Human natural killer (NK) cells express a series of activating receptors and coreceptors that are involved in recognition and killing of target cells. In this study, in an attempt to identify the cellular ligands for such triggering surface molecules, mice were immunized with NK-susceptible target cells. On the basis of a functional screening, four mAbs were selected that induced a partial down-regulation of the NK-mediated cytotoxicity against the immunizing target cells. As revealed by biochemical analysis, three of such mAbs recognized molecules of ∼70 kD. The other mAb reacted with two distinct molecules of ∼65 and 60 kD, respectively. Protein purification followed by tryptic digestion and mass spectra analysis, allowed the identification of the 70 kD and the 65/60 kD molecules as PVR (CD155) and Nectin-2 δ/α (CD112), respectively. PVR-Fc and Nectin-2-Fc soluble hybrid molecules brightly stained COS-7 cells transfected with the DNAM-1 (CD226) construct, thus providing direct evidence that both PVR and Nectin-2 represent specific ligands for the DNAM-1 triggering receptor. Finally, the surface expression of PVR or Nectin-2 in cell transfectants resulted in DNAM-1–dependent enhancement of NK-mediated lysis of these target cells. This lysis was inhibited or even virtually abrogated upon mAb-mediated masking of DNAM-1 (on NK cells) or PVR or Nectin-2 ligands (on cell transfectants).

The surface density of the triggering receptors responsible for the natural killer (NK)-mediated cytotoxicity is crucial for the ability of NK cells to kill susceptible target cells. In this study, we show that transforming growth factor β1 (TGFβ1) down-regulates the surface expression of NKp30 and in part of NKG2D but not that of other triggering receptors such as NKp46. The TGFβ1-mediated inhibition of NKp30 surface expression reflects gene regulation at the transcriptional level. NKp30 has been shown to represent the major receptor involved in the NK-mediated killing of dendritic cells. Accordingly, the TGFβ1-dependent down-regulation of NKp30 expression profoundly inhibited the NK-mediated killing of dendritic cells. On the contrary, killing of different NK-susceptible tumor cell lines was variably affected, reflecting the differential usage of NKp30 and/or NKG2D in the lysis of such tumors. Our present data suggest a possible mechanism by which TGFβ1-producing dendritic cells may acquire resistance to the NK-mediated attack.

After culture in interleukin (IL)-2, natural killer (NK) cells acquire an increased capability of mediating non–major histocompatibility complex (MHC)–restricted tumor cell lysis. This may reflect, at least in part, the de novo expression by NK cells of triggering receptors involved in cytolysis. In this study we identified a novel 44-kD surface molecule (NKp44) that is absent in freshly isolated peripheral blood lymphocytes but is progressively expressed by all NK cells in vitro after culture in IL-2. Different from other markers of cell activation such as CD69 or VLA.2, NKp44 is absent in activated T lymphocytes or T cell clones. Since NKp44 was not detected in any of the other cell lineages analyzed, it appears as the first marker specific for activated human NK cells. Monoclonal antibody (mAb)–mediated cross-linking of NKp44 in cloned NK cells resulted in strong activation of target cell lysis in a redirected killing assay. This data indicated that NKp44 can mediate triggering of NK cell cytotoxicity. mAb-mediated masking of NKp44 resulted in partial inhibition of cytolytic activity against certain (FcγR-negative) NK-susceptible target cells. This inhibition was greatly increased by the simultaneous masking of p46, another recently identified NK-specific triggering surface molecule. These data strongly suggest that NKp44 functions as a triggering receptor selectively expressed by activated NK cells that, together with p46, may be involved in the process of non-MHC-restricted lysis. Finally, we show that p46 and NKp44 are coupled to the intracytoplasmic transduction machinery via the association with CD3ζ or KARAP/DAP12, respectively; these associated molecules are tyrosine phosphorylated upon NK cell stimulation.

Natural killer (NK) cells are innate lymphoid cells (ILCs) contributing to immune responses to microbes and tumors. Historically, their classification hinged on a limited array of surface protein markers. Here, we used single-cell RNA sequencing (scRNA-seq) and cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) to dissect the heterogeneity of NK cells. We identified three prominent NK cell subsets in healthy human blood: NK1, NK2 and NK3, further differentiated into six distinct subgroups. Our findings delineate the molecular characteristics, key transcription factors, biological functions, metabolic traits and cytokine responses of each subgroup. These data also suggest two separate ontogenetic origins for NK cells, leading to divergent transcriptional trajectories. Furthermore, we analyzed the distribution of NK cell subsets in the lung, tonsils and intraepithelial lymphocytes isolated from healthy individuals and in 22 tumor types. This standardized terminology aims at fostering clarity and consistency in future research, thereby improving cross-study comparisons.

In this study, cancer cells were isolated from tumor specimens of nine glioblastoma patients. Glioblastoma cells, cultured under suitable culture conditions, displayed markers typical of neural stem cells, were capable of partial multilineage differentiation in vitro, and gave origin to infiltrating tumors when orthotopically injected in NOD/SCID mice. These cells, although resistant to freshly isolated NK cells, were highly susceptible to lysis mediated by both allogeneic and autologous IL-2 (or IL-15)-activated NK cells. Indeed, all stem cell-cultured glioblastoma cells analyzed did not express protective amounts of HLA class I molecules, while expressing various ligands of activating NK receptors that triggered optimal NK cell cytotoxicity. Importantly, glioblastoma stem cells expressed high levels of PVR and Nectin-2, the ligands of DNAM-1-activating NK receptor.

In this study, in an attempt to identify neuroblastoma-associated surface antigens, we generated mAbs against the ACN neuroblastoma cell line. A mAb was selected (5B14) that reacted with all neuroblastoma cell lines analyzed and allowed detection of tumor cell infiltrates in bone marrow aspirates from neuroblastoma patients. In cytofluorimetric analysis, unlike anti-disialoganglioside mAb, 5B14 mAb did not display reactivity with normal bone marrow hematopoietic cell precursors, thus representing a highly specific marker for identifying neuroblastoma cells. Molecular analysis revealed that the 5B14 mAb-reactive surface glycoprotein corresponded to the recently identified 4Ig-B7-H3 molecule. Remarkably, mAb-mediated masking of the 4Ig-B7-H3 molecule on cell transfectants or on freshly isolated neuroblastoma cells resulted in enhancement of natural killer-mediated lysis of these target cells. These data suggest that 4Ig-B7-H3 molecules expressed at the tumor cell surface can exert a protective role from natural killer-mediated lysis by interacting with a still undefined inhibitory receptor expressed on natural killer cells.