module 03 section 04 (TCR complex) Flashcards
what is the goal of T-cell development?
to generate mature, functional T-cells bearing TCRs that are capable of recognizing a broad range of antigens in the context of self-MHC
where does the development of mature T-cells from progenitor T-cells take place?
the thymus
mature T-cells express a heterodimeric TCR that is composed of __(1)__ or __(2)__
(1) one alpha and one beta chain
(2) one gamma and one delta chain
when progressing through development, progenitor T-cells undergo: (4)
lineage commitment, TCR gene rearrangements, proliferation and selection
are TCR heterodimeric domain structures classified as members of the Ig family? why or why not?
yes - structures are simialr to those of the Igs
what do all TCR chains contain in the extracellular domain?
2 Ig chains that are structurally homologous to the variable and constant chain domains of the Igs
how many domains does the membrane bound TCR consist of?
what are they?
3 - extracellular, transmembrane and cytoplasmic
what is the extracellular domain of the membrane bound TCR
heterodimer of a&b or g&d
but most have the TCR-ab heterodimer
how are the two heterodimer chains of the extracellular domain of TCR linked?
covalently by disulfide bonds
on each extracellular domain, TCR expresses:
oligosaccarides (CHO)
do T-cells expressing the TCR-gamma/delta heterodimer have a hightened or restricted ability to recognize antigens?
why?
are there any exceptions?
restricted - these cells lack CD4 and CD8 molecules on their surface
**except intraepithelial lymphocytes
where are intracellular lymphocytes?
what receptor do these cells express?
- subset of T-cells that are located in the gut epithelium
- express CD8+
what are the first lymphocytes to arise during development in the fetus?
intraepithelial lymphocytes - play a critical role in protection agaisnt infections
what is the funtion of the transmembrane domain of TCR?
to anchor TCR to the cell membrane together with the CD3 complex
what is the result when TCR lacks a cytoplasmic domain?
makes the TCR unable to trigger a singal by itself for cell activation
explain the cytoplasmic domain
TCR a/b and g/d are closely associated with the invarient proteins of the CD3 complex
what is the function of the cytoplasmic domain of TCR?
(1) signalling and cell activation (primary function)
(2) TCR assembly and expression on the cell surface
(3) stabilizing the TCR on the cell membrane
what does the CD3 complex consist of?
5 polypeptide transmembrane chains expressed as dimers: epsilon/delta, epsilon/gamma and sigma/sigma
what is required to interact for a signal to be transmitted by the TCR?
co-receptors
what are the 2 T-cell co-receptors expressed on a mature cell?
CD4 and CD8
what is the structure of a CD4 co-receptor?
monomer with 4 domains, structurally similar to Ig domains
what are 2 characteristics of the CD4 receptor?
(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
what is a clinical application of CD4 receptors?
HIV binds to CD4 to infect CD4+ helper T-cells
what is the structure of a CD8 co-receptor?
- aB heterodimer or aa homodimer
- chains are covalently attached by a disulfide bond
- has 1 Ig fold
what are 2 characteristics of the CD8 receptor?
(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
what determines the lineage of TCR gene rearrangement?
gene silencer elements embeded in the TCR gene segment
what are the two lineages that TCR gene rearrangement can commit to ?
alpha/beta or gamma/delta
explain the process of TCR rearrangement
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
once the α-VJ rearrangement has completed, what happens to the δ gene segment?
it’s deleted
how does TCR antigen specificity arise?
is this simialr or different to B-cell receptor specificity?
similar to b-cell receptors, TCR antigen specificity is the result of gene rearrangement
where is the mouse TCR-γ chain DNA located?
chromosome 13
explain TCR-γ chain gene rearrangement
it involves the joining of Vγ to Jγ forming the VJγ
what happens after TCR-γ gene rearrangement?
transcription and splicing of the VJγ exon to Cγ genes generates mRNA - which is then translated to yeild the TCR-γ chain
in addition to γ-silencers, what is another important structure involved in determining TCR gene rearrangement?
pseudogenes
what is a pseudogene
a nonfunctional gene due to mutation
how is the mRNA that is translated to yeild TCR β/α/… chain generated?
transcription and splicing of the VDJβ (VJα) exon to Cβ(α) generates the mRNA that is translated to yeild the TCR-β(α) chain
how can cells with nonprodutive VDJβ rearrangements in the Cβ1 locus be rescued?
ONLY by a subsequent rearrangement in the Cβ2 locus
describe how subsuquent rearrangement in the Cβ2 locus can rescue nonproductive rearrangements in the Cβ1 locus
involves another Vβ gene segment rearranging to a DJβ segment in the Cβ2 locus, deleting the Cβ1 locus and the nonproductively rearranged gene
why can Cβ1 locus only be rescued by Cβ2?
DJ rearrangement ONLY occurs wihtin the respective Cβ locus
i.e. Dβ1 cannot rearrange with Jβ2
where are the mouse TCR-α and TCR-δ genes located?
chromosome 14
where are the α gene segments V, J, and C located relative to the δ gene segment?
at the two ends of the δ gene segment (V on left end and J, C on the right)
what are the implications of the unusual location of Cδ between Vα and Jα?
results in the deletion of Cδ upon Vα-Jα rearrangement, meaning the cell looses its ability to create the TCR-δ chain
how many Jα segments are there in the TCR-α chain gene?
~50
what is permitted by the presence of many Jα segments?
TCR-α chain genes can undergo several successive rearrangement events to “leapfrog” over nonproductively rearranged VJα segments
how long can the repeated rearrangements in the TCR-α gene continue?
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
if there is productive VJα rearrangement, will there ever be production of the δ chain? why or why not?
no never - it will be excised via DNA looping
does TCR germ-line DNA contain fewer V gene segments than Ig germ-line DNA, or vice versa?
what are the implications of this?
- 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
how many V regions are present on the α, β, γ and δ chains?
α = 100 β = 25 γ = 7 δ = 10
how many D regions are present on the α, β, γ and δ chains?
α = 0 β = 2 γ = 0 δ = 2
how many J regions are present on the α, β, γ and δ chains?
α = 50 β = 12 γ = 3 δ = 2
how many V regions are present on the Ig heavy and light chain?
heavy = 300 light = 300
how many D regions are present on the heavy and light chains?
heavy = 12 light = 0
how many J regions are present on the heavy and light chains?
heavy = 4 light = 4
explain how the alternative joining of D gene segments differs btwn TCR and Igs
- 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)
explain how junctional flexibility differs btwn TCR and Igs
- it doesnt - junctional flexibility is exhibited during gene rearrangement in both Igs and TCRs
- it can generate many nonproductive rearrangements, but also increases diversity
how does junctional flexibility increase diversity?
by encoding several alternative amino acids at each junction
compare N-region nucleotide addition for Igs vs TCRs
- 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
compare somatic mutations for TCR and Igs
no somatic mutations for TCR, but can have somatic mutations for Igs
why are there no somatic mutations for TCR genes?
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)
who do somatic mutations occur for Igs?
- 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
compare allelic exclusion for TCRs vs Igs
Igs can and theres evidence to suggest that beta chain can and alpha chain can’t (no evidence for gamma or delta yet)
T-cell receptors contain ____ that bind to the MHC-peptide complex
complementary-determining regions (CDR)
how many CDRs do T-cell receptors contain? what are they
3! - CDR-1, CDR-2 and CDR-3
where is CDR-1 located and what does it bind?
- located in the variable region of the TCR
- interacts with the peptide in the MHC-peptide complex
where is CDR-2 located and what does it bind?
- located in the variable region of the TCR
- interacts with the alpha regions of the MHC in the MHC-peptide complex
where is CDR-3 located and what does it bind?
- located in the D-J region of the TCR
- interacts with the peptide in the MHC-peptide binding groove
