Emilie Narni-Mancinelli

Tuning of Natural Killer Cell Reactivity by NKp46 and Helios Calibrates T Cell Responses

Natural Killer Controls Cytolytic natural killer (NK) cells participate in both antimicrobial and antitumor immunity. Their responsiveness is tuned through signals received through a variety of inhibitory and activating receptors expressed on their cell surface. Narni-Mancinelli et al. (p. 344) now show that signaling through the activating receptor NKp46 paradoxically keeps NK cell responses in check. NK cells from mice with disrupted NKp46 expression were hyperresponsive to stimulation and better protected against viral infection. NK cell responses, which are part of the early response to infection, may thus need to be carefully tuned to ensure optimal initiation of adaptive immunity and formation of protective long-lived memory cells. The activating receptor NKp46 is important for keeping the responses of natural killer cells in check. Natural killer (NK) cells are lymphocytes involved in antimicrobial and antitumoral immune responses. Using N-ethyl-N-nitrosourea mutagenesis in mice, we identified a mutant with increased resistance to viral infections because of the presence of hyperresponsive NK cells. Whole-genome sequencing and functional analysis revealed a loss-of-function mutation in the Ncr1 gene encoding the activating receptor NKp46. The down-regulation of NK cell activity by NKp46 was associated with the silencing of the Helios transcription factor in NK cells. NKp46 was critical for the subsequent development of antiviral and antibacterial T cell responses, which suggests that the regulation of NK cell function by NKp46 allows for the optimal development of adaptive immune responses. NKp46 blockade enhanced NK cell reactivity in vivo, which could enable the design of immunostimulation strategies in humans.

Tuning the threshold of natural killer cell responses.

Natural killer cells are lymphocytes of the innate immune system that can kill an array of tumor and infected cells and secrete cytokines that participate in the shaping of the adaptive immune response. While it was believed that NK cell effector responses are acquired during maturation and then fixed, it appears that the threshold of NK cell responsiveness is more adaptable than originally thought. We review here how the local context provides several signals that impact on NK cell differentiation, responsiveness and shapes the antiviral and immunoregulatory outcome of NK cell activation.

Complement factor P is a ligand for the natural killer cell–activating receptor NKp46

Cross-talk between innate lymphoid cells and the alternative complement pathway protects against invasive bacterial infections. Inter-innate cooperation The different branches of the immune system work together like a well-oiled machine, but how this coordination occurs is less well understood. Narni-Mancinelli et al. have found one such mechanism—cross-talk between the alternative complement pathway and natural killer (NK) cells and innate lymphoid cells (ILCs). They report that complement factor P (CFP), a positive regulator of the alternative complement pathway, binds NKp46, which is expressed on subsets of NK cells and ILC1 and ILC3. Patients lacking CFP are more susceptible to Neisseria meningitidis infection, and in mice, this CFP protection was dependent on NKp46 and group 1 ILCs. These data suggest that ILCs and the alternative complement pathway cooperate to fight off bacterial infection. Innate lymphoid cells (ILCs) are involved in immune responses to microbes and various stressed cells, such as tumor cells. They include group 1 [such as natural killer (NK) cells and ILC1], group 2, and group 3 ILCs. Besides their capacity to respond to cytokines, ILCs detect their targets through a series of cell surface–activating receptors recognizing microbial and nonmicrobial ligands. The nature of some of these ligands remains unclear, limiting our understanding of ILC biology. We focused on NKp46, which is highly conserved in mammals and expressed by all mature NK cells and subsets of ILC1 and ILC3. We show here that NKp46 binds to a soluble plasma glycoprotein, the complement factor P (CFP; properdin), the only known positive regulator of the alternative complement pathway. Consistent with the selective predisposition of patients lacking CFP to lethal Neisseria meningitidis (Nm) infections, NKp46 and group 1 ILCs bearing this receptor were found to be required for mice to survive Nm infection. Moreover, the beneficial effects of CFP treatment for Nm infection were dependent on NKp46 and group 1 NKp46+ ILCs. Thus, group 1 NKp46+ ILCs interact with the complement pathway, via NKp46, revealing a cross-talk between two partners of innate immunity in the response to an invasive bacterial infection.

NK Cell Genesis: A Trick of the Trail

In this issue of Immunity, Gordon etxa0al. (2012) analyzed the role of the transcription factors T-bet and Eomesodermin in natural killer (NK) cell development, revealing a distinct spatiotemporal requirement of these factors for NK cell maturation.

FHL2 Regulates Natural Killer Cell Development and Activation during Streptococcus pneumoniae Infection

Recent in silico studies suggested that the transcription cofactor LIM-only protein FHL2 is a major transcriptional regulator of mouse natural killer (NK) cells. However, the expression and role of FHL2 in NK cell biology are unknown. Here, we confirm that FHL2 is expressed in both mouse and human NK cells. Using FHL2−/− mice, we found that FHL2 controls NK cell development in the bone marrow and maturation in peripheral organs. To evaluate the importance of FHL2 in NK cell activation, FHL2−/− mice were infected with Streptococcus pneumoniae. FHL2−/− mice are highly susceptible to this infection. The activation of lung NK cells is altered in FHL2−/− mice, leading to decreased IFNγ production and a loss of control of bacterial burden. Collectively, our data reveal that FHL2 is a new transcription cofactor implicated in NK cell development and activation during pulmonary bacterial infection.

Activating and inhibitory receptors expressed on innate lymphoid cells

Innate lymphoid cells (ILCs) are innate immune cells located in lymphoid and non-lymphoid tissues. They are particularly abundant at mucosal and barrier surfaces. Three major ILC subsets are present in humans and mice: group 1 ILCs (comprising natural killer (NK) cells and ILC1s), ILC2s, and ILC3s. ILCs are involved in the maintenance of homeostasis and the regulation of immunity. This review focuses on the extensive array of activating and inhibitory receptors expressed by ILCs for communication with other cell types and their environment in health and disease.

Structural Insights into the Inhibitory Mechanism of an Antibody against B7-H6, a Stress-Induced Cellular Ligand for the Natural Killer Cell Receptor NKp30

Antibodies have been shown to block signaling through cell surface receptors using several mechanisms. The two most common are binding to the ligand-binding site of the receptor and, conversely, binding to the receptor-binding site of the ligand. Here, we investigated the inhibitory mechanism of an antibody (17B1.3) against human B7-H6, a stress-induced cellular ligand for the natural killer (NK) cell receptor NKp30. Binding of this antibody to B7-H6, a transmembrane protein expressed on tumor and other stressed cells, but not on normal cells, prevents NK cell activation via NKp30. We determined the crystal structure of antibody 17B1.3 in complex with the ectodomain of B7-H6 to 2.5Å resolution. Surprisingly, 17B1.3 binds to a site on B7-H6 that is completely distinct from the binding site for NKp30, such that 17B1.3 does not block the NKp30-B7-H6 interaction. We then asked whether 17B1.3 prevents signaling by binding to a putative site for B7-H6 dimerization. However, structure-based mutations designed to disrupt potential B7-H6 dimerization through this site did not diminish NKp30-mediated cell activation. We conclude that the bulky 17B1.3 antibody most likely acts by sterically interfering with close cell-cell contacts at the NK cell-target cell interface that are required for NK cell activation. A similar inhibitory mechanism may apply to other antibodies, including therapeutic antibodies that block signaling through cell surface receptors whose ligands are also cell surface proteins.

Delivering Three Punches to Knockout Intracellular Bacteria

Cytotoxic lymphocytes kill bacteria-infected cells, but the mechanisms at work remain unclear. Walch etxa0al. show that these lymphocytes deliver a toxic molecular trio in a two-step process, penetrating first the infected cells and then delivering bactericidal granzymes into the intracytoplasmic bacteria.

A point mutation in the Ncr1 signal peptide impairs the development of innate lymphoid cell subsets

ABSTRACT NKp46 (CD335) is a surface receptor shared by both human and mouse natural killer (NK) cells and innate lymphoid cells (ILCs) that transduces activating signals necessary to eliminate virus-infected cells and tumors. Here, we describe a spontaneous point mutation of cysteine to arginine (C14R) in the signal peptide of the NKp46 protein in congenic Ly5.1 mice and the newly generated NCRB6C14R strain. Ly5.1C14R NK cells expressed similar levels of Ncr1 mRNA as C57BL/6, but showed impaired surface NKp46 and reduced ability to control melanoma tumors in vivo. Expression of the mutant NKp46C14R in 293T cells showed that NKp46 protein trafficking to the cell surface was compromised. Although Ly5.1C14R mice had normal number of NK cells, they showed an increased number of early maturation stage NK cells. CD49a+ILC1s were also increased but these cells lacked the expression of TRAIL. ILC3s that expressed NKp46 were not detectable and were not apparent when examined by T-bet expression. Thus, the C14R mutation reveals that NKp46 is important for NK cell and ILC differentiation, maturation and function. Significance Innate lymphoid cells (ILCs) play important roles in immune protection. Various subsets of ILCs express the activating receptor NKp46 which is capable of recognizing pathogen derived and tumor ligands and is necessary for immune protection. Here, we describe a spontaneous point mutation in the signal peptide of the NKp46 protein in congenic Ly5.1 mice which are widely used for tracking cells in vivo. This Ncr1 C14R mutation impairs NKp46 surface expression resulting in destabilization of Ncr1 and accumulation of NKp46 in the endoplasmic reticulum. Loss of stable NKp46 expression impaired the maturation of NKp46+ ILCs and altered the expression of TRAIL and T-bet in ILC1 and ILC3, respectively.

Killer ILCs in the Fat

Innate lymphoid cells (ILCs) residing in adipose tissue participate in the pathogenesis of obesity, but theirxa0contribution toward adipose tissue homeostasis in the lean state is unclear. Boulenouar etxa0al. (2017) now report that heterogenous type 1 ILCs in adipose tissues regulate macrophage homeostasis through cytotoxicity.