T Cell Development, Generationnof Receptor Diversity Flashcards
Describe the stages in lymphocyte development/maturation
Haematopeoisis - Commitment
Proliferation
Selection
Differentiation into distinct functional effector Sub populations
What are the key factors in lymphocyte development?
Stem cell factors
Cytokines - (IL-7, IL-3)
Tissue specific signals - notch and thymic stromal cells
Describe the journey of T cells through development
T cell progenitors develop in the bone marrow and migrate to the thymus
Positive and negative selection in the thymus
Induction of GATA3 - Commitment to the T cell lineage
Mature T cells migrate to the peripheral lymphoid organs
Activated T cells migrate to sites of infectiion
How are successive stages in T cell development marked?
By changes in surface receptors
Describe the changes in cell surface receptors that occurs in T cell development
1 week after arrival of precursors into the thymus, progenitors commit to the T cell lineage
These progenitor cells express early markers of T cell lineage (CD2 and Thy1)
Because of the absence of CD4 and CD8 early developing T cells are called DN (double negatives)
DN cells rearrange the T cell receptor locus
DN are charectrid by the expression of both CD4 and CD8 and later just one or the other
Describe the structure of a T cell receptor
Heterodimer
Two transmembrane polypeptide chains - covalently linked by disulphide bonds
Two types alpha-beta and gamma-delta
Each polypeptide chain has:
- Each chain has one N terminal
- variable domain with short stretches of animo acid sequences that are highly variable between receptors - form CDRs or complimentary determining regions
- one constant domain
- a hydrophobic transmembrane reigon
- short signalling cytoplasmic region
The 3 CDRs of the alpha chain and 3 of the beta chain form the peptide-MHC binding site
What’s the difference between TCR and immunoglobulin?
The activation is different
TCR activation is from membrane bound proteins and there is NO somatic mutation or isotope switching
How does a TCR signalling complex form?
The C regions have cysteines residues that bring the chains together
Charged residues in the transmembrane reigon bind
to CD3 and zeta chain to form the TCR signalling complex
CD3 and zeta allow for transduction of signals upon MHC binding
Describe the structure of a MHC molecule
- Extracellular peptide binding cleft
- Ig-like domain
- Cytoplasmic tail
MHC class 2 has a conserved CD4 binding site
MHC class 1 has a conserved CD8 binding site
How are MHC molecules polymorphic?
There are multiple variants of each gene within the population
How are MHC molecules polygenic?
Contains several different Class 1 and class 2 genes.
This every induviduals possesses a set of MHC molecules with different ranges of peptide binding specificities
Describe the MHC peptide interactions
Acquire peptides while assembled inside the cell
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 foreign and self peptide
MHC class 2 bind to longer peptide than class 1
Where are MHC class 1 located?
All cells but erythrocytes
Where are MHC class 2 located?
Antigen presenting cells
Describe the pathway of antigen processing and presentation on top of MHC class 2
- Uptake of extracellular proteins into vesicular compartments of APC
- Processing of internalised proteins in endosomal/lysosomal vesicles
- Biosynthesis and transport of class 2 MHC molecules to endosome
- Association of processed peptides with class 2 MHC molecules in vesicles
- Expression of peptide MHC complexes on cell surface
Describe the genetic processes that produces the TCR
Germ line alpha chain DNA > rearranged alpha chain DNA
Germ line beta chain DNA > rearranged beta chain DNA
Protein product alpha beta heterodimer
What are the properties of the TCR?
Only one form of TCR expressed on each T cell -
Each T cell and it’s daughter cells have only one TCR and one specificity for antigen - T cell clone
Infinite number of different versions of TCR each with a unique antigen binding site
A TCR has only one antigen binding site
What is the TCR gene in germ line configuration?
T cell receptor gene segments are arranged by the same enzymes as immunoglobulin
- Rag 1 and Rag 2
TCR concentration diversity in the thirst hyper aria blue region CDR3
What are the roles of RAG 1 and 2 genes in biosynthesis of TCR?
They mediate the recombination events leading to rearrangement.
Describe the structure and function of alpha TCR chain genes
Thy do not have D gene segments
They rerearranged only after the beta chain gene has been rearranged
Successive rearrangements may be attempted until. Productive rearrangement has been made.
What is each T cell receptor made of?
Alpha
- variable and control
Beta.
- variable and control
How can junction diversity occur?
During the joining of different gene segments, the addition or removal of nucleotides may create new sequences at junctions.
Mediated by TdT terminal deooxynucleotidyl transferase
What are the checkpoints for T cell development?
What is beta selection?
Suppression of expression of the RAG genes - allows no more rearrangements in this stage - allelic exclusion
Allelic exclusion ensures that only one TCR beta chain gene is expressed
These events are known as beta selection
Also induces further expression of CD4 and CD8
What are the results of successful signalling of a preTCR?
Halts further b him rearrangents
Induces expression of CD4 and CD8
Initiates alpha chain rearrangements
