Natural Killer (NK) Cells

Abstract

Natural killer (NK) cells belong to the innate immune system. They are a heterogeneous population of lymphocytes found in the spleen and circulating in the blood, but there are also noncirculating tissue‐resident NK cells in certain organs. NK cells were first discovered because they can kill tumour cells. However, it is less well appreciated that they are also critical in the host defence against infections, particularly against viruses, and help orchestrate a productive immune response involving not only innate immunity but also adaptive immunity. To perform these functions, they express (1) a variety of cell surface receptors to recognise their cellular targets and activate (or not) target killing and cytokine production and (2) cytokine receptors that also stimulate secretion of other cytokines.

Key Concepts

  • NK cells are innate lymphocytes that can kill tumour and infected cells and produce cytokines.
  • NK‐cell killing is guided by the ‘missing‐self’ hypothesis whereby target cells expressing normal surface levels of major histocompatibility complex (MHC) class I molecules are spared, whereas target cells with decreased levels of MHC class I (missing‐self) are killed.
  • NK‐cell killing is due to the balance and integration of signals from NK‐cell inhibitory receptors for target cell MHC class I molecules and activation receptors that recognise ligands expressed on target cells.
  • Mouse and human NK‐cell receptors for MHC class I molecules belong to two different structural types, as an outstanding example of convergent evolution.
  • NK cells are related to innate lymphoid cells (ILCs).
  • Several organs contain noncirculating tissue‐resident NK cells and conventional NK cells that represent circulating NK cells found in blood and spleen.

Keywords: NK cells; missing‐self; MHC class I; inhibitory receptors; innate immunity

Figure 1. Stages of cytotoxicity. (a) Recognition stage: NK cells survey the environment for stressed target cells; once identified, they adhere to the target cell using adhesion receptors. This initiates activation of the NK cell. (b) Effector stage: The effector stage is marked by the formation of the immunological synapse and polarisation of the lytic granules directed towards the target cell. The granules fuse with the plasma membrane of the NK cell, turning ‘inside‐out’, allowing delivery of granule contents, including perforin which forms a pore by polymerising in the plasma membrane of the target. Granzymes are also delivered which induce target cell apoptosis. (c) Termination stage: During the termination stage, the target cells undergo apoptosis and the NK cell disengages and continues to survey the microenvironment.
Figure 2. Assays to measure NK‐cell function. (a) Chromium‐51 release assay. The target cells are labelled with radioactive chromium (51Cr), washed and incubated with different numbers of NK cells for 4 h. The radioactivity is measured in the cell‐free supernatant of the cultures and graphed as the percentage of specific cytotoxicity or lysis, derived by the formula described in the text versus effector:target (E:T) ratio. (b) CD107a degranulation assay. Target cells are incubated with NK cells in which CD107a is expressed on the granule membrane oriented towards the lumen. Upon degranulation, granules fuse with the plasma membrane, turning the granule membrane ‘inside‐out’ and exposing CD107a on the NK‐cell surface. CD107a expression on individual NK cells can be detected by flow cytometry as an indication that degranulation had occurred, as shown here by a putative flow cytometry dot plot of CD107a versus NK1.1 expression on CD3ϵ cells. The NK1.1+ cells are the NK cells in this graph. (c) Intracellular cytokine staining. NK cells are incubated with stimuli, such as target cells. Prevention of cytokine secretion by addition of BFA to the cocultures allows the accumulation of the cytokine which is then detected by fixing and permeabilising the cells for cytokine staining and flow cytometry. A putative flow cytometry dot plot shows expression of IFNγ and NK1.1 on CD3ϵ cells. The NK1.1+ cells are the NK cells in this graph.
Figure 3. Activation and inhibitory receptors of NK cells. (a) Missing‐self hypothesis. NK cells have the capability of distinguishing between healthy and stressed cells by sensing the levels of MHC‐I expression on the target. NK cells express inhibitory receptors that engage target cell MHC‐I molecules and turn off the NK cells. Pathologic processes such as viral infection may down‐regulate MHC‐I levels so that the inhibitory receptor are no longer engaged, freeing the NK cell to lyse the target cell if an NK‐cell activation receptor detects a ligand on the target. If both inhibitory and activation receptors are engaged, inhibition usually dominates. (b) Activation receptors on NK cells contain charged transmembrane residues that enable association with transmembrane adaptor proteins, such as CD3ζ, FcϵRIγ or DAP12, which contain ITAMs. Signalling via the activation receptor results in ITAM phosphorylation by Src family of kinases and subsequent downstream activation of cytotoxicity, cytokine production and proliferation. All inhibitory receptors have ITIMs in the cytoplasmic tails that are phosphorylated upon ligand engagement, resulting in recruitment and activation of cytoplasmic tyrosine phosphatases, such as SHP‐1. The NKG2D activation receptor is associated with DAP10, which has the potential to phosphorylate Grb2/PI3K, activating another downstream pathway that stimulates NK‐cell functions.
Figure 4. Development of NK cells. (a) The common lymphoid progenitor (CLP) gives rise to NK cells, T and B cells. The ID2 expressing cells mark the NK/ILC progenitor cell, with the ID2low cells giving rise to NKp and the ID2high cells give rise to the LTi/ILCP which go on to develop into ILC1, ILC2, ILC3/LTi depending on the transcription factors that are expressed. The conventional NK cell requires transcription factors NFIL‐3, Tbet and Eomesodermin for full maturation. The thymic NK cells require GATA‐3. The tissue‐resident NK cells in the liver require Tbet for their development, while it is still unclear what transcription factor(s) is required for the development of the uterine NK cells. (b) In the bone marrow, the NK precursor expresses CD122 on the surface. This initiates the cell down an NK developmental pathway and is marked by the expression of NK1.1 activation marker. The cell expresses additional markers such as DX5, CD11b and the Ly49s as it continues to mature. A mature NK cell migrates to the spleen.
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Further Reading

Artis D and Spits H (2015) The biology of innate lymphoid cells. Nature 517: 293–301.

Orange JS (2013) Natural killer cell deficiency. Journal of Allergy and Clinical Immunology 132: 515–525; quiz 526.

Yokoyama WM (2013) Natural killer cells. In: Paul WE (ed) Fundamental Immunology, pp. 395–431, chap. 17. Philadelphia: Lippincott Williams & Wilkins.

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Sojka, Dorothy K, Piersma, Sytse J, and Yokoyama, Wayne M(Apr 2016) Natural Killer (NK) Cells. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001220.pub3]