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)
What do cytotoxic T cells require for activation?
- Cytotoxic T cell (TC, CTL) activation is a bit of a mystery
- Activation of naïve TC requires recognition of cognate antigen presented by MHC I on DC and help from an activated TH through the provision of cytokines to boost that response
- These interactions can happen in the lymph nodes
It is unlikely that all three (MHC class I, dendritic cell and cytotoxic T cell) will get together at the same place, same time:
Describe sequential activation
In the course of an infection you can get both helper T cells and cytotoxic T cells that respond to intracellular antigens through cross presentation and dendritic cells that helper T cells which send signals (licence) the dendritic cells which make them better at activating cytotoxic T cells
1 .DC activates TH
2 .TH gives DC the ‘licence’ to activate
3 .DC can now activate TC that recognise their cognate antigen in the licensed DC
- Licensing mechanism is unknown
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
Why kill target cells by apoptosis?
- Apoptosis is controlled
Apoptotic cells reveal signals for macrophages by labelling the outside of their membranes
- Apoptosis is tidy
TC can get rid of infected cells without any collateral damage, i.e. no danger signal
- Apoptosis destroys viral DNA
Viral DNA and assembled viruses will be destroyed by macs (who eat the apoptotic cells)
Explain the function of memory cytotoxic T cells:
When will they quickly be reactivated?
Do they provide long or short term protection?
- Once activated and their duties done, most T cells die
- Apoptosis by ACID – activation-induced cell death
- This makes room for new T cells that are appropriate for new infectious outbreaks
- But we set aside some activated (experienced) T cells as memory cells
- Memory cells will be quickly reactivated if we meet the same viral enemy again
- It’s unknown how T cells decide to become memory cells or how they manage to stay alive once they have decided
- Memory T cells provide long term protection against future attacks (this is why we can quickly fight off infections with viruses we’ve seen before and how many vaccines work)
Give a summary of cytotoxic T cells
- TCR has ligand binding (αβ) and signalling (CD3) domains
- TCR needs co-receptors for signalling (CD8 or CD4)
- CD8 and CD4 differentiate Killers (look for intracellular infections) from Helpers (help with extracellular infections)
- Killer T cell activation requires DC, TC and TH – sequential interaction?
- TC enter into hand-to-hand combat, deliver perforin and granzymes to target cells
- TC induce apoptosis in targets (tidy death)
- Most activated T cells will die by ACID, some become memory cells ready for subsequent infections (secondary responses)
Why do we want target cells to be killed by apoptosis rather than necrosis?
In necrosis the end result is DNA fragamentaion doesnt distingush between your DNA and intruder DNA, lots of infected virus protein would also be released
How does the activation of natural killer cells differ from that of cytotoxic T cells?
Cytotoxic T cells actively seek out cells that are bearing viral (intraceullular) antigen
Natural killer cells look for the lack of MHC class I
