Lecture 13 - Development of the T cell repertoire

Question 1

Describe T cell receptors

Answer 1

Evolved to recognise ‘internal epitopes’ buried within a pathogenic molecule
Closely related to immunoglobulin genes in germline layout and structure
Any one T cell can express up to 30,000 T cell receptors (TcRs)
All these TcRs on a giving T cell will be specific for the same peptide (epitope).
One binding site for antigen (unlike two for immunoglobulins/antibodies)
NEVER secreted from T cells (unlike immunoglobulins that can be secreted)
T cell receptors rearrange only during development- this occurs in the thymus

B cell receptor - Y shaped 2 antigen binding sites, TCR only has one

Question 2

What are the types of T cell receptors?

Answer 2

abTcR:

• Major population of T cells
• Found in blood, secondary lymphoid tissues
• Role clearly defined
• MHC restricted

gdTcR:

• Minor population of T cells
• Found predominantly in epidermis and epithelia of reproductive and intestinal track
• Role not absolutely clear
• May not be MHC restricted

Question 3

Describe abT cell Receptor Interaction with both peptide and MHC

Answer 3

-TcR must make contact with both peptide
and MHC molecule; hence said to be
MHC-restricted
-Aligns diagonally over the peptide-MHC
complex
-TcR alpha chain lies over the alpha 2 domain
of MHC class I + the amino terminal of peptide
-TcR beta chain lies over the alpha 1 domain
of MHC class I and the COOH terminal of
peptide.
-Hypervariable regions of both TcR chains meet
over the central amino acids of the peptide

Question 4

Describe the abT cell receptor locus

Answer 4

Key Features:
-Similar in layout to the B cell receptor
TcRb chain genes have V segments (V), D segments (D) and J segments (J) = Immunoglobulin heavy chain
TcRa chain has V segments, J segments (no D segments) = immunoglobulin light chain
-BUT unlike immunoglobulins, there is a limited number of C segments. This is because the T cell receptor solely acts as a receptor. It is never secreted like immunoglobulins in which the C segments defines the immunoglobulins function (C’ fixation, ADCC etc)

Question 5

Describe the mechanism for rearranging the abT cell receptor components

Answer 5

similar to that of immunoglobulins 
TcRa chain = V recombines with J
TcRb chain= D recombines with J then DJ with V
Key Points:
RSS 
23/12 rule
Excision of intervening
      sequences (TRECS)
Controlled by RAG1 and
      RAG 2 genes
Rearrangement occurs only during T cell development in the thymus

Question 6

Describe how variability in the T cell receptor can be added further by addition of nucleotides
at junctions

Answer 6

T cell receptor
P and N nucleotides added to V D and J junctions (TcRb)
Also added at V and J junctions (TcRa) [less so in Ig light chains]
Combination of rearrangement choices and junctional variability is the formation of 10 to the 18 TcRs
Because the DNA sequence is cut, this facilitates the ability to add/take away nucleotides to generate more diversity - TdT will do this just like for BCRs

Question 7

Why do Antibody genes rearrange in B cells and T cell receptor genes in T cells?

Answer 7

DNA accessibility – Recombination requires open DNA
In B cells, the antibody genes are in open chromatin
In T cells, the T cell receptor genes are in open chromatin

Question 8

What is central tolerance?

Answer 8

The removal of T cells with TcRs that cannot interact with host MHC or has a TcR that binds host MHC-peptide with high affinity
Only occurs in primary lymphoid organs- bone marrow for B cells and thymus for T cells
Results in T cells and B cells that recirculate in the body having a receptor specific for foreign (non-self) molecules.

Question 9

Describe development and education of T cells in the thymus

Answer 9

Pluripotent HSCs transform to common lymphoid progenitors (CLPS) in bone marrow
CLPs have B and T cell potential. Enter thymus via blood vessels at the cortical/medullary junction
Signals from thymic stromal cells commit CLPs to T cell lineage
Committed cells move to cortex where they undergo positive selection.
Positively selected cells move to medulla where they undergo negative selection.
Only cells that pass positive and negative selection allowed to leave thymus and recirculate in blood.
vast majority of thymocytes will die
T cell development in the thymus can be divided into stages based on the expression
of CD4 and CD8 molecules
CLP -> double negative -> double positive (positive selection) -> single positive (negative selection) -> exit thymus

Question 10

Explain the importance of the cortex and its role in positive selection

Answer 10

positively selects which developing T cells will survive and move to the next stage of development
Important for educating T cells as to
your MHC haplotype
Developing DP cells interact with thymic cortical epithelial cells (cTECS)
cTECs express both MHC class I and II + self-peptides in the peptide binding cleft
Strength of binding determines fate of
developing T cell; live or die
MHC class II/peptide = DP -> CD4+ SP
MHC class I/peptide = DP -> CD8+ SP

Question 11

Describe how sequential rearrangement of the TcR locus occurs in cortex

Answer 11

TcRb locus rearranges (D-> J, V->DJ)
A surrogate TcRa binds and stabilises TcRb

TcRa locus rearranges (V->J)
Positive signals via TcR-MHC molecules causes down regulation of CD4 or CD8

Stabilised TcRb triggers upregulation of CD4 and CD8

Question 12

Describe the negative selection of T cells

Answer 12

Cells that survive positive selection move to the medulla, here the next stage of education occurs –Negative selection
Negative selection is the removal of any T cell that carries a receptor for a self-peptide from the T cell repertoire
Key features:
-SP (CD4+ or CD8+) cells move from cortex to medulla
-Medullary epithelial cells (mTECs) express both
MHC class I and II molecules with self-peptides in
the peptide binding cleft
-SP cells bind mTECs and depending on signal strength
they live or die
-SP cells that survive negative selection, leave the thymus and recirculate around body blood/secondary lymphoid organs

Question 13

Describe how Autoimmune Regulator (AIRE) controls Tissue Specific Antigen (TSA) expression

Answer 13

mTECS express AIRE resulting
in expression of molecules (antigens) from
our peripheral tissues.

Here, insulin (red/white arrows) is
expressed on the surface of mTEC (yellow/
green) ready to interact with TcRs specific
for insulin

High affinity interaction will lead to death
of the T cell bearing insulin specific TcRs

Defects in AIRE production/activity leads to T cell-mediated attack of our own tissues.