Retargeting NK-92 for anti-melanoma activity by a TCR-like single-domain antibody

Enhanced non-viral gene delivery via calcium phosphate/DNA co-precipitates with low-voltage pulse electroporation in NK-92 cells for immunocellular therapy

Achieving high cell transfection efficiency is essential for various cell types in numerous disease applications. However, the efficient introduction of genes into natural killer (NK) cells remains a challenge. In this study, we proposed a design strategy for delivering exogenous genes into the NK cell line, NK-92, using a modified non-viral gene transfection method. Calcium phosphate/DNA nanoparticles (pDNA-CaP NPs) were prepared using co-precipitation methods and combined with low-voltage pulse electroporation to facilitate NK-92 transfection. The results demonstrated that the developed pDNA-CaP NPs exhibited a uniform diameter of approximately 393.9 nm, a DNA entrapment efficiency of 65.8%, and a loading capacity of 15.9%. Furthermore, at three days post-transfection, both the transfection efficiency and cell viability of NK-92 were significantly improved compared to standalone plasmid DNA (pDNA) electroporation or solely relying on the endocytosis pathway of pDNA-CaP NPs. This study provides valuable insights into a novel approach that combines calcium phosphate nanoparticles with low-voltage electroporation for gene delivery into NK-92 cells, offering potential advancements in cell therapy.

CAR NK Cell Therapy for the Treatment of Metastatic Melanoma: Potential & Prospects

Melanoma is among the most lethal forms of cancer, accounting for 80% of deaths despite comprising just 5% of skin cancer cases. Treatment options remain limited due to the genetic and epigenetic mechanisms associated with melanoma heterogeneity that underlie the rapid development of secondary drug resistance. For this reason, the development of novel treatments remains paramount to the improvement of patient outcomes. Although the advent of chimeric antigen receptor-expressing T (CAR-T) cell immunotherapies has led to many clinical successes for hematological malignancies, these treatments are limited in their utility by their immune-induced side effects and a high risk of systemic toxicities. CAR natural killer (CAR-NK) cell immunotherapies are a particularly promising alternative to CAR-T cell immunotherapies, as they offer a more favorable safety profile and have the capacity for fine-tuned cytotoxic activity. In this review, the discussion of the prospects and potential of CAR-NK cell immunotherapies touches upon the clinical contexts of melanoma, the immunobiology of NK cells, the immunosuppressive barriers preventing endogenous immune cells from eliminating tumors, and the structure and design of chimeric antigen receptors, then finishes with a series of proposed design innovations that could improve the efficacy CAR-NK cell immunotherapies in future studies.

Next-generation chimeric antigen receptors for T- and natural killer-cell therapies against cancer

Adoptive cellular therapy using chimeric antigen receptor (CAR) T cells has led to a paradigm shift in the treatment of various hematologic malignancies. However, the broad application of this approach for myeloid malignancies and solid cancers has been limited by the paucity and heterogeneity of target antigen expression, and lack of bona fide tumor-specific antigens that can be targeted without cross-reactivity against normal tissues. This may lead to unwanted on-target off-tumor toxicities that could undermine the desired antitumor effect. Recent advances in synthetic biology and genetic engineering have enabled reprogramming of immune effector cells to enhance their selectivity toward tumors, thus mitigating on-target off-tumor adverse effects. In this review, we outline the current strategies being explored to improve CAR selectivity toward tumor cells with a focus on natural killer (NK) cells, and the progress made in translating these strategies to the clinic.

Nanobodies in cell-mediated immunotherapy: On the road to fight cancer

The immune system is essential in recognizing and eliminating tumor cells. The unique characteristics of the tumor microenvironment (TME), such as heterogeneity, reduced blood flow, hypoxia, and acidity, can reduce the efficacy of cell-mediated immunity. The primary goal of cancer immunotherapy is to modify the immune cells or the TME to enable the immune system to eliminate malignancies successfully. Nanobodies, known as single-domain antibodies, are light chain-free antibody fragments produced from Camelidae antibodies. The unique properties of nanobodies, including high stability, reduced immunogenicity, enhanced infiltration into the TME of solid tumors and facile genetic engineering have led to their promising application in cell-mediated immunotherapy. They can promote the cancer therapy either directly by bridging between tumor cells and immune cells and by targeting cancer cells using immune cell-bound nanobodies or indirectly by blocking the inhibitory ligands/receptors. The T-cell activation can be engaged through anti-CD3 and anti-4-1BB nanobodies in the bispecific (bispecific T-cell engagers (BiTEs)) and trispecific (trispecific T-cell engager (TriTEs)) manners. Also, nanobodies can be used as natural killer (NK) cell engagers (BiKEs, TriKEs, and TetraKEs) to create an immune synapse between the tumor and NK cells. Nanobodies can redirect immune cells to attack tumor cells through a chimeric antigen receptor (CAR) incorporating a nanobody against the target antigen. Various cancer antigens have been targeted by nanobody-based CAR-T and CAR-NK cells for treating both hematological and solid malignancies. They can also cause the continuation of immune surveillance against tumor cells by stopping inappropriate inhibition of immune checkpoints. Other roles of nanobodies in cell-mediated cancer immunotherapy include reprogramming macrophages to reduce metastasis and angiogenesis, as well as preventing the severe side effects occurring in cell-mediated immunotherapy. Here, we highlight the critical functions of various immune cells, including T cells, NK cells, and macrophages in the TME, and discuss newly developed immunotherapy methods based on the targeted manipulation of immune cells and TME with nanobodies.

CAR-NK as a Rapidly Developed and Efficient Immunotherapeutic Strategy against Cancer

Chimeric antigen receptor (CAR)-modified T cell therapy has been rapidly developing in recent years, ultimately revolutionizing immunotherapeutic strategies and providing significant anti-tumor potency, mainly in treating hematological neoplasms. However, graft-versus-host disease (GVHD) and other adverse effects, such as cytokine release syndromes (CRS) and neurotoxicity associated with CAR-T cell infusion, have raised some concerns about the broad application of this therapy. Natural killer (NK) cells have been identified as promising alternative platforms for CAR-based therapies because of their unique features, such as a lack of human leukocyte antigen (HLA)-matching restriction, superior safety, and better anti-tumor activity when compared with CAR-T cells. The lack of CRS, neurotoxicity, or GVHD, in the case of CAR-NK therapy, in addition to the possibility of using allogeneic NK cells as a CAR platform for "off-the-shelf" therapy, opens new windows for strategic opportunities. This review underlines recent design achievements in CAR constructs and summarizes preclinical studies' results regarding CAR-NK therapies' safety and anti-tumor potency. Additionally, new approaches in CAR-NK technology are briefly described, and currently registered clinical trials are listed.

Chimeric antigen receptor-engineered NK cells: new weapons of cancer immunotherapy with great potential

Chimeric antigen receptor (CAR)-engineered T (CAR-T) cells have obtained prominent achievement in the clinical immunotherapy of hematological malignant tumors, leading to a rapid development of cellular immunotherapy in cancer treatment. Scientists are also aware of the prospective advantages of CAR engineering in cellular immunotherapy. Due to various limitations such as the serious side effects of CAR-T therapy, researchers began to investigate other immune cells for CAR modification. Natural killer (NK) cells are critical innate immune cells with the characteristic of non-specifically recognizing target cells and with the potential to become "off-the-shelf" products. In recent years, many preclinical studies on CAR-engineered NK (CAR-NK) cells have shown their remarkable efficacy in cancer therapy and their superiority over autologous CAR-T cells. In this review, we summarize the generation, mechanisms of anti-tumor activity and unique advantages of CAR-NK cells, and then analyze some challenges and recent clinical trials about CAR-NK cells therapy. We believe that CAR-NK therapy is a promising prospect for cancer immunotherapy in the future.

Assessment of T Cell Receptor Complex Expression Kinetics in Natural Killer Cells

Among the polypeptides that comprise the T cell receptor (TCR), only CD3ζ is found in Natural Killer (NK) cells, where it transmits signals from activating receptors such as CD16 and NKp46. NK cells are potent immune cells that recognize target cells through germline-encoded activating and inhibitory receptors. Genetic engineering of NK cells enables tumor-specific antigen recognition and, thus, has a significant promise in adoptive cell therapy. Ectopic expression of engineered TCR components in T cells leads to mispairing with the endogenous components, making a knockout of the endogenous TCR necessary. To circumvent the mispairing of TCRs or the need for knockout technologies, TCR complex expression has been studied in NK cells. In the current study, we explored the cellular processing of the TCR complex in NK cells. We observed that in the absence of CD3 subunits, the TCR was not expressed on the surface of NK cells and vice versa. Moreover, a progressive increase in surface expression of TCR between day three and day seven was observed after transduction. Interestingly, the TCR complex expression in NK92 cells was enhanced with a proteasome inhibitor (bortezomib) but not a lysosomal inhibitor (chloroquine). Additionally, we observed that the TCR complex was functional in NK92 cells as measured by estimating CD107a as a degranulation marker, IFNγ cytokine production, and killing assays. NK92 cells strongly degranulated when CD3ε was engaged in the presence of TCR, but not when only CD3 was overexpressed. Therefore, our findings encourage further investigation to unravel the mechanisms that prevent the surface expression of the TCR complex.

A Single-Domain TCR-like Antibody Selective for the Qa-1b/Qdm Peptide Complex Enhances Tumoricidal Activity of NK Cells via Blocking the NKG2A Immune Checkpoint

The NKG2A/HLA-E axis is an immune checkpoint that suppresses immune effector activity in the tumor microenvironment. In mice, the ligand for the NKG2A/CD94 inhibitory receptor is the nonclassical MHC molecule Qa-1^b, the HLA-E ortholog, which presents the peptide AMAPRTLLL, referred to as Qdm (for Qa-1 determinant modifier). This dominant peptide is derived from the leader sequences of murine classical MHC class I encoded by the H-2D and -L loci. To broaden our understanding of Qa-1^b/Qdm peptide complex biology and its tumor protective role, we identified a TCR-like Ab from a single domain VHH library using yeast surface display. The TCR-like Ab (EXX-1) binds only to the Qa-1^b/Qdm peptide complex and not to Qa-1^b alone or Qa-1^b loaded with control peptides. Conversely, currently available Abs to Qa-1^b bind independent of peptide loaded. Flow cytometric results revealed that EXX-1 selectively bound to Qa-1^b/Qdm-positive B16F10, RMA, and TC-1 mouse tumor cells but only after pretreatment with IFN-γ; no binding was observed following genetic knockdown of Qa-1^b or Qdm peptide. Furthermore, EXX-1 Ab blockade promoted NK cell-mediated tumor cell lysis in vitro. Our findings show that EXX-1 has exquisite binding specificity for the Qa-1^b/Qdm peptide complex, making it a valuable research tool for further investigation of the Qa-1^b/Qdm peptide complex expression and regulation in healthy and diseased cells and for evaluation as an immune checkpoint blocking Ab in syngeneic mouse tumor models.

Reprogramming the tumor microenvironment by genome editing for precision cancer therapy

The tumor microenvironment (TME) is essential for immune escape by tumor cells. It plays essential roles in tumor development and metastasis. The clinical outcomes of tumors are often closely related to individual differences in the patient TME. Therefore, reprogramming TME cells and their intercellular communication is an attractive and promising strategy for cancer therapy. TME cells consist of immune and nonimmune cells. These cells need to be manipulated precisely and safely to improve cancer therapy. Furthermore, it is encouraging that this field has rapidly developed in recent years with the advent and development of gene editing technologies. In this review, we briefly introduce gene editing technologies and systematically summarize their applications in the TME for precision cancer therapy, including the reprogramming of TME cells and their intercellular communication. TME cell reprogramming can regulate cell differentiation, proliferation, and function. Moreover, reprogramming the intercellular communication of TME cells can optimize immune infiltration and the specific recognition of tumor cells by immune cells. Thus, gene editing will pave the way for further breakthroughs in precision cancer therapy.

CAR-NK cells for cancer immunotherapy: from bench to bedside

Natural killer (NK) cells are unique innate immune cells and manifest rapid and potent cytotoxicity for cancer immunotherapy and pathogen removal without the requirement of prior sensitization or recognition of peptide antigens. Distinguish from the T lymphocyte-based cythotherapy with toxic side effects, chimeric antigen receptor-transduced NK (CAR-NK) cells are adequate to simultaneously improve efficacy and control adverse effects including acute cytokine release syndrome (CRS), neurotoxicity and graft-versus-host disease (GVHD). Moreover, considering the inherent properties of NK cells, the CAR-NK cells are “off-the-shelf” product satisfying the clinical demand for large-scale manufacture for cancer immunotherapy attribute to the cytotoxic effect via both NK cell receptor-dependent and CAR-dependent signaling cascades. In this review, we mainly focus on the latest updates of CAR-NK cell-based tactics, together with the opportunities and challenges for cancer immunotherapies, which represent the paradigm for boosting the immune system to enhance antitumor responses and ultimately eliminate malignancies. Collectively, we summarize and highlight the auspicious improvement in CAR-NK cells and will benefit the large-scale preclinical and clinical investigations in adoptive immunotherapy.

CAR-NK Cells: From Natural Basis to Design for Kill

Chimeric antigen receptors (CARs) are fusion proteins with an extracellular antigen recognition domain and numerous intracellular signaling domains that have been genetically modified. CAR-engineered T lymphocyte-based therapies have shown great success against blood cancers; however, potential fatal toxicity, such as in cytokine release syndrome, and high costs are some shortcomings that limit the clinical application of CAR-engineered T lymphocytes and remain to overcome. Natural killer (NK) cells are the focal point of current immunological research owing to their receptors that prove to be promising immunotherapeutic candidates for treating cancer. However, to date, manipulation of NK cells to treat malignancies has been moderately successful. Recent progress in the biology of NK cell receptors has greatly transformed our understanding of how NK cells recognize and kill tumor and infected cells. CAR-NK cells may serve as an alternative candidate for retargeting cancer because of their unique recognition mechanisms, powerful cytotoxic effects especially on cancer cells in both CAR-dependent and CAR-independent manners and clinical safety. Moreover, NK cells can serve as an ‘off-the-shelf product’ because NK cells from allogeneic sources can also be used in immunotherapies owing to their reduced risk of alloreactivity. Although ongoing fundamental research is in the beginning stages, this review provides an overview of recent developments implemented to design CAR constructs to stimulate NK activation and manipulate NK receptors for improving the efficiency of immunotherapy against cancer, summarizes the preclinical and clinical advances of CAR-NK cells against both hematological malignancies and solid tumors and confronts current challenges and obstacles of their applications. In addition, this review provides insights into prospective novel approaches that further enhance the efficiency of CAR-NK therapies and highlights potential questions that require to be addressed in the future.

Importance of T, NK, CAR T and CAR NK Cell Metabolic Fitness for Effective Anti-Cancer Therapy: A Continuous Learning Process Allowing the Optimization of T, NK and CAR-Based Anti-Cancer Therapies

Simple Summary

Cancer treatments are evolving at a very rapid pace. Some of the most novel anti-cancer medicines under development rely on the modification of immune cells in order to transform them into potent tumor-killing cells. However, the tumor microenvironment (TME) is competing for nutrients with these harnessed immune cells and therefore paralyzes their metabolic effective and active anti-cancer activities. Here we describe strategies to overcome these hurdles imposed on immune cell activity, which lead to therapeutic approaches to enhance metabolic fitness of the patient’s immune system with the objective to improve their anti-cancer capacity.

Abstract

Chimeric antigen receptor (CAR) T and CAR NK cell therapies opened new avenues for cancer treatment. Although original successes of CAR T and CAR NK cells for the treatment of hematological malignancies were extraordinary, several obstacles have since been revealed, in particular their use for the treatment of solid cancers. The tumor microenvironment (TME) is competing for nutrients with T and NK cells and their CAR-expressing counterparts, paralyzing their metabolic effective and active states. Consequently, this can lead to alterations in their anti-tumoral capacity and persistence in vivo. High glucose uptake and the depletion of key amino acids by the TME can deprive T and NK cells of energy and building blocks, which turns them into a state of anergy, where they are unable to exert cytotoxic activity against cancer cells. This is especially true in the context of an immune-suppressive TME. In order to re-invigorate the T, NK, CAR T and CAR NK cell-mediated antitumor response, the field is now attempting to understand how metabolic pathways might change T and NK responses and functions, as well as those from their CAR-expressing partners. This revealed ways to metabolically rewire these cells by using metabolic enhancers or optimizing pre-infusion in vitro cultures of these cells. Importantly, next-generation CAR T and CAR NK products might include in the future the necessary metabolic requirements by improving their design, manufacturing process and other parameters. This will allow the overcoming of current limitations due to their interaction with the suppressive TME. In a clinical setting, this might improve their anti-cancer effector activity in synergy with immunotherapies. In this review, we discuss how the tumor cells and TME interfere with T and NK cell metabolic requirements. This may potentially lead to therapeutic approaches that enhance the metabolic fitness of CAR T and CAR NK cells, with the objective to improve their anti-cancer capacity.

The emerging role of off-the-shelf engineered natural killer cells in targeted cancer immunotherapy

Natural killer (NK) cells are innate lymphocytes that recognize and clear infected and transformed cells. The importance of NK cells in tumor surveillance underlies the development of NK cell therapy as cancer treatment. The NK-92 cell line has been successfully modified to express high-affinity CD16 receptor for antibody-dependent cellular cytotoxicity and/or chimeric antigen receptors (CARs) that can recognize antigens expressed on tumor cells and mediate NK cell activation. Since there is no need for human leukocyte antigen matching or prior exposure to the tumor antigens, NK-92 provides an opportunity for the development of next-generation off-the-shelf cell therapy platforms. CAR-engineered NK-92 cells have demonstrated robust antitumor activity in in vitro and in vivo preclinical studies, propelling the clinical development of CAR NK-92 cells. Preliminary phase 1 data indicate that CAR NK-92 can be safely administered in the clinic. In this review, we provide an overview of recent advances in the research and clinical application of this novel cell immunotherapy.

CAR-NK Cells for Cancer Therapy: Molecular Redesign of the Innate Antineoplastic Response

The Chimeric Antigen Receptor (CAR) has arisen as a powerful synthetic biology-based technology with demonstrated versatility for implementation in T and NK cells. Despite CAR T cell successes in clinical trials, several challenges remain to be addressed regarding adverse events and long-term efficacy. NK cells present an attractive alternative with intrinsic advantages over T cells for treating solid and liquid tumors. Early preclinical and clinical trials suggest at least two major advantages: improved safety and an off-the-shelf application in patients due to its HLA independence. Due to the early stages of CAR NK translation to clinical trials, limited data is currently available. By analyzing these results, it seems that CAR NK cells could offer a reduced probability of Cytokine Release Syndrome (CRS) or Graft versus Host Disease (GvHD) in cancer patients, reducing safety concerns. Furthermore, NK cell therapy approaches may be boosted by combining it with immunological checkpoint inhibitors and by implementing genetic circuits to direct CAR-bearing cell behavior. This review provides a description of the CAR technology for modifying NK cells and the translation from preclinical studies to early clinical trials in this new field of immunotherapy.

NK Cells Armed with Chimeric Antigen Receptors (CAR): Roadblocks to Successful Development

In recent years, cell-based immunotherapies have demonstrated promising results in the treatment of cancer. Chimeric antigen receptors (CARs) arm effector cells with a weapon for targeting tumor antigens, licensing engineered cells to recognize and kill cancer cells. The quality of the CAR-antigen interaction strongly depends on the selected tumor antigen and its expression density on cancer cells. CD19 CAR-engineered T cells approved by the Food and Drug Administration have been most frequently applied in the treatment of hematological malignancies. Clinical challenges in their application primarily include cytokine release syndrome, neurological symptoms, severe inflammatory responses, and/or other off-target effects most likely mediated by cytotoxic T cells. As a consequence, there remains a significant medical need for more potent technology platforms leveraging cell-based approaches with enhanced safety profiles. A promising population that has been advanced is the natural killer (NK) cell, which can also be engineered with CARs. NK cells which belong to the innate arm of the immune system recognize and kill virally infected cells as well as (stressed) cancer cells in a major histocompatibility complex I independent manner. NK cells play an important role in the host’s immune defense against cancer due to their specialized lytic mechanisms which include death receptor (i.e., Fas)/death receptor ligand (i.e., Fas ligand) and granzyme B/perforin-mediated apoptosis, and antibody-dependent cellular cytotoxicity, as well as their immunoregulatory potential via cytokine/chemokine release. To develop and implement a highly effective CAR NK cell-based therapy with low side effects, the following three principles which are specifically addressed in this review have to be considered: unique target selection, well-designed CAR, and optimized gene delivery.

Preclinical studies of chimeric antigen receptor-modified natural killer cells in cancer immunotherapy: a review

INTRODUCTION

As one of the most efficacious methods of cancer immunotherapy, chimeric antigen receptor-modified immune cells have recently drawn enormous attention. After the great success achieved with CAR-T-cells in cancer treatment both in preclinical setting and in the clinic, other types of immune cells, including natural killer (NK)-cells and macrophages, have been evaluated for their anti-cancer effects along with their potential superiority against CAR-T-cells, especially in terms of safety. First introduced by Tran et al. almost 26 years ago, CAR-NK-cells are now being considered as efficient immunotherapeutic modalities in various types of cancers, not only in preclinical setting but also in numerous phase I and II clinical studies.

AREAS COVERED

In this review, we aim to provide a comprehensive survey of the preclinical studies on CAR-NK-cells' development, with an evolutional approach on CAR structures and their associated signaling moieties. Current NK-cell sources and modes of gene transfer are also reviewed.

EXPERT OPINION

CAR-NK-cells have appeared as safe and effective immunotherapeutic tools in preclinical settings; however, designing CAR structures with an eye on their specific biology, along with choosing the optimal cell source and gene transfer method require further investigation to support clinical studies.

Adoptive NK Cell Therapy: A Promising Treatment Prospect for Metastatic Melanoma

Simple Summary

The incidence of metastatic melanoma has been increasing over the past years with current therapies showing limited efficacy to cure the disease. Therefore, other options are being investigated, such as adoptive cell therapy (ACT) where activated immune cells are infused into a patient to attack melanoma. Natural killer (NK) cells are part of the innate immune system and extremely suitable for this kind of therapy since they show minimal toxicities in the clinical setting. In this review, we focus on current strategies for NK cell therapy and the development of new approaches that hold great promise for the treatment of advanced melanoma.

Abstract

Adoptive cell therapy (ACT) represents a promising alternative approach for patients with treatment-resistant metastatic melanoma. Lately, tumor infiltrating lymphocyte (TIL) therapy and chimeric antigen receptor (CAR)-T cell therapy have shown improved clinical outcome, compared to conventional chemotherapy or immunotherapy. Nevertheless, they are limited by immune escape of the tumor, cytokine release syndrome, and manufacturing challenges of autologous therapies. Conversely, the clinical use of Natural Killer (NK) cells has demonstrated a favorable clinical safety profile with minimal toxicities, providing an encouraging treatment alternative. Unlike T cells, NK cells are activated, amongst other mechanisms, by the downregulation of HLA class I molecules, thereby overcoming the hurdle of tumor immune escape. However, impairment of NK cell function has been observed in melanoma patients, resulting in deteriorated natural defense. To overcome this limitation, “activated” autologous or allogeneic NK cells have been infused into melanoma patients in early clinical trials, showing encouraging clinical benefit. Furthermore, as several NK cell-based therapeutics are being developed for different cancers, an emerging variety of approaches to increase migration and infiltration of adoptively transferred NK cells towards solid tumors is under preclinical investigation. These developments point to adoptive NK cell therapy as a highly promising treatment for metastatic melanoma in the future.

Chimeric Antigen Receptor-Engineered Natural Killer (CAR NK) Cells in Cancer Treatment; Recent Advances and Future Prospects

Natural Killer (NK) cells are critical members of the innate immunity lymphocytes and have a critical role in host defense against malignant cells. Adoptive cell therapy (ACT) using chimeric antigen receptor (CAR) redirects the specificity of the immune cell against a target-specific antigen. ACT has recently created an outstanding opportunity for cancer treatment. Unlike CAR-armored T cells which hadnsome shortcomings as the CAR-receiving construct, Major histocompatibility complex (MHC)-independency, shorter lifespan, the potential to produce an off-the-shelf immune product, and potent anti-tumor properties of the NK cells has introduced NK cells as a potent alternative target for expression of CAR. Here, we aim to provide an updated overview on the current improvements in CAR NK design and immunobiology and describe the potential of CAR-modified NK cells as an alternative “off-the-shelf” carrier of CAR. We also provide lists for the sources of NK cells in the process of CAR NK cell production, different methods for transduction of the CAR genetic sequence to NK cells, the differences between CAR T and CAR NK, and CAR NK-targeted tumor antigens in current studies. Additionally, we provide data on recently published preclinical and clinical studies of CAR NK therapy and a list of finished and ongoing clinical trials. For achieving CAR NK products with higher efficacy and safety, we discuss current challenges in transduction and expansion of CAR NK cells, CAR NK therapy side effects, and challenges that limit the optimal efficacy of CAR NK cells and recommend possible solutions to enhance the persistence, function, safety, and efficacy of CAR NK cells with a special focus on solid tumors.

Materials for Improving Immune Cell Transfection

Chimeric antigen receptor T cell (CAR-T) therapy holds great promise for preventing and treating deadly diseases such as cancer. However, it remains challenging to transfect and engineer primary immune cells for clinical cell manufacturing. Conventional tools using viral vectors and bulk electroporation suffer from low efficiency while posing risks like viral transgene integration and excessive biological perturbations. Emerging techniques using microfluidics, nanoparticles, and high-aspect-ratio nanostructures can overcome these challenges, and on top of that, provide universal and high-throughput cargo delivery. Herein, the strengths and limitations of traditional and emerging materials for immune cell transfection, and commercial development of these tools, are discussed. To enhance the characterization of transfection techniques and uptake by the clinical community, a list of in vitro and in vivo assays to perform, along with relevant protocols, is recommended. The overall aim, herein, is to motivate the development of novel materials to meet rising demand in transfection for clinical CAR-T cell manufacturing.

Renaissance of armored immune effector cells, CAR-NK cells, brings the higher hope for successful cancer therapy

In recent decades, a new method of cellular immunotherapy was introduced based on engineering and empowering the immune effector cells. In this type of immunotherapy, the immune effector cells are equipped with chimeric antigen receptor (CAR) to specifically target cancer cells. In much of the trials and experiments, CAR-modified T cell immunotherapy has achieved very promising therapeutic results in the treatment of some types of cancers and infectious diseases. However, there are also some considerable drawbacks in the clinical application of CAR-T cells although much effort is in progress to rectify the issues. In some conditions, CAR-T cells initiate over-activated and strong immune responses, therefore, causing unexpected side-effects such as systemic cytokine toxicity (i.e., cytokine release syndrome), neurotoxicity, on-target, off-tumor toxicity, and graft-versus-host disease (GvHD). To overcome these limitations in CAR-T cell immunotherapy, NK cells as an alternative source of immune effector cells have been utilized for CAR-engineering. Natural killer cells are key players of the innate immune system that can destroy virus-infected cells, tumor cells, or other aberrant cells with their efficient recognizing capability. Compared to T cells, CAR-transduced NK cells (CAR-NK) have several advantages, such as safety in clinical use, non-MHC-restricted recognition of tumor cells, and renewable and easy cell sources for their preparation. In this review, we will discuss the recent preclinical and clinical studies, different sources of NK cells, transduction methods, possible limitations and challenges, and clinical considerations.

Chimeric antigen receptor-engineered natural killer cells for cancer immunotherapy

Natural killer (NK) cells are a critical component of the innate immune system. Chimeric antigen receptors (CARs) re-direct NK cells toward tumor cells carrying corresponding antigens, creating major opportunities in the fight against cancer. CAR NK cells have the potential for use as universal CAR cells without the need for human leukocyte antigen matching or prior exposure to tumor-associated antigens. Exciting data from recent clinical trials have renewed interest in the field of cancer immunotherapy due to the potential of CAR NK cells in the production of "off-the-shelf" anti-cancer immunotherapeutic products. Here, we provide an up-to-date comprehensive overview of the recent advancements in key areas of CAR NK cell research and identify under-investigated research areas. We summarize improvements in CAR design and structure, advantages and disadvantages of using CAR NK cells as an alternative to CAR T cell therapy, and list sources to obtain NK cells. In addition, we provide a list of tumor-associated antigens targeted by CAR NK cells and detail challenges in expanding and transducing NK cells for CAR production. We additionally discuss barriers to effective treatment and suggest solutions to improve CAR NK cell function, proliferation, persistence, therapeutic effectiveness, and safety in solid and liquid tumors.

NK Cell Adoptive Immunotherapy of Cancer: Evaluating Recognition Strategies and Overcoming Limitations

Natural killer (NK) cells, the primary effector cells of the innate immune system, utilize multiple strategies to recognize tumor cells by (1) detecting the presence of activating receptor ligands, which are often upregulated in cancer; (2) targeting cells that have a loss of major histocompatibility complex (MHC); and (3) binding to antibodies that bind to tumor-specific antigens on the tumor cell surface. All these strategies have been successfully harnessed in adoptive NK cell immunotherapies targeting cancer. In this review, we review the applications of NK cell therapies across different tumor types. Similar to other forms of immunotherapy, tumor-induced immune escape and immune suppression can limit NK cell therapies' efficacy. Therefore, we also discuss how these limitations can be overcome by conferring NK cells with the ability to redirect their tumor-targeting capabilities and survive the immune-suppressive tumor microenvironment. Finally, we also discuss how future iterations can benefit from combination therapies with other immunotherapeutic agents.

Mechanisms of NK cell dysfunction in the tumor microenvironment and current clinical approaches to harness NK cell potential for immunotherapy

NK cells are innate immune cells with inherent capabilities in both recognizing and killing cancer cells. NK cell phenotypes and functional alterations are being described with increasing precision among patients harboring various cancer types, emphasizing the critical role that NK cells play in antitumor immune responses. In addition, advances in understanding NK cell biology have improved our knowledge of such alterations, thereby expanding the potential exploitation of NK cells' anticancer capabilities. In this review, we present an overview of (1) the various types of NK cell alterations that may contribute to immune evasion in cancer patients and (2) the various strategies to improve NK cell-based anticancer immunotherapies, including pharmacologic modulation and/or genetic modification.

The Landscape of CAR-T Cell Clinical Trials against Solid Tumors-A Comprehensive Overview

Simple Summary

Certain immune cells, namely T cells, of cancer patients can be genetically manipulated to express so-called chimeric antigen receptors (CARs), which enables these cells to kill the tumor cells after recognition by the receptor. This therapy is very successful in the treatment of hematologic tumors such as lymphoma or leukemia. However, tumors growing as a solid mass are less susceptible to this kind of treatment. This review summarizes known data of all clinical trials using this therapy against solid tumors that are registered at clinicaltrials.gov.

Abstract

CAR-T cells showed great potential in the treatment of patients with hematologic tumors. However, the clinical efficacy of CAR-T cells against solid tumors lags behind. To obtain a comprehensive overview of the landscape of CAR-T cell clinical trials against this type of cancer, this review summarizes all the 196 studies registered at clinicaltrials.gov. Special focus is on: (1) geographical distribution; (2) targeted organs, tumor entities, and antigens; (3) CAR transfer methods, CAR formats, and extra features introduced into the T cells; and (4) patient pretreatments, injection sites, and safety measurements. Finally, the few data on clinical outcome are reported. The last assessment of clinicaltrials.gov for the data summarized in this paper was on 4 August 2020.

Boosting Natural Killer Cell-Mediated Targeting of Sarcoma Through DNAM-1 and NKG2D

Sarcomas are malignancies of mesenchymal origin that occur in bone and soft tissues. Many are chemo- and radiotherapy resistant, thus conventional treatments fail to increase overall survival. Natural Killer (NK) cells exert anti-tumor activity upon detection of a complex array of tumor ligands, but this has not been thoroughly explored in the context of sarcoma immunotherapy. In this study, we investigated the NK cell receptor/ligand immune profile of primary human sarcoma explants. Analysis of tumors from 32 sarcoma patients identified the proliferative marker PCNA and DNAM-1 ligands CD112 and/or CD155 as commonly expressed antigens that could be efficiently targeted by genetically modified (GM) NK cells. Despite the strong expression of CD112 and CD155 on sarcoma cells, characterization of freshly dissociated sarcomas revealed a general decrease in tumor-infiltrating NK cells compared to the periphery, suggesting a defect in the endogenous NK cell response. We also applied a functional screening approach to identify relevant NK cell receptor/ligand interactions that induce efficient anti-tumor responses using a panel NK-92 cell lines GM to over-express 12 different activating receptors. Using GM NK-92 cells against primary sarcoma explants (n = 12) revealed that DNAM-1 over-expression on NK-92 cells led to efficient degranulation against all tested explants (n = 12). Additionally, NKG2D over-expression showed enhanced responses against 10 out of 12 explants. These results show that DNAM-1⁺ or NKG2D⁺ GM NK-92 cells may be an efficient approach in targeting sarcomas. The degranulation capacity of GM NK-92 cell lines was also tested against various established tumor cell lines, including neuroblastoma, Schwannoma, melanoma, myeloma, leukemia, prostate, pancreatic, colon, and lung cancer. Enhanced degranulation of DNAM-1⁺ or NKG2D⁺ GM NK-92 cells was observed against the majority of tumor cell lines tested. In conclusion, DNAM-1 or NKG2D over-expression elicited a dynamic increase in NK cell degranulation against all sarcoma explants and cancer cell lines tested, including those that failed to induce a notable response in WT NK-92 cells. These results support the broad therapeutic potential of DNAM-1⁺ or NKG2D⁺ GM NK-92 cells and GM human NK cells for the treatment of sarcomas and other malignancies.

CAR-NK for tumor immunotherapy: Clinical transformation and future prospects

Recently, the use of chimeric antigen receptor-modified T (CAR-T)-cells in the treatment of hematological tumors has been successful and has become a clinical hotspot in tumor immunotherapy. However, their wide application is limited by inherent risks such as graft-versus-host disease (GvHD) and the amount of time it takes to produce CAR-T cells. Natural killer (NK) cells can be xenografted and have the potential to become off-the-shelf products, making CAR-NK cell therapies universal products. These products may be safer than CAR-T cell therapy. Considering that the fundamental researche is still in its infancy, this review focuses on clinical achievements and new strategies for improving the safety and efficacy of CAR-NK cell therapy, as well as the corresponding challenges.

Opportunities and Challenges for Antibodies against Intracellular Antigens

Therapeutic antibodies are one most significant advances in immunotherapy, the development of antibodies against disease-associated MHC-peptide complexes led to the introduction of TCR-like antibodies. TCR-like antibodies combine the recognition of intracellular proteins with the therapeutic potency and versatility of monoclonal antibodies (mAb), offering an unparalleled opportunity to expand the repertoire of therapeutic antibodies available to treat diseases like cancer. This review details the current state of TCR-like antibodies and describes their production, mechanisms as well as their applications. In addition, it presents an insight on the challenges that they must overcome in order to become commercially and clinically validated.

Multifunctional nanoparticles for genetic engineering and bioimaging of natural killer (NK) cell therapeutics

Recently, natural killer (NK)-based immunotherapy has attracted attention as a next-generation cell-based cancer treatment strategy due to its mild side effects and excellent therapeutic efficacy. Here, we describe multifunctional nanoparticles (MF-NPs) capable of genetically manipulating NK cells and tracking them in vivo through non-invasive magnetic resonance (MR) and fluorescence optical imaging. The MF-NPs were synthesized with a core-shell structure by conjugation of a cationic polymer labeled with a near-infrared (NIR) fluorescent molecule, with the aid of a polydopamine (PDA) coating layer. When administered to NKs, the MF-NPs exhibited excellent cytocompatibility, efficiently delivered genetic materials into the immune cells, and induced target protein expression. In particular, the MF-NPs could induce the expression of EGFR targeting chimeric antigen receptors (EGFR-CARs) on the NK cell surface, which improved the cells' anti-cancer cytotoxic effect both in vitro and in vivo. Finally, when NK cells labeled with MF-NPs were injected into live mice, MF-NP-labeled NK cells could be successfully imaged using fluorescence and MR imaging devices. Our findings indicate that MF-NPs have great potential for application of NK cells, as well as other types of cell therapies involving genetic engineering and in vivo monitoring of cell trafficking.

CAR-Based Strategies beyond T Lymphocytes: Integrative Opportunities for Cancer Adoptive Immunotherapy

Chimeric antigen receptor (CAR)-engineered T lymphocytes (CAR Ts) produced impressive clinical results against selected hematological malignancies, but the extension of CAR T cell therapy to the challenging field of solid tumors has not, so far, replicated similar clinical outcomes. Many efforts are currently dedicated to improve the efficacy and safety of CAR-based adoptive immunotherapies, including application against solid tumors. A promising approach is CAR engineering of immune effectors different from αβT lymphocytes. Herein we reviewed biological features, therapeutic potential, and safety of alternative effectors to conventional CAR T cells: γδT, natural killer (NK), NKT, or cytokine-induced killer (CIK) cells. The intrinsic CAR-independent antitumor activities, safety profile, and ex vivo expansibility of these alternative immune effectors may favorably contribute to the clinical development of CAR strategies. The proper biological features of innate immune response effectors may represent an added value in tumor settings with heterogeneous CAR target expression, limiting the risk of tumor clonal escape. All these properties bring out CAR engineering of alternative immune effectors as a promising integrative option to be explored in future clinical studies.

Engineering antigen-specific NK cell lines against the melanoma-associated antigen tyrosinase via TCR gene transfer

Introduction of Chimeric Antigen Receptors to NK cells has so far been the main practical method for targeting NK cells to specific surface antigens. In contrast, T cell receptor (TCR) gene delivery can supply large populations of cytotoxic T-lymphocytes (CTL) targeted against intracellular antigens. However, a major barrier in the development of safe CTL-TCR therapies exists, wherein the mispairing of endogenous and genetically transferred TCR subunits leads to formation of TCRs with off-target specificity. To overcome this and enable specific intracellular antigen targeting, we have tested the use of NK cells for TCR gene transfer to human cells. Our results show that ectopic expression of TCR α/β chains, along with CD3 subunits, enables the functional expression of an antigen-specific TCR complex on NK cell lines NK-92 and YTS, demonstrated by using a TCR against the HLA-A2-restricted tyrosinase-derived melanoma epitope, Tyr_368-377 . Most importantly, the introduction of a TCR complex to NK cell lines enables MHC-restricted, antigen-specific killing of tumor cells both in vitro and in vivo. Targeting of NK cells via TCR gene delivery stands out as a novel tool in the field of adoptive immunotherapy which can also overcome the major hurdle of "mispairing" in TCR gene therapy.

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.

Targeting the MHC Ligandome by Use of TCR-Like Antibodies

Monoclonal antibodies (mAbs) are valuable as research reagents, in diagnosis and in therapy. Their high specificity, the ease in production, favorable biophysical properties and the opportunity to engineer different properties make mAbs a versatile class of biologics. mAbs targeting peptide–major histocompatibility molecule (pMHC) complexes are often referred to as “TCR-like” mAbs, as pMHC complexes are generally recognized by T-cell receptors (TCRs). Presentation of self- and non-self-derived peptide fragments on MHC molecules and subsequent activation of T cells dictate immune responses in health and disease. This includes responses to infectious agents or cancer but also aberrant responses against harmless self-peptides in autoimmune diseases. The ability of TCR-like mAbs to target specific peptides presented on MHC allows for their use to study peptide presentation or for diagnosis and therapy. This extends the scope of conventional mAbs, which are generally limited to cell-surface or soluble antigens. Herein, we review the strategies used to generate TCR-like mAbs and provide a structural comparison with the analogous TCR in pMHC binding. We further discuss their applications as research tools and therapeutic reagents in preclinical models as well as challenges and limitations associated with their use.

Engineering lymphocytes with RNAi

Lymphocytes are the gatekeepers of the body's immune system and are involved in pathogenesis if their surveillance is stalled by inhibitory molecules or when they act as mediators for viral entry. Engineering lymphocytes in order to restore their functions is an unmet need in immunological disorders, cancer and in lymphotropic viral infections. Recently, the FDA approved several therapeutic antibodies for blocking inhibitory signals on T cells. This has revolutionized the field of solid tumor care, together with chimeric antigen receptor T cell (CAR-T) therapy that did the same for hematological malignancies. RNA interference (RNAi) is a promising approach where gene function can be inhibited in almost all types of cells. However, manipulation of genes in lymphocyte subsets are difficult due to their hard-to-transfect nature and in vivo targeting remains challenging as they are dispersed throughout the body. The ability of RNAi molecules to gain entry into cells is almost impossible without delivery strategy. Nanotechnology approaches are rapidly growing and their impact in the field of drug and gene delivery applications to transport payloads inside cells have been extensively studied. Here we discuss various technologies available for RNAi delivery to lymphocytes. We shed light on the importance of targeting molecules in order to target lymphocytes in vivo. In addition, we discuss recent developments of RNAi delivery to lymphocyte subsets, and detail the potential implication for the future of molecular medicine in leukocytes implicated diseases.

Natural Killer Cells and Current Applications of Chimeric Antigen Receptor-Modified NK-92 Cells in Tumor Immunotherapy

Natural killer (NK) cells are innate immune cells that can be activated rapidly to target abnormal and virus-infected cells without prior sensitization. With significant advancements in cell biology technologies, many NK cell lines have been established. Among these cell lines, NK-92 cells are not only the most widely used but have also been approved for clinical applications. Additionally, chimeric antigen receptor-modified NK-92 cells (CAR-NK-92 cells) have shown strong antitumor effects. In this review, we summarize established human NK cell lines and their biological characteristics, and highlight the applications of NK-92 cells and CAR-NK-92 cells in tumor immunotherapy.

Novel alternatives to extracellular vesicle-based immunotherapy - exosome mimetics derived from natural killer cells

Exosomes are endogenous nanocarriers that can deliver biological information between cells. They are secreted by all cell types, including immune cells such as natural killer (NK) cells. However, mammalian cells release low quantities of exosomes, and the purification of exosomes is difficult. Here, nanovesicles were developed by extrusion of NK cells through filters with progressively smaller pore sizes to obtain exosome mimetics (NK-EM). The anti-tumour effect of the NK-EM was confirmed in vitro and in vivo. The morphological features of the NK-EM were revealed by transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA) and Western blot. In vitro, the cytotoxicity of the NK-EM to cancer cells (glioblastoma, breast carcinoma, anaplastic thyroid cancer and hepatic carcinoma) was assessed using bioluminescence imaging (BLI) and CCK-8 assay. For in vivo study, a xenograft glioblastoma mouse model was established. The anti-tumour activity of NK-EM was confirmed in vivo by the significant decreases of BLI, size and weight (all p < .001) of the tumour compared with the control group. Moreover, NK-EM cytotoxicity for glioblastoma cells that related with decreased levels of the cell survival markers p-ERK and p-AKT, and increased levels of apoptosis protein markers cleaved-caspase 3, cytochrome-c and cleaved-PARP was confirmed. All those results suggest that NK-EM exert stronger killing effects to cancer cells compared with the traditional NK-Exo, at the same time, the tumour targeting ability of the NK-EM was obtained in vivo. Therefore, NK-EM might be a promising immunotherapeutic agent for treatment of cancer.

Chimeric antigen receptor (CAR)-transduced natural killer cells in tumor immunotherapy

Natural killer (NK) cells are potential effector cells in cell-based cancer immunotherapy, particularly in the control of hematological malignancies. The chimeric antigen receptor (CAR) is an artificially modified fusion protein that consists of an extracellular antigen recognition domain fused to an intracellular signaling domain. T cells genetically modified with a CAR have demonstrated remarkable success in the treatment of hematological cancers. Compared to T cells, CAR-transduced NK cells (CAR-NK) exhibit several advantages, such as safety in clinical use, the mechanisms by which they recognize cancer cells, and their abundance in clinical samples. Human primary NK cells and the NK-92 cell line have been successfully transduced to express CARs against both hematological cancers and solid tumors in pre-clinical and clinical trials. However, many challenges and obstacles remain, such as the ex vivo expansion of CAR-modified primary NK cells and the low transduction efficiency of NK cells. Many strategies and technologies have been developed to improve the safety and therapeutic efficacy in CAR-based immunotherapy. Moreover, NK cells express a variety of activating receptors (NKRs), such as CD16, NKG2D, CD226 and NKp30, which might specifically recognize the ligands expressed on tumor cells. Based on the principle of NKR recognition, a strategy that targets NKRs is rapidly emerging. Given the promising clinical progress described in this review, CAR- and NKR-NK cell-based immunotherapy are likely promising new strategies for cancer therapy.

VEGFR2-specific FnCAR effectively redirects the cytotoxic activity of T cells and YT NK cells

T and NK cells armed with chimeric antigen receptors (CAR) are promising tools for the specific elimination of cancer cells. In most CAR designs implemented to date, the recognition of target cells is mediated by single-chain variable fragments (scFvs) derived from murine monoclonal antibodies. This format, however, has a number of limitations, including its relatively large size and potential immunogenicity in humans. In this study, we explored the feasibility of using human fibronectin type III domains (Fn3) as the antigen recognition domain in CARs. Human Fn3 domains have lower predicted immunogenicity compared to mouse-derived sequences, and a reduced molecular weight compared to scFvs. We created a functional CAR using a VEGFR2-specific Fn3 module replacing the conventional scFv. The resulting FnCAR specifically potentiates the cytotoxic activity of human T cells and YT NK cells in the presence of VEGFR2-positive targets. These findings demonstrate that Fn3 domains can be used in CARs for antigen recognition.

A role for multiple chimeric antigen receptor-expressing leukocytes in antigen-specific responses to cancer

While adoptive immunotherapy using chimeric antigen receptor (CAR)-modified T cells can induce remission of some tumors, the role of other CAR-modified leukocytes is not well characterized. In this study, we characterize the function of leukocytes including natural killer (NK) cells, macrophages and CAR T cells from transgenic mice expressing a CAR under the control of the pan-hematopoietic promoter, vav, and determine the ability of these mice to respond to ERB expressing tumors. We demonstrate the anti-tumor functions of leukocytes, including antigen specific cytotoxicity and cytokine secretion. The adoptive transfer of CAR T cells provided a greater survival advantage in the E0771ERB tumor model than their wildtype (WT) counterparts. In addition, CAR NK cells and CAR T cells also mediated increased survival in the RMAERB tumor model. When challenged with Her2 expressing tumors, F38 mice were shown to mount an effective immune response, resulting in tumor rejection and long-term survival. This was shown to be predominantly dependent on both CD8⁺ T cells and NK cells. However, macrophages and CD4⁺ T cells were also shown to contribute to this response. Overall, this study highlights the use of the vav-CAR mouse model as a unique tool to determine the anti-tumor function of various immune subsets, either alone or when acting alongside CAR T cells in adoptive immunotherapy.

Engineering Hematopoietic Cells for Cancer Immunotherapy: Strategies to Address Safety and Toxicity Concerns

Advances in cancer immunotherapies utilizing engineered hematopoietic cells have recently generated significant clinical successes. Of great promise are immunotherapies based on chimeric antigen receptor-engineered T (CAR-T) cells that are targeted toward malignant cells expressing defined tumor-associated antigens. CAR-T cells harness the effector function of the adaptive arm of the immune system and redirect it against cancer cells, overcoming the major challenges of immunotherapy, such as breaking tolerance to self-antigens and beating cancer immune system-evasion mechanisms. In early clinical trials, CAR-T cell-based therapies achieved complete and durable responses in a significant proportion of patients. Despite clinical successes and given the side effect profiles of immunotherapies based on engineered cells, potential concerns with the safety and toxicity of various therapeutic modalities remain. We discuss the concerns associated with the safety and stability of the gene delivery vehicles for cell engineering and with toxicities due to off-target and on-target, off-tumor effector functions of the engineered cells. We then overview the various strategies aimed at improving the safety of and resolving toxicities associated with cell-based immunotherapies. Integrating failsafe switches based on different suicide gene therapy systems into engineered cells engenders promising strategies toward ensuring the safety of cancer immunotherapies in the clinic.

Engineering Natural Killer Cells for Cancer Immunotherapy

The past several years have seen tremendous advances in the engineering of immune effector cells as therapy for cancer. While chimeric antigen receptors (CARs) have been used extensively to redirect the specificity of autologous T cells against hematological malignancies with striking clinical results, studies of CAR-modified natural killer (NK) cells have been largely preclinical. In this review, we focus on recent advances in NK cell engineering, particularly on preclinical evidence suggesting that NK cells may be as effective as T cells in recognizing and killing targets after genetic modification. We will discuss strategies to introduce CARs into both primary NK cells and NK cell lines in an effort to provide antigen specificity, the challenges of manufacturing engineered NK cells, and evidence supporting the effectiveness of this approach from preclinical and early-phase clinical studies using CAR-engineered NK cells. CAR-NK cells hold great promise as a novel cellular immunotherapy against refractory malignancies. Notably, NK cells can provide an "off-the-shelf" product, eliminating the need for a personalized and patient-specific product that plagues current CAR-T cell therapies. The ability to more potently direct NK cell-mediated cytotoxicity against refractory tumors through the expression of CAR is likely to contribute to the recent paradigm shift in cancer treatment.

The Rise of Allogeneic Natural Killer Cells As a Platform for Cancer Immunotherapy: Recent Innovations and Future Developments

Natural killer (NK) cells are critical immune effector cells in the fight against cancer. As NK cells in cancer patients are highly dysfunctional and reduced in number, adoptive transfer of large numbers of cytolytic NK cells and their potential to induce relevant antitumor responses are widely explored in cancer immunotherapy. Early studies from autologous NK cells have failed to demonstrate significant clinical benefit. In this review, the clinical benefits of adoptively transferred allogeneic NK cells in a transplant and non-transplant setting are compared and discussed in the context of relevant NK cell platforms that are being developed and optimized by various biotech industries with a special focus on augmenting NK cell functions.

Chimeric Antigen Receptor-Engineered NK-92 Cells: An Off-the-Shelf Cellular Therapeutic for Targeted Elimination of Cancer Cells and Induction of Protective Antitumor Immunity

Significant progress has been made in recent years toward realizing the potential of natural killer (NK) cells for cancer immunotherapy. NK cells can respond rapidly to transformed and stressed cells and have the intrinsic potential to extravasate and reach their targets in almost all body tissues. In addition to donor-derived primary NK cells, also the established NK cell line NK-92 is being developed for adoptive immunotherapy, and general safety of infusion of irradiated NK-92 cells has been established in phase I clinical trials with clinical responses observed in some of the cancer patients treated. To enhance their therapeutic utility, NK-92 cells have been modified to express chimeric antigen receptors (CARs) composed of a tumor-specific single chain fragment variable antibody fragment fused via hinge and transmembrane regions to intracellular signaling moieties such as CD3ζ or composite signaling domains containing a costimulatory protein together with CD3ζ. CAR-mediated activation of NK cells then bypasses inhibitory signals and overcomes NK resistance of tumor cells. In contrast to primary NK cells, CAR-engineered NK-92 cell lines suitable for clinical development can be established from molecularly and functionally well-characterized single cell clones following good manufacturing practice-compliant procedures. In preclinical in vitro and in vivo models, potent antitumor activity of NK-92 variants targeted to differentiation antigens expressed by hematologic malignancies, and overexpressed or mutated self-antigens associated with solid tumors has been found, encouraging further development of CAR-engineered NK-92 cells. Importantly, in syngeneic mouse tumor models, induction of endogenous antitumor immunity after treatment with CAR-expressing NK-92 cells has been demonstrated, resulting in cures and long-lasting immunological memory protecting against tumor rechallenge at distant sites. Here, we summarize the current status and future prospects of CAR-engineered NK-92 cells as off-the-shelf cellular therapeutics, with special emphasis on ErbB2 (HER2)-specific NK-92 cells that are approaching clinical application.

Immunosenescence: limitations of natural killer cell-based cancer immunotherapy

Cancer is primarily considered a disease of old age. Immunosenescence refers to the age-associated changes in the immune system, and its contribution to the increased risk of cancer in old individuals has been discussed for many years. Natural killer (NK) cells are cytotoxic innate immune cells specialized in defence against tumour and virus-infected cells. NK cell cytotoxicity is the result of a fine balance between activating and inhibitory receptors. Several activating receptors have been identified that recognize different ligands frequently found over-expressed on tumour cells or virus-infected cells. The most important NK cell inhibitory receptors interact with major histocompatibility complex class I molecules expressed on almost all nucleated cells preventing NK cell-mediated lysis of healthy cells. NK cell immunosenescence is characterized by a redistribution of NK cell subsets, a diminished expression of several activating receptors and lower per-cell cytotoxicity. Altered expression of activating receptors has also been described in young and elderly cancer patients probably due to chronic exposure to ligands on tumour cells. Thus, the effect of both age and cancer may act synergistically to diminish NK cell-mediated tumour immunosurveillance. Different strategies harnessing the power of NK cells to target tumour cells have been designed including adoptive therapy with autologous or allogeneic expanded NK cells. In addition, checkpoint blockade of inhibitory receptors and the use of agonist antibodies to stimulate activating receptors are emerging areas of research. In this context, the effect of immunosenescence should be considered to improve the efficiency of cancer immunotherapy.

Improving efficacy of cancer immunotherapy by genetic modification of natural killer cells

Natural killer (NK) cells are members of the innate immune system that recognize target cells via activating and inhibitory signals received through cell receptors. Derived from the lymphoid lineage, NK cells are able to produce cytokines and exert a cytotoxic effect on viral infected and malignant cells. It is their unique ability to lyse target cells rapidly and without prior education that renders NK cells a promising effector cell for adoptive cell therapy. However, both viruses and tumors employ evasion strategies to avoid attack by NK cells, which represent biological challenges that need to be harnessed to fully exploit the cytolytic potential of NK cells. Using genetic modification, the function of NK cells can be enhanced to improve their homing, cytolytic activity, in vivo persistence and safety. Examples include gene modification to express chemokine, high-affinity Fc receptor and chimeric antigen receptors, suicide genes and the forced expression of cytokines such as interleukin (IL)-2 and IL-15. Preclinical studies have clearly demonstrated that such approaches are effective in improving NK-cell function, homing and safety. In this review, we summarize the recent advances in the genetic manipulations of NK cells and their application for cellular immunotherapeutic strategies.

Natural Killer Cells for Immunotherapy - Advantages of the NK-92 Cell Line over Blood NK Cells

Natural killer (NK) cells are potent cytotoxic effector cells for cancer therapy and potentially for severe viral infections. However, there are technical challenges to obtain sufficient numbers of functionally active NK cells from a patient’s blood since they represent only 10% of the lymphocytes and are often dysfunctional. The alternative is to obtain cells from a healthy donor, which requires depletion of the allogeneic T cells to prevent graft-versus-host reactions. Cytotoxic cell lines have been established from patients with clonal NK-cell lymphoma. Those cells can be expanded in culture in the presence of IL-2. Except for the NK-92 cell line, though, none of the other six known NK cell lines has consistently and reproducibly shown high antitumor cytotoxicity. Only NK-92 cells can easily be genetically manipulated to recognize specific tumor antigens or to augment monoclonal antibody activity through antibody-dependent cellular cytotoxicity. NK-92 is also the only cell line product that has been infused into patients with advanced cancer with clinical benefit and minimal side effects.

Natural Killer Cell Recognition of Melanoma: New Clues for a More Effective Immunotherapy

Natural killer (NK) cells participate in the early immune response against melanoma and also contribute to the development of an adequate adaptive immune response by their crosstalk with dendritic cells and cytokine secretion. Melanoma resistance to conventional therapies together with its high immunogenicity justifies the development of novel therapies aimed to stimulate effective immune responses against melanoma. However, melanoma cells frequently escape to CD8 T cell recognition by the down-regulation of major histocompatibility complex (MHC) class I molecules. In this scenario, NK cells emerge as potential candidates for melanoma immunotherapy due to their capacity to recognize and destroy melanoma cells expressing low levels of MHC class I molecules. In addition, the possibility to combine immune checkpoint blockade with other NK cell potentiating strategies (e.g., cytokine induction of activating receptors) has opened new perspectives in the potential use of adoptive NK cell-based immunotherapy in melanoma.