Lecture 32 - NK Cells 2 Flashcards
Describe the observation of improved BM transplant outcome in leukaemia
KIR-Ligand Mismatch
(Donor NK cell KIRs and host HLA mis-match)
Certain populations of donor NK cells will kill:
• Residual host DCs
→ Less Graft vs. Host disease
• Residual host T cells
→ Less rejection
• Host AML cells
→ Less relapse, clearance of tumour
This is because the NK cells do not get the appropriate inhibition signals from the host leukaemic cells, and thus are able to selectively kill them
Describe the mechanism of NK cell killing of AML cells after BM transplant
Leukaemic cells:
• Appear to lack inhibitory ligand (due to KIR mismatch)
• Have the activation ligand
NK cells become activated against leukaemic cells
Killing of cells through toxic granule release
Describe the multiple cell types that NK cells from the donor can affect
- Residual host DCs
- Residual host T cells
• Removal of patient derived T cells that would recognise the incoming graft as foreign - Host AML cells
Describe Graft versus Host Disease (GvHD)
How does mis-match attenuate GvHD?
Incoming T cells in graft start to recognise the patient as foreign
Positive selection of T cells in the thymus occurs on cortical epithelial cells
Cortical epithelial cells are patient derived (since they do not come from BM)
Negative selection of T cells in thymus occurs on DCs
DCs are BM derived and thus will be donor defied after a BM transplant
What the host perceives as foreign is dictated by the donor derived DCs
If there are residual DCs in the periphery (i.e. they haven’t all been depleted) the developing T cells won’t be tolerant of them
These DCs stimulate immune responses against the patient’s original HLA (now perceived as allogeneic)
We now get T cells that are auto-reactive against tissues all over the body
In mismatch:
• The incoming NK cells kill residual patient DCs
• These DCs can no longer prime T cell responses against host tissue
• Hence no GvHD
What are the inhibitory ligands on host cells?
Various MHC class I molecules: • HLA-A3/11 • HLA-Bw4 • HLA-C1 • HLA-C2
What are the inhibitory receptors on NK cells?
- KIR (e.g. CD158a-f)
- Ly49
- CD94/NKG2A
What are the activating receptors on NK cells?
What are the ligands for activation?
- CD2
- 2B4
- Ligands: widespread expression on host cells
(Natural cytotoxicity receptors) • NKp30 • NKp44 • NKp46 Ligands: • Unclear except: • NKp30 - B7-H6
- Activating forms of CD158/Ly49
- Ligands: unknown
- NKG2D receptor
- Ligands:
- MIC-A
- MIC-B
- ULBP(1-4)
• CD16
Compare function and structural features of CD158(L) and CD158(S)
CD158(L): inhibition receptors
• Have ITIMs
• Long cytoplasmic tail
• When stimulated, bring about inhibition of NK cells
CD158(S): activation receptors • Lack ITIMs • Short cytoplasmic tail • Associate with ITAM adaptor molecules • When stimulated, bring about activation of NK cells
Both found on NK cells
Describe NKG2D
• Where it is expressed
• Function
• Ligands (in general)
Activating receptor expressed on all NK cells and certain T cell subsets
Function:
• Very potent activation of NK cells
( • Co-stimulation of T cells)
Ligands:
•MIC-A
• MIC-B
• ULBP (1-4)
Describe the general structural features of the ligands for NKG2D
Give some examples in mice and in humans
Ligands:
• Multiple ligands
• Indicating that this receptor is promiscuous
Structural features: • MHC class I-like (have alpha helix binding cleft over β-sheet platform structure as seen on the top of MHC I molecule)
- Do not bind peptides
- Because alpha helices closer together
• Do not associate with β2-microglobulin
Examples:
– In humans –
- MIC-A/B
• MHC like proteins - ULBPs
• Family of proteins
– In mice –
- RAE
- H60
Describe how NKG2D binds to its ligands
Remember: the ligands are similar in structure to MHC I:
Alpha helices form cleft with a beta sheet platform
NKG2D binds over the two alpha helices
Sits slightly diagonally
Describe the features of expression of NKG2D ligands
Where / when are they / aren’t these ligands expressed?
NKG2D is a very potent activation receptor on NK cells
We don’t want the receptor to be activated inappropriately
This, ligand expression is tightly regulated:
Features:
1. Non-constitutive
• Low or absent in most adult tissues
2. Inducible • Heat shock Infection: • CMV • M. tuberculosis • Pathogenic E. coli • Tumours
What does expression of NKG2D ligands on cells indicate?
Expression of these ligands indicates metabolic state of stress
Describe the pathway that leads to the expression NKG2D ligands
- Viral infection
- Cell stress; DNA damage
• Cleaved dsDNA
• Stalled replication forks
→ DNA damage repair pathway activated
- Activation of protein kinases ATM or ATR
- ATM or ATR induce NKG2D ligand expression
Describe how the ‘Missing self’ hypothesis of NK cell activation has been challenged
Missing self hypothesis:
• Absence of MHC class I on a cell will lead to activation of NK cells
• NK cells kill that target cell
Now:
• NKG2D ligand expression can override the presence of MHC class I (i.e. override the inhibitory signal)
• NK cell recognition of these ligands with NKG2D receptor
• Leading to NK cell killing of that cell
Compare responses to normal viral infections and CMV infection
– Normal viral infection –
- First few days
• Increase, decrease in viral titre
• Expansion of NK cell activity - Later on:
• CTL response kicks in and clears virus
– CMV viral infection –
(Latent infection)
- Initially:
• Viral titre increase
• Decrease around day 7 - Later on:
• Virus moves to salivary gland, and persists for years
Describe the basis of resistance to mCMV in mice, and how this was discovered
Genetic basis: Ly49H gene
• Member of Ly49 family
• Activating receptor on NK cells
• Recognises viral glycoprotein directly
- BALB/c mice: (Ly49H-)
• High titre in liver & spleen
• Cleared from salivary gland - C57BL/6 mice (Ly49H+)
• Low titre in liver and spleen
• Chronic infection of the salivary glands - A cross of these two mice:
• Low splenic titre
• (predicted chronic infection)
→ Dominant effect of these genes
What is Ly49H?
What does it recognise?
Describe its function
This is a gene that encodes an activating receptor on NK cells
Recognises:
• m157
• A glycoprotein expressed by mCMV
• MHC class I-like structure
Function:
1. m157 expressed in CMV infected cells
- Activation of receptor through ligation of viral glycoprotein
- DAP12 activation:
• Contains ITAMs
• Signals through DAP12 - Activation of NK cells
- Lysis of CMV infected cells
→ Reduction in splenic and liver loads
Describe preferential expansion of Ly49H+ NK cells in MCMV infection
What happens after this?
What is the implication of this?
- Preferential expansion
After infection with MCMV, there is an expansion of those NK cells which express Ly49H
Ly49H- NK cells increase a little bit, but not much
- Contraction
• After the response, the NK cells contract to a level above the original set point
• Memory NK cells are retained
Implication:
• This is like clonal expansion
• This is not what we typically think of as an innate response (Counter-Dogma)
• The contraction is also very similar to CTL responses
• Looks like Memory response?
Do NK cells exhibit memory responses?
“Memory” NK cells have a heightened magnitude of response in comparison to naïve NK cells
This is measured in the IFN-γ response
Why would a virus express m157 (a ligand for Ly49H)?
Compare infection with:
• Absence of Ly49H
• Presence of Ly49H
Describe the mechanism
Expression of Ly49H decreases viral load in liver and spleen, but somehow it enhances persistence
Absence of Ly49H: • Elevated viral load • More Ag • Robust T cell responses • Better long term viral clearance
Presence of Ly49H:
• Better acute control of viral replication
• Less pathology
• Enhanced survival (due to decreased acute infection)
• Increased killing of DCs
• Impaired T cell responses
• Viral persistence
Mechanism of enhanced persistence:
• Less virus and Ag leads to a decreased T cell response
• Virus is able to persist (in salivary gland)
Why is Ly49H and m157 an evolutionary win-win?
For host:
• Decreased acute infection
For virus:
• Increased transmission, because the host survives
Which cytokine is needed for (oligo)clonal expansion of NK cells?
IL-12
In which animals in Ly49H found?
Not found in humans
Only in mice
What is the significance of CD94-NKG2C+ NK cells?
These NK cells are present in increased numbers in individuals who are CMV seropositive
It could be that these cells could have a heightened functionality, in the way that the Ly49H+ NK cells (in mice) do
What is the importance of cytokines in NK responses?
NK responses are dependent on innate cytokines, such as: • Type I interferon • IL-15 • IL-12 • IL-18
This indicates they are important in acute viral infection
The role of the MHC receptors must then be more important later in the viral infection or in chronic viral infections
What is the other nomenclature for the KIRs on NK cells?
CD158(a-f)
Which mice have NK cells with Ly49H, and which don’t?
BALB/c mice: Ly49H-
C57BL/6 mice: Ly49H+
