Lecture 8 - CD8 T cells Flashcards
What are the two ways to present processed peptides?
1) Cytosolic processing of endogenous peptides
- virus that can replicate inside cell [icked up and targeted to proteasome, MHCI, APC
2) Cross presentation of endogenous proteins
- external virals picked up by endosome and released into cytosol, tageted to proteasome, MHC, APC
Why do CD8 cells require CD4 cells to help them become activated?
- CD8 cannot make IL-2 therefore cannot grow and proliferate as don’t have necessary cytokines and APC doesn’t contain enough to activate either
- also need 4-IBBL
How do CD4 cells help CD8 cells become activatied?
1) CD4 produces CD40L binds to CD40 on APC
2) This upregulates MH/P and costimulatory molecules allowing CD8 to bind effectively and connects 4-IBB and 4-IBBL (on APC)
3) signal combined with CD4 producton IL-2 triggers differentiation into CTL, growth and proliferation
4) CD8 cells transform into cytotoxic T cell killers and kill cell that express peptide/MHC complex specific forTCR
What are the two ways which cells can die?
1) Necrosis
2) Apoptosis
What are the features of Necrosis?
-dying by physical or chemical injury e.g. deprivation of oxygen kills heart cells
‘outside in death’ e.g. complement cells bind to outside of cell create pore leading to loos of osmosis
What are the features of Apoptosis?
normal cellular response necessary for tissue remodelling, cell dies from within.
proteins chew up DNA in cell stops functioning
Is necrosis or apoptosis preferred and why?
Necrosis leads to release of cellular components that can cause inflammation (dangerous)
Apoptosis leads to cellular constituents becoming encapsulated (invisible to immune system)then targeted to phagocytes that remove before causing inflammation
How do CTLs kill targets?
By induction of apoptosis = programmed cell death
- series of biochemical event and activation of proteases (capsases) leads to…
- nuclear blebbing
- chromatin condensation and DNA fragmentation
- shedding of membrane vesicles
What are the properties of CTL killing?
- rapid
- highly specific
- can serially kill many target cells, very efficient e.g. Type I diabetes B cells produce insulin killed by CTLs, number of CD4s in eyelid 10000 CTLs 100
Why are CTLs so efficient?
Contain preformed cytotoxic enzymes
What preformed cytotoxic enzymes do CTLs contain?
1) Preforin delivers contents of granules to target cell cytosol
2) Granzymes - serine proteases that activate apoptosis once in cytoplasm
3) Granulysis: anti-microbial activity, binds to membrane of mitochondria and damage contents
How do Perforin and Granzymes work together to kill cell?
1) perforin and granzyme and scaffolding protein serglycin (stabilises) bind together and form a multimeric compex which has to be delivered into infected cell
2) delivery of complex into cell triggers DNA fragmentation by activation of caspase cascade and release of pro-apoptotic molecules from mitochondria
3) complex binds surface of mitochondria + creates a pore increasing permeability of the membrane
4) cytochrome C compound released binds to Apoptotic protease activating factor (Apaf-1)
5) Apaf-1 binds to pro-caspase 9 which is cleaved to become active caspase 9 (apoptosome)
6) release of small mitochondria-derived activator of capsases (SMAC) enables other capsases to be activated
What are capsases and where are they present?
Capsases - seeds of self destruction
- present in all nucleated cells in inactive form (procaspases)
- quickly activated
- start self amplifying in an irreversible protease cascade
- cause cleavage of inhibitor of DNAase leads to cell death
What does capsase 3 do?
binds to caspase activated DNAase complex removes inhibitor, DNA degraded
What causes cell shrinkage and ‘blebbing’?
combined mitochondrial and DNA damage
How do phagocytes identify apoptotic bodies ‘blebs’?
1) reorganisation of cell membrane in apoptotic bodies leads to
2) phosphatidylserine relocates from extracellular surface to plasma membrane
3) macrophages carrying receptors that recognise phosphatidylserine enable engulfment of apoptotic body
How do CTL make sure that only infected cells are destroyed?
Focus release of perforin-granzyme complex at immunological synapse:
POLARISATION OF CTL TOWARDS TARGET
1) CD8 T cell binds to target cell creates an immunological synapse and strong signal
2) Leads to reorganisation of CD8 T cell lytic granules and secretary molecules lining up along immunologiccal synapse (polarisation)
3) when polarisation occurs CD8 cell recieves signal to release contents into infected cell
Why is chance killing of an uninfected cell by CTLs rare?
cannot create a strong enough attachment at the immunological synapse
How are CTLs serial killers?
bind to infected cell, deliver cytotoxic content, detach, attach to another infected cell
How does CTLs prevent self destruction by own perforin and granzyme enzymes?
The Immunological synapse polarises moleculs away from own mitochondria
What do interferons produced by CTLs do?
impede viral replication
alter the immunoproteasome (more peptides on surface of infected cell)
upregulate TAP1 and TAP2 for transport of peptides to MHC
How can effector T cells destroy infected cells that do not express co stimulatory molecules e.g. neurons?
effector T cells can bind to target and kill if only recieves signal 1 can respond to specific antigen in absence of costimulation
Why is it important that effector T cells can bind and kill cells that don’t express co stimulatory molecules?
1) effector CD8 cells must be able to act on any virally infected cell
2) effector CD4 cells must be able to activate B cells and macrophages that have taken up Antigen even if not expressing costim molecules
