Lec 14 - antigen processing

Question 1

how does MHC class I and II differ in antigen processing prior to presentation?

Answer 1

I - cytosolic/endogenous (I for Inside) processing for CD8+ T cells
II - exogenous processing for CD4+ T cells

Question 2

how do endogenous pathogens arise? what pathogens

Answer 2

pathogens mediate own entry into the cell (via endocytosis or membrane fusion) - usually viruses, some intracellular bacteria and parasite

Question 3

how are endogenous peptides generated?

Answer 3

by proteasomes (protease complexes) (get fed intracellular proteins tagged w/ ubiquitin - polyubiquitination is proteasome degradation signal) in CYTOPLASM
(proteasomes degrade proteins into peptide fragments)

Question 4

what is the 1st step of the endogenous pathway?

Answer 4

MHC class I alpha chain ONLY held by chaperone calnexin in ER (endoplasmic reticulum)

Question 5

what is the 2nd step for the endogenous pathway?

Answer 5

betamicroglobulin binds to alpha chain which makes calnexin release MHC alpha chain

calreticulin+ERp57 (chaperones) makes alpha chain and beta microglobulin interact more

partly folded MHC I binds to chaperone tapasin, linked to TAP

in parallel, proteins are made in cytosol (and ubiquitinated)

Question 6

what is step 3 in endogenous pathway?

Answer 6

polyubiquinated proteins get degraded by proteasome in cytosol (AFTER MHC I molecule partly folded)

peptide fragments brought into ER by TAP (like a tap, lets peptide fragments trickle through)

ERAAP trims peptides to fit into binding groove
trial and error (aa of peptide must fit w/ aa of binding groove)

once right peptide fits, MHC will be properly folded (all chains click into place)

Question 7

what is step 4 in the endogenous pathway?

Answer 7

folding complete of peptide bound MHC = pMHC
release from TAP, MHC I vesicle formation, targeted for cell membrane

Question 8

how do exogenous pathogens arise?

Answer 8

pathogen entry mediated by immune cells via phagocytosis - extracellular bacteria, parasites, fungi

Question 9

how are exogenous peptides generated?

Answer 9

internalised antigen in endocytic vesicles (endosomes) fuses with lysosome to form phagolysosome where antigen degradation occurs

at same time as MHC II molecules produced

Question 10

where is MHC-II molecule formed? pMHCII?

Answer 10

ER, cytosol

Question 11

what is the structure of MHC-II? purpose?

Answer 11

has an invariant chain (Ii) (w/ CLIP (class-II associated invariant chain peptide)) which binds to peptide binding groove
li tail guides MHC-II to endocytic vesicles, once in vesicle, proteolytic activity degrades li leaving CLIP only - CLIP prevents peptides from binding too early in ER

Question 12

what happens to li?

Answer 12

initially degraded by proteolytic activity w/in endocytic compartments

Question 13

what is step 1 in the exogenous pathway?

Answer 13

li in complex with MHC-II in ER in Er and still in endocytic vesicle

Question 14

what is step 2 in the exogenous pathway?

Answer 14

due to acidification of endosome, li is cleaved by proteases leaving CLIP bound to MHCII

Question 15

what is step 3 in the exogenous pathway?

Answer 15

endosome fuses w/ vesicle containing degraded peptide - CLIP still blocking access

Question 16

what is step 4 in the exogenous pathway?

Answer 16

HLA-DM always bound to and stabilises MHC-II after peptides in vesicle, HLA-DM releases CLIP
peptide can now bind peptide binding groove
pMHC-II targeted to cell surface

Question 17

what can MHC class I molecules present other than pathogen antigens?

Answer 17

self-peptides
antigens of cells w/ defective proteins or near their expiration date (for turnover of proteins and cells)

Question 18

what are key differences between the endogenous and exogenous pathway?

Answer 18

endo:
pathogen mediated entry
MHC molecule produced first and peptide generation after

exo:
phagocytosis/endocytosis
MHC molecule production and peptide generation at the same time

Question 19

what does HLA-DM stand for?

Answer 19

Human Leukocyte antigen-DM

Question 20

how many MHC molecules can be found in a vesicle?

Answer 20

many (more than one)

Question 21

how does HLA-DM know when to remove CLIP?

Answer 21

pH drops in vesicle and at low pH, HLA-DM changes conformation of MHC II, releasing CLIP
CLIP also has low affinity to MHC