MHC II

Question 1

How do MHC-I peptides get generated?

Answer 1

Many come from defective ribosomal products (DriPs)

Question 2

How are DriPs formed? Are they just human or viral?

Answer 2

• defectively folded proteins that often occur as a result of improperly spliced mRNAs or ribosomal frameshifts
• can be either viral or human proteins

Question 3

How are misfolded proteins labeled for destruction?

Answer 3

misfolded proteins can be ubiquitylated

Question 4

What is ubiquitylation?

Answer 4

• covalent addition of ubiquitin molecules to lyseine residues in misfolded proteins
• can form chains
• requires ATP and a series of enzymes

Question 5

Where are ubiquitylated proteins targeted?

Answer 5

26S proteasome

Question 6

What is the make-up of a 26S proteasome? What do the different subunits do?

Answer 6

20S core + 2(19S subunits)
- 19S “lid” binds ubiquitylated proteins, recycles ubiquitin and unfolds proteins that are not fully unfolded
- 20S core has proteases that chop up proteins into peptides

Question 7

What is the immunoproteasome, where is it expressed, are there factors that upregulate its expression?

Answer 7

presented in pAPCs

the 20S core of the proteasome has some special subunits = the immunoproteasome

IFNg and TNFa upregulate the subunit expression in response to viral infection

Question 8

What are the genes and proteins that make up the special subunits in the 20S portion the of the immunoproteasome? Where are these genes located?

Answer 8

1. LMP2 -> Bi
2. LMP7 -> B5i
3. LMP10 -> B2i

genes are found in the MHC region of the genome

Question 9

What are the differences between the peptides that the immunoproteasome creates vs normal proteasomes?

Answer 9

immunoproteasome increases the frequency of peptides that are the optimal size (8-10a.a) to bind to MHC-I which have C-term hydrophobic residues

Question 10

What complex can IFNg generate and what does it replace and do?

Answer 10

generates the 11S PA28 proteasome-activator complex

replaces the 19S subunits

allows of faster peptide generation

Question 11

What is the TAP transporter, what is it subunits (and where are these genes found), where does it get its energy, what does it transport into the RER?

Answer 11

TAP = transporter associated with Ag processing

subunits = TAP1 + TAP2 (genes in the MHC region)

energy = ATP

transports peptides that are 8-16 a.a in size that often have a hydrophobic C-term residue

Question 12

What does ERAP do, and where is it located?

Answer 12

located = RER

function = trims peptides transported into the RER by TAP at the N-term to fit into MHC-I

Question 13

Why doe ERAP have a low affinity for peptides < 8 a.a?

Answer 13

1. doesn’t change the C-term
2. doesn’t make too small

Question 14

Describe MHC-I peptide loading.

Answer 14

1. class-I alpha chain and B2M are synthesized into the RER lumen
2. Calnexin and ERp57 help partially fold the alpha chain
3. when B2M binds to the alpha chain, Calnexin is release and replaced with calreticulin and tapasin
4. this complex associated with TAP = PLC
5. when peptide is loaded, the MHC-I + peptide complex dissociates from the rest of the peptide loading complex
6. MHC-I + peptide complexes exit the RER and travel to the cell surface
7. peptides are presented to CD8+ T cells

Question 15

What causes Bare lymphocyte syndrome type 1?

Answer 15

mutations in TAP1 or 2 genes causes patients to lack or express very little MHC-I on their cell surface

Question 16

What are the consequences of Bare lymphocyte syndrome type 1?

Answer 16

• Number of CD8+ T cells are heavily reduced as MHC-I expression is critical for development of CD8+ T cells
• patients deal with many viral infections but have problems with respiratory bacterial infections
• repeated respiratory infections -> damage of the respiratory system
• develop necrotizing skin lesions due to excessive NK and gamma/delta T cell activation

Question 17

How are exogenous proteins made into peptides for MHC-II loading?

Answer 17

1. protein is internalized
2. internalized Ag.s are degraded by peptidases in vesicles as pH of vesicles decrease
3. endocytosed Ag.s/peptides end up in MHC-II late endosome for peptide loading (MIIC)

Question 18

Describe MHC-II peptide loading.

Answer 18

1. class II alpha and beta chains are synthesized into the RER
2. MHC-II is complexed with an invariant chain (Ii) protein
3. MHC-II/Ii complex moves to acidic endosomal vesicle
4. Ii is cleaved due to the activation of acidic proteases (cathepsins)
5. CLIP (class-II associated invariant chain peptide; a small peptide from Ii) still remains bound to the peptide binding groove to prevent peptides from binding to the peptide binding groove
6. vesicles containing internalized Ag that have been broken down by proteases fuse with MHC-II/CLIP vesicles (MIIC late endosome)
7. non-classical HLA-DM (present in the MIIC late endosome) binds the MHC-II/CLIP complex and promotes the release of CLIP and the binding of internalized peptide
8. once peptide is loaded, MHC-ll/peptide complexes are trafficked to the surface to display Ag to CD4+ T cells

Question 19

What are four functions of the invariant chain (Ii)?

Answer 19

1. binds all MHC-II molecules
2. prevents ER peptides from associating with MHC-II peptide binding groove
3. helps fold MHC-II
4. directs MHC-II out of the RER, through GA, towards acidic endosomal compartments

Question 20

What causes Bare lymphocyte syndrome, type 2?

Answer 20

mutations in TFs that promote MHC-II promotion -> cells lacking MHC-II expression

Question 21

What are the consequences of Bare lymphocyte syndrom, type 2?

Answer 21

1. severly lack CD4+ T cells as MHC-II expression is critical for CD4+ T cell development
2. SCID like phenotype:
   - less Th cells to help: B cells respond to T-dep Ag, macrophages kill intracellular bacteria, Tc cells get activated

Question 22

What is cross presentation? What is it important for?

Answer 22

in some pAPCs (e.g. DCs) exogenous Ag. can get into the MHC-I pathway, which is important for DCs to activate naive CD8+ T cells