Natural killer cells: from CD3NKp46+ to post-genomics meta-analyses

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The original definition of NK cells was based on their ‘natural’ cytolytic response against tumor cells and virus-infected cells in the absence of specific immunization. However, the term ‘natural killer’ reflects neither the education/maturation requirements before NK cells can kill nor the entirety of their biological functions. In light of new functional assays, genetic models and genomics analysis, we propose a more accurate definition of NK cells. This definition includes the phenotypical identification of NK cells as CD3NKp46+ cells across mammalian species. In general, this attempt to redefine NK cells also highlights the need to update the operational definition of cell types in the post-genomic area.

Introduction

Natural killer (NK) cells were identified more than 30 years ago as ‘small lymphocytes of undefined nature exerting spontaneous selective cytotoxic activity against Moloney leukemia cells’ — a phenomenon designated as to ‘natural cytotoxicity’ [1]. This definition was purely functional and did not exclude the contribution of several cell lineages to this cytolytic function. Later, several cell surface markers were discovered, the expression of which correlated with most of the observed natural cytotoxicity against the classical tumor cell targets (e.g. YAC-1 in the mouse and K562 in humans). This led to the proposal in 1986 that ‘NK cells formed a lineage of lymphocytes which specialized function was the natural cytotoxicity’ [2]. In this article, we revisit the basics of the biology of NK cells, including their phenotype, their transcriptional signature and their effector function, in an attempt to propose an updated and operational definition of this type of innate lymphocytes.

Section snippets

NK cell phenotype

To date, the cell surface markers used to define NK cells in human, mouse and rat have been quite different. In humans, NK cells are classically defined as CD56+CD3 lymphocytes. In NK cells, CD56 corresponds to the 140 kDa isoform of the neural cell adhesion molecule (N-CAM) [3]. However, N-CAM is also expressed by T-cell subsets, muscle cells and neurons, but it is not expressed by mouse NK cells. In the rat, NK cells express the activating receptor NKR-P1A, but this molecule is also expressed

NK cell transcriptional signatures

The development of transcriptional analysis has allowed the classification of cell types to be refined at the pan-genomic scale [12]. The compilation of several DNA micro-array data led to attempts at the definition of the NK cell transcriptional signature (i.e. the set of transcripts that distinguish NK cells from other cell types), in particular from T, B and NK/T lymphocytes [13, 14, 15, 16, 17••]. Using this set of data, human genes with a pattern of expression similar to that of NKp46

NK cell phylogeny

NK cells were once considered to be primitive weapons: ‘evolutionary forerunners of the cytolytic T cells’ [19]. In this scheme, ‘the acquisition by an NK cell of a clonally distributed receptor capable of fine discrimination between self and non-self might have been sufficient to make it a cytolytic T cell’ [19]. This definition was based on the assumption that NK cells were nothing but a cytolytic machinery controlled by some ancestral non-clonally distributed recognition mechanisms, and is

The ‘natural’ effector functions of NK cells

Can NK cells be defined in functional terms? In particular, can they be defined by their natural killing activity? The development of a novel single cell assay to measure cell cytotoxicity revealed that most cells with an apparently mature NK cell phenotype in the human blood or the mouse spleen are poorly capable of cytotoxicity against prototypical NK cell tumor targets (i.e. K562 and YAC-1, respectively) and therefore are inappropriately termed ‘killers’ (Figure 2) [24••]. This lack of

NK cell ‘paralogs’

Natural cytotoxicity is not an exclusive property of NK cells. Indeed, various T-cell subsets are capable of natural cytotoxicity against classical NK cell targets [2, 10, 33]. Recent findings show that under chronic activation discrete T-cell subsets might partially lose CD3–TCR surface expression and acquire the expression of several NK cell receptors such as NKp46 [34]. In steady-state conditions, a subset of mouse T cells also ‘hide’ in the NK cell gate (i.e. they express so little CD3–TCR

Defining the NK cell lineage

Most NK cells are thought to arise in the bone marrow [42]. In addition to this, a fraction of peripheral NK cells could arise within lymph nodes from migrated hematopoietic precursors [43••]. Importantly, lymph node human NK cells are mostly CD56bright and express the α chain of the IL-7 receptor (CD127), whereas peripheral blood NK cells are mostly CD56dim and do not express CD127 [44]. A similar population has been identified in mouse lymph nodes and thymus (Figure 4), leading to the

Conclusions

The definition of a cell type remains notoriously difficult to establish precisely and unambiguously, as illustrated above. Caution should be taken when names are given to classify a population, as names imprint the representation of a given cell type in the scientific community and beyond. Within the 411 cell types that have been recently proposed in humans [45], NK cells are certainly more complex than originally predicted. An updated definition of NK cells can be proposed to take into

Update

The work cited in the main body of text as TW et al., unpublished, has now been accepted for publication [48].

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We would like to thank Louis du Pasquier, as well as all the members of the ‘NK cells and Innate Immunity’ laboratory, CIML, Marseille. The laboratory is supported by European Union FP6, LSHB-CT-2004-503319-Allostem, Ligue Nationale contre le Cancer (‘Equipe labellisée La Ligue’), Agence Nationale de la Recherche (‘Réseau Innovation Biotechnologies’ and ‘Microbiologie Immunologie – Maladies Emergentes’), INSERM, CNRS and Ministère de l’Enseignement Supérieur et de la Recherche.

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