module 07 section 03 (antigen-specific CMI) Flashcards
the process of antiviral T-cell activation is delayed in:
the generation of antigen-specific cytotoxic T-cells
compared to the inital nonspecific immune response by NK cells, do cytotoxic T-cells generate a more or less effective and lasting response against the virally infected cell? why?
more - due to their specificity
once immunocomptent naive cytotoxic T-cells are circulating in the organism, they require 3 signals to kill a target cell, what are they?
adhesion
antigen presentation
activation
explain adhesion as a signal
- initial interaction of T-cell with the targets involves nonspecific adhesion molecules: formed by LFA-1 (integrin) on the T-cell and ICAM-1 (glycoprotein) on the target cell
- this interaction btwn adhesion molecules ensures that the T-cell remains in contact w the target cell
explain antigen presentation as a signal
the TCR-peptide:MHC complex, in association with CD28 and B7, provides a signal to activate the cytotoxic T-cell
explain cytotoxic T-cell activation as a signal
- poised or partially activated cytotoxic T-cells express high levels of the IL-2R B-chain (CD122) and low levels of the IL-2R a-chain (p55a)
- poised cytotoxic T-cells secrete small amounts of Il-2 but not enough for full activation of the cell
- the poised cell requires additional IL-2 from CD4+ Th1 cells
define “poised or partially activated cytotoxic T-cell”
- poised: on resting CD8+ cytotoxic cells, the B-chains and y-chains are expressed consititutively (continuously) and bind IL-2 with moderate affinity
- partially activated: due to CD28-B7 interaction
what is high-affinity IL-2R?
three-chain receptor structures produced only on activated T-cells (IL-2aBy chain)
what permits the formation of high-affinity IL-2R?
secretion of IL-2 from CD4+ helper Th1 cells increases p55a expression for IL-2R, permitting the formation of high-affinity IL-2R
what is the result of increased IL-2R expression? what does this allow for?
- binds more IL-2 secreted from Th1 cells
- allows for clonal expansion of activated cytotoxic T-cells (now fully functioning)
what is clonal expansion of activated cytotoxic T-cells responsible for?
the strength and specificity of the cell-mediated immune response
what is the result of clonal expansion?
- increased number of cytotoxic T-cells that are specific for a processed antigen ecpressed on MHC class I
- these can travel through the body and fight infection
small amounts of IL-2 secretion by poised cytotoxic cells = which type of signalling?
(paracrine, autocrine, endocrine)
autocrine
IL-2 secretion from helper T-cells = which type of singalling?
(paracrine, autocrine, endocrine)
paracrine
during clonal expansion, activated cytotoxic T-cells develop: (2)
(1) cytoplasmic granules that contain perforins, serine esterases, granzymes, toxins, etc.
(2) cytotoxic cytokines that include IFNy, TNFB
what happens to the cytoplasmic granules developed by cytotoxic t-cells during clonal expansion?
they are exocytosed during interactions with specific target cells
what happens to the cytotoxic cytokines developed by cytotoxic t-cells during clonal expansion?
can inhibit viral replication and are involved in the activation/recruitment of macriophages
explain the experiment that Zinkernagel and Doherty used to demonstrate how antigen recognition by cytotoxic T-cells exhibits MHC class I restriction
- mouse infected with lymphocytic choriomeningitis virus
- introduction of spleen cells from infected mouse containing cytotoxic T-cells into:
- a group of non-infected cells of the same strain
- a group of LCM-infected cells of the same haplotype
- a group of LCM-infected cells of a different haplotype - only infected cells of the same haplotype underwent lysis, demonstrating that antigen recognition by CTL exhibits MHC class I restriction
two responses can be elicted when cytotoxic T-cells interact with processed antigenic peptide presented by MHC on a target cell, what are they?
- no recognition
- peptide/MHC specific recognition
explain no recognition
if the TCR of a cytotoxic T-cell doesn’t recognize both the peptide and the MHC complex presented on the target cell it won’t recognize the target cell, so it will separate from the target cell (no killing)
explain peptide/MHC specific recognition
- when the peptide:MHC complex is recognized by TCR a series of events activate the cytotoxic T-cell functions
- cytotoxic T-cell kills the target cell
what happens when the cytotoxic T-cell is activated?
it becomes polarized
polarization of the T-cell results in what?
conformational changes of the components of the T-cell that promote pore formation in the target cell
what’s step 1 of T-cell polarization?
- peptide:MHC class I complex on target cell is recognized by TCR and is stabilized by coreceptor CD8
- LFA-1 (t-cell) and ICAM-1 (target) act as adhesion molecules
what’s step 2 of T-cell polarization?
- binding of TCR:MHC complex and adhesion molecules singals reorientation of the effector molecules in the T-cell towards the target cell
- effector molecules include the MTOC, cytolytic granules and the Golgi
what’s step 3 of T-cell polarization?
reorganization of effector molecules results in delivery of a “lethal hit” of cytotoxic granules to the target cell at the point of cell contact
what’s step 4 of T-cell polarization?
- perforins in the cytotoxic granules are released in a directed fashion to the target cell
- at the site of cell-cell contact, the perforins undergo Ca2+ dependent polymerization to form pores in the target cell membranes
what’s step 5 of T-cell polarization?
- pore formation permits entry of granules, which initiates target cell death by apoptosis
- membrane damage to the target cell results in a visible cavity, influx of h2o, and swelling of the target cell, ultimately killing it
what is familial hemophagocytic lymphohistiocytosis?
inherited perforin deficiency where the patients cytotoxic T-cells and Nk cells don’t have the capacity to create preforin that’s secreted as granules
what results from perforin deficiency?
cytotoxic T-cells stays intact but its ability to kill target cells (i.e., its effector function) is considerably reduced
what takes over to eliminate target cells during perforin deficiency?
Fas/FasL mediated apoptosis
do activated cytotoxic T-cells express Fas ligand or Fas?
Fas ligand - membrane bound protein (trimeric)
what does Fas ligand bind to?
target protein - Fas
where is Fas expressed?
many somatic cells
what does the FasL/Fas interaction result in?
activation of capase enzymes resulting in apoptosis of the target cell
how is apoptotic cell debris created as a result of FasL/Fas mediated apoptosis removed? what does this prevent?
- by macrophages
- prevents inflammatory response
Fas is a member of which receptor family?
TNF
describe the overall FasL/Fas reaction
- FasL binds 3 Fas proteins on somatic cell surface
- results in activated caspases inside the cell
- results in a proteolytic cascade
- end product of the cascade migrates to the nuclues degrades the chromatin, (i.e. cutting DNA into small pieces)
- ultimately kills the cell
summary: describe the cytotoxic T-cell killing process
- adhesion: LFA-1 and ICAM-1 interaction
- antigen recognition: TCR:peptide/MHC complex binding
- movement of granules: reorientation of cytoskeleton and cytoplasmic components
- killing of target cell: “lethal hit”, pore formation, cell apoptosis
do cytotoxic T-cells act on cancerous cells similarly or differently than they act on virally infected cells? explain.
similar - cancerous cells express self antigens on MHC allowing cytotoxic T-cells to recognize them
summary: what is the key molecule that mediates cell-mediated cytotoxicity?
perforins
