8. Specialised Cells: CD8+ T Cells

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Learning Outcomes: L17

The properties of CD8+ T cells/CTL Three cell activation model and IL-2 requirement Restriction to MHC class I Divergent need for co-stimulation depending on activation status; naïve versus effector The process of CTL killing Performed cytolytic granules, generation of new granules via Th1 help Importance of pro-and anti-apoptotic molecules in stability of mitochondria Cytochrome c release and formation of apoptosome Triggering of initiator and effector caspases leading to apoptosis of cell Clearance of apoptotic bodies by macrophage Fas-mediated fratricide

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Cytotoxic T Cell Killers (CTLs)

CD8 cells transform into cytotoxic T cell killer (CTLs) and kill cells that express peptide-MHC complexes specific for TCR on CD8 T cells Properties of CTL killing: Rapid Highly specific- important for non-renewing cells eg neurons Forms immunological synapse with infected cell Can serially kill many target cells At site of infection, do not need co-stimulatory signal

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Apoptosis vs Necrosis

Apoptosis: Physiological Cellular condensation Nuclear fragmentation Rapid phagocytosis Lack of inflammation Internal components remain encapsulated, wont create response to own tissue

Necrosis: Pathological Organelles swell Membrane rupture Leakage of cell contents - taken up by innate immune cells and cause marked inflammation Consequence: pathogenic bystander activation for inducing autoimmunity Immune response initially to pathogen subsequently spreads to host tissue (epitope spreading)

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Consequence of Necrosis

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Viral Infection: Innate immune system activated to infection, produce IFNs that dampen virus ability to replicate IFNs damage surrounding healthy cells-release internal components of healthy cells that can be picked up by APC in context of inflammation Present healthy cell internal components to a T cell recognising self Autoimmunity

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CTLs Kill Targets via Apoptosis

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Programmed cell death Governed by series of biochemical events and activation of serine proteases (caspases) lead to: Nuclear blebbing Chromatin condensation, DNA fragmentation Shedding of membrane vesicles

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CTLs Efficient Killing Machines

Contain preformed cytoxic granules: Perforin- delivers contents of granules to target cell cytosol Granzymes- serine proteases that activate apoptosis once in cytoplasm Granulysins- antimicrobial activity

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CD4-APC-CD8 Interaction: CD8 Activation

APC presents molecules to both CD8(MHC-I) and CD4(MHC-II) Brings CD4 and CD8 into close proximity CD4 co stimulatory molecule CD40L binds CD40 on APC-> upregulation of MHC-peptide complexes and costimulatory molecules (for CD8) CD8+ T cell co-stimulatory 4-1BB binds 4-1BBL on APC and provides survival signal for activated CD8 T cell CTL replenishes granules with weapons in by receiving signals from Th1 cell by IFNϒ IL-2 drives proliferation IFNϒ drives differentiation to effector status cytotoxic T cell (CTL)

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CD4-APC-CD8 Interaction: CD8 Activation

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Granzyme B + Perforin Expression

Granzyme B and Perforin mRNA expression is controlled by TFs T-bet and Eomes: IFNϒ produced by Th1 cell binds IFNϒR on effector CD8 T cell Combined signals via immunological synapse +IFNϒRs trigger TF T-bet T-bet activates transcription of granzyme B and perforin In absence of T-bet, Eomes can substitute

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Granzyme B + Perforin Expression

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Events at Site of Infection

Whilst CD8 T cell activation in LN, at site of infection: Innate cells are producing interferons-> modify proteosome inside infective tissue Replace certain catalytic subunits Peptides produced that are more receptive to binding MHC-I= more stable immunologcial synapse with T cells IFNgam modifications of top and bottom of immunoproteasome increases rate of peptides released- widens Innate cells produce cytokines that change epithelial cell lining Change adhesion molecules specific for T cells Once T cell activated in LN, downregulates chemokine R keeping it in LN, upregulate chemokine receptors specific for epithelial cells: entry of activated cells only at site of infection

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2 Signal Activation Model

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Need 2 signals to generate effector CD8 T cell in LN In infected tissue, only need 1 signal to make immunological synapse and release weapons

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How CD8 T Cells Kill Cells

Perforin and granzymes work together to kill target cell: form multimeric complex stabilised by scaffold molecule serglycin

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How CD8 T Cells Kill Cells

Formation of immunological synapse vital to prevent unwarranted death of healthy cells: Reorganisation of Golgi apparatus and MTs at strongest point of contact in immunological synapse Tight binding to target cell via TCR and MHC-I-peptide Signals into CTL to polarise its granules in direction of target cell Accumulation of granzyme and perforin molecules at site of immunological synapse Tubule extends into target cell and delivers molecules (undirectional)