Genetic modification of natural killer cells for adoptive cellular 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.

Redirecting natural killer cells to potentiate adoptive immunotherapy in solid tumors through stabilized Y-type bispecific aptamer

Modulating interactions between immune effector cells and tumor cells in vivo using a bispecific aptamer (Ap) is a promising strategy for cancer immunotherapy. However, it remains a technical challenge owing to the complex and dynamic internal environment accompanied by severe degradation. Herein, by using a Y-shaped DNA scaffold, a bispecific and stabilized Y-type Ap is designed to redirect natural killer (NK) cells to enhance adoptive immunotherapy of hepatocellular carcinoma (HCC) solid tumors. Y-type Ap is constituted by the HCC-specific Ap TLS11a linked with the CD16-specific Ap through a Y-shaped DNA scaffold. Owing to the rigid structure, Y-type Ap shows high stability in 10% serum for over 72 h and resistance to denaturation by 8 M urea. Additionally, the Y-type Ap exhibits more potent avidity to bind with NK cells and tumor cells both in vitro and in vivo, resulting in higher cytokine secretion and excellent antitumor efficiency. Collectively, this study offers a translational platform for constructing stable bispecific Ap, offering considerable potential to enhance adoptive immunotherapy of solid tumors.

T-cell receptor-based therapy: an innovative therapeutic approach for solid tumors

T-cell receptor (TCR)-based adoptive therapy employs genetically modified lymphocytes that are directed against specific tumor markers. This therapeutic modality requires a structured and integrated process that involves patient screening (e.g., for HLA-A*02:01 and specific tumor targets), leukapheresis, generation of transduced TCR product, lymphodepletion, and infusion of the TCR-based adoptive therapy. In this review, we summarize the current technology and early clinical development of TCR-based therapy in patients with solid tumors. The challenges of TCR-based therapy include those associated with TCR product manufacturing, patient selection, and preparation with lymphodepletion. Overcoming these challenges, and those posed by the immunosuppressive microenvironment, as well as developing next-generation strategies is essential to improving the efficacy and safety of TCR-based therapies. Optimization of technology to generate TCR product, treatment administration, and patient monitoring for adverse events is needed. The implementation of novel TCR strategies will require expansion of the TCR approach to patients with HLA haplotypes beyond HLA-A*02:01 and the discovery of novel tumor markers that are expressed in more patients and tumor types. Ongoing clinical trials will determine the ultimate role of TCR-based therapy in patients with solid tumors.

Natural killer cells and cancer therapy, what we know and where we are going

Natural killer (NK) cells are among the significant components of innate immune system and they have come to the first line of defense against tumor cells developing inside the body. CD56lo/CD16+ NK cells are highly cytotoxic and CD56hi NK cells can produce cytokines and perform a regulatory function. Specific features of NK cells have made them a unique choice for cancer immunotherapy. Simple interventions like cytokine-injection to boost the internal NK cells were the first trials to target these cells. Nowadays, many other types of intervention are under investigation, such as adoptive NK cell immunotherapy. In this paper, we will discuss the biology and function of NK cells in cancer immunosurveillance and therapeutic approaches against cancer via using NK cells.

A Distinct Subset of Highly Proliferative and Lentiviral Vector (LV)-Transducible NK Cells Define a Readily Engineered Subset for Adoptive Cellular Therapy

Genetic engineering is an important tool for redirecting the function of various types of immune cells and their use for therapeutic purpose. Although NK cells have many beneficial therapeutic features, genetic engineering of immune cells for targeted therapy focuses mostly on T cells. One of the major obstacles for NK cell immunotherapy is the lack of an efficient method for gene transfer. Lentiviral vectors have been proven to be a safe tool for genetic engineering, however lentiviral transduction is inefficient for NK cells. We show in this study that lentiviral vectors pseudotyped with a modified baboon envelope glycoprotein can transduce NK cells 20-fold or higher in comparison to VSV-G pseudotyped lentiviral vector. When we investigated the mechanism of transduction, we found that activated NK cells expressed baboon envelope receptor ASCT-2. Further analysis revealed that only a subset of NK cells could be expanded and transduced with an expression profile of NK56bright, CD16dim, TRAILhigh, and CX3CR1neg. Using CD19-CAR, we could show that CD19 redirected NK cells efficiently and specifically kill cell lines expressing CD19. Taken together, the results from this study will be important for future genetic modification and for redirecting of NK cell function for therapeutic purpose.

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.

Present and Future of Allogeneic Natural Killer Cell Therapy

Natural killer (NK) cells are innate lymphocytes that are capable of eliminating tumor cells and are therefore used for cancer therapy. Although many early investigators used autologous NK cells, including lymphokine-activated killer cells, the clinical efficacies were not satisfactory. Meanwhile, human leukocyte antigen (HLA)-haploidentical hematopoietic stem cell transplantation revealed the antitumor effect of allogeneic NK cells, and HLA-haploidentical, killer cell immunoglobulin-like receptor ligand-mismatched allogeneic NK cells are currently used for many protocols requiring NK cells. Moreover, allogeneic NK cells from non-HLA-related healthy donors have been recently used in cancer therapy. The use of allogeneic NK cells from non-HLA-related healthy donors allows the selection of donor NK cells with higher flexibility and to prepare expanded, cryopreserved NK cells for instant administration without delay for ex vivo expansion. In cancer therapy with allogeneic NK cells, optimal matching of donors and recipients is important to maximize the efficacy of the therapy. In this review, we summarize the present state of allogeneic NK cell therapy and its future directions.

A new hope in immunotherapy for malignant gliomas: adoptive T cell transfer therapy

Immunotherapy emerged as a promising therapeutic approach to highly incurable malignant gliomas due to tumor-specific cytotoxicity, minimal side effect, and a durable antitumor effect by memory T cells. But, antitumor activities of endogenously activated T cells induced by immunotherapy such as vaccination are not sufficient to control tumors because tumor-specific antigens may be self-antigens and tumors have immune evasion mechanisms to avoid immune surveillance system of host. Although recent clinical results from vaccine strategy for malignant gliomas are encouraging, these trials have some limitations, particularly their failure to expand tumor antigen-specific T cells reproducibly and effectively. An alternative strategy to overcome these limitations is adoptive T cell transfer therapy, in which tumor-specific T cells are expanded ex vivo rapidly and then transferred to patients. Moreover, enhanced biologic functions of T cells generated by genetic engineering and modified immunosuppressive microenvironment of host by homeostatic T cell expansion and/or elimination of immunosuppressive cells and molecules can induce more potent antitumor T cell responses and make this strategy hold promise in promoting a patient response for malignant glioma treatment. Here we will review the past and current progresses and discuss a new hope in adoptive T cell therapy for malignant gliomas.

Bringing natural killer cells to the clinic: ex vivo manipulation

Recently, there has been a substantial gain in our understanding of the role that natural killer (NK) cells play in mediating innate host immune responses against viruses and cancer. Although NK cells have long been known to be capable of killing cancer cells independently of antigen recognition, the full therapeutic potential of NK cell-based immunotherapy has yet to be realized. Here we review novel methods to activate and expand human NK cells ex vivo for adoptive transfer in humans, focusing on the important phenotypic and functional differences observed among freshly isolated, cytokine activated, and ex vivo-expanded NK populations.

Inhibition of intracellular antiviral defense mechanisms augments lentiviral transduction of human natural killer cells: implications for gene therapy

Adoptive immunotherapy with genetically modified natural killer (NK) cells is a promising approach for cancer treatment. Yet, optimization of highly efficient and clinically applicable gene transfer protocols for NK cells still presents a challenge. In this study, we aimed at identifying conditions under which optimum lentiviral gene transfer to NK cells can be achieved. Our results demonstrate that stimulation of NK cells with interleukin (IL)-2 and IL-21 supports efficient transduction using a VSV-G pseudotyped lentiviral vector. Moreover, we have identified that inhibition of innate immune receptor signaling greatly enhances transduction efficiency. We were able to boost the efficiency of lentiviral genetic modification on average 3.8-fold using BX795, an inhibitor of the TBK1/IKKɛ complex acting downstream of RIG-I, MDA-5, and TLR3. We have also observed that the use of BX795 enhances lentiviral transduction efficiency in a number of human and mouse cell lines, indicating a broadly applicable, practical, and safe approach that has the potential of being applicable to various gene therapy protocols.

Expression of IL-15 in NK cells results in rapid enrichment and selective cytotoxicity of gene-modified effectors that carry a tumor-specific antigen receptor

Natural killer (NK) cells hold promise for adoptive cancer immunotherapy but are dependent on cytokines such as interleukin (IL)-2 for growth and cytotoxicity. Here, we investigated the consequences of ectopic expression of IL-15 in human NK cells. IL-2 and IL-15 belong to the common γ chain family of cytokines and have overlapping activities. Transduction of clinically applicable NK-92 cells with lentiviral vectors encoding human IL-15 resulted in predominantly intracellular expression of the cytokine, and STAT5 activation, proliferation and cytotoxicity of the producer cells in the absence of IL-2. Growth of non-transduced bystander cells was not supported, allowing rapid enrichment of gene-modified cells solely by IL-2 withdrawal. This was also the case upon transduction of NK-92 and NKL cells with a bicistronic lentiviral vector encoding IL-15 and a chimeric antigen receptor (CAR) targeting the pancarcinoma antigen EpCAM. Effector cells co-expressing CAR and IL-15 continued to proliferate in the absence of exogenous cytokines and displayed high and selective cell-killing activity against EpCAM-expressing breast carcinoma cells that were resistant to the natural cytotoxicity of unmodified NK cells. This strategy facilitates rapid isolation and continuous expansion of retargeted NK cells and may extend their potential clinical utility.

Chimeric antigen receptors for T-cell based therapy

The Chimeric Antigen Receptor (CAR) consists of an antibody-derived targeting domain fused with T-cell signaling domains that, when expressed by a T-cell, endows the T-cell with antigen specificity determined by the targeting domain of the CAR. CARs can potentially redirect the effector functions of a T-cell towards any protein and nonprotein target expressed on the cell surface as long as an antibody or similar targeting domain is available. This strategy thereby avoids the requirement of antigen processing and presentation by the target cell and is applicable to nonclassical T-cell targets like carbohydrates. This circumvention of HLA-restriction means that the CAR T-cell approach can be used as a generic tool broadening the potential of applicability of adoptive T-cell therapy. Proof-of-principle studies focusing upon the investigation of the potency of CAR T-cells have primarily focused upon the genetic modification of human and mouse T-cells for therapy. This chapter focuses upon methods to modify T-cells from both species to generate CAR T-cells for functional testing.