NK cells and cd T cells are distinct subsets of lymphocytes that contextually share multiple phenotypic and functional characteristics. However, the acquisition and the extent of these similarities remain poorly understood. Here, using T cell receptor d locus-histone 2B-enhanced GFP (Tcrd-H2BEGFP) reporter mice, we show that germline transcription of Tcrd occurs in all maturing NK cells. We also describe a population of mouse NK-like cells that are indistinguishable from "bona fide" NK cells using standard protocols. Requirements for V(D)J recombination and a functional thymus, along with very low-level expression of surface TCRcd but high intracellular CD3, define these cells as cd T cells. "NK-like cd T cells" are CD127 + , have a memory-activated phenotype, express multiple NK cell receptors and readily produce interferon-c in response to IL-12/IL-18 stimulation. The close phenotypic resemblance between NK cells and NK-like cd T cells is a source of experimental ambiguity in studies bridging NK and T cell biology, such as those on thymic NK cell development. Instead, it ascribes chronic TCRcd engagement as a means of acquiring NK-like function.
IntroductionClassification of NK cells and some cd T cell subsets as innate immune cells is argued by their recognition and response to self ligands and microbial molecules without any requirement for prior specific immunization [1][2][3][4][5]. NK cells are mediators of immunity against various infectious diseases and tumor cells, performing direct cell-mediated cytotoxicity and producing cytokines in response to cell-associated and soluble stimuli [6,7]. They additionally shape adaptive immune responses, mainly through the production of cytokines [8]. Their activation is regulated through germ-line-encoded receptors including NKG2D, the natural cytotoxicity receptors and inhibitory receptors for MHC class I, many of which recognize self-encoded ligands [9][10][11]. Most cd T cell subsets are unconventional T cells that -similar to CD1d-restricted Va14i NKT cells, MR1-restricted MAIT cells and certain populations of CD8 + T cells -are not restricted to classical MHC class I-or MHC class II-bearing specific peptides [2,3]. They have heterogeneous effector functions associated with differential TCRcd usage. Roles have been described in immunity against infectious pathogens and tumors [12] and cd T cells have been strongly implicated in immunosuppressive regulation of the immune system and in the process of wound healing [13,14]. Using TCR transfer experiments, or direct ligand binding, TCRcd ligands have been described for a small number of subsets [5]. In the mouse, cd T cell subsets recognize the non-classical MHC class I molecules T10/T22 [15], and an unknown ligand expressed by keratinocytes [16]. In humans, some Vd1 cd T cells recognize non-classical MHC class I molecules MICA and MICB, and Vc9Vd2 cd T cells have been reported to recognize self and bacterial phosphoantigens, and a complex formed between F1-ATP synthase and apolipoprotein A-1 [5,[17][18][19][20].Du...