Mechanisms of lymphocyte-mediated cytotoxicity
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Mechanisms of lymphocyte-mediated cytotoxicity
Dr. Ronald SmeltzMedical Sciences Building
Room [email protected]
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Outline
• Identify the effector cells of cytotoxicity
• NK cell recognition of target cells
• CTL differentiation into cytotoxic T cells
• Mechanisms of cytotoxicity– Granules– Receptor-mediated– Signaling cytotoxicity
• Transition of cytotoxic T cells into memory cells
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Examples
• Pathogens
• Tumors
• Transplantation
• Homeostasis– Tolerance– Elimination of antigen-bearing dendritic cells
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Cells that mediate cytotoxicity
• Natural killer (NK) cells
• CD8+ cytotoxic T cells (CTL)
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NK cell recognition of target cells
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NK cell recognition
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The “Missing Self” Hypothesis
• States that altered expression/down-regulation of MHC Class I on target cells leads to spontaneous NK-mediated destruction of the target cell
• Down-regulation of MHC Class I OR over-expression of NK cell activating molecules leads to NK cell-mediated killing of target cell
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The “Altered Self” hypothesis
Activation of NK cells is the net effect of inhibitory and activating signals
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• Ly49 (mouse) H-2K, H-2D• KIR (human) HLA-A, HLA-B, HLA-C
• CD94/NKG2 Qa-1b
HLA-E
• NKG2D Rae-1MIC-A,MIC-B
• NKp ??
NK cell recognition molecules
NK cell receptors Target cell ligands
Genetic p
oly
morp
hism
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Ly49 Family (mouse)
• Most Ly49 members are inhibitory receptors, some are activating receptors
• Bind to Class I– Inhibitory receptors bind Class I with high
affinity• Example: Ly49A
– Activating receptors bind Class I with low affinity, but bind additional ligands with high affinity
• Examples: Ly49D, Ly49H
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KIR family (humans)
• KIR (Killer cell Ig-like receptors):– Immunoglobulin (Ig)-like domains
– Two types of KIR• Long: “L”, inhibitory
– 1-2 ITIM motifs• Short: “S”, activating
– No ITIMS, no cytoplasmic domains
– Similar to Ly49 family, inhibitory KIR molecules bind Class I with high affinity
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Similarities between Ly49/KIR
• Expressed on NK cells, activated CD8+ T cells
• Bind to determinants of MHC Class I expressed by target cell
• Inhibitory receptors have cytoplasmic ITIMs
• Activating receptors bind to ITAM-bearing DAP12 adaptor proteins– Asp- in transmembrane domain of DAP; Lys+ in transmembrane domain
of activating receptor
• Different structures, very similar functions!
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CD94/NKG2
• Inhibitory and activating receptors– CD94/NKG2A heterodimer: Inhibitory
• NKG2A has a cytoplasmic ITIM
– CD94/NKG2C heterodimer: – CD94/NKG2E heterodimer
• Activating• NKG2C must associate with DAP12
• Recognize HLA-E (Qa-1b in mouse) on target cell – Binds leader peptides derived from other
MHC class I alleles (HLA-A,B,C (humans), H-2 (mouse), HLA-G
CD94/NKG2 interactions exhibit peptide specificity!
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NKG2D: Activating receptor
Structurally linked but not encoded by the MHC locus
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Pan NK cell activating receptors
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Summary
Ly49/KIRLy49/KIR
polymorphic MHC encoded-peptide
H-2/HLA-A,B,C-peptide
CD94/NKG2CD94/NKG2
polymorphic MHC encoded+peptide
Qa-1/HLA-E+peptide
NKG2D non-poly Non-MHC encodedRae/MUC ligands
Rae/MUC ligands
NKp30,40,46 non-poly ?? ??
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CTL differentiation into cytotoxic T cells
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Requirements for generating CD8+ cytotoxic T cells
• Extrinsic factors:– Antigen-bearing DC (Signal 1)– Co-stimulation (Signal 2)
• CD27L/OX40L and CD27L/4-1BBL• CD40/CD40L
– CD4 help
– Cytokines (Signal 3)• Inverse relationship b/w inflammation and help
• Intrinsic factors – Transcription factors
• T-bet, Eomes
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See posted PDF for missing slide
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See posted PDF for missing slide
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See posted PDF for missing slide
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See posted PDF for missing slide
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Mechanisms of cytotoxicity
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Effector molecules of cytotoxicity
• Used by NK cells and CTL!
• Granule exocytosis pathway• Perforin, Granzymes• Trigger apoptosis
– Caspase-dependent/independent
• Fas/Fas-L pathway• Receptor-mediated death, caspase-dependent
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• Granzymes, perforin Cause apoptosis• Calreticulin Inhibitor of perforin• Serglycin Complex with granzymes• Cathepsins
– Cat C Processes granzymes– Cat B Protection
Lytic granules:Secretory lysosomes
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Mediators of cytotoxicity
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Delivery of granules
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The immunological synapse
Hours
20-30 minutes
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Movement of granules
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1. Membrane reorganization
2. Cytoskeletal polarization
3. Docking
4. Exocytosis
5. Binding
6. Entry
Summary
C2 domain of perforin acts as a lipid-recognition domain; pH-dependent
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Effects of Granzyme B
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Granzyme B mode of action
Bcl-2
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Granzyme A mode of action
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
The granzymes synergistically promote cytotoxicity in a perforin-dependent manner
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Fas-Fas-L
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Similarities between CTL and NK cells
• Importance of MHC class I molecules– Peptide requirements
• CD8, Ly49/KIR, CD94/NKG2
• Formation of immunological synapse
• Effector molecules, lytic granules
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Differences between CTL and NK cells
• NK cells:– Innate– Pre-formed effector molecules– Surveillance
• CTL:– Adaptive – Must synthesize effector molecules de novo– Restricted circulation
• Synapse
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Transition of cytotoxic T cells into memory cells
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Annual Reviews
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Marking memory
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Linear progression
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Asymmetric model
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Reading
• Chapters 7, 10 of Janeway Book (7th edition)• Suggested readings posted on MICR505
website