CD27 defines phenotypically and functionally different human NK cell subsets

The immunobiology of CD27 and OX40 and their potential as targets for cancer immunotherapy

In recent years, monoclonal antibodies (mAbs) able to reinvigorate antitumor T-cell immunity have heralded a paradigm shift in cancer treatment. The most high profile of these mAbs block the inhibitory checkpoint receptors PD-1 and CTLA-4 and have improved life expectancy for patients across a range of tumor types. However, it is becoming increasingly clear that failure of some patients to respond to checkpoint inhibition is attributable to inadequate T-cell priming. For full T-cell activation, 2 signals must be received, and ligands providing the second of these signals, termed costimulation, are often lacking in tumors. Members of the TNF receptor superfamily (TNFRSF) are key costimulators of T cells during infection, and there has been an increasing interest in harnessing these receptors to augment tumor immunity. We here review the immunobiology of 2 particularly promising TNFRSF target receptors, CD27 and OX40, and their respective ligands, CD70 and OX40L, focusing on their role within a tumor setting. We describe the influence of CD27 and OX40 on human T cells based on in vitro studies and on the phenotypes of several recently described individuals exhibiting natural deficiencies in CD27/CD70 and OX40. Finally, we review key literature describing progress in elucidating the efficacy and mode of action of OX40- and CD27-targeting mAbs in preclinical models and provide an overview of current clinical trials targeting these promising receptor/ligand pairings in cancer.

T-cell differentiation and CD56+ levels in polypoidal choroidal vasculopathy and neovascular age-related macular degeneration

Polypoidal choroidal vasculopathy (PCV) and neovascular age-related macular degeneration (AMD) are prevalent age-related diseases characterized by exudative changes in the macula. Although they share anatomical and clinical similarities, they are also distinctly characterized by their own features, e.g. vascular abnormalities in PCV and drusen-mediated progression in neovascular AMD. PCV remains etiologically uncharacterized, and ongoing discussion is whether PCV and neovascular AMD share the same etiology or constitute two substantially different diseases. In this study, we investigated T-cell differentiation and aging profile in human patients with PCV, patients with neovascular AMD, and age-matched healthy control individuals. Fresh venous blood was prepared for flow cytometry to investigate CD4⁺ and CD8⁺ T-cell differentiation (naïve, central memory, effector memory, effector memory CD45ra⁺), loss of differentiation markers CD27 and CD28, and expression of aging marker CD56. Patients with PCV were similar to the healthy controls in all aspects. In patients with neovascular AMD we found significantly accelerated T-cell differentiation (more CD28^-CD27^- cells) and aging (more CD56⁺ cells) in the CD8⁺ T-cell compartment. These findings suggest that PCV and neovascular AMD are etiologically different in terms of T cell immunity, and that neovascular AMD is associated with T-cell immunosenescence.

Emerging targets in cancer immunotherapy

The first generation of immune checkpoint inhibitors (anti-CTLA-4 and anti-PD-1/PD-L1) targeted natural immune homeostasis pathways, co-opted by cancers, to drive anti-tumor immune responses. These agents led to unprecedented results in patients with previously incurable metastatic disease and may become first-line therapies for some advanced cancers. However, these agents are efficacious in only a minority of patients. Newer strategies are becoming available that target additional immunomodulatory mechanisms to activate patients' own anti-tumor immune responses. Herein, we present a succinct summary of emerging immune targets with reported pre-clinical efficacy that have progressed to active investigation in clinical trials. These emerging targets include co-inhibitory and co-stimulatory markers of the innate and adaptive immune system. In this review, we discuss: 1) T lymphocyte markers: Lymphocyte Activation Gene 3 [LAG-3], T-cell Immunoglobulin- and Mucin-domain-containing molecule 3 [TIM-3], V-domain containing Ig Suppressor of T cell Activation [VISTA], T cell ImmunoGlobulin and ITIM domain [TIGIT], B7-H3, Inducible T-cell Co-stimulator [ICOS/ICOS-L], CD27/CD70, and Glucocorticoid-Induced TNF Receptor [GITR]; 2) macrophage markers: CD47/Signal-Regulatory Protein alpha [SIRPα] and Indoleamine-2,3-Dioxygenase [IDO]; and 3) natural killer cell markers: CD94/NKG2A and the Killer Immunoglobulin-like receptor [KIR] family. Finally, we briefly highlight combination strategies and potential biomarkers of response and resistance to these cancer immunotherapies.

IL15 Infusion of Cancer Patients Expands the Subpopulation of Cytotoxic CD56bright NK Cells and Increases NK-Cell Cytokine Release Capabilities

The cytokine IL15 is required for survival and activation of natural killer (NK) cells as well as expansion of NK-cell populations. Here, we compare the effects of continuous IL15 infusions on NK-cell subpopulations in cancer patients. Infusions affected the CD56^bright NK-cell subpopulation in that the expansion rates exceeded those of CD56^dim NK-cell populations with a 350-fold increase in their total cell numbers compared with 20-fold expansion for the CD56^dim subset. CD56^bright NK cells responded with increased cytokine release to various stimuli, as expected given their immunoregulatory functions. Moreover, CD56^bright NK cells gained the ability to kill various target cells at levels that are typical for CD56^dim NK cells. Some increased cytotoxic activities were also observed for CD56^dim NK cells. IL15 infusions induced expression changes on the surface of both NK-cell subsets, resulting in a previously undescribed and similar phenotype. These data suggest that IL15 infusions expand and arm CD56^bright NK cells that alone or in combination with tumor-targeting antibodies may be useful in the treatment of cancer. Cancer Immunol Res; 5(10); 929-38. ©2017 AACR.

Acquired Senescent T-Cell Phenotype Correlates with Clinical Severity in GATA Binding Protein 2-Deficient Patients

GATA binding protein 2 (GATA2) deficiency is a rare disorder of hematopoiesis, lymphatics, and immunity caused by spontaneous or autosomal dominant mutations in the GATA2 gene. Clinical manifestations range from neutropenia, lymphedema, deafness, to severe viral and mycobacterial infections, bone marrow failure, and acute myeloid leukemia. Patients also present with monocytopenia, dendritic cell, B- and natural killer (NK)-cell deficiency. We studied the T-cell and NK-cell compartments of four GATA2-deficient patients to assess if changes in these lymphocyte populations could be correlated with clinical phenotype. Patients with more severe clinical complications demonstrated a senescent T-cell phenotype whereas patients with lower clinical score had undetectable changes relative to controls. In contrast, patients’ NK-cells demonstrated an immature/activated phenotype that did not correlate with clinical score, suggesting an intrinsic NK-cell defect. These studies will help us to determine the contribution of T- and NK-cell dysregulation to the clinical phenotype of GATA2 patients, and may help to establish the most accurate therapeutic options for these patients. Asymptomatic patients may be taken into consideration for hematopoietic stem cell transplantation when dysregulation of T-cell and NK-cell compartment is present.

NK cell dysfunction in chronic lymphocytic leukemia is associated with loss of the mature cells expressing inhibitory killer cell Ig-like receptors

A prospective analysis of natural killer (NK) cell phenotype and function was performed on fresh peripheral blood samples from untreated patients with B-cell chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL). Compared to healthy controls, CD56^dim NK cells in CLL patients displayed reduced expression of the NKG2D activating receptor and increased CD27 expression, which indicates declines in mature cells. In addition, NK cells from CLL patients showed reduced degranulation responses toward transformed B cells alone or with rituximab and were more sensitive to activation-induced cell death. We further noted a striking reduction in the frequency and viability of NK cells expressing the inhibitory killer cell Ig-like receptors (KIR)2DL1 and/or KIR3DL1, which progressed over time in most patients. Comparisons between a CLL patient and healthy monozygotic twin were consistent with our results in the larger cohorts. Functional and biomarker alterations were less pronounced on NK cells from SLL patients, which have lower tumor burden in peripheral blood than CLL, but significant reduction in degranulation under ADCC conditions and lower frequency and viability of KIR-expressing NK cells were still evident in SLL. We conclude that mature KIR-expressing NK cells respond to the high circulating B cell tumor burden in CLL, but undergo activation-induced apoptosis. Consequently, CLL patients may benefit from therapies that augment NK cell survival and function.

Stimulating CD27 to quantitatively and qualitatively shape adaptive immunity to cancer

The capacity of the immune system to recognize and respond to tumors has been appreciated for over 100 years. However, clinical success has largely depended on the elucidation of the positive and negative regulators of effector cells after their activation via the antigen cell receptor. On the one hand, effector cells upregulate checkpoint molecules that are thought to play a role in limiting immunopathology. On the other, second and third waves of costimulation are often required to promote the expansion, survival and differentiation of effector cells. While it is clear that the immune system can be unleashed by blocking checkpoint molecules, this approach is most effective when pre-existing responses exist in patients' tumors. Thus, coordinating checkpoint blockade with costimulation could potentially expand the patient population that receives benefit from cancer immunotherapy. This review will discuss how the costimulatory molecule CD27 sculpts immunity and preclinical/clinical data indicating its potential for cancer immunotherapy and its clinical translation.

Circulating NK cells and their subsets in Behçet's disease

Behçet's disease (BD) is an autoinflammatory, chronic relapsing/remitting disease of unknown aetiology with both innate and acquired immune cells implicated in disease pathogenesis. Peripheral blood natural killer (NK) cells and their CD56^Dim /CD56^Bright subsets were surface phenotyped using CD27 and CD16 surface markers in 60 BD patients compared to 60 healthy controls (HCs). Functional potential was assessed by production of interferon (IFN)-γ, granzyme B, perforin and the expression of degranulation marker CD107a. The effects of disease activity (BD^Active versus BD^Quiet ) and BD medication on NK cells were also investigated. Peripheral blood NK cells (P < 0·0001) and their constituent CD56^Dim (P < 0·0001) and CD56^Bright (P = 0·0015) subsets were depleted significantly in BD patients compared to HCs, and especially in those with active disease (BD^Active ) (P < 0·0001). BD patients taking azathioprine also had significantly depleted NK cells compared to HCs (P < 0·0001). A stepwise multivariate linear regression model confirmed BD activity and azathioprine therapy as significant independent predictor variables of peripheral blood NK percentage (P < 0·001). In general, CD56^Dim cells produced more perforin (P < 0·0001) and granzyme B (P < 0·01) expressed higher CD16 levels (P < 0·0001) compared to CD56^Bright cells, confirming their increased cytotoxic potential with overall higher NK cell CD107a expression in BD compared to HCs (P < 0·01). Interestingly, IFN-γ production and CD27 expression were not significantly different between CD56^Dim /CD56^Bright subsets. In conclusion, both BD activity and azathioprine therapy have significant independent depletive effects on the peripheral blood NK cell compartment.

Human immunity against EBV-lessons from the clinic

The mammalian immune system has evolved over many millennia to be best equipped to protect the host from pathogen infection. In many cases, host and pathogen have coevolved, each acquiring sophisticated ways of inducing or protecting from disease. Epstein-Barr virus (EBV) is a human herpes virus that infects >90% of individuals. Despite its ubiquity, infection by EBV is often subclinical; this invariably reflects the necessity of the virus to preserve its host, balanced with sophisticated host immune mechanisms that maintain viral latency. However, EBV infection can result in various, and often fatal, clinical sequelae, including fulminant infectious mononucleosis, hemophagocytic lymphohistiocytosis, lymphoproliferative disease, organomegaly, and/or malignancy. Such clinical outcomes are typically observed in immunosuppressed individuals, with the most extreme cases being Mendelian primary immunodeficiencies (PIDs). Although these conditions are rare, they have provided critical insight into the cellular, biochemical, and molecular requirements for robust and long-lasting immunity against EBV infection. Here, we review the virology of EBV, mechanisms underlying disease pathogenesis in PIDs, and developments in immune cell–mediated therapy to treat disorders associated with or induced by EBV infection.

The role of CD27 in anti-viral T-cell immunity

CD27 is a co-stimulatory immune-checkpoint receptor, constitutively expressed on a broad range of T-cells (αβ and γδ), NK-cells and B-cells. Ligation of CD27 with CD70 results in potent co-stimulatory effects. In mice, co-stimulation of CD8⁺ T-cells through CD27 promotes immune activation and enhances primary, secondary, memory and recall responses towards viral infections. Limited in vitro human studies support mouse experiments and show that CD27 co-stimulation enhances antiviral T-cell immunity. Given the potent co-stimulatory effects of CD27, manipulating CD27 signalling is of interest for viral, autoimmune and anti-tumour immunotherapies. This review focuses on the role of CD27 co-stimulation in anti-viral T-cell immunity and discusses clinical studies utilising the CD27 co-stimulation pathway for anti-viral, anti-tumour and autoimmune immunotherapy.

Monocyte/Macrophage: NK Cell Cooperation-Old Tools for New Functions

Monocyte/macrophage and natural killer (NK) cells are partners from a phylogenetic standpoint of innate immune system development and its evolutionary progressive interaction with adaptive immunity. The equally conservative ways of development and differentiation of both invertebrate hemocytes and vertebrate macrophages are reviewed. Evolutionary conserved molecules occurring in macrophage receptors and effectors have been inherited by vertebrates after their common ancestor with invertebrates. Cytolytic functions of mammalian NK cells, which are rooted in immune cells of invertebrates, although certain NK cell receptors (NKRs) are mammalian new events, are characterized. Broad heterogeneity of macrophage and NK cell phenotypes that depends on surrounding microenvironment conditions and expression profiles of specific receptors and activation mechanisms of both cell types are discussed. The particular tissue specificity of macrophages and NK cells, as well as their plasticity and mechanisms of their polarization to different functional subtypes have been underlined. The chapter summarized studies revealing the specific molecular mechanisms and regulation of NK cells and macrophages that enable their highly specific cross-cooperation. Attention is given to the evolving role of human monocyte/macrophage and NK cell interaction in pathogenesis of hypersensitivity reaction-based disorders, including autoimmunity, as well as in cancer surveillance and progression.

Combined immunodeficiency and Epstein-Barr virus-induced B cell malignancy in humans with inherited CD70 deficiency

Abolhassani et al. show that CD70 deficiency is a novel cause of combined immunodeficiency and EBV-associated diseases, reminiscent of CD27 deficiency. CD70–CD27 interactions play a nonredundant role regulating humoral- and cell-mediated immunity in humans, especially for control of EBV.

In this study, we describe four patients from two unrelated families of different ethnicities with a primary immunodeficiency, predominantly manifesting as susceptibility to Epstein-Barr virus (EBV)–related diseases. Three patients presented with EBV-associated Hodgkin’s lymphoma and hypogammaglobulinemia; one also had severe varicella infection. The fourth had viral encephalitis during infancy. Homozygous frameshift or in-frame deletions in CD70 in these patients abolished either CD70 surface expression or binding to its cognate receptor CD27. Blood lymphocyte numbers were normal, but the proportions of memory B cells and EBV-specific effector memory CD8⁺ T cells were reduced. Furthermore, although T cell proliferation was normal, in vitro–generated EBV-specific cytotoxic T cell activity was reduced because of CD70 deficiency. This reflected impaired activation by, rather than effects during killing of, EBV-transformed B cells. Notably, expression of 2B4 and NKG2D, receptors implicated in controlling EBV infection, on memory CD8⁺ T cells from CD70-deficient individuals was reduced, consistent with their impaired killing of EBV-infected cells. Thus, autosomal recessive CD70 deficiency is a novel cause of combined immunodeficiency and EBV-associated diseases, reminiscent of inherited CD27 deficiency. Overall, human CD70–CD27 interactions therefore play a nonredundant role in T and B cell–mediated immunity, especially for protection against EBV and humoral immunity.

The Ebola Interferon Inhibiting Domains Attenuate and Dysregulate Cell-Mediated Immune Responses

Ebola virus (EBOV) infections are characterized by deficient T-lymphocyte responses, T-lymphocyte apoptosis and lymphopenia. We previously showed that disabling of interferon-inhibiting domains (IIDs) in the VP24 and VP35 proteins effectively unblocks maturation of dendritic cells (DCs) and increases the secretion of cytokines and chemokines. Here, we investigated the role of IIDs in adaptive and innate cell-mediated responses using recombinant viruses carrying point mutations, which disabled IIDs in VP24 (EBOV/VP24m), VP35 (EBOV/VP35m) or both (EBOV/VP35m/VP24m). Peripheral blood mononuclear cells (PBMCs) from cytomegalovirus (CMV)-seropositive donors were inoculated with the panel of viruses and stimulated with CMV pp65 peptides. Disabling of the VP35 IID resulted in increased proliferation and higher percentages of CD4+ T cells secreting IFNγ and/or TNFα. To address the role of aberrant DC maturation in the IID-mediated suppression of T cell responses, CMV-stimulated DCs were infected with the panel of viruses and co-cultured with autologous T-lymphocytes. Infection with EBOV/VP35m infection resulted in a significant increase, as compared to wt EBOV, in proliferating CD4+ cells secreting IFNγ, TNFα and IL-2. Experiments with expanded CMV-specific T cells demonstrated their increased activation following co-cultivation with CMV-pulsed DCs pre-infected with EBOV/VP24m, EBOV/VP35m and EBOV/VP35m/VP24m, as compared to wt EBOV. Both IIDs were found to block phosphorylation of TCR complex-associated adaptors and downstream signaling molecules. Next, we examined the effects of IIDs on the function of B cells in infected PBMC. Infection with EBOV/VP35m and EBOV/VP35m/VP24m resulted in significant increases in the percentages of phenotypically distinct B-cell subsets and plasma cells, as compared to wt EBOV, suggesting inhibition of B cell function and differentiation by VP35 IID. Finally, infection with EBOV/VP35m increased activation of NK cells, as compared to wt EBOV. These results demonstrate a global suppression of cell-mediated responses by EBOV IIDs and identify the role of DCs in suppression of T-cell responses.

CD27 natural killer cell subsets play different roles during the pre-onset stage of experimental autoimmune encephalomyelitis

NK cells participate in the development of human multiple sclerosis (MS) and mouse experimental autoimmune encephalomyelitis (EAE), but the roles of different NK cell subsets in disease onset remain poorly understood. In this study, murine NK cells were divided into CD27high and CD27low/− subsets. The CD27high subset was decreased and the CD27low/− subset was increased in lymphoid organs during the pre-onset stage of EAE. Compared with the counterpart in naïve mice, the CD27high subset showed lower expression of Ly49D, Ly49H and NKG2D, and less production of IFN-γ, whereas the CD27low/− subset showed similar expression of the above mentioned surface receptors but higher cytotoxic activity in EAE mice. Compared with the CD27high subset, the CD27low/− subset exhibited increased promotion of DC maturation and no significant inhibition of T cells proliferation and Th17 cells differentiation in vitro. Additionally, adoptive transfer of the CD27low/− subset, but not the CD27high subset, exacerbated the severity of EAE. Collectively, our data suggest the CD27 NK cell subsets play different roles in controlling EAE onset, which provide a new understanding for the regulation of NK cell subsets in early autoimmune disease.

Human Circulating and Tissue-Resident CD56(bright) Natural Killer Cell Populations

Two human natural killer (NK) cell subsets are usually distinguished, displaying the CD56^dimCD16⁺ and the CD56^brightCD16^−/+ phenotype. This distinction is based on NK cells present in blood, where the CD56^dim NK cells predominate. However, CD56^bright NK cells outnumber CD56^dim NK cells in the human body due to the fact that they are predominant in peripheral and lymphoid tissues. Interestingly, within the total CD56^bright NK cell compartment, a major phenotypical and functional diversity is observed, as demonstrated by the discovery of tissue-resident CD56^bright NK cells in the uterus, liver, and lymphoid tissues. Uterus-resident CD56^bright NK cells express CD49a while the liver- and lymphoid tissue-resident CD56^bright NK cells are characterized by co-expression of CD69 and CXCR6. Tissue-resident CD56^bright NK cells have a low natural cytotoxicity and produce little interferon-γ upon monokine stimulation. Their distribution and specific phenotype suggest that the tissue-resident CD56^bright NK cells exert tissue-specific functions. In this review, we examine the CD56^bright NK cell diversity by discussing the distribution, phenotype, and function of circulating and tissue-resident CD56^bright NK cells. In addition, we address the ongoing debate concerning the developmental relationship between circulating CD56^bright and CD56^dim NK cells and speculate on the position of tissue-resident CD56^bright NK cells. We conclude that distinguishing tissue-resident CD56^bright NK cells from circulating CD56^bright NK cells is a prerequisite for the better understanding of the specific role of CD56^bright NK cells in the complex process of human immune regulation.

Specific phenotype and function of CD56-expressing innate immune cell subsets in human thymus

Whereas innate immune cells, such as NK and innate lymphoid cells (ILCs), have been characterized in different human tissues, knowledge on the thymic CD56-expressing cell subsets is limited. In this study, the rare subpopulations of thymic CD56⁺CD3^- cells from samples of >100 patients have been successfully analyzed. The results revealed fundamental differences between thymic and peripheral blood (PB) CD56⁺CD3^- cells. Thymic tissues lacked immunoregulatory CD56^highCD16^dim NK cells but showed two Eomes⁺CD56^dim subsets on which common NK cell markers were significantly altered. CD56^dimCD16^high cells expressed high amounts of NKG2A, NKG2D, and CD27 with low CD57. Conversely, CD56^dimCD16^dim cells displayed high CD127 but low expression of KIR, NKG2D, and natural cytotoxicity receptors (NCRs). Thymic CD56⁺CD3^- cells were able to gain cytotoxicity but were especially immunoregulatory cells, producing a broad range of cytokines. Finally, one population of thymic CD56⁺ cells resembled conventional NK cells, whereas the other represented a novel, noncanonical NK subset.

Human Lymphoid Tissues Harbor a Distinct CD69+CXCR6+ NK Cell Population

Knowledge of human NK cells is based primarily on conventional CD56(bright) and CD56(dim) NK cells from blood. However, most cellular immune interactions occur in lymphoid organs. Based on the coexpression of CD69 and CXCR6, we identified a third major NK cell subset in lymphoid tissues. This population represents 30-60% of NK cells in marrow, spleen, and lymph node but is absent from blood. CD69(+)CXCR6(+) lymphoid tissue NK cells have an intermediate expression of CD56 and high expression of NKp46 and ICAM-1. In contrast to circulating NK cells, they have a bimodal expression of the activating receptor DNAX accessory molecule 1. CD69(+)CXCR6(+) NK cells do not express the early markers c-kit and IL-7Rα, nor killer cell Ig-like receptors or other late-differentiation markers. After cytokine stimulation, CD69(+)CXCR6(+) NK cells produce IFN-γ at levels comparable to CD56(dim) NK cells. They constitutively express perforin but require preactivation to express granzyme B and exert cytotoxicity. After hematopoietic stem cell transplantation, CD69(+)CXCR6(+) lymphoid tissue NK cells do not exhibit the hyperexpansion observed for both conventional NK cell populations. CD69(+)CXCR6(+) NK cells constitute a separate NK cell population with a distinct phenotype and function. The identification of this NK cell population in lymphoid tissues provides tools to further evaluate the cellular interactions and role of NK cells in human immunity.

Murine peripheral NK-cell populations originate from site-specific immature NK cells more than from BM-derived NK cells

Murine NK cells can be divided by the expression of two cell surface markers, CD27 and Mac-1 (a.k.a. CD11b), into four separate subsets. These subsets suggest a linear development model: CD27(-) Mac-1(-) → CD27(+) Mac-1(-) → CD27(+) Mac-1(+) → CD27(-) Mac-1(+) . Here, we used a combination of BrdU labeling experiments and mathematical modeling to gain insights regarding NK-cell development in mouse bone marrow (BM), spleen and liver. The modeling results that best fit the experimental data show that the majority of NK cells already express CD27 upon entering the NK-cell developmental pathway. Additionally, only a small fraction of NK cells exit the BM to other sites, suggesting that peripheral NK-cell populations originate from site-specific immature NK cells more than from BM-derived mature NK cells.

Natural Killer Cells and Liver Fibrosis

In the 40 years since the discovery of natural killer (NK) cells, it has been well established that these innate lymphocytes are important for early and effective immune responses against transformed cells and infections with different pathogens. In addition to these classical functions of NK cells, we now know that they are part of a larger family of innate lymphoid cells and that they can even mediate memory-like responses. Additionally, tissue-resident NK cells with distinct phenotypical and functional characteristics have been identified. Here, we focus on the phenotype of different NK cell subpopulations that can be found in the liver and summarize the current knowledge about the functional role of these cells with a special emphasis on liver fibrosis. NK cell cytotoxicity can contribute to liver damage in different forms of liver disease. However, NK cells can limit liver fibrosis by killing hepatic stellate cell-derived myofibroblasts, which play a key role in this pathogenic process. Therefore, liver NK cells need to be tightly regulated in order to balance these beneficial and pathological effects.

HIV-Specific Antibody-Dependent Cellular Cytotoxicity (ADCC) -Mediating Antibodies Decline while NK Cell Function Increases during Antiretroviral Therapy (ART)

Understanding alterations in HIV-specific immune responses during antiretroviral therapy (ART), such as antibody-dependent cellular cytotoxicity (ADCC), is important in the development of novel strategies to control HIV-1 infection. This study included 53 HIV-1 positive individuals. We evaluated the ability of effector cells and antibodies to mediate ADCC separately and in combination using the ADCC-PanToxiLux assay. The ability of the peripheral blood mononuclear cells (PBMCs) to mediate ADCC was significantly higher in individuals who had been treated with ART before seroconversion, compared to the individuals initiating ART at a low CD4⁺ T cell count (<350 cells/μl blood) and the ART-naïve individuals. The frequency of CD16 expressing natural killer (NK) cells correlated with both the duration of ART and Granzyme B (GzB) activity. In contrast, the plasma titer of antibodies mediating ADCC declined during ART. These findings suggest improved cytotoxic function of the NK cells if initiating ART early during infection, while the levels of ADCC mediating antibodies declined during ART.

Natural Killer Cell Immunomodulation: Targeting Activating, Inhibitory, and Co-stimulatory Receptor Signaling for Cancer Immunotherapy

There is compelling clinical and experimental evidence to suggest that natural killer (NK) cells play a critical role in the recognition and eradication of tumors. Efforts at using NK cells as antitumor agents began over two decades ago, but recent advances in elucidating NK cell biology have accelerated the development of NK cell-targeting therapeutics. NK cell activation and the triggering of effector functions is governed by a complex set of activating and inhibitory receptors. In the early phases of cancer immune surveillance, NK cells directly identify and lyse cancer cells. Nascent transformed cells elicit NK cell activation and are eliminated. However, as tumors progress, cancerous cells develop immunosuppressive mechanisms that circumvent NK cell-mediated killing, allowing for tumor escape and proliferation. Therapeutic intervention aims to reverse tumor-induced NK cell suppression and sustain NK cells’ tumorlytic capacities. Here, we review tumor–NK cell interactions, discuss the mechanisms by which NK cells generate an antitumor immune response, and discuss NK cell-based therapeutic strategies targeting activating, inhibitory, and co-stimulatory receptors.

Practical NK cell phenotyping and variability in healthy adults

Human natural killer (NK) cells display a wide array of surface and intracellular markers that indicate various states of differentiation and/or levels of effector function. These NK cell subsets exist simultaneously in peripheral blood and may vary among individuals. We examined variety among selected NK cell receptors expressed by NK cells from normal donors, as well as the distribution of select NK cell subsets and NK cell receptor expression over time in several individual donors. Peripheral blood mononuclear cells were evaluated using flow cytometry via fluorochrome-conjugated antibodies against a number of NK cell receptors. Results were analyzed for both mean fluorescence intensity (MFI) and the percent positive cells for each receptor. CD56(bright) and CD56(dim) NK cell subsets were also considered separately, as was variation in receptor expression in NK cell subsets over time in selected individuals. Through this effort, we provide ranges of NK cell surface receptor expression for a local adult population as well as provide insight into intra-individual variation.

Overexpression of IL-15 promotes tumor destruction via NK1.1+ cells in a spontaneous breast cancer model

BACKGROUND

Natural Killer (NK) cells play an important role in tumor prevention, but once tumors form, the numbers as well as the cytotoxic functions of NK cells are reduced. IL-15 is a cytokine that increases and activates NK cells. Here we will examine the anti-tumor role of IL-15 in a spontaneous breast cancer model.

METHODS

To achieve this, Polyoma Middle T (MT) mice that form spontaneous breast cancer were crossed with mice that either overexpress IL-15 (IL-15 transgenic (TG)) or mice that lack IL-15 (IL-15 knockout (KO)). We compared survival curves and tumor formation in IL-15 KO/MT, MT and IL-15 TG/MT groups. In addition, the phenotype, activation and contribution of NK cells and CD8 T cells to tumor formation were examined in each of these mouse strains via flow cytometry, ELISA, adoptive transfer and antibody depletion experiments.

RESULTS

IL-15KO/MT tumors formed and progressed to endpoint more quickly than MT tumors. These tumors displayed little apoptosis and poor CD8 T cell infiltration. In contrast, IL-15 TG/MT mice had increased survival and the tumors displayed extensive cell death, high proportions of activated NK cells and a higher infiltration of CD8 T cells than MT tumors. CD8 T cells in IL-15 TG/MT tumors were capable of secreting IFNγ, possessed markers of memory, did not display an exhausted phenotype and were frequently NK1.1+. Long-term antibody depletion studies in IL-15 TG/MT mice revealed that NK1.1+, but not CD8 T cells, were critical for tumor destruction. Lastly, human NK cells, when exposed to a similar cytokine environment as that found in IL-15TG/MT tumors, were capable of killing human breast cancer cells.

CONCLUSIONS

This study reveals that high levels of IL-15 can promote tumor destruction and reduce metastasis in breast cancer via effects on NK1.1+ cells. Our results suggest that strategies aimed at increasing NK cell activation may be effective against solid epithelial cancers.

HIV-specific ADCC improves after antiretroviral therapy and correlates with normalization of the NK cell phenotype

BACKGROUND

Natural killer (NK) cell phenotype and function have recently gained much attention as playing crucial roles in antibody-dependent cellular cytotoxicity (ADCC). We investigated NK cell function, as measured by ADCC, in HIV-1-positive individuals before and 6 months after highly active antiretroviral therapy (HAART) initiation.

METHOD

The ability of antibodies and NK cells to mediate ADCC was investigated separately and in combination in an autologous model. The NK cell subset distribution and NK cell phenotype (ie, expression of maturation and activation markers within NK cell subsets) were analyzed.

RESULTS

The ability of NK cells to mediate ADCC was significantly increased after only 6 months of HAART and was not explained by a normalization of NK cell subsets (CD56 CD16 and CD56 CD16 NK cells) but rather by normalization in the frequency of NK cells expressing CCR7 and CD27. For individuals with no increase in ADCC after 6 months of HAART, the frequency of NK cells expressing NKp46 was downregulated. The ability of antibodies to mediate ADCC alone and in combination in an autologous model was not improved.

CONCLUSIONS

HAART improves the ability of NK cells to mediate ADCC after 6 months. This improvement does not correlate with general immune restoration, as measured by CD4 T-cell counts, but rather to a decrease in the frequency of NK cells expressing CCR7 and CD27.

Expression of the immune regulation antigen CD70 in osteosarcoma

Osteosarcoma is the most frequent bone cancer in children and young adults. The outcome of patients with advanced disease is dismal. Exploitation of tumor-immune cell interactions may provide novel therapeutic approaches. CD70-CD27 interactions are important for the regulation of adaptive immunity. CD70 expression has been reported in some solid cancers and implicated in tumor escape from immunosurveillance. In this study, expression of CD70 and CD27 was analyzed in osteosarcoma cell lines and tumor specimens. CD70 protein was expressed on most osteosarcoma cell lines (5/7) and patient-derived primary osteosarcoma cultures (4/6) as measured by flow cytometry. In contrast, CD70 was detected on few Ewing sarcoma cell lines (5/15) and was virtually absent from neuroblastoma (1/7) and rhabdomyosarcoma cell lines (0/5). CD70(+) primary cultures were derived from CD70(+) osteosarcoma lesions. CD70 expression in osteosarcoma cryosections was heterogeneous, restricted to tumor cells and not attributed to infiltrating CD3(+) T cells as assessed by immunohistochemistry/immunofluorescence. CD70 was detected in primary (1/5) but also recurrent (2/4) and metastatic (1/3) tumors. CD27, the receptor for CD70, was neither detected on tumor cells nor on T cells in CD70(+) or CD70(-) tumors, suggesting that CD70 on tumor cells is not involved in CD27-dependent tumor-immune cell interactions in osteosarcoma. CD70 gene expression in diagnostic biopsies of osteosarcoma patients did not correlate with the occurrence of metastasis and survival (n = 70). Our data illustrate that CD70 is expressed in a subset of osteosarcoma patients. In patients with CD70(+) tumors, CD70 may represent a novel candidate for antibody-based targeted immunotherapy.

Influenza vaccine induces intracellular immune memory of human NK cells

Influenza vaccines elicit antigen-specific antibodies and immune memory to protect humans from infection with drift variants. However, what supports or limits vaccine efficacy and duration is unclear. Here, we vaccinated healthy volunteers with annual vaccine formulations and investigated the dynamics of T cell, natural killer (NK) cell and antibody responses upon restimulation with heterologous or homologous influenza virus strains. Influenza vaccines induced potential memory NK cells with increased antigen-specific recall IFN-γ responses during the first 6 months. In the absence of significant changes in other NK cell markers (CD45RO, NKp44, CXCR6, CD57, NKG2C, CCR7, CD62L and CD27), influenza vaccines induced memory NK cells with the distinct feature of intracellular NKp46 expression. Indeed, surface NKp46 was internalized, and the dynamic increase in NKp46(intracellular)⁺CD56^dim NK cells positively correlated with increased IFN-γ production to influenza virus restimulation after vaccination. In addition, anti-NKp46 antibodies blocked IFN-γ responses. These findings provide insights into a novel mechanism underlying vaccine-induced immunity and NK-related diseases, which may help to design persisting and universal vaccines in the future.

Multifunctional human CD56 low CD16 low natural killer cells are the prominent subset in bone marrow of both healthy pediatric donors and leukemic patients

We phenotypically and functionally characterized a distinct CD56(low) natural killer cell subset based on CD16 expression levels in bone marrow and peripheral blood of healthy children and pediatric patients with acute lymphoblastic leukemia. Our findings demonstrate for the first time that CD56(low)CD16(low) natural killer cells are more abundant in bone marrow than in peripheral blood and that their frequency is further increased in children with acute lymphoblastic leukemia. Bone marrow and peripheral blood CD56(low)CD16(low) natural killer cells compared with CD56(low)CD16(high) natural killer cells express lower levels of killer inhibitory receptors, higher levels of CD27, CD127, CD122, CD25, but undetectable levels of CD57, suggesting that they have a higher proliferative and differentiation potential. Moreover, CD56(low)CD16(low) natural killer cells display higher levels of CXCR4 and undetectable levels of CX3CR1 and can be consistently and rapidly mobilized in peripheral blood in response to CXCR4 antagonist. Unlike CD56(low)CD16(high), both bone marrow and peripheral blood CD56(low)CD16(low) natural killer cells release IFNγ following cytokine stimulation, and represent the major cytotoxic natural killer cell population against K562 or acute lymphoblastic leukemia target cells. All these data suggest that CD56(low)CD16(low) natural killer cells are multifunctional cells, and that the presence of hematologic malignancies affects their frequency and functional ability at both tumor site and in the periphery.

Organ-specific phenotypic and functional features of NK cells in humans

Natural killer (NK) cells kill virus-infected and tumor target cells without prior sensitization. Each NK cell expresses a multitude of activating and inhibitory receptors, and the interplay of signals determines the outcome of NK cell activity. NK cell-mediated cytolysis of target cell involves polarized degranulation at effector-target interface. Peripheral blood NK cell constitutes about 10% of lymphocytes, and approximately 90% of peripheral blood NK cells are CD56(dim)CD16(+); however, there is a distinct subset of NK cells, CD56(bright)CD16(-), expressed by certain lymphoid organs which are able to produce large amounts of cytokines including interferon-γ, tumor necrosis factor, and granulocyte-macrophage colony-stimulating factor, but the cytotoxicity is attained only on their prolonged activation. In this review, we discuss the accumulated data on distinct phenotypes of NK cells in human uterus, liver, intestine, skin, and lung and also attempt to correlate their phenotype with corresponding activity and functions, with significant stress on the role of NK cells in pathology in the specific organs. Our detailed understanding of altered NK cell activity in different organs and their inherent cytotoxic activity against tumor target cells will help us design better immunotherapeutic strategies in NK cell-mediated cancer therapies.

T cells and costimulation in cancer

Optimal T cell response is dependent not only on T cell receptor activation, but also on additional signaling from coreceptors. The main coreceptors include B7 and tumor necrosis factor family members. They exert costimulatory or coinhibitory effects, and their balance determines the fate of T cell response. In normal conditions, costimulators facilitate the development of protective immune response, whereas coinhibitors dampen inflammation to avoid organ/tissue damage from excessive immune reaction. In the tumor microenvironment, the balance is garbled: inhibitory pathways predominate, and T cell response is impaired. The importance of cosignaling in the tumor immune response has been experimentally and clinically demonstrated. New therapeutic strategies targeting T cell cosignaling, especially coinhibitory molecules, are under active experimental and clinical investigation. This review summarizes the functions of main T cell cosignaling axes and discusses their clinical application.

CD27(+)CD56Bright natural killer cells may be involved in spontaneous clearance of acute hepatitis C in HIV-positive patients

OBJECTIVE

The objective of this study was to analyse the potential role of CD27 in natural killer (NK) cell-mediated control of hepatitis C virus (HCV) infection in HIV-positive patients.

DESIGN

Frequency of CD27-expressing CD56 NK cells was analysed in HIV mono-infected individuals and HIV-positive patients with acute or chronic hepatitis C. Anti-HCV activity of CD27(+) and CD27(-) NK cells was compared.

METHODS

NK cell mediated inhibition of HCV replication was analysed using the HUH7 HCV Replicon model. NK cell phenotype and interferon (IFN) secretion was studied by flowcytometry.

RESULTS

High frequency of CD27(+)CD56 NK cells is associated with spontaneous clearance of acute hepatitis C in HIV-positive patients. Accordingly, we found CD27(+)CD56 NK cells to display strong anti-HCV activity.

CONCLUSION

Our results underline the important role of NK cells in modulating outcome of HCV infection.

Subsets of human natural killer cells and their regulatory effects

Human natural killer (NK) cells have distinct functions as NK(tolerant) , NK(cytotoxic) and NK(regulatory) cells and can be divided into different subsets based on the relative expression of the surface markers CD27 and CD11b. CD27⁺ NK cells, which are abundant cytokine producers, are numerically in the minority in human peripheral blood but constitute the large population of NK cells in cord blood, spleen, tonsil and decidua tissues. Recent data suggest that these NK cells may have immunoregulatory properties under certain conditions. In this review, we will focus on these new NK cell subsets and discuss how regulatory NK cells may serve as rheostats or sentinels in controlling inflammation and maintaining immune homeostasis in various organs.

CD56(bright)perforin(low) noncytotoxic human NK cells are abundant in both healthy and neoplastic solid tissues and recirculate to secondary lymphoid organs via afferent lymph

As limited information is available regarding the distribution and trafficking of NK cells among solid organs, we have analyzed a wide array of tissues derived from different human compartments. NK cells were widely distributed in most solid tissues, although their amount varied significantly depending on the tissue/organ analyzed. Interestingly, the distribution appeared to be subset specific, as some tissues were preferentially populated by CD56(bright)perforin(low) NK cells, with others by the CD56(dim)perforin(high) cytotoxic counterpart. Nevertheless, most tissues were highly enriched in CD56(bright)perforin(low) cells, and the distribution of NK subsets appeared in accordance with tissue gene expression of chemotactic factors, for which receptors are differently represented in the two subsets. Remarkably, chemokine expression pattern of tissues was modified after neoplastic transformation. As a result, although the total amount of NK cells infiltrating the tissues did not significantly change upon malignant transformation, the relative proportion of NK subsets infiltrating the tissues was different, with a trend toward a tumor-infiltrating NK population enriched in noncytotoxic cells. Besides solid tissues, CD56(bright)perforin(low) NK cells were also detected in seroma fluids, which represents an accrual of human afferent lymph, indicating that they may leave peripheral solid tissues and recirculate to secondary lymphoid organs via lymphatic vessels. Our results provide a comprehensive mapping of NK cells in human tissues, demonstrating that discrete NK subsets populate and recirculate through most human tissues and that organ-specific chemokine expression patterns might affect their distribution. In this context, chemokine switch upon neoplastic transformation might represent a novel mechanism of tumor immune escape.

Natural killer cells in patients with allergic diseases

Natural killer (NK) cells not only exert cytotoxic activity against tumor cells or infected cells but also act to regulate the function of other immune cells through secretion of cytokines and chemokines or cell contact-dependent mechanisms. NK cells are able to polarize in vitro into 2 functional distinct subsets, NK1 or NK2 cells, which are analogous to the T-cell subsets TH1 or TH2. In addition, a regulatory NK cell subset has been described that secretes IL-10, shows antigen-specific T-cell suppression, and suppresses IgE production. Although it has been demonstrated that NK cells play important roles in autoimmunity, cancer, transplantation, and pregnancy, the role of NK cells in allergy has not been extensively discussed. This review aims to discuss our understanding of NK cells and NK cell subsets in allergic inflammation and IgE regulation.

CD27 engagement by a soluble CD70 protein enhances non-cytolytic antiviral activity of CD56bright natural killer cells by IFN-γ secretion

We investigated regulation of human NK cell function by CD27 engagement using a recombinant soluble CD70 protein. CD27 was preferentially expressed on CD56(bright) NK cells, and soluble CD70 protein bound to CD27(+)CD56(bright) NK cells. While soluble CD70 protein enhanced IFN-γ secretion by CD56(bright) NK cells in the presence of IL-12, it augmented neither cytolytic activity nor proliferation of NK cells. Thus, we next asked if soluble CD70 protein could be used to induce non-cytolytic antiviral activity of NK cells using an in vitro hepatitis C virus (HCV) infection system. Soluble CD70 protein stimulated NK cells to suppress HCV replication by enhancing NK cell IFN-γ secretion without killing infected cells. Taken together, we demonstrate that CD27 engagement by a soluble CD70 protein enhances non-cytolytic antiviral activity of CD56(bright) NK cells by IFN-γ secretion. Thus, this soluble CD70 protein may be useful for the treatment of viral infections such as HCV infection.

The host environment is responsible for aging-related functional NK cell deficiency

NK cells play an important role in immunity against infection and tumors. Aging-related functional NK cell deficiency is well documented in humans and mice. However, the mechanism for this is poorly understood. Using an adoptive transfer approach in mice, we found that NK cells from both young and aged mice responded vigorously to priming by pathogen-derived products after being cotransferred into young mice. In contrast, NK cells from young mice responded poorly to priming by pathogen-derived products after being transferred to aged mice. In addition to defects in NK cell priming, maturation of NK cells under steady-state conditions is also impaired in aged mice, resulting in a decreased proportion of CD27(-) mature NK cells. We found that bone marrow from young and aged mice gave rise to CD27(-) mature NK cells similarly in young mixed bone marrow chimeric mice. Furthermore, by using a novel bone marrow transfer approach without irradiation, we found that after being transferred to aged mice, bone marrow from young mice gave rise to NK cells with maturation defects. Finally, we found that aging-related functional NK cell deficiency was completely reversed by injecting soluble IL-15/IL-15Rα complexes. In contrast, blockade of IL-10 signaling, which broadly augments inflammatory responses to pathogen-derived products, had little effect on aging-related defects in NK cell priming. These data demonstrate that the aged host environment is responsible for aging-related functional NK cell deficiency. Additionally, our data suggest that IL-15 receptor agonists may be useful tools in treating aging-related functional NK cell deficiency.

Agonist anti-human CD27 monoclonal antibody induces T cell activation and tumor immunity in human CD27-transgenic mice

The CD70/CD27 pathway plays a significant role in the control of immunity and tolerance, and previous studies demonstrated that targeting murine CD27 (mCD27) with agonist mAbs can mediate antitumor efficacy. We sought to exploit the potential of this pathway for immunotherapy by developing 1F5, a fully human IgG1 mAb to human CD27 (hCD27) with agonist activity. We developed transgenic mice expressing hCD27 under control of its native promoter for in vivo testing of the Ab. The expression and regulation of hCD27 in hCD27-transgenic (hCD27-Tg) mice were consistent with the understood biology of CD27 in humans. In vitro, 1F5 effectively induced proliferation and cytokine production from hCD27-Tg-derived T cells when combined with TCR stimulation. Administration of 1F5 to hCD27-Tg mice enhanced Ag-specific CD8(+) T cell responses to protein vaccination comparably to an agonist anti-mCD27 mAb. In syngeneic mouse tumor models, 1F5 showed potent antitumor efficacy and induction of protective immunity, which was dependent on CD4(+) and CD8(+) T cells. The requirement of FcR engagement for the agonistic and antitumor activities of 1F5 was demonstrated using an aglycosylated version of the 1F5 mAb. These data with regard to the targeting of hCD27 are consistent with previous reports on targeting mCD27 and provide a rationale for the clinical development of the 1F5 mAb, for which studies in advanced cancer patients have been initiated under the name CDX-1127.

Human NK cell receptors/markers: a tool to analyze NK cell development, subsets and function

Natural killer (NK) cells are important components of the innate immunity and play a key role in host defense by virtue of their ability to release cytokines and to mediate cytolytic activity against tumor cells and virus-infected cells. NK cells were first described more than 30 years ago on the basis of their peculiar functional capabilities. Subsequently, thanks to the production of a variety of monoclonal antibodies, it became possible to identify surface receptors and markers expressed by NK cells as well as to characterize their functional properties. Here, we provide a brief historical overview about the discovery of human NK cell receptors and we delineate the main phenotypic features of differentiating and mature NK cells in healthy donors as well as their alterations in certain pathologic conditions.

CD11b(-)CD27(-) NK cells are associated with the progression of lung carcinoma

NK cells are a major component of the antitumour immune response that limits tumour progression. However, it has been reported that tumour-infiltrating NK (TINK) cells from patients with non-small-cell lung carcinoma (NSCLC) exhibit profound defects in degranulation and IFN-γ production. In support of this notion, we report a novel mechanism associated with tumour escape from NK cell-mediated antitumour immunity in lung carcinoma. In this study, we investigated the phenotypic profile of TINK cells based on the expression of the NK-cell maturation markers CD11b and CD27. Interestingly, we found a substantial CD11b⁻CD27⁻ (DN) NK-cell population harboured within the tumour tissues. The presence of this CD11b⁻CD27⁻ NK subset indicated that the TINK cells were of an immature and inactive phenotype. Remarkably, we determined that the presence of DN NK cells had an impact on the clinical outcomes of patients with NSCLC, as the frequency of tumour-infiltrating DN NK cells was positively correlated with the tumour stage and tumour size. We further used a murine Lewis lung cancer (LLC) model to confirm the correlation between the frequency of tumour-infiltrating DN NK cells and the progression of lung carcinoma. Together, our findings demonstrate that the tumour microenvironment may render TINK cells less tumouricidal and thereby contribute to cancer progression.

Natural killer cells in liver disease

Natural killer (NK) cells are enriched in lymphocytes within the liver and have unique phenotypic features and functional properties, including tumor necrosis factor-related apoptosis-inducing ligand-dependent cytotoxicity and specific cytokine profiles. As a key component of innate immunity in the liver, NK cells perform critical roles in host defense against pathogens and tumors through their natural cytotoxicity and cytokine production, and they also act as regulatory cells by engaging in reciprocal interactions with other types of liver cells through cell-to-cell contact and the production of cytokines. Accumulating evidence from the last decade suggests that NK cells play an important role in controlling viral hepatitis, liver fibrosis, and liver tumorigenesis, but also contribute to the pathogenesis of liver injury and inflammation. The characterization of intrahepatic NK cell functions has not only helped us to better understand the pathogenesis of liver disease, but has also revealed new therapeutic targets for managing this disease.

Identification of cytolytic CD161- CD56+ regulatory CD8 T cells in human peripheral blood

We previously developed methods for establishing CD8 regulatory T cell (Treg) clones from normal human peripheral blood and demonstrated that these clones were capable of killing T cell receptor (TCR)-activated autologous CD4 T cells. Based on phenotypic and functional characterization of the CD8 Treg clones, we have identified a corresponding population of endogenous CD8 Treg in normal human peripheral blood. These cells appear morphologically as large lymphocytes with abundant cytoplasm and have the following unique phenotype: CD3⁺CD8⁺CD161⁻CD56⁺. The majority of CD8 Treg express CD45RA and CD62L with low or negative expression of CD45RO, CD25, CD27, CD28 and CCR7. The expression of CD94 and NKG2a on CD8 Treg was elevated compared to conventional CD8 T cells. Following in vitro activation, this T cell subset is capable of killing TCR-activated CD4 T cells. These studies identify an endogenous CD8 Treg population in humans and it will now be possible to characterize these cells in a variety of clinical conditions.

Porcine CD8αdim/-NKp46high NK cells are in a highly activated state

Natural Killer (NK) cells play a crucial role in the early phase of immune responses against various pathogens. In swine so far only little information about this lymphocyte population exists. Phenotypical analyses with newly developed monoclonal antibodies (mAbs) against porcine NKp46 recently revealed that in blood NKp46- and NKp46+ cells with NK phenotype exist with comparable cytotoxic properties. In spleen a third NKp46-defined population with NK phenotype was observed that was characterised by a low to negative CD8α and increased NKp46 expression. In the current study it is shown that this NKp46high phenotype was correlated with an increased expression of CD16 and CD27 compared to the CD8α+NKp46- and NKp46+ NK-cell subsets in spleen and blood. Additionally NKp46high NK cells expressed elevated levels of the chemokine receptor CXCR3 on mRNA level. Functional analyses revealed that splenic NKp46high NK cells produced much higher levels of Interferon-γ and Tumor Necrosis Factor-α upon stimulation with cytokines or phorbol-12-myristate-13-acetate/Ionomycin compared to the other two subsets. Furthermore, cross-linking of NKp46 by NKp46-specific mAbs led to a superior CD107a expression in the NKp46high NK cells, thus indicating a higher cytolytic capacity of this subset. Therefore porcine splenic NKp46high NK cells represent a highly activated subset of NK cells and may play a profound role in the immune surveillance of this organ.

Natural killer cells promote immune tolerance by regulating inflammatory TH17 cells at the human maternal-fetal interface

Natural killer (NK) cells accumulate at the maternal-fetal interface in large numbers, but their exact roles in successful pregnancy remain poorly defined. Here, we provide evidence that T(H)17 cells and local inflammation can occur at the maternal-fetal interface during natural allogenic pregnancies. We found that decidual NK cells promote immune tolerance and successful pregnancy by dampening inflammatory T(H)17 cells via IFN-γ secreted by the CD56(bright)CD27(+) NK subset. This NK-cell-mediated regulatory response is lost in patients who experience recurrent spontaneous abortions, which results in a prominent T(H)17 response and extensive local inflammation. This local inflammatory response further affects the regulatory function of NK cells, leading to the eventual loss of maternal-fetal tolerance. Thus, our data identify NK cells as key regulatory cells at the maternal-fetal interface by suppressing T(H)17-mediated local inflammation.

Natural killer cells in non-hematopoietic malignancies

Natural killer (NK) cells belong to the innate immune system and were initially described functionallywise by their spontaneous cytotoxic potential against transformed or virus-infected cells. A delicate balance between activating and inhibiting receptors regulates NK cell tolerance. A better understanding of tissue resident NK cells, of NK cell maturation stages and migration patterns has evolved allowing a thoughtful evaluation of their modus operandi. While evidence has been brought up for their relevance as gate keepers in some hematopoietic malignancies, the role of NK cells against progression and dissemination of solid tumors remains questionable. Hence, many studies pointed out the functional defects of the rare NK cell infiltrates found in tumor beds and the lack of efficacy of adoptively transferred NK cells in patients. However, several preclinical evidences suggest their anti-metastatic role in a variety of mouse tumor models. In the present review, we discuss NK cell functions according to their maturation stage and environmental milieu, the receptor/ligand interactions dictating tumor cell recognition and recapitulate translational studies aimed at deciphering their prognostic or predictive role against human solid malignancies.

Natural killer cell distribution and trafficking in human tissues

Few data are available regarding the recirculation of natural killer (NK) cells among human organs. Earlier studies have been often impaired by the use of markers then proved to be either not sufficiently specific for NK cells (e.g., CD57, CD56) or expressed only by subsets of NK cells (e.g., CD16). At the present, available data confirmed that human NK cells populate blood, lymphoid organs, lung, liver, uterus (during pregnancy), and gut. Several studies showed that NK cell homing appears to be subset-specific, as secondary lymphoid organs and probably several solid tissues are preferentially inhabited by CD56^brightCD16^neg/dull non-cytotoxic NK cells. Similar studies performed in the mouse model showed that lymph node and bone marrow are preferentially populated by CD11b^dull NK cells while blood, spleen, and lung by CD27^dull NK cells. Therefore, an important topic to be addressed in the human system is the contribution of factors that regulate NK cell tissue homing and egress, such as chemotactic receptors or homeostatic mechanisms. Here, we review the current knowledge on NK cell distribution in peripheral tissues and, based on recent acquisitions, we propose our view regarding the recirculation of NK cells in the human body.

Influence of congenital human cytomegalovirus infection and the NKG2C genotype on NK-cell subset distribution in children

Human cytomegalovirus (HCMV) has been reported to reshape the NK-cell receptor (NKR) distribution, promoting an expansion of CD94/NKG2C(+) NK and T cells. The role of NK cells in congenital HCMV infection is ill-defined. Here we studied the expression of NKR (i.e., NKG2C, NKG2A, LILRB1, CD161) and the frequency of the NKG2C gene deletion in children with past congenital infection, both symptomatic (n = 15) and asymptomatic (n = 11), including as controls children with postnatal infection (n = 11) and noninfected (n = 20). The expansion of NKG2C(+) NK cells in HCMV-infected individuals appeared particularly marked and was associated with an increased number of LILRB1(+) NK cells in cases with symptomatic congenital infection. Increased numbers of NKG2C(+), NKG2A(+), and CD161(+) T cells were also associated to HCMV infection. The NKG2C deletion frequency was comparable in children with congenital HCMV infection and controls. Remarkably, the homozygous NKG2C(+/+) genotype appeared associated with increased absolute numbers of NKG2C(+) NK cells. Moreover, HCMV-infected NKG2C(+/+) children displayed higher absolute numbers of NKG2A(+) and total NK cells than NKG2C(+/-) individuals. Our study provides novel insights on the impact of HCMV infection on the homeostasis of the NK-cell compartment in children, revealing a modulatory influence of NKG2C copy number.