L15 - T-cells: Generation of Receptor Repertoire Diversity Flashcards
Describe the stages of T-lymphocyte development?
COMMITMENT:
Multipotent progenitor become comitted precursors (pro- tcell)
PROLIFERATION:
Pro-tcells proliferate into pre-tcells
SELECTION:
t cells are selected for the type of receptor they express/produce
DIFFERENTIATION:
Differentiation into distinct functional effector subpopulations
What are the key factors that determine the process of lymphocyte development?
Stem cell factors (c-KIT)
Cytokines (IL-7 and IL-3)
Tissue specific signals (notch and thymic stromal cells)
Describe the different anatomic sites the T-cells pass through and the stages that they are at?
stem cells and pro-lymphocyte = bone marrow
pre-lymphocyte to immature lymphocyte = thymus
mature lymphocyte = peripheral lymphoid organs and tissue
Where do lymphocytes commit to the T-cell lineage?
In the Thymus
Describe the histology of the Thymus?
Cortex is the edge and medulla is within the cortex.
There is a dense network of stromal cells (specificlly epithelial cells) and T-lymphocytes
Describe the journey of t-cells through development
- T-cell progenitors develop in the bone marrow and migrate to the thymus:
T-cell precursors rearrange its T cells receptor genes in the thymus
- Positive and negative selection in the thymus:
Immature T-cells that recognise self-MHC recieve signals for survival. Those that interact strongly with self antigen are removed form the repertoire.
- Mature T-cells migrate to the peripheral lymphoid organs:
Mature T-cells encounter foreign antigens in the peripheral lymphoid organ and are activated
- Activated T-cells migrate to sites of infection:
Activated T-cells proliferate and eliminate the infection
Describe how the key factors play a role in T-cell development?
Notch signals from the thymic stroma - cells commit to the t- cell lineage and differentiate into early precursers
This induces transcription factor GATA3 which causes commitment to lineage
How long after entering Thymus, do T-cells commit to the lineage?
1 week after arrival of precursors into the thymus progenitors commit to the T cell lineage
What early markers are expressed on T cells?
(CD2 and Thy1)
What is the absence of CD8 and CD4 marker on T-cells called?
DN (double negatives)
What are developing T-cells called?
Thymocytes
How do we know T-cells are post DN stage?
They express both CD4 and CD8 and later just one or the other
Describe what the T cell receptor consists of?
T cells express high levels of TCR
TCR is a heterodimer consisting of two transmembrane polypeptide chains covelantly linked to each other by disulphide bonds
What are the two types of T- cell receptors?
One receptor has alpha-beta chains, and the other has gamma-delta chains
Describe what each chain on the T-cell receptor consists of?
has one Ig-like N terminal variable domain (V) and one Ig-like constant domain (C), a hydrophobic transmembrane region and a short signalling cytoplasmic region.
What do the Variable regions of the chains contain?
both contain short stretches of amino acids sequences that is highly variable between receptors.
These regions form the complementary determining regions. The 3 CDRS from alpha chain and the 3 from the beta chain form the peptide MHC binding site
Describe the other components of the membrane that help TCR?
charged amino acids allow the receptor to interact with accessory molecules like CD3.
Zeta chain also interacts with the TCR
How are other components bought together in the TCR complex.
The constant regions have cysteines residues that bring the chains together.
What is the entire structure of TCR, Zeta chain and CD3 together called?
TCR Signalling complex
What is the function of CD3 and Zeta chain in the TCR complex?
Allow for the transduction of signals upon MHC- peptide binding
What is an MHC and how many classes?
Major Histocopatibility Complex
MHC Class 1 and MHC Class 2
What is a HLA?
• HLA Human Leukocyte Antigen
What are MHC Class 1 Molecules?
MHC class I molecules present peptide
antigens derived from pathogens that
replicate inside the cell, such as viruses.
What are MHC Class 2 Molecules?
MHC class II molecules present peptides from
pathogens and antigens that are present
outside the cell taken up by endocytic vesicles
of phagocytic cells.
Why are MHC described as Polymorphic?
The MHC is highly polymorphic. There are multiple variants of each gene
within the population.
Why are MHC described as Polygenic?
The MHC is polygenic, it 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
Describe the structure of MHC?
Has 1 extracellular peptide binding cleft
Ig like domain
Cytoplasmic tail
Which binding site does MHC Class 2 have?
CD4 binding site
Which binding site does MHC Class 1 have?
CD8 binding site
Do MHC 2 or MHC 1 peptides bind to longer peptides?
MHC class 2
What types of peptides can MHC molecules bind to?
they can bind and display foreign and self peptides
Describe what cells express MHC class 1 and MHC class 2?
MHC Class 1 = all cells but erythrocytes
MHC Class 2 = Antigen presenting cells
Describe the pathway of antigen processing and presentation on top of MHC Class 2?
- Uptake of Extracellular protein antigens into vesicular compartments of APC
- Proccessing of internalise proteins in lysosomal vesicles
- APC Cell assembles MHC class 2 molecule.
- Transport of class 2 MHC molecules to lysosome with antigen
- Association of processed peptides with class 2 MHC molecules in vesicles
- Export of vesicle and expression of peptide MHC complexes on cell surface
How is the variable part of the chain formed?
composed of sequences chosen from a group of sequences that are encoded in the germ line
By which order is the Rearranged B chain DNA formed from the germ line
A D fragment is joined with a J fragment, and it then adds a V fragment to the DJ Fragment
Describe the properties of TCR on each T-cell?
only 1 form of TCR is expressed on each T-cell. When it prolifertaes, its daughter cells have only 1 TCR and specificity for antigen.
What enzymes are T-cells receptor genes rearranged by?
Rag 1 and Rag 2
Where do T-cells concentrate diversity?
in the 3rd Hyper-variable region called CDR3
How are alpha-beta and gamma-delta T-cell receptors generated?
both by gene rearrangement
Describe the TCR alpha chain gene rearrangement?
- doesn’t have a D gene segment
- They are arranged after the B-chain gene locus rearrangement
- Successive rearrangements may be attempted until a productive rearrangement has been achieved (one that works with the Beta chain)
How does the T-cell add extra diversity and what is it called?
Called Junctional Diversity
- during the joining of different gene segments, addition or removal of nucleotides may create new sequences at junctions
How is Junctional Diversity mediated?
TdT terminal Deoxynucleotidyl Transferase
Where does Gene rearrangement of TCR take place?
Takes place in the Cortex of the Thyymus
What do CD4- CD8- double negative cells express?
They both express CD44 and CD25
Describe the 4 stages of CD44/CD25 expression?
DN1:
TCR germline - no rearrangment (high esoression of CD44, low expression of CD25.
DN2 AND DN3:
Rearrangment of Beta locus
DN4:
Successful beta arrangment and pre-TCR
What happens after successful beta chain rearrangment?
It signals to the surrounding stromal thymus cells.
It sends the beta chain to the surface, with a surrogate chain (it mimicks the alpha chain).
Signalling through the pre-TCR
suppresses expression of the
RAG genes.
So, no more rearrangement at
this stage, this is allelic
exclusion.
Allelic exclusion ensures that
only one TCRβ chain gene is
expressed.
These events together are
known as β-selection
What are the key points that suggest the successful signalling of a pre TCR?
Successful signaling of a PreTCR:
• Halts further b chain rearrangements
- Induces expression of CD4 and CD8
- Initiates alpha chain rearrangement
