module 03 section 04 (TCR complex)

Question 1

what is the goal of T-cell development?

Answer 1

to generate mature, functional T-cells bearing TCRs that are capable of recognizing a broad range of antigens in the context of self-MHC

Question 2

where does the development of mature T-cells from progenitor T-cells take place?

Answer 2

the thymus

Question 3

mature T-cells express a heterodimeric TCR that is composed of __(1)__ or __(2)__

Answer 3

(1) one alpha and one beta chain

(2) one gamma and one delta chain

Question 4

when progressing through development, progenitor T-cells undergo: (4)

Answer 4

lineage commitment, TCR gene rearrangements, proliferation and selection

Question 5

are TCR heterodimeric domain structures classified as members of the Ig family? why or why not?

Answer 5

yes - structures are simialr to those of the Igs

Question 6

what do all TCR chains contain in the extracellular domain?

Answer 6

2 Ig chains that are structurally homologous to the variable and constant chain domains of the Igs

Question 7

how many domains does the membrane bound TCR consist of?

what are they?

Answer 7

3 - extracellular, transmembrane and cytoplasmic

Question 8

what is the extracellular domain of the membrane bound TCR

Answer 8

heterodimer of a&b or g&d

but most have the TCR-ab heterodimer

Question 9

how are the two heterodimer chains of the extracellular domain of TCR linked?

Answer 9

covalently by disulfide bonds

Question 10

on each extracellular domain, TCR expresses:

Answer 10

oligosaccarides (CHO)

Question 11

do T-cells expressing the TCR-gamma/delta heterodimer have a hightened or restricted ability to recognize antigens?

why?

are there any exceptions?

Answer 11

restricted - these cells lack CD4 and CD8 molecules on their surface
**except intraepithelial lymphocytes

Question 12

where are intracellular lymphocytes?

what receptor do these cells express?

Answer 12

• subset of T-cells that are located in the gut epithelium

- express CD8+

Question 13

what are the first lymphocytes to arise during development in the fetus?

Answer 13

intraepithelial lymphocytes - play a critical role in protection agaisnt infections

Question 14

what is the funtion of the transmembrane domain of TCR?

Answer 14

to anchor TCR to the cell membrane together with the CD3 complex

Question 15

what is the result when TCR lacks a cytoplasmic domain?

Answer 15

makes the TCR unable to trigger a singal by itself for cell activation

Question 16

explain the cytoplasmic domain

Answer 16

TCR a/b and g/d are closely associated with the invarient proteins of the CD3 complex

Question 17

what is the function of the cytoplasmic domain of TCR?

Answer 17

(1) signalling and cell activation (primary function)
(2) TCR assembly and expression on the cell surface
(3) stabilizing the TCR on the cell membrane

Question 18

what does the CD3 complex consist of?

Answer 18

5 polypeptide transmembrane chains expressed as dimers: epsilon/delta, epsilon/gamma and sigma/sigma

Question 19

what is required to interact for a signal to be transmitted by the TCR?

Answer 19

co-receptors

Question 20

what are the 2 T-cell co-receptors expressed on a mature cell?

Answer 20

CD4 and CD8

Question 21

what is the structure of a CD4 co-receptor?

Answer 21

monomer with 4 domains, structurally similar to Ig domains

Question 22

what are 2 characteristics of the CD4 receptor?

Answer 22

(1) marker for helper T-cells

(2) D1 and D2 domains bind to a conserved site on the B2 domain of the MHC class II molecules on APCs

Question 23

what is a clinical application of CD4 receptors?

Answer 23

HIV binds to CD4 to infect CD4+ helper T-cells

Question 24

what is the structure of a CD8 co-receptor?

Answer 24

• aB heterodimer or aa homodimer
• chains are covalently attached by a disulfide bond
• has 1 Ig fold

Question 25

what are 2 characteristics of the CD8 receptor?

Answer 25

(1) marker for cytotoxic T-cells | (2) binds to a site at the base of the a3 domain of MHC class I molecules on APCs

Question 26

what determines the lineage of TCR gene rearrangement?

Answer 26

gene silencer elements embeded in the TCR gene segment

Question 27

what are the two lineages that TCR gene rearrangement can commit to ?

Answer 27

alpha/beta or gamma/delta

Question 28

explain the process of TCR rearrangement

Answer 28

start with TCR-γ, (1) then if γ-silencers are turned off, the VDJ rearrangement occurs in the γ chain, then the VJ rearrangement happens in the δ chain, leading to the production of TCR-γδ - 1-5% of cells express this (2) if γ-silencers are on, the VDJ rearrangement occurs in the β chain, the VJ rearrangement occurs in the α chain, leading to the production of TCR-αβ

Question 29

once the α-VJ rearrangement has completed, what happens to the δ gene segment?

Answer 29

it's deleted

Question 30

how does TCR antigen specificity arise? | is this simialr or different to B-cell receptor specificity?

Answer 30

similar to b-cell receptors, TCR antigen specificity is the result of gene rearrangement

Question 31

where is the mouse TCR-γ chain DNA located?

Answer 31

chromosome 13

Question 32

explain TCR-γ chain gene rearrangement

Answer 32

it involves the joining of Vγ to Jγ forming the VJγ

Question 33

what happens after TCR-γ gene rearrangement?

Answer 33

transcription and splicing of the VJγ exon to Cγ genes generates mRNA - which is then translated to yeild the TCR-γ chain

Question 34

in addition to γ-silencers, what is another important structure involved in determining TCR gene rearrangement?

Answer 34

pseudogenes

Question 35

what is a pseudogene

Answer 35

a nonfunctional gene due to mutation

Question 36

how is the mRNA that is translated to yeild TCR β/α/... chain generated?

Answer 36

transcription and splicing of the VDJβ (VJα) exon to Cβ(α) generates the mRNA that is translated to yeild the TCR-β(α) chain

Question 37

how can cells with nonprodutive VDJβ rearrangements in the Cβ1 locus be rescued?

Answer 37

ONLY by a subsequent rearrangement in the Cβ2 locus

Question 38

describe how subsuquent rearrangement in the Cβ2 locus can rescue nonproductive rearrangements in the Cβ1 locus

Answer 38

involves another Vβ gene segment rearranging to a DJβ segment in the Cβ2 locus, deleting the Cβ1 locus and the nonproductively rearranged gene

Question 39

why can Cβ1 locus only be rescued by Cβ2?

Answer 39

DJ rearrangement ONLY occurs wihtin the respective Cβ locus | i.e. Dβ1 cannot rearrange with Jβ2

Question 40

where are the mouse TCR-α and TCR-δ genes located?

Answer 40

chromosome 14

Question 41

where are the α gene segments V, J, and C located relative to the δ gene segment?

Answer 41

at the two ends of the δ gene segment (V on left end and J, C on the right)

Question 42

what are the implications of the unusual location of Cδ between Vα and Jα?

Answer 42

results in the deletion of Cδ upon Vα-Jα rearrangement, meaning the cell looses its ability to create the TCR-δ chain

Question 43

how many Jα segments are there in the TCR-α chain gene?

Answer 43

~50

Question 44

what is permitted by the presence of many Jα segments?

Answer 44

TCR-α chain genes can undergo several successive rearrangement events to "leapfrog" over nonproductively rearranged VJα segments

Question 45

how long can the repeated rearrangements in the TCR-α gene continue?

Answer 45

until a productive rearrangement leads to positive selection OR until there are no more Vα or Jα segments available for rarrangement, leading to cell death

Question 46

if there is productive VJα rearrangement, will there ever be production of the δ chain? why or why not?

Answer 46

no never - it will be excised via DNA looping

Question 47

does TCR germ-line DNA contain fewer V gene segments than Ig germ-line DNA, or vice versa? what are the implications of this?

Answer 47

- TCR germ-line contains far fewer (i.e. there are more Ig variable genes than TCR) - can still make a large number of random combinations for all TCR chains just as IG heavy/light chains do

Question 48

how many V regions are present on the α, β, γ and δ chains?

Answer 48

``` α = 100 β = 25 γ = 7 δ = 10 ```

Question 49

how many D regions are present on the α, β, γ and δ chains?

Answer 49

``` α = 0 β = 2 γ = 0 δ = 2 ```

Question 50

how many J regions are present on the α, β, γ and δ chains?

Answer 50

``` α = 50 β = 12 γ = 3 δ = 2 ```

Question 51

how many V regions are present on the Ig heavy and light chain?

Answer 51

``` heavy = 300 light = 300 ```

Question 52

how many D regions are present on the heavy and light chains?

Answer 52

``` heavy = 12 light = 0 ```

Question 53

how many J regions are present on the heavy and light chains?

Answer 53

``` heavy = 4 light = 4 ```

Question 54

explain how the alternative joining of D gene segments differs btwn TCR and Igs

Answer 54

- due to the arrangement of RSS in TCR germ line DNA, alternative joining of D segments can occur (in the beta and delta chain ONLY) - also one D segment can join another (VDDJ) creating considerable diversity - this cannot occur in the Igs (they must obey the one-turn two-turn rule)

Question 55

explain how junctional flexibility differs btwn TCR and Igs

Answer 55

- it doesnt - junctional flexibility is exhibited during gene rearrangement in both Igs and TCRs - it can generate many nonproductive rearrangements, but also increases diversity

Question 56

how does junctional flexibility increase diversity?

Answer 56

by encoding several alternative amino acids at each junction

Question 57

compare N-region nucleotide addition for Igs vs TCRs

Answer 57

- in both Ig (heavy chain ONLY) and TCR genes, nucleotides may be added at the juncitons btwn some gene segments during rearrangement (as many as 6) - generates up to 5461 possible combinations assuming the random selection of nucleotides

Question 58

compare somatic mutations for TCR and Igs

Answer 58

no somatic mutations for TCR, but can have somatic mutations for Igs

Question 59

why are there no somatic mutations for TCR genes?

Answer 59

to ensure that T-cell specificity does not change after thymic selection (therefore reducing the possibilty that a random mutation may generate a self-reactive T-cell)

Question 60

who do somatic mutations occur for Igs?

Answer 60

- largely random process - they occur in germinal centers where B-cells proliferate - these mutations target rearranged V regions located in the DNA sequence, influcencing the VJ and VDJ rearrangement processes

Question 61

compare allelic exclusion for TCRs vs Igs

Answer 61

Igs can and theres evidence to suggest that beta chain can and alpha chain can't (no evidence for gamma or delta yet)

Question 62

T-cell receptors contain ____ that bind to the MHC-peptide complex

Answer 62

complementary-determining regions (CDR)

Question 63

how many CDRs do T-cell receptors contain? what are they

Answer 63

3! - CDR-1, CDR-2 and CDR-3

Question 64

where is CDR-1 located and what does it bind?

Answer 64

- located in the variable region of the TCR | - interacts with the peptide in the MHC-peptide complex

Question 65

where is CDR-2 located and what does it bind?

Answer 65

- located in the variable region of the TCR | - interacts with the alpha regions of the MHC in the MHC-peptide complex

Question 66

where is CDR-3 located and what does it bind?

Answer 66

- located in the D-J region of the TCR | - interacts with the peptide in the MHC-peptide binding groove