The interaction between CD300a and phosphatidylserine inhibits tumor cell killing by NK cells.

The activity of NK cells is controlled by inhibitory and activating receptors. The inhibitory receptors interact mostly with MHC class I proteins, however, inhibitory receptors such as CD300a, which bind to non-MHC class I ligands, also exist. Recently, it was discovered that phosphatidylserine (PS) is a ligand for CD300a and that the interaction between PS expressed on apoptotic cells and CD300a inhibits the uptake of apoptotic cells by phagocytic cells. Whether PS can inhibit NK-cell activity through CD300a is unknown. Here, we have generated specific antibodies directed against CD300a and we used these mAbs to demonstrate that various NK-cell clones express different levels of CD300a. We further demonstrated that both CD300a and its highly homologous molecule CD300c bind to the tumor cells equally well and that they recognize PS and additional unknown ligand(s) expressed by tumor cells. Finally, we showed that blocking the PS-CD300a interaction resulted in increased NK-cell killing of tumor cells. Collectively, we demonstrate a new tumor immune evasion mechanism that is mediated through the interaction between PS and CD300a and we suggest that CD300c, similarly to CD300a, also interacts with PS.

Neuraminidase-mediated, NKp46-dependent immune-evasion mechanism of influenza viruses.

Natural killer (NK) cells play an essential role in the defense against influenza virus, one of the deadliest respiratory viruses known today. The NKp46 receptor, expressed by NK cells, is critical for controlling influenza infections, as influenza-virus-infected cells are eliminated through the recognition of the viral hemagglutinin (HA) protein by NKp46. Here, we describe an immune-evasion mechanism of influenza viruses that is mediated by the neuraminidase (NA) protein. By using various NA blockers, we show that NA removes sialic acid residues from NKp46 and that this leads to reduced recognition of HA. Furthermore, we provide in vivo and in vitro evidence for the existence of this NA-mediated, NKp46-dependent immune-evasion mechanism and demonstrate that NA inhibitors, which are commonly used for the treatment of influenza infections, are useful not only as blockers of virus budding but also as boosters of NKp46 recognition.

MiR-10b downregulates the stress-induced cell surface molecule MICB, a critical ligand for cancer cell recognition by natural killer cells.

Natural killer cells (NK) are a component of innate immunity well known for their potent ability to kill virus-infected or neoplastically transformed cells following stimulation of the NK cell receptor NKG2D. One of the various ligands of NKG2D is MICB, a stress-induced ligand that has been found to be upregulated on the surface of tumor cells. However, there is little knowledge about how this upregulation may occur or how it may be selected against in tumors as a mechanism of immune escape. Here, we report that the metastasis-associated microRNA (metastamir) miR-10b directly binds to the 3' untranslated region of MICB and downregulates its expression. Notably, antagonizing miR-10b action enhanced NKG2D-mediated killing of tumor cells in vitro and enhanced clearance of tumors in vivo. Conversely, overexpression of miR-10b downregulated MICB and impaired elimination of tumor cells. Together, our results define MICB as a novel immune target of miR-10b, implying a direct link between metastasis capability and immune escape from NK cells.

An identical miRNA of the human JC and BK polyoma viruses targets the stress-induced ligand ULBP3 to escape immune elimination.

The human polyoma viruses JCV and BKV establish asymptomatic persistent infection in 65%-90% of humans but can cause severe illness under immunosuppressive conditions. The mechanisms by which these viruses evade immune recognition are unknown. Here we show that a viral miRNA identical in sequence between JCV and BKV targets the stress-induced ligand ULBP3, which is a protein recognized by the killer receptor NKG2D. Consequently, viral miRNA-mediated ULBP3 downregulation results in reduced NKG2D-mediated killing of virus-infected cells by natural killer (NK) cells. Importantly, when the activity of the viral miRNA was inhibited during infection, NK cells killed the infected cells more efficiently. Because NKG2D is also expressed by various T cell subsets, we propose that JCV and BKV use an identical miRNA that targets ULBP3 to escape detection by both the innate and adaptive immune systems, explaining how these viruses remain latent without being eliminated by the immune system.

The human cytomegalovirus microRNA miR-UL112 acts synergistically with a cellular microRNA to escape immune elimination.

Although approximately 200 viral microRNAs are known, only very few share similar targets with their host's microRNAs. A notable example of this is the stress-induced ligand MICB, which is targeted by several distinct viral and cellular microRNAs. Through the investigation of the microRNA-mediated immune-evasion strategies of herpesviruses, we initially identified two new cellular microRNAs that targeted MICB and were expressed differently both in healthy tissues and during melanocyte transformation. We show that coexpression of various pairs of cellular microRNAs interfered with the downregulation of MICB, whereas the viral microRNAs optimized their targeting ability to efficiently downregulate MICB. Moreover, we demonstrate that through site proximity and possibly inhibition of translation, a human cytomegalovirus (HCMV) microRNA acts synergistically with a cellular microRNA to suppress MICB expression during HCMV infection.

Expression and function of CD300 in NK cells.

The killing activity of NK cells is regulated by signals derived from inhibitory and activating NK cell receptors, including the CD300 family of proteins. CD300a was reported to be expressed on all NK cells and to deliver an inhibitory signal upon binding to a yet unknown ligand/s. The CD300a protein contains four ITIMs and is highly homologous to CD300c. Little is known about the function and distribution of these two receptors and the identity of their ligand/s. In this article, we show that CD300a is indeed an inhibitory receptor expressed by human NK cells, but surprisingly, we show that not all NK clones are inhibited in a CD300a-dependent manner. We demonstrate, using a panel of 13 new anti-CD300a and CD300c Abs that we generated, that CD300a and CD300c are indistinguishable on the surface of NK cells. Using mutational-analysis survey, we show that tyrosine 267 located in the third ITIM motif of the CD300a protein is important for the inhibitory function of CD300a.

Inhibitory NK receptor recognition of HLA-G: regulation by contact residues and by cell specific expression at the fetal-maternal interface.

The non-classical HLA-G protein is distinguished from the classical MHC class I molecules by its expression pattern, low polymorphism and its ability to form complexes on the cell surface. The special role of HLA-G in the maternal-fetal interface has been attributed to its ability to interact with specific receptors found on maternal immune cells. However this interaction is restricted to a limited number of receptors. In this study we elucidate the reason for this phenomenon by comparing the specific contact residues responsible for MHC-KIR interactions. This alignment revealed a marked difference between the HLA-G molecule and other MHC class I molecules. By mutating these residues to the equivalent classical MHC residues, the HLA-G molecule regained an ability of interacting with KIR inhibitory receptors found on NK cells derived either from peripheral blood or from the decidua. Functional NK killing assays further substantiated the binding results. Furthermore, double immunofluorescent staining of placental sections revealed that while the conformed form of HLA-G was expressed in all extravillous trophoblasts, the free heavy chain form of HLA-G was expressed in more distal cells of the column, the invasion front. Overall we suggest that HLA-G protein evolved to interact with only some of the NK inhibitory receptors thus allowing a control of inhibition, while permitting appropriate NK cell cytokine and growth factor production necessary for a viable maternal fetal interface.

Methods to identify and characterize different NK cell receptors and their ligands.

Different cellular immune responses are modulated by the cross talk between activating and inhibitory signaling pathways initiated via different cell surface receptors. Similarly, the killing of NK cells is controlled by multiple activating and inhibitory surface receptors. In humans, the major NK triggering receptors, identified so far, include NKp80, 2B4 NKG2D, and CD16 and the natural cytotoxic receptors (collectively named NCRs) include NKp46, NKp44, and NKp30. The two major families of MHC-specific inhibitory receptors identified in humans are the Ig superfamily (KIR and LIR) and the C-type lectin (CD94/NKG2A) receptor superfamily. The different inhibitory receptors show diverse specificity and discriminate between different class I MHC proteins. Much is known about the function and expression patterns of the different NK cell receptors, but the ligand identity of several of the activating NK cell receptors is yet to be discovered. This chapter introduces several research tools that can be used to uncover the identities of different ligands for NK cell receptors.

Endometrial NK cells are special immature cells that await pregnancy.

NK cells populate the human endometrium before pregnancy. Unlike decidual NK cells that populate the decidua during pregnancy, the NK cells present in the human endometrium, before pregnancy, have not been fully characterized. In this study, we provide a detailed analysis of the origin, phenotype, and function of endometrial NK cells (eNK). We show that eNK cells have a unique receptor repertoire. In particular, they are negative for NKp30 and chemokine receptor expression, which distinguishes them from any other NK subset described so far. We further show that eNK cells lack NK-specific functional phenotype and activity such as cytokine secretion and cytotoxicity, before IL-15 stimulation. Following such stimulation, endometrial NK cells acquire phenotype and function that are similar to those of decidual NK cells. We therefore suggest that eNK cells are inactive cells (before IL-15 activation and in relation to the known NK activity) that are present in the endometrium before conception, waiting for pregnancy.