T cell receptor, antigen recognition Flashcards
How do T cells differ from B cells? Development.
Location?
Thymus (T cell)
- positive and negative selection happen here
Bone marrow (B cells)
- negative selection
- BUT for positive selection they go to secondary lymphoid organs
Receptor
B vs T cells
similar mechanism but different structure
- both derived from gene rearrangement
- B cell has four chains
- T cell has two chains
Isotype switching?
Affinity maturation?
T vs B cells
Isotype switching
- T cells do not undergo isotype switching or affinity maturation.
The constant region of the T cell recepor does not have specific functions, like the Fc (constant region of heavy chain) on B cell receptors
How do B and T cells recognize antigens
T cells require MHC
B cell receptors recognize antigens directly (do not need MHC) - they can simply bind to native antigens
Function of T vs B cells
T cells - either kill infected host cells or make cytokines
B cells - make antibodies
Beta chain - somatic recombination
VDJ
like the heavy chain of B cell receptor
alpha chain - somatic recombination
VJ
like the light chain of B cell receptors
VDJ segment encode
antigen binding site on the beta chain of T cell receptor
VJ segment encode
antigen binding site on the alpha chain of the T cell receptor
Where is there more varible segments (V) in T cell receptors?
Alpha
D segments in immunoglobin and T cell receptors
Only in Heavy chain (B cell)
- has more than in T cell receptors
Only in beta (T cell )
Joining segments
About equal in B cells between H and lambda/kappa
Very numerous in the alpha chain of T cell receptor
Junctional diversity
Much higher in T cell receptors, compared to in B cell receptors
Which type of receptor has more total diversity
Significanly higher in T cells receptors.
5 orders of magnitude difference
Two classes of T cell receptors
- Alpha + Beta = most common
- Gamma + delta
This decision is made during rearrangement
Alpha + beta T cell receptor
- Most common / majority of Conventional T cells
- Recognize MHC/peptide
- High degree of receptor diversity
- Cells abundant in all secondary lymphoid tissues
Both types of T cell receptors:
- Always transmembrane
- Short cytoplasmic tails
- Constant region - does not participate in antigen binding
- Variable regions - antigen binding site
1. variable regions are made up of two chains
Gamma / delta
T cell receptor
- Smaller subset of T cells
- Recognition specificity not fully clear, MHC class Ib
- Lower degree of receptor diversity
- Cells abundant in gut mucosa
Types of alpha/beta T cells receptors
CD4+ T helper cells
CD8+ Cytolytic T cells
CD4+ T helper cells
- Upon activation, produce cytokines to “help” other cells clear infection
- Recognize MHC class II via CD4 interaction
- Antigens presented to these T cells tend to be from extracellular sources
- Depending on the cytokines produced, these cells can be further sub-divided
CD8+ Cytolytic T cells (CTL)
- Upon activation, kill target cells through several mechanisms
- Recognize MHC class I via CD8 interaction
- Antigens presented to these cells end to ocme from cytoplasmic sources
CD4 structure
Monomeric protein consisting of D1-D4
CD8 structure
Dimer
consiting of alpha and beta
CD8 binds the _ domain of _
Alpha3
MHC Class I
CD4 binds the _ domain of _
Beta2
MHC Class II
TCR complex
The TCR needs the help of CD3 signaling complex to transmit signals for activation
CD3 (signaling adapting molecule) also required for cell surface expression.
CD3 initiates a signaling transduction cascade
CD3 chains
Conists of 6 chains
- 2 epsilon
- 2 zeta
- 1 gamma
- 1 delta
CD3 complex
accompanies the actual T cell receptor (alpha and beta chain), but does not participate in antigen binding
Not part of the T cell receptor, although it helps with the signaling for that receptor
Steps in T cell receptor formation
-
Beta, gamma and delta chains attempt rearrangement simultaneously. Order of gene segment selection is similar to B cells
- if beta is succesful, then it is tested with the pre-TCR (beta+pTalpha) - surrogate light chain
- if gamma and delta are both successful, then rearrangement is complete and the cell will be a gamma/delta. - Alpha, gamma, delta chains attempt rearrangement simultaneously
- if alpha is succesful, then it is combined with beta and selection can begin
- If gamma and delta are both succesful, then rearrangement is complete and the cells will be a gamma/delta
surrogate alpha chain in T cell receptor
Pre - TCR
(beta+pTalpha)
tests if the beta chain is able to pair with alpha.
A common double negative T cell receptor progenitor gives rise to A;B and Y;D T cells
- uncomitted proginitor (CD34) and travel from BM to thymus
- Committed double negative T cell progeniator (express CD2)
- Beta, gamma and delta rearrangement
- if beta is succesful, the beta chain will be tested with the preTcell receptor surrogate chain
- if succesful, rearrangement on alpha, delta and gamma occur. If gamma and delta rearranges before alpha, the cell becomes a gamma/delta T cells
- If the gamma delta rearrangement happened before beta, the cell becomes a y;d cell
Expression of CD4 and CD8 during rearrangement
When cells enter the thymus, they express neither CD4 or CD8. (making them a double negative cell)
When they have succesfully come to the preTcell receptor stage (not yet committed), they will upregulate both CD4 and CD8 (making them double positive).
When the cell has comitted to be a CD4 or CD8, they are (single positive, because they only express either CD4 or CD8)
After they have gone through the entire selection process, they become a single positive cell.
if the gamma/delta rearrangement is succesful
the gamma:delta cell matures, leaves the thymus and migrates to peripheral tissue
NO pre-TCR stage on gamma;delta cells
If the beta chain rearrangement is succesful
Pre TCR testing
then, the pre-T cell resumes rearrangement of alpha, gamma and delta.
Why do y;d cells not go through a stage where they express a Pre-TCR?
because both receptor are rearranged at the same time.
No time/room for a pre T cell receptor test.
Pre-TCR serves a parallel purpose to the Pre-BCR
- Test the ability of a rearranged B chain to form a complex with the pre T-alpha chain
- Stops rearrangement at the other beta locus (second chromosome) - allelic exclusion
Pre T cell receptor
vs
T cell receptor
pTalpha is present in the Pre T cell receptor, whereas in a T cell receptor alpha has taken pTalpha’s place.
pTalpha forms with beta to make sure that the beta is fully formed and ready to pair with alpha.
Productive rearrangement of the beta locus
only two attempts can be made to achieve a productive rearrangement
Each chromosome **has two chances to generate a rearrangemnt of the beta chain locus. **
total = four attempts to make a beta chain
the alpha chain locus can sustain many attempts at a functional rearrangement
important implication for A:B vs Y:D TCR rearrangement
the delta gene segments are situated within the alpha gene locus, so rearrangement of the alpha chain results in deletion of the delta loci, so no chance to make a Y:D TCR.
delta regions
VDJ
gamma regions
VJ
rearrangement of an alpha chain gene results in
the elimination of delta chain locus.
RAG expression
reflects the timing of rearrangement of the beta and alpha chains.
Rearrangement:
- V-J beta (first checkpoint)
- V-DJ beta (first checkpoint)
- V-J alpha (second checkpoint)
CD4, CD8 function
Co-receptor
After the Pre-T cell receptor has been sucessfully tested.
RAG1-2 function
Lymphoid specific recombinase
Rearrangment of b,y,d
Rearrangment of a,y,d
when are CD4 and CD8 expressed
When T cells begin rearrangement they are CD4 and CD8 double negative
When the Pre-TCR is tested and proven to be succesful, CD4 and CD8 will be upregulated (double positive)
After selection is complete, and the T cell is ready to enter periphery, only CD4 or CD8 is expressed
CD8 T cell + virus infected host cell
killing of the virus infected cell
shuts off viral infection
CD4 T cell help + macrophage
Macrophage may express MHCII complexes which are recognized by CD4-T cell and the CD4-T cell secretes cytokines such as IFNy which activates the macrophage.
The macrophage is now able to kill bacteria and secrete cytokines to increase inflammation
CD4 T cells + B cell
Important for B cell activation
CD40/CD40L interaction + cytokines.
These signals instruct the B cells to perform isotype switching and differntiation.
This cell may now become a B cell that secretes antibodies/
