Stimulating natural killer cells to protect against cancer: recent developments

Current cancer immunotherapies have begun to target cell types involved in innate immunity, such as natural killer (NK) cells that recognize and kill tumor cells. Recent advances in the study of NK cell biology have generated interest in manipulating these cells to generate anti-tumor responses. A rise in the number of activated NK cells has been shown to prevent and treat cancer in many preclinical models and is a positive clinical factor in human tumors. This article will focus on recent research on the ability of IL-15 and Toll-like receptor ligands to stimulate NK cell activity against cancer. The potential of these therapies, both alone and in conjunction with traditional and other vaccine platforms, will be reviewed. The current status of these therapies in clinical trials will also be discussed. Targeting these cell types in the context of human cancers may be an essential factor in future cancer treatments.     

Cellular and molecular immunology of lung cancer: therapeutic implications

Although the incidence of lung cancer is declining, the prognosis remains poor. This is likely due to lack of early detection and only recent developments in selective cancer therapies. Key immune cells involved in the pathogenesis of lung cancer include CD4⁺ T lymphocytes, macrophages, dendritic cells and NK cells. The growing understanding of these cells indicates a highly complex and intertwined network of their involvement in each stage of lung cancer. Immune cell types and numbers affect prognosis and could offer an opportunity for clinical therapeutic applications. However, an incomplete understanding of immune cell involvement and the underlying processes in lung cancer still remain. Deeper investigation focusing on the role of the immune cells will further the understanding of lung carcinogenesis and develop novel therapeutic approaches for the treatment and management of patients with more specialized and selective lung cancer.     

Natural killer cell-mediated immunosurveillance of human cancer

The contribution of natural killer (NK) cells to immunosurveillance of human cancer remains debatable. Here, we discuss advances in several areas of human NK cell research, many of which support the ability of NK cells to prevent cancer development and avoid relapse following adoptive immunotherapy. We describe the molecular basis for NK cell recognition of human tumor cells and provide evidence for NK cell-mediated killing of human primary tumor cells ex vivo. Subsequently, we highlight studies demonstrating the ability of NK cells to migrate to, and reside in, the human tumor microenvironment where selection of tumor escape variants from NK cells can occur. Indirect evidence for NK cell immunosurveillance against human malignancies is provided by the reduced incidence of cancer in individuals with high levels of NK cell cytotoxicity, and the significant clinical responses observed following infusion of human NK cells into cancer patients. Finally, we describe studies showing enhanced tumor progression, or increased cancer incidence, in patients with inherited and acquired defects in cellular cytotoxicity. All these observations have in common that they, either indirectly or directly, suggest a role for NK cells in mediating immunosurveillance against human cancer. This opens up for exciting possibilities with respect to further exploring NK cells in settings of adoptive immunotherapy in human cancer.     

NK cell-based immunotherapy for malignant diseases

Natural killer (NK) cells play critical roles in host immunity against cancer. In response, cancers develop mechanisms to escape NK cell attack or induce defective NK cells. Current NK cell-based cancer immunotherapy aims to overcome NK cell paralysis using several approaches. One approach uses expanded allogeneic NK cells, which are not inhibited by self histocompatibility antigens like autologous NK cells, for adoptive cellular immunotherapy. Another adoptive transfer approach uses stable allogeneic NK cell lines, which is more practical for quality control and large-scale production. A third approach is genetic modification of fresh NK cells or NK cell lines to highly express cytokines, Fc receptors and/or chimeric tumor-antigen receptors. Therapeutic NK cells can be derived from various sources, including peripheral or cord blood cells, stem cells or even induced pluripotent stem cells (iPSCs), and a variety of stimulators can be used for large-scale production in laboratories or good manufacturing practice (GMP) facilities, including soluble growth factors, immobilized molecules or antibodies, and other cellular activators. A list of NK cell therapies to treat several types of cancer in clinical trials is reviewed here. Several different approaches to NK-based immunotherapy, such as tissue-specific NK cells, killer receptor-oriented NK cells and chemically treated NK cells, are discussed. A few new techniques or strategies to monitor NK cell therapy by non-invasive imaging, predetermine the efficiency of NK cell therapy by in vivo experiments and evaluate NK cell therapy approaches in clinical trials are also introduced.     

Ontogeny and Expansion of Human Natural Killer Cells: Clinical Implications

Our knowledge of NK cells and their critical role in the innate immune system has increased enormously since their discovery several decades ago. However, it is only within the last 10 years that rational cytokine therapies, such as those utilizing low doses of IL-2, have been successful in expanding NK cells in patients with cancer and/or immunodeficiency. Such experiences in vivo have highlighted the importance of basing immunotherapeutic strategies on the known cellular and molecular properties of the targeted cell population. Recent advances in our understanding of the physiologic factors and events that orchestrate NK cell ontogeny, including IL-15 and receptor tyrosine kinase ligands to c-kit and fit3, provide novel therapeutic possibilities for cytokine therapy. This review summarizes our current understanding of human NK cell ontogeny, and links this knowledge to ongoing and future clinical strategies for the endogenous expansion of NK cells in patients with cancer and/or immunodeficiency.     

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

Natural killer (NK) cells kill virus-infected and tumor target cells without prior sensitization. Each NK cell expresses a multitude of activating and inhibitory receptors, and the interplay of signals determines the outcome of NK cell activity. NK cell-mediated cytolysis of target cell involves polarized degranulation at effector–target interface. Peripheral blood NK cell constitutes about 10 % of lymphocytes, and approximately 90 % of peripheral blood NK cells are CD56^dimCD16⁺; however, there is a distinct subset of NK cells, CD56^brightCD16^?, expressed by certain lymphoid organs which are able to produce large amounts of cytokines including interferon-γ, tumor necrosis factor, and granulocyte–macrophage colony-stimulating factor, but the cytotoxicity is attained only on their prolonged activation. In this review, we discuss the accumulated data on distinct phenotypes of NK cells in human uterus, liver, intestine, skin, and lung and also attempt to correlate their phenotype with corresponding activity and functions, with significant stress on the role of NK cells in pathology in the specific organs. Our detailed understanding of altered NK cell activity in different organs and their inherent cytotoxic activity against tumor target cells will help us design better immunotherapeutic strategies in NK cell-mediated cancer therapies.     

Natural killer cells: diversity in search of a niche

Over the years, the enigmatic lifestyle of natural killer (NK) cells has fascinated immunologists. Elucidating the principles governing NK cell biology is a constant source of surprise, as these lymphocytes often break the rules of lymphocyte 'behavior' established by their T and B cell counterparts. In this Focus issue, experts address how NK cells have secured a fundamental place in the immune system, how they 'see' their targets and how they participate in and influence immune responses. We now need to understand how environmental 'niches' condition the generation and function of distinct NK cell subsets in vivo. Ultimately, we can hope that unraveling the mysteries of how these atypical lymphocytes function will have an impact on human disease therapies.     

Ontogeny and expansion of human natural killer cells: clinical implications

Our knowledge of NK cells and their critical role in the innate immune system has increased enormously since their discovery several decades ago. However, it is only within the last 10 years that rational cytokine therapies, such as those utilizing low doses of IL-2, have been successful in expanding NK cells in patients with cancer and/or immunodeficiency. Such experiences in vivo have highlighted the importance of basing immunotherapeutic strategies on the known cellular and molecular properties of the targeted cell population. Recent advances in our understanding of the physiologic factors and events that orchestrate NK cell ontogeny, including IL-15 and receptor tyrosine kinase ligands to c-kit and flt3, provide novel therapeutic possibilities for cytokine therapy. This review summarizes our current understanding of human NK cell ontogeny, and links this knowledge to ongoing and future clinical strategies for the endogenous expansion of NK cells in patients with cancer and/or immunodeficiency.     

Cellular therapy to treat haematological and other malignancies: progress and pitfalls

The recent Food and Drug Administration (FDA) approval of a cellular therapy to treat castration resistant prostate cancer has reinforced the potential of cellular therapy to consolidate current pharmacological approaches to treating cancer. The emergence of the cell manufacturing facility to facilitate clinical translation of these new methodologies allows greater access to these novel therapies. Here we review different strategies currently being explored to treat haematological malignancies with a focus on adoptive allogeneic or autologous transfer of antigen specific T cells, NK cells or dendritic cells. These approaches all aim to generate immunological responses against overexpressed tissue antigens, mismatched minor histocompatability antigens or tumour associated antigens. Current successes and limitations of these different approaches will be discussed with an emphasis on challenges encountered in generating long term engraftment, antigen selection and implementation as well as therapeutic immune monitoring of clinical responses, with examples from recent clinical trials.     

The identification of factors in seminal plasma responsible for suppression of natural killer cell activity.

We have identified two components of human seminal plasma which suppress natural killer (NK) cell activity in vitro. Dialysis and gel filtration experiments have shown both components to be of low molecular weight. The first will suppress NK cells following a short period of pretreatment, but this suppression is dependent upon the presence of bovine serum in the medium and is directly related to a loss of cell viability as measured by trypan blue dye exclusion. We suggest that this molecule is a polyamine. The second factor will not suppress NK activity following pretreatment of lymphocytes, but is a potent suppressor when added for the duration of the assay. This suppression is completely bovine serum independent, unrelated to toxicity and appears to be mediated by prostaglandin E2. The relevance of these results to a clinical situation is discussed.     

The role of natural killer cells in tumor control—effectors and regulators of adaptive immunity

Natural killer (NK) cells are the primary effector cells of the innate immune system and have a well-established role in tumor rejection in a variety of spontaneous and induced cancer models. NK cell function is regulated by a complex balance of inhibitory and activating signals that allow them to selectively target and kill cells that display an abnormal pattern of cell surface molecules, while leaving normal healthy cells unharmed. In this review we discuss NK cell function, the role of NK cells in cancer therapies, the emerging concept of bi-directional cross-talk between NK cells and dendritic cells, and the implications of these interactions for tumor immunotherapy.     

The role of natural killer cells in tumor control--effectors and regulators of adaptive immunity

Natural killer (NK) cells are the primary effector cells of the innate immune system and have a well-established role in tumor rejection in a variety of spontaneous and induced cancer models. NK cell function is regulated by a complex balance of inhibitory and activating signals that allow them to selectively target and kill cells that display an abnormal pattern of cell surface molecules, while leaving normal healthy cells unharmed. In this review we discuss NK cell function, the role of NK cells in cancer therapies, the emerging concept of bi-directional cross-talk between NK cells and dendritic cells, and the implications of these interactions for tumor immunotherapy.     

Role of human natural killer cells in health and disease.

Natural killer (NK) cells, the CD3- CD56+ CD16+ subset of peripheral blood lymphocytes, have long been known to be involved in non-major histocompatibility complex-restricted natural immunity to virally infected and malignant target cells. The association of abnormalities in NK cell numbers or functions with a broad spectrum of human diseases has been more clearly defined in recent years as a result of the improved knowledge of NK cell physiology and advances in monitoring of NK cell functions in health and disease. The ability to reliably measure changes in NK activity and/or numbers during the course of disease or response to treatment has focused attention on the role of the NK cell in disease pathogenesis. The improved understanding of NK cell deficiency in disease has opened a way for therapies specifically designed to improve NK cell function. The therapeutic use of biologic response modifiers capable of augmenting NK cell activity in vivo and of adoptive transfer of highly enriched, activated autologous NK cells in diseases such as cancer and AIDS is being evaluated. The importance of NK cells in health and the consequences of NK cell deficiency or excess are likely to be more extensively monitored in the future.     

Natural Killer Cells： Biology and Clinical Use in Cancer Therapy

Natural killer (NK) cells have the ability to mediate both bone marrow rejection and promote engraftment,aswell as the ability to elicit potent anti-tumor effects.However the clinical results for these processes are stillelusive.Greater understanding of NK cell biology,from activating and inhibitory receptor functions to the roleof NK cells in allogeneic transplantation,needs to be appreciated in order to draw out the clinical potential ofNK cells.Mechanisms of bone marrow cell (BMC) rejection are known to be dependant on inhibitory receptorsspecific for major histocompatibility complex (MHC) molecules and on activating receptors that have manypotential ligands.The modulation of activating and inhibitory receptors may hold the key to clinical successinvolving NK cells.Pre-clinical studies in mice have shown that different combinations of activating andinhibitory receptors on NK cells can reduce graft-versus-host disease (GVHD),promote engraftment,andprovide superior graft-versus-tumor (GVT) responses.Recent clinical data have shown that the use ofKIR-ligand incompatibility produces tremendous graft-versus-leukemia effect in patients with acute myeloidleukemia at high risk of relapse.This review will attempt to be a synthesis of current knowledge concerning NKcells,their involvement in BMT,and their use as an immunotherapy for cancer and other hematologicmalignancies.Cellular & Molecular Immunology.2004;1(1):12-21.     

NK cell MHC class I specific receptors (KIR): from biology to clinical intervention

The natural killer (NK) cell effector response towards infected cells or tumoural cells is guided by the integration of activating and inhibitory signals sensed by NK cell surface receptors. Major histocompatibility complex class I specific inhibitory receptors expressed by NK cells have two distinct roles: while allowing self tolerance, they are also needed for the acquisition of NK cell functional competence, a process termed education. In the context of allotransplantation, NK cell alloreactivity, arising from the expression on donor NK cells of inhibitory killer Ig-like receptors (KIRs) that do not recognize human leukocyte antigen from the patient, has shown clinical benefit for leukaemia patients. Based on these genetic studies, a blocking antibody directed against KIRs, as well as allogeneic NK cell infusions are now tested in clinical trials in various oncology indications. They offer promising immunotherapeutic approaches for the treatment of cancer patients.     

Mechanisms of neovascularization and resistance to anti-angiogenic therapies in glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most malignant brain tumor and highly resistant to intensive combination therapies. GBM is one of the most vascularized tumors and vascular endothelial growth factor (VEGF) produced by tumor cells is a major factor regulating angiogenesis. Successful results of preclinical studies of anti-angiogenic therapies using xenograft mouse models of human GBM cell lines encouraged clinical studies of anti-angiogenic drugs, such as bevacizumab (Avastin), an anti-VEGF antibody. However, these clinical studies have shown that most patients become resistant to anti-VEGF therapy after an initial response. Recent studies have revealed some resistance mechanisms against anti-VEGF therapies involved in several types of cancer. In this review, we address mechanisms of angiogenesis, including unique features in GBMs, and resistance to anti-VEGF therapies frequently observed in GBM. Enhanced invasiveness is one such resistance mechanism and recent works report the contribution of activated MET signaling induced by inhibition of VEGF signaling. On the other hand, tumor cell-originated neovascularization including tumor-derived endothelial cell-induced angiogenesis and vasculogenic mimicry has been suggested to be involved in the resistance to anti-VEGF therapy. Therefore, these mechanisms should be targeted in addition to anti-angiogenic therapies to achieve better results for patients with GBM.     

Role of natural killer cell function in dendritic cell-based vaccines

Recent studies have elucidated the functional links between natural killer (NK) cells and, demonstrating the reciprocal activation of these cell types through NK-DC interactions. The subsets of cells and molecular pathways involved in such interactions have been defined, and the possible anatomical sites of these interactions have also been reported. Murine experiments have demonstrated that injection of mature DCs induces rapid recruitment of NK cells to lymph nodes and that these NK cells provide interferon-gamma for Type 1 priming. Thus, there is an increasing body of in vivo evidence indicating that NK-DC interactions during the early phase of innate immunity can impact the quality and magnitude of the subsequent adaptive immune response. Importantly, these studies imply that NK cells might not serve merely as cytotoxic lymphocytes combating viral pathogens and malignant tumors, but must also be considered as important immunoregulatory cells with a significant influence on adaptive immunity. In contrast to the large volume of knowledge obtained through basic research, there is a relative paucity of information regarding NK cell function in adaptive immunity from clinical trials, as few DC vaccine studies have attempted to evaluate the nonspecific, yet potentially clinically relevant, NK response to immunization. In this article, the authors will review studies focusing on NK-DC interactions and highlight the most recent clinical findings relating to the potential role of NK cells in DC-based vaccine therapy.     

HLA class I downregulation is associated with enhanced NK‐cell killing of melanoma cells with acquired drug resistance to BRAF inhibitors

The frequent development of drug resistance to targeted therapies in cancer patients has stimulated interest in strategies counteracting resistance. Combining immunotherapies with targeted therapies is one such strategy. In this context, we asked whether human NK cells can target melanoma cells that have acquired resistance to selective inhibitors targeting activating mutants of the B‐Raf kinase (BRAF inhibitors, BRAFi). We generated drug‐resistant cell variants in vitro from human BRAF‐mutant melanoma cell lines MEL‐HO, COLO‐38, SK‐MEL‐37, 1520 and from primary melanoma cells freshly isolated from two patients. All drug‐resistant cell variants remained susceptible to lysis by IL‐2‐activated NK cells; and two BRAFi‐resistant lines (BRAFi‐R) became significantly more susceptible to NK‐cell lysis than their parental lines. This was associated with significant HLA class I antigen downregulation and PD‐L1 upregulation on the drug‐resistant lines. Although blocking HLA class I enhanced the extent of lysis of both BRAFi‐R and parental cells to NK‐cell‐mediated lysis, antibody‐mediated inhibition of PD1–PD‐L1 interactions had no detectable effect. HLA class I antigen expression on BRAFi‐R melanoma variants thus appears to play a major role in their susceptibility to NK‐cell cytotoxicity. These findings suggest that NK‐cell‐based immunotherapy may be a viable approach to treat melanoma patients with acquired resistance to BRAF inhibitors.