module 07 section 03 (antigen-specific CMI)

Question 1

the process of antiviral T-cell activation is delayed in:

Answer 1

the generation of antigen-specific cytotoxic T-cells

Question 2

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?

Answer 2

more - due to their specificity

Question 3

once immunocomptent naive cytotoxic T-cells are circulating in the organism, they require 3 signals to kill a target cell, what are they?

Answer 3

adhesion
antigen presentation
activation

Question 4

explain adhesion as a signal

Answer 4

• 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

Question 5

explain antigen presentation as a signal

Answer 5

the TCR-peptide:MHC complex, in association with CD28 and B7, provides a signal to activate the cytotoxic T-cell

Question 6

explain cytotoxic T-cell activation as a signal

Answer 6

• 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

Question 7

define “poised or partially activated cytotoxic T-cell”

Answer 7

• 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

Question 8

what is high-affinity IL-2R?

Answer 8

three-chain receptor structures produced only on activated T-cells (IL-2aBy chain)

Question 9

what permits the formation of high-affinity IL-2R?

Answer 9

secretion of IL-2 from CD4+ helper Th1 cells increases p55a expression for IL-2R, permitting the formation of high-affinity IL-2R

Question 10

what is the result of increased IL-2R expression? what does this allow for?

Answer 10

• binds more IL-2 secreted from Th1 cells

- allows for clonal expansion of activated cytotoxic T-cells (now fully functioning)

Question 11

what is clonal expansion of activated cytotoxic T-cells responsible for?

Answer 11

the strength and specificity of the cell-mediated immune response

Question 12

what is the result of clonal expansion?

Answer 12

• 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

Question 13

small amounts of IL-2 secretion by poised cytotoxic cells = which type of signalling?
(paracrine, autocrine, endocrine)

Answer 13

autocrine

Question 14

IL-2 secretion from helper T-cells = which type of singalling?
(paracrine, autocrine, endocrine)

Answer 14

paracrine

Question 15

during clonal expansion, activated cytotoxic T-cells develop: (2)

Answer 15

(1) cytoplasmic granules that contain perforins, serine esterases, granzymes, toxins, etc.
(2) cytotoxic cytokines that include IFNy, TNFB

Question 16

what happens to the cytoplasmic granules developed by cytotoxic t-cells during clonal expansion?

Answer 16

they are exocytosed during interactions with specific target cells

Question 17

what happens to the cytotoxic cytokines developed by cytotoxic t-cells during clonal expansion?

Answer 17

can inhibit viral replication and are involved in the activation/recruitment of macriophages

Question 18

explain the experiment that Zinkernagel and Doherty used to demonstrate how antigen recognition by cytotoxic T-cells exhibits MHC class I restriction

Answer 18

1. mouse infected with lymphocytic choriomeningitis virus
2. 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
3. only infected cells of the same haplotype underwent lysis, demonstrating that antigen recognition by CTL exhibits MHC class I restriction

Question 19

two responses can be elicted when cytotoxic T-cells interact with processed antigenic peptide presented by MHC on a target cell, what are they?

Answer 19

• no recognition

- peptide/MHC specific recognition

Question 20

explain no recognition

Answer 20

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)

Question 21

explain peptide/MHC specific recognition

Answer 21

• 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

Question 22

what happens when the cytotoxic T-cell is activated?

Answer 22

it becomes polarized

Question 23

polarization of the T-cell results in what?

Answer 23

conformational changes of the components of the T-cell that promote pore formation in the target cell

Question 24

what’s step 1 of T-cell polarization?

Answer 24

• 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

Question 25

what’s step 2 of T-cell polarization?

Answer 25

• 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

Question 26

what’s step 3 of T-cell polarization?

Answer 26

reorganization of effector molecules results in delivery of a “lethal hit” of cytotoxic granules to the target cell at the point of cell contact

Question 27

what’s step 4 of T-cell polarization?

Answer 27

• 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

Question 28

what’s step 5 of T-cell polarization?

Answer 28

• 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

Question 29

what is familial hemophagocytic lymphohistiocytosis?

Answer 29

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

Question 30

what results from perforin deficiency?

Answer 30

cytotoxic T-cells stays intact but its ability to kill target cells (i.e., its effector function) is considerably reduced

Question 31

what takes over to eliminate target cells during perforin deficiency?

Answer 31

Fas/FasL mediated apoptosis

Question 32

do activated cytotoxic T-cells express Fas ligand or Fas?

Answer 32

Fas ligand - membrane bound protein (trimeric)

Question 33

what does Fas ligand bind to?

Answer 33

target protein - Fas

Question 34

where is Fas expressed?

Answer 34

many somatic cells

Question 35

what does the FasL/Fas interaction result in?

Answer 35

activation of capase enzymes resulting in apoptosis of the target cell

Question 36

how is apoptotic cell debris created as a result of FasL/Fas mediated apoptosis removed? what does this prevent?

Answer 36

• by macrophages

- prevents inflammatory response

Question 37

Fas is a member of which receptor family?

Answer 37

TNF

Question 38

describe the overall FasL/Fas reaction

Answer 38

• 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

Question 39

summary: describe the cytotoxic T-cell killing process

Answer 39

• 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

Question 40

do cytotoxic T-cells act on cancerous cells similarly or differently than they act on virally infected cells? explain.

Answer 40

similar - cancerous cells express self antigens on MHC allowing cytotoxic T-cells to recognize them

Question 41

summary: what is the key molecule that mediates cell-mediated cytotoxicity?

Answer 41

perforins