Human NK cells: From surface receptors to clinical applications

Therapeutic potential of natural killer cells in neuroimmunological diseases

Natural killer (NK) cells, a major component of the innate immune system, have prominent immunoregulatory, antitumor proliferation, and antiviral activities. NK cells act as a double-edged sword with therapeutic potential in neurological autoimmunity. Emerging evidence has identified NK cells are involved in the development and progression of neuroimmunological diseases such as multiple sclerosis, neuromyelitis optica spectrum disorders, autoimmune encephalitis, Guillain-Barré Syndrome, chronic inflammatory demyelinating polyneuropathy, myasthenia gravis, and idiopathic inflammatory myopathy. However, the regulatory mechanisms and functional roles of NK cells are highly variable in different clinical states of neuroimmunological diseases and need to be further determined. In this review, we summarize the evidence for the heterogenic involvement of NK cells in the above conditions. Further, we describe cutting-edge NK-cell-based immunotherapy for neuroimmunological diseases in preclinical and clinical development and highlight challenges that must be overcome to fully realize the therapeutic potential of NK cells.

Immune Cells in the Tumor Microenvironment of Soft Tissue Sarcomas

Soft tissue sarcomas (STSs) are a rare heterogeneous group of malignant neoplasms characterized by their aggressive course and poor response to treatment. This determines the relevance of research aimed at studying the pathogenesis of STSs. By now, it is known that STSs is characterized by complex relationships between the tumor cells and immune cells of the microenvironment. Dynamic interactions between tumor cells and components of the microenvironment enhance adaptation to changing environmental conditions, which provides the high aggressive potential of STSs and resistance to antitumor therapy. Today, active research is being conducted to find effective antitumor drugs and to evaluate the possibility of using therapy with immune cells of STS. The difficulty in assessing the efficacy of new antitumor options is primarily due to the high heterogeneity of this group of malignant neoplasms. Studying the role of immune cells in the microenvironment in the progression STSs and resistance to antitumor therapies will provide the discovery of new biomarkers of the disease and the prediction of response to immunotherapy. In addition, it will help to initially divide patients into subgroups of good and poor response to immunotherapy, thus avoiding wasting precious time in selecting the appropriate antitumor agent.

PD-1 is expressed in cytotoxic granules of NK cells and rapidly mobilized to the cell membrane following recognition of tumor cells

The contribution of the T cell-related inhibitory checkpoint PD-1 to the regulation of NK cell activity is still not clear with contradictory results concerning its expression and role in the modulation of NK cell cytotoxicity. We provide novel key findings on the mechanism involved in the regulation of PD-1 expression on NK cell membrane and its functional consequences for the elimination of cancer cells. In contrast to freshly isolated NK cells from cancer patients, those from healthy donors did not express PD-1 on the cell membrane. However, when healthy NK cells were incubated with tumor target cells, membrane PD-1 expression increased, concurrent with the CD107a surface mobilization. This finding suggested that PD-1 was translocated to the cell membrane during NK cell degranulation after contact with target cells. Indeed, cytosolic PD-1 was expressed in freshly-isolated-NK cells and partly co-localized with CD107a and GzmB, confirming that membrane PD-1 corresponded to a pool of preformed PD-1. Moreover, NK cells that had mobilized PD-1 to the cell membrane presented a significantly reduced anti-tumor activity on PD-L1-expressing-tumor cells in vitro and in vivo, which was partly reversed by using anti-PD-1 blocking antibodies. Our results indicate that NK cells from healthy individuals express cytotoxic granule-associated PD-1, which is rapidly mobilized to the cell membrane after interaction with tumor target cells. This novel finding helps to understand how PD-1 expression is regulated on NK cell membrane and the functional consequences of this expression during the elimination of tumor cells, which will help to design more efficient NK cell-based cancer immunotherapies.

Practical Considerations and Workflow in Utilizing KIR Genotyping in Transplantation Medicine

This chapter is intended to serve as a practical guide for establishing a workflow using sequence-specific polymorphism PCR (SSP-PCR) for killer cell immunoglobulin-like receptor (KIR) genotyping in a clinical setting, especially in allogeneic hematopoietic stem cell transplantation (HSCT). As clinical evidence accumulates on the application of KIR and HLA genetics to guide donor selection in HSCT, there is an increasing need for KIR genotyping in clinical settings, and thus medical institutes may need to build this capability. Among the various KIR genotyping approaches now available, SSP-PCR methods are well-established and are the most cost-effective and will likely be the method of choice especially when expenses will be passed on to the patient. The protocol described in this chapter developed by Vilches et al. features small amplicon PCR and is suitable for KIR genotyping using FFPE-derived DNA as well as DNA extracted from blood samples. Setting up a laboratory workflow for in-house KIR genotyping is relatively straightforward; in this chapter, considerations for KIR genotyping to guide clinical decisions are discussed.In HSCT, a main objective of KIR genotyping is to apply the genetic analysis to predict donor and recipient combinations that have the most potential to produce NK cell alloresponses either through the missing-self mechanism or by action associated with activating KIR. The desired effects are reduction in acute GVHD and relapse rates and enhancement of overall survival. The information herein may also be useful to clinical laboratories considering the application of KIR genotyping in areas such as solid organ transplantation, NK cell-based treatment in other forms of cancer and autoimmune diseases, humanized antibody treatment, regenerative medicine, and reproductive medicine. Some background knowledge on KIR genetics will be necessary in managing a KIR genotyping platform. This chapter aims to address the main difficulties often encountered by physicians in understanding the KIR system, such as basic aspects of the nomenclature of KIR genes and haplotypes, genotypes, and determining presence/absence of KIR ligands in the patient and donor from the extensively diversified HLA class I allotypes. In describing the workflow, emphasis has been placed on the processes after genotype PCR and gel image acquisition: haplotype inference, generating B content scores, deduction of KIR ligands from HLA typing results, and the emerging algorithms for donor selection based on KIR and HLA genetics.

New Orders to an Old Soldier: Optimizing NK Cells for Adoptive Immunotherapy in Hematology

NK (Natural Killer) cell-mediated adoptive immunotherapy has gained attention in hematology due to the progressing knowledge of NK cell receptor structure, biology and function. Today, challenges related to NK cell expansion and persistence in vivo as well as low cytotoxicity have been mostly overcome by pioneering trials that focused on harnessing NK cell functions. Recent technological advancements in gene delivery, gene editing and chimeric antigen receptors (CARs) have made it possible to generate genetically modified NK cells that enhance the anti-tumor efficacy and represent suitable “off-the-shelf” products with fewer side effects. In this review, we highlight recent advances in NK cell biology along with current approaches for potentiating NK cell proliferation and activity, redirecting NK cells using CARs and optimizing the procedure to manufacture clinical-grade NK and CAR NK cells for adoptive immunotherapy.

Tumor and Peripheral Immune Status in Soft Tissue Sarcoma: Implications for Immunotherapy

Simple Summary

Soft Tissue Sarcomas are a rare and heterogeneous group of tumors, which have a characteristic complexity, leading to a difficult diagnosis and a lack of response to treatment. The aim of this review is to summarize the role of immune cells, soluble plasmatic factors, immune checkpoints; and the expression of immune-related genes predicting survival, response to therapy, and potential immunotherapeutic agents or targets in Soft Tissue Sarcomas.

Abstract

Soft Tissue Sarcomas (STS) are a heterogeneous and rare group of tumors. Immune cells, soluble factors, and immune checkpoints are key elements of the complex tumor microenvironment. Monitoring these elements could be used to predict the outcome of the disease, the response to therapy, and lead to the development of new immunotherapeutic approaches. Tumor-infiltrating B cells, Natural Killer (NK) cells, tumor-associated neutrophils (TANs), and dendritic cells (DCs) were associated with a better outcome. On the contrary, tumor-associated macrophages (TAMs) were correlated with a poor outcome. The evaluation of peripheral blood immunological status in STS could also be important and is still underexplored. The increased lymphocyte-to-monocyte ratio (LMR) and neutrophil-to-lymphocyte ratio (NLR), higher levels of monocytic myeloid-derived suppressor cells (M-MDSCs), and Tim-3 positive CD8 T cells appear to be negative prognostic markers. Meanwhile, NKG2D-positive CD8 T cells were correlated with a better outcome. Some soluble factors, such as cytokines, chemokines, growth factors, and immune checkpoints were associated with the prognosis. Similarly, the expression of immune-related genes in STS was also reviewed. Despite these efforts, only very little is known, and much research is still needed to clarify the role of the immune system in STS.

Ovarian Cancer-Associated Ascites Have High Proportions of Cytokine-Responsive CD56bright NK Cells

Ovarian cancer is the most lethal gynecological malignancy, with serous histotype as the most prevalent epithelial ovarian cancer (EOC). Peritoneal ascites is a frequent comorbidity in advanced EOC. EOC-associated ascites provide a reliable sampling source for studying lymphocytes directly from tumor environment. Herein, we carried out flow cytometry-based analysis to readdress issues on NK and T lymphocyte subsets in women with advanced EOC, additionally evaluating phenotypic modulation of their intracellular pathways involved in interleukin (IL)-2 and IL-15 signaling. Results depicted ascites as an inflammatory and immunosuppressive environment, presenting significantly (p < 0.0001) higher amounts of IL-6 and IL-10 than in the patients' blood, as well as significantly (p < 0.05) increased expression of checkpoint inhibitory receptors (programmed death protein-1, PD-1) and ectonucleotidase (CD39) on T lymphocytes. However, NK lymphocytes from EOC-associated ascites showed higher (p < 0.05) pS6 phosphorylation compared with NK from blood. Additionally, in vitro treatment of lymphocytes with IL-2 or IL-15 elicited significantly (p < 0.001) phosphorylation of the STAT5 protein in NK, CD3 and CD8 lymphocytes, both from blood and ascites. EOC-associated ascites had a significantly (p < 0.0001) higher proportion of NK CD56bright lymphocytes than blood, which, in addition, were more responsive (p < 0.05) to stimulation by IL-2 than CD56dim NK. EOC-associated ascites allow studies on lymphocyte phenotype modulation in the tumor environment, where inflammatory profile contrasts with the presence of immunosuppressive elements and development of cellular self-regulating mechanisms.

The Immune Microenvironment of Malignant Pleural Mesothelioma: A Literature Review

Malignant pleural mesothelioma (MPM) is a rare and aggressive tumour with a poor prognosis, associated with asbestos exposure. Nowadays, treatment is based on chemotherapy with a median overall survival of less than two years. This review highlights the main characteristics of the immune microenvironment in MPM with special emphasis on recent biological advances. The MPM microenvironment is highly infiltrated by tumour-associated macrophages, mainly M2-macrophages. In line with infiltration by M2-macrophages, which contribute to immune suppression, other effectors of innate immune response are deficient in MPM, such as dendritic cells or natural killer cells. On the other hand, tumour infiltrating lymphocytes (TILs) are also found in MPM, but CD4+ and CD8+ TILs might have decreased cytotoxic effects through T-regulators and high expression of immune checkpoints. Taken together, the immune microenvironment is particularly heterogeneous and can be considered as mainly immunotolerant or immunosuppressive. Therefore, identifying molecular vulnerabilities is particularly relevant to the improvement of patient outcomes and the assessment of promising treatment approaches.

Regulation of MHC I Molecules in Glioblastoma Cells and the Sensitizing of NK Cells

Immunotherapy has been established as an important area in the therapy of malignant diseases. Immunogenicity sufficient for immune recognition and subsequent elimination can be bypassed by tumors through altered and/or reduced expression levels of major histocompatibility complex class I (MHC I) molecules. Natural killer (NK) cells can eliminate tumor cells in a MHC I antigen presentation-independent manner by an array of activating and inhibitory receptors, which are promising candidates for immunotherapy. Here we summarize the latest findings in recognizing and regulating MHC I molecules that affect NK cell surveillance of glioblastoma cells.

Dissecting the NK Cell Population in Hematological Cancers Confirms the Presence of Tumor Cells and Their Impact on NK Population Function

The lymphocyte lineage natural killer (NK) cell is part of the innate immune system and protects against pathogens and tumor cells. NK cells are the main cell effectors of the monoclonal antibodies (mAbs) that mediates antibody-dependent cell cytotoxicity (ADCC). Hence, it is relevant to understand NK physiology and status to investigate the biological effect of mAbs in the clinic. NK cells are heterogeneous with multiple subsets that may have specific activity against different attacks. The presence of viral-sculpted NK cell populations has already been described, but the presence of cancer-sculpted NK cells remains unknown. Cancer induces a broad NK cell dysfunction, which has not been linked to a specific population. Here, we investigated the NK cell population by Uniform Manifold Approximation and Projection (UMAP) embed maps in Hodgkin lymphoma (HL) and acute myeloid leukemia (AML) patients at diagnosis and at least 30 days after treatment, which correlates with tumor cell clearance. We found that the NK lineage largely responded to the tumor by generating antitumor NK cells and renewing the population with a subset of immature NK cells. However, we failed to identify a specific "memory-like" subset with the NK cell markers used. Moreover, in patients in relapse, we found essentially the same NK populations as those found at diagnosis, suggesting that NK cells equally respond to the first or second tumor rise. Finally, we observed that previous cytomegalovirus (CMV) infection largely affects the tumor-associated changes in NK population, but the CMV-associated CD57⁺NKG2C⁺ NK cell population does not appear to play any role in tumor immunity.

Echinococcus multilocularis vesicular fluid induces the expression of immune checkpoint proteins in vitro

AIMS

Alveolar echinococcosis is a severe chronic helminthic infection that mimics a tumour-like disease. This study aimed at investigating in vitro interactions between Echinococcus multilocularis vesicular fluid (VF) and different immune checkpoints (PD-1/PD-L1, CTLA-4, LAG-3 and TIM-3).

METHODS AND RESULTS

Peripheral blood mononuclear cells (PBMC) from healthy blood donors were isolated by Ficoll. Natural killer (NK) cells were selected. Each type of cell was stimulated individually with E. multilocularis-VF. Expression of the different immune checkpoints was measured by flow cytometry on day 3 and day 6; all supernatants were used for immunoassays. Cells and supernatants from 22 healthy donors were analysed. A significant increase of PD-1, PD-L1, LAG-3 and TIM-3 was observed upon E. multilocularis-VF exposure for NK cells on day 3 (P < .05, Wilcoxon signed-rank test). A significant increase of PD-L1 and CTLA-4 was observed upon E. multilocularis-VF exposure for T cells on day 6 (P < .05, Wilcoxon signed-rank test), which was associated with increased levels of Th1 and Th2 cytokines P < .05, Wilcoxon signed-rank test).

CONCLUSION

These preliminary data suggest that immune checkpoints could be a way for E. multilocularis to modulate the host immune response during alveolar echinococcosis.

Optimization of electroporation and other non-viral gene delivery strategies for T cells

CAR-T therapy is a particularly effective treatment for some types of cancer that uses retroviruses to deliver the gene for a chimeric antigen receptor (CAR) to a patient's T cells ex vivo. The CAR enables the T cells to bind and eradicate cells with a specific surface marker (e.g., CD19⁺ B cells) after they are transfused back into the patient. This treatment was proven to be particularly effective in treating non-Hodgkin's lymphoma (NHL) and acute lymphoblastic leukemia (ALL), but the current CAR-T cell manufacturing process has a few significant drawbacks. For example, while lentiviral and gammaretroviral transduction are both relatively effective, the process of producing viral vectors is time-consuming and costly. Additionally, patients must undergo follow up appointments for several years to monitor them for any unanticipated side effects associated with the virus. Therefore, several studies have endeavored to find alternative non-viral gene delivery methods that are less expensive, more precise, simple, and safe. This review focuses on the current state of the most promising non-viral gene delivery techniques, including electroporation and transfection with cationic polymers or lipids.

Cathepsin G and its Dichotomous Role in Modulating Levels of MHC Class I Molecules

Cathepsin G (CatG) is involved in controlling numerous processes of the innate and adaptive immune system. These features include the proteolytic activity of CatG and play a pivotal role in alteration of chemokines as well as cytokines, clearance of exogenous and internalized pathogens, platelet activation, apoptosis, and antigen processing. This is in contrast to the capability of CatG acting in a proteolytic-independent manner due to the net charge of arginine residues in the CatG sequence which interferes with bacteria. CatG is a double-edged sword; CatG is also responsible in pathophysiological conditions, such as autoimmunity, chronic pulmonary diseases, HIV infection, tumor progression and metastasis, photo-aged human skin, Papillon-Lefèvre syndrome, and chronic inflammatory pain. Here, we summarize the latest findings for functional responsibilities of CatG in immunity, including bivalent regulation of major histocompatibility complex class I molecules, which underscore an additional novel role of CatG within the immune system.

IL-27 impact on NK cells activity: Implication for a robust anti-tumor response in chronic lymphocytic leukemia

Interleukin 27 (IL-27) belongs to IL-12 cytokine family, has shown anti-tumor potential in several solid tumors, as well as hematologic malignancies. IL-27 can inhibit tumor growth and progression through direct and indirect mechanisms, such as inhibition of proliferation, angiogenesis, induction of apoptosis in tumor cells, and anti-tumor immune response. B-CLL is characterized by remarkable immune perturbation, which leads to disease complications and reduced effectiveness of the treatment. Natural killer cells (NK) are considered as an important arm for the elimination of transformed cells. However, NK cells have shown significant impairment in patients with CLL. Here we analyzed the activity of recombinant human (rh) IL-27-stimulated NK cells in bone marrow (BM) and peripheral blood (PB) of CLL patients using cell surface flow cytometry assessment, and cytotoxicity assay. We showed that rhIL-27 can increase CD69 on NK cells both in BM and PB. Interestingly, BM-NK cells treated with rhIL-27 exhibited a significant increase in degranulation and NK cell-mediated cytotoxicity as compared with untreated NK cells, whereas it did not improve NK cell activity of PB. These observations added further explanation to the anti-tumor activity of IL-27 and also could pave the way to adoption immunostimulatory adjuvant for therapies in CLL.

Non-Genetically Improving the Natural Cytotoxicity of Natural Killer (NK) Cells

The innate lymphocyte lineage natural killer (NK) is now the target of multiple clinical applications, although none has received an agreement from any regulatory agency yet. Transplant of naïve NK cells has not proven efficient enough in the vast majority of clinical trials. Hence, new protocols wish to improve their medical use by producing them from stem cells and/or modifying them by genetic engineering. These techniques have given interesting results but these improvements often hide that natural killers are mainly that: natural. We discuss here different ways to take advantage of NK physiology to improve their clinical activity without the need of additional modifications except for in vitro activation and expansion and allograft in patients. Some of these tactics include combination with monoclonal antibodies (mAb), drugs that change metabolism and engraftment of specific NK subsets with particular activity. Finally, we propose to use specific NK cell subsets found in certain patients that show increase activity against a specific disease, including the use of NK cells derived from patients.

Recalling the Biological Significance of Immune Checkpoints on NK Cells: A Chance to Overcome LAG3, PD1, and CTLA4 Inhibitory Pathways by Adoptive NK Cell Transfer?

Immune checkpoint receptors (IC) positively or negatively regulate the activation of the host immune response, preventing unwanted reactions against self-healthy tissues. In recent years the term IC has been mainly used for the inhibitory ICs, which are critical to control Natural Killer (NK) and Cytotoxic CD8⁺ T cells due to its high cytotoxic potential. Due to the different nature of the signals that regulate T and NK cell activation, specific ICs have been described that mainly regulate either NK cell or T cell activity. Thus, strategies to modulate NK cell activity are raising as promising tools to treat tumors that do not respond to T cell-based immunotherapies. NK cell activation is mainly regulated by ICs and receptors from the KIR, NKG2 and NCRs families and the contribution of T cell-related ICs is less clear. Recently, NK cells have emerged as contributors to the effect of inhibitors of T cell-related ICs like CTLA4, LAG3 or the PD1/PD-L1 axes in cancer patients, suggesting that these ICs also regulate the activity of NK cells under pathological conditions. Strikingly, in contrast to NK cells from cancer patients, the level of expression of these ICs is low on most subsets of freshly isolated and in vitro activated NK cells from healthy patients, suggesting that they do not control NK cell tolerance and thus, do not act as conventional ICs under non-pathological conditions. The low level of expression of T cell-related ICs in “healthy” NK cells suggest that they should not be restricted to the detrimental effects of these inhibitory mechanisms in the cancer microenvironment. After a brief introduction of the regulatory mechanisms that control NK cell anti-tumoral activity and the conventional ICs controlling NK cell tolerance, we will critically discuss the potential role of T cell-related ICs in the control of NK cell activity under both physiological and pathological (cancer) conditions. This discussion will allow to comprehensively describe the chances and potential limitations of using allogeneic NK cells isolated from a healthy environment to overcome immune subversion by T cell-related ICs and to improve the efficacy of IC inhibitors (ICIs) in a safer way.

Development of Automated Separation, Expansion, and Quality Control Protocols for Clinical-Scale Manufacturing of Primary Human NK Cells and Alpharetroviral Chimeric Antigen Receptor Engineering

In cellular immunotherapies, natural killer (NK) cells often demonstrate potent antitumor effects in high-risk cancer patients. But Good Manufacturing Practice (GMP)-compliant manufacturing of clinical-grade NK cells in high numbers for patient treatment is still a challenge. Therefore, new protocols for isolation and expansion of NK cells are required. In order to attack resistant tumor entities, NK cell killing can be improved by genetic engineering using alpharetroviral vectors that encode for chimeric antigen receptors (CARs). The aim of this work was to demonstrate GMP-grade manufacturing of NK cells using the CliniMACS^® Prodigy device (Prodigy) with implemented applicable quality controls. Additionally, the study aimed to define the best time point to transduce expanding NK cells with alpharetroviral CAR vectors. Manufacturing and clinical-scale expansion of primary human NK cells were performed with the Prodigy starting with 8-15.0 × 10⁹ leukocytes (including 1.1–2.3 × 10⁹ NK cells) collected by small-scale lymphapheresis (n = 3). Positive fraction after immunoselection, in-process controls (IPCs), and end product were quantified by flow cytometric no-wash, single-platform assessment, and gating strategy using positive (CD56/CD16/CD45), negative (CD14/CD19/CD3), and dead cell (7-aminoactinomycine [7-AAD]) discriminators. The three runs on the fully integrated manufacturing platform included immunomagnetic separation (CD3 depletion/CD56 enrichment) followed by NK cell expansion over 14 days. This process led to high NK cell purities (median 99.1%) and adequate NK cell viabilities (median 86.9%) and achieved a median CD3+ cell depletion of log −3.6 after CD3 depletion and log −3.7 after immunomagnetic CD3 depletion and consecutive CD56 selection. Subsequent cultivation of separated NK cells in the CentriCult^® chamber of Prodigy resulted in approximately 4.2–8.5-fold NK cell expansion rates by adding of NK MACS^® basal medium containing NK MACS^® supplement, interleukin (IL)-2/IL-15 and initial IL-21. NK cells expanded for 14 days revealed higher expression of natural cytotoxicity receptors (NKp30, NKp44, NKp46, and NKG2D) and degranulation/apoptotic markers and stronger cytolytic properties against K562 compared to non-activated NK cells before automated cultivation. Moreover, expanded NK cells had robust growth and killing activities even after cryopreservation. As a crucial result, it was possible to determine the appropriate time period for optimal CAR transduction of cultivated NK cells between days 8 and 14, with the highest anti-CD123 CAR expression levels on day 14. The anti-CD123 CAR NK cells showed retargeted killing and degranulation properties against CD123-expressing KG1a target cells, while basal cytotoxicity of non-transduced NK cells was determined using the CD123-negative cell line K562. Time-lapse imaging to monitor redirected effector-to-target contacts between anti-CD123 CAR NK and KG1a showed long-term effector–target interaction. In conclusion, the integration of the clinical-scale expansion procedure in the automated and closed Prodigy system, including IPC samples and quality controls and optimal time frames for NK cell transduction with CAR vectors, was established on 48-well plates and resulted in a standardized GMP-compliant overall process.

Human CAR NK Cells: A New Non-viral Method Allowing High Efficient Transfection and Strong Tumor Cell Killing

CAR-NK cells may represent a valuable tool, complementary to CAR-T cells, in adoptive immunotherapy of leukemia and solid tumors. However, gene transfer to human NK cells is a challenging task, particularly with non-virus-based techniques. Here, we describe a new procedure allowing efficient electroporation-based transfection of plasmid DNA, including CAR and CCR7 genes, in resting or cytokine-expanded human NK cell populations and NK-92 cell line. This procedure may offer a suitable platform for a safe and effective use of CAR-NK cells in adoptive immunotherapy of cancer.

Innate Lymphoid Cells: Expression of PD-1 and Other Checkpoints in Normal and Pathological Conditions

Innate lymphoid cells (ILCs) belong to a family of immune cells. Recently, ILCs have been classified into five different groups that mirror the function of adaptive T cell subsets counterparts. In particular, NK cells mirror CD8⁺ cytotoxic T cells while ILC1, ILC2, ILC3, and Lymphoid tissue inducer (LTi)-like cells reflect the function of CD4⁺T helper (Th) cells (Th1, Th2, and Th17 respectively). ILCs are involved in innate host defenses against pathogens and tumors, in lymphoid organogenesis, and in tissue remodeling/repair. In recent years, important molecular inducible checkpoints (PD-1, TIM3, and TIGIT) were shown to control/inactivate different immune cell types. The expression of many of these receptors has been detected on NK cells and subsets of tissue-resident ILCs in both physiological and pathological conditions, including cancer. In particular, it has been demonstrated that the interaction between PD-1⁺ immune cells and PD-L1/PD-L2+ tumor cells may compromise the anti-tumor effector function leading to tumor immune escape. However, while the effector function of NK cells in tumor is well-established, limited information exists on the other ILC subsets. We will summarize what is known to date on the expression and function of these checkpoint receptors on NK cells and ILCs, with a particular focus on the recent data that reveal an essential contribution of the blockade of PD-1 and TIGIT on NK cells to the immunotherapy of cancer. A better information regarding the presence and the function of different ILCs and of the inhibitory checkpoints in pathological conditions may offer important clues for the development of new immune therapeutic strategies.

Complementing the Cancer-Immunity Cycle

Reactivation of cytotoxic CD8+ T-cell responses has set a new direction for cancer immunotherapy. Neutralizing antibodies targeting immune checkpoint programmed cell death protein 1 (PD-1) or its ligand (PD-L1) have been particularly successful for tumor types with limited therapeutic options such as melanoma and lung cancer. However, reactivation of T cells is only one step toward tumor elimination, and a substantial fraction of patients fails to respond to these therapies. In this context, combination therapies targeting more than one of the steps of the cancer-immune cycle may provide significant benefits. To find the best combinations, it is of upmost importance to understand the interplay between cancer cells and all the components of the immune response. This review focuses on the elements of the complement system that come into play in the cancer-immunity cycle. The complement system, an essential part of innate immunity, has emerged as a major regulator of cancer immunity. Complement effectors such as C1q, anaphylatoxins C3a and C5a, and their receptors C3aR and C5aR1, have been associated with tolerogenic cell death and inhibition of antitumor T-cell responses through the recruitment and/or activation of immunosuppressive cell subpopulations such as myeloid-derived suppressor cells (MDSCs), regulatory T cells (Tregs), or M2 tumor-associated macrophages (TAMs). Evidence is provided to support the idea that complement blocks many of the effector routes associated with the cancer-immunity cycle, providing the rationale for new therapeutic combinations aimed to enhance the antitumor efficacy of anti-PD-1/PD-L1 checkpoint inhibitors.

Presence of innate lymphoid cells in pleural effusions of primary and metastatic tumors: Functional analysis and expression of PD-1 receptor

The tumor microenvironment (TM) contains a wide variety of cell types and soluble factors capable of suppressing immune responses. While the presence of NK cells in pleural effusions (PE) has been documented, no information exists on the presence of other innate lymphoid cell (ILC) subsets and on the expression of programmed cell death‐1 (PD‐1) in NK and ILC. The presence of ILC was assessed in PE of 54 patients (n = 33 with mesothelioma, n = 15 with adenocarcinoma and n = 6 with inflammatory pleural diseases) by cell staining with suitable antibody combinations and cytofluorimetric analysis. The cytokine production of ILC isolated from both PE and autologous peripheral blood was analyzed upon cell stimulation and intracytoplasmic staining. We show that, in addition to NK cells, also ILC1, ILC2 and ILC3 are present in malignant PE and that the prevalent subset is ILC3. PE‐ILC subsets produced their typical sets of cytokines upon activation. In addition, we analyzed the PD‐1 expression on NK/ILC by multiparametric flow‐cytometric analysis, while the expression of PD‐1 ligand (PD‐L1) was evaluated by immunohistochemical analysis. Both NK cells and ILC3 expressed functional PD‐1, moreover, both tumor samples and malignant PE‐derived tumor cell lines were PD‐L1⁺ suggesting that the interaction between PD‐1⁺ILC and PD‐L1⁺tumor cells may hamper antitumor immune responses mediated by NK and ILC.

What's new?

Pleural tumors result in effusions that are not well characterized. In this study, the authors found that pleural effusions from patients with primary mesothelioma or metastatic adenocarcinoma contain NK cells and other innate lymphoid cells (ILC). These immune cells were capable of expressing normal cytokines, including the checkpoint protein PD‐1. However, the tumor cells were found to express the ligand PD‐L1. These results suggest a PD‐1‐mediated inhibitory effect on lymphoid cells with potential anti‐tumor activity. Better understanding of this inhibition in the tumor microenvironment may lead to new targets for checkpoint‐inhibitor therapies.

Immune evasion by TGFβ-induced miR-183 repression of MICA/B expression in human lung tumor cells

Immune escape is a hallmark of cancer. In human lung cancer, we have identified a unique microRNA (miR)-based pathway employed by tumor cells to repress detection by immune cells via the NKG2D-MICA/B receptor-ligand system. MICA/B is readily induced by cell transformation and serves as a danger signal and ligand to alert NK and activated CD8⁺ T cells. However, immunohistochemical analysis indicated that human lung adenocarcinoma and squamous cell carcinoma specimens express little MICA/B while high levels of miR-183 were detected in both tumor types in a TCGA database. Human lung tumor cell lines confirmed the reverse relationship in expression of MICA/B and miR-183. Importantly, a miR-183 binding site was identified on the 3'untranslated region (UTR) of both MICA and MICB, suggesting its role in MICA/B regulation. Luciferase reporter constructs bearing the 3'UTR of MICA or MICB in 293 cells supported the function of miR-183 in repressing MICA/B expression. Additionally, anti-sense miR-183 transfection into H1355 or H1299 tumor cells caused the upregulation of MICA/B. Abundant miR-183 expression in tumor cells was traced to transforming growth factor-beta (TGFβ), as evidenced by antisense TGFβ transfection into H1355 or H1299 tumor cells which subsequently lost miR-183 expression accompanied by MICA/B upregulation. Most significantly, anti-sense miR-183 transfected tumor cells became more sensitive to lysis by activated CD8⁺ T cells that express high levels of NKG2D. Thus, high miR-183 triggered by TGFβ expressed in lung tumor cells can target MICA/B expression to circumvent detection by NKG2D on immune cells.

PD-1 in human NK cells: evidence of cytoplasmic mRNA and protein expression

Under physiological conditions, PD-1/PD-L1 interactions regulate unwanted over-reactions of immune cells and contribute to maintain peripheral tolerance. However, in tumor microenvironment, this interaction may greatly compromise the immune-mediated anti-tumor activity. PD-1⁺ NK cells have been detected in high percentage in peripheral blood and ascitic fluid of ovarian carcinoma patients. To acquire information on PD-1 expression and physiology in human NK cells, we analyzed whether PD-1 mRNA and protein are present in resting, surface PD-1⁻, NK cells from healthy donors. Both different splicing isoforms of PD-1 mRNA and a cytoplasmic pool of PD-1 protein were detected. Similar results were obtained also from both in vitro-activated and tumor-associated NK cells. PD-1 mRNA and protein were higher in CD56^dim than in CD56^bright NK cells. Confocal microscopy analyses revealed that PD-1 protein is present in virtually all NK cells analyzed. The present findings are compatible with a rapid surface expression of PD-1 in NK cells in response to appropriate, still undefined, stimuli.

Unrelated donor vs HLA-haploidentical α/β T-cell- and B-cell-depleted HSCT in children with acute leukemia

Traditionally, hematopoietic stem cell transplantation (HSCT) from both HLA-matched related and unrelated donors (UD) has been used for treating children with acute leukemia (AL) in need of an allograft. Recently, HLA-haploidentical HSCT after αβ T-cell/B-cell depletion (αβhaplo-HSCT) was shown to be effective in single-center studies. Here, we report the first multicenter retrospective analysis of 127 matched UD (MUD), 118 mismatched UD (MMUD), and 98 αβhaplo-HSCT recipients, transplanted between 2010 and 2015, in 13 Italian centers. All these AL children were transplanted in morphological remission after a myeloablative conditioning regimen. Graft failure occurred in 2% each of UD-HSCT and αβhaplo-HSCT groups. In MUD vs MMUD-HSCT recipients, the cumulative incidence of grade II to IV and grade III to IV acute graft-versus-host disease (GVHD) was 35% vs 44% and 6% vs 18%, respectively, compared with 16% and 0% in αβhaplo-HSCT recipients (P < .001). Children treated with αβhaplo-HSCT also had a significantly lower incidence of overall and extensive chronic GVHD (P < .01). Eight (6%) MUD, 32 (28%) MMUD, and 9 (9%) αβhaplo-HSCT patients died of transplant-related complications. With a median follow-up of 3.3 years, the 5-year probability of leukemia-free survival in the 3 groups was 67%, 55%, and 62%, respectively. In the 3 groups, chronic GVHD-free/relapse-free (GRFS) probability of survival was 61%, 34%, and 58%, respectively (P < .001). When compared with patients given MMUD-HSCT, αβhaplo-HSCT recipients had a lower cumulative incidence of nonrelapse mortality and a better GRFS (P < .001). These data indicate that αβhaplo-HSCT is a suitable therapeutic option for children with AL in need of transplantation, especially when an allele-matched UD is not available.

Expansion of allogeneic NK cells with efficient antibody-dependent cell cytotoxicity against multiple tumors

Monoclonal antibodies (mAbs) have significantly improved the treatment of certain cancers. However, in general mAbs alone have limited therapeutic activity. One of their main mechanisms of action is to induce antibody-dependent cell-mediated cytotoxicity (ADCC), which is mediated by natural killer (NK) cells. Unfortunately, most cancer patients have severe immune dysfunctions affecting NK activity. This can be circumvented by the injection of allogeneic, expanded NK cells, which is safe. Nevertheless, despite their strong cytolytic potential against different tumors, clinical results have been poor.

Methods: We combined allogeneic NK cells and mAbs to improve cancer treatment. We generated expanded NK cells (e-NK) with strong in vitro and in vivo ADCC responses against different tumors and using different therapeutic mAbs, namely rituximab, obinutuzumab, daratumumab, cetuximab and trastuzumab.

Results: Remarkably, e-NK cells can be stored frozen and, after thawing, armed with mAbs. They mediate ADCC through degranulation-dependent and -independent mechanisms. Furthermore, they overcome certain anti-apoptotic mechanisms found in leukemic cells.

Conclusion: We have established a new protocol for activation/expansion of NK cells with high ADCC activity. The use of mAbs in combination with e-NK cells could potentially improve cancer treatment.

NK cells and multiple myeloma-associated endothelial cells: molecular interactions and influence of IL-27

Angiogenesis represents a hallmark of tumor progression in Multiple Myeloma (MM), a still incurable malignancy. Here we analyzed the activity of cytokine-stimulated NK cells against tumor-associated endothelial cells isolated from bone marrow aspirates of MM patients with active disease (MMECs). We show that NK cells activated with optimal doses of IL-15 killed MMECs thanks to the concerted action of multiple activating receptors. In particular, according to the high expression of PVR and Nectin-2 on MMECs, DNAM-1 actively participated in target recognition. Interestingly, in MMECs the surface density of PVR was significantly higher than that detected in endothelium from patients with MM in complete remission or with monoclonal gammopathy of undetermined significance (MGUS). Importantly, IL-27, which unlike IL-15 does not display pro-angiogenic properties, maintained or increased the NK cell functions induced by suboptimal concentrations of IL-15. NK cell properties included killing of MMECs, IFN-γ production as well as a peculiar increase of NKp46 expression on NK cell surface. Finally, IL-27 showed a striking capability of up-regulating the expression of PD-L2 and HLA-I on tumor endothelium, whereas it did not modify that of PD-L1 and HLA-II.

Our results suggest that cytokine-activated endogenous or adoptively transferred NK cells might support conventional therapies improving the outcome of MM patients.

NK cell activation and recovery of NK cell subsets in lymphoma patients after obinutuzumab and lenalidomide treatment

Obinutuzumab (OBZ) shows stronger antibody-dependent cell cytotoxicity (ADCC) compared to rituximab and improved clinical activity for treating certain CD20⁺ neoplasia. However, the efficacy of monoclonal antibody (mAb) as a monotherapy is limited. Natural Killer (NK) cells are mediators of ADCC. Hematological cancer patients possess antitumor NK cells that are unable to control disease, possibly because they are dysfunctional. The immunomodulatory drug lenalidomide (LEN) could be a treatment to restore exhausted NK cell cytotoxic functions. The clinical trial GALEN is a Phase Ib/II study of OBZ combined with LEN for the treatment of relapsed/refractory follicular and aggressive (DLBCL and MCL) B-cell Lymphoma. During treatment, we analyzed specific aspects of NK cell biology. Treatment reversed the immature NK phenotype of patients and increased expression of NK activating receptors. Inhibitory receptors were either unchanged or decreased. There was a strong NK response at the end of the 1st cycle: NK number and intracellular granzyme B (GrzB) expression decreased, degranulation increased and NK responded better to allogeneic target challenge. Moreover, the interaction of NK cells with B cell targets, measured by trogocytosis, decreased during treatment. At the end of treatment, when target cells had been wiped out, the proportion of reactive NK cells (CD69⁺, CD45RARO⁺, CD107a⁺, CD19⁺) strongly decreased. Because all patients received LEN and OBZ, it was uncertain which drug was responsible of our observations, or even if a combination of both products was necessary for the described effects on this lymphocyte lineage.

Echinococcus multilocularis vesicular fluid inhibits activation and proliferation of natural killer cells

Alveolar echinococcosis is a severe chronic helminthic disease that mimics slow-growing liver cancer. The immune evasion strategy of Echinococcus multilocularis Leuckart, 1863 remains poorly understood. The aim of this study was to investigate in vitro the impact of E. multilocularis vesicular fluid (Em-VF) on peripheral blood mononuclear cells (PBMC) and on natural killer (NK) cells. PBMC and NK cells were exposed to Em-VF (1 µg/ml) during six days. The effect of Em-VF was assessed on CD69, viability and proliferation, and on and transforming growth factor β (TGF-β), interferon γ (IFN-γ), interleukin 17 (IL-17) and interleukin 10, using flow cytometry and ELISA, respectively. Exposure to Em-VF had no bearing on PBMC's viability, proliferation and expression of CD69. In contrast, higher levels of IL-17 at day three and of TGF-β at day six were observed in PBMC supernatant after exposure to Em-VF (p < 0.05, Wilcoxon signed-rank test). Exposure to Em-VF induced a significant decrease of CD69 expression of NK cells at day three and a significant decrease of proliferation of NK cells at day six (p < 0.05, Wilcoxon signed-rank test). In contrast, NK cells viability and levels of cytokines did not vary significantly over Em-VF stimulation. Exposure to Em-VF had a significant bearing on activation and proliferation of NK cells. NK cells may play an important role in the immune response of the host against E. multilocularis.

Enhanced antitumor activity of gemcitabine by polysaccharide-induced NK cell activation and immune cytotoxicity reduction in vitro/vivo

The polysaccharide SEP has been reported to activate NK and T cells via TLR2/4. Here, the combination of gemcitabine (GEM) and SEP against HepG-2 was investigated. SEP apparently enhanced antitumor activity of gemcitabine against liver cancer through stimulating NKG2D and DAP10/Akt pathway to activate NK cells. The NKG2D upregulation could improve the sensitivity of NK-92 cells targeting to its ligand MICA expressed on HepG-2 cells. Meanwhile, GEM up-regulated MICA expression and attenuated soluble MICA secretion through inhibiting ADAM10 expression, which in turn enhanced the cytotoxicity of NK-92 cells against cancer cells. SEP remarkably enhanced GEM antitumor activity with an inhibitory rate of 79.1% in an H22-bearing mouse model. Moreover, SEP reversed atrophy and apoptosis caused by GEM in both spleen and bone marrow through suppressing ROS secretion in vivo. The data indicated that the combination of SEP and GEM is a potential chemo-immunotherapy strategy for liver cancer treatment clinically.

Redirected Primary Human Chimeric Antigen Receptor Natural Killer Cells As an "Off-the-Shelf Immunotherapy" for Improvement in Cancer Treatment

Primary human natural killer (NK) cells recognize and subsequently eliminate virus infected cells, tumor cells, or other aberrant cells. However, cancer cells are able to develop tumor immune escape mechanisms to undermine this immune control. To overcome this obstacle, NK cells can be genetically modified to express chimeric antigen receptors (CARs) in order to improve specific recognition of cancer surface markers (e.g., CD19, CD20, and ErbB2). After target recognition, intracellular CAR domain signaling (CD3ζ, CD28, 4-1BB, and 2B4) leads to activation of PI3K or DNAX proteins (DAP10, DAP12) and finally to enhanced cytotoxicity, proliferation, and/or interferon γ release. This mini-review summarizes both the first preclinical trials with CAR-engineered primary human NK cells and the translational implications for “off-the-shelf immunotherapy” in cancer treatment. Signal transduction in NK cells as well as optimization of CAR signaling will be described, becoming more and more a focal point of interest in addition to redirected T cells. Finally, strategies to overcome off-target effects will be discussed in order to improve future clinical trials and to avoid attacking healthy tissues.

The biology of uveal melanoma

Uveal melanoma (UM), a rare cancer of the eye, is distinct from cutaneous melanoma by its etiology, the mutation frequency and profile, and its clinical behavior including resistance to targeted therapy and immune checkpoint blockers. Primary disease is efficiently controlled by surgery or radiation therapy, but about half of UMs develop distant metastasis mostly to the liver. Survival of patients with metastasis is below 1 year and has not improved in decades. Recent years have brought a deep understanding of UM biology characterized by initiating mutations in the G proteins GNAQ and GNA11. Cytogenetic alterations, in particular monosomy of chromosome 3 and amplification of the long arm of chromosome 8, and mutation of the BRCA1-associated protein 1, BAP1, a tumor suppressor gene, or the splicing factor SF3B1 determine UM metastasis. Cytogenetic and molecular profiling allow for a very precise prognostication that is still not matched by efficacious adjuvant therapies. G protein signaling has been shown to activate the YAP/TAZ pathway independent of HIPPO, and conventional signaling via the mitogen-activated kinase pathway probably also contributes to UM development and progression. Several lines of evidence indicate that inflammation and macrophages play a pro-tumor role in UM and in its hepatic metastases. UM cells benefit from the immune privilege in the eye and may adopt several mechanisms involved in this privilege for tumor escape that act even after leaving the niche. Here, we review the current knowledge of the biology of UM and discuss recent approaches to UM treatment.

Cell surface cathepsin G activity differs between human natural killer cell subsets

Natural killer (NK) cells are critical in diverse defense mechanisms, including elimination of viral infected cells and destruction of tumor cells. NK cells are characterized by the ability to initiate apoptosis in target cells when their cell surface major histocompatibility complex class I (MHC I) repertoire is missing. On the other hand, NK cells are not activated when MHC I or non-classical MHC molecules are found on the respective cells. It was demonstrated that cathepsin G (CatG) binds to the cell surface of NK cells; however, the distribution of this protease on the cell surface of NK cell subsets has not been identified. Here, we show that CatG cell surface level differs between NK cell subsets. CatG was determined on the protein- and activity level (activity-based probe MARS116) by using flow cytometry. Thus, MARS116 is a novel reporter of cell surface CatG activity and can be used to differentiate between distinct NK cell subsets.