Cell-mediated Cytotoxicity Dr. Diebel 5/14/14 Flashcards
Mediators of cell mediated immunity
Antigen non-specific effector cells
-NK cells. Produce cytotoxins (performs and granzymes), IFN-gamma, TNF, Fas ligand (FASL). Mechanism of killing is cytotoxic granule release and FASL-FAS interactions. NK cells bind NK receptor and tonic activating ligand and recognize lack of MHC I or NK receptor/tonic activating ligand and stress-induced activating ligand.
Antigen specific effector cells
CD8+ cells (cytotoxic T cells, T cells, CTLs, TC cells). Produce cytotoxins (performs and granzymes), IFN-gamma, TNF, Fas ligand (FASL). Mechanism of killing is cytotoxic granule release and FASL-FAS interactions. CTL TCR and MHC I bind along w/ CD8 mediating cytotoxicity.
CD8+ T cells (CTLs)
Naive T cells are called CTL precursors (CTL-P) to indicate their immature state.
A CTL-P matures after being activated by an interaction w/ a Th1 CD4+ T cell
Maturation required three sequential signals
- Antigen-specific signal transmitted by TCR upon recognition of the proper peptide:MHC I complex present on an APC
- Co-stimulatory signal is transmitted by CD28:B7 interaction between CTL-P cell and licensed APC
- Signal induced by the IL-2 (needed for differentiation) secreted by a Th1 CD4+ T cell results in the proliferation and differentiation of the antigen-activated CTL-P cell to a fully active CTL
Naive vs. Mature CTL
Naive
Does not express IL-2 or IL-2R only occurs after activation
Expresses CD45RA
Express low levels of cell adhesion molecules CD2 and LFA-1
No Cytotoxic activity
Mature
Expresses high affinity IL-2R and requires high level of IL-2 to proliferate
Synthesizes low levels of IL-2
Expresses CD45RO
Expresses High levels of the adhesion molecules CD2 and LFA-1
Exhibits cytotoxicity
Memory
May not require CD4 help to reactivate. Requires only low levels of IL-2 which can be produced by the activated CTLs)
CTL killing: Binding of the target cell
TCR-CD3 complex on the CTL recognizes the peptide:MHC class I complex on the target cell
LFA-1 on the CTL binds to ICAMs on the target cell
Antigen activation converts the LFA-1 from a low affinity state to a high affinity state for better binding. After about 5-10 minutes the LFA-1 returns to a low-affinity state, resulting in the dissociation of the CTL from the target cell.
CTL killing mechanisms
- Perforin and granzyme secretions
- perforin forms a pore on target cell membranes
- granzyme molecules activate apoptosis by cleavage of caspases - Fas ligand protein on the cell membrane surface
- Membrane bound FasL binds to Fas on the membrane of the target cells and initiates killing
- Activates apoptosis by cleavage of caspases - CTLs can also kill by TNF production and secretion
CTL Killing: Pathways to apoptosis
Granzyme is a serene protease that is specific to apoptosi.
Granzyme B can go to caspase 3 OR Bid–>mitochondria–>cytochrome C.
FASL–>FASL–>FDD–>(cleaved) caspase 8–>Bid–> mitchondria–>cytochrome C
NK cells
Play a major role in killing virus-infected cells, intracellular pathogen-infected cells, and tumor cells.
NK cells produce a number of important cytokines including IFN-gamma.
- IFN-gamma tilts the immune response toward Th1 cells by inhibiting Th2 cells and inducing IL-12 production by macrophages and DC.
- IFN-gamma can activate macrophages (M1) and NK Cells
NK cell activity is stimulated by IFN-alpha, IFN-beta, IFN-gamma, TNF-alpha, and IL-25
NK cells and early defense against viruses
Type 1 interferon through TLR3–>IRF3(NF-kB)–>IFN-beta
TLR7/8–>IFN-alpha–IRF7(NF-kB)–>IFN-alpha
Type 1 interferon will draw in NK cells to hold down the fort and supply signals to get CD8 into the area.
NK cells versus CTLs
NK cells express CD16 and FcRIII
NK cells do not need to be educated in the thymus
NK cells do not undergo gene rearrangement of receptor genes (don’t make TCR or CD3)
NK cell killing is not MHC restricted
NK cell killing is similar to CTL killing
- FasL expressed on surface can kill cells expressing Fas
- Perforin and granzyme released from granules
- TNF expressed on the surface and excreted
NK cell receptors
NK cells have both activation and inhibitory (higher affinity than activating) receptors.
NK cell receptors fall into two categories.
- Lectin-like receptors
- bind proteins rather than polysaccharides - Immunoglobulin-like receptors (KIR=killer cell immunoglobulin-like receptor)
- bind to most MHC I molecules
Lectin-like receptors of NK cells
non-inhibitory receptor
-CD94 and NKG2C. When they bind nothing is stopping them.
Inhibitory receptor
-CD94 and NKG2A. ITIM which inhibits down stream activation.
ITAM (CD3, CD79a/b) is a class of activating domains
Killer immunoglobulin-like receptors
non-inhibitory receptors
- short
- KIR-2D
- KIR-3D
Inhibitory receptors
- Long
- KIR-2D
- KIR-3D
- ITIM
NK Cell receptors and ligands
KIR family (MHC I)
Receptor Ligand
KIR2DL1 HLA-C2
KIR2DS1 HLA-C2 (activating)
KIR3DL1 HLA-Bw4
KIR3DL2 HLA-A3 and A11
KIR2DL2/3 HLA-C1
C-type lectin-like family
CD94-NKG2A HLA-E
Certain NK receptors predispose to certain diseases (e.g. HLA-Bw4 and HIV is not as severe)
HLA-E
In the ER sometimes a leader sequence interacts w/ HLA-E which goes onto the surface. Target is NKG2A.
Advantage would be in a cell that is having trouble getting MHC I out.
TNF released by?
NK cells, monocytes, and macrophages