Cytotoxic T cells Flashcards
What do cytotoxic T cells look like?
Where do cytotoxic T cells come from?
- Like B cells, T cells are ‘born’ in the bone marrow from
- B and T cells are descended from HSC (hematopoetic stem cells )(haematopoietic stem cells)
- BUT B cells stay in the bone marrow whilst T cells go to the thymus to mature
- T cell development is a bit more complicated than B cell
What distinguishes T cells from B cells?
How are they similar?
- The T cell receptor (TCR) distinguishes T cells from B cells. Instead of a long heavy chain and a shorter light chain, TCRs have two chains with a transmembrane region (anchored in the membrane of the T cells)
- The T cell receptor resembles the B cell receptor; membrane bound Ig. There is an antigen binding site on one end and a constant domain at the other end
The T cell receptor (TCR) resembles the B cell receptor (BCR; membrane-bound Ig):
Describe its structure and composition
- The T-cell receptor is a membrane-bound heterodimer composed of an α chain of 40-50 kDa and a β chain of 35-46 kDa.
- The extracellular portion of each chain consists of a C domain and a V domain.
- The three-dimensional structure formed by the four domains of the T-cell receptor resembles the antigen-binding fragment of an antibody (BCR).
TCR can be αβ or γδ
- Each TCR is made up of two proteins (αβ or γδ)
- TCR protein chains are made by gene rearrangement (just like BCR)
- T cell competition is between αβ and γδ chains - whoever is successfully rearranged and expressed first is expressed on the cell surface
- T cells are either αβ or γδ (no mixtures)
How many circulating T cells have αβ TCR?
What do αβ T cells express?
Where are αβ T cells educated?
- 95% circulating T cells have αβ TCR
- αβ T cells express co-receptors (either CD4 or CD8)
- αβ T cells TCR are diverse (like BCR)
- αβ T cells are educated in the thymus
What do γδ cells not express?
What surfaces are they important in protecting?
Do they mature in the thymus?
Are they diverse?
- γδ T cells seem to be important in protecting mucosal surfaces
- Have a very broad number of antigens they will respond to
- γδ T cells don’t mature in the thymus
- γδ T cells TCR not as diverse - limited gene rearrangement
- γδ T cells keep watch, tuned in to specific invaders
Explain how T cells follow a developmental route through the thymus:
Where do they proliferate?
Where do they exit?
What happens at each stage of T cell development?
T cell precursors move to the outer cortex, proliferate, pass through thymic medulla and exit at the cortico-medullary junction
The stages in T cell development are defined by the expression of cell surface markers:
- Immature T cell that has just left the bone marrow →DN double negative thymocyte (carries neither CD4- nor CD8-)
- Upon entry into the thymus→DP double positive thymocyte (CD4+ CD8+) during this phase they interact with star shaped (self) antigen bearing cells and the T cells can see if CD4 or CD8 is a better fit
- By the time they leave the thymus they commit to being one type of T cell→ SP single positive thymocyte (CD4+ or CD8+)
- During their journey through the thymus T cells are selected (to get cells with the ideal functions)
Is thymic selection based on a single interaction?
How many thymocytes develop into T cells?
- Thymic selection requires multiple interactions between multiple cell types
- Only about 2% of all thymocytes develop into T cells
- Successful selection = self MHC-restricted, self-tolerant T cells
- Affinity is key
Explain Positive and negative thymic selection-Thymic selection processes:
- Select for cells that recognise self MHC
Positive selection
- Select against cells that recognise self antigen
Negative selection
You want T cells to be able to recognise self from non-self but you dont want them to recognise self too strongly
- Affinity too high→negatively selected→delected (2%-5%)
- Affinity too low→fail to be positively selected→”deah by neglect”-apoptosis (90%-96%)
- Intermediate affinity→ positevely selected→ survive (2%-5%)
Explain TCR signalling
- Like the BCR, receptor clustering is crucial for signalling
- The T cell requires TCR and CD3 for signalling
- CD3 is made up of four different proteins:
γ-Gamma
δ-Delta
ε-Epsilon
ζ-Zeta (starts and transduces the signal)
- CD3 proteins are anchored in the cell membrane with long cytoplasmic tails to allow signalling
- TCR clustering recruits kinases by CD3 tails and signal sent to nucleus
T cell signalling requires co-receptors
- T cells express either CD8 or CD4 co-receptors which allows the immune systems to check that the right cells are interacting with each other
- CD8 and CD4 expression differentiates killer and helper T cells
- Cytotoxic and helper T cells have very different functions:
- CD8+ T cells (Killers) need to examine intracellular peptides so will only interact with (MHC I)
- CD 4+ T cells (Helpers) need to examine extracellular peptides (MHC II)
- If a cytotoxic T cell got confused and recognised a peptide in MHC II, it would kill the APC (and we like APC)
What does the expression of these co-receptors make sure of?
The expression of these co-receptors make sure that the killers stick to MHC I and kill the right cells and the helpers stick to MHC II and provide the right help
CD4 and CD8 bind MHC
MHC class II interacts with CD4
MHC class I interacts with CD8
What are the functions of cytotoxic T cells?
- Recognise antigen combined with class I MHC (altered self cells)
- Recieve signal for activation
With CD4 T cell help: Cytokines (IL-2)
Without help: activation by potent APCs
- Releases cytotoxic proteins:
Perforin: forms a pore for delivery of granzymes
Granzymes: programmes cell to die (induces apoptosis)
- Granulysin: stabs the cell (creates holes in target membrane and destroys it)
Cytotoxic T cell activation:
What is required for full cytotoxic T cell activation?
What is the role of clonal expansion?
Cytokines (especially IL-2) from helper T cells are required for full CTL activation
- CTL-P (cytotoxic lymphocye precursor) has seen its antigen but needs to be fully activated and it gets that boost in activation from IL-2 that is produced by helper T cells (previously activated by an APC carrying MHC class II)
- IL-2 from the T-helper cells (specifically TH1 cells) bind to IL-2 receptors on CD8 positive T cell and that cell can then be fully activated
- That cell will go through a round of clonal expansion in which it makes multiples rounds of its cells, all of which with the same T cell receptor and affinity for a particular antigen
- Those cytotoxic T cells can now go out and kill viral infected cells
Explain cytotoxic T cell killing of infected cells
- Activated TC rapidly proliferate to build up their numbers
- Then they will leave the blood at the site of infection
- Hand-to-hand combat with infected cells:
- TCR identifies target (peptide in MHC Class I) as being infected with a virus
- Upregulation of adhesion molecules on TC which hold the target close
- TC delivers (secretes) perforin and granzyme into the target cell
- Perforin is a protein like C9 ( the pore-forming subunit of the MAC) – pokes holes in cell membranes (allows granzyme to enter cytoplasm)
- Granzyme induces apoptosis through multiple pathways e.g. activation of caspase-3 and caspase-8 and the generation of the apoptosome and finally the fragmentation of all the genetic material inside the cell
- Infected cell dies
Summarise Killer T cell induction of apoptosis
- Granzyme and perforin (stored in special granules)
- TC delivers granzyme and perforin into target cell in a vesicle
- Induction of apoptosis
