Involvement of natural cytotoxicity receptors in human natural killer cell-mediated lysis of neuroblastoma and glioblastoma cell lines

A Multi-Color Flow Cytometric Assay for Quantifying Dinutuximab Binding to Neuroblastoma Cells in Tumor, Bone Marrow, and Blood

GD2, a disialoganglioside, is present on the surface of most neuroblastomas, as well as on some other cancers, such as melanoma and osteogenic sarcoma. The anti-GD2 antibody ch14.18 (dinutuximab) has an FDA-registered indication for use as maintenance therapy for high-risk neuroblastoma with cytokines and 13-cis-retinoic acid after myeloablative therapy. Recent studies using immunohistochemistry of tumor or tumor cells in marrow have shown that some neuroblastomas are negative for GD2. Dinutuximab and other anti-GD2 antibodies are increasingly used in combination with cytotoxic chemotherapy for treating relapsed neuroblastoma, so it is important to be able to identify patients with tumor cells with low GD2 expression, as such patients may experience toxicity but not benefit from the antibody therapy. As the most common clinical samples available for relapsed neuroblastoma are bone marrow aspirates, we developed a method to quantify dinutuximab binding density and the frequency of neuroblastoma cells positive for the antibody in bone marrow aspirates. Here, we describe a multi-color flow cytometry assay that employs non-GD2 antibodies to identify neuroblastoma cells in a mixed population (tumor, bone marrow, or blood) and an anti-GD2 antibody to quantify both the frequency and density of GD2 expression on neuroblastoma cells.

Polymorphisms of Killer Ig-like Receptors and the Risk of Glioblastoma

PURPOSE

The immune responses of natural killer (NK) cells against cancer cells vary by patient. Killer Ig-like receptors (KIRs), which are some of the major receptors involved in regulating NK cell activity for killing cancer cells, have significant genetic variation. Numerous studies have suggested a potential association between the genetic variation of KIR genes and the risk of development or prognosis of various cancer types. However, an association between genetic variations of KIR genes and glioblastoma (GB) remains uncertain. We sought to evaluate the association of genetic variations of KIRs and their ligand genes with the risk of GB development in Koreans.

METHODS

A case-control study was performed to identify the odds ratios (ORs) of KIR genes and Classes A, B, and, C of the human leukocyte antigen (HLA) for GB. The GB group was comprised of 77 patients with newly diagnosed IDH-wildtype GB at our institution, and the control group consisted of 200 healthy Korean volunteers.

RESULTS

There was no significant difference in the frequency of KIR genes and KIR haplotypes between the GB and control groups. Genetic variations of KIR-2DL1, 3DL1, and 3DS1 with their ligand genes (HLA-C2, HLA-Bw4/6, and Bw4, respectively) had effects on the risk of GB in Korean patients. The frequency of KIR-2DL1 with HLA-C2 (OR 2.05, CI 1.19-3.52, p = 0.009), the frequency of KIR-3DL1 without HLA-Bw4 (80I) (OR 8.36, CI 4.06-17.18, p < 0.001), and the frequency of KIR-3DL1 with Bw6 (OR 4.54, CI 2.55-8.09, p < 0.001) in the GB group were higher than in the control group. In addition, the frequency of KIR-2DL1 without HLA-C2 (OR 0.44, CI 0.26-0.75, p = 0.003), the frequency of KIR-3DL1 with HLA-Bw4 (80T) (OR 0.13, CI 0.06-0.27, p < 0.001), the frequency of KIR-3DL1 without Bw6 (OR 0.27, CI 0.15-0.49, p < 0.001), and the frequency of KIR-3DS1 with Bw4 (80I) (OR 0.03, CI 0.00-0.50, p < 0.001) in the GB group were lower than in the control group.

CONCLUSIONS

This study suggests that genetic variations of KIRs and their ligand genes may affect GB development in the Korean population. Further investigations are needed to demonstrate the different immune responses for GB cells according to genetic variations of KIR genes and their ligand genes.

Neuroblastoma Tumor-Associated Mesenchymal Stromal Cells Regulate the Cytolytic Functions of NK Cells

Neuroblastoma tumor-associated mesenchymal stromal cells (NB-TA-MSC) have been extensively characterized for their pro-tumorigenic properties, while their immunosuppressive potential, especially against NK cells, has not been thoroughly investigated. Herein, we study the immune-regulatory potential of six primary young and senescent NB-TA-MSC on NK cell function. Young cells display a phenotype (CD105+/CD90+/CD73+/CD29+/CD146+) typical of MSC cells and, in addition, express high levels of immunomodulatory molecules (MHC-I, PDL-1 and PDL-2 and transcriptional-co-activator WWTR1), able to hinder NK cell activity. Notably, four of them express the neuroblastoma marker GD2, the most common target for NB immunotherapy. From a functional point of view, young NB-TA-MSC, contrary to the senescent ones, are resistant to activated NK cell-mediated lysis, but this behavior is overcome using anti-CD105 antibody TRC105 that activates antibody-dependent cell-mediated cytotoxicity. In addition, proliferating NB-TA-MSC, but not the senescent ones, after six days of co-culture, inhibit proliferation, expression of activating receptors and cytolytic activity of freshly isolated NK. Inhibitors of the soluble immunosuppressive factors L-kynurenine and prostaglandin E2 efficiently counteract this latter effect. Our data highlight the presence of phenotypically heterogeneous NB-TA-MSC displaying potent immunoregulatory properties towards NK cells, whose inhibition could be mandatory to improve the antitumor efficacy of targeted immunotherapy.

A Transcriptional Signature of PDGF-DD Activated Natural Killer Cells Predicts More Favorable Prognosis in Low-Grade Glioma

The binding of platelet-derived growth factor D (PDGF-DD) to the NKp44 receptor activates a distinct transcriptional program in primary IL-2 expanded human natural killer (NK) cells. We were interested in knowing if the PDGF-DD-NKp44 pathway of NK cell activation might play a clinically relevant role in anti-tumor immunity. In order to address this question, we determined transcriptional signatures unique to resting, IL-2 expanded, and PDGF-DD activated, NK cells, in addition to different T cell subsets, and established the abundance of these immune cell phenotypes in The Cancer Genome Atlas (TCGA) low-grade glioma (LGG) dataset using CIBERSORT. Our results show that LGG patient tumors enriched for either the PDGF-DD activated NK cell or memory CD8⁺ T cell phenotypes are associated with a more favorable prognosis. Combined cell phenotype analyses revealed that patients with LGG tumors enriched for the PDGF-DD activated NK cell phenotype and the CD4⁺ T helper cell phenotype had a more favorable prognosis. High expression of transcripts encoding members of the killer cell lectin-like receptor (KLR) family, such as KLRK1 and KLRC2, KLRC3 and KLRC4 in LGG tumors were associated with more favorable prognosis, suggesting that these NK cell family receptors may play a prominent role in LGG anti-tumor immunity. Finally, many of the TCGA findings were reciprocated in LGG patients from the Chinese Glioma Genome Atlas (CGGA) dataset. Our results provide transcriptomic evidence that PDGF-DD activated NK cells and KLR family receptors may play an important clinical role in immune surveillance of LGG.

Pediatric Tumors-Mediated Inhibitory Effect on NK Cells: The Case of Neuroblastoma and Wilms' Tumors

Simple Summary

Neuroblastoma (NB) and Wilms’ tumor (WT) are the most common childhood solid extracranial tumors. The current treatments consist of a combination of surgery and chemotherapy or radiotherapy in high-risk patients. Such treatments are responsible for significant adverse events requiring long-term monitoring. Thus, a main challenge in NB and WT treatment is the development of novel therapeutic strategies to eliminate or minimize the adverse effects. The characterization of the immune environment could allow for the identification of new therapeutic targets. Herein, we described the interaction between these tumors and innate immune cells, in particular natural killer cells and monocytes. The detection of the immunosuppressive activity of specific NB and WT tumor cells on natural killer cells and on monocytes could offer novel cellular and molecular targets for an effective immunotherapy of NB and WT.

Abstract

Natural killer (NK) cells play a key role in the control of cancer development, progression and metastatic dissemination. However, tumor cells develop an array of strategies capable of impairing the activation and function of the immune system, including NK cells. In this context, a major event is represented by the establishment of an immunosuppressive tumor microenvironment (TME) composed of stromal cells, myeloid-derived suppressor cells, tumor-associated macrophages, regulatory T cells and cancer cells themselves. The different immunoregulatory cells infiltrating the TME, through the release of several immunosuppressive molecules or by cell-to-cell interactions, cause an impairment of the recruitment of NK cells and other lymphocytes with effector functions. The different mechanisms by which stromal and tumor cells impair NK cell function have been particularly explored in adult solid tumors and, in less depth, investigated and discussed in a pediatric setting. In this review, we will compare pediatric and adult solid malignancies concerning the respective mechanisms of NK cell inhibition, highlighting novel key data in neuroblastoma and Wilms’ tumor, two of the most frequent pediatric extracranial solid tumors. Indeed, both tumors are characterized by the presence of stromal cells acting through the release of immunosuppressive molecules. In addition, specific tumor cell subsets inhibit NK cell cytotoxic function by cell-to-cell contact mechanisms likely controlled by the transcriptional coactivator TAZ. These findings could lead to a more performant diagnostic approach and to the development of novel immunotherapeutic strategies targeting the identified cellular and molecular targets.

Identification of neuroblastoma cell lines with uncommon TAZ+/mesenchymal stromal cell phenotype with strong suppressive activity on natural killer cells

Background

Neuroblastoma (NB) is the most common, extracranial childhood solid tumor arising from neural crest progenitor cells and is a primary cause of death in pediatric patients. In solid tumors, stromal elements recruited or generated by the cancer cells favor the development of an immune-suppressive microenvironment. Herein, we investigated in NB cell lines and in NB biopsies, the presence of cancer cells with mesenchymal phenotype and determined the immune-suppressive properties of these tumor cells on natural killer (NK) cells.

Methods

We assessed the mesenchymal stromal cell (MSC)-like phenotype and function of five human NB cell lines and the presence of this particular subset of neuroblasts in NB biopsies using flow-cytometry, immunohistochemistry, RT-qPCR, cytotoxicity assays, western blot and silencing strategy. We corroborated our data consulting a public gene-expression dataset.

Results

Two NB cell lines, SK-N-AS and SK-N-BE(2)C, exhibited an unprecedented MSC phenotype (CD105⁺/CD90⁺/CD73⁺/CD29⁺/CD146⁺/GD2⁺/TAZ⁺). In these NB-MSCs, the ectoenzyme CD73 and the oncogenic/immune-regulatory transcriptional coactivator TAZ were peculiar markers. Their MSC-like nature was confirmed by their adipogenic and osteogenic differentiation potential. Immunohistochemical analysis confirmed the presence of neuroblasts with MSC phenotype (CD105⁺/CD73⁺/TAZ⁺). Moreover, a public gene-expression dataset revealed that, in stage IV NB, a higher expression of TAZ and CD105 strongly correlated with a poorer outcome.

Among the NB-cell lines analyzed, only NB-MSCs exhibited multifactorial resistance to NK-mediated lysis, inhibition of activating NK receptors, signal adaptors and of NK-cell cytotoxicity through cell-cell contact mediated mechanisms. The latter property was controlled partially by TAZ, since its silencing in NB cells efficiently rescued NK-cell cytotoxic activity, while its overexpression induced opposite effects in non-NB-MSC cells.

Conclusions

We identified a novel NB immunoregulatory subset that: (i) displayed phenotypic and functional properties of MSC, (ii) mediated multifactorial resistance to NK-cell-induced killing and (iii) efficiently inhibited, in coculture, the cytotoxic activity of NK cells against target cells through a TAZ-dependent mechanism. These findings indicate that targeting novel cellular and molecular components may disrupt the immunomodulatory milieu of the NB microenvironment ameliorating the response to conventional treatments as well as to advanced immunotherapeutic approaches, including adoptive transfer of NK cells and chimeric antigen receptor T or NK cells.

The Role of NK Cells and Innate Lymphoid Cells in Brain Cancer

The brain is considered an immune privileged site due to the high selectivity of the blood-brain barrier which restricts the passage of molecules and cells into the brain parenchyma. Recent studies have highlighted active immunosurveillance mechanisms in the brain. Here we review emerging evidence for the contribution of innate lymphoid cells (ILCs) including natural killer (NK) cells to the immunosurveillance of brain cancers focusing on glioblastoma, one of the most aggressive and most common malignant primary brain tumors diagnosed in adults. Moreover, we discuss how the local tissue microenvironment and unique cellular interactions influence ILC functions in the brain and how these interactions might be successfully harnessed for cancer immunotherapy using insights gained from the studies of autoimmunity, aging, and CNS injury.

Molecular Heterogeneity and Immunosuppressive Microenvironment in Glioblastoma

Glioblastoma (GBM) is the most aggressive primary brain tumor in adults, with a poor prognosis, despite surgical resection combined with radio- and chemotherapy. The major clinical obstacles contributing to poor GBM prognosis are late diagnosis, diffuse infiltration, pseudo-palisading necrosis, microvascular proliferation, and resistance to conventional therapy. These challenges are further compounded by extensive inter- and intra-tumor heterogeneity and the dynamic plasticity of GBM cells. The complex heterogeneous nature of GBM cells is facilitated by the local inflammatory tumor microenvironment, which mostly induces tumor aggressiveness and drug resistance. An immunosuppressive tumor microenvironment of GBM provides multiple pathways for tumor immune evasion. Infiltrating immune cells, mostly tumor-associated macrophages, comprise much of the non-neoplastic population in GBM. Further understanding of the immune microenvironment of GBM is essential to make advances in the development of immunotherapeutics. Recently, whole-genome sequencing, epigenomics and transcriptional profiling have significantly helped improve the prognostic and therapeutic outcomes of GBM patients. Here, we discuss recent genomic advances, the role of innate and adaptive immune mechanisms, and the presence of an established immunosuppressive GBM microenvironment that suppresses and/or prevents the anti-tumor host response.

Tissue-Resident NK Cells: Development, Maturation, and Clinical Relevance

Natural killer (NK) cells belong to type 1 innate lymphoid cells (ILC1) and are essential in killing infected or transformed cells. NK cells mediate their effector functions using non-clonotypic germ-line-encoded activation receptors. The utilization of non-polymorphic and conserved activating receptors promoted the conceptual dogma that NK cells are homogeneous with limited but focused immune functions. However, emerging studies reveal that NK cells are highly heterogeneous with divergent immune functions. A distinct combination of several activation and inhibitory receptors form a diverse array of NK cell subsets in both humans and mice. Importantly, one of the central factors that determine NK cell heterogeneity and their divergent functions is their tissue residency. Decades of studies provided strong support that NK cells develop in the bone marrow. However, evolving evidence supports the notion that NK cells also develop and differentiate in tissues. Here, we summarize the molecular basis, phenotypic signatures, and functions of tissue-resident NK cells and compare them with conventional NK cells.

Inhibitory checkpoints in human natural killer cells: IUPHAR Review 28

Immune checkpoint inhibitors have revolutionized cancer therapy leading to exceptional success. However, there is still the need to improve their efficacy in non‐responder patients. Natural killer (NK) cells represent the first line of defence against tumours, due to their ability to release immunomodulatory cytokines and kill target cells that have undergone malignant transformation. Harnessing NK cell response will open new possibilities to improve control of tumour growth. In this respect inhibitory checkpoints expressed on these innate lymphocytes represents a promising target for next‐generation immunotherapy. In this review, we will summarize recent evidences on the expression of NK cells receptors in cancer, with a focus on the inhibitory checkpoint programmed cell death protein 1 (PD‐1). We will also highlight the strength and limitations of the blockade of PD‐1 inhibitory pathway and suggest new combination strategies that may help to unleash more efficiently NK cell anti‐tumour response.

NK cells to cure cancer

Natural Killer (NK) cells are innate lymphocytes able to mediate immune-surveillance and clearance of viral infected and tumor-transformed cells. Growing experimental and clinical evidence highlighted a dual role of NK cells either in the control of cancer development/progression or in promoting the onset of immune-suppressant tumor microenvironments. Indeed, several mechanisms of NK cell-mediated tumor escape have been described and these includes cancer-induced aberrant expression of activating and inhibitory receptors (i.e. NK cell immune checkpoints), impairments of NK cell migration to tumor sites and altered NK cell effector-functions. These phenomena highly contribute to tumor progression and metastasis formation. In this review, we discuss the latest insights on those NK cell receptors and related molecules that are currently being implemented in clinics either as possible prognostic factors or therapeutic targets to unleash NK cell anti-tumor effector-functions in vivo. Moreover, we address here the major recent advances in regard to the genetic modification and ex vivo expansion of anti-tumor specific NK cells used in innovative adoptive cellular transfer approaches.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Spotlight on dinutuximab in the treatment of high-risk neuroblastoma: development and place in therapy

Neuroblastoma (NB) is a pediatric cancer of the sympathetic nervous system which accounts for 8% of childhood cancers. Most NBs express high levels of the disialoganglioside GD2. Several antibodies have been developed to target GD2 on NB, including the human/mouse chimeric antibody ch14.18, known as dinutuximab. Dinutuximab used in combination with granulocyte-macrophage colony-stimulating factor, interleukin-2, and isotretinoin (13-cis-retinoic acid) has a US Food and Drug Administration (FDA)-registered indication for treating high-risk NB patients who achieved at least a partial response to prior first-line multi-agent, multimodality therapy. The FDA registration resulted from a prospective randomized trial assessing the benefit of adding dinutuximab + cytokines to post-myeloablative maintenance therapy for high-risk NB. Dinutuximab has also shown promising antitumor activity when combined with temozolomide and irinotecan in treating NB progressive disease. Clinical activity of dinutuximab and other GD2-targeted therapies relies on the presence of the GD2 antigen on NB cells. Some NBs have been reported as GD2 low or negative, and such tumor cells could be nonresponsive to anti-GD2 therapy. As dinutuximab relies on complement and effector cells to mediate NB killing, factors affecting those components of patient response may also decrease dinutuximab effectiveness. This review summarizes the development of GD2 antibody-targeted therapy, the use of dinutuximab in both up-front and salvage therapy for high-risk NB, and the potential mechanisms of resistance to dinutuximab.

Splice variants of human natural cytotoxicity receptors: novel innate immune checkpoints

The natural cytotoxicity receptors (NCRs; NKp30, NKp44, and NKp46) were first defined as activating receptors on human NK cells that are important in recognition of and response to tumors. A flurry of recent research, however, has revealed that differential splicing can occur during transcription of each of the NCR genes, resulting in some transcripts that encode receptor isoforms with inhibitory functions. These alternative transcripts can arise in certain tissue microenvironments and appear to be induced by cytokines. Evidence indicates that some of the inhibitory NCRs are triggered by specific ligands, such as the interaction of the inhibitory isoform of NKp44 with PCNA on the surface of tumor cells. Here, we review the different NCR splice variants, cytokines that modulate their expression, their functional impacts on innate immune cells, and their differential expression in the contexts of cancer, pregnancy, and infections. The recent discovery of these inhibitory NCR isoforms has revealed novel innate immune checkpoints, many of which still lack defined ligands and clear mechanisms driving their expression. These NCR checkpoint pathways offer exciting potential therapeutic targets to manipulate innate immune functions under defined pathological conditions, such as cancer, pregnancy disorders, and pathogen exposure.

Microenvironment in neuroblastoma: isolation and characterization of tumor-derived mesenchymal stromal cells

Background

It has been proposed that mesenchymal stromal cells (MSCs) promote tumor progression by interacting with tumor cells and other stroma cells in the complex network of the tumor microenvironment. We characterized MSCs isolated and expanded from tumor tissues of pediatric patients diagnosed with neuroblastomas (NB-MSCs) to define interactions with the tumor microenvironment.

Methods

Specimens were obtained from 7 pediatric patients diagnosed with neuroblastoma (NB). Morphology, immunophenotype, differentiation capacity, proliferative growth, expression of stemness and neural differentiation markers were evaluated. Moreover, the ability of cells to modulate the immune response, i.e. inhibition of phytohemagglutinin (PHA) activated peripheral blood mononuclear cells (PBMCs) and natural killer (NK) cytotoxic function, was examined. Gene expression profiles, known to be related to tumor cell stemness, Wnt pathway activation, epithelial-mesenchymal transition (EMT) and tumor metastasis were also evaluated. Healthy donor bone marrow-derived MSCs (BM-MSC) were employed as controls.

Results

NB-MSCs presented the typical MSC morphology and phenotype. They showed a proliferative capacity superimposable to BM-MSCs. Stemness marker expression (Sox2, Nanog, Oct3/4) was comparable to BM-MSCs. NB-MSC in vitro osteogenic and chondrogenic differentiation was similar to BM-MSCs, but NB-MSCs lacked adipogenic differentiation capacity. NB-MSCs reached senescence phases at a median passage of P7 (range, P5-P13). NB-MSCs exhibited greater immunosuppressive capacity on activated T lymphocytes at a 1:2 (MSC: PBMC) ratio compared with BM-MSCs (p = 0.018). NK cytotoxic activity was not influenced by co-culture, either with BM-MSCs or NB-MSCs. Flow-cytometry cell cycle analysis showed that NB-MSCs had an increased number of cells in the G0-G1 phase compared to BM-MSCs. Transcriptomic profiling results indicated that NB-MSCs were enriched with EMT genes compared to BM-MSCs.

Conclusions

We characterized the biological features, the immunomodulatory capacity and the gene expression profile of NB-MSCs. The NB-MSC gene expression profile and their functional properties suggest a potential role in promoting tumor escape, invasiveness and metastatic traits of NB cancer cells. A better understanding of the complex mechanisms underlying the interactions between NB cells and NB-derived MSCs should shed new light on potential novel therapeutic approaches.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-5082-2) contains supplementary material, which is available to authorized users.

Adoptive immunotherapy with haploidentical natural killer cells and Anti-GD2 monoclonal antibody m3F8 for resistant neuroblastoma: Results of a phase I study

Natural killer (NK) cell-mediated antibody-dependent toxicity is a potent mechanism of action of the anti-GD2 murine monoclonal antibody 3F8 (m3F8). Killer immunoglobulin-like receptor (KIR) and HLA genotypes modulate NK activity and are key prognostic markers in m3F8-treated patients with neuroblastoma. Endogenous NK-cells are suppressed in the setting of high tumor burden and chemotherapy. Allogeneic NK-cells however, demonstrate potent anti-neuroblastoma activity. We report on the results of a phase I clinical trial of haploidentical NK-cells plus m3F8 administered to patients with high-risk neuroblastoma after conditioning chemotherapy. The primary objective was to determine the maximum tolerated NK-cell dose (MTD). Secondary objectives included assessing anti-neuroblastoma activity and its relationship to donor-recipient KIR/HLA genotypes, NK function, and donor NK chimerism. Patients received a lymphodepleting regimen prior to infusion of haploidentical CD3-CD56+ NK-cells, followed by m3F8. Overall and progression free survival (PFS) were assessed from the time of first NK-cell dose. Univariate Cox regression assessed relationship between dose and outcomes. Thirty-five patients received NK-cells at one of five dose levels ranging from <1×10⁶ to 50×10⁶ CD3-CD56+cells/kg. One patient experienced grade 3 hypertension and grade 4 pneumonitis. MTD was not reached. Ten patients (29%) had complete or partial response; 17 (47%) had no response; and eight (23%) had progressive disease. No relationship was found between response and KIR/HLA genotype or between response and FcγRIII receptor polymorphisms. Patients receiving >10×10⁶ CD56+cells/kg had improved PFS (HR: 0.36, 95%CI: 0.15-0.87, p = 0.022). Patient NK-cells displayed high NKG2A expression, leading to inhibition by HLA-E-expressing neuroblastoma cells. Adoptive NK-cell therapy in combination with m3F8 is safe and has anti-neuroblastoma activity at higher cell doses.

Natural Killer Cells Control Tumor Growth by Sensing a Growth Factor

Many tumors produce platelet-derived growth factor (PDGF)-DD, which promotes cellular proliferation, epithelial-mesenchymal transition, stromal reaction, and angiogenesis through autocrine and paracrine PDGFRβ signaling. By screening a secretome library, we found that the human immunoreceptor NKp44, encoded by NCR2 and expressed on natural killer (NK) cells and innate lymphoid cells, recognizes PDGF-DD. PDGF-DD engagement of NKp44 triggered NK cell secretion of interferon gamma (IFN)-γ and tumor necrosis factor alpha (TNF-α) that induced tumor cell growth arrest. A distinctive transcriptional signature of PDGF-DD-induced cytokines and the downregulation of tumor cell-cycle genes correlated with NCR2 expression and greater survival in glioblastoma. NKp44 expression in mouse NK cells controlled the dissemination of tumors expressing PDGF-DD more effectively than control mice, an effect enhanced by blockade of the inhibitory receptor CD96 or CpG-oligonucleotide treatment. Thus, while cancer cell production of PDGF-DD supports tumor growth and stromal reaction, it concomitantly activates innate immune responses to tumor expansion.

Veritable antiviral capacity of natural killer cells in chronic HBV infection: an argument for an earlier anti-virus treatment

Background

There is limited information on innate immunity, especially natural killer (NK) cell function, in different chronic hepatitis B (CHB) stages. Therefore, we examined whether the clinical staging strategy accurately reflects veritable NK cell immunity.

Methods

A total of 237 eligible CHB patients and 22 healthy controls were enrolled in our study. Demographic and clinical data were collected, and the CHB phases (immune active-IA, immune tolerant phase-IT, inactive CHB-IC, and grey zone-GZ) were classified according to the latest American Association for the Study of Liver Disease guidelines. Peripheral blood mononuclear cells from patients and healthy controls were tested for NK cell frequency, phenotype and function using flow cytometry.

Results

A significant decrease in activating receptor NKp44 and NKp46 expression and significant increase of exhaustion molecule Tim-3 expression were observed in NK cells from CHB patients. Reduced cytokine secretion and preserved or elevated cytotoxic function were also observed. Patients in the IT group exhibited comparable cytokine secretion and cytolytic capacity as age-matched IA patients. NK cell anti-viral functions were preserved in GZ patients. Some of the NK cell function in patients who were excluded from treatment by the current treatment guidelines was less compromised than patients who qualified for treatment.

Conclusion

Our findings provide evidence of veritable NK cell immunity during different natural history phases in treatment-naïve patients with chronic HBV Infection. Chronic HBV infection hindered NK cell function in CHB patients. However, the presumed IT and GZ statuses of CHB patients based on the clinical parameters may not accurately reflect the inner immune status of these patients and should be reconsidered. Some patients excluded from treatment by the current treatment guidelines may be able to be selected as candidates for treatment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-017-1318-1) contains supplementary material, which is available to authorized users.

Targeting Multiple Tumors Using T-Cells Engineered to Express a Natural Cytotoxicity Receptor 2-Based Chimeric Receptor

Recent developments in cancer treatment are demonstrating the increasing and powerful potential of immunotherapeutic strategies. In this regard, the adoptive transfer of tumor-specific T-lymphocytes approaches can lead to tumor regression in cancer patients. More recently, the use of T-cells genetically engineered to express cancer-specific receptors such as the anti-CD19 chimeric antigen receptor (CAR) continues to show promise for the treatment of hematological malignancies. Still, there is a crucial need to develop efficient CAR-T cell approaches for the treatment of solid tumors. It has been shown that other lymphocytes such as natural killer (NK) cells can demonstrate potent antitumor function—nonetheless, their use in immunotherapy is rather limited due to difficulties in expanding these cells to therapeutically relevant numbers and to suppression by endogenous inhibitory mechanisms. Cancer recognition by NK cells is partly mediated by molecules termed natural cytotoxicity receptors (NCRs). In the present study, we hypothesize that it is possible to endow T-cells with an NK recognition pattern, providing them with a mean to recognize tumor cells, in a non-MHC restricted way. To test this, we genetically modified human T-cells with different chimeric receptors based on the human NCR2 molecule and then assessed their antitumor activity in vitro and in vivo. Our results show that expression in primary lymphocytes of an NCR2-derived CAR, termed s4428z, confers T-cells with the ability to specifically recognize heterogeneous tumors and to mediate tumor cytotoxicity in a mouse model. This study demonstrates the benefit of combining tumor recognition capability of NK cells with T cell effectiveness to improve cancer immunotherapy.

Regulation of the Functions of Natural Cytotoxicity Receptors by Interactions with Diverse Ligands and Alterations in Splice Variant Expression

The natural cytotoxicity receptor (NCR) family is constituted by NKp46, NKp44, and NKp30 in humans, which are expressed mainly on natural killer (NK) cells and are encoded by the ncr1, ncr2, and ncr3 genes, respectively. NCRs have classically been defined as activating receptors that trigger cytotoxicity and cytokine responses by NK cells upon engaging with ligands on tumor cells. Several new findings, however, have challenged this model and identified alternative mechanisms regulating the function of NCRs. Recent reports indicate that ligand matters, since the interaction of NKp44 with distinct ligands on target cells can either activate or inhibit NK cells. Also, the NCRs have been found to interact with distinct specificities to various heparan sulfate glycosaminoglycans, which are complex polysaccharides found in extracellular matrix or on cell surface heparan sulfate proteoglycans (HSPGs). The NCRs can engage with HSPGs in trans as a co-ligand on the target cells or in cis on the NK cell surface to regulate receptor–ligand interactions and NK cell activation. A number of splice variants of ncr2 and ncr3 have also been identified, and a predominant expression of certain variants results in inhibitory signaling through NKp44 and NKp30. Several recent studies have found that the selective expression of some of these inhibitory splice variants can significantly influence outcome in the contexts of cancer, infection, and pregnancy. These findings establish that NCR functions are more diverse than originally thought, and better understanding of their splice variant expression profiles and ligand interactions are needed to establish their functional regulation in the context of human health.

Granule-Dependent Natural Killer Cell Cytotoxicity to Fungal Pathogens

Natural killer (NK) cells kill or inhibit the growth of a number of fungi including Cryptococcus, Candida, Aspergillus, Rhizopus, and Paracoccidioides. Although many fungi are not dangerous, invasive fungal pathogens, such as Cryptococcus neoformans, cause life-threatening disease in individuals with impaired cell-mediated immunity. While there are similarities to cell-mediated killing of tumor cells, there are also important differences. Similar to tumor killing, NK cells directly kill fungi in a receptor-mediated and cytotoxic granule-dependent manner. Unlike tumor cell killing where multiple NK cell-activating receptors cooperate and signal events that mediate cytotoxicity, only the NKp30 receptor has been described to mediate signaling events that trigger the NK cell to mobilize its cytolytic payload to the site of interaction with C. neoformans and Candida albicans, subsequently leading to granule exocytosis and fungal killing. More recently, the NKp46 receptor was reported to bind Candida glabrata adhesins Epa1, 6, and 7 and directly mediate fungal clearance. A number of unanswered questions remain. For example, is only one NK cell-activating receptor sufficient for signaling leading to fungal killing? Are the signaling pathways activated by fungi similar to those activated by tumor cells during NK cell killing? How do the cytolytic granules traffic to the site of interaction with fungi, and how does this process compare with tumor killing? Recent insights into receptor use, intracellular signaling and cytolytic granule trafficking during NK cell-mediated fungal killing will be compared to tumor killing, and the implications for therapeutic approaches will be discussed.

Natural killer cell subsets and receptor expression in peripheral blood mononuclear cells of a healthy Korean population: Reference range, influence of age and sex, and correlation between NK cell receptors and cytotoxicity

BACKGROUND

The purpose of this study was to identify CD56^bright and CD56^dim natural killer (NK) cell subsets and analyze their receptors expression in a healthy Korean population, and to determine whether receptor expression correlates with age, sex, and cytotoxicity.

MATERIALS AND METHODS

We performed multicolor flow cytometry assays to analyze the expression of various NK cell receptors (CD16, NKG2A, NKG2C, NKG2D, CD57, DNAM-1, CD8a, CD62L, NKp30, and NKp46) on both CD3^-/CD56^dim and CD3^-/CD56^bright NK cells in whole-blood samples from 122 healthy donors. The expression of these receptors was compared according to age (<30years, n=22, 30-60years, n=73 and >60years, n=27) and gender (male, n=61, female, n=61). NK cell cytotoxicity assays were performed with peripheral blood mononuclear cells (PBMCs) from 18 individuals. The results were compared to the expression levels of NKp30 and NKp46 receptors.

RESULTS

A normal reference range for NK cell receptor expression in two NK cell subsets was established. NKp46 and NKG2D expression gradually decreased with age (p<0.01 and p<0.05, respectively) whereas NK cell proportion and numbers, frequencies of CD56^dim cells, and CD57 expression increased with age (p<0.01 in all cases). Men showed greater NK cell proportion and numbers, frequencies of CD56^dim cells, and CD57 expression compared to those of women (p<0.05 and p<0.001; p<0.01 and p<0.01, respectively). Notably, the expression of NKp46 was negatively correlated with NK cell frequency (r=-0.42, p<0.001). Furthermore, NK cell cytotoxicity was found to positively correlate with NCR expression (p=0.02), but not NK cell proportion (p=0.80).

CONCLUSION

We have established a profile of NK cell surface receptors for a Korean population, and revealed that age and gender have an effect on the expression of NK cell receptors in the population. Our data might explain why neither NK cell numbers nor proportions correlate with NK cell cytotoxicity.

Altered Expression of Natural Cytotoxicity Receptors and NKG2D on Peripheral Blood NK Cell Subsets in Breast Cancer Patients

Human natural killer (NK) cells are considered professional cytotoxic cells that are integrated into the effector branch of innate immunity during antiviral and antitumoral responses. The purpose of this study was to examine the peripheral distribution and expression of NK cell activation receptors from the fresh peripheral blood mononuclear cells of 30 breast cancer patients prior to any form of treatment (including surgery, chemotherapy, and radiotherapy), 10 benign breast pathology patients, and 24 control individuals. CD3⁻CD56^dimCD16^bright NK cells (CD56^dim NK) and CD3⁻CD56^brightCD16^dim/− NK cells (CD56^bright NK) were identified using flow cytometry. The circulating counts of CD56^dim and CD56^bright NK cells were not significantly different between the groups evaluated, nor were the counts of other leukocyte subsets between the breast cancer patients and benign breast pathology patients. However, in CD56^dim NK cells, NKp44 expression was higher in breast cancer patients (P = .0302), whereas NKp30 (P = .0005), NKp46 (P = .0298), and NKG2D (P = .0005) expression was lower with respect to healthy donors. In CD56^bright NK cells, NKp30 (P = .0007), NKp46 (P = .0012), and NKG2D (P = .0069) expression was lower in breast cancer patients compared with control group. Only NKG2D in CD56^bright NK cells (P = .0208) and CD56^dim NK cells (P = .0439) showed difference between benign breast pathology and breast cancer patients. Collectively, the current study showed phenotypic alterations in activation receptors on CD56^dim and CD56^bright NK cells, suggesting that breast cancer patients have decreased NK cell cytotoxicity.

New immunotherapeutic strategies for the treatment of neuroblastoma

The prognosis of high-risk neuroblastoma (NB) is still poor, in spite of aggressive multimodal treatment. Recently, adjuvant immunotherapy with anti-GD2 antibodies combined with IL-2 or GM-CSF has been shown to improve survival. Several other immunotherapy strategies proved efficacy in preclinical models of NB, including different types of vaccines, adoptive cell therapies and combined approaches. The remarkable differences in the immunobiology of syngeneic models and human NB may, at least in part, limit the translation of preclinical therapies to a clinical setting. Nonetheless, several preliminary evidences suggest that new antibodies, cancer vaccines and adoptive transfer of lymphocytes, genetically engineered to acquire NB specificity, may result in clinical benefit, and clinical studies are currently ongoing.

Clinical impact of the NKp30/B7-H6 axis in high-risk neuroblastoma patients

The immunosurveillance mechanisms governing high-risk neuroblastoma (HR-NB), a major pediatric malignancy, have been elusive. We identify a potential role for natural killer (NK) cells, in particular the interaction between the NK receptor NKp30 and its ligand, B7-H6, in the metastatic progression and survival of HR-NB after myeloablative multimodal chemotherapy and stem cell transplantation. NB cells expressing the NKp30 ligand B7-H6 stimulated NK cells in an NKp30-dependent manner. Serum concentration of soluble B7-H6 correlated with the down-regulation of NKp30, bone marrow metastases, and chemoresistance, and soluble B7-H6 contained in the serum of HR-NB patients inhibited NK cell functions in vitro. The expression of distinct NKp30 isoforms affecting the polarization of NK cell functions correlated with 10-year event-free survival in three independent cohorts of HR-NB in remission from metastases after induction chemotherapy (n = 196, P < 0.001), adding prognostic value to known risk factors such as N-Myc amplification and age >18 months. We conclude that the interaction between NKp30 and B7-H6 may contribute to the fate of NB patients and that both the expression of NKp30 isoforms on circulating NK cells and the concentration of soluble B7-H6 in the serum may be clinically useful as biomarkers for risk stratification.

Cancer Stem Cells and Their Interaction with the Tumor Microenvironment in Neuroblastoma

Neuroblastoma, a solid tumor arising from neural crest cells, accounts for over 15% of all pediatric cancer deaths. The interaction of neuroblastoma cancer-initiating cells with their microenvironment likely plays an integral role in the maintenance of resistant disease and tumor relapse. In this review, we discuss the interaction between neuroblastoma cancer-initiating cells and the elements of the tumor microenvironment and how these interactions may provide novel therapeutic targets for this difficult to treat disease.

Natural cytotoxicity receptor splice variants orchestrate the distinct functions of human natural killer cell subtypes

The natural cytotoxicity receptors NKp46/NCR1, NKp44/NCR2 and NKp30/NCR3 are critical for natural killer (NK) cell functions. Their genes are transcribed into several splice variants whose physiological relevance is not yet fully understood. Here we report that decidua basalis NK (dNK) cells of the pregnant uterine mucosa and peripheral blood NK (pNK) cells, two functionally distinct subsets of the physiological NK cell pool, display differential expression of NKp30/NCR3 and NKp44/NCR2 splice variants. The presence of cytokines that are enriched within the decidual microenvironment is sufficient to convert the splice variant profile of pNK cells into one similar to that of dNK cells. This switch is associated with decreased cytotoxic function and major adaptations to the secretome, hallmarks of the decidual phenotype. Thus, NKp30/NCR3 and NKp44/NCR2 splice variants delineate functionally distinct NK cell subsets. To our knowledge, this is the first conclusive evidence underlining the physiological importance of NCR splice variants.

Decidual natural killer (NK) cells from the pregnant uterus play an important role in the physiology of pregnancy and differ functionally from peripheral blood NK cells. Siewiera et al. reveal that this is partly due to the differential expression of splice variants of natural cytotoxicity receptors by these two cell subsets.

An NCR1-based chimeric receptor endows T-cells with multiple anti-tumor specificities

The Ral (Ras-like) GTP-binding proteins (RalA and RalB), as effectors of the proto-oncogene Natural killer (NK) cells are an important component of the anti-tumor response. Tumor recognition by NK cells was found to be partly triggered by molecules termed natural cytotoxic receptors (NCRs). Adoptive transfer of genetically-engineered tumor-reactive T-lymphocytes can mediate remarkable tumor regressions mostly in melanoma and leukemia patients. Yet, the application of such treatments to other cancers is needed and dependent on the isolation of receptors that could facilitate efficient recognition of these malignancies. Herein, we aimed at combining NK tumor recognition capability with the genetic modification of T-cells to provide the latter with a means to recognize several tumors in a non-MHC restricted way. Consequently, we generated and evaluated several chimeric receptors based on the extracellular domain of NCR1 (NKp46) fused to multiple signaling moieties and assess their antitumor activity when retrovirally expressed in T-cells. Following co-culture with different tumors, primary human T-lymphocytes expressing a chimeric NCR1 molecule recognized target cells derived from lung, cervical carcinoma, leukemia and pancreatic cancer. In addition, this receptor mediated an upregulation of surface activation markers and significant antitumor cytotoxicity both in vitro and in vivo. These results have meaningful implications for the immunotherapeutic treatment of cancer using gene-modified T-cells.

Increased frequencies of the killer immunoglobulin-like receptor genes KIR2DL2 and KIR2DS2 are associated with neuroblastoma

Neuroblastoma is the most common extra-cranial solid tumour in children. Natural killer (NK) cells are innate lymphocytes that are known to mediate the direct cytotoxicity of neuroblastoma tumour cells. Natural variation in the highly polymorphic killer immunoglobulin-like receptors (KIR) and their cognate human leukocyte antigen (HLA) class I ligands results in considerable diversity in NK cell function. As the early onset of neuroblastoma suggests the contribution of genetic factors, we investigated if individual KIR genes, combined KIR gene haplotypes or compound KIR-HLA ligand genotypes could influence susceptibility to neuroblastoma. Genotype analysis of the KIR genes as well as their three major HLA class I ligand groups, HLA-C1, HLA-C2 and HLA-Bw4, was carried out in a cohort of 201 neuroblastoma patients compared with 240 healthy control subjects using polymerase chain reaction with sequence-specific primers. We found a significant increase in the frequency of KIR2DL2 (P = 0.019) as well as KIR2DS2 (P = 0.008) in patients with neuroblastoma compared with the healthy control group. While the incidence of the least inhibitory compound KIR-HLA-C genotype, KIR2DL3 in the presence of HLA-C1 was slightly reduced in neuroblastoma patients, this did not reach statistical significance (P = 0.069). In summary, while KIR-HLA compound genotypes have previously been implicated in predicting treatment outcomes in neuroblastoma, here we show that the presence of the individual KIR genes, KIR2DL2 and KIR2DS2, irrespective of HLA-C genotype is associated with the onset of this embryonal malignancy.

NKp44 and Natural Cytotoxicity Receptors as Damage-Associated Molecular Pattern Recognition Receptors

Natural killer (NK) cells are a key constituent of the innate immune system, protecting against bacteria, virally infected cells, and cancer. Recognition and protective function against such cells are dictated by activating and inhibitory receptors on the surface of the NK cell, which bind to specific ligands on the surface of target cells. Among the activating receptors is a small class of specialized receptors termed the natural cytotoxicity receptors (NCRs) comprised of NKp30, NKp46, and NKp44. The NCRs are key receptors in the recognition and termination of virally infected and tumor cells. Since their discovery over 10 years ago, ligands corresponding to the NCRs have largely remained elusive. Recent identification of the cellular ligands for NKp44 and NKp30 as exosomal proliferating cell nuclear antigen (PCNA) and HLA-B-associated transcript 3 (BAT3), respectively, implicate that NCRs may function as receptors for damage-associated molecular pattern (DAMP) molecules. In this review, we focus on NKp44, which surprisingly recognizes two distinct ligands resulting in either activation or inhibition of NK cell effector responses in response to tumor cells. The inhibitory function of NKp44 requires further study as it may play a pivotal role in placentation in addition to being exploited by tumors as a mechanism to escape NK cell killing. Finally, we suggest that the NCRs are a class of pattern recognition receptors, which recognize signals of genomic instability and cellular stress via interaction with the c-terminus of DAMP molecules localized to the surface of target cells by various co-ligands.

NK cells with KIR2DS2 immunogenotype have a functional activation advantage to efficiently kill glioblastoma and prolong animal survival

Glioblastomas (GBMs) are lethal brain cancers that are resistant to current therapies. We investigated the cytotoxicity of human allogeneic NK cells against patient-derived GBM in vitro and in vivo, as well as mechanisms mediating their efficacy. We demonstrate that KIR2DS2 immunogenotype NK cells were more potent killers, notwithstanding the absence of inhibitory killer Ig-like receptor (KIR)-HLA ligand mismatch. FACS-sorted and enriched KIR2DS2(+) NK cell subpopulations retained significantly high levels of CD69 and CD16 when in contact with GBM cells at a 1:1 ratio and highly expressed CD107a and secreted more soluble CD137 and granzyme A. In contrast, KIR2DS2(-) immunogenotype donor NK cells were less cytotoxic against GBM and K562, and, similar to FACS-sorted or gated KIR2DS2(-) NK cells, significantly diminished CD16, CD107a, granzyme A, and CD69 when in contact with GBM cells. Furthermore, NK cell-mediated GBM killing in vitro depended upon the expression of ligands for the activating receptor NKG2D and was partially abrogated by Ab blockade. Treatment of GBM xenografts in NOD/SCID mice with NK cells from a KIR2DS2(+) donor lacking inhibitory KIR-HLA ligand mismatch significantly prolonged the median survival to 163 d compared with vehicle controls (log-rank test, p = 0.0001), in contrast to 117.5 d (log-rank test, p = 0.0005) for NK cells with several inhibitory KIR-HLA ligand mismatches but lacking KIR2DS2 genotype. Significantly more CD56(+)CD16(+) NK cells from a KIR2DS2(+) donor survived in nontumor-bearing brains 3 wk after infusion compared with KIR2DS2(-) NK cells, independent of their proliferative capacity. In conclusion, KIR2DS2 identifies potent alloreactive NK cells against GBM that are mediated by commensurate, but dominant, activating signals.

Interleukin-2-stimulated natural killer cells are less susceptible to mycophenolate mofetil than non-activated NK cells: possible consequences for immunotherapy

In a clinical phase I/II trial, pediatric patients with high-risk malignancies were treated with ex vivo IL-2-stimulated donor natural killer (NK) cells after transplantation with haploidentical stem cells. To evaluate the potential negative effects of the immunosuppressive drug mycophenolate mofetil (MMF) used for immunotherapy, the functionality and signaling of ex vivo NK cells was investigated. Our results show that during NK cell expansion, long-term (9 days) incubation with mycophenolic acid (MPA), the active metabolite of MMF, in therapeutically relevant concentrations led to the severe inhibition of NK cell proliferation. This correlated with a significantly reduced cytokine/chemokine secretion and the inhibited acquisition of surface receptors regarding cytotoxicity (e.g., NKp30, NKp44, NKp46, NKG2D), adhesion/migration (e.g., ICAM-1/CD54, LFA-1/CD11a, CD62L, CXCR3) and activation (e.g., CD25). Moreover, MPA prevented phosphorylation of the central signaling molecules STAT-3/-4/-5, AKT and ERK1/2. In contrast, short-term (24 h) MPA incubation of IL-2-stimulated NK cells had no or only marginal effects on the activated NK cell phenotype, including receptor expression, cytokine/chemokine secretion and intracellular signaling. Further, short-term MPA incubation only moderately affected the highly cytotoxic activity of previously IL-2-stimulated NK cells. In conclusion, while long-term MPA incubation significantly compromised ex vivo NK cell functionality, previously IL-2-activated NK cells seemed to be rather resistant to short-term MPA treatment. This finding supports the use of IL-2-activated NK cells as immunotherapy, especially for patients treated with MMF after haploidentical stem cell transplantation.

NK cell receptor NKp46 regulates graft-versus-host disease

Hematopoietic stem cell transplantation (HSCT) is often the only curative treatment for a wide variety of hematologic malignancies. Donor selection in these diseases is crucial, given that transplanted cells can mediate not only the desired graft-versus-leukemia effect but also graft-versus-host disease (GVHD). Here, we demonstrate that in the absence of NKp46, a major killer receptor expressed by human and mouse natural killer (NK) cells, GVHD is greatly exacerbated, resulting in rapid mortality of the transplanted animals because of infection with commensal bacteria. Furthermore, we demonstrate that the exacerbated GVHD is the result of an altered ability of immune cells to respond to stimulation by immature dendritic cells. Because high and low expression of NKp46 on NK cells is observed in different individuals, our data indicate that choosing NKp46-high donors for the treatment of different hematologic malignancies might lead to better tumor eradication while minimizing GVHD.

Natural killer cells and neuroblastoma: tumor recognition, escape mechanisms, and possible novel immunotherapeutic approaches

Neuroblastoma (NB) is the most common extra-cranial solid tumor of childhood and arises from developing sympathetic nervous system. Most primary tumors localize in the abdomen, the adrenal gland, or lumbar sympathetic ganglia. Amplification in tumor cells of MYCN, the major oncogenic driver, patients' age over 18 months, and the presence at diagnosis of a metastatic disease (stage IV, M) identify NB at high risk of treatment failure. Conventional therapies did not significantly improve the overall survival of these patients. Moreover, the limited landscape of somatic mutations detected in NB is hampering the development of novel pharmacological approaches. Major efforts aim to identify novel NB-associated surface molecules that activate immune responses and/or direct drugs to tumor cells and tumor-associated vessels. PVR (Poliovirus Receptor) and B7-H3 are promising targets, since they are expressed by most high-risk NB, are upregulated in tumor vasculature and are essential for tumor survival/invasiveness. PVR is a ligand of DNAM-1 activating receptor that triggers the cytolytic activity of natural killer (NK) cells against NB. In animal models, targeting of PVR with an attenuated oncolytic poliovirus induced tumor regression and elimination. Also B7-H3 was successfully targeted in preclinical studies and is now being tested in phase I/II clinical trials. B7-H3 down-regulates NK cytotoxicity, providing NB with a mechanism of escape from immune response. The immunosuppressive potential of NB can be enhanced by the release of soluble factors that impair NK cell function and/or recruitment. Among these, TGF-β1 modulates the cytotoxicity receptors and the chemokine receptor repertoire of NK cells. Here, we summarize the current knowledge on the main cell surface molecules and soluble mediators that modulate the function of NK cells in NB, considering the pros and cons that must be taken into account in the design of novel NK cell-based immunotherapeutic approaches.

Primitive neuroectodermal tumor in an ovarian cystic teratoma: natural killer and neuroblastoma cell analysis

In the present study, we report an extremely rare case of a 31-year-old woman with neuroblastoma arising in an ovarian cystic teratoma. We analyzed the expression of activating receptors on natural killer (NK) cells derived from the patient's peripheral blood and peritoneal fluid. In addition, we investigated the presence of specific ligands recognized by different NK cell receptors on tumor cells. We show that NK cells isolated from peritoneal fluid expressed certain triggering receptors including DNAM-1 (CD226) and CD16 with lower intensity as compared to peripheral blood NK cells. Remarkably, at variance with most cases of childhood neuroblastoma, the tumor cells from this patient expressed substantial amounts of HLA class-I molecules. These molecules are known to be protective against NK cell-mediated lysis. In addition, neuroblastoma cells expressed B7-H3 (CD276), another surface molecule that inhibits NK cell function. Finally, this tumor did not express the PVR (CD155) and nectin-2 (CD112) ligands for the DNAM-1 activating NK receptor, which plays a crucial role in NK/neuroblastoma interactions. Altogether, these findings indicate that the neuroblastoma cells of this patient express an NK-resistant surface phenotype, which is at least in part similar to that previously described in a fraction of childhood neuroblastoma.

Natural ligands and antibody-based fusion proteins: harnessing the immune system against cancer

The insight that the immune system is able to eradicate tumor cells inspired the development of targeted immunotherapies. These novel approaches aim to trigger immune molecules and receptors, including CD3 on T cells and NKG2D and NKp30 on natural killer (NK) cells, to harness the immune system against cancer. In cancer patients, overcoming immune suppression induced by malignant cells or by the tumor microenvironment remains the major challenge to the clinical efficacy of immunotherapies. Recombinant constructs have been developed in various formats either utilizing natural ligands (immunoligands) or antibody-derived components (immunoconstructs) to circumvent mechanisms that counteract an effective antitumor immune response.

Novel interaction between proliferating cell nuclear antigen and HLA I on the surface of tumor cells inhibits NK cell function through NKp44

NK cell function is closely regulated by numerous inhibitory and activating receptors binding corresponding ligands on the surface of target cells, providing vital first line defenses against infections and cancer. NKp44, originally discovered as an activating NK cell receptor, was recently found to elicit inhibitory effects on NK cell effector function through recognition of cell surface PCNA. Other reports have pointed to potential associations between NKp44 and HLA I molecules, as well as HLA I and Damage Associated Molecular Pattern molecules (DAMPs) on the surface of tumor cells. In this report, we have identified novel interaction between HLA I and PCNA on the surface of human tumor cells by confocal microscopy and immunoprecipitation. In addition to previous reports, we show PCNA on the cell surface where novel association with HLA I does not require the presence of NKp44 expressing NK cells and occurs with endogenous PCNA. The association of HLA I and PCNA forms the inhibitory ligand for NKp44, resulting in inhibition of NK cell cytotoxicity. We further postulate NCR ligands are composed of DAMP molecules localized to the cell surface, colocalizing with HLA I, and potentially heparin sulfate proteoglycans.

Tumor autoantibodies as biomarkers for predicting ovarian cancer recurrence

Ovarian cancer (OVCA) has a high incidence of recurrence and a high rate of mortality. We performed a pilot study to evaluate the usefulness of tumor autoantibodies to tumor associated antigens (TAA) to predict OVCA recurrence. A validation study with 56 antigens, previously identified in the initial phase of the study, along with 13 known tumor antigens on protein arrays was performed on an independent cohort of recurrent and non-recurrent OVCA patients. Statistical analyses revealed that a panel of 3 antigens predicted recurrence at a median time of 9.07 months prior to clinical recurrence in a study population, where majority of patients had CA125 values less than 35 U/ml, with an average sensitivity, specificity and accuracy of 94.7%, 86.7% and 93.3% respectively. One of the top 3 antigens has been associated with the development of polymyositis (PM) which has been shown in some cases to precede the occurrence of ovarian carcinoma. Our results indicate that these 3 antigens have potential for predicting recurrence at an early time and may have better prognostic utility than CA125 alone for early therapeutic intervention. These biomarkers could guide us to identify those patients that could benefit most from maintenance or consolidation therapy.

Ex vivo activation of CD56(+) immune cells that eradicate neuroblastoma

Despite the use of intensive contemporary multimodal therapy, the overall survival of patients with high-risk neuroblastoma is still less than 50%. Therefore, immunotherapy without cross-resistance and overlapping toxicity has been proposed. In this study, we report the development of a novel strategy to specifically activate and expand human CD56(+) (NCAM1) natural killer (NK) immune cells from normal donors and patients with neuroblastoma. Enriched CD56(+) cells from peripheral blood were mixed with CD56(-) fraction at 1:1 ratio and cultured in the presence of OKT3, interleukin (IL)-2, and -15 for five days and then without OKT3 for 16 more days. The final products contained more than 90% CD56(+) cells and could kill neuroblastoma cells effectively that were originally highly resistant to nonprocessed NK cells. Mechanistically, cytolysis of neuroblastoma was mediated through natural cytotoxicity receptor (NCR), DNAX accessory molecule-1 (DNAM-1; CD226), perforin, and granzyme B. Successful clinical scale-up in a good manufacturing practices (GMP)-compliant bioreactor yielded effector cells that in a neuroblastoma xenograft model slowed tumor growth and extended survival without GVHD. Investigation of CD56(+) cells from patients with neuroblastoma revealed a similar postactivation phenotype and lytic activity. Our findings establish a novel and clinically expedient strategy to generate allogeneic or autologous CD56(+) cells that are highly cytotoxic against neuroblastoma with minimal risk of GVHD.

Mimicking an induced self phenotype by coating lymphomas with the NKp30 ligand B7-H6 promotes NK cell cytotoxicity

Induced self expression of the NKp30 ligand B7-H6 facilitates NK cell-mediated elimination of stressed cells. A fusion protein consisting of the ectodomain of B7-H6 and the CD20 single-chain fragment variable 7D8 was generated to mimic an induced self phenotype required for NK cell-mediated target cell elimination. B7-H6:7D8 had bifunctional properties as reflected by its ability to simultaneously bind to the CD20 Ag and to the NKp30 receptor. B7-H6:7D8 bound by CD20(+) lymphoma cells activated human NK cells and triggered degranulation. Consequently, the immunoligand B7-H6:7D8 induced killing of lymphoma-derived cell lines as well as fresh tumor cells from chronic lymphocytic leukemia or lymphoma patients. B7-H6:7D8 was active at nanomolar concentrations in a strictly Ag-specific manner and required interaction with both CD20 and NKp30. Remarkably, NK cell cytotoxicity was further augmented by concomitant activation of Fcγ receptor IIIa or NK group 2 member D. Thus, B7-H6:7D8 acted synergistically with the CD20 Ab rituximab and the immunoligand ULBP2:7D8, which was similarly designed as B7-H6:7D8 but engaging the NK group 2 member D receptor. In conclusion, to our knowledge, B7-H6:7D8 represents the first Ab-based molecule stimulating NKp30-mediated NK cell cytotoxicity for therapeutic purposes and provides proof of concept that Ag-specific NKp30 engagement may represent an innovative strategy to enhance antitumoral NK cell cytotoxicity.

Unlicensed NK cells target neuroblastoma following anti-GD2 antibody treatment

Survival outcomes for patients with high-risk neuroblastoma (NB) have significantly improved with anti-disialoganglioside GD2 mAb therapy, which promotes NK cell activation through antibody-dependent cell-mediated cytotoxicity. NK cell activation requires an interaction between inhibitory killer cell immunoglobulin-like receptors (KIRs) and HLA class I ligands. NK cells lacking KIRs that are specific for self HLA are therefore "unlicensed" and hyporesponsive. mAb-treated NB patients lacking HLA class I ligands for their inhibitory KIRs have significantly higher survival rates, suggesting that NK cells expressing KIRs for non-self HLA are mediating tumor control in these individuals. We found that, in the presence of mAb, both licensed and unlicensed NK cells are highly activated in vitro. However, HLA class I expression on NB cell lines selectively inhibited licensed NK cell activity, permitting primarily unlicensed NK cells to mediate antibody-dependent cell-mediated cytotoxicity. These results indicate that unlicensed NK cells play a key antitumor role in patients undergoing mAb therapy via antibody-dependent cell-mediated cytotoxicity, thus explaining the potent "missing KIR ligand" benefit in patients with NB.

The stalk domain and the glycosylation status of the activating natural killer cell receptor NKp30 are important for ligand binding

The natural cytotoxicity receptors are a unique set of activating proteins expressed mainly on the surface of natural killer (NK) cells. The human natural cytotoxicity receptor family comprises the three type I membrane proteins NKp30, NKp44, and NKp46. Especially NKp30 is critical for the cytotoxicity of NK cells against different targets including tumor, virus-infected, and immature dendritic cells. Although the crystal structure of NKp30 was recently solved (Li, Y., Wang, Q., and Mariuzza, R. A. (2011) J. Exp. Med. 208, 703-714; Joyce, M. G., Tran, P., Zhuravleva, M. A., Jaw, J., Colonna, M., and Sun, P. D. (2011) Proc. Natl. Acad. Sci. U.S.A. 108, 6223-6228), a key question, how NKp30 recognizes several non-related ligands, remains unclear. Therefore, we investigated the parameters that impact ligand recognition of NKp30. Based on various NKp30-hIgG1-Fc fusion proteins, which were optimized for minimal background binding to cellular Fcγ receptors, we identified the flexible stalk region of NKp30 as an important but so far neglected module for ligand recognition and related signaling of the corresponding full-length receptor proteins. Moreover, we found that the ectodomain of NKp30 is N-linked glycosylated at three different sites. Mutational analyses revealed differential binding affinities and signaling capacities of mono-, di-, or triglycosylated NKp30, suggesting that the degree of glycosylation could provide a switch to modulate the ligand binding properties of NKp30 and NK cell cytotoxicity.

Immunology and immunotherapy of neuroblastoma

PURPOSE

This review demonstrates the importance of immunobiology and immunotherapy research for understanding and treating neuroblastoma.

PRINCIPAL RESULTS

The first suggestions of immune system-neuroblastoma interactions came from in vitro experiments showing that lymphocytes from patients were cytotoxic for their own tumor cells and from evaluations of tumors from patients that showed infiltrations of immune system cells. With the development of monoclonal antibody (mAb) technology, a number of mAbs were generated against neuroblastoma cells lines and were used to define tumor associated antigens. Disialoganglioside (GD2) is one such antigen that is highly expressed by virtually all neuroblastoma cells and so is a useful target for both identification and treatment of tumor cells with mAbs. Preclinical research using in vitro and transplantable tumor models of neuroblastoma has demonstrated that cytotoxic T lymphocytes (CTLs) can specifically recognize and kill tumor cells as a result of vaccination or of genetic engineering that endows them with chimeric antigen receptors. However, CTL based clinical trials have not progressed beyond pilot and phase I studies. In contrast, anti-GD2 mAbs have been extensively studied and modified in pre-clinical experiments and have progressed from phase I through phase III clinical trials. Thus, the one proven beneficial immunotherapy for patients with high-risk neuroblastoma uses a chimeric anti-GD2 mAb combined with IL-2 and GM-CSF to treat patients after they have received intensive cyto-reductive chemotherapy, irradiation, and surgery. Ongoing pre-clinical and clinical research emphasizes vaccine, adoptive cell therapy, and mAb strategies. Recently it was shown that the neuroblastoma microenvironment is immunosuppressive and tumor growth promoting, and strategies to overcome this are being developed to enhance anti-tumor immunotherapy.

CONCLUSIONS

Our understanding of the immunobiology of neuroblastoma has increased immensely over the past 40 years, and clinical translation has shown that mAb based immunotherapy can contribute to improving treatment for high-risk patients. Continued immunobiology and pre-clinical therapeutic research will be translated into even more effective immunotherapeutic strategies that will be integrated with new cytotoxic drug and irradiation therapies to improve survival and quality of life for patients with high-risk neuroblastoma.

Involvement of activating NK cell receptors and their modulation in pathogen immunity

Natural Killer (NK) cells are endowed with cell-structure-sensing receptors providing inhibitory protection from self-destruction (inhibitory NK receptors, iNKRs, including killer inhibitory receptors and other molecules) and rapid triggering potential leading to functional cell activation by Toll-like receptors (TLRs), cytokine receptors, and activating NK cell receptors including natural cytotoxicity receptors (NCRs, i.e., NKp46, NKp46, and NKp44). NCR and NKG2D recognize ligands on infected cells which may be endogenous or may directly bind to some structures derived from invading pathogens. In this paper, we address the known direct or indirect interactions between activating receptors and pathogens and their expression during chronic HIV and HCV infections.

A novel Ncr1-Cre mouse reveals the essential role of STAT5 for NK-cell survival and development

We generated a transgenic mouse line that expresses the Cre recombinase under the control of the Ncr1 (p46) promoter. Cre-mediated recombination was tightly restricted to natural killer (NK) cells, as revealed by crossing Ncr1-iCreTg mice to the eGFP-LSLTg reporter strain. Ncr1-iCreTg mice were further used to study NK cell-specific functions of Stat5 (signal transducers and activators of transcription 5) by generating Stat5(f/f) Ncr1-iCreTg animals. Stat5(f/f) Ncr1-iCreTg mice were largely devoid of NK cells in peripheral lymphoid organs. In the bone marrow, NK-cell maturation was abrogated at the NK cell-precursor stage. Moreover, we found that in vitro deletion of Stat5 in interleukin 2-expanded NK cells was incompatible with NK-cell viability. In vivo assays confirmed the complete abrogation of NK cell-mediated tumor control against B16F10-melanoma cells. In contrast, T cell-mediated tumor surveillance against MC38-adenocarcinoma cells was undisturbed. In summary, the results of our study show that STAT5 has a cell-intrinsic role in NK-cell development and that Ncr1-iCreTg mice are a powerful novel tool with which to study NK-cell development, biology, and function.