An introduction to T cells and the T cell receptor

Question 1

Describe the recognition process by T-cells

Answer 1

Must be able to recognise some feature of the pathogen from outside the infected cell!
- This obviously can’t be done by antibodies since they recognise intact proteins
* It is achieved by a complex process of antigen proteolysis (processing) inside the infected cell
- Peptides are presented on the cell surface by MHC class I or class II molecules
* The MHC-peptide combination is recognised by the T cell receptor (TCR)
- Just like antibodies, the repertoire of possible TCR specificities is huge
* Recognition activates signalling processes inside the T cell leading to functional responses
* T cells can be either short-lived “effector” cells or they can become “memory cells”

Question 2

What type of immunity are T-cells?

Answer 2

• Essential for cell-mediated immunity
• The Listeria model (an intracellular bacteria)

Question 3

How do T-cells activate macrophages?

Answer 3

Question 4

How is the requirement for T-cells pathogen-specific?

Answer 4

• The nature of the cell-mediated immune response
  depends upon the pathogen
• Killing of listeria and some other intracellular
  bacteria require T cells and macrophages
• Killing of parasites by T cells requires IgE and
  eosinophils and/or mast cells
• Killing of many virally-infected cells by T-cells
  does not require any other cell types

Question 5

What are the several different types of T-cells?
List their main functions and the pathogens they target.

Answer 5

Question 6

Describe MHC restriction

Answer 6

cells have to recognise both peptide and the MHC allele presenting it

Will only recognise antigen if it is presented by self MHC

Question 7

What would happen without the need for
dual antigen/MHC recognition?

Answer 7

• Toxic shock syndrome e.g. caused by the staphylococcal syndrome toxin-1 which acts as a superantigen
• The superantigen binds directly to both MHC class II and T-cell receptor, triggering multiple T cells to produce cytokines

Question 8

Describe the T-cell receptor (TCR)

Answer 8

• T cells all possess receptors which, like surface
  immunoglobulins (the antigen receptors on B-cells), have a very large range of specificities
• Like Ig, this is due to highly variable amino acid sequence of the variable (antigen-binding) region

Question 9

Describe the anatomy of the thymus

Answer 9

Question 10

What are the similarities and differences between the T-cell receptor structure and immunoglobulin structure?

Answer 10

• Highly variable antigen-binding domains attached to constant regions
• Clearly they share a common evolutionary origin
• BUT - there are important differences:
• Antibodies can have multiple binding sites; TCR has one
• TCRs do not bind to native antigens but only to processed (cleaved) peptides (7-20 amino acids) bound in the cleft of MHC-encoded proteins.
  *, Unlike antibodies, the TCR is not a direct effector

Question 11

What determines TCR diversity?

Answer 11

Rearrangement of TCR genes generates
diversity as for Immunoglobulins

• No complete gene exists in the germline
• As for Igs, TCR genes are encoded in separate segments (Vs, Js and a C for TCRalpha and Vs Js, Ds and Cs for TCRBeta).

Question 12

Where are TCR genes rearrangement?

What does it require?

Answer 12

TCR genes are rearranged during T-
lymphocyte development in the thymus

• Gene rearrangement is essentially as for
  immunoglobulin genes, requiring RAG genes and signalling sequences at the appropriate sites to allow the looping out or inversion of intervening DNA to bring single V, D, J and C’s together

Question 13

Describe how junctional diversity is prominent in TCR genes

Answer 13

• Insertion of non-coded bases and variation in exact joining site is more important in TCRs than in Igs.
• This results in diverse amino acid sequences in the CDR-3 loops (α green; β magenta), most of which make contact with the presented peptide

Question 14

Where do T-cells originate?
Describe their maturation

Answer 14

• Derive from haematopoietic stem cells in bone marrow
• Emigrate to the thymus (thymus-dependent lymphocytes = T cells)
• Acquire their TCR in the thymus and become CD4+ or CD8+ cells
• Naïve T cells migrate to secondary lymphoid tissue (lymph nodes)
• Naïve T cells interact with peptides presented on MHC molecules by antigen-presenting cells (APCs)
• Productive interaction with a T cell expressing a high-affinity TCR leads to clonal selection and amplification of
  the T cell
• Selected and amplified cells leave lymph nodes

Question 15

Where is the selection of the T-cell receptor?

Answer 15

The T cell repertoire is selected in the thymus during development

Question 16

What happens to CD4−CD8− cells when they first enter the thymus?

Answer 16

• The newly arrived cells in the thymus are CD4−CD8−cells, and are termed early thymic progenitor (ETP) cells; they do not express the TCR genes. As a result of being CD4- and CD8- they are called DOUBLE NEGATIVE cells
• In the thymus these cells upregulate CD25 (the IL2 receptor) and the recombination genes RAG1 and RAG2 and they re-arrange the TCRβ locus

Question 17

What happens to the CD4−CD8− cells after having rearranged the TCRβ?

Answer 17

• Having rearranged the TCRβ gene the cells then “try it out”!
• The T cells express an invariant α-chain called pre-Tα alongside the TCRβ gene
• If the rearranged β-chain successfully pairs with the invariant α-chain, signals are produced which cease the rearrangement of the β-chain (and silence the alternate allele)
• If the pre-TCR forms, these cells undergo a round of proliferation and then begin to re-arrange the TCRα locus
• Concomitant with this, the production of a pre-TCR signals cells to start to transcribe the genes for CD4 and CD8. BOTH genes are stimulated, leading to the production of CD4+CD8+ DOUBLE POSITIVE cells

Question 18

T cell receptors produced can be grouped into 3 classes:

Answer 18

• Potentially useful in interacting with foreign pathogens (want these)
• Pretty useless in that they don’t interact with anything
• Potentially actually harmful - reacting with self-antigens ( you don’t want the latter two)

Question 19

How do TCR-positive, CD4+/CD8+ Double-positive thymocytes receive survival signals?

Answer 19

-They migrate deep into the thymic cortex, where they are presented with self-antigens.
-These self-antigens are expressed by thymic cortical epithelial cells (CTECs) on MHC molecules on the cell surface.
-Only those thymocytes that interact with MHC-I or MHC-II will receive a vital “survival signal”.
-cTECs are unusual in expressing both MHC class I and MHC class II genes

Question 20

Describe the Selection of the T cell repertoire for alpha beta T-cells

Answer 20

• For ab T cells, the TCR is first expressed during development in the thymus (so this is where selection occurs)
• T cells undergo “negative selection” eliminating self-reactive cells
• T cells undergo “positive selection” of cells with TCRs capable of interacting with self-MHC

Question 21

Describe positive selection for cells that do not interact strongly with MHC

Answer 21

Cells that do not interact strongly enough with MHC peptide complexes will die by “death by neglect” (no survival signal).
This process ensures that the selected T-cells will have an MHC affinity that can serve useful functions in the body (i.e., the cells must be able to interact with MHC and peptide complexes to effect immune responses).
The vast majority of developing thymocytes will die during this process. The process of positive selection takes a number of days

Question 22

Describe positive selection for those that interact well with MHC I and MHC II

Answer 22

• A thymocyte’s fate is determined during positive selection.
• Double-positive cells (CD4+/CD8+) that interact well with MHC class II molecules will eventually become CD4+ cells, whereas thymocytes that interact well with MHC class I molecules mature into CD8+ cells.
• A T-cell becomes a CD4+ cell by down-regulating the expression of its CD8 cell surface receptors.
• If the cell does not lose its signal, it will continue downregulating CD8 and become a CD4+, single-positive cell.

Question 23

What happens to thymocytes that may cause autoimmunity?

Answer 23

Positive selection does not remove thymocytes that may cause autoimmunity. The potentially autoimmune cells are removed by the process of negative selection, which occurs in the
thymic medulla

Question 24

Describe negative selection

Answer 24

• Negative selection removes thymocytes that are capable of strongly binding with “self” MHC peptides. Thymocytes that survive positive selection migrate towards the boundary of the
  cortex and medulla in the thymus. While in the medulla, they are again presented with a self-antigen presented on the MHC complex of medullary thymic epithelial cells (mTECs).
• Thymocytes that interact too strongly with the self-antigen receive an apoptotic signal that leads to cell death. However, some of these cells are selected to become Treg cells. The
  remaining cells exit the thymus as mature naïve T cells. This process is an important component of central tolerance and serves to prevent the formation of self-reactive T cells that are capable of inducing autoimmune diseases in the host.

Question 25

Two important features of negative
thymic selection

Answer 25

• This system only works if it can eliminate self-antigens that are expressed in all tissues; the thymus is a specialised organ and should not be able to do this! The problem is solved by mTECs expressing a transcription of the factor called AIRE, which allows promiscuous expression of self-antigens from all tissues of the body on MHC class I
• This leaves the problem of how you negatively select T-cells that strongly recognise self-antigens presented in MHC class II. This is solved by thymic dendritic cells which can phagocytose mTECs; this allows for presentation of self-antigens on MHC class II molecules

Question 26

What happens to mature T cells?

Answer 26

“Mature” T cells enter the circulation
* After the development of a rearranged TCR T cells leave the thymus and relocate in lymph nodes
* Known as ‘naïve’ until they first encounter their target antigen
* Encounter takes place in secondary lymphoid tissues
* Following the encounter and proliferation, they exit from lymph tissue as “effector” T cells and migrate to the site of infection

Question 27

What attracts naïve T cells into the lymph
nodes?

Answer 27

• The chemokine receptor CCR7 on the naïve T cell surface recognises CCL21 produced by specialised blood vessels called high endothelial venules (HEV)
• CCR7 also recognises CCL19 produced by dendritic cells in the lymph node

Question 28

Describe the entry of Naive T cells into the lymph node

Answer 28

Entry into the lymph node depends on chemokines and adhesion molecules
* 1) L selectin on naïve T cell induces rolling
* 2) Chemokine CCL21 recognition activates
integrins to enhance their binding
* 3) T cell continues to follow
chemokine gradient into
lymph node

Question 29

Describe naive T cells in the lymph node

Answer 29

Naïve T cells can make contact with thousands of dendritic cells in the lymphoid tissues
every day

Naïve T cells that do not encounter their specific antigen exit through the efferent lymphatics, re-enter the blood and continue circulating

If they encounter specific
antigen, they cease to migrate

Question 30

How are effector T-cells guided to sites of
infection

Answer 30

Effector T cells are guided to sites of infection by chemokines and newly expressed adhesion molecules
* Effector T cells cease production of L-selectin
(so no longer home to lymph nodes)
* Express VLA-4, which recognises VCAM-1
* VCAM-1 is found on activated vascular
endothelium when inflammation is triggered

Question 31

How do effector T cells find targets?

Answer 31

• Non-specific adhesion proteins allow initial binding to potential targets
• If the target has the correct antigen/MHC
  combination T cell receptor signalling occurs
• TCR signalling induces conformational
  change in adhesion proteins – strengthens binding
• Focussed release of effector molecules

Question 32

Describe gamma-delta T-cells

Answer 32

• Discovered during studies of V-D-J recombination
• Their T cell receptor 2 distinct chains, which form gd heterodimer
• Less variable than the conventional TCR, together with CD3; may play a role in innate immunity
• Found in peripheral sites (skin, lung, intestine, liver)
  gd T cells may be able to recognize antigens in an MHC-independent fashion, for example:
• gd T cells can be found in normal numbers in the MHC class I and class II-deficient mice;