endosomes and lysosomes. Fusion of the exocytic vesicles w/ these compartments brings together the MHC class II proteins w/ processed peptides derived from extracellular proteins

*these fused structures, called MHC class II compartments contain proteolytic enzymes, class II MHC, the invariant chain and HLA-DM

• The invariant chain is acted upon by proteolytic enzymes leaving only a small remnant called the Class II-assoc. invariant chain peptide (CLIP)

*CLIP sits in the peptide binding groove in the same way as a peptide would. The groove becomes vacant through the action of HLA-DM, which appears to catalyse the removal of CLIP

*once vacant, it appears that the peptide binding groove is filled w/ peptide and that HLA-DM may facilitate this process

• Class II MHC are stabilized by the bound peptide and these stable complexes are delivered to the surface of the cell

Question 20

Endogenous antigen processing

Answer 20

• This pathway is used for processing of cytosolic proteins for presentation at the cell surface on Class I MHC. These proteins would include viral gene products, proteins from phagocytosed microbes that have either leaked out of or been transported from vesicles into the cytoplasm or mutated/altered host cell genes
• A portion of all proteins synthesized within a cell and are present in the cytoplasm are targeted for destruction

*proteins are complexed w/ ubiquitin

*targets these proteins for transport to a proteosome

*proteosome: 28 subunit cylindrical protease complex

> degrades protein into peptide

> peptides transported to endoplasmic reticulum by TAP transporter

• Meanwhile, newly synthesized MHC class I and Beta2 microglobulin are transported to the ER where they form a losse assoc. w/ TAP and other chaperone molecules
• Tapasin form a bridge b/w Class I and TAP transporter and allows transfer of a peptide into the peptide binding cleft
• Class I MHC: Beta2 microglobulin complex then becomes properly folded and is exported to the cell surface