Cell Mediated Cytotoxicity

Question 1

What’s the major antigen NON-specific effector cell in the cell-mediated immunity world? Antigen specific?

Answer 1

natural killer cells

cytotoxic T cells

Question 2

What are the three types ot cytotoxic lymphocytes?

Answer 2

natrual killes
natural killer T cells
CTLs

Question 3

What are the three sequential signals required for CTL activation?

Answer 3

1. antigen-specific signal delivered by the TCR upon recognition of proper peptide on MHC 1 (from APC or tissue cell)
2. co-stimulatory signal transmitted by CD28:B7 interaction
3. SIgnal induced by IL-2 secreted from a Th1 (or Th17) CD4+ cell

Question 4

What are the two ways an APC can become licensed?

Answer 4

through engagement with an activated CD40l+ help T cell or thorugh engagement with pathogens via TLRs

Question 5

Which is more efficient: when the licensing of the APC and the activation of the CD8 cell occurs sequentially or simultaneously? Why?

Answer 5

It’s more efficient when it’s simultaneous because the CD4 cell that helped license the APC will secrete huge amounts of IL-2, which is necessary for the CD8 cell to fully differentiate - so it happens simultaneously, there’s already a pool of IL-2 for the CD8 cell to enjoy

Question 6

CTL-P cells lack expression of what cytokine and it’s receptor compared to mature CTLs?

Answer 6

IL-2 and the IL-2R

Question 7

Why don’t CTL-Ps have cytotoxic activity?

Answer 7

They don’t have the appropriate granules yet

Question 8

How can CD45 be used to differentiate between a naive CTL-P and a mature CTL?

Answer 8

CTL-Ps express CD45RA and mature CTLs express CD45RO

Question 9

Why don’t memory CTLs require Th1 CD4+ T cell help to reactivate?

Answer 9

Because memory CTLs only need low levels of IL-2 to reactivate, and they can produce that themselves

Question 10

What are the steps involved to CTL-mediated killing of target cells?

Answer 10

1. the TCR/CD3 complex recognizes the peptide on MHC1 on the target cell and binds
2. LFA-1 (an adhesion molecule) binds to ICAMs on the target cell to help the adjesion junction so you can transfer granules
3. Perforin molecules form a pore on the target cell membrane
4. Granzyme molecules activate apoptosis by cleavage of caspases
5. Fas ligand protein binds to Fas on the membrane of target cells and also initiates apoptosis
6. CTL produces TNF which also feeds into apoptosis pathway
7. CTL lets go and target cell dies

Question 11

How is the CTL able to bind tightly and then let go before the cell is even killed?

Answer 11

When you get antigen activation, the LFA-1 converts from a low affinity state to a high affinity state for strong binding

this is transient though

5-10 minutes later, the LFA-1 returns to a low-affinity state, resulting in the dissotiation of the CTL from the target cell

Question 12

So…what were those three ways CTLs initiate apoptosis again?

Answer 12

1. secrete perforin and granzymes
2. Fas/FasL binding
3. TNF secretion

Question 13

What sort of molecule is a granzyme? WHat does it do specifically?

Answer 13

a serine protease - so it cleaves at serine residues specific for proteins along the apoptosis pathway

for example, granzyme B bill cleave Bid which gives a pro-apoprotic signal to the mitochondria to release cytochrome C and will also cleave procaspase 3 to lead to apoptosis as well

Question 14

How does Fas’Fas-L binding lead to apoptosis?

Answer 14

It will actiate death domains (FADD) which will cleave caspase-8 to initiate apoptosis

Question 15

What cytokines will stimulate NK activity?

Answer 15

IFN alpha, beta and gamma
TNF alpha
IL-15

Question 16

What cytokine is released by NK cells and what’s the purpose of it in that context?

Answer 16

IFN-gamma
It will tilt the immune response toward Th1 cells by inhibiting Th2 cells and inducing IL-12 production by macropahges and dendritic cells (specifically M1-angry macrophages)

Question 17

Which start working first in a viral infection: NKs or CTLs?

Answer 17

NKs - remember they’re not antigen-specific, so they just need the Type 1 IFNs from the viral infected cells

Question 18

What do NK cells use to bind to the constant portion of antibody stuck on target cells?

Answer 18

Fc gamma RIII

Question 19

True or false: NK cells undergo rearrangement of receptor genes?

Answer 19

false - they don’t

Question 20

True or false: NK cells are not MHC restricted.

Answer 20

True - remember it doesn’t even need antigen

Question 21

Is NK cell killing different from CTL killing?

Answer 21

Nope - pretty much identical

Question 22

What are the two categories of NK cell receptors?

Answer 22

1. lectin-like receptors

2. immunoglobulin-like receptors (KIR = killer cell immunoglobulin-like receptors)

Question 23

What do the lectin-like receptors bind? THe KIRs?

Answer 23

lectin - bind proteins
KIR - bind to most MHC class 1 molecules

Question 24

Which have stronger affinity for their ligands: NK activation receptors or inhibition receptors?

Answer 24

inhibition receptors

Question 25

If a receptor is associated with an ITIM, is it inhibiting or activating? With an ITAM?

Answer 25

ITIM = inhibiting
ITAM = activating

Question 26

FOr the KIRs - which are the ones that have the long cytoplasmic tails - the activating or the inhibiting?

Answer 26

The long cytoplasmic tails contain the ITIMs (inhibiting0

The ones with ITAMs just have a short tail with a charged lysine residue

Question 27

If an NK cell has an activating receptor bind it’s ligand, but there’s lots of MHC1 on the target cell, will the NK kill it?

Answer 27

no - MHC1 bind to inhibitory receptors with greater affinity than the activating receptor iwll bind its ligand, so there will be no killing

Question 28

Why then are NK cells so great for killing virally infected cells?

Answer 28

Viruses often down-regulate MHC1 expression in an attempt to evade the CTL killing, but this triggers death from NK cells

Question 29

What makes HLA-E special and what effect does it have?

Answer 29

It’s similar to MHC1, but not quite

It doesn’t really present peptides that were synthesized in the cell - it just presents a single “leader peptide” that it picks up as it’s synthesized in the RER

The leader peptide is the target for a c-type lectin-like receptor which is an INHIBITORY SIGNAL

So even if a healthy cell is having an issue with antigen presentation, it won’t necessarily be killed needlessly because the HLA-E doesn’t do antigen presentation per se

Question 30

Activation of an ITIM will inhibit any activating signals present through interaction with which molecule?

Answer 30

SHP-1

Question 31

Although NK T cell express a TCR and CD3, how are they different from CTLs?

Answer 31

The TCR is INVARIANT and they do NOT recognize MHC-bound peptides
also don’t have memory (probaby)

Question 32

If NK T cells don’t recognize MHC-bound peptides, what do they recognize?

Answer 32

they recognize glycolipids presented by a molecule called CD1d

Question 33

Antibody-dependent Cell mediated cytotoxicity entails what?

Answer 33

effector cells binding to antigen via the antibody through the Fc receptor and then killing occurs thorugh cytolytic enzymes released by the effector cells