Selection of T cell receptor repertoire and CD4/CD8 lineage commitment Flashcards
For context, where do T cells come from?
- Multipotent Lymphoid: Progenitors Migrate from the Bone Marrow to the Thymus
- In the Thymus, the Lymphoid Progenitors Differentiate to pre-T Cells and are Educated to Differentiate Self from Non-self
- Positively Selected T Cells Emigrate from the Thymus to Mediate and Effect the Cognate Immune Response
Describe how T cell migration occurs from BM to thymus?
- Thymus secretes a series of cytokines : thymosin, thymotaxin, thympoietin, thymic factors
- These travel to the BM, where T cells recognise these and follow trail along blood vessel to thymus along concentration gradient (high in thymus)
What is hassall’s corpuscle?
Site of aggregation of mature lymphocytes found within the medulla
State the different development stages of the thymocytes in the parts of the thymus?
Thymocytes at different developmental stages are found in distinct parts of the thymus
Step 1: Immature CD3- 4- 8- double negative thymocytes
Step 2: Immature CD3+ 4+ 8+double positive thymocytes
Step 3: Mature CD4+ 8- and CD8+ 4- thymocytes
How can T cell development be characterised? VD
- Flow cytometry
- Allows to see sub-populations with larger cells based on the expression of cell surface markers
Using flow cytometry, what can Double Negative’s be further subdivided into? VD
- DN’s can be subdivided into DN1 through to DN4
alpha chain of IL2R
What type of T cell appears in the fetal thymus first?
Double negative cells appear in the fetal thymus before double positive cells
Describe the T cell populations during development - thymus and spleen in flow cytometry?
VD
- Spleen only contains a single positive CD4+ or CD8+ cells
Lineage commitment
~ Diagram showing T cell development and migration - key concept to understand here is gamma:delta has barely any steps in differentiation to become a mature T cell
T cell development and migration
VD
Between gamma:delta and alpha:beta thymocytes, which are favoured during early fetal development? VD for graph
Gamma:delta T cells are favored during early fetal development
Describe how antigen recognition by gamma:delta T cells is different to alpha:beta?
- Gamma:delta T cells bearing specific receptors end up in skin (Vg5), gut (Vg2), uterus (Vg6), etc.
- Gamma:delta T cells are not MHC restricted
- Antigen is recognized directly, more like an antibody
- In some cases ligands for the gamma:delta TCR are self proteins upregulated under stress conditions
- In humans, circulating gamma:delta cells recognize a phospholipid antigen from Mycobacterium tuberculosis
- Play a role in cancer surveillance
Descrbe the proportion of T cells between TCR alpha:beta and TCR:gamma:delta lineage?
- TCR alpha:beta lineage comprises the majority of T cells
- A:B = 90%
- Gamma:delta = 10%
What factor in T cell development correlates with progression through development?
Rearragement: First is beta chain during Development, then alpha chain during Development progression
What does a Double positive thymocyte require to progress to SP (single positive) stage?
Functional TCR-alpha chain rearrangement
• CD4 and MHC II (To be a CD4+ cell)
• CD8, MHC I and TAP (To be a CD8+ cell)
• ERK signaling
• Calcineurin signaling
State the proportion of cells that fail to complete thymocyte maturation and briefly describe the enzyme required within the process of removing it?
- Most cells fail to complete thymocyte maturation (95%)
- These don’t enter + ve or -ve selection
- These cells undergo apoptosis programmed cell death (DNA fragmentation)
- via enzyme caspase 8
- Causes chromatin disintegration, DNA break up -> cell death
- Removed by macrophages
What are the possible fates awaiting alpha/beta T cell selection (DP Thymocyte)? VD
- Dependent on signal intensity and abiltiy to bind to MHC
- Either Negative selection, positive selection CD4 SP, death by neglect or positive selection CD8 SP
State what cell and level of APC is MHC-I and MHC_II cells in the thymus expressed on?
- MHC-1: Expressed on thymic stromal cells and low level on APC (DC and macrophages)
- MHC-II: Expressed on thymic medullary stromal cells and high level on APC
Positive selection of CD4 or CD8 SP T cells - self restriction
Describe the purpose of positive selection?
- Positive selection ensures that only T cells which are useful and can engage in recognition, are selected
- Specifically DP cells that are able to bind to MHC/HLA
- DP CD4/CD8 cells bind to MHC-I or MHC-II on thymic epithelial cells - it is a random event which one binds•
- Following adequate binding of CD4:MHC-II, CD8 is downregulated and vIce versa
- From here on, the SP CD4 or CD8 T cells are ready for negative selection
- Unselected cells die by apoptosis
Negative Selection
Describe the purpose of negative selection?
- Negative selection ensures that self-reactive cells are removed, as they would cause autoimmunity •
- This is determined based on affinity of TCR for presented self-peptide: high - kill him, low - keep him
- Self-peptide is presented via thymus - remember they don’t represent all self antigens
- This ensures that remaining T cells are only reactive to foreign peptides
- Self-reactive cells are not removed immediately but go through further TCR rearrangements (second chance) - before they are eventually removed if still self-reactive
How are T cells negatively selected against self-antigens not present in thymus?
- The Transcription Factor Autoimmune Regulator (AIRE)
- Mediates Ectopic Gene Expression in the Thymic Medullary Stroma of other tissue specific genes… kidney, heart, liver, lungs, gut, … apart from brain and testes
- Allows -ve selection against most bodily self proteins
- This is known as promiscuous gene expression - about 10% of all genes in thymus are expressed this way
- This eliminates many self-reactive T cells
What 2 factors control thymocyte selection?
- Antigen presentation
- MHC expression
Describe the role of cell types
- Certain cell types control different cell development fates e.g. + ve, -ve selection, receptor editing
- Examples include:
- DC
- Medullary Epithelial cells
- Cortical Epithelial cells
- Endothelial cells
What type of cells will develop in the thymus if there is high expression of CD25 and Foxp3 and state its role?
- Regulatory T cells
- Do not proliferate in response to MHC self-peptide complexes
- T Regs accumulate in Hassall corpuscles and later migrate to different tissues
- Main role: dampen T cell response
What occurs once the T cells leave the thymus? (PART 1)
- T cells that pass both positive and negative selection become conventional T cells
- They migrate to secondary lymphoid organs looking for their target antigen
- ‘Immunological synapse’
What occurs once the T cells leave the thymus? (PART 2)
Possible scenarios:
- If they encounter specific antigen, they get activated, proliferate and become effector T cells
- Some become memory T cells
- If they don’t find the target they, eventually die by apoptosis after period of circulation
Take home messages
- TC chains undergo V (D)J recombination to generate diversity; they also exhibit allelic exclusion
- TCRbeta chain is selected with an invariant pTalpha chain at the DN3 stage
- TCRalpha chain is selected with pre-existing TCRbeta chain at the DP stage
- DP Thymocytes Undergo Positive and Negative Selection to Generate a Population of Mature T Lymphocytes that can Recognize Self MHC with Intermediate Affinity
- Recent Evidence Indicates that Positive and Negative Selection Are Mediated by Distinct Pathways
- APC in the thymus may present self antigen that effect negative selection
Questions you should be able to answer by the end of the lecture
How do you generate a T cell population that is self-MHC restricted?
T cells that are compatible with the hosts genetic makeup
Questions you should be able to answer by the end of the lecture
How do you ensure that those diverse T cell receptors are not- self reactive?
Don’t target the host
Questions you should be able to answer by the end of the lecture
How do you coordinate lineage specification with MH specificity and coreceptor expression?
- alpha/beta vs. gamma/delta T cell
- CD4 vs. CD8
