T cell development and generation of repertoire diversity Flashcards
What is the process of haematopoiesis where the result is a fully matured T cell?
Common precursor –> Pro T cell –> Pre- T Cell –> Fully matured T lymphocyte
- Involves the transition from a multipotent progenitor commitment into the lymphoid lineage - at this point can go into either B cells or T cells
- Proliferation then happens from Pre T-cell to fully matured T cell
How does a multipotent haemoatopoietic stem cell give rise to the T cell lineages?
What are the stages of T cell maturation?
What is the thymus?
gland above the heart and has many precursors that are committed to the T cell lineage
- Has many lobes
- Has cortex and medulla
How do T-cell progentiors commit to the T cell lineage?
Notch signals by thymic stroma make progenitors commit to T cell lineage
Notch signals induce activation of transcription factor GATA3 – essential for lineage commitment and development of precursors which results in intense proliferation of T cells.
What is the journey from a T-cell precuroser to being activated to eliminate infection?
Once the cell passes positive and negative selection - cells can leave the thymus and get in contact with antigen presenting cells in the lymph nodes and spleen and become activated cells that can carry out the affect function, either activate macrophages or kill a virally infected cell
How can you see what part of development the T cell is in from its surface receptors?
Express early markers of the T cell lineage - CD2 and Thy1
- But Do not express any of markers that define T cells later in development - eg. CD4 or 8
Early developing T cells are called DN (double negatives) - due to the absence of CD4 and CD8
- At the DN stage, developing T cells (AKA thymocytes) re-arrange the TCR locus
** stages post DN are characterized by the expression of both CD4 and CD8
How can you see T cell lineages on flow cytometry?
Describe the T-cell receptor
Heterodimer consisting of two transmembrane polypeptide chains covantely linked to each other by disulphide bones
Upon successful rearrangement and in the periphery (if selected) T cells express high levels of TCR
2 types:
- Alpha-beta
- Gamma-delta
Each chain consists of one Ig-like N terminal variable domain (V) and one Ig-like constant domain (C ), hydrophobic transmembrane region and a short signalling cytoplasmic region
- V regions of both chains contain short stretches of amino acid sequence that is highly variable between receptors
- These regions form the CDRs (complementary determining regions)
- 3 CDRs of the alpha chain and 3 CDRs of the beta chain form the peptide-MHC binding site
- These regions form the CDRs (complementary determining regions)
How is the TCR similar to the immunoglobulin?
What is the TCR signalling complex?
The TCR interacts with accessory molecules such as CD3 as well as the zeta chain
These are essential to send signals downstream of the receptor to activate the T cell binding of the peptide MHC complexes in or using the variable regions of the antigen binding site
CD3 and zeta chain, together with the TCR = TCR signalling complex
What is the process that determines if a T cell is functional or not?
Some parts of the peptide bind to the T cell receptor (T cell contact residue of peptide), other parts bind to the MHC (anchor residue of peptide)
–> this process determines whether a T cell is functional or not (due the binding) - molecular principle of MHC restriction
In development, T cells need to be selected to be able to bind self MHC
What is the MHC?
Major histocompatibility complex
Class 1 - present peptide antigens from pathogens that replicate inside the cell (viruses)
- Has a conserved CD8 binding site
Class 2 - present peptides from pathogens and antigens that are present outside the cell taken up by phagocytosis
- 2 chains
- Extracellular peptide binding cleft
- Cytoplasmic tail
- Has a conserved CD4 binding site
How are MHC molecules polymorphic and polygenic?
Polymorphic - multiple variants of each gene within the population (different locations of polymorphic residues or amino acid and how that determines the interaction with peptides)
Polygenic - contains several different MHC class I and class II genes. Thus every individual possesses a set of MHC molecules with different ranges of peptide binding specificities
What happens in MHC-peptide interactions at the residue location?
Each MHC has one cleft that binds one peptide at the time but can bind different peptides (due to high diversity)
Peptides that bind one MHC share structural features that promote binding
Peptide-MHC interactions are saturable with low off rate
Very small number of MHC-peptide complexes can activate a T cell
MHC molecules can bind and display both foreign and self peptides
MHC class 2 binds to longer peptides than class 1
Where are MHC class 1 and class 2 found?
What is the pathway of antigen processing and presentation on top of MHC class 2?
Protein antigen is phagocytoses and goes through endocytic pathway to be destroyed by enzymes within lysosomes
At the same time, the cell assembles MHC class 2 molecules in ER and blocks the peptide binding grove by the association with LI and CLIP chain
MHC class 2 molecules is exported to the Golgi and then vesicles fuses with the lysosome
Molecule is released and exchanged by a particular peptide that has affinity for the diverse binding grove - when this happens, the MHC class 2 molecule is exported into the plasma membrane for presentation onto CD4+ T cell
- This process prevents endogenous peptides from binding to the MHC class 2 molecule in the context of antigen presentation
What is the TCR made up of in arrangement?
TCR is made up of a and b chain and both have each a variable and constant domain. Variable domain is composed of sequences chosen from a group of sequences encoded in the germ line.
- First the D fragment is combined with a J fragment
- Then joining a particular V fragment with the D-J fragment making a rearranged product.
- This is joined with a C product to form a Beta chain.
- After this, the same happens with the alpha chain to form the rearranged alpha form.
**The alpha chain does not have a D fragment, only beta does
How does rearrangment happen simultaneoulsy with the maturation of the T cell receptor?
How does beta chain rearrangment happen?
- Initial process of rearrangement mediated by Rag that joins D with J fragment.
- Then a V fragment joins the D-J fragment that were recombined. The primary transcript is formed.
- The middle elements are spliced out of the primary transcript to form a beta chain.
- The L sequence tells the cell where to send the polypeptide chain.
How does alpha chain rearrangment occur?
- No D fragment so V and J fragments join.
- Production and splicing of the transcript.
- Alpha chain is formed and then combines with the beta chain to form the TCR.
How is each TCR made up of different gene segements?
How does TCRa chain gene differ from beta?
They do not have D gene segments
They are rearranged only after the TCRb chain gene locus has been rearranged.
Successive rearrangements may be attempted until a productive rearrangement has been achieved.
Describe the early development of ab T cells in the thymus
