T cell development - Generation of receptor repertoire diversity Flashcards
What are the main events in lymphocyte development?
- Commitment
- Proliferation
- Selection
- Differentiation into distinct functional effector (mature cell to perform a function) subpopulations
What key factors are needed for lymphocyte development?
Key factors: Stem cell factors (c-KIT)
Cytokines (IL-7, IL-3)
Tissue specific signals (Notch and thymic stromal cells)
What do multipotent HSCs give rise to?
What are the two types of T-cells (they are named after there receptor)?
Multipotent HSCs give rise to distinct B and T cell lineages
Alpha beta
gamma delta t-cells
Describe the stages of T-cell development
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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
Mature T-cells migrate to the peripheral lymphoid organs
Activated T-cells migrate to the sites of infection
What are the successive stages in T cell development?
- 1 week after arrival of precursors into the thymus progenitors commit to the T cell lineage
- Express early markers of the T cell lineage (CD2 and Thy1)
- Do not express any of the markers that define T cells later in development or after in the periphery (CD3, CD4, CD8)
- Because of the absence of CD4 and CD8 early developing T cells are called DN (double negatives)
- At DN stage developing T cells (thymocytes) re-arrange the TCR locus
- Stages post DN are characterized by the expression of both CD4 and CD8 and later just one or the other
Describe the acquisition of either CD4 or CD8 receptors
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Describe the structure of a T-cell receptor
- Upon successful rearrangement and in the periphery (if selected) T cells express high levels of TCR
- TCR is a heterodimer consisting of two transmembrane polypeptide chains covalently linked to each other by disulphide bonds
- Two types alpha-beta and gamma-delta
- Each chain has one Ig-like N terminal variable domain (V) and one Ig-like constant domain (C), a hydrophobic transmembrane region and a short signaling cytoplasmic region
- The V regions of both chains contain short stretches of amino acid sequence that is highly variable between receptors. These regions form the CDRs or complementary determining regions. The 3 CDRs of the alpha chain and 3 of the beta chain form the peptide-MHC binding site
Compare and contrast TCR and Ig
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What does the binding of the charged residues of the transmembrane region allow?
- The C regions have cysteines residues that bring the chains together
- Charged residues in the transmembrane region bind to CD3 and the zeta chain to form the TCR signaling complex
- CD3 and zeta allow for the transduction of signals upon MHC-peptide binding
How do most T-cells recognise peptides and no other molecules?
Only peptides bind to MHC molecules
How do T-cells recognise linear peptides and not conformational determinants of protein antigens?
Linear peptides bind to the clefts of MHC molecules, and protein conformation is lost during the generation of these peptides
How do T-cells recognise cell associated and not soluble antigens?
Most T-cells recognise only peptide MHC complexes and MHC molecules are membrane proteins that display stably bind peptides on cell surfaces
What do cells do MCH class 1 and 2 express peptides to?
MHC 1 = Cytotoxic T-cell (CD8+)
• MHC class I molecules present peptide antigens derived from pathogens that replicate inside the cell, such as viruses.
MCH2- T-helper cell (CD4+)
• MHC class II molecules present peptides from pathogens and antigens that are present outside the cell taken up by endocytic vesicles of phagocytic cells.
What is MHC?
MHC is the Major Histocompatibility Complex
How are MHC molecules polymorphic and polygenic?
- The MHC is highly polymorphic. There are multiple variants of each gene within the population.
- 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
- Thus polymorphism and polygeny lead to a high degree of polymorphism
Describe Pathway of antigen processing and presentation on top of MHC class II
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Describe the overview of TCR rearrangement
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Describe TCR gene in germ-line configuration before rearrangement
The T-cell receptor gene segments are arranged in a similar pattern to immunoglobulin gene segments and are rearranged by the same enzymes; Rag 1 and Rag 2
T-cell receptors concentrate diversity in the third hypervariable region CDR3.
: T-cell receptors are also generated by gene rearrangement.
What do RAG 1 and RAG 2 enzymes do?
- RAG 1 and 2 genes mediate the recombination events leading to rearrangement”
- This process is antigen-independent
How are the alpha chains different to the beta chains on TCR?
- They do not have D gene segments
- They are rearranged only after the TCRβ chain gene locus has been rearranged.
- Successive rearrangements may be attempted until a productive rearrangement has been achieved
What creates even more diversity when creating TCRs?
Describe the two checkpoints in T-cell development
During the joining of different gene segments, addition (or removal) of nucleotides may create new sequences at junctions.
Mediated by TdT terminal deoxynucleotidyl
transferase
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What is allelic exclusion?
- 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 does Successful signaling of a PreTCR do?
- Halts further b chain rearrangements
- Induces expression of CD4 and CD8
- Initiates alpha chain rearrangement
