CX3CR1 regulates the maintenance of KLRG1+ NK cells into the bone marrow by promoting their entry into circulation

Protein phosphatase 2A inhibitor modulates natural killer cell homeostasis in peripheral tissues

Although natural killer (NK) cell responses to tumor and viral infection have been studied, the mechanisms underlying NK cell homeostasis in vivo remain unclear. In this study, we demonstrate the pharmacological action of cytostatin, a protein phosphatase 2A (PP2A) specific inhibitor (PP2Ai), on NK cells in regulating NK cell homeostasis in the peripheral tissues. We found that PP2Ai treatment decreased NK cell percentages in the bone marrow and secondary lymphoid tissues while increasing NK cell percentages in peripheral tissues such as the lung and liver. In the peripheral tissues of PP2Ai-treated mice, Ki-67 expression and BrdU uptake in NK cells were upregulated, and an initial increase in the pre-mature CD11bhiCD27hi NK subset was observed, followed by an increase in the terminally differentiated mature CD11bhiCD27lo NK subset. In addition, bone marrow Ki-67+ NK cells predominantly expressed CX3CR1 in the PP2Ai-treated mice and were further mobilized to the peripheral tissues. Among various target molecules of PP2A, we found that the upregulation of c-Myc pathway and its phosphorylation, along with its downstream cyclin E expression and G1/S cell cycle transition in PP2Ai-treated mice NK cells. Our results suggest that PP2Ai modulates NK cell proliferation through c-Myc and cyclin E, leading to their maturation and trafficking from the bone marrow to the peripheral tissues.

Natural Killer Cell Dysfunction In Human Bladder Cancer Is Caused By Tissue-Specific Suppression of SLAMF6 Signaling

NK cells are innate lymphocytes critical for surveillance of viruses and tumors, however the mechanisms underlying NK cell dysfunction in cancer are incompletely understood. We assessed the effector function of NK cells from bladder cancer patients and found severe dysfunction in NK cells derived from tumors versus peripheral blood. While both peripheral and tumor-infiltrating NK cells exhibited conserved patterns of inhibitory receptor over-expression, this did not explain the observed defects in NK surveillance in bladder tumors. Rather, TME-specific TGF-β and metabolic perturbations such as hypoxia directly suppressed NK cell function. Specifically, an oxygen-dependent reduction in signaling through SLAMF6 was mechanistically responsible for poor NK cell function, as tumor-infiltrating NK cells cultured ex vivo under normoxic conditions exhibited complete restoration of function, while deletion of SLAMF6 abrogated NK cell cytolytic function even under normoxic conditions. Collectively, this work highlights the role of tissue-specific factors in dictating NK cell function, and implicates SLAMF6 signaling as a rational target for immuno-modulation to improve NK cell function in bladder cancer.

Metabolic programming of organ-specific natural killer cell responses

Cells of the mammalian innate immune system have evolved to protect the host from various environmental or internal insults and injuries which perturb the homeostatic state of the organism. Among the lymphocytes of the innate immune system are natural killer (NK) cells, which circulate and survey host tissues for signs of stress, including infection or transformation. NK cells rapidly eliminate damaged cells in the blood or within tissues through secretion of cytolytic machinery and production of proinflammatory cytokines. To perform these effector functions while traversing between the blood and tissues, patrolling NK cells require sufficient fuel to meet their energetic demands. Here, we highlight the ability of NK cells to metabolically adapt across tissues, during times of nutrient deprivation and within tumor microenvironments. Whether at steady state, or during viral infection and cancer, NK cells readily shift their nutrient uptake and usage in order to maintain metabolism, survival, and function.

Recruitment and Maintenance of CX3CR1+CD4+ T Cells during Helminth Infection

Distinct subsets of T lymphocytes express CX3CR1 under inflammatory conditions, but little is known about CX3CR1+CD4+ T cells during type 2 inflammation in helminth infections. In this study, we used a fate-mapping mouse model to characterize CX3CR1+CD4+ T cells during both acute Nippostrongylus brasiliensis and chronic Schistosoma mansoni murine models of helminth infections, revealing CX3CR1+CD4+ T cells to be an activated tissue-homing subset with varying capacity for cytokine production. Tracking these cells over time revealed that maintenance of CX3CR1 itself along with a TH2 phenotype conferred a survival advantage in the inflamed tissue. Single-cell RNA sequencing analysis of fate-mapped CX3CR1+CD4+ T cells from both the peripheral tissue and the spleen revealed a considerable level of diversity and identified a distinct population of BCL6+TCF-1+PD1+CD4+ T cells in the spleen during helminth infections. Conditional deletion of BCL6 in CX3CR1+ cells resulted in fewer CX3CR1+CD4+ T cells during infection, indicating a role in sustaining CD4+ T cell responses to helminth infections. Overall, our studies revealed the behavior and heterogeneity of CX3CR1+CD4+ T cells during type 2 inflammation in helminth infections and identified BCL6 to be important in their maintenance.

Chimeric antigen receptor-based natural killer cell immunotherapy in cancer: from bench to bedside

Immunotherapy has rapidly evolved in the past decades in the battle against cancer. Chimeric antigen receptor (CAR)-engineered T cells have demonstrated significant success in certain hematologic malignancies, although they still face certain limitations, including high costs and toxic effects. Natural killer cells (NK cells), as a vital component of the immune system, serve as the "first responders" in the context of cancer development. In this literature review, we provide an updated understanding of NK cell development, functions, and their applications in disease therapy. Furthermore, we explore the rationale for utilizing engineered NK cell therapies, such as CAR-NK cells, and discuss the differences between CAR-T and CAR-NK cells. We also provide insights into the key elements and strategies involved in CAR design for engineered NK cells. In addition, we highlight the challenges currently encountered and discuss the future directions in NK cell research and utilization, including pre-clinical investigations and ongoing clinical trials. Based on the outstanding antitumor potential of NK cells, it is highly likely that they will lead to groundbreaking advancements in cancer treatment in the future.

Control of the Development, Distribution, and Function of Innate-Like Lymphocytes and Innate Lymphoid Cells by the Tissue Microenvironment

Recently, considerable attention has been directed toward innate-like T cells (ITCs) and innate lymphoid cells (ILCs) owing to their indispensable contributions to immune responses, tissue homeostasis, and inflammation. Innate-like T cells include NKT cells, MAIT cells, and γδ T cells, whereas ILCs include NK cells, type 1 ILCs (ILC1s), type 2 ILCs (ILC2s), and type 3 ILCs (ILC3s). Many of these ITCs and ILCs are distributed to specific tissues and remain tissue-resident, while others, such as NK cells and some γδ T cells, circulate through the bloodstream. Nevertheless, recent research has shed light on novel subsets of innate immune cells that exhibit characteristics intermediate between tissue-resident and circulating states under normal and pathological conditions. The local microenvironment frequently influences the development, distribution, and function of these innate immune cells. This review aims to consolidate the current knowledge on the functional heterogeneity of ITCs and ILCs, shaped by local environmental cues, with particular emphasis on IL-15, which governs the activities of the innate immune cells involved in type 1 immune responses.

Chronic chromosome instability induced by Plk1 results in immune suppression in breast cancer

Chromosome instability (CIN) contributes to resistance to therapies and tumor evolution. Although natural killer (NK) cells can eliminate cells with complex karyotypes, high-CIN human tumors have an immunosuppressive phenotype. To understand which CIN-associated molecular features alter immune recognition during tumor evolution, we overexpress Polo-like kinase 1 (Plk1) in a Her2+ breast cancer model. These high-CIN tumors activate a senescence-associated secretory phenotype (SASP), upregulate PD-L1 and CD206, and induce non-cell-autonomous nuclear factor κB (NF-κβ) signaling, facilitating immune evasion. Single-cell RNA sequencing from pre-neoplastic mammary glands unveiled the presence of Arg1+ macrophages, NK cells with reduced effector functions, and increased resting regulatory T cell infiltration. We further show that high PLK1-expressing human breast tumors display gene expression patterns associated with SASP, NF-κβ signaling, and immune suppression. These findings underscore the need to understand the immune landscape in CIN tumors to identify more effective therapies, potentially combining immune checkpoint or NF-κβ inhibitors with current treatments.

Exploiting innate immunity for cancer immunotherapy

Immunotherapies have revolutionized the treatment paradigms of various types of cancers. However, most of these immunomodulatory strategies focus on harnessing adaptive immunity, mainly by inhibiting immunosuppressive signaling with immune checkpoint blockade, or enhancing immunostimulatory signaling with bispecific T cell engager and chimeric antigen receptor (CAR)-T cell. Although these agents have already achieved great success, only a tiny percentage of patients could benefit from immunotherapies. Actually, immunotherapy efficacy is determined by multiple components in the tumor microenvironment beyond adaptive immunity. Cells from the innate arm of the immune system, such as macrophages, dendritic cells, myeloid-derived suppressor cells, neutrophils, natural killer cells, and unconventional T cells, also participate in cancer immune evasion and surveillance. Considering that the innate arm is the cornerstone of the antitumor immune response, utilizing innate immunity provides potential therapeutic options for cancer control. Up to now, strategies exploiting innate immunity, such as agonists of stimulator of interferon genes, CAR-macrophage or -natural killer cell therapies, metabolic regulators, and novel immune checkpoint blockade, have exhibited potent antitumor activities in preclinical and clinical studies. Here, we summarize the latest insights into the potential roles of innate cells in antitumor immunity and discuss the advances in innate arm-targeted therapeutic strategies.

Interplay of Chemokines Receptors, Toll-like Receptors, and Host Immunological Pathways

A comprehensive framework has been established for understanding immunological pathways, which can be categorized into eradicated and tolerable immune responses. Toll-like receptors (TLRs) are associated with specific immune responses. TH1 immunity is related to TLR7, TLR8, and TLR9, while TH2 immunity is associated with TLR1, TLR2, and TLR6. TH22 immunity is linked to TLR2, TLR4, and TLR5, and THαβ (Tr1) immunity is related to TLR3, TLR7, and TLR9. The chemokine receptor CXCR5 is a marker of follicular helper T cells, and other chemokine receptors can also be classified within a framework based on host immunological pathways. On the basis of a literature review on chemokines and immunological pathways, the following associations were identified: CCR5 with TH1 responses, CCR1 with TH1-like responses, CCR4 (basophils) and CCR3 (eosinophils) with TH2 and TH9 responses, CCR10 with TH22 responses, CCR6 with TH17 responses, CXCR3 with THαβ responses, CCR8 with regulatory T cells (Treg), and CCR2 with TH3 responses. These findings contribute to the identification of biomarkers for immune cells and provide insights into host immunological pathways. Understanding the chemokine and Toll-like receptor system is crucial for comprehending the function of the innate immune system, as well as adaptive immune responses.

Alzheimer's disease alters the transcriptomic profile of natural killer cells at single-cell resolution

Alzheimer's disease (AD) is the most common dementia without an effective cure at least partially due to incomplete understanding of the disease. Inflammation has emerged as a central player in the onset and progression of AD. As innate lymphoid cells, natural killer (NK) cells orchestrate the initiation and evolution of inflammatory responses. Yet, the transcriptomic features of NK cells in AD remain poorly understood. We assessed the diversity of NK cells using web-based single-cell RNA sequencing data of blood NK cells from patients with AD and control subjects and flow cytometry. We identified a contraction of NK cell compartment in AD, accompanied by a reduction of cytotoxicity. Unbiased clustering revealed four subsets of NK cells in AD, i.e., CD56bright NK cells, CD56dim effector NK cells, adaptive NK cells, and a unique NK cell subset that is expanded and characterized by upregulation of CX3CR1, TBX21, MYOM2, DUSP1, and ZFP36L2, and negatively correlated with cognitive function in AD patients. Pseudo-temporal analysis revealed that this unique NK cell subset was at a late stage of NK cell development and enriched with transcription factors TBX21, NFATC2, and SMAD3. Together, our study identified a distinct NK cell subset and its potential involvement in AD.

Natural killer cell homing and trafficking in tissues and tumors: from biology to application

Natural killer (NK) cells, a subgroup of innate lymphoid cells, act as the first line of defense against cancer. Although some evidence shows that NK cells can develop in secondary lymphoid tissues, NK cells develop mainly in the bone marrow (BM) and egress into the blood circulation when they mature. They then migrate to and settle down in peripheral tissues, though some special subsets home back into the BM or secondary lymphoid organs. Owing to its success in allogeneic adoptive transfer for cancer treatment and its "off-the-shelf" potential, NK cell-based immunotherapy is attracting increasing attention in the treatment of various cancers. However, insufficient infiltration of adoptively transferred NK cells limits clinical utility, especially for solid tumors. Expansion of NK cells or engineered chimeric antigen receptor (CAR) NK cells ex vivo prior to adoptive transfer by using various cytokines alters the profiles of chemokine receptors, which affects the infiltration of transferred NK cells into tumor tissue. Several factors control NK cell trafficking and homing, including cell-intrinsic factors (e.g., transcriptional factors), cell-extrinsic factors (e.g., integrins, selectins, chemokines and their corresponding receptors, signals induced by cytokines, sphingosine-1-phosphate (S1P), etc.), and the cellular microenvironment. Here, we summarize the profiles and mechanisms of NK cell homing and trafficking at steady state and during tumor development, aiming to improve NK cell-based cancer immunotherapy.

NK cell surveillance of hematological malignancies. Therapeutic implications and regulation by chemokine receptors

NK cells are circulating innate lymphoid cells that constantly move from bloodstream into tissues, exerting several functions including tumor surveillance. For this reason, NK cells are considered attractive target for cancer immunotherapy. Several strategies are employed to harness NK cell efficacy especially in hematological tumors, including adoptive transfer, genetic manipulation to overexpress chimeric antigen receptors and cytokine or immunomodulatory drug treatments of ex-vivo cultivated and expanded NK cells. Several chemokine receptors support NK cell tissue homing and are required for efficient tumor infiltration. Nevertheless, chemokine receptor expression is often insufficient, or their respective ligands may not be expressed in the tumor microenvironment, thus limiting NK cell localization at the tumor site. Therefore, strategies to implement expression or promote the function of the correct chemokine receptor/ligand axes have been employed in the last years with promising results in preclinical models. In this review, we discuss how chemokine receptors and their ligands regulate the trafficking and localization of NK cells in hematological tumors and how the chemokine function can be manipulated to improve current therapeutic approaches.

Chemokine networks modulating natural killer cell trafficking to solid tumors

Natural killer (NK) cell-based cell therapy has been emerging as a powerful weapon in the treatment of multiple malignancies. However, the inadequate infiltration of the therapeutic NK cells into solid tumors remains a big challenge to their clinical utility. Chemokine networks, which play essential roles in the migration of lymphocytes, have been recognized as critical in driving the intratumoral infiltration of NK cells via interactions between soluble chemokines and their receptors. Often, such interactions are complex and disease-specific. In the context of NK cells, chemokine receptors of note have included CCR2, CCR5, CCR7, CXCR3, and CX3CR1. The immunobiology of chemokine-receptor interactions has fueled the development of approaches that hope to improve the infiltration of NK cells into the microenvironment of solid tumors. Stimulation of NK cells ex vivo in the presence of various cytokines (such as IL-2, IL-15, and IL-21) and genetic engineering of NK cells have been utilized to alter the chemokine receptor profile and generate NK cells with higher infiltrating capacity. Additionally, the immune-suppressive tumor microenvironment has also been targeted, by introducing, either directly or indirectly, chemokine ligands which NK cells are able to respond to, ultimately creating a more hospitable niche for NK cell trafficking. Such strategies have promoted the infiltration and activity of infused NK cells into multiple solid tumors. In this review, we discuss how chemokine receptors and their ligands coordinate and how they can be manipulated to regulate the trafficking, distribution, and residence of NK cells in solid tumors.

Tbet promotes CXCR6 expression in immature natural killer cells and natural killer cell egress from the bone marrow

Tbet-deficient mice have reduced natural killer (NK) cells in blood and spleen, but increased NK cells in bone marrow and lymph nodes, a phenotype that is thought to be the result of defective migration. Here, we revisit the role of Tbet in NK cell bone marrow egress. We definitively show that the accumulation of NK cells in the bone marrow of Tbet-deficient Tbx21^-/- animals occurs because of a migration defect and identify a module of genes, co-ordinated by Tbet, which affects the localization of NK cells in the bone marrow. Cxcr6 is approximately 125-fold underexpressed in Tbx21^-/- , compared with wild-type, immature NK cells. Immature NK cells accumulate in the bone marrow of CXCR6-deficient mice, and CXCR6-deficient progenitors are less able to reconstitute the peripheral NK cell compartment than their wild-type counterparts. However, the CXCR6 phenotype is largely confined to immature NK cells, whereas the Tbet phenotype is present in both immature and mature NK cells, suggesting that genes identified as being more differentially expressed in mature NK cells, such as S1pr5, Cx3cr1, Sell and Cd69, may be the major drivers of the phenotype.

NK Cell Function Regulation by TGF-β-Induced Epigenetic Mechanisms

TGF-β is a potent immunosuppressive cytokine that severely affects the function of NK cells. Tumor cells can take advantage of this ability, enriching their surrounding microenvironment with TGF-β. TGF-β can alter the expression of effector molecules and of activating and chemokine receptors, influence metabolism, induce the NK cell conversion toward the less cytolytic ILC1s. These and other changes possibly occur by the induction of complex gene expression programs, involving epigenetic mechanisms. While most of these programs are at present unexplored, the role of certain transcription factors, microRNAs and chromatin changes determined by TGF-β in NK cells start to be elucidated in human and/or mouse NK cells. The deep understanding of these mechanisms will be useful to design therapies contributing to restore the full NK function.

Targeting of CXCR3 improves anti-myeloma efficacy of adoptively transferred activated natural killer cells

Background

The peculiar multiple myeloma microenvironment, characterized by up-regulated levels of several inflammatory chemokines, including the CXCR3 receptor ligands CXCL9 and CXCL10, limits NK cell positioning into the bone marrow by interfering with CXCR4 function. It is still unclear if the consequent reduced influx of transferred cells into the tumor represents a potential limiting factor for the success of NK cell-based adoptive therapy. We hypothesize that inhibition of CXCR3 function on NK cells will result in increased tumor clearance, due to higher NK cell bone marrow infiltration.

Methods

Since different activation protocols differently affect expression and function of homing receptors, we analyzed the bone marrow homing properties and anti-tumor efficacy of NK cells stimulated in vitro with two independent protocols. NK cells were purified from wild-type or Cxcr3^−/− mice and incubated with IL-15 alone or with a combination of IL-12, IL-15, IL-18 (IL-12/15/18). Alternatively, CXCR3 function was neutralized in vivo using a specific blocking antibody. NK cell functional behavior and tumor growth were analyzed in bone marrow samples by FACS analysis.

Results

Both activation protocols promoted degranulation and IFN-γ production by donor NK cells infiltrating the bone marrow of tumor-bearing mice, although IL-15 promoted a faster but more transient acquisition of functional capacities. In addition, IL-15-activated cells accumulated more in the bone marrow in a short time but showed lower persistence in vivo. Targeting of CXCR3 increased the bone marrow homing capacity of IL-15 but not IL12/15/18 activated NK cells. This effect correlated with a superior and durable myeloma clearance capacity of transferred cells in vivo.

Conclusions

Our results demonstrate that in vitro activation affects NK cell anti-myeloma activity in vivo by regulating their BM infiltration. Furthermore, we provided direct evidence that CXCR3 restrains NK cell anti-tumor capacity in vivo according to the activation protocol used, and that the effects of NK cell-based adoptive immunotherapy for multiple myeloma can be improved by increasing their bone marrow homing through CXCR3 inhibition.

Toxoplasma gondii infection drives conversion of NK cells into ILC1-like cells

Innate lymphoid cells (ILCs) were originally classified based on their cytokine profiles, placing natural killer (NK) cells and ILC1s together, but recent studies support their separation into different lineages at steady-state. However, tumors may induce NK cell conversion into ILC1-like cells that are limited to the tumor microenvironment and whether this conversion occurs beyond this environment remains unknown. Here, we describe Toxoplasma gondii infection converts NK cells into ILC1-like cells that are distinct from both steady-state NK cells and ILC1s in uninfected mice. These cells were Eomes-dependent, indicating that NK cells can give rise to Eomes^- Tbet-dependent ILC1-like cells that circulate widely and persist independent of ongoing infection. Moreover, these changes appear permanent, as supported by epigenetic analyses. Thus, these studies markedly expand current concepts of NK cells, ILCs, and their potential conversion.

Bone Marrow NK Cells: Origin, Distinctive Features, and Requirements for Tissue Localization

NK cell maturation is a continuous process, which initiates in the bone marrow and proceeds in peripheral tissues, where NK cells follow distinct differentiation routes. Drastic phenotypic changes are observed during progression from precursors to mature NK cells, including changes of expression and functionalities of several chemoattractant receptors. Upon differentiation, mature NK cells migrate outside the bone marrow; as well, peculiar subsets of NK cells can also home back to or localize in this anatomic compartment to play specific functions. In humans, NK cells with a tissue resident phenotype have been identified in bone marrow, sharing similarities with tissue resident memory CD8+ T cells; while in mouse, long-lived NK cells undergo homeostatic proliferation in this site during viral infections. The mechanisms underlying NK cell subset localization in the bone marrow have only recently started to be investigated, especially in pathological settings such as tumors or infections. In this review, we discuss the phenotype and function of NK cells as well as their requirements for bone marrow maintenance and/or homing.

Focused Ultrasound Improves NK-92MI Cells Infiltration Into Tumors

The efficiency of natural killer (NK) cells, adoptively transferred, for treatment against solid tumors is hindered by their difficulty to enter tumors from the blood circulation as well as their inability to prolong viability in the absence of IL-2. Among different sources of NK cells, we used genetically modified NK-92MI cells, a suitable candidate which can release IL-2 to maintain their viability and overcome undesirable side effects caused by systemic administration of exogenous IL-2. In this study, we evaluated whether the combination of focused ultrasound (FUS) and microbubbles can improve adoptively NK-92MI cell infiltration into ovarian tumors through biodistribution, immunofluorescence, and flow cytometry. The treatment effects of using this strategy twice a week were explored. The potential molecular mechanism of FUS assisting NK cell therapy was also initially explored through evaluating the expression of ICAM1 and CX3CL1 by qRT-PCR. Our results indicated that FUS and microbubbles can improve NK-92MI cells’ infiltration into tumors, and the combination of FUS and NK-92MI cells had a better treatment effect compared to the PBS group, but not compared to the NK-92MI group. The qRT-PCR results also showed that CX3CL1 may be involved in the process of FUS-assisted NK cell infiltration. These results indicate that further optimization of the FUS-assisted strategy is still needed to achieve therapeutic benefit.

Molecular Mechanisms Directing Migration and Retention of Natural Killer Cells in Human Tissues

A large body of data shows that Natural Killer (NK) cells are immune effectors exerting a potent cytolytic activity against tumors and virus infected cells. The discovery and characterization of several inhibitory and activating receptors unveiled most of the mechanisms allowing NK cells to spare healthy cells while selectively attacking abnormal tissues. Nevertheless, the mechanisms ruling NK cell subset recirculation among the different compartments of human body have only lately started to be investigated. This is particularly true for pathological settings such as tumors or infected tissues but also for para-physiological condition like pregnant human uterine mucosa. It is becoming evident that the microenvironment associated to a particular clinical condition can deeply influence the migratory capabilities of NK cells. In this review we describe the main mechanisms and stimuli known to regulate the expression of chemokine receptors and other molecules involved in NK cell homing to either normal or pathological/inflamed tissues, including tumors or organs such as lung and liver. We will also discuss the role played by the chemokine/chemokine receptor axes in the orchestration of physiological events such as NK cell differentiation, lymphoid organ retention/egress and recruitment to decidua during pregnancy.

Inflammation as a central mechanism in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by cognitive decline and the presence of two core pathologies, amyloid β plaques and neurofibrillary tangles. Over the last decade, the presence of a sustained immune response in the brain has emerged as a third core pathology in AD. The sustained activation of the brain's resident macrophages (microglia) and other immune cells has been demonstrated to exacerbate both amyloid and tau pathology and may serve as a link in the pathogenesis of the disorder. In the following review, we provide an overview of inflammation in AD and a detailed coverage of a number of microglia-related signaling mechanisms that have been implicated in AD. Additional information on microglia signaling and a number of cytokines in AD are also reviewed. We also review the potential connection of risk factors for AD and how they may be related to inflammatory mechanisms.

Lung-resident natural killer cells control pulmonary tumor growth in mice

Accumulating evidence indicates the importance of natural killer (NK) cells in controlling tumor growth and metastasis. NK cell subsets display diversities in their function and tissue distribution and Mac-1hi CD27lo NK cells are the predominant population of lung-resident NK cells. Although the lung is a major organ where primary tumor develops and cancer cells metastasize, there is no clear evidence whether circulating NK cells and/or tissue-resident NK cells control tumor growth in the lung. In the present study, we examined an antitumor function of lung-resident NK cells to control pulmonary tumor growth. In an orthotopic lung tumor model, NK cells controlled pulmonary tumor growth, and mature circulating NK cell subsets were increased in tumor-bearing lungs through a C-X-C motif chemokine receptor 3 (CXCR3)-dependent mechanism. Although such increase in migratory NK cell subsets can be blocked by anti-CXCR3 treatment, there was no difference in pulmonary tumor growth in anti-CXCR3-treated mice compared with control mice. In addition to pulmonary tumor growth, lung-resident NK cells, but not migratory NK cells, play a dominant role in controlling metastatic growth of cancer cells in lung. These results strongly indicate an importance of lung-resident NK cells for controlling pulmonary tumor growth.

Rapid reconstitution of NK1 cells after allogeneic transplantation is associated with a reduced incidence of graft-versus-host disease

The balance between immunostimulation and immunoregulation in T cell immunity is achieved by maintaining specific ratios of Th1, Th2, Th3 and Tr1 cells. Here, we investigate levels of type 1 (IFN-gamma; NK1), type 2 (IL-13; NK2), type 3 (TGF-beta; NK3) and regulatory (IL-10; NKr) cytokines in peripheral blood to assess the cytokine profiles of natural killer (NK) cells following human allogeneic hematopoietic stem cell transplantation (allo-HSCT). NK2 and NK3 cell expansion was observed after allo-HSCT; levels of NKr cells reached donor levels at day 15, though levels of NK1 cells were consistently lower than donor levels until day 60 after allo-HSCT. Multivariate analysis showed that a higher level of NK1 cells by day 15 was associated with a lower overall risk of acute graft-versus-host disease (GVHD) (HR 0.157, P=0.010) as well as II-IV acute GVHD (HR 0.260, P=0.059). Furthermore, higher levels of NK1 cells by day 15 were correlated with lower rates of cytomegalovirus (CMV) reactivation (HR 0.040, 0.005-0.348, P=0.003). These results indicate that rapid reconstitution of NK cells, especially NK1 cells, can help prevent the development of GVHD as well as CMV reactivation after allogeneic transplantation.

Tissue-specific Role of CX3CR1 Expressing Immune Cells and Their Relationships with Human Disease

Chemokine (C-X3-C motif) ligand 1 (CX₃CL1, also known as fractalkine) and its receptor chemokine (C-X3-C motif) receptor 1 (CX₃CR1) are widely expressed in immune cells and non-immune cells throughout organisms. However, their expression is mostly cell type-specific in each tissue. CX₃CR1 expression can be found in monocytes, macrophages, dendritic cells, T cells, and natural killer (NK) cells. Interaction between CX₃CL1 and CX₃CR1 can mediate chemotaxis of immune cells according to concentration gradient of ligands. CX₃CR1 expressing immune cells have a main role in either pro-inflammatory or anti-inflammatory response depending on environmental condition. In a given tissue such as bone marrow, brain, lung, liver, gut, and cancer, CX₃CR1 expressing cells can maintain tissue homeostasis. Under pathologic conditions, however, CX₃CR1 expressing cells can play a critical role in disease pathogenesis. Here, we discuss recent progresses of CX₃CL1/CX₃CR1 in major tissues and their relationships with human diseases.

TGF-β1 Downregulates the Expression of CX3CR1 by Inducing miR-27a-5p in Primary Human NK Cells

Activity of human natural killer (NK) cells against cancer cells is deeply suppressed by TGF-β1, an immunomodulatory cytokine that is released and activated in the tumor microenvironment. Moreover, our previous data showed that TGF-β1 modifies the chemokine receptor repertoire of NK cells. In particular, it decreases the expression of CX₃CR1 that drives these effectors toward peripheral tissues, including tumor sites. To identify possible mechanisms mediating chemokine receptors modulation, we analyzed the microRNA profile of TGF-β1-treated primary NK cells. The analysis pointed out miR-27a-5p as a possible modulator of CX₃CR1. We demonstrated the functional interaction of miR-27a-5p with the 3′ untranslated region (3′UTR) of CX₃CR1 mRNA by two different experimental approaches: by the use of a luciferase assay based on a reporter construct containing the CX₃CR1 3′UTR and by transfection of primary NK cells with a miR-27a-5p inhibitor. We also showed that the TGF-β1-mediated increase of miR-27a-5p expression is a consequence of miR-23a-27a-24-2 cluster induction. Moreover, we demonstrated that miR-27a-5p downregulates the surface expression of CX₃CR1. Finally, we showed that neuroblastoma cells induced in resting NK cells a downregulation of the CX₃CR1 expression that was paralleled by a significant increase of miR-27a-5p expression. Therefore, the present study highlights miR-27a-5p as a pivotal TGF-β1-induced regulator of CX₃CR1 expression.

Sex differences in microglial CX3CR1 signalling determine obesity susceptibility in mice

Female mice are less susceptible to the negative metabolic consequences of high-fat diet feeding than male mice, for reasons that are incompletely understood. Here we identify sex-specific differences in hypothalamic microglial activation via the CX3CL1-CX3CR1 pathway that mediate the resistance of female mice to diet-induced obesity. Female mice fed a high-fat diet maintain CX3CL1-CX3CR1 levels while male mice show reductions in both ligand and receptor expression. Female Cx3cr1 knockout mice develop 'male-like' hypothalamic microglial accumulation and activation, accompanied by a marked increase in their susceptibility to diet-induced obesity. Conversely, increasing brain CX3CL1 levels in male mice through central pharmacological administration or virally mediated hypothalamic overexpression converts them to a 'female-like' metabolic phenotype with reduced microglial activation and body-weight gain. These data implicate sex differences in microglial activation in the modulation of energy homeostasis and identify CX3CR1 signalling as a potential therapeutic target for the treatment of obesity.

GM-CSF Inhibits c-Kit and SCF Expression by Bone Marrow-Derived Dendritic Cells

Stem cell factor (SCF), the ligand of c-kit, is a key cytokine for hematopoiesis. Hematopoietic precursors express c-kit, whereas differentiated cells of hematopoietic lineage are negative for this receptor, with the exception of NK cells, mast cells, and a few others. While it has long been recognized that dendritic cells (DCs) can express c-kit, several questions remain concerning the SCF/c-kit axis in DCs. This is particularly relevant for DCs found in those organs wherein SCF is highly expressed, including the bone marrow (BM). We characterized c-kit expression by conventional DCs (cDCs) from BM and demonstrated a higher proportion of c-kit⁺ cells among type 1 cDC subsets (cDC1s) than type 2 cDC subsets (cDC2s) in both humans and mice, whereas similar levels of c-kit expression were observed in cDC1s and cDC2s from mouse spleen. To further study c-kit regulation, DCs were generated with granulocyte-macrophage colony-stimulating factor (GM-CSF) from mouse BM, a widely used protocol. CD11c⁺ cells were purified from pooled non-adherent and slightly adherent cells collected after 7 days of culture, thus obtaining highly purified BM-derived DCs (BMdDCs). BMdDCs contained a small fraction of c-kit⁺ cells, and by replating them for 2 days with GM-CSF, we obtained a homogeneous population of c-kit⁺ CD40^hi MHCII^hi cells. Not only did BMdDCs express c-kit but they also produced SCF, and both were striking upregulated if GM-CSF was omitted after replating. Furthermore, a small but significant reduction in BMdDC survival was observed upon SCF silencing. Incubation of BMdDCs with SCF did not modulate antigen presentation ability of these cells, nor it did regulate their membrane expression of the chemokine receptor CXCR4. We conclude that the SCF/c-kit-mediated prosurvival circuit may have been overlooked because of the prominent use of GM-CSF in DC cultures in vitro, including those human DC cultures destined for the clinics. We speculate that DCs more prominently rely on SCF in vivo in some microenvironments, with potential implications for graft-versus-host disease and antitumor immunity.

CXCR3/CXCL10 Axis Regulates Neutrophil-NK Cell Cross-Talk Determining the Severity of Experimental Osteoarthritis

Several immune cell populations are involved in cartilage damage, bone erosion, and resorption processes during osteoarthritis. The purpose of this study was to investigate the role of NK cells in the pathogenesis of experimental osteoarthritis and whether and how neutrophils can regulate their synovial localization in the disease. Experimental osteoarthritis was elicited by intra-articular injection of collagenase in wild type and Cxcr3^-/- 8-wk old mice. To follow osteoarthritis progression, cartilage damage, synovial thickening, and osteophyte formation were measured histologically. To characterize the inflammatory cells involved in osteoarthritis, synovial fluid was collected early after disease induction, and the cellular and cytokine content were quantified by flow cytometry and ELISA, respectively. We found that NK cells and neutrophils are among the first cells that accumulate in the synovium during osteoarthritis, both exerting a pathogenic role. Moreover, we uncovered a crucial role of the CXCL10/CXCR3 axis, with CXCL10 increasing in synovial fluids after injury and Cxcr3^-/- mice being protected from disease development. Finally, in vivo depletion experiments showed that neutrophils are involved in an NK cell increase in the synovium, possibly by expressing CXCL10 in inflamed joints. Thus, neutrophils and NK cells act as important disease-promoting immune cells in experimental osteoarthritis and their functional interaction is promoted by the CXCL10/CXCR3 axis.

Dysregulation of Chemokine/Chemokine Receptor Axes and NK Cell Tissue Localization during Diseases

Chemokines are small chemotactic molecules that play key roles in physiological and pathological conditions. Upon signaling via their specific receptors, chemokines regulate tissue mobilization and trafficking of a wide array of immune cells, including natural killer (NK) cells. Current research is focused on analyzing changes in chemokine/chemokine receptor expression during various diseases to interfere with pathological trafficking of cells or to recruit selected cell types to specific tissues. NK cells are a heterogeneous lymphocyte population comprising several subsets endowed with distinct functional properties and mainly representing distinct stages of a linear development process. Because of their different functional potential, the type of subset that accumulates in a tissue drives the final outcome of NK cell-regulated immune response, leading to either protection or pathology. Correspondingly, chemokine receptors, including CXCR4, CXCR3, and CX₃CR1, are differentially expressed by NK cell subsets, and their expression levels can be modulated during NK cell activation. At first, this review will summarize the current knowledge on the contribution of chemokines to the localization and generation of NK cell subsets in homeostasis. How an inappropriate chemotactic response can lead to pathology and how chemokine targeting can therapeutically affect tissue recruitment/localization of distinct NK cell subsets will also be discussed.

Natural killer cell recognition of in vivo drug-induced senescent multiple myeloma cells

Recognition of tumor cells by the immune system is a key step in cancer eradication. Melphalan is an alkylating agent routinely used in the treatment of patients with multiple myeloma (MM), but at therapeutic doses it leads to an immunosuppressive state due to lymphopenia. Here, we used a mouse model of MM to investigate the ability of in vivo treatment with low doses of melphalan to modulate natural killer (NK) cell activity, which have been shown to play a major role in the control of MM growth. Melphalan treatment was able to enhance the surface expression of the stress-induced NKG2D ligands RAE-1 and MULT-1, and of the DNAM-1 ligand PVR (CD155) on MM cells, leading to better tumor cell recognition and killing by NK cells, as highlighted by NK cell increased degranulation triggered by melphalan-treated tumor cells. Remarkably, NK cell population was not affected by the melphalan dose used, but rather displayed activation features as indicated by CD107a and CD69 expression. Furthermore, we showed that low doses of melphalan fail to induce tumor cell apoptosis, but promote the in vivo establishment of a senescent tumor cell population, harboring high levels of the stress-induced ligands RAE-1 and PVR. Taken together our data support the concept of using chemotherapy in order to boost antitumor innate immune responses and report the possibility to induce cellular senescence of tumor cells in vivo.

CX3CR1-dependent recruitment of mature NK cells into the central nervous system contributes to control autoimmune neuroinflammation

Fractalkine receptor (CX3CR1)-deficient mice develop very severe experimental autoimmune encephalomyelitis (EAE), associated with impaired NK cell recruitment into the CNS. Yet, the precise implications of NK cells in autoimmune neuroinflammation remain elusive. Here, we investigated the pattern of NK cell mobilization and the contribution of CX3CR1 to NK cell dynamics in the EAE. We show that in both wild-type and CX3CR1-deficient EAE mice, NK cells are mobilized from the periphery and accumulate in the inflamed CNS. However, in CX3CR1-deficient mice, the infiltrated NK cells displayed an immature phenotype contrasting with the mature infiltrates in WT mice. This shift in the immature/mature CNS ratio contributes to EAE exacerbation in CX3CR1-deficient mice, since transfer of mature WT NK cells prior to immunization exerted a protective effect and normalized the CNS NK cell ratio. Moreover, mature CD11b(+) NK cells show higher degranulation in the presence of autoreactive 2D2 transgenic CD4(+) T cells and kill these autoreactive cells more efficiently than the immature CD11b(-) fraction. Together, these data suggest a protective role of mature NK cells in EAE, possibly through direct modulation of T cells inside the CNS, and demonstrate that mature and immature NK cells are recruited into the CNS by distinct chemotactic signals.

Multiple Myeloma Impairs Bone Marrow Localization of Effector Natural Killer Cells by Altering the Chemokine Microenvironment

Natural killer (NK) cells are key innate immune effectors against multiple myeloma, their activity declining in multiple myeloma patients with disease progression. To identify the mechanisms underlying NK cell functional impairment, we characterized the distribution of functionally distinct NK cell subsets in the bone marrow of multiple myeloma-bearing mice. Herein we report that the number of KLRG1(-) NK cells endowed with potent effector function rapidly and selectively decreases in bone marrow during multiple myeloma growth, this correlating with decreased bone marrow NK cell degranulation in vivo. Altered NK cell subset distribution was dependent on skewed chemokine/chemokine receptor axes in the multiple myeloma microenvironment, with rapid downmodulation of the chemokine receptor CXCR3 on NK cells, increased CXCL9 and CXCL10, and decreased CXCL12 expression in bone marrow. Similar alterations in chemokine receptor/chemokine axes were observed in patients with multiple myeloma. Adoptive transfer experiments demonstrated that KLRG1(-) NK cell migration to the bone marrow was more efficient in healthy than multiple myeloma-bearing mice. Furthermore, bone marrow localization of transferred CXCR3-deficient NK cells with respect to wild type was enhanced in healthy and multiple myeloma-bearing mice, suggesting that CXCR3 restrains bone marrow NK cell trafficking. Our results indicate that multiple myeloma-promoted CXCR3 ligand upregulation together with CXCL12 downmodulation act as exit signals driving effector NK cells outside the bone marrow, thus weakening the antitumor immune response at the primary site of tumor growth.

PTEN regulates natural killer cell trafficking in vivo

Phosphatase and tensin homolog (PTEN) is a critical negative regulator of the phosphoinositide-3 kinase pathway, members of which play integral roles in natural killer (NK) cell development and function. However, the functions of PTEN in NK cell biology remain unknown. Here, we used an NK cell-specific PTEN-deletion mouse model to define the ramifications of intrinsic NK cell PTEN loss in vivo. In these mice, there was a significant defect in NK cell numbers in the bone marrow and peripheral organs despite increased proliferation and intact peripheral NK cell maturation. Unexpectedly, we observed a significant expansion of peripheral blood NK cells and the premature egress of NK cells from the bone marrow. The altered trafficking of NK cells from peripheral organs into the blood was due to selective hyperresponsiveness to the blood localizing chemokine S1P. To address the importance of this trafficking defect to NK cell immune responses, we investigated the ability of PTEN-deficient NK cells to traffic to a site of tumor challenge. PTEN-deficient NK cells were defective at migrating to distal tumor sites but were more effective at clearing tumors actively introduced into the peripheral blood. Collectively, these data identify PTEN as an essential regulator of NK cell localization in vivo during both homeostasis and malignancy.

IFN-γ induces aberrant CD49b⁺ NK cell recruitment through regulating CX3CL1: a novel mechanism by which IFN-γ provokes pregnancy failure

Interferon-γ (IFN-γ), a pleiotropic lymphokine, has important regulatory effects on many cell types. Although IFN-γ is essential for the initiation of uterine vascular modifications and maintenance of decidual integrity, IFN-γ administration can also cause pregnancy failure in many species. However, little is known about the effector mechanisms involved. In this study, using an IFN-γ-induced abortion mouse model, we reported that no Dolichos biflorus agglutinin lectin-positive uterine natural killer (uNK) cells were observed in the uteri from IFN-γ-induced abortion mice. By contrast, the percentage of CD3⁻CD49b⁺ NK cells in the uterus and blood from a foetal resorption group was significantly higher than that of the control group. Similarly, significantly upregulated expression of CD49b (a pan-NK cell marker), CX3CL1 and CX3CR1 (CX3CL1 receptor) was detected in the uteri of IFN-γ-induced abortion mice. Using isolated uterine stromal cells, we showed that upregulated expression of CX3CL1 by IFN-γ was dependent on a Janus family kinase 2-signal transducers and activators of transcription 1 (JAK2-STAT1) pathway. We further demonstrated the chemotactic activity of CX3CL1 in uterine stromal cell conditioned medium on primary splenic NK cells. Finally, we observed increased recruitment of CD49b⁺ NK cells into the endometrium after exogenous CX3CL1 administration. Collectively, our findings indicate that IFN-γ can significantly increase uterine CX3CL1 expression via activation of the JAK2-STAT1 pathway, thus inducing CD49b⁺ NK cell uterine homing, and eventually provoke foetal loss. Thus, we provide a new line of evidence correlating the deleterious effects of IFN-γ on pregnancy with the aberrant regulation of CX3CL1 and CD49b⁺ NK cells.

Multiple levels of chemokine receptor regulation in the control of mouse natural killer cell development

Chemokines play a fundamental role in lymphocyte development, mainly attributable to the control of the correct localization in the proper microenvironments of cells undergoing maturation. Natural killer (NK) cell development occurs in the bone marrow (BM) where their localization is regulated by the balance of chemokine function in cell retention into tissues and mobilization into circulation. In addition, NK cells from several extra-medullary tissues are phenotypically and functionally different from their circulating counterpart suggesting that maturation can be completed in organs other than BM. Indeed, a role of chemokines in NK cell localization into tissues during homeostatic conditions is also documented. In this review, we summarize the current notion related to the relevance of several chemokine/chemokine receptor axes in NK cell development with a focus on the regulation of their expression and function.