Confinement of activating receptors at the plasma membrane controls natural killer cell tolerance

Killer instincts: natural killer cells as multifactorial cancer immunotherapy

Natural killer (NK) cells integrate heterogeneous signals for activation and inhibition using germline-encoded receptors. These receptors are stochastically co-expressed, and their concurrent engagement and signaling can adjust the sensitivity of individual cells to putative targets. Against cancers, which mutate and evolve under therapeutic and immunologic pressure, the diversity for recognition provided by NK cells may be key to comprehensive cancer control. NK cells are already being trialled as adoptive cell therapy and targets for immunotherapeutic agents. However, strategies to leverage their naturally occurring diversity and agility have not yet been developed. In this review, we discuss the receptors and signaling pathways through which signals for activation or inhibition are generated in NK cells, focusing on their roles in cancer and potential as targets for immunotherapies. Finally, we consider the impacts of receptor co-expression and the potential to engage multiple pathways of NK cell reactivity to maximize the scope and strength of antitumor activities.

Allelic variation of KIR and HLA tunes the cytolytic payload and determines functional hierarchy of NK cell repertoires

The functionality of natural killer (NK) cells is tuned during education and is associated with remodeling of the lysosomal compartment. We hypothesized that genetic variation in killer cell immunoglobulin-like receptor (KIR) and HLA, which is known to influence the functional strength of NK cells, fine-tunes the payload of effector molecules stored in secretory lysosomes. To address this possibility, we performed a high-resolution analysis of KIR and HLA class I genes in 365 blood donors and linked genotypes to granzyme B loading and functional phenotypes. We found that granzyme B levels varied across individuals but were stable over time in each individual and genetically determined by allelic variation in HLA class I genes. A broad mapping of surface receptors and lysosomal effector molecules revealed that DNAM-1 and granzyme B levels served as robust metric of the functional state in NK cells. Variation in granzyme B levels at rest was tightly linked to the lytic hit and downstream killing of major histocompatibility complex-deficient target cells. Together, these data provide insights into how variation in genetically hardwired receptor pairs tunes the releasable granzyme B pool in NK cells, resulting in predictable hierarchies in global NK cell function.

Crosstalk of Mast Cells and Natural Killer Cells with Neurons in Chemotherapy-Induced Peripheral Neuropathy

Chemotherapy-induced peripheral neuropathy (CIPN) is a major comorbidity of cancer. Multiple clinical interventions have been studied to effectively treat CIPN, but the results have been disappointing, with no or little efficacy. Hence, understanding the pathophysiology of CIPN is critical to improving the quality of life and clinical outcomes of cancer patients. Although various mechanisms of CIPN have been described in neuropathic anti-cancer agents, the neuroinflammatory process involving cytotoxic/proinflammatory immune cells remains underexamined. While mast cells (MCs) and natural killer (NK) cells are the key innate immune compartments implicated in the pathogenesis of peripheral neuropathy, their role in CIPN has remained under-appreciated. Moreover, the biology of proinflammatory cytokines associated with MCs and NK cells in CIPN is particularly under-evaluated. In this review, we will focus on the interactions between MCs, NK cells, and neuronal structure and their communications via proinflammatory cytokines, including TNFα, IL-1β, and IL-6, in peripheral neuropathy in association with tumor immunology. This review will help lay the foundation to investigate MCs, NK cells, and cytokines to advance future therapeutic strategies for CIPN.

Functional prominence of natural killer cells and natural killer T cells in pregnancy and infertility: A comprehensive review and update

During pregnancy, complicated connections are formed between a mother and a fetus. In a successful pregnancy, the maternal-fetal interface is affected by dynamic changes, and the fetus is protected against the mother's immune system. Natural killer (NK) cells are one of the immune system cells in the female reproductive system that play an essential role in the physiology of pregnancy. NK cells not only exist in peripheral blood (PB) but also can exist in the decidua. Studies have suggested multiple roles for these cells, including decidualization, control of trophoblast growth and invasion, embryo acceptance and maintenance by the mother, and facilitation of placental development during pregnancy. Natural killer T (NKT) cells are another group of NK cells that play a crucial role in the maintenance of pregnancy and regulation of the immune system during pregnancy. Studies show that NK and NKT cells are not only effective in maintaining pregnancy but also can be involved in infertility-related diseases. This review focuses on NK and NKT cells biology and provides a detailed description of the functions of these cells in implantation, placentation, and immune tolerance during pregnancy and their role in pregnancy complications.

PD-1 expression on mouse intratumoral NK cells and its effects on NK cell phenotype

Although PD-1 was shown to be a hallmark of T cells exhaustion, controversial studies have been reported on the role of PD-1 on NK cells. Here, we found by flow cytometry and single cell RNA sequencing analysis that PD-1 can be expressed on MHC class I-deficient tumor-infiltrating NK cells in vivo. We also demonstrate distinct alterations in the phenotype of PD-1-deficient NK cells and a more mature phenotype which might reduce their capacity to migrate and kill in vivo. Tumor-infiltrating NK cells that express PD-1 were highly associated with the expression of CXCR6. Furthermore, our results demonstrate that PD-L1 molecules in membranes of PD-1-deficient NK cells migrate faster than in NK cells from wild-type mice, suggesting that PD-1 and PD-L1 form cis interactions with each other on NK cells. These data demonstrate that there may be a role for the PD-1/PD-L1 axis in tumor-infiltrating NK cells in vivo.

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NK cells from PD-1 deficient mice have a more mature phenotype

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Elimination of MHC-I-deficient cells is impaired in PD-1^−/− mice

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PD-1 expression on NK cells is associated with surface expression of CXCR6

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PD-1/PD-L1 interactions on NK cells may occur in cis

MHC Class I–Dependent Shaping of the NK Cell Ly49 Receptor Repertoire Takes Place Early during Maturation in the Bone Marrow

MHC class I (MHC I) expression in the host influences NK cells in a process termed education. The result of this education is reflected in the responsiveness of NK cells at the level of individual cells as well as in the repertoire of inhibitory MHC I–specific receptors at the NK cell system level. The presence of MHC I molecules in the host environment gives rise to a skewed receptor repertoire in spleen NK cells where subsets expressing few (one or two) inhibitory receptors are expanded whereas subsets with many (three or more) receptors are contracted. It is not known whether this MHC I–dependent skewing is imposed during development or after maturation of NK cells. In this study, we tested the hypothesis that the NK cell receptor repertoire is shaped already early during NK cell development in the bone marrow. We used mice with a repertoire imposed by a single MHC I allele, as well as a C57BL/6 mutant strain with exaggerated repertoire skewing, to investigate Ly49 receptor repertoires at different stages of NK cell differentiation. Our results show that NK cell inhibitory receptor repertoire skewing can indeed be observed in the bone marrow, even during the earliest developmental steps where Ly49 receptors are expressed. This may partly be accounted for by selective proliferation of certain NK cell subsets, but other mechanisms must also be involved. We propose a model for how repertoire skewing is established during a developmental phase in the bone marrow, based on sequential receptor expression as well as selective proliferation.

Combinatorial Expression of NK Cell Receptors Governs Cell Subset Reactivity and Effector Functions but Not Tumor Specificity

NK cell receptors allow NK cells to recognize targets such as tumor cells. Many of them are expressed on a subset of NK cells, independently of each other, which creates a vast diversity of receptor combinations. Whether these combinations influence NK cell antitumor responses is not well understood. We addressed this question in the C57BL/6 mouse model and analyzed the individual effector response of 444 mouse NK cell subsets, defined by combinations of 12 receptors, against tumor cell lines originating from different tissues and mouse strains. We found a wide range of reactivity among NK subsets, but the same hierarchy of responses was observed for the different tumor types, showing that the repertoire of NK cell receptors does not encode for different tumor specificities but for different intrinsic reactivities. The coexpression of CD27, NKG2A, and DNAM-1 identified subsets with relative cytotoxic specialization, whereas reciprocally, CD11b and KLRG1 defined the best IFN-γ producers. The expression of educating receptors Ly49C, Ly49I, and NKG2A was also strongly correlated with IFN-γ production, but this effect was suppressed by unengaged receptors Ly49A, Ly49F, and Ly49G2. Finally, IL-15 coordinated NK cell effector functions, but education and unbound inhibitory receptors retained some influence on their response. Collectively, these data refine our understanding of the mechanisms governing NK cell reactivity, which could help design new NK cell therapy protocols.

Autologous NK cells as consolidation therapy following stem cell transplantation in multiple myeloma

Few approaches have been made toward exploring autologous NK cells in settings of cancer immunotherapy. Here, we demonstrate the feasibility of infusing multiple doses of ex vivo activated and expanded autologous NK cells in patients with multiple myeloma (MM) post-autologous stem cell transplantation. Infused NK cells were detected in circulation up to 4 weeks after the last infusion. Elevations in plasma granzyme B levels were observed following each consecutive NK cell infusion. Moreover, increased granzyme B levels were detected in bone marrow 4 weeks after the last infusion. All measurable patients had objective, detectable responses after NK cell infusions in terms of reduction in M-component and/or minimal residual disease. The present study demonstrates that autologous NK cell-based immunotherapy is feasible in a setting of MM consolidation therapy. It opens up the possibility for usage of autologous NK cells in clinical settings where patients are not readily eligible for allogeneic NK cell-based immunotherapies.

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Infusing activated and expanded autologous NK cells in patients with MM is possible

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Infused NK cells are detected in circulation for up to 4 weeks

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Elevated granzyme B levels are observed following each consecutive NK cell infusion

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Objective, detectable responses after NK cell infusions are seen in patients

Epigenetic regulation of natural killer cell memory

Immunological memory is the underlying mechanism by which the immune system remembers previous encounters with pathogens to produce an enhanced secondary response upon re-encounter. It stands as the hallmark feature of the adaptive immune system and the cornerstone of vaccine development. Classic recall responses are executed by conventional T and B cells, which undergo somatic recombination and modify their receptor repertoire to ensure recognition of a vast number of antigens. However, recent evidence has challenged the dogma that memory responses are restricted to the adaptive immune system, which has prompted a reevaluation of what delineates "immune memory." Natural killer (NK) cells of the innate immune system have been at the forefront of these pushed boundaries, and have proved to be more "adaptable" than previously thought. Like T cells, we now appreciate that their "natural" abilities actually require a myriad of signals for optimal responses. In this review, we discuss the many signals required for effector and memory NK cell responses and the epigenetic mechanisms that ultimately endow their enhanced features.

Dynamic variability in SHP-1 abundance determines natural killer cell responsiveness

[Figure: see text].

NKG2A expression identifies a subset of human Vδ2 T cells exerting the highest antitumor effector functions

Human Vδ2 cells are innate-like γδ T effectors performing potent immune surveillance against tumors. The constitutive expression of NKG2A identifies a subset of Vδ2 T cells licensed with an intrinsic hyper-responsiveness against cancer. Indeed, the transcriptomic profiles of NKG2A⁺ and NKG2A^- cells characterize two distinct "intralineages" of Vδ2 T lymphocytes that appear early during development, keep their phenotypes, and show self-renewal capabilities in adult life. The hyper-responsiveness of NKG2A⁺ Vδ2 T cells is counterbalanced by the inhibitory signaling delivered by human leukocyte antigen E (HLA-E) expressed on malignant cells as a tumor-escape mechanism. However, either masking or knocking out NKG2A restores the capacity of Vδ2 T cells to exert the highest effector functions even against HLA-E⁺ tumors. This is highly relevant in the clinic, as the different degrees of engagement of the NKG2A-HLA-E checkpoint in hepatocellular carcinoma, glioblastoma, and non-small cell lung cancer directly impact patients' overall survival. These findings open avenues for developing combined cellular and immunologic anticancer therapies.

Licensing Natural Killers for Antiviral Immunity

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-bearing receptors (IRs) enable discrimination between self- and non-self molecules on the surface of host target cells. In this regard, they have a vital role in self-tolerance through binding and activating intracellular tyrosine phosphatases which can inhibit cellular activation. Yet, self-MHC class I (MHC I)-specific IRs are versatile in that they can also positively impact lymphocyte functionality, as exemplified by their role in natural killer (NK) cell education, often referred to as ’licensing‘. Recent discoveries using defined mouse models of cytomegalovirus (CMV) infection have revealed that select self-MHC I IRs can increase NK cell antiviral defenses as well, whereas other licensing IRs cannot, or instead impede virus-specific NK responses for reasons that remain poorly understood. This review highlights a role for self-MHC I ‘licensing’ IRs in antiviral immunity, especially in the context of CMV infection, their impact on virus-specific NK cells during acute infection, and their potential to affect viral pathogenesis and disease.

Phase I Trial of Prophylactic Donor-Derived IL-2-Activated NK Cell Infusion after Allogeneic Hematopoietic Stem Cell Transplantation from a Matched Sibling Donor

Simple Summary

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a curative option for high-risk hematologic malignancies. However, disease recurrence after allo-HSCT remains a critical issue, underlining the need to develop maintenance therapy. In this context, NK cell-based immunotherapies could enhance graft-versus-tumor effect without triggering graft-versus-host disease. In this prospective phase I clinical trial, we demonstrated the safety of donor-derived NK cell infusion as a prophylactic treatment after allo-HSCT for patients with hematological malignancies. This opens perspectives for future developments of NK cell based therapeutic strategies after allo-HSCT with low incidence of GVHD, representing an advantage over post-transplant T cell modulations that are commonly used in clinical routine.

Abstract

Background: NK cell-based immunotherapy to prevent relapse after allogeneic transplantation is an appealing strategy because NK cells can provide strong antitumor effect without inducing graft-versus-host disease (GVHD). Thus, we designed a phase-I clinical trial evaluating the safety of a prophylactic donor-derived ex vivo IL-2 activated NK cell (IL-2 NK) infusion after allo-HSCT for patients with hematologic malignancies. Methods: Donor NK cells were purified and cultured ex vivo with IL-2 before infusion, at three dose levels. To identify the maximum tolerated dose was the main objective. In addition, we performed phenotypical and functional characterization of the NK cell therapy product, and longitudinal immune monitoring of NK cell phenotype in patients. Results: Compared to unstimulated NK cells, IL-2 NK cells expressed higher levels of activating receptors and exhibited increased degranulation and cytokine production in vitro. We treated 16 patients without observing any dose-limiting toxicity. At the last follow up, 11 out of 16 treated patients were alive in complete remission of hematologic malignancies without GVHD features and immunosuppressive treatment. Conclusions: Prophylactic donor-derived IL-2 NK cells after allo-HSCT is safe with low incidence of GVHD. Promising survivals and IL-2 NK cell activated phenotype may support a potential clinical efficacy of this strategy.

The transcription factor Bcl11b promotes both canonical and adaptive NK cell differentiation

Epigenetic landscapes can provide insight into regulation of gene expression and cellular diversity. Here, we examined the transcriptional and epigenetic profiles of seven human blood natural killer (NK) cell populations, including adaptive NK cells. The BCL11B gene, encoding a transcription factor (TF) essential for T cell development and function, was the most extensively regulated, with expression increasing throughout NK cell differentiation. Several Bcl11b-regulated genes associated with T cell signaling were specifically expressed in adaptive NK cell subsets. Regulatory networks revealed reciprocal regulation at distinct stages of NK cell differentiation, with Bcl11b repressing RUNX2 and ZBTB16 in canonical and adaptive NK cells, respectively. A critical role for Bcl11b in driving NK cell differentiation was corroborated in BCL11B-mutated patients and by ectopic Bcl11b expression. Moreover, Bcl11b was required for adaptive NK cell responses in a murine cytomegalovirus model, supporting expansion of these cells. Together, we define the TF regulatory circuitry of human NK cells and uncover a critical role for Bcl11b in promoting NK cell differentiation and function.

Crosstalk Between Trophoblasts and Decidual Immune Cells: The Cornerstone of Maternal-Fetal Immunotolerance

The success of pregnancy relies on the fine adjustment of the maternal immune system to tolerate the allogeneic fetus. Trophoblasts carrying paternal antigens are the only fetal-derived cells that come into direct contact with the maternal immune cells at the maternal–fetal interface. The crosstalk between trophoblasts and decidual immune cells (DICs) via cell–cell direct interaction and soluble factors such as chemokines and cytokines is a core event contributing to the unique immunotolerant microenvironment. Abnormal trophoblasts–DICs crosstalk can lead to dysregulated immune situations, which is well known to be a potential cause of a series of pregnancy complications including recurrent spontaneous abortion (RSA), which is the most common one. Immunotherapy has been applied to RSA. However, its development has been far less rapid or mature than that of cancer immunotherapy. Elucidating the mechanism of maternal–fetal immune tolerance, the theoretical basis for RSA immunotherapy, not only helps to understand the establishment and maintenance of normal pregnancy but also provides new therapeutic strategies and promotes the progress of immunotherapy against pregnancy-related diseases caused by disrupted immunotolerance. In this review, we focus on recent progress in the maternal–fetal immune tolerance mediated by trophoblasts–DICs crosstalk and clinical application of immunotherapy in RSA. Advancement in this area will further accelerate the basic research and clinical transformation of reproductive immunity and tumor immunity.

The Role of the Cytoskeleton in Regulating the Natural Killer Cell Immune Response in Health and Disease: From Signaling Dynamics to Function

Natural killer (NK) cells are innate lymphoid cells, which play key roles in elimination of virally infected and malignant cells. The balance between activating and inhibitory signals derived from NK surface receptors govern the NK cell immune response. The cytoskeleton facilitates most NK cell effector functions, such as motility, infiltration, conjugation with target cells, immunological synapse assembly, and cytotoxicity. Though many studies have characterized signaling pathways that promote actin reorganization in immune cells, it is not completely clear how particular cytoskeletal architectures at the immunological synapse promote effector functions, and how cytoskeletal dynamics impact downstream signaling pathways and activation. Moreover, pioneering studies employing advanced imaging techniques have only begun to uncover the architectural complexity dictating the NK cell activation threshold; it is becoming clear that a distinct organization of the cytoskeleton and signaling receptors at the NK immunological synapse plays a decisive role in activation and tolerance. Here, we review the roles of the actin cytoskeleton in NK cells. We focus on how actin dynamics impact cytolytic granule secretion, NK cell motility, and NK cell infiltration through tissues into inflammatory sites. We will also describe the additional cytoskeletal components, non-muscle Myosin II and microtubules that play pivotal roles in NK cell activity. Furthermore, special emphasis will be placed on the role of the cytoskeleton in assembly of immunological synapses, and how mutations or downregulation of cytoskeletal accessory proteins impact NK cell function in health and disease.

Peripheral natural killer cells in chronic hepatitis B patients display multiple molecular features of T cell exhaustion

Antiviral effectors such as natural killer (NK) cells have impaired functions in chronic hepatitis B (CHB) patients. The molecular mechanism responsible for this dysfunction remains poorly characterised. We show that decreased cytokine production capacity of peripheral NK cells from CHB patients was associated with reduced expression of NKp30 and CD16, and defective mTOR pathway activity. Transcriptome analysis of patients NK cells revealed an enrichment for transcripts expressed in exhausted T cells suggesting that NK cell dysfunction and T cell exhaustion employ common mechanisms. In particular, the transcription factor TOX and several of its targets were over-expressed in NK cells of CHB patients. This signature was predicted to be dependent on the calcium-associated transcription factor NFAT. Stimulation of the calcium-dependent pathway recapitulated features of NK cells from CHB patients. Thus, deregulated calcium signalling could be a central event in both T cell exhaustion and NK cell dysfunction occurring during chronic infections.

Dual effects of natural killer cells in transplantation for leukemia

Natural killer (NK) cells were originally considered to belong to the innate immune system to play a protective role against tumor cells and viral infections. In human, they can recognize self and non-self HLA class 1 as their ligand. So, analyzing the outcomes of allogeneic hematopoietic stem cell transplantation is a good opportunity to know the antitumor effects and regulatory effects of NK cells through HLA class 1 matching and mismatching of donor and recipient. In this review, I looked back on the main analysis results of the past transplants, summarized our reports consisting of many cases in a single ethnic, and showed that NK cells might work oppositely depending on the type of leukemia. New treatment strategies based on these concepts may offer individualized treatment options and ultimately increase offer the possibility of a cure for patients with leukemia.

Paths taken towards NK cell-mediated immunotherapy of human cancer-a personal reflection

The discovery that NK cells are able to specifically recognize cells lacking the expression of self‐MHC class I molecules provided the first insight into NK cell recognition of tumour cells. It started a flourishing field of NK cell research aimed at exploring the molecular nature of NK cell receptors involved in tumour cell recognition. While much of the important early work was conducted in murine experimental model systems, studies of human NK cells rapidly followed. Over the years, human NK cell research has swiftly progressed, aided by new detailed molecular information on human NK cell development, differentiation, molecular specificity, tissue heterogeneity and functional capacity. NK cells have also been studied in many different diseases aside from cancer, including viral diseases, autoimmunity, allergy and primary immunodeficiencies. These fields of research have all, indirectly or directly, provided further insights into NK cell‐mediated recognition of target cells and paved the way for the development of NK cell‐based immunotherapies for human cancer. Excitingly, NK cell‐based immunotherapy now opens up for novel strategies aimed towards treating malignant diseases, either alone or in combination with other drugs. Reviewed here are some personal reflections of select contributions leading up to the current state‐of‐the‐art in the field, with a particular emphasis on contributions from our own laboratory. This review is part of a series of articles on immunology in Scandinavia, published in conjunction with the 50th anniversary of the Scandinavian Society for Immunology.

Umbilical Cord Blood and iPSC-Derived Natural Killer Cells Demonstrate Key Differences in Cytotoxic Activity and KIR Profiles

Natural killer (NK) cells derived or isolated from different sources have been gaining in importance for cancer therapies. In this study, we evaluate and compare key characteristics between NK cells derived or isolated from umbilical cord blood, umbilical cord blood hematopoietic stem/progenitor cells, peripheral blood, and induced pluripotent stem cells (iPSCs). Specifically, we find CD56⁺ NK cells isolated and expanded directly from umbilical cord blood (UCB56) and NK cells derived from CD34⁺ hematopoietic stem/progenitors in umbilical cord blood (UCB34) differ in their expression of markers associated with differentiation including CD16, CD2, and killer Ig-like receptors (KIRs). UCB56-NK cells also displayed a more potent cytotoxicity compared to UCB34-NK cells. NK cells derived from iPSCs (iPSC-NK cells) were found to have variable KIR expression, with certain iPSC-NK cell populations expressing high levels of KIRs and others not expressing KIRs. Notably, KIR expression on UCB56 and iPSC-NK cells had limited effect on cytotoxic activity when stimulated by tumor target cells that express high levels of cognate HLA class I, suggesting that in vitro differentiation and expansion may override the KIR-HLA class I mediated inhibition when used across HLA barriers. Together our results give a better understanding of the cell surface receptor, transcriptional, and functional differences between NK cells present in umbilical cord blood and hematopoietic progenitor-derived NK cells which may prove important in selecting the most active NK cell populations for treatment of cancer or other therapies.

NK Cell Development in Times of Innate Lymphoid Cell Diversity

After being described in the 1970s as cytotoxic cells that do not require MHC-dependent pre-activation, natural killer (NK) cells remained the sole member of innate lymphocytes for decades until lymphoid tissue-inducer cells in the 1990s and helper-like innate lymphoid lineages from 2008 onward completed the picture of innate lymphoid cell (ILC) diversity. Since some of the ILC members, such as ILC1s and CCR6^- ILC3s, share specific markers previously used to identify NK cells, these findings provoked the question of how to delineate the development of NK cell and helper-like ILCs and how to properly identify and genetically interfere with NK cells. The description of eomesodermin (EOMES) as a lineage-specifying transcription factor of NK cells provided a candidate that may serve as a selective marker for the genetic targeting and identification of NK cells. Unlike helper-like ILCs, NK cell activation is, to a large degree, regulated by the engagement of activating and inhibitory surface receptors. NK cell research has revealed some elegant mechanisms of immunosurveillance, coined "missing-self" and "induced-self" recognition, thus complementing "non-self recognition", which is predominantly utilized by adaptive lymphocytes and myeloid cells. Notably, the balance of activating and inhibitory signals perceived by surface receptors can be therapeutically harnessed for anti-tumor immunity mediated by NK cells. This review aims to summarize the similarities and the differences in development, function, localization, and phenotype of NK cells and helper-like ILCs, with the purpose to highlight the unique feature of NK cell development and regulation.

Do ABC transporters regulate plasma membrane organization?

The plasma membrane (PM) spatiotemporal organization is one of the major factors controlling cell signaling and whole-cell homeostasis. The PM lipids, including cholesterol, determine the physicochemical properties of the membrane bilayer and thus play a crucial role in all membrane-dependent cellular processes. It is known that lipid content and distribution in the PM are not random, and their transversal and lateral organization is highly controlled. Mainly sphingolipid- and cholesterol-rich lipid nanodomains, historically referred to as rafts, are extremely dynamic “hot spots” of the PM controlling the function of many cell surface proteins and receptors. In the first part of this review, we will focus on the recent advances of PM investigation and the current PM concept. In the second part, we will discuss the importance of several classes of ABC transporters whose substrates are lipids for the PM organization and dynamics. Finally, we will briefly present the significance of lipid ABC transporters for immune responses.

Fluorescence strategies for mapping cell membrane dynamics and structures

Fluorescence spectroscopy has been a cornerstone of research in membrane dynamics and organization. Technological advances in fluorescence spectroscopy went hand in hand with discovery of various physicochemical properties of membranes at nanometric spatial and microsecond timescales. In this perspective, we discuss the various challenges associated with quantification of physicochemical properties of membranes and how various modes of fluorescence spectroscopy have overcome these challenges to shed light on the structure and organization of membranes. Finally, we discuss newer measurement strategies and data analysis tools to investigate the structure, dynamics, and organization of membranes.

Tuning of human NK cells by endogenous HLA-C expression

NK cells are primarily responsible for detecting malignant or pathogen-infected cells, and their function is influenced both by stress-associated activating signals and opposing inhibitory signals from receptors that recognize self MHC. The receptors that produce this inhibitory signal shift from the NKG2A:HLA-E system to that of KIR:HLA as the NK cells mature. This maturation is associated with an increase in lytic activity, as well as an increase in HLA-C protein levels controlled by the NK-specific HLA-C promoter, NK-Pro. We propose that modulation of the translatability of HLA-C transcripts in NK cells constitutes an evolutionary mechanism to control cis inhibitory signaling by HLA-C, which fine tunes NK cell activity. Furthermore, the high degree of variability in KIR receptor affinity for HLA alleles, as well as the variable expression levels of both KIR and HLA, suggest an evolutionary requirement for the tuning of NK lytic activity. Various data have demonstrated that mature NK cells may gain or lose lytic activity when placed in different environments. This indicates that NK cell activity may be more a function of constant tuning by inhibitory signals, rather than a static, irreversible "license to kill" granted to mature NK cells. Inhibitory signaling controls the filling of the cytolytic granule reservoir, which becomes depleted if there are insufficient inhibitory signals, leading to a hyporesponsive NK cell. We propose a novel model for the tuning of human NK cell activity via cis interactions in the context of recent findings on the mechanism of NK education.

NK cells for cancer immunotherapy

Natural killer (NK) cells can swiftly kill multiple adjacent cells if these show surface markers associated with oncogenic transformation. This property, which is unique among immune cells, and their capacity to enhance antibody and T cell responses support a role for NK cells as anticancer agents. Although tumours may develop several mechanisms to resist attacks from endogenous NK cells, ex vivo activation, expansion and genetic modification of NK cells can greatly increase their antitumour activity and equip them to overcome resistance. Some of these methods have been translated into clinical-grade platforms and support clinical trials of NK cell infusions in patients with haematological malignancies or solid tumours, which have yielded encouraging results so far. The next generation of NK cell products will be engineered to enhance activating signals and proliferation, suppress inhibitory signals and promote their homing to tumours. These modifications promise to significantly increase their clinical activity. Finally, there is emerging evidence of increased NK cell-mediated tumour cell killing in the context of molecularly targeted therapies. These observations, in addition to the capacity of NK cells to magnify immune responses, suggest that NK cells are poised to become key components of multipronged therapeutic strategies for cancer.

IL-2 And IL-15 Induced NKG2D, CD158a and CD158b Expression on T, NKT- like and NK Cell Lymphocyte Subsets from Regional Lymph Nodes of Melanoma Patients

Regional lymph nodes (LN)s represent important immunological barriers in spreading of malignant tumors. However, they are the most frequent early metastatic site in melanoma. Immunomodulatory agents including cytokines have been included in therapy of melanoma and have shown severe side effects and toxicity. In this sense, there is a growing need for bringing these agents to further in vitro testing that may enlighten aspects of their regional application. Therefore, the aim of this study was to investigate the effect of interleukin (IL)-2 and IL-15, the two cytokines with similar immune-enhancing effects, on the expression of activating NKG2D, inhibitory CD158a and CD158b receptors on CD8+ T, NKT-like and NK cell lymphocyte subsets from regional LNs of melanoma patients. In this study, we showed significant effects of IL-2 and IL-15 cytokine treatments on the expression of activating NKG2D and on inhibitory CD158a and CD158b receptors on lymphocytes, CD8+ T, NKT-like and NK cell lymphocyte subsets originating from regional LNs of melanoma patients. Furthermore, IL-2 and IL-15 by inducing the expression of NKG2D activating receptor on innate and on adaptive lymphocyte subsets and by augmenting NK cell antitumor cytotoxicity that correlated with the cytokine-induced NKG2D expression, increased antitumor potential of immune cells in regional LNs of melanoma patients irrespective of LN involvement. These findings indicate the importance of immune cell population from regional LNs of melanoma patients in the development of immune intervention strategies that may if applied locally increase antitumor potential to the level that controls tumor progressions.

Pair Correlation Analysis Maps the Dynamic Two-Dimensional Organization of Natural Killer Cell Receptors at the Synapse

In living systems, the contact between cells is the basis of recognition, differentiation, and orchestration of an immune response. Obstacles and barriers to biomolecular motion, especially for receptors at cellular synapses, critically control these functions by creating an anisotropic environment. Whereas conventional fluorescence fluctuation methods, such as fluorescence correlation spectroscopy or fluorescence recovery after photobleaching, can only measure the isotropic diffusion of molecules, the two-dimensional pair correlation function (2D-pCF) approach probes the anisotropic paths at different spatial locations within an image, allowing the creation of high-resolution maps that can visualize and quantify how molecules move in a living cell. In this work, we show how the 2D-pCF method maps the environment in cellular synapses as perceived by natural killer (NK) cell receptors. In cultured human HLA null 721.221 cells, 2D-pCF reveals the motion of inhibitory receptor HLA-Cw4-YFP coexpressed with KIR3DL1 to be highly directional around specific loci, while these restrictions were absent in the case of HLA-B51-YFP coexpressed with KIR2DL1. Further, in freshly isolated educated (H-2Dd) and uneducated (MHC-/-) primary murine NK cells, the 2D-pCF method shows significant differences in the paths taken by activating receptor NKp46 and inhibitory receptor Ly49A in educated compared to uneducated cells. Altogether, we demonstrate that the 2D-pCF method is very powerful in informing about the spatial organization of motion in cells. Our data support the hypothesis that flexibility in the spatial arrangement of membrane receptors, that is, the absence of barriers, is crucial for NK cell function.

Genetic diversity affects the nanoscale membrane organization and signaling of natural killer cell receptors

Genetic diversity in human natural killer (NK) cell receptors is linked to resistance and susceptibility to many diseases. Here, we tested the effect of this diversity on the nanoscale organization of killer cell immunoglobulin-like receptors (KIRs). Using superresolution microscopy, we found that inhibitory KIRs encoded by different genes and alleles were organized differently at the surface of primary human NK cells. KIRs that were found at low abundance assembled into smaller clusters than those formed by KIRs that were more highly abundant, and at low abundance, there was a greater proportion of KIRs in clusters. Upon receptor triggering, a structured interface called the immune synapse assembles, which facilitates signal integration and controls NK cell responses. Here, triggering of low-abundance receptors resulted in less phosphorylation of the downstream phosphatase SHP-1 but more phosphorylation of the adaptor protein Crk than did triggering of high-abundance receptors. In cells with greater KIR abundance, SHP-1 dephosphorylated Crk, which potentiated NK cell spreading during activation. Thus, genetic variation modulates both the abundance and nanoscale organization of inhibitory KIRs. That is, as well as the number of receptors at the cell surface varying with genotype, the way in which these receptors are organized in the membrane also varies. Essentially, a change in the average surface abundance of a protein at the cell surface is a coarse descriptor entwined with changes in local nanoscale clustering. Together, our data indicate that genetic diversity in inhibitory KIRs affects membrane-proximal signaling and, unexpectedly, the formation of activating immune synapses.

Regulation of MHC class I-independent NK cell education by SLAM family receptors

Seven members of signaling lymphocytic activation molecule (SLAM) family receptors (SFRs) are ubiquitously expressed on hematopoietic cells and they play critical roles in immune cell differentiation and activation. The engagement of these receptors transmits intracellular signaling mainly by recruiting SLAM-associated protein (SAP) and its related adaptors, EWS-FLI1-activated transcript-2 (EAT-2) and EAT-2-related transducer (ERT). The critical roles of SFRs and SAP-family adaptors are highlighted by the discovery that SAP is mutated in human X-linked lymphoproliferative (XLP1) disease in which the contact between T and B cells in germinal center and cytotoxic lymphocytes (NK cells and CD8⁺ T cells) function are severely compromised. These immune defects are closely associated with the defective antibody production and the high incidence of lymphoma in the patients with XLP1. In addition to these well-known functions, SLAM-SAP family is involved in NK cell education, a process describing NK cell functional competence. In this chapter, we will mainly discuss these unappreciated roles of SAP-dependent and SAP-independent SFR signaling in regulating MHC-I-independent NK cell education.

NK Cell Hyporesponsiveness: More Is Not Always Better

Natural Killer (NK) cells are a type of cytotoxic lymphocytes that play an important role in the innate immune system. They are of particular interest for their role in elimination of intracellular pathogens, viral infection and tumor cells. As such, numerous strategies are being investigated in order to potentiate their functions. One of these techniques aims at promoting the function of their activating receptors. However, different observations have revealed that providing activation signals could actually be counterproductive and lead to NK cells’ hyporesponsiveness. This phenomenon can occur during the NK cell education process, under pathological conditions, but also after treatment with different agents, including cytokines, that are promising tools to boost NK cell function. In this review, we aim to highlight the different circumstances where NK cells become hyporesponsive and the methods that could be used to restore their functionality.

Membrane Organization and Physical Regulation of Lymphocyte Antigen Receptors: A Biophysicist's Perspective

Receptors at the membrane of immune cells are the central players of innate and adaptative immunity, providing effective defence mechanisms against pathogens or cancer cells. Their function is intimately linked to their position at and within the membrane which provides accessibility, mobility as well as membrane proximal cytoskeleton anchoring, all of these elements playing important roles in the final function and links to cellular actions. Understanding how immune cells integrate the specific signals received at their membrane to take a decision remains an immense challenge and a very active field of fundamental and applied research. Recent progress in imaging and micromanipulation techniques have led to an unprecedented refinement in the description of molecular structures and supramolecular assemblies at the immune cell membrane, and provided a glimpse into their dynamics and regulation by force. Several key elements have been scrutinized such as the roles of relative sizes of molecules, lateral organisation, motion in the membrane of the receptors, but also physical cues such as forces, mediated by cellular substrates of different rigidities or applied by the cell itself, in conjunction with its partner cell. We review here these recent discoveries associated with a description of the biophysical methods used. While a conclusive picture integrating all of these components is still lacking, mainly due to the implication of diverse and different mechanisms and spatio-temporal scales involved, the amount of quantitative data available opens the way for physical modelling and numerical simulations and new avenues for experimental research.

Remodeling of secretory lysosomes during education tunes functional potential in NK cells

Inhibitory signaling during natural killer (NK) cell education translates into increased responsiveness to activation; however, the intracellular mechanism for functional tuning by inhibitory receptors remains unclear. Secretory lysosomes are part of the acidic lysosomal compartment that mediates intracellular signalling in several cell types. Here we show that educated NK cells expressing self-MHC specific inhibitory killer cell immunoglobulin-like receptors (KIR) accumulate granzyme B in dense-core secretory lysosomes that converge close to the centrosome. This discrete morphological phenotype is independent of transcriptional programs that regulate effector function, metabolism and lysosomal biogenesis. Meanwhile, interference of signaling from acidic Ca²⁺ stores in primary NK cells reduces target-specific Ca²⁺-flux, degranulation and cytokine production. Furthermore, inhibition of PI(3,5)P₂ synthesis, or genetic silencing of the PI(3,5)P₂-regulated lysosomal Ca²⁺-channel TRPML1, leads to increased granzyme B and enhanced functional potential, thereby mimicking the educated state. These results indicate an intrinsic role for lysosomal remodeling in NK cell education.

PD-1 mediates functional exhaustion of activated NK cells in patients with Kaposi sarcoma

Programmed Death-1 (PD-1), an inhibitory receptor expressed by activated lymphocytes, is involved in regulating T- and B-cell responses. PD-1 and its ligands are exploited by a variety of cancers to facilitate tumor escape through PD-1-mediated functional exhaustion of effector T cells. Here, we report that PD-1 is upregulated on Natural Killer (NK) cells from patients with Kaposi sarcoma (KS). PD-1 was expressed in a sub-population of activated, mature CD56dimCD16pos NK cells with otherwise normal expression of NK surface receptors. PD-1pos NK cells from KS patients were hyporesponsive ex vivo following direct triggering of NKp30, NKp46 or CD16 activating receptors, or short stimulation with NK cell targets. PD-1pos NK cells failed to degranulate and release IFNγ, but exogenous IL-2 or IL-15 restored this defect. That PD-1 contributed to NK cell functional impairment and was not simply a marker of dysfunctional NK cells was confirmed in PD-1-transduced NKL cells. In vitro, PD-1 was induced at the surface of healthy control NK cells upon prolonged contact with cells expressing activating ligands, i.e. a condition mimicking persistent stimulation by tumor cells. Thus, PD-1 appears to plays a critical role in mediating NK cell exhaustion. The existence of this negative checkpoint fine-tuning NK activation highlights the possibility that manipulation of the PD-1 pathway may be a strategy for circumventing tumor escape not only from the T cell-, but also the NK-cell mediated immune surveillance.

Educated natural killer cells show dynamic movement of the activating receptor NKp46 and confinement of the inhibitory receptor Ly49A

Educated natural killer (NK) cells have inhibitory receptors specific for self major histocompatibility complex (MHC) class I molecules and kill cancer cells more efficiently than do NK cells that do not have such receptors (hyporesponsive NK cells). The mechanism behind this functional empowerment through education has so far not been fully described. In addition, distinctive phenotypic markers of educated NK cells at the single-cell level are lacking. We developed a refined version of the image mean square displacement (iMSD) method (called iMSD carpet analysis) and used it in combination with single-particle tracking to characterize the dynamics of the activating receptor NKp46 and the inhibitory receptor Ly49A on resting educated versus hyporesponsive murine NK cells. Most of the NKp46 and Ly49A molecules were restricted to microdomains; however, individual NKp46 molecules resided in these domains for shorter periods and diffused faster on the surface of educated, compared to hyporesponsive, NK cells. In contrast, the movement of Ly49A was more constrained in educated NK cells compared to hyporesponsive NK cells. Either disrupting the actin cytoskeleton or adding cholesterol to the cells prohibited activating signaling, suggesting that the dynamics of receptor movements within the cell membrane are critical for the proper activation of NK cells. The faster and more dynamic movement of NKp46 in educated NK cells may facilitate a swifter response to interactions with target cells.

Natural Killer Cell Education and the Response to Infection and Cancer Therapy: Stay Tuned

The functional capacities of natural killer (NK) cells differ within and between individuals, reflecting considerable genetic variation. 'Licensing/arming', 'disarming', and 'tuning' are models that have been proposed to explain how interactions between MHC class I molecules and their cognate inhibitory receptors - Ly49 in mice and KIR in humans - 'educate' NK cells for variable reactivity and sensitivity to inhibition. In this review we discuss recent progress toward understanding the genetic, epigenetic, and molecular features that titrate NK effector function and inhibition, and the impact of variable NK cell education on human health and disease.

Human NK Cells Downregulate Zap70 and Syk in Response to Prolonged Activation or DNA Damage

The extent of NK cell activity during the innate immune response affects downstream immune functions and, ultimately, the outcome of infectious or malignant disease. However, the mechanisms that terminate human NK cell responses have yet to be defined. When activation receptors expressed on NK cell surfaces bind to ligands on diseased cells, they initiate a signal that is propagated by a number of intracellular kinases, including Zap70 and Syk, eventually leading to NK cell activation. We assayed Zap70 and Syk content in NK cells from healthy human donors and identified a subset of NK cells with unusually low levels of these two kinases. We found that this Zap70^lowSyk^low subset consisted of NK cells expressing a range of surface markers, including CD56^hi and CD56^low NK cells. Upon in vitro stimulation with target cells, Zap70^lowSyk^low NK cells failed to produce IFN-γ and lysed target cells at one third the capacity of Zap70^hiSyk^hi NK cells. We determined two independent in vitro conditions that induce the Zap70^lowSyk^low phenotype in NK cells: continuous stimulation with activation beads and DNA damage. The expression of inhibitory receptors, including NKG2A and inhibitory killer Ig-like receptors (KIRs), was negatively correlated with the Zap70^lowSyk^low phenotype. Moreover, expression of multiple KIRs reduced the likelihood of Zap70 downregulation during continuous activation, regardless of whether NK cells had been educated through KIR-HLA interactions in vivo. Our findings show that human NK cells are able to terminate their functional activity without the aid of other immune cells through the downregulation of activation kinases.

High mTOR activity is a hallmark of reactive natural killer cells and amplifies early signaling through activating receptors

NK cell education is the process through which chronic engagement of inhibitory NK cell receptors by self MHC-I molecules preserves cellular responsiveness. The molecular mechanisms responsible for NK cell education remain unclear. Here, we show that mouse NK cell education is associated with a higher basal activity of the mTOR/Akt pathway, commensurate to the number of educating receptors. This higher activity was dependent on the SHP-1 phosphatase and essential for the improved responsiveness of reactive NK cells. Upon stimulation, the mTOR/Akt pathway amplified signaling through activating NK cell receptors by enhancing calcium flux and LFA-1 integrin activation. Pharmacological inhibition of mTOR resulted in a proportional decrease in NK cell reactivity. Reciprocally, acute cytokine stimulation restored reactivity of hyporesponsive NK cells through mTOR activation. These results demonstrate that mTOR acts as a molecular rheostat of NK cell reactivity controlled by educating receptors and uncover how cytokine stimulation overcomes NK cell education.

The cells of the immune system patrol the body to detect and destroy harmful microbes and diseased cells. Natural killer cells are immune cells with a natural capacity to kill infected or cancerous cells, as their name suggests. Importantly, they do so while sparing the surrounding healthy cells.

As natural killer cells mature they go through an “education” process to learn to distinguish between normal and abnormal cells. During education, the natural killer cells interact continuously with nearby healthy cells. However, it remains unknown how these interactions change the natural killer cells, or how these changes control their killing activity.

Marçais et al. now show that a protein called mTOR is essential to the education of natural killer cells. Comparing natural killer cells that had or had not completed the education process revealed that mTOR is more active in the educated cells. Moreover, inhibiting the activity of mTOR caused educated natural killer cells to lose their ability to identify diseased cells, while stimulating mTOR activity in uneducated natural killer cells mimicked the education process, allowing them to recognize and eliminate diseased host cells.

Certain nutrients are known to control the activity of mTOR, which suggests these nutrients could also affect how natural killer cells develop. In addition, manipulating the activity of mTOR could be used to control the response of natural killer cells to diseased host cells, and so could form part of treatments for cancer and infectious diseases. However, given that mTOR plays numerous roles within different body cells, any potential therapies that are developed would need to be able to manipulate mTOR specifically in natural killer cells.

Immune selection during tumor checkpoint inhibition therapy paves way for NK-cell "missing self" recognition

The ability of NK cells to specifically recognize cells lacking expression of self-MHC class I molecules was discovered over 30 years ago. It provided the foundation for the "missing self" hypothesis. Research in the two past decades has contributed to a detailed understanding of the molecular mechanisms that determine the specificity and strength of NK cell-mediated "missing self" responses to tumor cells. However, in light of the recent remarkable breakthroughs in clinical cancer immunotherapy, the cytolytic potential of NK cells still remains largely untapped in clinical settings. There is abundant evidence demonstrating partial or complete loss of HLA class I expression in a wide spectrum of human tumor types. Such loss may result from immune selection of escape variants by tumor-specific CD8 T cells and has more recently also been linked to acquired resistance to checkpoint inhibition therapy. In the present review, we discuss the early predictions of the "missing self" hypothesis, its molecular basis and outline the potential for NK cell-based adoptive immunotherapy to convert checkpoint inhibitor therapy-resistant patients into clinical responders.

Contribution of inhibitory receptor TIGIT to NK cell education

Engagement of inhibitory receptors by cognate host MHC-I molecules triggers NK cell education, resulting in functional maturation and allowing NK cells to sense missing-self. However, NK cells also express inhibitory receptors for non-MHC-I ligands and their role in NK cell education is poorly understood. TIGIT is a recently identified inhibitory receptor that recognizes a non-MHC-I ligand CD155. Here, we demonstrated that TIGIT⁺ NK cells from wild-type mice exerted augmented responsiveness to various stimuli, including targets that lacked expression of CD155 ligand. TIGIT⁺ NK cells derived from CD155-deficient hosts, however, exhibited functional impairment, indicating that the engagement of TIGIT receptor by host CD155 promoted NK cell functional maturation. Furthermore, TIGIT deficiency impaired NK cell-mediated missing-self recognition and rejection of CD155^- targets, such as allogenic splenocytes and certain tumor cells, in an MHC-I-independent and CD226-unrelated manner. Thus, TIGIT-CD155 pathway is also involved in the acquisition of optimal NK cell effector function, representing a novel MHC-I-independent education mechanism for NK cell tolerance and activation.

A user's guide for characterizing plasma membrane subdomains in living cells by spot variation fluorescence correlation spectroscopy

Due to the intrinsic molecular Brownian agitation within plasma membrane and the vast diversity of membrane components, it is expected that the plasma membrane organization is highly heterogeneous with the formation of local complex multicomponent assemblies of lipids and proteins on different time scales. Still, deciphering this lateral organization on living cells and on the appropriate length and temporal scales has been challenging but is crucial to advance our knowledge on the biological function of the plasma membrane. Among the methodological developments based on biophotonics, the spot variation FCS (svFCS), a fluorescent correlation spectroscopy (FCS)-based method, has allowed the significant progress in the characterization of cell membrane lateral organization at the suboptical level, including to providing compelling evidence for the in vivo existence of lipid-dependent nanodomains. The aim of this chapter is to serve as a guide for setting and applying the svFCS methodology to study the plasma membrane of both adherent and nonadherent cell types.

Decidual natural killer cells and the immune microenvironment at the maternal-fetal interface

During early pregnancy, an orchestrated evolutionary maternal adaption toward tolerance of the semiallogeneic fetus is required to ensure decidualization and early embryo development. Remodeling of the immune system involves natural killer cells (NKs), macrophages, T cells and dendritic cells (DCs) altering the microenvironment in the deciduas. In particular, a unique population of NK cells with a CD56^brightCD16^- phenotype in the decidua has been proposed to play a key role in the maternal adaptation to pregnancy. However, there is a tendency for pregnancy immunology to reflect transplantation immunology regarding the assumption that the maternal immune system should be suppressed. This tendency is misleading. We discuss how the immune system is formed in early deciduas and the interactions between maternal NK cells and fetal growth. We propose that the maternal immune response must not be fully suppressed and is even necessary for the local response of uterine NK cells.

Analysis of Ly49 gene transcripts in mature NK cells supports a role for the Pro1 element in gene activation, not gene expression

The variegated expression of murine Ly49 loci has been associated with the probabilistic behavior of an upstream promoter active in immature cells, the Pro1 element. However, recent data suggest that Pro1 may be active in mature natural killer (NK) cells and function as an enhancer element. To assess directly if Pro1 transcripts are present in mature Ly49-expressing NK cells, RNA-sequencing of the total transcript pool was performed on freshly isolated splenic NK cells sorted for expression of either Ly49G or Ly49I. No Pro1 transcripts were detected from the Ly49a, Ly49c or Ly49i genes in mature Ly49(+) NK cells that contained high levels of Pro2 transcripts. Low levels of Ly49g Pro1 transcripts were found in both Ly49G(+) and Ly49G(-) populations, consistent with the presence of a small population of mature NK cells undergoing Ly49g gene activation, as previously demonstrated by culture of splenic NK cells in interleukin-2. Ly49 gene reporter constructs containing Pro1 failed to show any enhancer activity of Pro1 on Pro2 in a mature Ly49-expressing cell line. Taken together, the results are consistent with Pro1 transcription having a role in gene activation in developing NK, and argue against a role for Pro1 in Ly49 gene transcription by mature NK cells.

Plasma Membrane Organization of Epidermal Growth Factor Receptor in Resting and Ligand-Bound States

The spatial arrangement of the epidermal growth factor receptor (EGFR) on the cellular plasma membrane is one of the prime factors that control its downstream signaling pathways and related functions. However, the molecular organization, which spans the scale from nanometers to micrometer-size clusters, has not been resolved in detail, mainly due to a lack of techniques with the required spatiotemporal resolution. Therefore, we used imaging total internal reflection-fluorescence correlation spectroscopy to investigate EGFR dynamics on live CHO-K1 plasma membranes in resting and ligand-bound states. In combination with the fluorescence correlation spectroscopy diffusion law, this provides information on the subresolution organization of EGFR on cell membranes. We found that overall EGFR organization is sensitive to both cholesterol and the actin cytoskeleton. EGFR in the resting state is partly trapped in cholesterol-containing domains, whereas another fraction exhibits cholesterol independent trapping on the membrane. Disruption of the cytoskeleton leads to a broader range of EGFR diffusion coefficients and a reduction of hop diffusion. In the ligand-bound state we found a dose-dependent behavior. At 10 ng/mL EGF the EGFR is endocytosed and recycled to the membrane, whereas diffusion and organization do not change significantly. At 100 ng/mL EGF the EGFR forms clusters, which are subsequently internalized, whereas outside the clusters diffusivity increases and the organization of the receptor remains unchanged. After disruption of cholesterol-containing domains or actin cytoskeleton, EGF induces microscopic EGFR clusters on the membrane and endocytosis is inhibited.

IL-2 in the tumor microenvironment is necessary for Wiskott-Aldrich syndrome protein deficient NK cells to respond to tumors in vivo

To kill target cells, natural killer (NK) cells organize signaling from activating and inhibitory receptors to form a lytic synapse. Wiskott-Aldrich syndrome (WAS) patients have loss-of-function mutations in the actin regulator WASp and suffer from immunodeficiency with increased risk to develop lymphoreticular malignancies. NK cells from WAS patients fail to form lytic synapses, however, the functional outcome in vivo remains unknown. Here, we show that WASp KO NK cells had decreased capacity to degranulate and produce IFNγ upon NKp46 stimulation and this was associated with reduced capacity to kill MHC class I-deficient hematopoietic grafts. Pre-treatment of WASp KO NK cells with IL-2 ex vivo restored degranulation, IFNγ production, and killing of MHC class I negative hematopoietic grafts. Moreover, WASp KO mice controlled growth of A20 lymphoma cells that naturally produced IL-2. WASp KO NK cells showed increased expression of DNAM-1, LAG-3, and KLRG1, all receptors associated with cellular exhaustion and NK cell memory. NK cells isolated from WAS patient spleen cells showed increased expression of DNAM-1 and had low to negative expression of CD56, a phenotype associated with NK cells exhaustion. Finally, in a cohort of neuroblastoma patients we identified a strong correlation between WASp, IL-2, and patient survival.

NK cell education via nonclassical MHC and non-MHC ligands

Natural killer (NK) cell education, a process for achieving functional maturation and self-tolerance, has been previously defined by the interaction between self-major histocompatibility complex class I (MHC-I) molecules and their specific inhibitory receptors. Over the past several years, growing evidence has highlighted the important roles of nonclassical MHC-I and non-MHC-I molecules in NK cell education. Herein, we review the current knowledge of NK cell education, with a particular focus on nonclassical MHC-I- and non-MHC-I-dependent education, and compare them with the classical MHC-I-dependent education theory. In addition, we update and extend this theory by presenting the 'Confining Model', discussing cis and trans characteristics, reassessing quantity and quality control, and elucidating the redundancy of NK cell education in tumor and virus infection.

Ly49C Impairs NK Cell Memory in Mouse Cytomegalovirus Infection

NK cells possess inhibitory receptors that are responsible for self-MHC class I recognition; beyond their inhibitory function, accumulating evidence indicates that such receptors confer NK cell functional competence through an unclear process termed "licensing." Ly49C is the main self-specific inhibitory Ly49 receptor in H-2(b) C57BL/6 (B6) mice. We used B6 Ly49C-transgenic and B6 β2 microglobulin (β2m)-knockout Ly49C-transgenic mice to investigate the impact of licensing through this inhibitory receptor in precursor and mature NK cells. We found that self-specific inhibitory receptors affected NK cell precursor survival and proliferation at particular developmental stages in an MHC class I-dependent manner. The presence of Ly49C impacted the NK cell repertoire in a β2m-dependent manner, with reduced Ly49A(+), Ly49G2(+), and Ly49D(+) subsets, an increased DNAM-1(+) subset, and higher NKG2D expression. Licensed NK cells displayed a skewed distribution of the maturation stages, which was characterized by differential CD27 and CD11b expression, toward the mature phenotypes. We found that Ly49C-mediated licensing induced a split effect on NK cell functions, with increased cytokine-production capabilities following engagement of various activating receptors while cytotoxicity remained unchanged. Analysis of licensed NK cell functions in vivo, in a system of mouse CMV infection, indicated that licensing did not play a major role in the NK cell antiviral response during acute infection, but it strongly impaired the generation and/or persistence of memory NK cells. This study unravels multifaceted effects of licensing on NK cell populations and their functions.

Newtonian cell interactions shape natural killer cell education

Newton's third law of motion states that for every action on a physical object there is an equal and opposite reaction. The dynamic change in functional potential of natural killer (NK) cells during education bears many features of such classical mechanics. Cumulative physical interactions between cells, under a constant influence of homeostatic drivers of differentiation, lead to a reactive spectrum that ultimately shapes the functionality of each NK cell. Inhibitory signaling from an array of self‐specific receptors appear not only to suppress self‐reactivity but also aid in the persistence of effector functions over time, thereby allowing the cell to gradually build up a functional potential. Conversely, the frequent non‐cytolytic interactions between normal cells in the absence of such inhibitory signaling result in continuous stimulation of the cells and attenuation of effector function. Although an innate cell, the degree to which the fate of the NK cell is predetermined versus its ability to adapt to its own environment can be revealed through a Newtonian view of NK cell education, one which is both chronological and dynamic. As such, the development of NK cell functional diversity is the product of qualitatively different physical interactions with host cells, rather than simply the sum of their signals or an imprint based on intrinsically different transcriptional programs.