lecture 1- MHC & Antigen processing

Question 1

what is antigen processing

Answer 1

series of intracellular events in which antigen presenting cells make antigens available to T cells

• involves uptake of antigens (proteins), their degradation to peptides, building of the peptides to MHC class I or MHC class II, and transport to the cell surface

Question 2

what is antigen presentation

Answer 2

presentation of MHC-peptide complexes on cell surface for stimulation of T cells

• performed by antigen presenting cells: dendritic, macrophages, B cells

Question 3

CD8 = ___ T cells
MHC class ___

Answer 3

cytotoxic

MHC class I

Question 4

CD4 = ___ T cells
MHC Class ___

Answer 4

helper
MHC Class II

Question 5

name 2 antigen processing pathways

Answer 5

1- endocytic
2- cytosolic

Question 6

endocytic pathway also called ___ or ___ pathway

Answer 6

exogenous, MHC Class II

Question 7

cytosolic pathway also called ___ or ___ pathway

Answer 7

endogenous, MHC Class I

Question 8

describe endocytic pathway

Answer 8

1- antigen taken up from extracellular space to intracellular vesicles
2- endosome pH drops, which activates proteases in the endosome to cleave antigen(protein) into peptides
3- vesicles containing peptides fuse with vesicles containing MHC Class II molcules

Question 9

how does MHC Class II get to processed peptides in the endocytic pathway?

Answer 9

1- invariant chain blocks binding of peptides to MHC Class II molecules in the ER
2- IN VESICLES- invariant chain is cleaved, leaving the CLIP fragment bound
3- CLIP blocks binding of peptides to MHC Class II in vesicles
4- HLA-DM facilitates release of CLIP, allowing peptides to bind

Question 10

___ presents peptide antigens in the endocytic pathway

Answer 10

MHC Class II

Question 11

___ directs MHC Class II away from typical secretory to endocytic pathway and blocks peptide loading in the ER

Answer 11

invariant chain

Question 12

___ acts as a chaperone or catalyst to facilitate exchange of CLIP with antigenic peptides

Answer 12

HLA-DM

Question 13

describe the cytoplasmic pathway

Answer 13

1- class I heavy chain is stabilized by calnexin until beta 2-microglobulin binds
2- calnexin is released and the heterodimer of MHC class I heavy chain and B2m forms the peptide-loading complex
3- proteasome degraded protein into peptides
4- a peptide delivered by TAP binds to MHC Class I heavy chain, forming mature MHC Class I molecule
5- MHC Class I molecule dissociates from the peptide-loading complex, and is exported from the ER by TAP

Question 14

___ presents antigenic peptides to T cells in cytoplasmic pathway

Answer 14

MHC Class I

Question 15

___ shuttles peptides from cytosol to ER in cytoplasmic pathway

Answer 15

TAP

Question 16

in endocytic pathway, peptide loading is done in the ___
in cytoplasmic pathway, peptide loading is done in the ___

Answer 16

not ER
ER

Question 17

the end result of antigen processing is ______

Answer 17

display of MHC peptide complex for TCR recognition

Question 18

MHC refers to the complex of genes that encode molecules on the cell surface that….

Answer 18

1- present peptides delivered from pathogens to T cells
2- compatibility for organ transplants
3- susceptibility to autoimmune diseases
4- known as HLA in humans and H-2 in mice

Question 19

describe MHC class I structure

Answer 19

• peptide binding cleft made of alpha chain (alpha 1 and alpha 2)- 9 AA’s long
• 1 transmembrane domain (alpha chain)
• B2 microglobulin- invariant, always the same
• variability in peptide binding cleft (T cells exposed to wide variety of peptides)
• when CD8 associates, binds alpha3

Question 20

describe MHC Class II structure

Answer 20

• peptide binding cleft made of 2 chains (alpha 1 and beta 1)- 10-24 AA’s in length
• 2 transmembrane domains
• variability in beta chain
• CD4 binds to beta2

Question 21

MHC is both ___ and ___, meaning…

Answer 21

polymorphic (more than 1 allele possible for each genetic locus) & polygenic (many MHC genes)

Question 22

name the 3 things needed by both MHC I and MHC II for stabilization on cell surface

Answer 22

MHC I: alpha chain, B2 microglobulin, peptide
MHC II: alpha chain, beta chain, peptide

Question 23

___ is having two identical alleles of the same gene

Answer 23

homozygous

Question 24

___ is having two distinct alleles of the same gene

Answer 24

heterozygous

Question 25

___ = the collective set of MHC alleles present on an individual chromosome

Answer 25

haplotype

Question 26

the human MHC is referred to as ___

Answer 26

HLA (human leukocyte antigens)

Question 27

name the human MHC Class I genes

Answer 27

HLA-A
HLA-B
HLA-C
(highly polymorphic)

Question 28

name the human MHC Class II genes

Answer 28

HLA-DP
HLA-DQ
HLA-DR

Question 29

molecules involved in ___ are also located in the MHC complex

Answer 29

antigen processing

Question 30

MHC class I is expressed by ___
describe its expression

Answer 30

all cells
expression is constitutive, but can be upregulated by interferon I (IFN-alpha and IFN-beta)

Question 31

MHC Class II is expressed by ___
describe its expression

Answer 31

antigen presenting cells- B cells, dendritic cells, macrophages
- expression is inducible above basal levels by interferon-gamma and CIITA

Question 32

CIITA is responsive to ___, moves to nucleus to upregulate ___

Answer 32

IFN-gamma
MHC Class II

Question 33

antigen presenting cells are categorized by their ability to express ___ and ___

Answer 33

MHC I
MHC II

Question 34

MHC genes are expressed ___

Answer 34

co-dominantly

Question 35

when a cell receives a signal from IFN-y, what happens to proteasome?

Answer 35

changes from constitutive proteasome to immunoproteasome, exchange of beta subunits and different caps — function enhanced

Question 36

immunoproteasome is better at generating ___

Answer 36

immunogenic peptides

Question 37

IFN-y is known to activate macrophages, but also enhances antigen processing by acting on ___ to improve peptide generation for binding to MHC Class I, and enhance overall expression of ___

Answer 37

immunoproteasome
MHC Class II

Question 38

what would you predict a cell lacking HLA-DM would look like?

Answer 38

HLA-DM helps exchange away the CLIP peptide and allow binding of antigenic peptide; without HLA-DM, cell surface will not be loaded with antigenic peptide, it would with CLIP in peptide-binding cleft, inefficient for MHC class II- would cripple its ability to present anything but CLIP (immune system is selected to not recognize CLIP btw)

Question 39

what would you predict a cell lacking invariant chain would look like?

Answer 39

MHC class I loads peptides in ER, MHC class II does not because invariant chain protects peptide binding cleft in ER, when invariant chain is cleaved, CLIP can bind and then exchange for antigenic peptide
- Also contains signal sequence in cytoplasmic domain that directs the MHC class II containing vesicle to intersect with the endocytic pathway which contains the pathogens and pathogen-derived peptides
- without invariant chain, alpha and beta chains do not assemble very well- instable, no expression of MHC class II on cell surface

Question 40

what if the cell lacks TAP1? Beta 2 microglobulin?

Answer 40

• Without B2, no MHC class I on cell surface
• Without TAP1, no transport- (directs peptides to ER normally), without it, peptides still generated but no mechanism to get into ER, cannot load peptides