Week 23 - Natural Killer cells

What are natural Killer Cells

Main circulating lymphocytes in the blood contributing to innate immunity.

Only have germline receptors → not antigen specific

Release lytic granules that kill some virus infected cells.

What are NK cells two distinctive functions in the innate immune system

• Kill cells infected with virus

  • Sacrafice of cell hosting the virus stops virus replication

• Maintain and even increase the state of inflammation in the infected tissues

How do NK cells maintain the state of inflammation in infected tissues

• Secretion of inflammatory cytokines that act mainly on resident macrophages and increase their capacity to secrete inflammatory cytokines and to phagocytose viral particles and microorganisms

No _____protein or________ receptor is exclusively expressed by NK cells

• Specific intracellular

• Cell surface receptor

What is the working definition of a NK cell

• A lymphocyte that expresses CD56, a protein of unknown function → expressed by all NK cells

• And lacks CD3 → present on T cells

People who are deficient in NK cells suffer from ……. And can be treated with _____ when they experience infections.

• Persistent Viral infections

  • Herpes

• Antiviral drugs used to treat infections

What percentage of the lymphocytes in the blood do NK cells make up

5 -25%

What are the two subpopulations of NK cells

— Divided on the basis of abundance of CD56 glycoprotein on cell surface and cytotoxic potential —

• CD56dim NK cells → more than 90% of blood NK cells

  • Have fewer CD56 molecules but have a greater capacity for killing cells

• CD56bright → Found in tissues rather than blood (around 90% of tissue NK cells are bright)

  • Reverse function of dim

What are the two types of NK cells based off

• Could possibly be due to different stages in NK development

What do NK cells originate from

— Like any other lymphocyte —

• Starts as a multi potent haemotopoietic Stem cell

• Turns into a Common lymphoid precurosor

• Differentiates into an ILC/NK precursor

• Then differentiates into a NK cell or Innate lymphoid cell

• Making up the Non antigen specific - Innate immune cells

What are innate lymphoid cells and where can they be found

— Type of immune cell that responds rapidly to tissue damage or invading pathogens —

• Periphery→ Mucosa

  • T cells usually reside in lymphatics

• Critical for homeostasis → early response to pathogens and amplification of CD4+ T- Helper adaptive immune response

What are cytotoxic T cells

Recognise foreign proteins on class I MHC

• all uncleared cells express class I MHC except neurones

• Class I MHC present Endogenous antigens

  • Those from inside the cell

• Kill infected cells

How do NK cells and Cytotoxic T cells work together

• CTLs recognise the antigen presented on MHC I

• Many pathogens downregulate expression of MHC I

• NK cells recognise downregulation of MHC I and Kill the cell

• They can also recognise stress molecules indicating infection

• NK cells then kill using performing / granzyme or FasL which CTLs use

What do NK cells express to prevent killing of self antigens

• Both inhibitory and Activating receptors

  • Inhibitory binds to MHC I

  • Activating binds to Stress ligands

Under normal conditions what can be expected from NK cells

• The inhibitory signal outweighs the activating signal

• NK cell inhibition leads to No killing

• As inhibitory receptor binds to MHC class I ligands preventing the activation signals being stronger

How do some pathogens or tumours try and escape killing by CD8+ CTLs and how do NK cells prevent them from escaping

• They down regulate MHC Class I

• NK cells get a weaker inhibitory signal as MHC I is being down regulated

• therefore the activating signal is stronger than inhibitory

• Therefore the NK cell has been activated to kill.

What happens if the Tumour or Virus doesn’t down regulated MHC class I , How do NK cells respond.

• Infected cells produce more stress signals

• The activating signals still outweigh the inhibitory signals

• NK cell is activated to Kill

What is ADCC

—Antibody dependent cell mediated cytotoxicity —

• Where NK cells recognise antibodies on surface of infected/ transformed cells

• IgG antibodies - bound by activating receptor FcgammaRII on NK cells

Are NK cells a type of Innate or adaptive immune cell

• Innate

What do NK cells recognise

• Cells which have downregulated Class I MHC (Missing self)

• Or Upregulated Stress ligands ( Stress induced Self)

What types of NK cell deficiencies are there

• Mutations Causing Total Absence of NK cells

  • Very Very Rare

  • Highly susceptible to viral infections

  • Increased risk of premature death

• Functional NK Deficiencies

  • More common

  • Deficiencies of NK activity such as Killy

  • Increased susceptibility to infections especially viral

What process does CTLs and NKs kill through

• Perforin and granzymes from granule release or FAS ligation

How does perforin and Granxymes kill infected cells

• Perfoin Forms a Pore

• Granzyme can enter through that pore and induce apoptosis via activation of caspase 3

How does FAS ligation cause cell death

• Direct signals to induce apoptosis

  • FasL ligation clusters intracellular domains, Recruits FADD and activates caspase 8

  • Initiating apoptosis

What are granzymes

• Seriene proteases

• Stored in NK cell granules

• Released on NK activation

• Enter target cells via perforin induced pores

What are the two families of NK receptors

• KIRs

  • Killer immunoglobulin like receptors

• KLRs

  • Killer lectin like receptors

What are the structures of KIR and KLR families of NK cells

• Both have activating and inhibitory receptors

• Inhibitor receptors have Tyrosine Based Inhibitory Motifs (ITIMs)

• Activating associate with CD3 or DAP12 containing an ITAM

What is the mechanism for signal initiation in NK receptors

• Phosphorylation of the tyrosine residues on ITAM by Src-Like kinases

• Leading to binding of Zap-70 kinase via phosphotyrosine binding SH2 domains

• Kinase activated and initiates signalling

Which motif is essential for suppression of activation in inhibitory receptors

• ITIM motifs

What is HLA-E

Non classical Class I MHC protein

• Only Presents the leader peptides of other Class I MHC proteins

What are leader sequences

Sequences that indicate proteins which should be targeted to the cell membrane

These are cleaved off in the endoplasmic reticulum

What binds to HLA-E

- CD94:NKG2A

How are expression of levels of all Class I MHC sensed by NK and CTLs

CD94 or NKG2A binds to the leader peptide HLA-E

What are Natural Cytoxicity Receptors

• NCRs

  • NKp30

  • NKp44

  • NKp46

• NKG2D

What is NKG2D and what does it do

• Exception in the NKG2 family

  • Forms a homodimer instead of a heterodimer

  • Ligands are Non classical class I MHC like proteins

  • Not expressed constituitvely

    • Only under conditions of stress

Overview of NK Receptors

• Large number of NK activating receptors

  • Ligands and roles poorly understood

• Some act like PRRs

  • Recognising viral PAMPs

• Others bind Stress Ligands

• Others bind IgG Fc

  • Mediating ADCC

How is an antiviral sate induced

• Infected cells release type I IFNs

  • Inducing antiviral state in neighbouring cells

    • Upregualting antiviral proteins

    • Downregulating everything PSE

  • Type I IFNs also activat NK cells → more effective killers

How does activation of NK cells occur through type I IFNs

• Virus infects cells and triggers the interferon response

• Type I interferon drives proliferation of NK cells

• Type I interferon drives differentiation into cytotoxic effector cells

• Effector NK cells kill virus infected cells by inducing apoptosis

What other cells do NK cells communicate with

— Other Immune Cells —

• Macophages

  • Activated via PAMPs

  • Releasing IL-8, IL-12, I-15

  • These Activate NK cells to proliferate and produce IFN-gamma

  • These further activate macrophages to make more IL-12 _> positive feedback loop

Are there memory NK cells

Yes

• Express higher levels of some receptors activated by original infection

Overall summary of NK cells

• Abundant innate immune cells that defend against viral infections and cancer

• Important source of cytokines - early IFNgamma

• Killing function highly regulated by inhibitory and activating receptors

• Inhibitory receptors recognise MHC class I molecules on target cells

• Activating receptors bind pathogen associated molecules as well as self ligands

• Balance between inhibition and activation crucial in determining NK cell activation