MHC and Antigen Processing

Question 1

What are the 6 isotypes of MHC class 1?

Answer 1

HLA-A, B, C, E, F, G

Question 2

What are the 5 isotypes of MHC class 2?

Answer 2

HLA-DP, DQ, DR, DM, and DO

Question 3

For the 6 isotypes of MHC class 1 and 5 isotypes of MHC class 2, which are highly polymorphic, polymorphic, oligomorphic, and monomorphic?

Answer 3

Highly polymorphic: HLA-A, HLA-B, HLA-C, and HLA-DR (beta chain)
Polymorphic: HLA-DP and HLA-DQ
Oligomorphic: HLA-E, HLA-G HLA-DM, HLA-DO (beta chain), and HLA-DR (alpha chain)
Monomorphic: HLA-F, and HLA-DO (alpha chain)

Question 4

What are the 2 components to inherited MHC diversity? Describe

Answer 4

Gene families: which consist of multiple, similar genes encoding the proteins
Genetic polymorphism: the presence, within the population, of multiple, alternative forms or alleles of a gene

Question 5

Which of the MHC isoforms are most involved in antigen presentation?

Answer 5

Those that are highly polymorphic
For CD4 T cells: HLA-A, B, and C
For CD8 T cells: HLA-DP, DQ, and DR

Question 6

For the purpose of organ transplantation which and how many HLA anitgens do you want to match at?

Answer 6

Must match at 6 HLA antigens, HLA-A, HLA-B, and HLA-DR. There are only 3 genes but we have 2 copies of each gene and they happen to be codominantly expressed

Question 7

What is a haplotype?

Answer 7

A group of genes within an organism that was inherited together from a single parent. With respect to MHC this is a linked cluster of polymorphic genes all encoded on chromosome 6

Question 8

What cells are you likely to find MHC class II expressed on?

Answer 8

1. Some activated T cells
2. B cells
3. Macrophages/APCs
4. Thymic epithelial cells
   * MHC II can be induced in certain cell types by cytokines but expression is not constitutive

Question 9

What cells are you likely to find MHC class I expressed on?

Answer 9

All nucleated cells in the body (so MHC I is not found in red blood cells)

Question 10

What is the structure of an MHC class I molecule?

Answer 10

1 peptide called the alpha chain (encoded by 1 gene) which contains 3 globular domains called alpha 1, 2, and 3. The alpha chain complexes with beta-2 microglobulin. The peptide binding groove is between the alpha 1 and 2 domains

Question 11

What is the structure of an MHC class II molecule?

Answer 11

Composed of 2 peptide chains called alpha and beta which both contain 2 globular domains called alpha 1 and 2 and beta 1 and 2, respectively. The peptide binding groove is located between the beta 1 and alpha 1 domains

Question 12

What are the secondary structural characteristics of both MHC I and II with respect to the peptide binding groove?

Answer 12

Peptide binding groove is composed of 2 alpha helices that flank a beta sheet. Together they contain most of the variability which allows for specificity of antigen.

Question 13

What is a key difference between MHC I and II in how they interact with antigen?

Answer 13

MHC I will bind peptide at both ends
MHC II will bind it just at one end
This means that the size of the peptides is more uniform for MHC I

Question 14

What do the non-polymorphic areas of MHC interact with?

Answer 14

Also called the invariant region they will interact with the coreceptor CD4 for class II or CD8 for class I

Question 15

What are 6 features of MHC?

Answer 15

1. Broad specificity: many different peptides can bind to the same MHC molecule
2. Each MHC molecule displays one peptide at a time
3. MHC molecules bind only to peptides: (cannot bind carbs, nucleic acids, or lipids)
4. Peptides are acquired during intracellular assembly
5. Stable surface expression of MHC molecule requires bound peptide
6. Very slow off-rate: MHC displays bound peptide long enough to be located by T cell

Question 16

What is MHC restriction?

Answer 16

T cells are only able to recognize antigen presented by MHC only in the context of that individuals particular MHC isoform

Question 17

What types of peptides does each class of MHC have a preference for?

Answer 17

MHC I: endogenous peptide
MHC II: exogenous peptide
*MHC can bind self peptide but this is not recognized by the immune system

Question 18

What are the direct and indirect mechanisms by which a phagocytic cell recognizes antigen for internalization?

Answer 18

Direct: Toll-Like Receptor (TLR), scavenger receptor, and mannose receptors
Indirect: Binding of C3b and/or the Fc region of Ig
Macrophages use direct and indirect methods and dendritic cells use just direct methods

Question 19

Describe the process by which exogenous antigen is processed and presented. 7 steps

Answer 19

1. Recognition by PRRs on the surface of professional APCs causes internalization of the antigen.
2. The endosome fuses with the lysosome which causes a decrease in pH and proteolytic enzymes, thus producing peptide fragments 8-25 AA long
3. Synthesis of MHC II occurs in the R.E.R. along with an invariant chain which occludes the peptide binding cleft.
4. MHC II + invariant chain are moved to the golgi where they are packaged into an exocytic vesicle which also contains HLA-DM and various proteases
5. The proteases remove most of the invariant chain leaving behind just a short peptide that is bound to the peptide binding cleft called Class II-associated invariant chain peptide (CLIP)
6. The phagolysosome fuses with the exocytic vesicle and subsequently HLA-DM catalyzes the removal of CLIP thus allow exogenous peptide to bind to MHC II.
7. Binding of peptide to MHC II stabilizes MHC II and the exocytic vesicle fuses with the plasma membrane thus completing the presentation process

Question 20

Describe the process by which endogenous protein/antigen is presented. 6 steps

Answer 20

1. Cytosolic proteins become ubiquitinated and then targeted by the proteasome
2. Peptides bind to the TAP transporter which is present on the R.E.R. membrane
3. Synthesis of MHC I and beta-2 microglobulin occurs in the R.E.R. where they form a loose association with the TAP transporter
4. Another protein called Tapasin forms a bridge between MHC I and TAP transporter to efficiently transfer peptide from TAP to MHC I
5. Binding of peptide to MHC I causes beta-2 microglobulin to properly fold thus stabilizing the complex
6. The MHC I molecule bound with peptide is then directly transported to the membrane for presentation

Question 21

What is cross-presentation? How does it occur?

Answer 21

The process by which a dendritic cell presents exogenous antigen either by class I or II MHC.
Involves the transfer of peptide from an endocytic vesicle to the cytoplasm (unclear exactly how this occurs)

Question 22

Why is cross presentation important?

Answer 22

It allows a dendritic cell to simultaneously activate by CD4+ and CD8+ T lymphocytes