Anti-NKG2A mAb Is a Checkpoint Inhibitor that Promotes Anti-tumor Immunity by Unleashing Both T and NK Cells

Different Features of Tumor-Associated NK Cells in Patients With Low-Grade or High-Grade Peritoneal Carcinomatosis

Peritoneal carcinomatosis (PC) is a rare disease defined as diffused implantation of neoplastic cells in the peritoneal cavity. This clinical picture occurs during the evolution of peritoneal tumors, and it is the main cause of morbidity and mortality of patients affected by these pathologies, though cytoreductive surgery with heated intra-peritoneal chemotherapy (CRS/HIPEC) is yielding promising results. In the present study, we evaluated whether the tumor microenvironment of low-grade and high-grade PC could affect the phenotypic and functional features and thus the anti-tumor potential of NK cells. We show that while in the peritoneal fluid (PF) of low-grade PC most CD56dim NK cells show a relatively immature phenotype (NKG2A+KIR-CD57-CD16dim), in the PF of high-grade PC NK cells are, in large majority, mature (CD56dimKIR+CD57+CD16bright). Furthermore, in low-grade PC, PF-NK cells are characterized by a sharp down-regulation of some activating receptors, primarily NKp30 and DNAM-1, while, in high-grade PC, PF-NK cells display a higher expression of the PD-1 inhibitory checkpoint. The compromised phenotype observed in low-grade PC patients corresponds to a functional impairment. On the other hand, in the high-grade PC patients PF-NK cells show much more important defects that only partially reflect the compromised phenotype detected. These data suggest that the PC microenvironment may contribute to tumor escape from immune surveillance by inducing different NK cell impaired features leading to altered anti-tumor activity. Notably, after CRS/HIPEC treatment, the altered NK cell phenotype of a patient with a low-grade disease and favorable prognosis was reverted to a normal one. Our present data offer a clue for the development of new immunotherapeutic strategies capable of restoring the NK-mediated anti-tumor responses in association with the CRS/HIPEC treatment to increase the effectiveness of the current therapy.

NK Cell Dysfunction and Checkpoint Immunotherapy

NK cells play important roles in the innate immune responses against tumors. The effector function of NK cells relies on the integration of activating and inhibitory signals. Emerging checkpoint receptors and molecules are being revealed to mediate NK cell dysfunction in the tumor microenvironment. Inhibition of some NK cell surface checkpoint receptors has displayed the potential to reverse NK cell dysfunction in tumors, and to boost anti-tumor immunity, both in clinical trials (anti-KIR and anti-NKG2A), and in preclinical studies (e.g., anti-TIGIT, and anti-CD96). To fully exploit the potential of NK-based checkpoint immunotherapy, more understanding of the regional features of NK cells in the tumor microenvironment is required. This will provide valuable information regarding the dynamic nature of NK cell immune response against tumors, as well as novel checkpoints or pathways to be targeted. In this Review, we discuss recent advances in the understanding of NK cell dysfunction in tumors, as well as emerging strategies of NK-based checkpoint immunotherapy for tumors.

A Natural Impact: NK Cells at the Intersection of Cancer and HIV Disease

Despite efficient suppression of plasma viremia in people living with HIV (PLWH) on cART, evidence of HIV-induced immunosuppression remains, and normally benign and opportunistic pathogens become major sources of co-morbidities, including virus-induced cancers. In fact, cancer remains a primary cause of death even in virally suppressed PLWH. Natural killer (NK) cells provide rapid early responses to HIV infection, contribute substantially to disease modulation and vaccine protection, and are also major therapeutic targets for cancer immunotherapy. However, much like other lymphocyte populations, recent burgeoning evidence suggests that in chronic conditions like HIV, NK cells can become functionally exhausted with impaired cytotoxic function, altered cytokine production and impaired antibody-dependent cell-mediated cytotoxicity. Recent work suggests functional anergy is likely due to low-level ongoing virus replication, increased inflammatory cytokines, or increased presence of MHC^low target cells. Indeed, HIV-induced loss of NK cell-mediated control of lytic EBV infection has been specifically shown to cause lymphoma and also increases replication of CMV. In this review, we will discuss current understanding of NK cell modulation of HIV disease, reciprocal exhaustion of NK cells, and how this may impact increased cancer incidences and prospects for NK cell-targeted immunotherapies. Finally, we will review the most recent evidence supporting adaptive functions of NK cells and highlight the potential of adaptive NK cells for cancer immunotherapy.

Naturally Killing the Silent Killer: NK Cell-Based Immunotherapy for Ovarian Cancer

Ovarian cancer (OC) is diagnosed in ~22,000 women in the US each year and kills 14,000 of them. Often, patients are not diagnosed until the later stages of disease, when treatment options are limited, highlighting the urgent need for new and improved therapies for precise cancer control. An individual's immune function and interaction with tumor cells can be prognostic of the response to cancer treatment. Current emerging therapies for OC include immunotherapies, which use antibodies or drive T cell-mediated cancer recognition and elimination. In OC, these have been limited by adverse side effects and tumor characteristics including inter- and intra-tumoral heterogeneity, lack of targetable antigens, loss of tumor human leukocyte antigen expression, high levels of immunosuppressive factors, and insufficient immune cell trafficking. Natural killer (NK) cells may be ideal as primary or collateral effectors to these nascent immunotherapies. NK cells exhibit multiple functions that combat immune escape and tumor relapse: they kill targets and elicit inflammation through antigen-independent pathways and detect loss of HLA as a signal for activation. NK cells are efficient mediators of tumor immune surveillance and control, suppressed by the tumor microenvironment and rescued by immune checkpoint blockade. NK cells are regulated by a variety of activating and inhibitory receptors and already known to be central effectors across an array of existing therapies. In this article, we highlight interactions between NK cells and OC and their potential to change the immunosuppressive tumor microenvironment and participate in durable immune control of OC.

Enriched HLA-E and CD94/NKG2A Interaction Limits Antitumor CD8+ Tumor-Infiltrating T Lymphocyte Responses

Immunotherapy treatments with anti-PD-1 boost recovery in less than 30% of treated cancer patients, indicating the complexity of the tumor microenvironment. Expression of HLA-E is linked to poor clinical outcomes in mice and human patients. However, the contributions to immune evasion of HLA-E, a ligand for the inhibitory CD94/NKG2A receptor, when expressed on tumors, compared with adjacent tissue and peripheral blood mononuclear cells, remains unclear. In this study, we report that epithelial-derived cancer cells, tumor macrophages, and CD141+ conventional dendritic cells (cDC) contributed to HLA-E enrichment in carcinomas. Different cancer types showed a similar pattern of enrichment. Enrichment correlated to NKG2A upregulation on CD8+ tumor-infiltrating T lymphocytes (TIL) but not on CD4+ TILs. CD94/NKG2A is exclusively expressed on PD-1high TILs while lacking intratumoral CD103 expression. We also found that the presence of CD94/NKG2A on human tumor-specific T cells impairs IL2 receptor-dependent proliferation, which affects IFNγ-mediated responses and antitumor cytotoxicity. These functionalities recover following antibody-mediated blockade in vitro and ex vivo Our results suggest that enriched HLA-E:CD94/NKG2A inhibitory interaction can impair survival of PD-1high TILs in the tumor microenvironment.

Strategies to Augment Natural Killer (NK) Cell Activity against Solid Tumors

The immune system plays a crucial role to prevent local growth and dissemination of cancer. Therapies based on activating the immune system can result in beneficial responses in patients with metastatic disease. Treatment with antibodies targeting the immunological checkpoint axis PD-1 / PD-L1 can result in the induction of anti-tumor T cell activation leading to meaningful long-lasting clinical responses. Still, many patients acquire resistance or develop dose-limiting toxicities to these therapies. Analysis of tumors from patients who progress on anti-PD-1 treatment reveal defective interferon-signaling and antigen presentation, resulting in immune escape from T cell-mediated attack. Natural killer (NK) cells are innate lymphocytes that can kill tumor cells without prior sensitization to antigens and can be activated to kill tumor cells that have an impaired antigen processing and presentation machinery. Thus, NK cells may serve as useful effectors against tumor cells that have become resistant to classical immune checkpoint therapy. Various approaches to activate NK cells are being increasingly explored in clinical trials against cancer. While clinical benefit has been demonstrated in patients with acute myeloid leukemia receiving haploidentical NK cells, responses in patients with solid tumors are so far less encouraging. Several hurdles need to be overcome to provide meaningful clinical responses in patients with solid tumors. Here we review the recent developments to augment NK cell responses against solid tumors with regards to cytokine therapy, adoptive infusion of NK cells, NK cell engagers, and NK cell immune checkpoints.

Tumor-derived exosomes (TDEs): How to avoid the sting in the tail

Exosomes are abundantly secreted extracellular vesicles that accumulate in the circulation and are of great interest for disease diagnosis and evaluation since their contents reflects the phenotype of their cell of origin. Tumor‐derived exosomes (TDEs) are of particular interest for cancer diagnosis and therapy, since most tumor demonstrate highly elevated exosome secretion rates and provide specific information about the genotype of a tumor and its response to treatment. TDEs also contain regulatory factors that can alter the phenotypes of local and distant tissue sites and alter immune cell functions to promote tumor progression. The abundance, information content, regulatory potential, in vivo half‐life, and physical durability of exosomes suggest that TDEs may represent a superior source of diagnostic biomarkers and treatment targets than other materials currently under investigation. This review will summarize current information on mechanisms that may differentially regulate TDE biogenesis, TDE effects on the immune system that promote tumor survival, growth, and metastasis, and new approaches understudy to counteract or utilize TDE properties in cancer therapies.

The Society for Immunotherapy of Cancer consensus statement on immunotherapy for the treatment of squamous cell carcinoma of the head and neck (HNSCC)

Head and neck cancers, including those of the lip and oral cavity, nasal cavity, paranasal sinuses, oropharynx, larynx and nasopharynx represent nearly 700,000 new cases and 380,000 deaths worldwide per annum, and account for over 10,000 annual deaths in the United States alone. Improvement in outcomes are needed for patients with recurrent and or metastatic squamous cell carcinoma of the head and neck (HNSCC). In 2016, the US Food and Drug Administration (FDA) granted the first immunotherapeutic approvals - the anti-PD-1 immune checkpoint inhibitors nivolumab and pembrolizumab - for the treatment of patients with recurrent squamous cell carcinoma of the head and neck (HNSCC) that is refractory to platinum-based regimens. The European Commission followed in 2017 with approval of nivolumab for treatment of the same patient population, and shortly thereafter with approval of pembrolizumab monotherapy for the treatment of recurrent or metastatic HNSCC in adults whose tumors express PD-L1 with a ≥ 50% tumor proportion score and have progressed on or after platinum-containing chemotherapy. Then in 2019, the FDA granted approval for PD-1 inhibition as first-line treatment for patients with metastatic or unresectable, recurrent HNSCC, approving pembrolizumab in combination with platinum and fluorouracil for all patients with HNSCC and pembrolizumab as a single agent for patients with HNSCC whose tumors express a PD-L1 combined positive score ≥ 1. These approvals marked the first new therapies for these patients since 2006, as well as the first immunotherapeutic approvals in this disease. In light of the introduction of these novel therapies for the treatment of patients with head and neck cancer, The Society for Immunotherapy of Cancer (SITC) formed an expert committee tasked with generating consensus recommendations for emerging immunotherapies, including appropriate patient selection, therapy sequence, response monitoring, adverse event management, and biomarker testing. These consensus guidelines serve as a foundation to assist clinicians' understanding of the role of immunotherapies in this disease setting, and to standardize utilization across the field for patient benefit. Due to country-specific variances in approvals, availability and regulations regarding the discussed agents, this panel focused solely on FDA-approved drugs for the treatment of patients in the U.S.

Anti-Cancer Effects of Pristimerin and the Mechanisms: A Critical Review

As a quinonemethide triterpenoid extracted from species of the Celastraceae and Hippocrateaceae, pristimerin has been shown potent anti-cancer effects. Specifically, it was found that pristimerin can affect many tumor-related processes, such as apoptosis, autophagy, migration and invasion, vasculogenesis, and drug resistance. Various molecular targets or signaling pathways are also involved, such as cyclins, reactive oxygen species (ROS), microRNA, nuclear factor kappa B (NF-κB), mitogen-activated protein kinase (MAPK), and PI3K/AKT/mammalian target of rapamycin (mTOR) pathways. In this review, we will focus on the research about pristimerin-induced anti-cancer activities to achieve a deeper understanding of the targets and mechanisms, which offer evidences suggesting that pristimerin can be a potent anti-cancer drug.

An Historical Overview: The Discovery of How NK Cells Can Kill Enemies, Recruit Defense Troops, and More

Natural killer (NK) cells were originally defined as effector lymphocytes of innate immunity characterized by the unique ability of killing tumor and virally infected cells without any prior priming and expansion of specific clones. The "missing-self" theory, proposed by Klas Karre, the seminal discovery of the first prototypic HLA class I-specific inhibitory receptors, and, later, of the Natural Cytotoxicity Receptors (NCRs) by Alessandro Moretta, provided the bases to understand the puzzling behavior of NK cells. Actually, those discoveries proved crucial also for many of the achievements that, along the years, have contributed to the modern view of these cells. Indeed, NK cells, besides killing susceptible targets, are now known to functionally interact with different immune cells, sense pathogens using TLR, adapt their responses to the local environment, and, even, mount a sort of immunological memory. In this review, we will specifically focus on the main activating NK receptors and on their crucial role in the ever-increasing number of functions assigned to NK cells and other innate lymphoid cells (ILCs).

Evaluation of NK cell cytotoxic activity against malignant cells by the calcein assay

The immune system exerts a tight cancer surveillance to eradicate nascent and developing tumors. Natural killer (NK) cells rely on their cytotoxic activity to eliminate abnormal or stressed cells, including those undergoing malignant transformation. Thereupon, NK cells, along with CD8⁺ T cells, are effector immune cells with a pivotal role in cancer immunosurveillance. Due to the outstanding effectivity of immunotherapy in cancer management, strategies aimed at boosting NK cell-mediated tumor recognition and elimination are being thoroughly studied, such as approaches to induce tumor surface expression of NK cell activating ligands. Additionally, the development of methods to evaluate NK cell cytotoxic responses towards malignant cells with distinct immunogenic profiles is of paramount relevance. In this chapter, we detail a simple but highly sensitive and reproducible methodological approach to quantify in vitro NK cell-mediated lysis of a given target cell, e.g. a tumor cell, based on calcein-AM staining and fluorescence detection. Further, this method can be employed to determine the impact of chemical modulators or antibodies blocking cell surface proteins on the antitumor capacity of NK cells, with the aim to improve anticancer immune responses and elucidate the underlying mechanisms regulating NK cell activity.

A cytofluorimetric assay to evaluate intracellular cytokine production by NK cells

Natural killer (NK) cells are innate lymphoid cells that, together with CD8⁺ T lymphocytes, are tightly correlated with immunesurveillance and the elimination of transforming and malignant cells both during early steps of tumorigenesis and metastasis. This capacity is due, but not limited to, their great cytotoxic capacity upon recognition of stress-related activating ligands of NK cells on the surface of tumor cells. Due to the emerging role of NK cells in the response to treatment with immune checkpoint blockers (ICBs) and other immunotherapies, it has become essential to deeply study this immune subset. As mentioned above, NK cell antitumor responses not only rely on their high cytotoxic capacity, cytokine production by this immune subset, such as interferon gamma (IFN-γ) or tumor necrosis factor (TNF-α), also plays a key role on NK cell function. In this chapter, we provide a detailed protocol to measure the intracellular expression of cytokines produced by NK cells upon stimulation. A step-by-step guide to isolate peripheral blood mononuclear cells, purify NK cells from whole blood, co-culture with tumor cell lines and evaluate their cytokine production capacity by flow cytometry is here provided.

Inhibition of the NKp44-PCNA Immune Checkpoint Using a mAb to PCNA

mAb-based blocking of the immune checkpoints involving the CTLA4-B7 and PD1-PDL1 inhibitory axes enhance T-cell-based adaptive immune responses in patients with cancer. We show here that antitumor responses by natural killer (NK) cells can be enhanced by a checkpoint-blocking mAb, 14-25-9, which we developed against proliferating cell nuclear antigen (PCNA). PCNA is expressed on the surface of cancer cells and acts as an inhibitory ligand for the NK-cell receptor, NKp44-isoform1. We tested for cytoplasmic- and membrane-associated PCNA by FACS- and ImageStream-based staining of cell lines and IHC of human cancer formalin fixed, paraffin embedded tissues. The mAb, 14-25-9, inhibited binding of chimeric NKp44 receptor to PCNA and mostly stained the cytoplasm and membrane of tumor cells, whereas commercial antibody (clone PC10) stained nuclear PCNA. NK functions were measured using ELISA-based IFNγ secretion assays and FACS-based killing assays. The NK92-NKp44-1 cell line and primary human NK cells showed increased IFNγ release upon coincubation with mAb 14-25-9 and various solid tumor cell lines and leukemias. Treatment with 14-25-9 also increased NK cytotoxic activity. In vivo efficacy was evaluated on patient-derived xenografts (PDX)-bearing NSG mice. In PDX-bearing mice, intravenous administration of mAb 14-25-9 increased degranulation (CD107a expression) of intratumorally injected patient autologous or allogeneic NK cells, as well as inhibited tumor growth when treated long term. Our study describes a mAb against the NKp44-PCNA innate immune checkpoint that can enhance NK-cell antitumor activity both in vitro and in vivo.

PD/1-PD-Ls Checkpoint: Insight on the Potential Role of NK Cells

The identification of inhibitory NK cell receptors specific for HLA-I molecules (KIRs and NKG2A) provided the molecular basis for clarifying the mechanism by which NK cells kill transformed cells while sparing normal cells. The direct interactions between inhibitory NK cell receptors and their HLA-I ligands enable NK cells to distinguish healthy from transformed cells, which frequently show an altered expression of HLA-I molecules. Indeed, NK cells can kill cancer cells that have lost, or under express, HLA-I molecules, but not cells maintaining their expression. In this last case, it is possible to use anti-KIR or anti-NKG2A monoclonal antibodies to block the inhibitory signals generated by these receptors and to restore the anti-tumor NK cell activity. These treatments fall within the context of the new immunotherapeutic strategies known as “immune checkpoint blockade.” These antibodies are currently used in clinical trials in the treatment of both hematological and solid tumors. However, a more complex scenario has recently emerged. For example, NK cells can also express additional immune checkpoints, including PD-1, that was originally described on T lymphocytes, and whose ligands (PD-Ls) are usually overexpressed on tumor cells. Thus, it appears that the activation of NK cells and their potentially harmful effector functions are under the control of different immune checkpoints and their simultaneous expression could provide additional levels of suppression to anti-tumor NK cell responses. This review is focused on PD-1 immune checkpoint in NK cells, its potential role in immunosuppression, and the therapeutic strategies to recover NK cell cytotoxicity and anti-tumor effect.

NK Cells in the Human Lungs

The lung offers one of the largest exchange surfaces of the individual with the elements of the environment. As a place of important interactions between self and non-self, the lung is richly endowed in various immune cells. As such, lung natural killer (NK) cells play major effector and immunoregulatory roles to ensure self-integrity. A better understanding of their abilities in health and diseases has been made possible over the past decade thanks to tremendous discoveries in humans and animals. By precisely distinguishing the different NK cell subsets and dissecting the ontogeny and differentiation of NK cells, both blood and tissue-resident NK populations now appear to be much more pleiotropic than previously thought. In light of these recent findings in healthy individuals, this review describes the different lung NK cell populations quantitatively, qualitatively, phenotypically, and functionally. Their identification, immunological diversity, and adaptive capacities are also addressed. For each of these elements, the impact of the mutual interactions of lung NK cells with environmental and microenvironmental factors are questioned in terms of functionality, competence, and adaptive capacities. As pulmonary diseases are major causes of morbidity and mortality worldwide, special attention is also given to the involvement of lung NK cells in various diseases, including infectious, inflammatory, autoimmune, and neoplastic lung diseases. In addition to providing a comprehensive overview of lung NK cell biology, this review also provides insight into the potential of NK cell immunotherapy and the development of targeted biologics.

Killer Ig-Like Receptors (KIRs): Their Role in NK Cell Modulation and Developments Leading to Their Clinical Exploitation

Natural killer (NK) cells contribute to the first line of defense against viruses and to the control of tumor growth and metastasis spread. The discovery of HLA class I specific inhibitory receptors, primarily of killer Ig-like receptors (KIRs), and of activating receptors has been fundamental to unravel NK cell function and the molecular mechanisms of tumor cell killing. Stemmed from the seminal discoveries in early '90s, in which Alessandro Moretta was the major actor, an extraordinary amount of research on KIR specificity, genetics, polymorphism, and repertoire has followed. These basic notions on NK cells and their receptors have been successfully translated to clinical applications, primarily to the haploidentical hematopoietic stem cell transplantation to cure otherwise fatal leukemia in patients with no HLA compatible donors. The finding that NK cells may express the PD-1 inhibitory checkpoint, particularly in cancer patients, may allow understanding how anti-PD-1 therapy could function also in case of HLA class Ineg tumors, usually susceptible to NK-mediated killing. This, together with the synergy of therapeutic anti-checkpoint monoclonal antibodies, including those directed against NKG2A or KIRs, emerging in recent or ongoing studies, opened new solid perspectives in cancer therapy.

The IRE1 endoplasmic reticulum stress sensor activates natural killer cell immunity in part by regulating c-Myc

Natural killer (NK) cells are critical mediators of host immunity to pathogens. Here, we demonstrate that the endoplasmic reticulum stress sensor inositol-requiring enzyme 1 (IRE1α) and its substrate transcription factor X-box-binding protein 1 (XBP1) drive NK cell responses against viral infection and tumors in vivo. IRE1α-XBP1 were essential for expansion of activated mouse and human NK cells and are situated downstream of the mammalian target of rapamycin signaling pathway. Transcriptome and chromatin immunoprecipitation analysis revealed c-Myc as a new and direct downstream target of XBP1 for regulation of NK cell proliferation. Genetic ablation or pharmaceutical blockade of IRE1α downregulated c-Myc, and NK cells with c-Myc haploinsufficency phenocopied IRE1α-XBP1 deficiency. c-Myc overexpression largely rescued the proliferation defect in IRE1α^-/- NK cells. Like c-Myc, IRE1α-XBP1 also promotes oxidative phosphorylation in NK cells. Overall, our study identifies a IRE1α-XBP1-cMyc axis in NK cell immunity, providing insight into host protection against infection and cancer.

NK cells to cure cancer

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

Beyond PD-1/PD-L1 Inhibition: What the Future Holds for Breast Cancer Immunotherapy

Cancer immunotherapy has altered the management of human malignancies, improving outcomes in an expanding list of diseases. Breast cancer - presumably due to its perceived low immunogenicity - is a late addition to this list. Furthermore, most of the focus has been on the triple negative subtype because of its higher tumor mutational load and lymphocyte-enriched stroma, although emerging data show promise on the other breast cancer subtypes as well. To this point the clinical use of immunotherapy is limited to the inhibition of two immune checkpoints, Programmed Cell Death Protein 1 (PD-1) and Cytotoxic T-lymphocyte-associated Protein 4 (CTLA-4). Consistent with the complexity of the regulation of the tumor - host interactions and their lack of reliance on a single regulatory pathway, combinatory approaches have shown improved efficacy albeit at the cost of increased toxicity. Beyond those two checkpoints though, a large number of co-stimulatory or co-inhibitory molecules play major roles on tumor evasion from immunosurveillance. These molecules likely represent future targets of immunotherapy provided that the promise shown in early data is translated into improved patient survival in randomized trials. The biological role, prognostic and predictive implications regarding breast cancer and early clinical efforts on exploiting these immune-related therapeutic targets are herein reviewed.

Blocking expression of inhibitory receptor NKG2A overcomes tumor resistance to NK cells

A key mechanism of tumor resistance to immune cells is mediated by expression of peptide-loaded HLA-E in tumor cells, which suppresses natural killer (NK) cell activity via ligation of the NK inhibitory receptor CD94/NKG2A. Gene expression data from approximately 10,000 tumor samples showed widespread HLAE expression, with levels correlating with those of KLRC1 (NKG2A) and KLRD1 (CD94). To bypass HLA-E inhibition, we developed a way to generate highly functional NK cells lacking NKG2A. Constructs containing a single-chain variable fragment derived from an anti-NKG2A antibody were linked to endoplasmic reticulum-retention domains. After retroviral transduction in human peripheral blood NK cells, these NKG2A Protein Expression Blockers (PEBLs) abrogated NKG2A expression. The resulting NKG2Anull NK cells had higher cytotoxicity against HLA-E-expressing tumor cells. Transduction of anti-NKG2A PEBL produced more potent cytotoxicity than interference with an anti-NKG2A antibody and prevented de novo NKG2A expression, without affecting NK cell proliferation. In immunodeficient mice, NKG2Anull NK cells were significantly more powerful than NKG2A+ NK cells against HLA-E-expressing tumors. Thus, NKG2A downregulation evades the HLA-E cancer immune-checkpoint, and increases the anti-tumor activity of NK cell infusions. Because this strategy is easily adaptable to current protocols for clinical-grade immune cell processing, its clinical testing is feasible and warranted.

Human NK cells: surface receptors, inhibitory checkpoints, and translational applications

NK cells play important roles in innate defenses against viruses and in the control of tumor growth and metastasis. The regulation/induction of NK cell function is mediated by an array of activating or inhibitory surface receptors. In humans, major activating receptors involved in target cell killing are the natural cytotoxicity receptors (NCRs) and NKG2D. Activating receptors recognize ligands that are overexpressed or expressed de novo upon cell stress, viral infection, or tumor transformation. The HLA-class I-specific inhibitory receptors, including KIRs recognizing HLA-class I allotypic determinants and CD94/NKG2A recognizing the class-Ib HLA-E, constitute a fail-safe mechanism to avoid unwanted NK-mediated damage to healthy cells. Other receptors such as PD-1, primarily expressed by activated T lymphocytes, are important inhibitory checkpoints of immune responses that ensure T-cell tolerance. PD-1 also may be expressed by NK cells in cancer patients. Since PD-1 ligand (PD-L1) may be expressed by different tumors, PD-1/PD-L1 interactions inactivate both T and NK cells. Thus, the reliable evaluation of PD-L1 expression in tumors has become a major issue to select patients who may benefit from therapy with mAbs disrupting PD-1/PD-L1 interactions. Recently, NKG2A was revealed to be an important checkpoint controlling both NK and T-cell activation. Since most tumors express HLA-E, mAbs targeting NKG2A has been used alone or in combination with other therapeutic mAbs targeting PD-1 or tumor antigens (e.g., EGFR), with encouraging results. The translational value of NK cells and their receptors is evidenced by the extraordinary therapeutic success of haploidentical HSCT to cure otherwise fatal high-risk leukemias.

Hepatic Natural Killer Cells: Organ-Specific Sentinels of Liver Immune Homeostasis and Physiopathology

The liver is considered a preferential tissue for NK cells residency. In humans, almost 50% of all intrahepatic lymphocytes are NK cells that are strongly imprinted in a liver-specific manner and show a broad spectrum of cellular heterogeneity. Hepatic NK (he-NK) cells play key roles in tuning liver immune response in both physiological and pathological conditions. Therefore, there is a pressing need to comprehensively characterize human he-NK cells to better understand the related mechanisms regulating their effector-functions within the dynamic balance between immune-tolerance and immune-surveillance. This is of particular relevance in the liver that is the only solid organ whose parenchyma is constantly challenged on daily basis by millions of foreign antigens drained from the gut. Therefore, the present review summarizes our current knowledge on he-NK cells in the light of the latest discoveries in the field of NK cell biology and clinical relevance.

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.

Setting traps for NKG2A gives NK cell immunotherapy a fighting chance

The equilibrium of signaling through activating and inhibitory receptors dictates whether a given NK cell will execute cellular cytotoxicity. In this issue of the JCI, Kamiya et al. describe a novel approach to efficiently inhibiting surface expression of the inhibitory receptor CD94/NK group 2 member A (NKG2A) through retention of the protein in the endoplasmic reticulum. In adoptive transfer experiments into tumor-bearing immunodeficient mice, NKG2Anull NK cells were significantly more effective at eliminating HLA-E-expressing tumor cells than NKG2A+ NK cells. This study provides proof of concept for a new immunotherapeutic approach using NKG2Anull NK cells.

NKG2A is a NK cell exhaustion checkpoint for HCV persistence

Exhaustion of cytotoxic effector natural killer (NK) and CD8⁺ T cells have important functions in the establishment of persistent viral infections, but how exhaustion is induced during chronic hepatitis C virus (HCV) infection remains poorly defined. Here we show, using the humanized C/O^Tg mice permissive for persistent HCV infection, that NK and CD8⁺ T cells become sequentially exhausted shortly after their transient hepatic infiltration and activation in acute HCV infection. HCV infection upregulates Qa-1 expression in hepatocytes, which ligates NKG2A to induce NK cell exhaustion. Antibodies targeting NKG2A or Qa-1 prevents NK exhaustion and promotes NK-dependent HCV clearance. Moreover, reactivated NK cells provide sufficient IFN-γ that helps rejuvenate polyclonal HCV CD8⁺ T cell response and clearance of HCV. Our data thus show that NKG2A serves as a critical checkpoint for HCV-induced NK exhaustion, and that NKG2A blockade sequentially boosts interdependent NK and CD8⁺ T cell functions to prevent persistent HCV infection.

Immune cells may become less responsive, or ‘exhausted’, upon chronic viral infection, but the underlying mechanism and crosstalk are still unclear. Here the authors show that, upon chronic hepatitis C virus (HCV) infection, natural killer cell exhaustion is induced by NKG2A signalling to instruct downstream exhaustion of CD8⁺ T cells and HCV persistence.

Host tissue determinants of tumour immunity

Although common evolutionary principles drive the growth of cancer cells regardless of the tissue of origin, the microenvironment in which tumours arise substantially differs across various organ sites. Recent studies have established that, in addition to cell-intrinsic effects, tumour growth regulation also depends on local cues driven by tissue environmental factors. In this Review, we discuss how tissue-specific determinants might influence tumour development and argue that unravelling the tissue-specific contribution to tumour immunity should help the development of precise immunotherapeutic strategies for patients with cancer.

The Head and Neck Squamous Cell Carcinoma Microenvironment as a Potential Target for Cancer Therapy

Similarly to other types of malignant tumours, the incidence of head and neck cancer is increasing globally. It is frequently associated with smoking and alcohol abuse, and in a broader sense also with prolonged exposure to these factors during ageing. A higher incidence of tumours observed in younger populations without a history of alcohol and tobacco abuse may be due to HPV infection. Malignant tumours form an intricate ecosystem of cancer cells, fibroblasts, blood/lymphatic capillaries and infiltrating immune cells. This dynamic system, the tumour microenvironment, has a significant impact on the biological properties of cancer cells. The microenvironment participates in the control of local aggressiveness of cancer cells, their growth, and their consequent migration to lymph nodes and distant organs during metastatic spread. In cancers originating from squamous epithelium, a similarity was demonstrated between the cancer microenvironment and healing wounds. In this review, we focus on the specificity of the microenvironment of head and neck cancer with emphasis on the mechanism of intercellular crosstalk manipulation for potential therapeutic application.

Enhancing Dendritic Cell Therapy in Solid Tumors with Immunomodulating Conventional Treatment

Dendritic cells (DCs) are the most potent antigen-presenting cells and are the key initiator of tumor-specific immune responses. These characteristics are exploited by DC therapy, where DCs are ex vivo loaded with tumor-associated antigens (TAAs) and used to induce tumor-specific immune responses. Unfortunately, clinical responses remain limited to a proportion of the patients. Tumor characteristics and the immunosuppressive tumor microenvironment (TME) of the tumor are likely hampering efficacy of DC therapy. Therefore, reducing the immunosuppressive TME by combining DC therapy with other treatments could be a promising strategy. Initially, conventional cancer therapies, such as chemotherapy and radiotherapy, were thought to specifically target cancerous cells. Recent insights indicate that these therapies additionally augment tumor immunity by targeting immunosuppressive cell subsets in the TME, inducing immunogenic cell death (ICD), or blocking inhibitory molecules. Therefore, combining DC therapy with registered therapies such as chemotherapy, radiotherapy, or checkpoint inhibitors could be a promising treatment strategy to improve the efficacy of DC therapy. In this review, we evaluate various clinical applicable combination strategies to improve the efficacy of DC therapy.

Targeting natural killer cells in solid tumors

Natural killer (NK) cells are innate lymphoid cells endowed with cytolytic activity and a capacity to secrete cytokines and chemokines. Several lines of evidence suggest that NK cells play an important role in anti-tumor immunity. Some therapies against hematological malignacies make use of the immune properties of NK cells, such as their ability to kill residual leukemic blasts efficiently after conditioning during haploidentical hematopoietic stem cell transplantation. However, knowledge on NK cell infiltration and the status of NK cell responsiveness in solid tumors is limited so far. The pro-angiogenic role of the recently described NK cell-like type 1 innate lymphoid cells (ILC1s) and their phenotypic resemblance to NK cells are confounding factors that add a level of complexity, at least in mice. Here, we review the current knowledge on the presence and function of NK cells in solid tumors as well as the immunotherapeutic approaches designed to harness NK cell functions in these conditions, including those that aim to reinforce conventional anti-tumor therapies to increase the chances of successful treatment.

Ex vivo Hsp70-Activated NK Cells in Combination With PD-1 Inhibition Significantly Increase Overall Survival in Preclinical Models of Glioblastoma and Lung Cancer

Heat shock protein 70 (Hsp70) which is expressed on the plasma membrane of highly aggressive tumors including non-small cell lung carcinoma and glioblastoma multiforme serves as a target for Hsp70-targeting NK cells. Herein, we aimed to investigate the antitumor effects of a combined therapy consisting of ex vivo Hsp70-peptide TKD/IL-2-activated NK cells in combination with mouse/human anti-PD-1 antibody in a syngeneic glioblastoma and a xenograft lung cancer mouse model. Mice with membrane Hsp70 positive syngeneic GL261 glioblastoma or human xenograft A549 lung tumors were sham-treated with PBS or injected with ex vivo TKD/IL-2-activated mouse/human NK cells and mouse/human PD-1 antibody either as a single regimen or in combination. Tumor volume was assessed by MR scanning and tumor-infiltrating CD8⁺ T, NK, and PD-1⁺ cells were quantified by immunohistochemistry (IHC). We could show that the adoptive transfer of ex vivo TKD/IL-2-activated mouse NK cells or the inhibition of PD-1 resulted in tumor growth delay and an improved overall survival (OS) in a syngeneic glioblastoma mouse model. A combination of both therapies was well-tolerated and significantly more effective with respect to both outcome parameters than either of the single regimens. A combined treatment in a xenograft lung cancer model showed identical effects in immunodeficient mice bearing human lung cancer after adoptive transfer of TKD/IL-2-activated human effector cells and a human PD-1 antibody. Tumor control was associated with a massive infiltration with CD8⁺ T and NK cells in both tumor models and a decreased in PD-1 expression on immune effector cells. In summary, a combined approach consisting of activated NK cells and anti-PD-1 therapy is safe and results in a long-term tumor control which is accompanied by a massive tumor immune cell infiltration in 2 preclinical tumor models.

Innate lymphoid cells: A potential link between microbiota and immune responses against cancer

The adaptive immune system plays a crucial role in anti-tumor surveillance. Enhancement of T cell responses through checkpoint blockade has become a major therapeutic avenue of intervention for several tumors. Because it shapes immune responses and regulates their amplitude and duration, the microbiota has a substantial impact on anti-tumor immunity. Innate lymphoid cells (ILCs) comprise a heterogeneous population of lymphocytes devoid of antigen-specific receptors that mirror T helper cells in their ability to secrete cytokines that activate immune responses. Ongoing studies suggest that ILCs contribute to anti-tumor responses. Moreover, since ILCs are present at barrier surfaces, they are stimulated by the microbiota and, reciprocally, influence the composition of the microbiota by regulating the surface barrier microenvironment. Thus, ILC-microbiota cross-talk may in part underpin the effects of the microbiota on anti-tumor responses. In this article, we review current evidence linking ILCs to cancer and discuss the potential impact of ILC-microbiota cross-talk in anti-tumor immune responses.

Circulating CD56bright NK cells inversely correlate with survival of melanoma patients

The roles of NK cells in human melanoma remain only partially understood. We characterized NK cells from peripheral blood ex vivo by flow cytometry obtained from late stage (III/IV) melanoma patients. Interestingly, we found that the abundance of CD56bright NK cells negatively correlate with overall patient survival, together with distant metastases, in a multivariate cox regression analysis. The patients' CD56bright NK cells showed upregulation of CD11a, CD38 and CD95 as compared to healthy controls, pointing to an activated phenotype as well as a possible immune regulatory role in melanoma patients. After stimulation in vitro, CD56bright NK cells produced less TNFα and GMCSF in patients than controls. Furthermore, IFNγ production by the CD56bright NK cells correlated inversely with overall survival. Our results highlight that abundance and function of CD56bright NK cells are associated with melanoma patient survival, emphasizing the potential of NK cell subsets for biomarker discovery and future therapeutic targeting.

Strengthening the AntiTumor NK Cell Function for the Treatment of Ovarian Cancer

The crosstalk between cancer cells and host cells is a crucial prerequisite for tumor growth and progression. The cells from both the innate and adaptive immune systems enter into a perverse relationship with tumor cells to create a tumor-promoting and immunosuppressive tumor microenvironment (TME). Epithelial ovarian cancer (EOC), the most lethal of all gynecological malignancies, is characterized by a unique TME that paves the way to the formation of metastasis and mediates therapy resistance through the deregulation of immune surveillance. A characteristic feature of the ovarian cancer TME is the ascites/peritoneal fluid, a malignancy-associated effusion occurring at more advanced stages, which enables the peritoneal dissemination of tumor cells and the formation of metastasis. The standard therapy for EOC involves a combination of debulking surgery and platinum-based chemotherapy. However, most patients experience disease recurrence. New therapeutic strategies are needed to improve the prognosis of patients with advanced EOC. Harnessing the body’s natural immune defenses against cancer in the form of immunotherapy is emerging as an innovative treatment strategy. NK cells have attracted attention as a promising cancer immunotherapeutic target due to their ability to kill malignant cells and avoid healthy cells. Here, we will discuss the recent advances in the clinical application of NK cell immunotherapy in EOC.

Natural killer cell transcriptional control, subsets, receptors and effector function

There have been considerable advances in characterization of the complexities of natural killer (NK) cell ligand recognition, activation, subsets and effector functions. The nature of the transcription factors that act to define distinctive functional programmes of NK cell subsets are now starting to be clarified as a consequence of studies in knockouts. Importantly, this is being extended to improved understanding of the nature of NK cell memory. As NK cell biology offers increasing resonance with analogous pathways in CD8 biology, some have made the case that immunology may sometimes benefit from a little less conceptual 'splitting' and a little more conceptual 'lumping.'

Negative regulation of innate lymphoid cell responses in inflammation and cancer

The immune system employs an array of effector cells to ensure tissue homeostasis and protection against pathogens. Lymphocytes belonging to both the adaptive and innate branches share several functions, comprising the ability to directly kill stressed or transformed cells, and to provide helper responses through specific production of cytokines. These properties are regulated by distinct sets of soluble molecules, receptors, and intracellular factors, which altogether tune the functional output of effector lymphocytes and their final activation state. In contrast to adaptive T cells, innate lymphoid cells (ILCs) do not require antigen receptors and are characterized for their ability to provide rapid immune responses. While the factors underlying functional diversification and the main principles leading to ILC activation have been dissected, our understanding of the mechanisms underlying termination of ILC effector functions is still in its infancy. Herein, we discuss the recent findings describing how ILC responses are turned off in the context of inflammation and cancer.

Born to Kill: NK Cells Go to War against Cancer

Preclinical and clinical data emerging over the past year demonstrate that cancer cells suppress the cytotoxic functions of natural killer cells by a variety of mechanisms. These findings reveal a new arsenal of actionable therapeutic targets to drive clinically relevant immune responses against cancer.

The Emergence of Natural Killer Cells as a Major Target in Cancer Immunotherapy

Immune 'checkpoint' inhibitors can increase the activity of tumor-resident cytotoxic lymphocytes and have revolutionized cancer treatment. Current therapies block inhibitory pathways in tumor-infiltrating CD8⁺ T cells and recent studies have shown similar programs in other effector populations such as natural killer (NK) cells. NK cells are critical for immunosurveillance, particularly the control of metastatic cells or hematological cancers. However, how NK cells specifically recognize transformed cells and dominant negative feedback pathways, as well as how tumors escape NK cell control, remains undefined. This review summarizes recent advances that have illuminated inhibitory checkpoints in NK cells, some of which are shared with conventional cytotoxic T lymphocytes. It also outlines emerging approaches aimed at unleashing the potential of NK cells in immunotherapy.

Exploiting NK Cell Surveillance Pathways for Cancer Therapy

Natural killer (NK) cells can evoke potent anti-tumour activity. This function is largely mediated through a battery of specialised cell-surface receptors which probe the tissue microenvironment for changes in surface and secretory phenotypes that may alert to the presence of infection or malignancy. These receptors have the potential to arouse the robust cytotoxic and cytokine-secreting functions of NK cells and so must be tightly regulated to prevent autoimmunity. However, such functions also hold great promise for clinical intervention. In this review, we highlight some of the latest breakthroughs in fundamental NK cell receptor biology that have illuminated our understanding of the molecular strategies NK cells employ to perceive malignant cells from normal healthy cells. Moreover, we highlight how these sophisticated tumour recognition strategies are being harnessed for cancer immunotherapies in the clinic.

Immune Checkpoint Inhibitors: Anti-NKG2A Antibodies on Board

Antibodies directed towards checkpoint inhibitors have unveiled extraordinary potential in cancer therapy. An article by the Vivier group (Cell 2018;175:1731-1743) shows that blocking the HLA-E-specific NKG2A inhibitory receptor, expressed by NK and inducible in T cells, results in benefits against poor prognosis tumors. Moreover, NKG2A and PD-1/PD-L1 mAb combinations unleash tumor-specific T cell proliferation and memory.