L6 - Adaptive immunity 3

Question 1

What happens during activation of T-cells

Answer 1

• Antigen presenting cells (APCs) determine which peptides will be presented on class I and class II MHC during initial activation
• T-cells need to be able to distinguish between external antigens (taken up by APCs) and internal antigens (infected cell)

Question 2

What is antigen processing?

Answer 2

• Enzymatic process of degrading proteins through proteases into antigenic peptides
• Antigen processing requires energy (ATP) and movement of endocytic vesicles

Question 3

Process of MHC class II presentation

Answer 3

• Extracellular antigen enters cell via endocytosis and an endocytic vesicle forms
• Peptide production in phagolysosome
• Peptide binding by MHC class II
• MHC class II presents peptide at cell surface

Question 4

Process of MHC class I presentation

Answer 4

• Intracellular antigen enters proteasome (antigen processing to peptides in proteasome)
• Peptide transport into endoplasmic reticulum
• Peptide binding by MHC class I
• MHC class I presents peptide at cell surface

Question 5

Specific immune cells for each antigen processing pathway

Answer 5

• Endogenous antigens in cytosol presented on class I MHC molecules to CD8 T cells and tumour cells
• Exogenous antigens in endosomes presented on class II MHC molecules to CD4 T cells

Question 6

Action of the proteasome

Answer 6

• Unfolds proteins and then cleaves proteins into peptides and amino acids

Question 7

How are peptides produced in the cytosol transported into the endoplasmic reticulum after proteasome processing

Answer 7

• Via the TAP transporter

Question 8

Features of TAP proteins

Answer 8

• TAP (transporters associated with antigen processing)
• TAP 1 and TAP 2 form heterodimer in membrane of ER to facilitate selective transport of peptides from cytoplasm into lumen of ER
• TAP pump preferentially transport peptides with a length of 8-15 amino acids

Question 9

Process of MHC class I molecule

Answer 9

• Class I heavy chain is stabilised by calnexin until B2-microglobulin binds
• Calnexin is released. The deterodimer of class I heavy chain and B2m forms the peptide-loading complex with calreticulin, tapasin, TAP and ERp57
• A peptide delivered by TAP binds to the class I heavy chain, forming the mature MHC class I molecule
• The class I molecule dissociates form the peptide-loading complex, and is exported from the endoplasmic reticulum

Question 10

Activation of CD8+ Tc by endogenous or intracellular antigens

Answer 10

• Effector CD8+ Tc (CTLs) are primarily needed for the eradication of infected cells
• CTLs can also be activated against cancer cells (tumour) targets ‘neo antigens’

Question 11

CTL killing of infected target cells

Answer 11

• Viruses must replicate inside cells and many bacteria and parasites live inside host cells
• Therefore antigens for stimulating CTLs come from inside the cell because they signal an intracellular infection

Question 12

Immune evasion - viruses can interfere with Class I MHC expression to escape killing by CTLs

Answer 12

• Herpes simplex virus (HSV) protein ICP47 can selectively bind to TAP and inhibit the transfer of peptides into ER

Question 13

How are peptides generated in the endocytic processing pathway

Answer 13

• Antigen is taken up into intracellular vesicles
• In early endosomes of neutral pH endosomal proteases are inactive
• Acidification of vesicles activates proteases to degrade anigen into peptide fragments
• Vesicles containing peptides fuse with vesicles containing MHC class II molecules

Question 14

Where are peptides bound to MHC class II molecules derived from

Answer 14

• Derived from engulfed pathogens (and internalised TM proteins)
• Acidification of endocytic vesicles activates proteases that degrade proteins into fragments
• These peptide fragments are loaded onto MHC class II molecules

Question 15

Trafficking of MHC class II molecules

Answer 15

• MHC class II alpha and beta chains associate in the ER
• In the trans golgi network, MHC class II is sorted into vesicles
• These vesicles deliver MHC class II to specialised compartments where peptide loading occurs

Question 16

What prevents MHC class II from binding ‘self’ peptides in the ER

Answer 16

• Invariant chain blocks binding of peptides to MHC class II molecules in the ER
• In vesicles, invariant chain is cleaved, leaving the CLIP fragment bound
• CLIP blocks binding of peptides to MHC class II in vesicles
• HLA-DM facilitates release of CLIP, allowing peptides to bind

Question 17

Function of HLA-DM

Answer 17

• HLA-DM acts like a chaperone for MHC class iI molecules and catalyses the release of CLIP once an antigenic peptide is present

Question 18

Exogenous pathway - Class II MHC peptide loading

Answer 18

• Class II MHC loading takes place in endosomes where acidic pH is required for protein degradation into peptides
• Invariant chain is degraded and CLIP is exchanged with foreign peptide

Question 19

CD4+ Th activation by exogenous antigens

Answer 19

• Foreign antigens/extracellular pathogens need to be taken up by APCs to get noticed by Th cells of the immune system
• This leads to activation of macrophage and the production of secreted antibody by plasma cells

Question 20

Viral inhibition of Class II MHC

Answer 20

• Adenovirus interferes with class II upregulation in APCs
• The HSV viral envelope protein, glycoprotein B, reduces MHC class II processing and inhibits the production of invariant chain peptide
• HIV interferes with Class II processing

Question 21

Pathogens that evade lysosomes

Answer 21

• Leishmania and mycobacteria (TB) prevent phagosome-lysosome fusion

Question 22

How are T-cell antigens kept apart?

Answer 22

• Class I and class II MHC molecules both traverse through ER to cell surface but load peptides in different cell compartments
• Control is through accessory proteins

Question 23

Accessory proteins that control T-cell antigens

Answer 23

• Class I requires TAP, tapasin, etc control

- Class II requires low pH for removal of li

Question 24

Summary of T-cell dependent B-cell response

Answer 24

• Antigen binding to BCR provides ‘signal 1’ to B cell
• Antigen is internalised, processed and antigenic peptides are displayed on MHC for T cell recognition
• Th (helper T cell) recognises antigen-MHC complex via the T cell antigen receptor(TCR): provides ‘signal 1’ to T-cell
• CD80 on B-cell binding to CD28 on T-cell provides ‘signal 2’ to T cell
• T-cell activation leads to up-regulation of CD40L which bind to CD40 providing signal 2 to B cell
• Cytokine production by activated T cell also help to activate B-cell
• B-cell proliferates and differentiates into antibody secreting B-cell (plasma cell)