The elimination of viruses and tumors by natural killer cells is mediated by specific natural killer cell receptors. To study the in vivo function of a principal activating natural killer cell receptor, NCR1 (NKp46 in humans), we replaced the gene encoding this receptor (Ncr1) with a green fluorescent protein reporter cassette. There was enhanced spread of certain tumors in 129/Sv but not C57BL/6 Ncr1(gfp/gfp) mice, and influenza virus infection was lethal in both 129/Sv and C57BL/6 Ncr1(gfp/gfp) mice. We noted accumulation of natural killer cells at the site of influenza infection by tracking the green fluorescent protein. Our results demonstrate a critical function for Ncr1 in the in vivo eradication of influenza virus.

MICA and MICB are stress-induced ligands recognized by the activating receptor NKG2D. A microRNA encoded by human cytomegalovirus downregulates MICB expression by targeting a specific site in the MICB 3' untranslated region. As this site is conserved among different MICB alleles and a similar site exists in the MICA 3' untranslated region, we speculated that these sites are targeted by cellular microRNAs. Here we identified microRNAs that bound to these MICA and MICB 3' untranslated region sequences and obtained data suggesting that these microRNAs maintain expression of MICA and MICB protein under a certain threshold and facilitate acute upregulation of MICA and MICB during cellular stress. These microRNAs were overexpressed in various tumors and we demonstrate here that they aided tumor avoidance of immune recognition.

The mechanism of action of natural killer (NK) cells in type 1 diabetes is still unknown. Here we show that the activating receptor NKp46 recognizes mouse and human ligands on pancreatic beta cells. NK cells appeared in the pancreas when insulitis progressed to type 1 diabetes, and NKp46 engagement by beta cells led to degranulation of NK cells. NKp46-deficient mice had less development of type 1 diabetes induced by injection of a low dose of streptozotocin. Injection of soluble NKp46 proteins into nonobese diabetic mice during the early phase of insulitis and the prediabetic stage prevented the development of type 1 diabetes. Our findings demonstrate that NKp46 is essential for the development of type 1 diabetes and highlight potential new therapeutic modalities for this disease.

The in vitro elimination of virus-infected and tumor cells by NK cells is regulated by a balance between signals conveyed via specific inhibitory and activating receptors. Whether NK cells and specifically the NK-activating receptor NKp46 (NCR1 in mice) are directly involved in tumor eradication in vivo is still largely unknown. Since the NKp46/NCR1 tumor ligands have not been identified yet, we use a screening technique to identify functional ligands for NKp46/NCR1 which is based on a cell reporter assay and discover a NCR1 ligand in the PD1.6 lymphoma line. To study whether NKp46/NCR1 is important for the eradication of PD1.6 lymphoma in vivo, we used the Ncr1 knockout Ncr1gfp/gfp mice generated by our group. Strikingly, all Ncr1 knockout mice developed growing PD1.6 tumors, whereas initial tumor growth was observed in the wild-type mice and tumors were completely rejected as time progressed. The growth of other lymphoma cell lines such as B10 and EL4 was equivalent between the Ncr1 knockout and wild-type mice. Finally, we show that PD1.6 lymphoma cells are less killed both in vitro and in vivo in the absence of NKp46/NCR1. Our results therefore reveal a crucial role for NKp46/NCR1 in the in vivo eradication of some lymphoma cells.

Periodontitis is a common human chronic inflammatory disease that results in the destruction of the tooth attachment apparatus and tooth loss. Although infections with periopathogenic bacteria such as Porphyromonas gingivalis ( P. gingivalis ) and Fusobacterium nucleatum ( F. nucleatum ) are essential for inducing periodontitis, the nature and magnitude of the disease is determined by the host's immune response. Here, we investigate the role played by the NK killer receptor NKp46 (NCR1 in mice), in the pathogenesis of periodontitis. Using an oral infection periodontitis model we demonstrate that following F. nucleatum infection no alveolar bone loss is observed in mice deficient for NCR1 expression, whereas around 20% bone loss is observed in wild type mice and in mice infected with P. gingivalis . By using subcutaneous chambers inoculated with F. nucleatum we demonstrate that immune cells, including NK cells, rapidly accumulate in the chambers and that this leads to a fast and transient, NCR1-dependant TNF-α secretion. We further show that both the mouse NCR1 and the human NKp46 bind directly to F. nucleatum and we demonstrate that this binding is sensitive to heat, to proteinase K and to pronase treatments. Finally, we show in vitro that the interaction of NK cells with F. nucleatum leads to an NCR1-dependent secretion of TNF-α. Thus, the present study provides the first evidence that NCR1 and NKp46 directly recognize a periodontal pathogen and that this interaction influences the outcome of F. nucleatum -mediated periodontitis.

NK cells interact with a wide variety of hazardous cells including pathogen-infected and tumor cells. NKp46 is a specific NK killer receptor that recognizes various influenza hemagglutinins and unknown tumor ligands. It was recently shown that NKp46 plays a significant role in the in vivo eradication of tumor cells; however, the role played by NKp46 in vivo with regard to tumor development is still unclear. In this study, we used the 3-methylcholanthrene (MCA)-induced fibrosarcoma model in NKp46-deficient mice to test the NKp46 recognition of carcinogen-induced tumors. We show that although the rate of MCA-induced tumor formation was similar in the presence and in the absence of NKp46, the expression of its unknown ligands was NKp46 dependent. The unknown NKp46 ligands were nearly absent in tumors that originated in wild-type mice, whereas they were detected in tumors that originated in the NKp46-deficient mice. We demonstrate that the interactions between NKp46 and its MCA tumor-derived ligands lead to the secretion of IFN-γ but not to the elimination of the MCA-derived tumor cells. In addition, we show that the in vivo growth of MCA-derived tumor cells expressing high levels of the NKp46 ligands is NKp46 and IFN-γ dependent. Thus, we present in this study a novel NKp46-mediated mechanism of tumor editing.

SUMMARYHematopoietic stem cell transplantation (HSCT) is often the only curative treatment for a wide variety of hematologic malignancies. Donor selection in these diseases is crucial, given that transplanted cells can mediate not only the desired graft-versus-leukemia effect but also graft-versus-host disease (GVHD). Here, we demonstrate that in the absence of NKp46, a major killer receptor expressed by human and mouse natural killer (NK) cells, GVHD is greatly exacerbated, resulting in rapid mortality of the transplanted animals because of infection with commensal bacteria. Furthermore, we demonstrate that the exacerbated GVHD is the result of an altered ability of immune cells to respond to stimulation by immature dendritic cells. Because high and low expression of NKp46 on NK cells is observed in different individuals, our data indicate that choosing NKp46-high donors for the treatment of different hematologic malignancies might lead to better tumor eradication while minimizing GVHD.

Natural killer (NK) cells are cytotoxic cells that are able to rapidly kill viruses, tumor cells, parasites, bacteria, and even cells considered “self”. The activity of NK cells is controlled by a fine balance of inhibitory and activating signals mediated by a complex set of different receptors. However, the function of NK cells is not restricted only to the killing of target cells, NK cells also possess other properties such as the secretion of proangiogenic factors during pregnancy. Here, we demonstrate another unique NK-cell activity, namely the regulation of T-cell mediated allergic responses, which is dependent on the NK-cell specific receptor NKp46 (Ncr1 in mice). Using mice in which the Ncr1 gene has been replaced with a green fluorescent protein, we demonstrate reduced delayed-type hypersensitivity and airway hypersensitivity. Interestingly, we show that this reduction in airway hypersensitivity is due to differences in the stimulation of T cells resulting in an altered cytokine profile.