NK cells and Gamma/Delta T cells Flashcards
What are NK cells?
A subset of innate lymphoid cells
What determines the phenotypic differences between ILC subsets?
Transcription factors
e.g. T-bet, GATA3 and RORgt
What are the type of gd T cells?
Vd1 - skin, lung, gut
Vd2 - blood, lymph node, spleen
Vd3 - blood (rare)
Describe NK cells
Large granular lymphocytes that are not T or B cells
Do not express TCR or BCR
Do not express cell surface marker CD56
CD3- CD56+
What do NK cells do?
Cytokine secretion
Cytotoxicity causing lysis of target cell
What defines NK cell subsets?
CD56 expression level
10% of NK cells are CD56bright, 90% are CD56dim
Describe CD56bright NK cells
Predominantly found in secondary lymphoid organs
Greater cytokine production
Low level of perforin/granzymes
Describe CD56dim NK cells
Low CD56 expression
Predominantly found in blood
Highly cytotoxic
High level of perforin/granzymes
What are the cytokines released by NK cells?
IFN-g
TNF-a
What does IFN-g released by NK cells do?
Activation, growth and differentiation of T, B, NK cells and macrophages
Promotes Th1 differentiation
Enhances MHC expression on APC
What does TNF-a released by NK cells do?
Inflammatory mediator
Regulates growth and differentiation of a wide variety of cells
Selectively cytotoxic for many transformed cells
What do NK cells release to kill target cells?
Lytic granules containing proteins that act on the target cell
What proteins do lytic granules of NK cells contain?
Perforin
Granzymes
Granulysin
What does perforin do?
Aids in delivering contents of granules into the cytoplasm of the target cell
What are granzymes and what do they do?
Serine proteases
Activate apoptosis once in the cytoplasm of the target cell
What does granulysin do?
Has antimicrobial actions and can induce apoptosis
Describe the role of NK cells in cancer
Medium and high cytolytic function of NK cells = reduced cancer risk (because NK cells are in abundance and can fight cancer cells)
Low cytolytic function = increased cancer risk
LAK cell therapy can be used for individuals with low cytolytic function
How do LAK cells work?
Lymphokine-activated killer cells
In the presence of IL-2 they are stimulated to kill tumour cells
They have many receptors/adhesion molecule on their surface for IL-2 so they are cultured to increase cell division into the cytotoxic line of cells and become activated to kill cancer cells
Describe the role of NK cells in infection
Low NK activity = increased risk of herpesvirus infections
How are NK cells activated in viral infection?
Virus infected cells release IFN-a and IFN-b
This induces resistance to viral replication in all cells
Increase MHC-I expression and antigen presentation in all cells
Activate NK cells to kill virus infected cells
How do NK cells make the decision to kill?
Fc receptor recognition of Ab/Ag complexes
Balance of activating/inhibitory receptors - detection of missing self and detection of induced self
What is missing self?
When there is an absence or altered expression of MHC-I molecules
Inhibitory receptors on NK cells cannot bind to MHC-I which leads to cell lysis of the target cell
What is induced self?
When target cells express activating ligands that bind to activating receptors and result in NK cell killing
However if MHC-I present and binds to inhibitory receptor then there is balance of activating/inhibitory receptors, resulting in no killing
Describe antibody-dependent cell-mediated cytotoxicity (ADCC)
Ab bind Ags on surface of target cells
CD16 (FcgRIII) on NK cells allows them to bind to Ab-coated targets
Cross-linking of Fc receptors signals the NK cell to kill the target cell
e.g. Rituximab and CD20 in Burkitt’s lymphoma
Describe how Rituximab causes killing of Burkitt cells
Rituximab is a monoclonal antibody
Binds to CD20 on Burkitt cells
CD16 on NK cells will bind to Rituximab and cause cross-linking of the Fc receptors
Burkitt cells are killed
How to inhibitory receptors signal?
Through ITIMs
How do activating receptors signal?
Through ITAMs
What receptors regulate NK cell function?
Killer Ig-like receptors (KIR)
Leukocyte Ig-like receptors (LILR)
Both are encoded in the leukocyte receptor complex (LRC) on chromosome 19
Describe KIR
Innate immune receptors
Regulate the activity of NK cells
Polymorphic
Individual KIR genes vary in their presence between individuals
Describe LILR
Innate immune receptors
Regulate functions of NK cells and APCs
Describe how KIRs function
When KIR recognise MHC-I they inhibit NK cells from releasing lytic granules
Some viruses downregulate MHC-I to evade cytotoxic T cells
If a target cell does not express MHC-I then there is no KIR inhibition
Lytic granules will then be released to lyse the target cell
Known as missing self
What do inhibitory KIR bind to?
The same face of MHC-I as the TCR
What do the KIRs recognise?
Subsets of MHC-I alleles
What do different MHC-I/KIR combinations show?
Disease associations
e.g. in HIV infection
What happens in the interaction of an NK cell with a healthy autologous cell?
No killing of healthy autologous cell
What happens in the interaction of an NK cell with a healthy allogeneic cell of similar HLA-C type?
No killing of allogeneic cell of similar HLA-C type
What happens in the interaction of an NK cell with a healthy allogeneic cell of different HLA-C type?
Killing of healthy allogeneic cell of different HLA-C type
Why are tumour cells susceptible to NK cells?
Tumour cells downgrade the expression of MHC-I
This means inhibitory receptors on NK cells will not bind to the tumour cell
Causes lysis of tumour cell
How are KIR receptors named?
Based on number of Ig domains
Whether they have a long or short cytoplasmic tail - inhibitory receptors have long tails, activating receptors have short tails
e.g. KIR3DL1 = 3 Ig domains, long cytoplasmic tail
KIR2DS1 = 2 Ig domains, short cytoplasmic tail
What do NK cells need to become mature?
Ligation of inhibitory receptors
This supposes that the inhibitory receptors play 2 opposing roles dependent on the stage of NK maturation - to license the cell and to stop cytotoxicity
How do inhibitory and activating receptors exist?
As heterodimers
What is the name of the inhibitory receptor?
CD94/NKG2A, B
KIR2DL
KIR3DL
What are the names of activating receptors?
CD94/NKG2C
CD94/NKG2E
KIR2DS
KIR3DS
What do inhibitory and activating receptors recognise?
HLA-E on target cell
What is HLA-E?
Non-classical MHC-I protein that presents leader peptides from other MHC-I molecules (HLA-A, B, C)
What does HLA-E bind to?
Lectin-like receptors NKG2A, B and C
NKG2A, B and C form heterodimers with CD94
Why do we need HLA-E?
Some immune cells can be HLA-E-restricted so HLA-E will present the peptides needed for recognition from other HLA class I allotypes
CD94/NKG2A is specific to HLA-E meaning it will not bind to other HLA class I allotypes
What is the structure of HLA-E?
a1, 2 and 3 subunits
b2m subunit
Describe LILRs
Encoded adjacent to KIR in the leukocyte receptor complex (LRC)
They have inhibitory motifs - LILRB (inhibitory)
Short tail and associate with adaptor proteins - LILRA (activatory)
Varying expression profiles
LILRB1 expressed by NK cells - powerful immune inhibitor
Describe NKG2D
Expressed on NK cells, gd T cells, CD8+ ab T cells and macrophages
Associates with DAP10 for signalling
Binds to MHC-I-like proteins, MICA, MICB, ULBPs
Activating
What happens when NKG2D recognises MICA, MIC or ULBPs?
Release of perforin
Release of IFN-g, TNF-a and GM-CSF
What inhibitory receptors recognise missing self?
KIR recognise classical MHC
LILR recognise classical and non-classical MHC
NKG2A, B and C recognise non-classical MHC
What activating receptors recognise induced self?
NKG2D recognise MICA, MICB and ULBPs
What are the natural cytotoxicity receptors?
NCR1, NCR2 and NCR3
What do NCRs do?
Provide activating signals to NK cells
What are the ligands for NCR1?
Viral hemagglutinin
What does NCR2 bind?
A ligand expressed on tumour cells and upregulated by viral (HIV) infection
What is the ligand for NCR3 (NKp30)?
BAT3 - a stress induced protein
What does target cell death or survival depend on?
Balance of activating and inhibitory signals
e.g. if 3 activating ligands bind activating but only 1 MHC binds to inhibitory receptor then target cell is lysed because more activation than inhibition
How does cytomegalovirus (CMV) evade immune response?
Produces UL16 which binds NKG2D ligands, retaining them in the cytoplasm
This prevents recognition b NK cells and co-recognition of CD8 T cells
How do tumours avoid immune response?
They produce soluble MIC as a decoy so that the CTLs do not bind to the membrane-bound MIC and cause lysis
Describe the role of HLA-G in immune invasion
Normally only expressed by trophoblast during pregnancy but is expressed by many tumours and can be upregulated in HIV infection
HLA-G is the highest affinity ligand for LILRB1
LILRB1 is expressed on B cells, NK cells, T cells and APCs and can inhibit the functions of each
What are some other innate lymphocytes?
B1 cells
Epithelial gd cells
NK T cells
Describe B1 cells
Make natural Ab - protect against infection with Strep pneumoniae
Ligands no MHC associated
Cannot be boosted
Describe epithelial gd cells
Produce cytokines rapidly
Ligands are MHC class IB associated
Cannot be boosted
Describe NK T cells
Produce cytokines rapidly
Ligands are lipids bound to CD1d
Cannot be boosted
What is the difference between NK T cells and CD8/CD4 T cells?
NK T cells express a biased T cell receptor repertoire
Most NK T cells use a particular TCR alpha chain rearrangement (Va24JaQ) in humans
What receptors do NK T cells express?
Some inhibitory and activation receptors - relevant in bacterial and viral infection
What do NK T cells do?
Kill tumour cells in vitro
Produce cytokines
What does the NK T cell TCR recognise?
Glycolipids in the context of CD1d (MHC I-like molecule)
e.g. alpha-galactoceramide
What is DC vaccination?
a-galactoceramide molecules are loaded onto DCs and this boosts the NK T cell population
It is used to kill tumour cells
Describe CD1
Encoded outside MHC
Specialised binding groove presents glycolipids or phospholipids
Many lipids and glycolipids differ between microbes and mammals
Recognised by specialised subsets of TCR - Va24/Vb11
What are gamma delta T cells?
Unconventional T cells
Mechanism of selection largely unknown
Generally enriched in mucosal and epithelial tissues
MHC independent
Activated by TCR and NK-receptor (NKG2D)
What are the functions of gd T cells?
Lysis of infected or stressed cells
Cytokine and chemokine production
B cell help and IgE production
Priming of ab T cells via antigen presentation
DC maturation
Regulation of stromal cell function via growth factor production
How was the gd TCR discovered?
Delta gene found by chance located within the alpha locus
Then gamma gene was discovered when trying to look for corresponding chain
Describe the loci of the gamma and delta genes
Limited V segments at both loci
Delta has numerous D gene segments
How does the gd TCR get its diversity?
Junctional diversity leads to potentially many CDR3 sequences
What percentage of T cells are gd?
Between 1-10% of peripheral blood T cells
What is the TCR responsible for according to animal models?
Localisation of the cell to tissues
Murine models - Vg3 localise to intestinal epidermis, Vg5 localise to epithelium
Humans - Vg9Vd2 in peripheral blood, Vd1 in epithelial tissue and resident in liver
May involve butyrophilin-like molecules (Btnl)
What molecule is expressed in intestinal epithelial cells of mice?
Btnl1 or Btnl6
What do Btnl1/Btnl6 do?
Activate and home intraepithelial cells for Vg7 T cells to intestinal epithelial cells
What molecule is expressed in thymocytes of mice?
Skint1
What does Skint1 do?
Induces maturation of Vg5 T cells in the mouse thymus
What doe intraepithelial cells for Vg4 in humans bind to?
BTNL3/BTNL8 expressed in intestinal epithelial cells
How do Vg9Vd2 T cells become activated in humans?
Bind butyrophilin itself which presents phosphoantigens
Particular motif binds the phosphoantigens, changing the external conformation
The Vg9Vd2 T cell can then recognise this conformation and become activated
What percentage of gd T cells are Vd1?
10-30% of gd T cells in the blood
Describe Vd1 T cells
More highly represented in tissue
Use TCR and NK receptors to identify tumour targets
Recognise CD1d/lipid complexes and NKG2D ligands
MHC recognition (allogeneic)
Role in homeostasis and stress surveillance
What percentage of normal PBLs are Vd2?
4% but can expand rapidly to 60%+ in infections
What do Vd2 T cells recognise?
Small phosphoantigens
e.g. isopentenyl pyrophosphate (IPP) and E-4-hydroxy-3-methyl-but-2-enyl-pyrophosphate (HMBPP) presented by butyrophilin3A1
Can also recognise NKG2D ligands
Describe butyrophilin
Part of the B7 receptor family-like proteins
2 extracellular Ig-like domains
Does not appear to present pAg on the usual, MHC-like way
What are the types of butyrophilin?
BTN3A1, A2 and A3
What is BTN3A1 essential for?
pAg recognition
What region of BTN3A1 binds pAg?
Intracellular region 30.2
What does binding of pAg to BTN3A1 do?
Changes the conformation of the extracellular portion of BTN3A1
Allows it to be recognised by Vg9Vd2 cells
What are effector functions of gd T cells?
Cytokine production - Th1-like prod IFN-g and TNF-a, Th17-like prod IL-17
Cytotoxicity against infected or transformed cells
Degranulate like NK cells, perforin, serine esterases, granulysin
Thought to be a rapid response in immunosurveillance
What drugs are used in gd T cell therapy?
Aminobisphosphonates
Pamidronate
Alendronate
Zoledronic acid
What does gd T cell therapy do?
Blocks mevalonate pathway and causes accumulation of IPP
IPP binds 30.2 region of BTN3A1 and causes activation of Vg9Vd2 T cells
What is BCG?
Bacille Calmetter-Guerin
A vaccine form of Mycobacterium bovis used to prevent tuberculosis and other mycobacterial infections
What is the significance of BCG in relation to gd T cells?
BCG activates Vg9Vd2 cells and these cells are better at killing tumours than zoledronic acid
