Rapid and potent induction of cell death and loss of NK cell cytotoxicity against oral tumors by F(ab')2 fragment of anti-CD16 antibody

Emerging combined CAR-NK cell therapies in cancer treatment: Finding a dancing partner

In recent decades, immunotherapy with chimeric antigen receptors (CARs) has revolutionized cancer treatment and given hope where other cancer therapies have failed. CAR-natural killer (NK) cells are NK cells that have been engineered ex vivo with a CAR on the cell membrane with high specificity for specific target antigens of tumor cells. The impressive results of several studies suggest that CAR-NK cell therapy has significant potential and successful performance in cancer treatment. Despite its effectiveness, CAR-NK cell therapy can have significant challenges when it comes to treating cancer. These challenges include tumor heterogeneity, antigen escape, an immunosuppressive tumor microenvironment, limited tissue migration from blood, exhaustion of CAR-NK cells, and inhibition by immunosuppressive checkpoint molecule signaling, etc. In CAR-T cell therapy, the use of combined approaches has shown encouraging outcomes for tumor regression and improved cancer treatment compared to single therapies. Therefore, to overcome these significant challenges in CAR-NK cells, innovative combination therapies of CAR-NK cells with other conventional therapies (e.g., chemotherapy and radiotherapy) or other immunotherapies are needed to counteract the above challenges and thereby increase the activity of CAR-NK cells. This review comprehensively discusses various cancer-treatment approaches in combination with CAR-NK cell therapy in the hope of providing valuable insights that may improve cancer treatment in the near future.

Co-Colorectal cancer stem cells employ the FADS1/DDA axis to evade NK cell-mediated immunosuppression after co-cultured with NK cells under hypoxia

Colorectal cancer (CRC) ranks as China's second most common cancer and fifth top cancer death cause. The study highlights the role of Natural Killer (NK) cells in targeting cancer stem cells (CSCs) that evade immune responses in CRC. Colorectal cancer stem cells (CCSCs) were stem from HT-29 cells and co-cultured with NK cells under normoxic or hypoxic conditions. The impact of this co-culture was evaluated using CCK8 assays for NK cell viability, ELISA for cytokine level changes, and flow cytometry for assessing NK cell apoptosis and activation. Comprehensive metabolomic and transcriptomic analyses were also performed to identify key genes and metabolites involved in the interaction between CCSCs and NK cells Co-culture of CCSCs with NK cells under hypoxia reduced NK cytotoxicity, increased NK apoptosis, and altered cytokine secretion by decreasing IFN-γ and TNF-α levels while increasing IL-6. Transcriptomic and metabolomic analysis identified 4 genes (FADS1, ALDH3A2, GCSH, MTCL1) and 3 metabolites (glyoxylic acid, spermine, DDA) as significant. Interfering with FADS1 counteracted the suppression of IFN-γ and TNF-α induced by CSC cells. Curiously, this inhibition caused by si-FADS1 could be neutralized by the addition of exogenous DDA. Co-culturing with NK cells notably increased spermine levels. Exogenous spermine resulted in a significant reduction in HT-29 cell death rates at 32 µM, 64 µM, and 128 µM, compared to NK cells without spermine. Our research explored CCSCs employed the FADS1/DDA axis to evade NK cell-mediated immunosuppression after co-cultured with NK cells under hypoxia.

Different glycosylation profiles of cystatin F alter the cytotoxic potential of natural killer cells

Cystatin F, a cysteine peptidase inhibitor, is a potent modulator of NK cytotoxicity. By inhibiting granule-mediated cytotoxicity pathway, cystatin F induces formation of non-functional NK cell stage, called split-anergy. We show that N-glycosylation determines the localization and cellular function of cystatin F. Cystatin F mostly exhibited high-mannose glycosylation in U-937 cells, both high-mannose and complex glycosylation in NK-92 and primary NKs, and predominantly complex glycosylation in super-charged NKs. Manipulating N-glycosylation with kifunensine increased high-mannose glycosylation of cystatin F and lysosome localisation, which decreased cathepsin C activity and reduced NK cytotoxicity. Mannose-6-phosphate could significantly reduce the internalization of extracellular cystatin F. By comparing NK cells with different cytotoxic potentials, we found that high-mannose cystatin F was strongly associated with lysosomes and cathepsin C in NK-92 cell line. In contrast, in highly cytotoxic super-charged NKs, cystatin F with complex glycosylation was associated with the secretory pathway and less prone to inhibit cathepsin C. Modulating glycosylation to alter cystatin F localisation could increase the cytotoxicity of NK cells, thereby enhancing their therapeutic potential for treating cancer patients.

Bi-Specific Killer Cell Engager Enhances NK Cell Activity against Interleukin-13 Receptor Alpha-2 Positive Gliomas

Glioblastoma (GBM) is a lethal brain tumor with limited therapeutic options. Bi-specific killer cell engagers (BiKEs) are novel immunotherapies designed to engage natural killer (NK) cells against cancer. We designed a BiKE molecule consisting of a single-domain CD16 antibody, an interleukin-15 linker, and a single-chain variable antibody against the glioma-associated antigen interleukin 13 receptor alpha 2 (IL13Rα2). Recombinant BiKE protein was expressed in HEK cells and purified. Flow cytometric analysis of co-cultures of peripheral blood-derived NK cells with GBM6 and GBM39 patient-derived xenograft lines revealed significantly increased activation of NK cells (CD25+CD69+) and increased glioma cell killing following BiKE treatment compared to controls (n = 4, p < 0.01). Glioma cell killing was also confirmed via immunofluorescence staining for cleaved caspase-3 (p < 0.05). In vivo, intracranial delivery of NK cells with BiKE extended median survival in mice bearing GBM6 (p < 0.01) and GBM12 (p < 0.01) tumors compared to controls. Finally, histological analysis of brain tissues revealed a higher frequency of peritumoral NK cells in mice treated with BiKE than with NK cells alone (p < 0.05). In conclusion, we demonstrate that a BiKE generated in a mammalian expression system is functional in augmenting NK cell targeting of IL13Rα2-positive gliomas.

Successes and Challenges in Taming the Beast: Cytotoxic Immune Effectors in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a neurological disease characterized by the progressive loss of motor neurons in the brain and spinal cord. No effective therapeutic strategies have been established thus far, and therefore there is a significant unmet need for effective therapeutics to arrest the disease and reverse the pathologies induced by it. Although the cause of ALS is not well-defined, it appears to be heterogenous. Currently over 20 genes have been found to be associated with ALS. Family history can only be found in 10% of ALS patients, but in the remaining 90% no association with family history is found. The most common genetic causes are expansion in the C9orf72 gene and mutations in superoxide dismutase 1, TDP-43, and FUS. In our recent study, we also found mutations in TDP43 and FUS in ALS patients. To understand the pathogenesis of the disease, we set ourselves the task of analyzing the phenotype and function of all key immune effectors in ALS patients, comparing them with either a genetically healthy twin or healthy individuals. Our study demonstrated a significant increase in functional activation of NK and CD8+ T cytotoxic immune effectors and release of significant IFN-γ not only by the effector cells but also in the serum of ALS patients. Longitudinal analysis of CD8+ T cell-mediated IFN-γ secretion from ALS patients demonstrated continued and sustained increase in IFN-γ secretion with periods of decrease which coincided with certain treatments; however, the effects were largely short-lived. N-acetyl cysteine (NAC), one of the treatments used, is known to block cell death; however, even though such treatment was able to block most of the proinflammatory cytokines, chemokines, and growth factor release, it was not able to block IFN-γ and TNF-α, the two cytokines we had demonstrated previously to induce differentiation of the cells. In this review, we discuss the contribution of cytotoxic effector cells, especially primary NK cells, supercharged NK cells (sNK), and the contribution of sNK cells in expansion and functional activation of CD8+ T cells to memory/effector T cells in the pathogenesis of ALS. Potential new targeted therapeutic strategies are also discussed.

NK cell upraise in the dark world of cancer stem cells

One of the obstacles in treating different cancers, especially solid tumors, is cancer stem cells (CSCs) with their ability in resistance to chemo/radio therapy. The efforts for finding advanced treatments to overcome these cells have led to the emergence of advanced immune cell-based therapy (AICBT). Today, NK cells have become the center of attention since they have been proved to show an appropriate cytotoxicity against different cancer types as well as the capability of detecting and killing CSCs. Attempts for reaching an off-the-shelf source of NK cells have been made and resulted in the emergence of chimeric antigen receptor natural killer cells (CAR-NK cells). The CAR technology has then been used for generating more cytotoxic and efficient NK cells, which has increased the hope for cancer treatment. Since utilizing this advanced technology to target CSCs have been published in few studies, the present study has focused on discussing the characteristics of CSCs, which are detected and targeted by NK cells, the advantages and restrictions of using CAR-NK cells in CSCs treatment and the probable challenges in this process.

ADCC against MICA/B Is Mediated against Differentiated Oral and Pancreatic and Not Stem-Like/Poorly Differentiated Tumors by the NK Cells; Loss in Cancer Patients due to Down-Modulation of CD16 Receptor

Tumor cells are known to upregulate major histocompatibility complex-class I chain related proteins A and B (MICA/B) expression under stress conditions or due to radiation exposure. However, it is not clear whether there are specific stages of cellular maturation in which these ligands are upregulated or whether the natural killer (NK) cells differentially target these tumors in direct cytotoxicity or antibody-dependent cell cytotoxicity (ADCC). We used freshly isolated primary and osteoclast (OCs)-expanded NK cells to determine the degree of direct cytotoxicity or of ADCC using anti-MICA/B monoclonal antibodies (mAbs) against oral stem-like/poorly-differentiated oral squamous cancer stem cells (OSCSCs) and Mia PaCa-2 (MP2) pancreatic tumors as well as their well-differentiated counterparts: namely, oral squamous carcinoma cells (OSCCs) and pancreatic PL12 tumors. By using phenotypic and functional analysis, we demonstrated that OSCSCs and MP2 tumors were primary targets of direct cytotoxicity by freshly isolated NK cells and not by ADCC mediated by anti-MICA/B mAbs, which was likely due to the lower surface expression of MICA/B. However, the inverse was seen when their MICA/B-expressing differentiated counterparts, OSCCs and PL12 tumors, were used in direct cytotoxicity and ADCC, in which there was lower direct cytotoxicity but higher ADCC mediated by the NK cells. Differentiation of the OSCSCs and MP2 tumors by NK cell-supernatants abolished the direct killing of these tumors by the NK cells while enhancing NK cell-mediated ADCC due to the increased expression of MICA/B on the surface of these tumors. We further report that both direct killing and ADCC against MICA/B expressing tumors were significantly diminished by cancer patients' NK cells. Surprisingly, OC-expanded NK cells, unlike primary interleukin-2 (IL-2) activated NK cells, were found to kill OSCCs and PL12 tumors, and under these conditions, we did not observe significant ADCC using anti-MICA/B mAbs, even though the tumors expressed a higher surface expression of MICA/B. In addition, differentiated tumor cells also expressed higher levels of surface epidermal growth factor receptor (EGFR) and programmed death-ligand 1(PDL1) and were more susceptible to NK cell-mediated ADCC in the presence of anti-EGFR and anti-PDL1 mAbs compared to their stem-like/poorly differentiated counterparts. Overall, these results suggested the possibility of CD16 receptors mediating both direct cytotoxicity and ADCC, resulting in the competitive use of these receptors in either direct killing or ADCC, depending on the differentiation status of tumor cells and the stage of maturation and activation of NK cells.

Fc Receptor is Involved in Nk Cell Functional Anergy Induced by Miapaca2 Tumor Cell Line

Impaired NK cytotoxicity has been linked to poor cancer prognosis, but its mechanisms are not clearly established. Increasing data demonstrate that NK cells lose cytotoxicity after interaction with NK cell-sensitive tumor cells. In this paper, we provide evidence that the human adenocarcinoma cell line MiaPaCa2 and TNFα and TGFβ-treated MiaPaCa2 cultures (MiaPaCa2-TT) induced functional anergy of NK cells via FGL2 protein. MiaPaCa2-TT cultures decreased expression of IFNγ, CD107a, DNAM-1, and stimulated expression of PD1 by NK cells, as well as inhibited their cytotoxic activity in a greater manner compared to the parental culture. More importantly, we found that co-cultivation with anergized NK cells decreased expression of IFNγ and CD107a by naïve NK cells, which supports the hypothesis of NK cell functional anergy transmission. The obtained results suggest a mechanism by which tumor cells may inhibit cytotoxic functions of tumor-infiltrating and circulating NK cells in cancer.

ABBREVIATIONS

CFSE: Carboxyfluorescein diacetate succinimidyl ester; CSCs: Cancer stem cells; FGL2: Fibrinogen-like protein 2; mAbs: Monoclonal antibodies; MiaPaCa2: Human adenocarcinoma cell line; MiaPaCa2-ТТ: Adenocarcinoma cell line MiaPaCa2 cells stimulated with TNFα and TGFβ-1; PI: Propidium iodide; TGFβ: Transforming growth factor beta; TME: Tumor microenvironment; TNFα: Tumor necrosis factor alfa.

The Potential of CD16 on Plasma-Derived Exosomes as a Liquid Biomarker in Head and Neck Cancer

Head and neck squamous cell carcinomas (HNSCC) are highly immune suppressive and aggressive malignancies. As part of the tumor microenvironment, exosomes contribute to this immune suppression. The Fc receptor CD16 is widely expressed on monocytes, neutrophils, and natural killer (NK) cells and is involved in antibody-dependent cell-mediated cytotoxicity (ADCC). Here, surface levels of CD16 on total exosomes and tumor-derived exosomes (TEX) from plasma of HNSCC patients were analyzed regarding their potential as liquid biomarkers for disease stage. Exosomes were isolated from plasma using mini size exclusion chromatography. TEX were enriched by immune affinity capture with CD44v3 antibodies. On-bead flow cytometry was used to measure CD16 levels on total exosomes and TEX. The results were correlated with clinicopathological parameters. Total exosomes from HNSCC patients had significantly higher CD16 levels compared to TEX. Further, CD16 surface levels of total exosomes, but not TEX, correlated with clinicopathological parameters. Patients with advanced tumor stages T3/4 and Union for International Cancer Control (UICC) stages III/IV had significantly higher CD16 levels on total exosomes compared to patients with early tumor stages T1/2 and UICC stages I/II, respectively. Overall, CD16 positive exosomes have the potential as liquid biomarkers for HNSCC tumor stage and aggressiveness.

MED15, transforming growth factor beta 1 (TGF-β1), FcγRIII (CD16), and HNK-1 (CD57) are prognostic biomarkers of oral squamous cell carcinoma

Owing to the high incidence and mortality of oral squamous cell carcinoma (OSCC), knowledge of its diagnostic and prognostic factors is of significant value. The biomarkers 'CD16, CD57, transforming growth factor beta 1 (TGF-β1), and MED15' can play crucial roles in tumorigenesis, and hence might contribute to diagnosis, prognosis, and treatment. Since there was no previous study on MED15 in almost all cancers, and since the studies on diagnostic/prognostic values of the other three biomarkers were a few in OSCC (if any) and highly controversial, this study was conducted. Biomarker expressions in all OSCC tissues and their adjacent normal tissues available at the National Tumor Bank (n = 4 biomarkers × [48 cancers + 48 controls]) were estimated thrice using qRT-PCR. Diagnostic values of tumors were assessed using receiver-operator characteristic (ROC) curves. Factors contributing to patients' survival over 10 years were assessed using multiple Cox regressions. ROC curves were used to estimate cut-off points for significant prognostic variables (α = 0.05). Areas under the curve pertaining to diagnostic values of all markers were non-significant (P > 0.15). Survival was associated positively with tumoral upregulation of TGF-β1 and downregulation of CD16, CD57, and MED15. It was also associated positively with younger ages, lower histological grades, milder Jacobson clinical TNM stages (and lower pathological Ns), smaller and thinner tumors, and surgery cases not treated with incisional biopsy (Cox regression, P < 0.05). The cut-off point for clinical stage -as the only variable with a significant area under the curve- was between the stages 2 and 3. Increased TGF-β1 and reduced CD16, CD57, and MED15 expressions in the tumor might independently favor the prognosis. Clinical TNM staging might be one of the most reliable prognostic factors, and stages above 2 can predict a considerably poorer prognosis.

Targeting Cancer Stem Cells by Genetically Engineered Chimeric Antigen Receptor T Cells

The term cancer stem cell (CSC) starts 25 years ago with the evidence that CSC is a subpopulation of tumor cells that have renewal ability and can differentiate into several distinct linages. Therefore, CSCs play crucial role in the initiation and the maintenance of cancer. Moreover, it has been proposed throughout several studies that CSCs are behind the failure of the conventional chemo-/radiotherapy as well as cancer recurrence due to their ability to resist the therapy and their ability to re-regenerate. Thus, the need for targeted therapy to eliminate CSCs is crucial; for that reason, chimeric antigen receptor (CAR) T cells has currently been in use with high rate of success in leukemia and, to some degree, in patients with solid tumors. This review outlines the most common CSC populations and their common markers, in particular CD133, CD90, EpCAM, CD44, ALDH, and EGFRVIII, the interaction between CSCs and the immune system, CAR T cell genetic engineering and signaling, CAR T cells in targeting CSCs, and the barriers in using CAR T cells as immunotherapy to treat solid cancers.

Natural Killer Cells: Diverse Functions in Tumor Immunity and Defects in Pre-neoplastic and Neoplastic Stages of Tumorigenesis

Natural killer (NK) cells are the key immune effectors with the ability to mediate selection and differentiation of a number of different cancer stem cells/undifferentiated tumors via lysis, and secreted or membrane-bound interferon (IFN)-γ and tumor necrosis factor (TNF)-α, respectively, leading to curtailment of tumor growth and metastasis. In this review, we present an overview of our recent findings on the biology and significance of NK cells in selection and differentiation of stem-like tumors using in vitro and in vivo studies conducted in humanized-BLT mice and in cancer patients. In addition, we present current advances in NK cell expansion and therapeutic delivery, and discuss the utility of allogeneic supercharged NK cells in the treatment of cancer patients. Moreover, we discuss the potential loss of NK cell numbers and function at the neoplastic and pre-neoplastic stages of tumorigenesis in induction and progression of pancreatic cancer. Therefore, because of their indispensable role in targeting cancer stem-like/undifferentiated tumors, NK cells should be placed high in the armamentarium of tumor immunotherapy. A combination of allogeneic supercharged NK cells with other immunotherapeutic strategies such as oncolytic viruses, antibody-dependent cellular cytotoxicity (ADCC)-inducing antibodies, checkpoint inhibitors, chimeric antigen receptor (CAR) T cells, CAR NK cells, and chemotherapeutic and radiotherapeutic strategies can be used for the ultimate goal of tumor eradication.

NK cells shape pancreatic and oral tumor microenvironments; role in inhibition of tumor growth and metastasis

We have recently shown that natural killer (NK) cells select and differentiate cancer stem cells (CSCs)/undifferentiated tumors via secreted and membrane bound IFN-gamma (IFN-γ) and TNF-alpha (TNF-α), preventing tumor growth and inducing remodeling of the tumor microenvironment. Since many conventional therapeutic strategies, including chemotherapy and radiotherapy remain fairly unsuccessful in treating CSCs/poorly differentiated tumors, there has been an increasing interest in NK cell-targeted immunotherapy for the treatment of aggressive tumors. In our recent studies, we used humanized-BLT (hu-BLT) mouse model with transplanted human bone marrow, liver and thymus to demonstrate the efficacy of adoptive transfer of ex vivo expanded, super-charged NK cells in selection and differentiation of stem-like tumors within the context of a fully reconstituted human immune system. Furthermore, we have demonstrated that CSCs differentiated with split-anergized NK cells prior to implantation in hu-BLT mice were not able to grow or metastasize. However, when NK cell-mediated tumor differentiation was blocked by the addition of antibodies to IFN-γ and TNF-α, tumors grew and metastasized. In this review, we present current advances in NK cell expansion and therapeutic delivery, and discuss the utility of allogeneic super-charged NK cells in treatment of cancer patients. In addition, NK suppression occurs not only at the stage of overt cancer, but also at the pre-neoplastic stage. Therefore, due to the indispensable role of NK cells in targeting CSCs/undifferentiated tumors and their role in differentiation of the tumors, NK cells should be placed high in the armamentarium of tumor immunotherapy.

Natural killer cells target and differentiate cancer stem-like cells/undifferentiated tumors: strategies to optimize their growth and expansion for effective cancer immunotherapy

Natural killer (NK) cells are known to select and differentiate cancer stem-like cells/undifferentiated tumors via lysis, and secreted/membrane bound IFN-γ and TNF-α respectively, resulting in the control of tumor growth. Several in vivo mouse models including humanized-BLT mice have been used to study the biology and significance of NK cells in selection/differentiation of stem-like tumors within the context of a reconstituted human immune system. In addition, we discuss the evidence and significance of NK cell loss at the pre-neoplastic stage. Therefore, because of their indispensable role in targeting CSCs/undifferentiated tumors, NK-cells should be placed high in the armamentarium of tumor therapy.

Regulation of split anergy in natural killer cells by inhibition of cathepsins C and H and cystatin F

Freshly isolated human primary NK cells induce preferential lysis of Oral Squamous Carcinoma Stem Cells (OSCSCs) when compared to differentiated Oral Squamous Carcinoma Cells (OSCCs), while anti-CD16 antibody and monocytes induce functional split anergy in primary NK cells by decreasing the cytotoxic function of NK cells and increasing the release of IFN-γ. Since NK92 cells have relatively lower levels of cytotoxicity when compared to primary NK cells, and have the ability to increase secretion of regulatory cytokines IL-10 and IL-6, we used these cells as a model of NK cell anergy to identify and to study the upstream regulators of anergy. We demonstrate in this paper that the levels of truncated monomeric cystatin F, which is known to inhibit the functions of cathepsins C and H, is significantly elevated in NK92 cells and in anergized primary NK cells. Furthermore, cystatin F co-localizes with cathepsins C and H in the lysosomal/endosomal vesicles of NK cells. Accordingly, the mature forms of aminopeptidases cathepsins C and H, which regulate the activation of effector granzymes in NK cells, are significantly decreased, whereas the levels of pro-cathepsin C enzyme is increased in anergized NK cells after triggering of the CD16 receptor. In addition, the levels of granzyme B is significantly decreased in anti-CD16mAb and target cell anergized primary NK cells and NK92 cells. Our study provides the cellular and molecular mechanisms by which target cells may utilize to inhibit the cytotoxic function of NK cells.

Resistance to cytotoxicity and sustained release of interleukin-6 and interleukin-8 in the presence of decreased interferon-γ after differentiation of glioblastoma by human natural killer cells

Natural killer (NK) cells are functionally suppressed in the glioblastoma multiforme (GBM) tumor microenvironment. We have recently shown that survival and differentiation of cancer stem-like cells (CSCs)/poorly differentiated tumors are controlled through two distinct phenotypes of cytotoxic and non-cytotoxic/split anergized NK cells, respectively. In this paper, we studied the function of NK cells against brain CSCs/poorly differentiated GBM and their NK cell-differentiated counterparts. Brain CSCs/poorly differentiated GBM, differentiated by split anergized NK supernatants (supernatants from NK cells treated with IL-2 + anti-CD16mAb) expressed higher levels of CD54, B7H1 and MHC-I and were killed less by the NK cells, whereas their CSCs/poorly differentiated counterparts were highly susceptible to NK cell lysis. Resistance to NK cells and differentiation of brain CSCs/poorly differentiated GBM by split anergized NK cells were mediated by interferon (IFN)-γ and tumor necrosis factor (TNF)-α. Brain CSCs/poorly differentiated GBM expressed low levels of TNFRs and IFN-γRs, and when differentiated and cultured with IL-2-treated NK cells, they induced increased secretion of pro-inflammatory cytokine interleukin (IL)-6 and chemokine IL-8 in the presence of decreased IFN-γ secretion. NK-induced differentiation of brain CSCs/poorly differentiated GBM cells was independent of the function of IL-6 and/or IL-8. The inability of NK cells to lyse GBM tumors and the presence of a sustained release of pro-inflammatory cytokines IL-6 and chemokine IL-8 in the presence of a decreased IFN-γ secretion may lead to the inadequacy of NK cells to differentiate GBM CSCs/poorly differentiated tumors, thus failing to control tumor growth.

Bisphosphonate-induced differential modulation of immune cell function in gingiva and bone marrow in vivo: role in osteoclast-mediated NK cell activation

The aim of this study is to establish osteoclasts as key immune effectors capable of activating the function of Natural Killer (NK) cells, and expanding their numbers, and to determine in vivo and in vitro effect of bisphosphonates (BPs) during NK cell interaction with osteoclasts and on systemic and local immune function. The profiles of 27 cytokines, chemokines and growth factors released from osteoclasts were found to be different from dendritic cells and M1 macrophages but resembling to untreated monocytes and M2 macrophages. Nitrogen-containing BPs Zoledronate (ZOL) and Alendronate (ALN), but not non-nitrogen-containing BPs Etidronate (ETI), triggered increased release of pro-inflammatory mediators from osteoclasts while all three BPs decreased pit formation by osteoclasts. ZOL and ALN mediated significant release of IL-6, TNF-` and IL-1β, whereas they inhibited IL-10 secretion by osteoclasts. Treatment of osteoclasts with ZOL inhibited NK cell mediated cytotoxicity whereas it induced significant secretion of cytokines and chemokines. NK cells lysed osteoclasts much more than their precursor cells monocytes, and this correlated with the decreased expression of MHC class I expression on osteoclasts. Intravenous injection of ZOL in mice induced pro-inflammatory microenvironment in bone marrow and demonstrated significant immune activation. By contrast, tooth extraction wound of gingival tissues exhibited profound immune suppressive microenvironment associated with dysregulated wound healing to the effect of ZOL which could potentially be responsible for the pathogenesis of Osteonecrosis of the Jaw (ONJ). Finally, based on the data obtained in this paper we demonstrate that osteoclasts can be used as targets for the expansion of NK cells with superior function for immunotherapy of cancer.

Adoptive transfer of osteoclast-expanded natural killer cells for immunotherapy targeting cancer stem-like cells in humanized mice

Based on data obtained from oral, pancreatic and lung cancers, glioblastoma, and melanoma, we have established that natural killer (NK) cells target cancer stem-like cells (CSCs). CSCs displaying low MHC class I, CD54, and PD-L1 are killed by cytotoxic NK cells and are differentiated by split anergized NK cells through both membrane bound and secreted forms of TNF-α and IFN-γ. NK cells select and differentiate both healthy and transformed stem-like cells, resulting in target cell maturation and shaping of their microenvironment. In our recent studies, we have observed that oral, pancreatic, and melanoma CSCs were capable of forming large tumors in humanized bone marrow, liver, thymus (hu-BLT) mice with fully reconstituted human immune system. In addition, major human immune subsets including NK cells, T cells, B cells, and monocytes were present in the spleen, bone marrow, peripheral blood, and tumor microenvironment. Similar to our previously published in vitro data, CSCs differentiated with split anergized NK cells prior to implantation in mice formed smaller tumors. Intravenous injection of functionally potent osteoclast-expanded NK cells inhibited tumor growth through differentiation of CSCs in humanized mice. In this review, we present current approaches, advances, and existing limitations in studying interactions of the immune system with the tumor, in particular NK cells with CSCs, using in vivo preclinical hu-BLT mouse model. In addition, we discuss the use of osteoclast-expanded NK cells in targeting cancer stem-like tumors in humanized mice-a strategy that provides a much-needed platform to develop effective cancer immunotherapies.

Split anergized Natural Killer cells halt inflammation by inducing stem cell differentiation, resistance to NK cell cytotoxicity and prevention of cytokine and chemokine secretion

The mechanism of suppression of NK cytotoxicity in cancer patients is not clearly established. In this paper we provide evidence that anergized NK cells induce differentiation of healthy Dental Pulp Stem Cells (DPSCs) or transformed Oral Squamous Cancer Stem Cells (OSCSCs) resulting in cell growth inhibition, resistance to NK cell-mediated cytotoxicity and prevention of inflammatory mediators secretion. Induction of cytotoxicity resistance in differentiated cells correlated with increased CD54 and MHC class I surface expression and mediated by the combination of IFN-γ and TNF-α since antibodies to both, but not each cytokine alone, was able to inhibit resistance. In contrast, inhibition of cytokine and chemokine release was mediated by IFN-γ since the addition of anti-IFN-γ antibody, and not anti-TNF-α, restored secretion of inflammatory mediators in NK cell cultures with differentiated DPSCs and OSCSCs. There was a gradual and time dependent decrease in MHC class I and CD54 expression which correlated with the restoration of NK cell cytotoxicity, augmentation of cytokine secretion and increased cell growth from days 0–12 post NK removal. Continuous presence of NK cells is required for the maintenance of cell differentiation since the removal of NK cell-mediated function reverses the phenotype and function of differentiated cells to their stem-like cells.

Differential Targeting of Stem Cells and Differentiated Glioblastomas by NK Cells

We have recently shown that Natural Killer (NK) cells control survival and differentiation of Cancer Stem-like Cells (CSCs) through two distinct phenotypes of cytotoxic and anergic NK cells, respectively. In this report, brain CSCs and their serum and NK cell differentiated counterparts were studied. Serum-differentiated brain CSCs were significantly less susceptible to NK cells and CTL direct cytotoxicity as well as NK cell mediated Antibody Dependent Cellular Cytotoxicity (ADCC), whereas their CSCs were highly susceptible. The levels of CD44 and EGFR were higher in brain tumor CSCs when compared to the serum-differentiated tumors. No differences could be observed for the expression of MHC class I between brain tumor stem cells and their serum-differentiated counterparts. Moreover, supernatants from the combination of IL-2 and anti-CD16mAb treated NK cells (anergized NK cells) induced resistance of brain tumor CSCs to NK cell mediated cytotoxicity. Unlike serum-differentiated CSCs, NK supernatant induced differentiation and resistance to cytotoxicity in brain CSCs correlated with the increased expression of CD54 and MHC class I. The addition of anti-MHC class I antibody moderately inhibited NK mediated cytotoxicity against untreated or serum-differentiated CSCs, whereas it increased cytotoxicity against NK supernatant differentiated tumors. Therefore, two distinct mechanisms govern serum and NK supernatant mediated differentiation of brain tumors.

Differential Cytotoxicity but Augmented IFN-γ Secretion by NK Cells after Interaction with Monocytes from Humans, and Those from Wild Type and Myeloid-Specific COX-2 Knockout Mice

The list of genes, which augment NK cell function when knocked out in neighboring cells is increasing, and may point to the fundamental function of NK cells targeting cells with diminished capability to differentiate optimally since NK cells are able to target less differentiated cells, and aid in their differentiation. In this paper, we aimed at understanding the effect of monocytes from targeted knockout of COX-2 in myeloid cells (Cox-2^flox/flox;LysM^Cre/+) and from control littermates (Cox-2^flox/flox;LysM^+/+) on ex vivo function of NK cells. Furthermore, we compared the effect of monocytes treated with and without lipopolysaccharide (LPS) on NK cells from mice and humans. NK cells purified from Cox-2^flox/flox;LysM^Cre/+ mice had heightened cytotoxic activity when compared to those obtained from control littermates. In addition, NK cells cultured with autologous Cox-2^flox/flox;LysM^Cre/+ monocytes and DCs, mouse embryonic fibroblasts from global knockout COX-2, but not with knockout of COX-2 in T cells, had increased cytotoxic function as well as augmented IFN-γ secretion when compared to NK cells from control littermates cultured with monocytes. LPS inhibited NK cell cytotoxicity while increasing IFN-γ secretion when cultured in the presence of monocytes from either Cox-2^flox/flox;LysM^Cre/+ or control littermates. In contrast to mice, NK cells from humans when cultured with monocytes lost cytotoxic function and gained ability to secrete large amounts of IFN-γ, a process, which we had previously coined as “split anergy.” Similar to mice, LPS potentiated the loss of human NK cell cytotoxicity while increasing IFN-γ secretion in the presence of monocytes. Greater loss of cytotoxicity and larger secretion of IFN-γ in NK cells induced by gene knockout cells may be important for the greater need of these cells for differentiation.

Induction of Split Anergy Conditions Natural Killer Cells to Promote Differentiation of Stem Cells through Cell-Cell Contact and Secreted Factors

In this paper, we provide evidence that anergized NK cells through secreted factors and direct cell–cell contact have the ability to induce differentiation of healthy dental pulp stem cells and stem cell of apical papillae as well as transformed oral squamous cancer stem cell (OSCSC) and Mia-Paca-2, poorly differentiated stem-like pancreatic tumors, resulting in their resistance to NK cell-mediated cytotoxicity. Induction of NK cell resistance and differentiation in the stem cells correlated with the increased expression of CD54, B7H1, and MHC class I, and mediated by the combination of membrane-bound or secreted IFN-γ and TNF-α from the NK cells since antibodies to both cytokines and not each one alone were able to inhibit differentiation or resistance to NK cells. Similarly, antibodies to both TNF-α and IFN-γ were required to prevent NK-mediated inhibition of cell growth, and restored the numbers of the stem cells to the levels obtained when stem cells were cultured in the absence of anergized NK cells. Interestingly, the effect of anti-IFN-γ antibody in the absence of anti-TNF-α antibody was more dominant for the prevention of increase in surface receptor expression since its addition abrogated the increase in CD54, B7H1, and MHC class I surface expression. Antibodies to CD54 or LFA-1 was unable to inhibit differentiation whereas antibodies to MHC class I but not B7H1 increased cytotoxicity of well-differentiated oral squamous carcinoma cells as well as OSCSCs differentiated by the IL-2 + anti-CD16 mAb-treated NK cells whereas it inhibited the cytotoxicity of NK cells against OSCSCs. Thus, NK cells may inhibit the progression of cancer by killing and/or differentiation of cancer stem cells, which severely halt cancer growth, invasion, and metastasis.

Natural killer cells as effectors of selection and differentiation of stem cells: role in resolution of inflammation

Evidence has previously been demonstrated for the role of NK cells in specific elimination of healthy stem cells (e.g. hMSC, hDPSC, hESC, hiPSC) as well as cancer stem cells, but not their differentiated counterparts. There is also a stage-wise susceptibility to NK cell-mediated cyto-toxicity in tumors, in which case the poorly-differentiated tumors are lysed much more than moderately-differentiated tumors. Well-differentiated tumors were lysed the least compared to either moderately- or poorly-differentiated tumors. It has also been reported that inhibition of differentiation or reversion of cells to a less-differentiated stage by blocking NF-κB or by gene deletion of COX2 significantly augmented NK cell cytotoxicity against both transformed and healthy cells. Additionally, the cytotoxic function of NK cells was severely inhibited against stem cells when they were cultured in the presence of monocytes. Therefore, it is proposed that CD16(+)CD56(dim)CD69(-) NK cells were important for the selection of stem cells, whereas the CD16(dim/-)CD56(dim/+)CD69(+) anergized NK cells were important for differentiation and eventual regeneration of the tissues and the resolution of inflammation, thus potentially serving as regulatory NK (NK(reg)) cells. The concept of 'split anergy' in NK cells and the generation of NK(reg) cells with regard to contributions to cell differentiation, tissue repair and regeneration and in tumor resistance are discussed in this review.

Tumor-infiltrating immune cells promoting tumor invasion and metastasis: existing theories

It is a commonly held belief that infiltration of immune cells into tumor tissues and direct physical contact between tumor cells and infiltrated immune cells is associated with physical destructions of the tumor cells, reduction of the tumor burden, and improved clinical prognosis. An increasing number of studies, however, have suggested that aberrant infiltration of immune cells into tumor or normal tissues may promote tumor progression, invasion, and metastasis. Neither the primary reason for these contradictory observations, nor the mechanism for the reported diverse impact of tumor-infiltrating immune cells has been elucidated, making it difficult to judge the clinical implications of infiltration of immune cells within tumor tissues. This mini-review presents several existing hypotheses and models that favor the promoting impact of tumor-infiltrating immune cells on tumor invasion and metastasis, and also analyzes their strength and weakness.

Dual functions of natural killer cells in selection and differentiation of stem cells; role in regulation of inflammation and regeneration of tissues

Accumulated evidence from our laboratory indicates that conditioned or anergized NK cells have the ability to induce resistance of healthy stem cells and transformed cancer stem cells through both secreted factors and direct cell-cell contact by inducing differentiation. Cytotoxic function of NK cells is suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. Furthermore, decreased peripheral blood NK cell function has been documented in many cancer patients. We have previously shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells (OSCCs). In addition, human embryonic stem cells (hESCs), human mesenchymal stem cells (hMSCs), human dental pulp stem cells (hDPSCs) and induced human pluripotent stem cells (hiPSCs) were all significantly more susceptible to NK cell mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or gene deletion of COX2 significantly augmented NK cell function. Furthermore, the induction of resistance of the stem cells to NK cell mediated cytotoxicity and their subsequent differentiation is amplified when either the stem cells or the NK cells were cultured in the presence of monocytes. Therefore, we propose that the two stages of NK cell maturation namely CD16+CD56dimCD69- NK cells are important for the lysis of stem cells or poorly differentiated cells whereas the CD16dim/-CD56dim/+CD69+NK cells are important for differentiation and eventual regeneration of the tissues and the resolution of inflammation, thus functionally serving as regulatory NK cells (NK(reg)). CD16 receptor on the NK cells were found to be the receptor with significant potential to induce NK cell anergy, however, our recent data indicated that NKp46 but not NKp30 or NKp44 were also able to induce significant anergy in NK cells, although the levels were less when compared to CD16 receptor triggering. The concept of split anergy in NK cells and generation of NK(reg) and its contribution to cell differentiation, tissue repair and regeneration and in tumor resistance will be discussed in this review.

Potential rescue, survival and differentiation of cancer stem cells and primary non-transformed stem cells by monocyte-induced split anergy in natural killer cells

Cytotoxic function of NK cells is largely suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. The aims of this review are to provide a rationale and a potential mechanism for immunosuppression in cancer and to demonstrate the significance of such immunosuppression in cellular differentiation and progression of cancer. We have recently shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells. In addition, human embryonic stem cells, mesenchymal stem cells (hMSCs), dental pulp stem cells (hDPSCs) and induced pluripotent stem cells were all significantly more susceptible to NK-cell-mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB significantly augmented NK-cell function. Total population of monocytes and those depleted of CD16+ subsets were able to substantially suppress NK-cell-mediated lysis of OSCSCs, hMSCs and hDPSCs. Overall, our results suggest that stem cells but not their differentiated counterparts are significant targets of the NK-cell cytotoxicity. The concept of split anergy in NK cells and its contribution to cell differentiation, tissue repair and regeneration and in tumor resistance and progression will be discussed in this review.

Tumor induced inactivation of natural killer cell cytotoxic function; implication in growth, expansion and differentiation of cancer stem cells

Accumulated evidence indicates that cytotoxic function of immune effectors is largely suppressed in the tumor microenvironment by a number of distinct effectors and their secreted factors. The aims of this review are to provide a rationale and a potential mechanism for immunosuppression in cancer and to demonstrate the significance of such immunosuppression in cellular differentiation and progression of cancer. To that end, we have recently shown that NK cells mediate significant cytotoxicity against primary oral squamous carcinoma stem cells (OSCSCs) as compared to their more differentiated oral squamous carcinoma cells (OSCCs). In addition, human embryonic stem cells (hESCs), Mesenchymal Stem Cells (hMSCs), dental pulp stem cells (hDPSCs) and induced pluripotent stem cells (hiPSCs) were all significantly more susceptible to NK cell mediated cytotoxicity than their differentiated counterparts or parental cells from which they were derived. We have also reported that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFκB or targeted knock down of COX2 in primary monocytes in vivo significantly augmented NK cell function. Total population of monocytes and those depleted of CD16(+) subsets were able to substantially prevent NK cell mediated lysis of OSCSCs, MSCs and DPSCs. Taken together, our results suggest that stem cells are significant targets of the NK cell cytotoxicity. The concept of split anergy in NK cells and its contribution to tissue repair and regeneration and in tumor resistance and progression will be discussed in this review.

Increased lysis of stem cells but not their differentiated cells by natural killer cells; de-differentiation or reprogramming activates NK cells

The aims of this study are to demonstrate the increased lysis of stem cells but not their differentiated counterparts by the NK cells and to determine whether disturbance in cell differentiation is a cause for increased sensitivity to NK cell mediated cytotoxicity. Increased cytotoxicity and augmented secretion of IFN-gamma were both observed when PBMCs or NK cells were co-incubated with primary UCLA oral squamous carcinoma stem cells (UCLA-OSCSCs) when compared to differentiated UCLA oral squamous carcinoma cells (UCLA-OSCCs). In addition, human embryonic stem cells (hESCs) were also lysed greatly by the NK cells. Moreover, NK cells were found to lyse human Mesenchymal Stem Cells (hMSCs), human dental pulp stem cells (hDPSCs) and human induced pluripotent stem cells (hiPSCs) significantly more than their differentiated counterparts or parental lines from which they were derived. It was also found that inhibition of differentiation or reversion of cells to a less-differentiated phenotype by blocking NFkappaB or targeted knock down of COX2 in monocytes significantly augmented NK cell cytotoxicity and secretion of IFN-gamma. Taken together, these results suggest that stem cells are significant targets of the NK cell cytotoxicity. However, to support differentiation of a subset of tumor or healthy untransformed primary stem cells, NK cells may be required to lyse a number of stem cells and/or those which are either defective or incapable of full differentiation in order to lose their cytotoxic function and gain the ability to secrete cytokines (split anergy). Therefore, patients with cancer may benefit from repeated allogeneic NK cell transplantation for specific elimination of cancer stem cells.

Strategies to rescue mesenchymal stem cells (MSCs) and dental pulp stem cells (DPSCs) from NK cell mediated cytotoxicity

BACKGROUND

The aim of this paper is to study the function of allogeneic and autologous NK cells against Dental Pulp Stem Cells (DPSCs) and Mesenchymal Stem Cells (MSCs) and to determine the function of NK cells in a three way interaction with monocytes and stem cells.

METHODOLOGY/PRINCIPAL FINDINGS

We demonstrate here that freshly isolated untreated or IL-2 treated NK cells are potent inducers of cell death in DPSCs and MSCs, and that anti-CD16 antibody which induces functional split anergy and apoptosis in NK cells inhibits NK cell mediated lysis of DPSCs and MSCs. Monocytes co-cultured with either DPSCs or MSCs decrease lysis of stem cells by untreated or IL-2 treated NK cells. Monocytes also prevent NK cell apoptosis thereby raising the overall survival and function of NK cells, DPSCs or MSCs. Both total population of monocytes and those depleted of CD16(+) subsets were able to prevent NK cell mediated lysis of MSCs and DPSCs, and to trigger an increased secretion of IFN-gamma by IL-2 treated NK cells. Protection of stem cells from NK cell mediated lysis was also seen when monocytes were sorted out from stem cells before they were added to NK cells. However, this effect was not specific to monocytes since the addition of T and B cells to stem cells also protected stem cells from NK cell mediated lysis. NK cells were found to lyse monocytes, as well as T and B cells.

CONCLUSION/SIGNIFICANCE

By increasing the release of IFN-gamma and decreasing the cytotoxic function of NK cells monocytes are able to shield stem cells from killing by the NK cells, resulting in an increased protection and differentiation of stem cells. More importantly studies reported in this paper indicate that anti-CD16 antibody can be used to prevent NK cell induced rejection of stem cells.