Segregation of HLA-C from ICAM-1 at NK cell immune synapses is controlled by its cell surface density

Maintaining the Balance: Regulation of NK Cell Activity

Natural Killer (NK) cells, integral components of the innate immune system, play a crucial role in the protection against intracellular threats. Their cytotoxic power requires that activation is tightly controlled, and in this, they take a unique position within the immune system. Rather than depending on the engagement of a single activating receptor, their activation involves a delicate balance between inhibitory and activating signals mediated through an array of surface molecules. Only when this cumulative balance surpasses a specific threshold do NK cells initiate their activity. Remarkably, the activation threshold of NK cells remains robust even when cells express vastly different repertoires of inhibitory and activating receptors. These threshold values seem to be influenced by NK cell interactions with their environment during development and after release from the bone marrow. Understanding how NK cells integrate this intricate pattern of stimuli is an ongoing area of research, particularly relevant for cellular therapies seeking to harness the anti-cancer potential of these cells by modifying surface receptor expression. In this review, we will explore some of the current dogmas regarding NK cell activation and discuss recent literature addressing advances in our understanding of this field.

Actin cytoskeleton remodeling at the cancer cell side of the immunological synapse: good, bad, or both?

Cytotoxic lymphocytes (CLs), specifically cytotoxic T lymphocytes and natural killer cells, are indispensable guardians of the immune system and orchestrate the recognition and elimination of cancer cells. Upon encountering a cancer cell, CLs establish a specialized cellular junction, known as the immunological synapse that stands as a pivotal determinant for effective cell killing. Extensive research has focused on the presynaptic side of the immunological synapse and elucidated the multiple functions of the CL actin cytoskeleton in synapse formation, organization, regulatory signaling, and lytic activity. In contrast, the postsynaptic (cancer cell) counterpart has remained relatively unexplored. Nevertheless, both indirect and direct evidence has begun to illuminate the significant and profound consequences of cytoskeletal changes within cancer cells on the outcome of the lytic immunological synapse. Here, we explore the understudied role of the cancer cell actin cytoskeleton in modulating the immune response within the immunological synapse. We shed light on the intricate interplay between actin dynamics and the evasion mechanisms employed by cancer cells, thus providing potential routes for future research and envisioning therapeutic interventions targeting the postsynaptic side of the immunological synapse in the realm of cancer immunotherapy. This review article highlights the importance of actin dynamics within the immunological synapse between cytotoxic lymphocytes and cancer cells focusing on the less-explored postsynaptic side of the synapse. It presents emerging evidence that actin dynamics in cancer cells can critically influence the outcome of cytotoxic lymphocyte interactions with cancer cells.

Driving natural killer cell-based cancer immunotherapy for cancer treatment: An arduous journey to promising ground

Immunotherapy represents one of the most effective strategies for cancer treatment. Recently, progress has been made in using natural killer (NK) cells for cancer therapy. NK cells can directly kill tumor cells without pre-sensitization and thus show promise in clinical applications, distinct from the use of T cells. Whereas, research and development on NK cell-based immunotherapy is still in its infancy, and enhancing the therapeutic effects of NK cells remains a key problem to be solved. An incompletely understanding of the mechanisms of action of NK cells, immune resistance in the tumor microenvironment, and obstacles associated with the delivery of therapeutic agents in vivo, represent three mountains that need to be scaled. Here, we firstly describe the mechanisms underlying the development, activity, and maturation of NK cells, and the formation of NK‑cell immunological synapses. Secondly, we discuss strategies for NK cell-based immunotherapy strategies, including adoptive transfer of NK cell therapy and treatment with cytokines, monoclonal antibodies, and immune checkpoint inhibitors targeting NK cells. Finally, we review the use of nanotechnology to overcome immune resistance, including enhancing the anti-tumor efficiency of chimeric antigen receptor-NK, cytokines and immunosuppressive-pathways inhibitors, promoting NK cell homing and developing NK cell-based nano-engagers.

Spatially resolved in silico modeling of NKG2D signaling kinetics suggests a key role of NKG2D and Vav1 Co-clustering in generating natural killer cell activation

Natural Killer (NK) cells provide key resistance against viral infections and tumors. A diverse set of activating and inhibitory NK cell receptors (NKRs) interact with cognate ligands presented by target host cells, where integration of dueling signals initiated by the ligand-NKR interactions determines NK cell activation or tolerance. Imaging experiments over decades have shown micron and sub-micron scale spatial clustering of activating and inhibitory NKRs. The mechanistic roles of these clusters in affecting downstream signaling and activation are often unclear. To this end, we developed a predictive in silico framework by combining spatially resolved mechanistic agent based modeling, published TIRF imaging data, and parameter estimation to determine mechanisms by which formation and spatial movements of activating NKG2D microclusters affect early time NKG2D signaling kinetics in a human cell line NKL. We show co-clustering of NKG2D and the guanosine nucleotide exchange factor Vav1 in NKG2D microclusters plays a dominant role over ligand (ULBP3) rebinding in increasing production of phospho-Vav1(pVav1), an activation marker of early NKG2D signaling. The in silico model successfully predicts several scenarios of inhibition of NKG2D signaling and time course of NKG2D spatial clustering over a short (~3 min) interval. Modeling shows the presence of a spatial positive feedback relating formation and centripetal movements of NKG2D microclusters, and pVav1 production offers flexibility towards suppression of activating signals by inhibitory KIR ligands organized in inhomogeneous spatial patterns (e.g., a ring). Our in silico framework marks a major improvement in developing spatiotemporal signaling models with quantitatively estimated model parameters using imaging data.

Natural Killer cells are lymphocytes of our innate immunity and provide important resistance against viral infections and tumors. NK cells scan the local environment with diverse activating and inhibitory NK cell receptors (NKRs) and remain tolerized or lyse target cells expressing cognate ligands to NKRs. NKRs have been found to form micron sized clusters (or microclusters) as they interact with cognate ligands, and mechanisms regarding how the formation and movements of these microclusters influence NK cell signaling and activation, specifically related to activating NKRs, are often unclear. To this end, we develop a predictive spatially resolved early-time NK cell signaling model to study the interplay between membrane-proximal biochemical signaling events and the kinetics of microclusters of activating NKG2D and inhibitory KIR2DL2 receptors. We used published TIRF imaging data to validate our in silico models and estimate model parameters. Predictions from multiple in silico models are tested against a variety of data obtained from published imaging experiments and immunoassays. Our analysis suggests co-clustering of NKG2D and the guanosine nucleotide exchange factor Vav1 in the microclusters plays a major role in enhancing downstream activating signals. The developed framework can be extended to describe spatiotemporal signaling for other activating NKRs including CD16.

The spatial distribution of target cell ligands determines NK cell degranulation

Cell-bound ligands are often viewed as moving passively in response to displacement of their cognate receptors. Verron et al provide an example of the distribution of ligands influencing the functional outcome of receptor stimulation.

NK cells integrate signals over large areas when building immune synapses but require local stimuli for degranulation

Immune synapses are large-scale, transient molecular assemblies that serve as platforms for antigen presentation to B and T cells and for target recognition by cytotoxic T cells and natural killer (NK) cells. The formation of an immune synapse is a tightly regulated, stepwise process in which the cytoskeleton, cell surface receptors, and intracellular signaling proteins rearrange into supramolecular activation clusters (SMACs). We generated artificial immune synapses (AIS) consisting of synthetic and natural ligands for the NK cell-activating receptors LFA-1 and CD16 by microcontact printing the ligands into circular-shaped SMAC structures. Live-cell imaging and analysis of fixed human NK cells in this reductionist system showed that the spatial distribution of activating ligands influenced the formation, stability, and outcome of NK cell synapses. Whereas engagement of LFA-1 alone promoted synapse initiation, combined engagement of LFA-1 and CD16 was required for the formation of mature synapses and degranulation. Organizing LFA-1 and CD16 ligands into donut-shaped AIS resulted in fewer long-lasting, symmetrical synapses compared to dot-shaped AIS. NK cells spreading evenly over either AIS shape exhibited similar arrangements of the lytic machinery. However, degranulation only occurred in regions containing ligands that therefore induced local signaling, suggesting the existence of a late checkpoint for degranulation. Our results demonstrate that the spatial organization of ligands in the synapse can affect its outcome, which could be exploited by target cells as an escape mechanism.

To Kill But Not Be Killed: Controlling the Activity of Mammalian Pore-Forming Proteins

Pore-forming proteins (PFPs) are present in all domains of life, and play an important role in host-pathogen warfare and in the elimination of cancers. They can be employed to deliver specific effectors across membranes, to disrupt membrane integrity interfering with cell homeostasis, and to lyse membranes either destroying intracellular organelles or entire cells. Considering the destructive potential of PFPs, it is perhaps not surprising that mechanisms controlling their activity are remarkably complex, especially in multicellular organisms. Mammalian PFPs discovered to date include the complement membrane attack complex (MAC), perforins, as well as gasdermins. While the primary function of perforin-1 and gasdermins is to eliminate infected or cancerous host cells, perforin-2 and MAC can target pathogens directly. Yet, all mammalian PFPs are in principle capable of generating pores in membranes of healthy host cells which-if uncontrolled-could have dire, and potentially lethal consequences. In this review, we will highlight the strategies employed to protect the host from destruction by endogenous PFPs, while enabling timely and efficient elimination of target cells.

The cancer-natural killer cell immunity cycle

Immunotherapy with checkpoint blockade induces rapid and durable immune control of cancer in some patients and has driven a monumental shift in cancer treatment. Neoantigen-specific CD8⁺ T cells are at the forefront of current immunotherapy strategies, and the majority of drug discovery and clinical trials revolve around further harnessing these immune effectors. Yet the immune system contains a diverse range of antitumour effector cells, and these must function in a coordinated and synergistic manner to overcome the immune-evasion mechanisms used by tumours and achieve complete control with tumour eradication. A key antitumour effector is the natural killer (NK) cells, cytotoxic innate lymphocytes present at high frequency in the circulatory system and identified by their exquisite ability to spontaneously detect and lyse transformed or stressed cells. Emerging data show a role for intratumoural NK cells in driving immunotherapy response and, accordingly, there have been renewed efforts to further elucidate and target the pathways controlling NK cell antitumour function. In this Review, we discuss recent clinical evidence that NK cells are a key immune constituent in the protective antitumour immune response and highlight the major stages of the cancer-NK cell immunity cycle. We also perform a new analysis of publicly available transcriptomic data to provide an overview of the prognostic value of NK cell gene expression in 25 tumour types. Furthermore, we discuss how the role of NK cells evolves with tumour progression, presenting new opportunities to target NK cell function to enhance cancer immunotherapy response rates across a more diverse range of cancers.

Human NK cell receptor KIR2DS4 detects a conserved bacterial epitope presented by HLA-C

Natural killer (NK) cells have an important role in immune defense against viruses and cancer. Activation of human NK cell cytotoxicity toward infected or tumor cells is regulated by killer cell immunoglobulin-like receptors (KIRs) that bind to human leukocyte antigen class I (HLA-I). Combinations of KIR with HLA-I are genetically associated with susceptibility to disease. KIR2DS4, an activating member of the KIR family with poorly defined ligands, is a receptor of unknown function. Here, we show that KIR2DS4 has a strong preference for rare peptides carrying a Trp at position 8 (p8) of 9-mer peptides bound to HLA-C*05:01. The complex of a peptide bound to HLA-C*05:01 with a Trp at p8 was sufficient for activation of primary KIR2DS4+ NK cells, independent of activation by other receptors and of prior NK cell licensing. HLA-C*05:01+ cells that expressed the peptide epitope triggered KIR2DS4+ NK cell degranulation. We show an inverse correlation of the worldwide allele frequency of functional KIR2DS4 with that of HLA-C*05:01, indicative of functional interaction and balancing selection. We found a highly conserved peptide sequence motif for HLA-C*05:01-restricted activation of human KIR2DS4+ NK cells in bacterial recombinase A (RecA). KIR2DS4+ NK cells were stimulated by RecA epitopes from multiple human pathogens, including Helicobacter, Chlamydia, Brucella, and Campylobacter. We predict that over 1,000 bacterial species could activate NK cells through KIR2DS4, and propose that human NK cells also contribute to immune defense against bacteria through recognition of a conserved RecA epitope presented by HLA-C*05:01.

Natural Killer Cells as Allogeneic Effectors in Adoptive Cancer Immunotherapy

Natural killer (NK) cells are attractive within adoptive transfer settings in cancer immunotherapy due to their potential for allogeneic use; their alloreactivity is enhanced under conditions of killer immunoglobulin-like receptor (KIR) mismatch with human leukocyte antigen (HLA) ligands on cancer cells. In addition to this, NK cells are platforms for genetic modification, and proliferate in vivo for a shorter time relative to T cells, limiting off-target activation. Current clinical studies have demonstrated the safety and efficacy of allogeneic NK cell adoptive transfer therapies as a means for treatment of hematologic malignancies and, to a lesser extent, solid tumors. However, challenges associated with sourcing allogeneic NK cells have given rise to controversy over the contribution of NK cells to graft-versus-host disease (GvHD). Specifically, blood-derived NK cell infusions contain contaminating T cells, whose activation with NK-stimulating cytokines has been known to lead to heightened release of proinflammatory cytokines and trigger the onset of GvHD in vivo. NK cells sourced from cell lines and stem cells lack contaminating T cells, but can also lack many phenotypic characteristics of mature NK cells. Here, we discuss the available published evidence for the varying roles of NK cells in GvHD and, more broadly, their use in allogeneic adoptive transfer settings to treat various cancers.

Inhibitory Receptor Crosslinking Quantitatively Dampens Calcium Flux Induced by Activating Receptor Triggering in NK Cells

Natural killer (NK) cell function is regulated by a balance between activating and inhibitory receptors, but the details of this receptor interplay are not extensively understood. We developed a flow cytometry-based assay system in which Ca²⁺ flux downstream of antibody-mediated activating receptor triggering was studied in the presence or absence of inhibitory receptor co-crosslinking. We show that the inhibitory influence on activating receptor-induced Ca²⁺ flux is quantitatively regulated, both on murine and human NK cells. Furthermore, both activating and inhibitory receptors operate in an additive way, suggesting that a fine-tuned balance between activating and inhibitory receptors regulate proximal NK cell signaling. We also demonstrate that murine NK cell expression of H2D^d lowered the capacity of Ly49A to deliver inhibitory signals after antibody crosslinking, suggesting that the cis interaction between H2D^d and Ly49A reduce the signaling capacity of Ly49A in this setting. Finally, we show that priming of NK cells by IL-15 rapidly augments Ca²⁺ flux after activating receptor signaling without attenuating the potential of inhibitory receptors to reduce Ca²⁺ flux. Our data shed new light on NK cell inhibition and raises new questions for further studies.

Natural killer cell cytotoxicity and its regulation by inhibitory receptors

Natural killer (NK) cells express an array of germ-line encoded receptors that are capable of triggering cytotoxicity. NK cells tend to express many members of a given family of signalling molecules. The presence of many activating receptors and many members of a given family of signalling molecules can enable NK cells to detect different kinds of target cells, and to mount different kinds of responses. This contributes also to the robustness of NK cells responses; cytotoxic functions of NK cells often remain unaffected in the absence of selected signalling molecules. NK cells express many MHC-I-specific inhibitory receptors. Signals from MHC-I-specific inhibitory receptors tightly control NK cell cytotoxicity and, paradoxically, maintain NK cells in a state of proper responsiveness. This review provides a brief overview of the events that underlie NK cell activation, and how signals from inhibitory receptors intercept NK cell activation to prevent inappropriate triggering of cytotoxicity.

Selection, tuning, and adaptation in mouse NK cell education

Natural killer (NK) cells recognize transformed cells with an array of germline-encoded inhibitory and activating receptors. Inhibitory Ly49 receptors bind major histocompatibility complex class I (MHC-I) molecules, providing a mechanism by which NK cells maintain self-tolerance yet eliminate cells expressing reduced levels of MHC-I. Additionally, MHC-I molecules are required for NK cell education, a process in which NK cells acquire responsiveness. In this review, we discuss three facets of MHC class I-dependent education of mouse NK cells: skewing of the inhibitory receptor repertoire, induction of functional responsiveness, and tuning in response to changes in MHC-I expression. We discuss prevailing models for education such as licensing and disarming and propose a model for positive selection of 'useful' NK cell subsets. Furthermore, we argue that both repertoire skewing and functional NK cell education may be altered in mature NK cells subject to changes in MHC-I input and suggest that this process may provide increased dynamics to the NK cell system.

NAP-2 Secreted by Human NK Cells Can Stimulate Mesenchymal Stem/Stromal Cell Recruitment

Strategies for improved homing of mesenchymal stem cells (MSCs) to a place of injury are being sought and it has been shown that natural killer (NK) cells can stimulate MSC recruitment. Here, we studied the chemokines behind this recruitment. Assays were performed with bone marrow human MSCs and NK cells freshly isolated from healthy donor buffy coats. Supernatants from MSC-NK cell co-cultures can induce MSC recruitment but not to the same extent as when NK cells are present. Antibody arrays and ELISA assays confirmed that NK cells secrete RANTES (CCL5) and revealed that human NK cells secrete NAP-2 (CXCL7), a chemokine that can induce MSC migration. Inhibition with specific antagonists of CXCR2, a receptor that recognizes NAP-2, abolished NK cell-mediated MSC recruitment. This capacity of NK cells to produce chemokines that stimulate MSC recruitment points toward a role for this immune cell population in regulating tissue repair/regeneration.

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Primary unstimulated human NK cells produce NAP-2 (CXCL7)

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NAP-2 is a chemokine that can promote recruitment of bone marrow MSCs

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Inhibiting the NAP-2 receptor CXCR2 abolishes NK cell-mediated MSC recruitment

Action and Traction: Cytoskeletal Control of Receptor Triggering at the Immunological Synapse

It is well known that F-actin dynamics drive the micron-scale cell shape changes required for migration and immunological synapse (IS) formation. In addition, recent evidence points to a more intimate role for the actin cytoskeleton in promoting T cell activation. Mechanotransduction, the conversion of mechanical input into intracellular biochemical changes, is thought to play a critical role in several aspects of immunoreceptor triggering and downstream signal transduction. Multiple molecules associated with signaling events at the IS have been shown to respond to physical force, including the TCR, costimulatory molecules, adhesion molecules, and several downstream adapters. In at least some cases, it is clear that the relevant forces are exerted by dynamics of the T cell actomyosin cytoskeleton. Interestingly, there is evidence that the cytoskeleton of the antigen-presenting cell also plays an active role in T cell activation, by countering the molecular forces exerted by the T cell at the IS. Since actin polymerization is itself driven by TCR and costimulatory signaling pathways, a complex relationship exists between actin dynamics and receptor activation. This review will focus on recent advances in our understanding of the mechanosensitive aspects of T cell activation, paying specific attention to how F-actin-directed forces applied from both sides of the IS fit into current models of receptor triggering and activation.

The limits of chemosensation vary across dimensions

Many biological processes rely on the ability of cells to measure local ligand concentration. However, such measurements are constrained by noise arising from diffusion and the stochastic nature of receptor–ligand interactions. It is thus critical to understand how accurately, in principle, concentration measurements can be made. Previous theoretical work has mostly investigated this in 3D under the simplifying assumption of an unbounded domain of diffusion, but many biological problems involve 2D concentration measurement in bounded domains, for which diffusion behaves quite differently. Here we present a theory of the precision of chemosensation that covers bounded domains of any dimensionality. We find that the quality of chemosensation in lower dimensions is controlled by domain size, suggesting a general principle applicable to many biological systems. Applying the theory to biological problems in 2D shows that diffusion-limited signalling is an efficient mechanism on time scales consistent with behaviour.

Theoretical studies on chemosensation often invoke a model of three dimensional unbounded diffusion, but many biological problems involve two-dimensional diffusion in a bounded domain. Here Bicknell et al. present a model for chemosensation that covers bounded domains of any dimension, and apply it to biological problems in two dimensions.

Identification of 14-3-3β gene as a novel miR-152 target using a proteome-based approach

Recent studies demonstrated that miR-152 overexpression down-regulates the nonclassical human leukocyte antigen (HLA) class I molecule HLA-G in human tumors thereby contributing to their immune surveillance. Using two-dimensional gel electrophoresis followed by MALDI-TOF mass spectrometry, the protein expression profile of HLA-G(+), miR-152(low) cells, and their miR-152-overexpressing (miR(high)) counterparts was compared leading to the identification of 24 differentially expressed proteins. These were categorized according to their function and localization demonstrating for most of them an important role in the initiation and progression of tumors. The novel miR-152 target 14-3-3 protein β/α/YWHAB (14-3-3β) is down-regulated upon miR-152 overexpression, although its overexpression was often found in tumors of distinct origin. The miR-152-mediated reduction of the 14-3-3β expression was accompanied by an up-regulation of BAX protein expression resulting in a pro-apoptotic phenotype. In contrast, the reconstitution of 14-3-3β expression in miR-152(high) cells increased the expression of the anti-apoptotic BCL2 gene, enhances the proliferative activity in the presence of the cytostatic drug paclitaxel, and causes resistance to apoptosis induced by this drug. By correlating clinical microarray data with the patients' outcome, a link between 14-3-3β and HLA-G expression was found, which could be associated with poor prognosis and overall survival of patients with tumors. Because miR-152 controls both the expression of 14-3-3β and HLA-G, it exerts a dual role in tumor cells by both altering the immunogenicity and the tumorigenicity.

Dynamics of natural killer cell receptor revealed by quantitative analysis of photoswitchable protein

Natural Killer (NK) cell activation is dynamically regulated by numerous activating and inhibitory surface receptors that accumulate at the immune synapse. Quantitative analysis of receptor dynamics has been limited by methodologies that rely on indirect measurements such as fluorescence recovery after photobleaching. Here, we report an apparently novel approach to study how proteins traffic to and from the immune synapse using NK cell receptors tagged with the photoswitchable fluorescent protein tdEosFP, which can be irreversibly photoswitched from a green to red fluorescent state by ultraviolet light. Thus, after a localized switching event, the movement of the photoswitched molecules can be temporally and spatially resolved by monitoring fluorescence in two regions of interest. By comparing images with mathematical models, we evaluated the diffusion coefficient of the receptor KIR2DL1 (0.23 ± 0.06 μm(2) s(-1)) and assessed how synapse formation affects receptor dynamics. Our data conclude that the inhibitory NK cell receptor KIR2DL1 is continually trafficked into the synapse, and remains surprisingly stable there. Unexpectedly, however, in NK cells forming synapses with multiple target cells simultaneously, KIR2DL1 at one synapse can relocate to another synapse. Thus, our results reveal a previously undetected intersynaptic exchange of protein.

Blocking oncogenic RAS enhances tumour cell surface MHC class I expression but does not alter susceptibility to cytotoxic lymphocytes

Mutations in the RAS family of oncogenes are highly prevalent in human cancer and, amongst its manifold effects, oncogenic RAS impairs the expression of components of the antigen presentation pathway. This allows evasion of cytotoxic T lymphocytes (CTL). CTL and natural killer (NK) cells are reciprocally regulated by MHC class I molecules and any gain in CTL recognition obtained by therapeutic inactivation of oncogenic RAS may be offset by reduced NK cell activation. We have investigated the consequences of targeted inactivation of oncogenic RAS on the recognition by both CTL and NK cells. Inactivation of oncogenic RAS, either by genetic deletion or inactivation with an inducible intracellular domain antibody (iDAb), increased MHC class I expression in human colorectal cell lines. The common RAS mutations, at codons 12, 13 and 61, all inhibited antigen presentation. Although MHC class I modulates the activity of both CTL and NK cells, the enhanced MHC class I expression resulting from inactivation of mutant KRAS did not significantly affect the in vitro recognition of these cell lines by either class of cytotoxic lymphocyte. These results show that oncogenic RAS and its downstream signalling pathways modulate the antigen presentation pathway and that this inhibition is reversible. However, the magnitude of these effects was not sufficient to alter the in vitro recognition of tumour cell lines by either CTL or NK cells.

Superresolution microscopy reveals nanometer-scale reorganization of inhibitory natural killer cell receptors upon activation of NKG2D

Natural killer (NK) cell responses are regulated by a dynamic equilibrium between activating and inhibitory receptor signals at the immune synapse (or interface) with target cells. Although the organization of receptors at the immune synapse is important for appropriate integration of these signals, there is little understanding of this in detail, because research has been hampered by the limited resolution of light microscopy. Through the use of superresolution single-molecule fluorescence microscopy to reveal the organization of the NK cell surface at the single-protein level, we report that the inhibitory receptor KIR2DL1 is organized in nanometer-scale clusters at the surface of human resting NK cells. Nanoclusters of KIR2DL1 became smaller and denser upon engagement of the activating receptor NKG2D, establishing an unexpected crosstalk between activating receptor signals and the positioning of inhibitory receptors. These rearrangements in the nanoscale organization of surface NK cell receptors were dependent on the actin cytoskeleton. Together, these data establish that NK cell activation involves a nanometer-scale reorganization of surface receptors, which in turn affects models for signal integration and thresholds that control NK cell effector functions and NK cell development.

Building tolerance by dismantling synapses: inhibitory receptor signaling in natural killer cells

Cell surface receptors bearing immunotyrosine-based inhibitory motifs (ITIMs) maintain natural killer (NK) cell tolerance to normal host tissues. These receptors are difficult to analyze mechanistically because they block activating responses in a rapid and comprehensive manner. The advent of high-resolution single cell imaging techniques has enabled investigators to explore the cell biological basis of the inhibitory response. Recent studies using these approaches indicate that ITIM-containing receptors function at least in part by structurally undermining the immunological synapse between the NK cell and its target. In this review, we discuss these new advances and how they might relate to what is known about the biochemistry of inhibitory signaling in NK cells and other cell types.

A peptide antagonist disrupts NK cell inhibitory synapse formation

Productive engagement of MHC class I by inhibitory NK cell receptors depends on the peptide bound by the MHC class I molecule. Peptide:MHC complexes that bind weakly to killer cell Ig-like receptors (KIRs) can antagonize the inhibition mediated by high-affinity peptide:MHC complexes and cause NK cell activation. We show that low-affinity peptide:MHC complexes stall inhibitory signaling at the step of Src homology protein tyrosine phosphatase 1 recruitment and do not go on to form the KIR microclusters induced by high-affinity peptide:MHC, which are associated with Vav dephosphorylation and downstream signaling. Furthermore, the low-affinity peptide:MHC complexes prevented the formation of KIR microclusters by high-affinity peptide:MHC. Thus, peptide antagonism of NK cells is an active phenomenon of inhibitory synapse disruption.

Human NK cell lytic granules and regulation of their exocytosis

Natural killer (NK) cells form a subset of lymphocytes that play a key role in immuno-surveillance and host defense against cancer and viral infections. They recognize stressed cells through a variety of germline-encoded activating cell surface receptors and utilize their cytotoxic ability to eliminate abnormal cells. Killing of target cells is a complex, multi-stage process that concludes in the directed secretion of lytic granules, containing perforin and granzymes, at the immunological synapse. Upon delivery to a target cell, perforin mediates generation of pores in membranes of target cells, allowing granzymes to access target cell cytoplasm and induce apoptosis. Therefore, lytic granules of NK cells are indispensable for normal NK cell cytolytic function. Indeed, defects in lytic granule secretion lead or are related to serious and often fatal diseases, such as familial hemophagocytic lymphohistiocytosis (FHL) type 2–5 or Griscelli syndrome type 2. A number of reports highlight the role of several proteins involved in lytic granule release and NK cell-mediated killing of tumor cells. This review focuses on lytic granules of human NK cells and the advancements in understanding the mechanisms controlling their exocytosis.

Illuminating the dynamics of signal integration in Natural Killer cells

Natural Killer (NK) cell responses are shaped by the integration of signals transduced from multiple activating and inhibitory receptors at their surface. Biochemical and genetic approaches have identified most of the key proteins involved in signal integration but a major challenge remains in understanding how the spatial and temporal dynamics of their interactions lead to NK cells responding appropriately when encountering ligands on target cells. Well over a decade of research using fluorescence microscopy has revealed much about the architecture of the NK cell immune synapse - the structured interface between NK cells and target cells - and how it varies when inhibition or activation is the outcome of signal integration. However, key questions - such as the proximity of individual activating and inhibitory receptors - have remained unanswered because the resolution of optical microscopy has been insufficient, being limited by diffraction. Recent developments in fluorescence microscopy have broken this limit, seeding new opportunities for studying the nanometer-scale organization of the NK cell immune synapse. Here, we discuss how these new technologies, super-resolution imaging and other novel light-based methods, can illuminate our understanding of NK cell biology.

The adaptor protein Crk controls activation and inhibition of natural killer cells

Natural killer (NK) cell inhibitory receptors recruit tyrosine phosphatases to prevent activation, induce phosphorylation and dissociation of the small adaptor Crk from cytoskeleton scaffold complexes, and maintain NK cells in a state of responsiveness to subsequent activation events. How Crk contributes to inhibition is unknown. We imaged primary NK cells over lipid bilayers carrying IgG1 Fc to stimulate CD16 and human leukocyte antigen (HLA)-E to inhibit through receptor CD94-NKG2A. HLA-E alone induced Crk phosphorylation in NKG2A(+) NK cells. At activating synapses with Fc alone, Crk was required for the movement of Fc microclusters and their ability to trigger activation signals. At inhibitory synapses, HLA-E promoted central accumulation of both Fc and phosphorylated Crk and blocked the Fc-induced buildup of F-actin. We propose a unified model for inhibitory receptor function: Crk phosphorylation prevents essential Crk-dependent activation signals and blocks F-actin network formation, thereby reducing constraints on subsequent engagement of activation receptors.

Mechanisms for size-dependent protein segregation at immune synapses assessed with molecular rulers

Immunological synapses are specialized intercellular contacts formed by several types of immune cells in contact with target cells or antigen-presenting cells. A late-stage immune synapse is commonly a bulls-eye pattern of immune cell receptor-ligand pairs surrounded by integrin complexes. Based on crystal structures, the intermembrane distance would be ∼15 nm for many immune cell receptor-ligand pairs, but ∼40 nm for integrin-ligand pairs. Close proximity of these two classes of intermembrane bonds would require significant membrane bending and such proteins can segregate according to their size, which may be key for receptor triggering. However, tools available to evaluate the intermembrane organization of the synapse are limited. Here, we present what we believe to be a novel approach to test the importance of size in the intercellular organization of proteins, using live-cell microscopy of a size-series of fluorescently-labeled molecules and quantum dots to act as molecular rulers. Small particles readily colocalized at the synapse with MHC class I bound to its cognate natural killer cell receptor, whereas particles larger than 15 nm were increasingly segregated from this interaction. Combined with modeling of the partitioning of the particles by scaled-particle adsorption theory, these molecular rulers show how membrane-bending elasticity can drive size-dependent exclusion of proteins within immune synapses.

Human NK cells differ more in their KIR2DL1-dependent thresholds for HLA-Cw6-mediated inhibition than in their maximal killing capacity

In this study we have addressed the question of how activation and inhibition of human NK cells is regulated by the expression level of MHC class I protein on target cells. Using target cell transfectants sorted to stably express different levels of the MHC class I protein HLA-Cw6, we show that induction of degranulation and that of IFN-γ secretion are not correlated. In contrast, the inhibition of these two processes by MHC class-I occurs at the same level of class I MHC protein. Primary human NK cell clones were found to differ in the amount of target MHC class I protein required for their inhibition, rather than in their maximum killing capacity. Importantly, we show that KIR2DL1 expression determines the thresholds (in terms of MHC I protein levels) required for NK cell inhibition, while the expression of other receptors such as LIR1 is less important. Furthermore, using mathematical models to explore the dynamics of target cell killing, we found that the observed delay in target cell killing is exhibited by a model in which NK cells require some activation or priming, such that each cell can lyse a target cell only after being activated by a first encounter with the same or a different target cell, but not by models which lack this feature.

Boltzmann energy-based image analysis demonstrates that extracellular domain size differences explain protein segregation at immune synapses

Immune synapses formed by T and NK cells both show segregation of the integrin ICAM1 from other proteins such as CD2 (T cell) or KIR (NK cell). However, the mechanism by which these proteins segregate remains unclear; one key hypothesis is a redistribution based on protein size. Simulations of this mechanism qualitatively reproduce observed segregation patterns, but only in certain parameter regimes. Verifying that these parameter constraints in fact hold has not been possible to date, this requiring a quantitative coupling of theory to experimental data. Here, we address this challenge, developing a new methodology for analysing and quantifying image data and its integration with biophysical models. Specifically we fit a binding kinetics model to 2 colour fluorescence data for cytoskeleton independent synapses (2 and 3D) and test whether the observed inverse correlation between fluorophores conforms to size dependent exclusion, and further, whether patterned states are predicted when model parameters are estimated on individual synapses. All synapses analysed satisfy these conditions demonstrating that the mechanisms of protein redistribution have identifiable signatures in their spatial patterns. We conclude that energy processes implicit in protein size based segregation can drive the patternation observed in individual synapses, at least for the specific examples tested, such that no additional processes need to be invoked. This implies that biophysical processes within the membrane interface have a crucial impact on cell:cell communication and cell signalling, governing protein interactions and protein aggregation.

Simulations of the NK cell immune synapse reveal that activation thresholds can be established by inhibitory receptors acting locally

NK cell activation is regulated by a balance between activating and inhibitory signals. To address the question of how these signals are spatially integrated, we created a computer simulation of activating and inhibitory NK cell immunological synapse (NKIS) assembly, implementing either a "quantity-based" inhibition model or a "distance-based" inhibition model. The simulations mimicked the observed molecule distributions in inhibitory and activating NKIS and yielded several new insights. First, the total signal is highly influenced by activating complex dissociation rates but not by adhesion and inhibitory complex dissociation rates. Second, concerted motion of receptors in clusters significantly accelerates NKIS maturation. Third, when the potential of a cis interaction between Ly49 receptors and MHC class I on murine NK cells was added to the model, the integrated signal as a function of receptor and ligand numbers was only slightly increased, at least up to the level of 50% cis-bound Ly49 receptors reached in the model. Fourth, and perhaps most importantly, the integrated signal behavior obtained when using the distance-based inhibition signal model was closer to the experimentally observed behavior, with an inhibition radius of the order 3-10 molecules. Microscopy to visualize Vav activation in NK cells on micropatterned surfaces of activating and inhibitory strips revealed that Vav is only locally activated where activating receptors are ligated within a single NK cell contact. Taken together, these data are consistent with a model in which inhibitory receptors act locally; that is, that every bound inhibitory receptor acts on activating receptors within a certain radius around it.

The role of HLA-G in immunity and hematopoiesis

The non-classical HLA class I molecule HLA-G was initially shown to play a major role in feto-maternal tolerance. Since this discovery, it has been established that HLA-G is a tolerogenic molecule which participates to the control of the immune response. In this review, we summarize the recent advances on (1) the multiple structures of HLA-G, which are closely associated with their role in the inhibition of NK cell cytotoxicity, (2) the factors that regulate the expression of HLA-G and its receptors, (3) the mechanism of action of HLA-G at the immunological synapse and through trogocytosis, and (4) the generation of suppressive cells through HLA-G. Moreover, we also review recent findings on the non-immunological functions of HLA-G in erythropoiesis and angiogenesis.

A human NK cell activation/inhibition threshold allows small changes in the target cell surface phenotype to dramatically alter susceptibility to NK cells

NK cell activation is negatively regulated by the expression of target cell MHC class I molecules. We show that this relationship is nonlinear due to an NK cell activation/inhibition threshold. Ewing's sarcoma family tumor cell monolayers, which were highly susceptible to NK cells in vitro, developed a highly resistant phenotype when cultured as three-dimensional multicellular tumor spheroid structures. This suggested that tumor architecture is likely to influence the susceptibility to NK cells in vivo. Resistance of the multicellular tumor spheroid was associated with the increased expression of MHC class I molecules and greatly reduced NK cell activation, implying that a threshold of NK cell activation/inhibition had been crossed. Reducing MHC class I expression on Ewing's sarcoma family tumor monolayers did not alter their susceptibility to NK cells, whereas increased expression of MHC class I rendered them resistant and allowed the threshold point to be identified. This threshold, as defined by MHC class I expression, was predictive of the number of NK-resistant target cells within a population. A threshold permits modest changes in the target cell surface phenotype to profoundly alter the susceptibility to NK cells. Whereas this allows for the efficient detection of target cells, it also provides a route for pathogens and tumors to evade NK cell attack.

Imaging immune surveillance of individual natural killer cells confined in microwell arrays

New markers are constantly emerging that identify smaller and smaller subpopulations of immune cells. However, there is a growing awareness that even within very small populations, there is a marked functional heterogeneity and that measurements at the population level only gives an average estimate of the behaviour of that pool of cells. New techniques to analyze single immune cells over time are needed to overcome this limitation. For that purpose, we have designed and evaluated microwell array systems made from two materials, polydimethylsiloxane (PDMS) and silicon, for high-resolution imaging of individual natural killer (NK) cell responses. Both materials were suitable for short-term studies (<4 hours) but only silicon wells allowed long-term studies (several days). Time-lapse imaging of NK cell cytotoxicity in these microwell arrays revealed that roughly 30% of the target cells died much more rapidly than the rest upon NK cell encounter. This unexpected heterogeneity may reflect either separate mechanisms of killing or different killing efficiency by individual NK cells. Furthermore, we show that high-resolution imaging of inhibitory synapse formation, defined by clustering of MHC class I at the interface between NK and target cells, is possible in these microwells. We conclude that live cell imaging of NK-target cell interactions in multi-well microstructures are possible. The technique enables novel types of assays and allow data collection at a level of resolution not previously obtained. Furthermore, due to the large number of wells that can be simultaneously imaged, new statistical information is obtained that will lead to a better understanding of the function and regulation of the immune system at the single cell level.

Matched sizes of activating and inhibitory receptor/ligand pairs are required for optimal signal integration by human natural killer cells

It has been suggested that receptor-ligand complexes segregate or co-localise within immune synapses according to their size, and this is important for receptor signaling. Here, we set out to test the importance of receptor-ligand complex dimensions for immune surveillance of target cells by human Natural Killer (NK) cells. NK cell activation is regulated by integrating signals from activating receptors, such as NKG2D, and inhibitory receptors, such as KIR2DL1. Elongating the NKG2D ligand MICA reduced its ability to trigger NK cell activation. Conversely, elongation of KIR2DL1 ligand HLA-C reduced its ability to inhibit NK cells. Whereas normal-sized HLA-C was most effective at inhibiting activation by normal-length MICA, only elongated HLA-C could inhibit activation by elongated MICA. Moreover, HLA-C and MICA that were matched in size co-localised, whereas HLA-C and MICA that were different in size were segregated. These results demonstrate that receptor-ligand dimensions are important in NK cell recognition, and suggest that optimal integration of activating and inhibitory receptor signals requires the receptor-ligand complexes to have similar dimensions.

Natural killer cell tolerance persists despite significant reduction of self MHC class I on normal target cells in mice

BACKGROUND

A major group of murine inhibitory receptors on Natural Killer (NK) cells belong to the Ly49 receptor family and recognize MHC class I molecules. Infected or transformed target cells frequently downmodulate MHC class I molecules and can thus avoid CD8(+) T cell attack, but may at the same time develop NK cell sensitivity, due to failure to express inhibitory ligands for Ly49 receptors. The extent of MHC class I downregulation needed on normal cells to trigger NK cell effector functions is not known.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we show that cells expressing MHC class I to levels well below half of the host level are tolerated in an in vivo assay in mice. Hemizygous expression (expression from only one allele) of MHC class I was sufficient to induce Ly49 receptor downmodulation on NK cells to a similar degree as homozygous expression, despite a strongly reduced cell surface level of MHC class I. Co-expression of weaker MHC class I ligands in the host did not have any further effect on the degree of Ly49 downmodulation. Furthermore, a single MHC class I allele could downmodulate up to three Ly49 receptors on individual NK cells. Only when NK cells simultaneously expressed several Ly49 receptors and hemizygous MHC class I levels, a putative threshold for Ly49 downmodulation was reached.

CONCLUSION

Collectively, our findings suggest that in interactions between NK cells and normal untransformed cells, MHC class I molecules are in most cases expressed in excess compared to what is functionally needed to ensure self tolerance and to induce maximal Ly49 downmodulation. We speculate that the reason for this is to maintain a safety margin for otherwise normal, autologous cells over a range of MHC class I expression levels, in order to ensure robustness in NK cell tolerance.

Tethering of intercellular adhesion molecule on target cells is required for LFA-1-dependent NK cell adhesion and granule polarization

Alpha(L)beta(2) integrin (LFA-1) has an important role in the formation of T cell and NK cell cytotoxic immunological synapses and in target cell killing. Binding of LFA-1 to ICAM on target cells promotes not only adhesion but also polarization of cytolytic granules in NK cells. In this study, we tested whether LFA-1-dependent NK cell responses are regulated by the distribution and mobility of ICAM at the surface of target cells. We show that depolymerization of F-actin in NK-sensitive target cells abrogated LFA-1-dependent conjugate formation and granule polarization in primary NK cells. Degranulation, which is not controlled by LFA-1, was not impaired. Fluorescence recovery after photobleaching experiments and particle tracking by total internal reflection fluorescence microscopy revealed that ICAM-1 and ICAM-2 were distributed in largely immobile clusters. ICAM clusters were maintained and became highly mobile after actin depolymerization. Moreover, reducing ICAM-2 mobility on an NK-resistant target cell through expression of ezrin, an adaptor molecule that tethers proteins to the actin cytoskeleton, enhanced LFA-1-dependent adhesion and granule polarization. Finally, although NK cells kept moving over freely diffusible ICAM-1 on a lipid bilayer, they bound and spread over solid-phase ICAM-1. We conclude that tethering, rather than clustering of ICAM, promotes proper signaling by LFA-1 in NK cells. Our findings suggest that the lateral diffusion of integrin ligands on cells may be an important determinant of susceptibility to lysis by cytotoxic lymphocytes.

Cytotoxic immunological synapses

One of the most fundamental activities of the adaptive immune system is to kill infected cells and tumor cells. Two distinct pathways mediate this process, both of which are facilitated by a cytotoxic immunological synapse. While traditionally thought of as innate immune cells, natural killer (NK) cells are now appreciated to have the capacity for long-term adaptation to chemical and viral insults. These cells integrate multiple positive and negative signals through NK cell cytotoxic or inhibitory synapses. The traditional CD8(+)alphabeta T-cell receptor-positive cells are among the best models for the concept of an immunological synapse, in which vectoral signaling is linked to directed secretion in a stable interface to induce apoptotic cell death in an infected cell. Large-scale molecular organization in synapses generated a number of hypotheses. Studies in the past 5 years have started to provide clear answers regarding the validity of these models. In vivo imaging approaches have provided some hints as to the physiologic relevance of these processes with great promise for the future. This review provides an overview of work on cytotoxic immunological synapses and suggests pathways forward in applying this information to the development of therapeutic agents.

Spatial organization and signal transduction at intercellular junctions

The coordinated organization of cell membrane receptors into diverse micrometre-scale spatial patterns is emerging as an important theme of intercellular signalling, as exemplified by immunological synapses. Key characteristics of these patterns are that they transcend direct protein-protein interactions, emerge transiently and modulate signal transduction. Such cooperativity over multiple length scales presents new and intriguing challenges for the study and ultimate understanding of cellular signalling. As a result, new experimental strategies have emerged to manipulate the spatial organization of molecules inside living cells. The resulting spatial mutations yield insights into the interweaving of the spatial, mechanical and chemical aspects of intercellular signalling.

Quantitative measurement of F-actin accumulation at the NK cell immunological synapse

NK cells are lymphocytes of the innate immune system that can kill target cells after activation signal-induced directional secretion of lytic granule contents. This process depends upon F-actin polymerization at the NK cell immunological synapse (NKIS), which is the dynamic organization of molecules at the interface between the NK cell and target cell. Although F-actin accumulation at the NKIS is easily visualized, the ability to quantify F-actin at the NKIS is required to understand how F-actin reorganization and accumulation enable NK cell function. Here, we demonstrate several novel algorithms for measuring the content of F-actin accumulated at the NKIS with special emphasis upon actin contributed by the NK cell. These algorithms do not rely upon overexpressing fluorescent proteins or preincubating cells with vital fluorescent dyes. Using models of the activating and inhibitory NKIS as well as NK cells expressing fluorescent protein--cell surface receptor fusion proteins, these algorithms were tested and were used to quantitatively demonstrate that F-actin accumulates at the activating, but not at the inhibitory NKIS. With these approaches, we have also established mathematical formulas that should prove valuable in the comprehensive quantitative evaluation of the NKIS and be more broadly applicable in the measurement of the accumulation of any fluorophore at an intercellular junction.

Inhibitory and regulatory immune synapses

Cell contact-dependent inhibition and regulation of immune responses play an essential role in balancing the need for rapid and efficient responses to a wide variety of pathological challenges, while at the same time maintaining self-tolerance. Much attention has been given to immune synapses that lead to the activation of, for example, cell-mediated cytotoxicity, and here we compare the supramolecular dynamics of synapses that lead to inhibition or regulatory functions. We focus on natural killer cells where such different synapses have been best studied. An emergent principle is that inhibition or regulatory responses are commonly achieved by selective recruitment of signalling proteins to the synapse and exclusion of membrane-proximal intracellular proteins needed for activation. We also discuss evidence that an inhibitory synapse triggers or maintains effector cells in a migratory configuration, which serves to break the synapse before the steps needed for effector cell activation can be completed. This model implies that the concept of kinetic-proofreading, previously used to describe activation of individual T-cell receptors, can also apply in determining the outcome of intercellular conjugation.

Membrane nanotubes facilitate long-distance interactions between natural killer cells and target cells

Membrane nanotubes are membranous tethers that physically link cell bodies over long distances. Here, we present evidence that nanotubes allow human natural killer (NK) cells to interact functionally with target cells over long distances. Nanotubes were formed when NK cells contacted target cells and moved apart. The frequency of nanotube formation was dependent on the number of receptor/ligand interactions and increased on NK cell activation. Most importantly, NK cell nanotubes contained a submicron scale junction where proteins accumulated, including DAP10, the signaling adaptor that associates with the activating receptor NKG2D, and MHC class I chain-related protein A (MICA), a cognate ligand for NKG2D, as occurs at close intercellular synapses between NK cells and target cells. Quantitative live-cell fluorescence imaging suggested that MICA accumulated at small nanotube synapses in sufficient numbers to trigger cell activation. In addition, tyrosine-phosphorylated proteins and Vav-1 accumulated at such junctions. Functionally, nanotubes could aid the lysis of distant target cells either directly or by moving target cells along the nanotube path into close contact for lysis via a conventional immune synapse. Target cells moving along the nanotube path were commonly polarized such that their uropods faced the direction of movement. This is the opposite polarization than for normal cell migration, implying that nanotubes can specifically drive target cell movement. Finally, target cells that remained connected to an NK cell by a nanotube were frequently lysed, whereas removing the nanotube using a micromanipulator reduced lysis of these target cells.

B-lymphoma cells escape rituximab-triggered elimination by NK cells through increased HLA class I expression

OBJECTIVE

Antibody-dependent cellular cytotoxicity (ADCC) by natural killer (NK) cells is a major effector mechanism of the monoclonal anti-CD20 antibody rituximab in eliminating B-cell lymphomas. Resistance to this treatment occurs, although CD20 antigen is expressed on the tumor cells.

MATERIALS AND METHODS

A model of ADCC was established by stimulating human bulk NK cells and inhibitory killer immunoglobulin receptor (KIR)-defined NK cells from human leukocyte antigen (HLA)-typed donors. NK-cell activation was triggered via stimulation of the Fc receptor with immunoglobulin G aggregates, rituximab-labeled HLA-defined CD20-positive B-lymphoblast cell lines or CD20-positive B-lymphoma cell lines. The effect of KIR ligation by anti-KIR antibodies and HLA, the HLA expression density and rituximab concentrations on the efficacy of ADCC were analyzed in granzyme B ELISPOT measuring NK-cell activation and fluorescein-activated cell sorting cytotoxicity assay.

RESULTS

HLA, but not CD20 expression density correlated with NK-cell activity against rituximab-labeled targets. ADCC was increased or decreased following HLA shielding or KIR activation by anti-KIR antibodies, respectively. Herein we show that rituximab-induced ADCC is attenuated upon ligation of KIR by HLA molecules expressed on human B-lymphoma target cells. Moreover, anti-KIR antibodies do not only block KIR/HLA interactions, but display agonistic effects at the KIR, which has to be considered for therapeutical applications.

CONCLUSION

KIR activation and HLA expression density are critical determinants for the efficacy of rituximab treatment. An explanation for the failure of rituximab treatment may be the protection of the tumor cells from ADCC by inhibiting NK-cell function with their surface HLA.

Natural killer cell signal integration balances synapse symmetry and migration

Imaging immune surveillance by natural killer (NK) cells has revealed that integration of activating and inhibitory signals determines whether or not NK cells stop to kill the target cell or retain a migratory configuration.

Natural killer (NK) cells discern the health of other cells by recognising the balance of activating and inhibitory ligands expressed by each target cell. However, how the integration of activating and inhibitory signals relates to formation of the NK cell immune synapse remains a central question in our understanding of NK cell recognition. Here we report that ligation of LFA-1 on NK cells induced asymmetrical cell spreading and migration. In contrast, ligation of the activating receptor NKG2D induced symmetrical spreading of ruffled lamellipodia encompassing a dynamic ring of f-actin, concurrent with polarization towards a target cell and a “stop” signal. Ligation of both LFA-1 and NKG2D together resulted in symmetrical spreading but co-ligation of inhibitory receptors reverted NK cells to an asymmetrical migratory configuration leading to inhibitory synapses being smaller and more rapidly disassembled. Using micropatterned activating and inhibitory ligands, signals were found to be continuously and locally integrated during spreading. Together, these data demonstrate that NK cells spread to form large, stable, symmetrical synapses if activating signals dominate, whereas asymmetrical migratory “kinapses” are favoured if inhibitory signals dominate. This clarifies how the integration of activating and inhibitory receptor signals is translated to an appropriate NK cell response.

Immune cells survey their local environment and an immunological response can be activated when an appropriate target cell or antigen-presenting cell is recognised by key cell surface molecules. Just how the multitude of protein–protein interactions work to regulate this decision is an ongoing question. Imaging technology has provided key insights, demonstrating that immune cell activation is often accompanied by the segregation of proteins at immune synapses. Natural killer (NK) cells are lymphocytes that can recognise and kill virally infected or tumour-transformed cells via the formation of a synapse that facilitates secretion of cytotoxic granules directed at the target cells. Key to understanding target cell recognition by NK cells is to establish how the balance of activating and inhibitory signals at the synapse leads to an appropriate response, e.g., to kill or spare a target cell. We found that when activating ligands are dominant on a target cell, NK cells stop migrating and spread lamellipodia across the target cell to form a large symmetrical synapse. If inhibitory signals dominate, the symmetry of the NK cell spreading response is broken and the stop signal is reversed, which reduces the time spent in contact with the target cell. Thus, NK cell activating and inhibitory signals regulate NK cell synapse symmetry and migration to determine whether an NK cell will kill or move on.

Killer Ig-like receptor (KIR) genotype predicts the capacity of human KIR-positive CD56dim NK cells to respond to pathogen-associated signals

IFN-gamma emanating from NK cells is an important component of innate defense against infection. In this study, we demonstrate that, following in vitro stimulation of human peripheral blood NK cells with a variety of microbial ligands, CD56(dim) as well as CD56(bright) NK cells contribute to the overall NK cell IFN-gamma response with, for most cell donors, IFN-gamma(+) CD56(dim) NK cells outnumbering IFN-gamma(+) CD56(bright) NK cells. We also observe that the magnitude of the human NK IFN-gamma response to microbial ligands varies between individuals; that the antimicrobial response of CD56(bright), but not CD56(dim), NK cells is highly correlated with that of myeloid accessory cells; and that the ratio of IFN-gamma(+) CD56(dim) to IFN-gamma(+) CD56(bright) NK cells following microbial stimulation differs between individuals but remains constant for a given donor over time. Furthermore, ratios of IFN-gamma(+) CD56(dim) to IFN-gamma(+) CD56(bright) NK cells for different microbial stimuli are highly correlated and the relative response of CD56(dim) and CD56(bright) NK cells is highly significantly associated with killer Ig-like receptor (KIR) genotype. These data reveal an influence of KIR genotype, possibly mediated via NK cell education, on the ability of NK cells to respond to nonviral infections and have implications for genetic regulation of susceptibility to infection in humans.

Role of initial protein phosphorylation events and localized release-activated calcium influx in B cell antigen receptor signaling

An increase in intracellular calcium concentration is one of the major initial steps in B cell activation following antigen receptor (BCR) ligation. We show herein that in C57BL/6 murine B lymphocytes and in model cell lines, BCR-mediated calcium ion (Ca(2+)) influx occurs via highly selective Ca(2+) release-activated channels, and stromal interaction molecule 1 (STIM1) plays an important role in this pathway. We also demonstrate the temporal relation between Ca(2+)-dependent signaling events and formation of the immune synapse. Our data indicate that cytoplasmic Ca(2+) levels in areas adjacent to the immune synapse differ from those in the rest of the cytoplasm. Finally, a comparison of phosphorylation patterns of BCR-triggered signaling proteins in the presence or absence of Ca(2+) revealed the unanticipated finding that initial BCR-triggered, Ca(2+)-dependent tyrosine phosphorylation events involve predominantly Ca(2+) released from intracellular stores and that influx-derived Ca(2+) is not essential. This suggests a different role for this phase of Ca(2+) influx.

Negative signaling by inhibitory receptors: the NK cell paradigm

Receptors carrying immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their cytoplasmic tail control a vast array of cellular responses, ranging from autoimmunity, allergy, phagocytosis of red blood cells, graft versus host disease, to even neuronal plasticity in the brain. The inhibitory function of many receptors has been deduced on the basis of cytoplasmic ITIM sequences. Tight regulation of natural killer (NK) cell cytotoxicity and cytokine production by inhibitory receptors specific for major histocompatibility complex class I molecules has served as a model system to study the negative signaling pathway triggered by an ITIM-containing receptor in the physiological context of NK-target cell interactions. Advances in our understanding of the molecular details of inhibitory signaling in NK cells have provided a conceptual framework to address how ITIM-mediated regulation controls cellular reactivity in diverse cell types.

Role of the inhibitory KIR ligand HLA-Bw4 and HLA-C expression levels in the recognition of leukemic cells by Natural Killer cells

Transplantation of acute myeloid leukemia (AML) patients with grafts from related haploidentical donors has been shown to result in a potent graft-versus-leukemia effect. This effect is mediated by NK cells because of the lack of activation of inhibitory killer cell immunoglobulin-like receptors (KIRs) which recognize HLA-Bw4 and HLA-C alleles. However, conflicting results have been reported about the impact of KIR ligand mismatching on the outcome of unrelated HLA-mismatched hematopoietic stem cells transplants (HSCT) to leukemic patients. The interpretation of these conflicting results is hampered by the scant information about the level of expression of HLA class I alleles on leukemic cells, although this variable may affect the activation of inhibitory KIRs. Therefore in the present study, utilizing a large panel of human monoclonal antibodies we have measured the level of expression of HLA-A, -B and -C alleles on 20 B-chronic lymphoid leukemic (B-CLL) cell preparations, on 16 B-acute lymphoid leukemic (B-ALL) cell preparations and on 19 AML cell preparations. Comparison of the level of HLA class I antigen expression on leukemic cells and autologous normal T cells identified selective downregulation of HLA-A and HLA-B alleles on 15 and 14 of the 20 B-CLL, on 2 and 5 of the 16 B-ALL and on 7 and 11 of the 19 AML patients tested, respectively. Most interestingly HLA-C alleles were markedly downregulated on all three types of leukemic cells; the downregulation was most pronounced on AML cells. The potential functional relevance of these abnormalities is suggested by the dose-dependent enhancement of NK cell activation caused by coating the HLA-HLA-Bw4 epitope with monoclonal antibodies on leukemic cells which express NK cell activating ligands. Our results suggest that besides the HLA and KIR genotype, expression levels of KIR ligands on leukemic cells should be included among the criteria used to select the donor-recipient combinations for HSCT.