IL-2 stimulated but not unstimulated NK cells induce selective disappearance of peripheral blood cells: concomitant results to a phase I/II study

Future directions in haploidentical hematopoietic stem cell transplantation

Outcomes of haploidentical hematopoietic stem cell transplantation (haplo-SCT) have improved over time. Graft failure and graft-versus-host disease (GVHD), which were important complications in major human leukocyte antigen (HLA)-disparity stem cell transplantation, have significantly decreased. These improvements have led to an exponential increase in the use of haploidentical donors for transplantation, as well as in the number of publications evaluating haplo-SCT outcomes. Many studies focused on factors important in donor selection, novel conditioning regimens or GVHD prophylaxis, the impact of donor-specific anti-HLA antibodies (DSA), as well as strategies to prevent disease relapse post-transplant. DSA represents an important limitation and multimodality desensitization protocols, including plasma exchange, rituximab, intravenous immunoglobulin and donor buffy coat infusion, can contribute to the successful engraftment in patients with high DSA levels and is currently the standard therapy for highly allosensitized individuals. With regards to donor selection, younger donors are preferred due to lower risk of complications and better transplant outcomes. Moreover, recent studies also showed that younger haploidentical donors may be a better choice than older-matched unrelated donors. Improvement of disease relapse remains a top priority, and several studies have demonstrated that higher natural killer (NK) cell numbers early post-transplant are associated with improved outcomes. Prospective studies have started to assess the role of NK cell administration in decreasing post-transplant relapse. These studies suggest that the incorporation of other cell products post-transplant, including the administration of chimeric antigen receptor T-cells, should be explored in the future.

Promising approach for targeting ROBO1 with CAR NK cells to combat ovarian cancer primary tumor cells and organoids

Aim: This study aimed to explore using peripheral blood mononuclear cell (PBMC)-derived chimeric antigen receptor (CAR) NK cells targeting ROBO1 as a personalized medicine approach for ovarian cancer. Methods: A two-step strategy generated ROBO1-targeted CAR NK cells from PBMCs of ovarian cancer patients. Efficacy was evaluated using xCELLigence RTCA, CCK-8 and Live/Dead fluorescence assays. Results: ROBO1-NK cells exhibited higher efficiency in eradicating primary ovarian cancer cells and lysing ovarian tumor organoids compared with primary NK cells without ROBO1-CAR modification. Conclusion: These findings highlight the potential of developing ROBO1-targeted CAR-NK cells from patients' PBMCs as a personalized treatment option for ovarian cancer.

Adoptive cellular therapy after hematopoietic stem cell transplantation

Effective cellular therapy using CD19 chimeric antigen receptor T-cells for the treatment of advanced B-cell malignancies raises the question of whether the administration of adoptive cellular therapy (ACT) posttransplant could reduce relapse and improve survival. Moreover, several early phase clinical studies have shown the potential beneficial effects of administration of tumor-associated antigen-specific T-cells and natural killer cells posttransplant for high-risk patients, aiming to decrease relapse and possibly improve survival. In this article, we present an in-depth review of ACT after transplantation, which has the potential to significantly improve the efficacy of this procedure and revolutionize this field.

Ex vivo expansion of natural killer cells for hematological cancer immunotherapy: a systematic review and meta-analysis

The present systematic review aimed to investigate natural killer (NK) cell ex vivo expansion protocols within the scope of clinical trials targeting hematological cancer and to conduct a meta-analysis to assess the effect of NK cell infusion on survival. Research articles of clinical studies in which cell products produced by ex vivo expansion, consisting of a certain amount of NK cells and infused to patients with hematological cancer, were included in the systematic review. We conducted a proportion analysis with random effects for product purity and viability values. Studies having control groups were included in the survival meta-analysis. Among 11.028 identified records, 21 were included in the systematic review. We observed statistically significant heterogeneity for viability (I2 = 97.83%, p < 0.001) and purity values (I2 = 99.95%, p < 0.001), which was attributed to the diversity among isolation and expansion protocols. In addition, the survival meta-analysis findings suggested that NK cell therapy favors disease-free survival (DFS) of patients with myeloid malignancies but limited to only two clinical studies (odds ratio = 3.40 (confidence interval:1.27-9.10), p = 0.01). While included protocols yielded cell products with acceptable viability, the utility of immunomagnetic methods; feeder cells such as K562 expressing membrane-bound IL15 and 4-1BBL or expressing membrane-bound IL21 and 4-1BBL might be preferable to achieve better purity. In conclusion, NK cell therapy has a potential to improve DFS of patients with myeloid malignancies.

Arabinogalactan G1-4A isolated from Tinospora cordifolia induces PKC/mTOR mediated direct activation of natural killer cells and through dendritic cell cross-talk

BACKGROUND

Tinospora cordifolia polysaccharide G1-4A activates antigen-presenting cells, but its effect on natural killer (NK) cells is not known. The objective of this study is to assess the effect of G1-4A on NK cells; direct effects as well as through dendritic cell (DC) cross-talk.

METHODS

NK cell phenotype and function were assessed in spleen cells treated in vitro with G1-4A or isolated from mice administered with G1-4A. Following treatment with G1-4A in vitro or in cells isolated from G1-4A treated mice (in vivo), activated NK cell phenotype was characterized as CD3^-NKp46⁺CD69⁺ cells by flow cytometry; NK cell function was evaluated by IFN-γ secretion (ELISA) and cytotoxicity assay (calcein release by target cells in effector: target cells co-culture assay).

RESULTS

Both in vitro as well as in vivoG1-4A treatment increased phenotypic and functional activation of NK cells. So, we wanted to determine if this was through NK-DC crosstalk or direct activation of NK cells. There was increased NK cell activation following co-culture with bone marrow derived DC matured withG1-4A in vitro or splenic DC isolated from G1-4A administered mice indicating crosstalk. G1-4A also increased activation of NK cells in (a) CD11c depleted splenic cells that was contact dependent and (b) purified NKp46⁺ cells that was abrogated by PKC/mTOR inhibitors indicating direct effects on NK cells.

CONCLUSION

In summary, treatment with G1-4A results in phenotypic and functional activation of NK cells directly as well as through NK-DC cross talk and has the potential to be used as an immunotherapeutic agent.

The infusion of ex vivo, interleukin-15 and -21-activated donor NK cells after haploidentical HCT in high-risk AML and MDS patients-a randomized trial

Clinical effect of donor-derived natural killer cell infusion (DNKI) after HLA-haploidentical hematopoietic cell transplantation (HCT) was evaluated in high-risk myeloid malignancy in phase 2, randomized trial. Seventy-six evaluable patients (aged 21-70 years) were randomized to receive DNKI (N = 40) or not (N = 36) after haploidentical HCT. For the HCT conditioning, busulfan, fludarabine, and anti-thymocyte globulin were administered. DNKI was given twice 13 and 20 days after HCT. Four patients in the DNKI group failed to receive DNKI. In the remaining 36 patients, median DNKI doses were 1.0 × 10⁸/kg and 1.4 × 10⁸/kg on days 13 and 20, respectively. Intention-to-treat analysis showed a lower disease progression for the DNKI group (30-month cumulative incidence, 35% vs 61%, P = 0.040; subdistribution hazard ratio, 0.50). Furthermore, at 3 months after HCT, the DNKI patients showed a 1.8- and 2.6-fold higher median absolute blood count of NK and T cells, respectively. scRNA-sequencing analysis in seven study patients showed that there was a marked increase in memory-like NK cells in DNKI patients which, in turn, expanded the CD8⁺ effector-memory T cells. In high-risk myeloid malignancy, DNKI after haploidentical HCT reduced disease progression. This enhanced graft-vs-leukemia effect may be related to the DNKI-induced, post-HCT expansion of NK and T cells. Clinical trial number: NCT02477787.

The establishment of polypeptide PSMA-targeted chimeric antigen receptor-engineered natural killer cells for castration-resistant prostate cancer and the induction of ferroptosis-related cell death

Background

The mortality of castration‐resistant prostate cancer (CRPC) is high due to lack of an effective treatment. Chimeric antigen receptor (CAR)‐based therapy is a promising immunotherapeutic strategy. Here, we aimed to design a novel CAR‐natural killer (NK) cells with a clinically significant tumoricidal effect on CRPC.

Methods

We constructed novel CAR‐NK92MI cells with a CD244‐based recombinant lentiviral vector. Different intracellular segments (CD244, NKG2D, or CD3ζ) were screened to identify the best candidate according to cell lysis assay and CD107a expression levels. To enhance the affinity of the CAR to the tumor antigen, we compared an antibody specific for prostate‐specific membrane antigen (anti‐PSMA) with PSMA‐targeted polypeptide (p‐PSMA), which was screened by phage display combinatorial library. Then, CAR‐NK92MI cells with both a high affinity for PSMA and a strong tumoricidal capacity were generated. In addition, we verified their tumor‐killing effect in vitro and in vivo. The release of cytokine by NK92MI cells was compared with that by CAR‐NK92MI cells through flow cytometry and enzyme‐linked immunosorbent assay. Moreover, ferroptosis‐related cell death was explored as a possible underlying mechanism.

Results

Three different CAR intracellular regions CAR1 (CD244), CAR2 (CD244, NKG2D) and CAR3 (CD244, NKG2D, and CD3ζ) were constructed. CAR2 was chosen to confer a stronger tumoricidal ability on CAR‐NK92MI cells. Compared with anti‐PSMA, p‐PSMA exhibited enhanced affinity for the tumor antigen. Thus, p‐PSMA‐CAR‐NK92MI cells, which expressed CAR with a polypeptide‐based antigen‐binding region, an intracellular CD244 and a NKG2D costimulatory domain, were generated. They could selectively and successfully kill PSMA⁺ target cells and exhibited specific lysis rate of 73.19% for PSMA‐positive C4‐2 cells and 33.04% for PSMA‐negative PC3 cells. Additionally, p‐PSMA‐CAR‐NK92MI cells had significantly higher concentrations of IFN‐γ, TNF‐α and granzyme B than NK92MI cells. In a CRPC cancer xenograft model, p‐PSMA‐CAR‐NK92MI cells significantly inhibited tumor growth and exerted a more consistent killing effect than NK92MI cells. Moreover, ferroptosis is a potential mechanism through which CAR‐NK92MI cells attack cancer cells, and is triggered by IFN‐γ.

Conclusions

p‐PSMA‐CAR‐NK92MI cells can effectively kill CRPC^PSMA+ cells in vitro and in vivo. This strategy may provide additional treatment options for patients with CRPC.

A general toxicity and biodistribution study of human natural killer cells by single or repeated intravenous dose in severe combined immune deficient mice

Natural killer (NK) cells are a part of the innate immune system and represent the first line of defense against infections and tumors. NK cells can eliminate tumor cells without major histocompatibility restriction and are independent of the expression of tumor-associated antigens. Therefore, they are considered an emerging tool for cancer immunotherapy. However, the general toxicity and biodistribution of NK cells after transplantation remain to be understood. This study was conducted to evaluate the general toxicity and biodistribution of human NK cells after single or repeated intravenous dosing in severely combined immunodeficient (SCID) mice. There were no test item-related toxicological changes in single and repeated administration groups. The no observed adverse effect level of human NK cells was 2 × 107 cells/head for both male and female SCID mice. Results from the biodistribution study showed that human NK cells were mainly distributed in the lungs, and a small number of the cells were detected in the liver, heart, spleen, and kidney of SCID mice, in both the single and repeated dose groups. Additionally, human NK cells were completely eliminated from all organs of the mice in the single dose group on day 7, while the cells persisted in mice in the repeated dose group until day 64. In conclusion, transplantation of human NK cells in SCID mice had no toxic effects. The cells were mainly distributed in the lungs and completely disappeared from the body over time after single or repeated intravenous administration.

Taking Lessons from CAR-T Cells and Going Beyond: Tailoring Design and Signaling for CAR-NK Cells in Cancer Therapy

Cancer immunotherapies utilize the capabilities of the immune system to efficiently target malignant cells. In recent years, chimeric antigen receptor (CAR) equipped T cells showed promising results against B cell lymphomas. Autologous CAR-T cells require patient-specific manufacturing and thus extensive production facilities, resulting in high priced therapies. Along with potentially severe side effects, these are the major drawbacks of CAR-T cells therapies. Natural Killer (NK) cells pose an alternative for CAR equipped immune cells. Since NK cells can be safely transferred from healthy donors to cancer patients, they present a suitable platform for an allogeneic “off-the-shelf” immunotherapy. However, administration of activated NK cells in cancer therapy has until now shown poor anti-cancer responses, especially in solid tumors. Genetic modifications such as CARs promise to enhance recognition of tumor cells, thereby increasing anti-tumor effects and improving clinical efficacy. Although the cell biology of T and NK cells deviates in many aspects, the development of CAR-NK cells frequently follows within the footsteps of CAR-T cells, meaning that T cell technologies are simply adopted to NK cells. In this review, we underline the unique properties of NK cells and their potential in CAR therapies. First, we summarize the characteristics of NK cell biology with a focus on signaling, a fine-tuned interaction of activating and inhibitory receptors. We then discuss why tailored NK cell-specific CAR designs promise superior efficacy compared to designs developed for T cells. We summarize current findings and developments in the CAR-NK landscape: different CAR formats and modifications to optimize signaling, to target a broader pool of antigens or to increase in vivo persistence. Finally, we address challenges beyond NK cell engineering, including expansion and manufacturing, that need to be addressed to pave the way for CAR-NK therapies from the bench to the clinics.

Arming Immune Cells for Battle: A Brief Journey through the Advancements of T and NK Cell Immunotherapy

Simple Summary

This review is intended to provide an overview on the history and recent advances of T cell and natural killer (NK) cell-based immunotherapy. While the thymus was discovered as the origin of T cells in the 1960s, and NK cells were first described in 1975, the clinical application of adoptive cell therapies (ACT) only began in the early 1980s with the first lymphokine activated killer (LAK) cell product for the treatment of cancer patients. Over the past decades, further immunotherapies have been developed, including ACT using cytokine-induced killer (CIK) cells, products based on the NK cell line NK-92 as well as specific T and NK cell preparations. Recent advances have successfully improved the effectiveness of T, NK, CIK or NK-92 cells towards tumor-targeting antigens generated by genetic engineering of the immune cells. Herein, we summarize the promising development of ACT over the past decades in the fight against cancer.

Abstract

The promising development of adoptive immunotherapy over the last four decades has revealed numerous therapeutic approaches in which dedicated immune cells are modified and administered to eliminate malignant cells. Starting in the early 1980s, lymphokine activated killer (LAK) cells were the first ex vivo generated NK cell-enriched products utilized for adoptive immunotherapy. Over the past decades, various immunotherapies have been developed, including cytokine-induced killer (CIK) cells, as a peripheral blood mononuclear cells (PBMCs)-based therapeutic product, the adoptive transfer of specific T and NK cell products, and the NK cell line NK-92. In addition to allogeneic NK cells, NK-92 cell products represent a possible “off-the-shelf” therapeutic concept. Recent approaches have successfully enhanced the specificity and cytotoxicity of T, NK, CIK or NK-92 cells towards tumor-specific or associated target antigens generated by genetic engineering of the immune cells, e.g., to express a chimeric antigen receptor (CAR). Here, we will look into the history and recent developments of T and NK cell-based immunotherapy.

Current progress in cancer immunotherapy based on natural killer cells

One of the most common diseases in the present era is cancer. The common treatment methods used to control cancer include surgery, chemotherapy, and radiotherapy. Despite progress in the treatment of cancers, there still is no definite therapeutic approach. Among the currently proposed strategies, immunotherapy is a new approach that can provide better outcomes compared with existing therapies. Employing natural killer (NK) cells is one of the means of immunotherapy. As innate lymphocytes, NK cells are capable of rapidly responding to cancer cells without being sensitized or restricted to the cognate antigen in advance, as compared to T cells that are tumor antigen-specific. Latest insights into the biology of NK cells have clarified the underlying molecular mechanisms of NK cell maturation and differentiation, as well as controlling their effector functions through the investigation of the ligands and receptors engaged in recognizing cancer cells by NK cells. Elucidating the fact that NK cells recognize cancer cells could similarly show the mechanism through which cancer cells possibly avoid NK cell-dependent immune surveillance. Additionally, the expectations for novel immunotherapies by targeting NK cells have increased through the latest clinical outcomes of T-cell-targeted cancer immunotherapy. For this emerging method, researchers are still attempting to develop protocols for conferring the best proliferation and expansion medium, activation pathways, utilization dosage, transferring methods, as well as reducing possible side effects in cancer therapy. This study reviews the NK cells, their proliferation and expansion methods, and their recent applications in cancer immunotherapy.

Use of Cell and Genome Modification Technologies to Generate Improved "Off-the-Shelf" CAR T and CAR NK Cells

The broad success of adoptive immunotherapy to treat human cancer has resulted in a paradigm shift in modern medicine. Modification of autologous and allogenic immune cells with chimeric antigen receptors (CAR) designed to target specific antigens on tumor cells has led to production of CAR T and CAR NK cell therapies, which are ever more commonly introduced into cancer patient treatment protocols. While allogenic T cells may offer advantages such as improved anti-tumor activity, they also carry the risk of adverse reactions like graft-versus-host disease. This risk can be mitigated by use of autologous immune cells, however, the time needed for T and/or NK cell isolation, modification and expansion may be too long for some patients. Thus, there is an urgent need for strategies to robustly produce “off-the-shelf” CAR T and CAR NK cells, which could be used as a bridging therapy between cancer diagnosis or relapse and allogeneic transplantation. Advances in genome modification technologies have accelerated the generation of designer cell therapy products, including development of “off-the-shelf” CAR T cells for cancer immunotherapy. The feasibility and safety of such approaches is currently tested in clinical trials. This review will describe cell sources for CAR-based therapies, provide background of current genome editing techniques and the applicability of these approaches for generation of universal “off-the-shelf” CAR T and NK cell therapeutics.

Natural Born Killers: NK Cells in Cancer Therapy

Cellular therapy has emerged as an attractive option for the treatment of cancer, and adoptive transfer of chimeric antigen receptor (CAR) expressing T cells has gained FDA approval in hematologic malignancy. However, limited efficacy was observed using CAR-T therapy in solid tumors. Natural killer (NK) cells are crucial for tumor surveillance and exhibit potent killing capacity of aberrant cells in an antigen-independent manner. Adoptive transfer of unmodified allogeneic or autologous NK cells has shown limited clinical benefit due to factors including low cell number, low cytotoxicity and failure to migrate to tumor sites. To address these problems, immortalized and autologous NK cells have been genetically engineered to express high affinity receptors (CD16), CARs directed against surface proteins (PD-L1, CD19, Her2, etc.) and endogenous cytokines (IL-2 and IL-15) that are crucial for NK cell survival and cytotoxicity, with positive outcomes reported by several groups both preclinically and clinically. With a multitude of NK cell-based therapies currently in clinic trials, it is likely they will play a crucial role in next-generation cell therapy-based treatment. In this review, we will highlight the recent advances and limitations of allogeneic, autologous and genetically enhanced NK cells used in adoptive cell therapy.

New avenues for melanoma immunotherapy: Natural Killer cells?

Human solid malignant tumours may be particularly resistant to conventional therapies. Among solid tumours, immunological features of cutaneous melanoma have been well characterized in the past and today melanoma patients are routinely treated with the anti-immune checkpoints immunotherapy that has completely changed metastatic melanoma treatment and prognosis. Two cytotoxic cell populations may lead to the physical elimination of tumour cell targets: cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Tumour recognition by CTLs depends on major histocompatibility complex (MHC) class I molecules, while NK cells recognize tumours expressing low or null levels of MHC class I molecules. Despite this well-established complementarity, NK cells are still left behind in the optimization of innovative immunotherapy approaches. NK cells are members of innate lymphoid cells (ILCs) that play a critical role in early host defence against invading pathogens and transformed cells. Recent findings suggest that NK cell frequencies directly correlate with the overall survival of ipilimumab-treated melanoma patients. Furthermore, in vitro and in vivo evidences indicate that NK cells can selectively kill cancer stem cells, reducing tumour size and delaying metastatic progression. The aim of this review is to provide a survey of the evidences indicating NK cells as an excellent candidate to complement the newest solid tumour immunotherapy approaches.

Recent progress in and challenges in cellular therapy using NK cells for hematological malignancies

NK cells have killing activity against leukemic cells and solid cancer cells that escape from T cell recognition because of the low expression level of HLA class I molecules. This characteristic feature of NK cell recognition of target cells in contrast to T cells provides a strategy to overcome tolerance in cancer and leukemia patients. A strong alloreactive NK cell-mediated anti-leukemia effect can be induced in haploidentical hematopoietic stem cell transplantation. Also, NK cells can be expanded by several methods for adoptive immunotherapy for hematological malignancies and other malignant diseases. We review the historical role of NK cells and recent approaches to enhance the functions of NK cells, including ex vivo expansion of autologous and allogenic NK cells, checkpoint receptor blockade, and the use of memory-like NK cells and CAR-NK cells, for treatment of hematological malignancies.

The future of cellular immunotherapy for childhood leukemia

PURPOSE OF REVIEW

Exciting translational discoveries in recent years have brought realized promise of immunotherapy for children with high-risk leukemias. This review summarizes the current immunotherapeutic landscape with a focus on key clinical trials for patients with acute lymphoblastic leukemia or acute myeloid leukemia.

RECENT FINDINGS

Chemotherapy resistance remains a major barrier to cure in children with high-risk leukemias. Immunotherapy approaches have potential to overcome this resistance given alternative mechanisms of action. Based upon preclinical activity and/or success in adult patients, recent clinical trials have demonstrated safety and efficacy of various mAb, antibody-drug conjugate, bispecific T-cell-engaging antibody, natural killer cell, and chimeric antigen receptor-redirected T-cell immunotherapies for children with acute lymphoblastic leukemia or acute myeloid leukemia. Food and Drug Administration approval of several of these immunotherapies has increased the pediatric leukemia therapeutic portfolio and improved clinical outcomes for previously incurable patients.

SUMMARY

Several antibody-based or cellular immunotherapy modalities have demonstrated appreciable efficacy in children with relapsed or chemotherapy-refractory leukemia via early-phase clinical trials. Some studies have also identified critical biomarkers of treatment response and resistance that merit further investigation. Continued preclinical and clinical evaluation of novel immunotherapies is imperative to improve cure rates for children with high-risk leukemias.

Targeting T cell malignancies using CAR-based immunotherapy: challenges and potential solutions

Chimeric antigen receptor (CAR) T cell therapy has been successful in treating B cell malignancies in clinical trials; however, fewer studies have evaluated CAR T cell therapy for the treatment of T cell malignancies. There are many challenges in translating this therapy for T cell disease, including fratricide, T cell aplasia, and product contamination. To the best of our knowledge, no tumor-specific antigen has been identified with universal expression on cancerous T cells, hindering CAR T cell therapy for these malignancies. Numerous approaches have been assessed to address each of these challenges, such as (i) disrupting target antigen expression on CAR-modified T cells, (ii) targeting antigens with limited expression on T cells, and (iii) using third party donor cells that are either non-alloreactive or have been genome edited at the T cell receptor α constant (TRAC) locus. In this review, we discuss CAR approaches that have been explored both in preclinical and clinical studies targeting T cell antigens, as well as examine other potential strategies that can be used to successfully translate this therapy for T cell disease.

The Synergistic Use of IL-15 and IL-21 for the Generation of NK Cells From CD3/CD19-Depleted Grafts Improves Their ex vivo Expansion and Cytotoxic Potential Against Neuroblastoma: Perspective for Optimized Immunotherapy Post Haploidentical Stem Cell Transplantation

Neuroblastoma (NB) is the most common solid extracranial tumor in childhood. Despite therapeutic progress, prognosis in high-risk NB is poor and innovative therapies are urgently needed. Therefore, we addressed the potential cytotoxic capacity of interleukin (IL)-activated natural killer (NK) cells compared to cytokine-induced killer (CIK) cells for the treatment of NB. NK cells were isolated from peripheral blood mononuclear cells (PBMCs) by indirect CD56-enrichment or CD3/CD19-depletion and expanded with different cytokine combinations, such as IL-2, IL-15, and/or IL-21 under feeder-cell free conditions. CIK cells were generated from PBMCs by ex vivo stimulation with interferon-γ, IL-2, OKT-3, and IL-15. Comparative analysis of expansion rate, purity, phenotype and cytotoxicity was performed. CD56-enriched NK cells showed a median expansion rate of 4.3-fold with up to 99% NK cell content. The cell product after CD3/CD19-depletion consisted of a median 43.5% NK cells that expanded significantly faster reaching also 99% of NK cell purity. After 10–12 days of expansion, both NK cell preparations showed a significantly higher median cytotoxic capacity against NB cells relative to CIK cells. Remarkably, these NK cells were also capable of efficiently killing NB spheroidal 3D culture in long-term cytotoxicity assays. Further optimization using a novel NK cell culture medium and a prolonged culturing procedure after CD3/CD19-depletion for up to 15 days enhanced the expansion rate up to 24.4-fold by maintaining the cytotoxic potential. Addition of an IL-21 boost prior to harvesting significantly increased the cytotoxicity. The final cell product consisted for the major part of CD16⁻, NCR-expressing, poly-functional NK cells with regard to cytokine production, CD107a degranulation and antitumor capacity. In summary, our study revealed that NK cells have a significantly higher cytotoxic potential to combat NB than CIK cell products, especially following the synergistic use of IL-15 and IL-21 for NK cell activation. Therefore, the use of IL-15+IL-21 expanded NK cells generated from CD3/CD19-depleted apheresis products seems to be highly promising as an immunotherapy in combination with haploidentical stem cell transplantation (SCT) for high-risk NB patients.

Distinct Effects of Immunosuppressive Drugs on the Anti-Aspergillus Activity of Human Natural Killer Cells

As the prognosis of invasive aspergillosis remains unacceptably poor in patients undergoing hematopoietic stem cell transplantation (HSCT), there is a growing interest in the adoptive transfer of antifungal effector cells, such as Natural Killer (NK) cells. Because immunosuppressive agents are required in most HSCT recipients, knowledge of the impact of these compounds on the antifungal activity of NK cells is a prerequisite for clinical trials. We, therefore, assessed the effect of methylprednisolone (mPRED), cyclosporin A (CsA) and mycophenolic acid (MPA) at different concentrations on proliferation, apoptosis/necrosis, and the direct and indirect anti-Aspergillus activity of human NK cells. Methylprednisolone decreased proliferation and increased apoptosis of NK cells in a significant manner. After seven days, a reduction of viable NK cells was seen for all three immunosuppressants, which was significant for MPA only. Cyclosporin A significantly inhibited the direct hyphal damage by NK cells in a dose-dependent manner. None of the immunosuppressive compounds had a major impact on the measured levels of interferon-γ, granulocyte-macrophage colony-stimulating factor and RANTES (regulated on activation, normal T cell expressed and secreted; CCL5). Our data demonstrate that commonly used immunosuppressive compounds have distinct effects on proliferation, viability and antifungal activity of human NK cells, which should be considered in designing studies on the use of NK cells for adoptive antifungal immunotherapy.

Antigen-independent killer cells prepared for adoptive immunotherapy: One source, divergent protocols, diverse nomenclature

Adoptive cell therapy (ACT) using tumor antigen-independent killer cells has been widely used in clinical trials of cancer treatment. Circumventing the need for identification of a particular tumor-associated antigen on tumor cells, the approach has opened possibilities for the extension of ACT immunotherapy to patients with a wide variety of cancer types. Namely, Natural Killer (NK), Lymphokine-activated Killer (LAK) cells and Cytokine-induced killer (CIK) cells are the most commonly used cell types in antigen-independent adoptive immunotherapy of cancer. They all originate from peripheral blood mononuclear cells and share several common features in their killing mechanisms. However, despite broad application in clinical settings, the boundaries between these cell types are not very clearly defined. The current study aims to review different aspects of these cell populations in terms of phenotypical characteristic and preparation media, to clarify how the boundaries are set.

Adoptive cellular immunotherapy for refractory childhood cancers: a single center experience

Prognosis of refractory childhood cancers despite multimodal treatment strategies remains poor. Here, we report a single center experience encountered in 18 patients with refractory solid malignancies treated with adoptive cellular immunotherapy (ACI) from haploidentical or matched donors following hematopoietic stem cell transplantation. While seven patients were in partial and six in complete remission (CR), five patients suffered from relapsed diseases at the time of ACI. 1.5-year probabilities of overall survival (OS) and progression-free survival (PFS) were 19.5% and 16.1% for all patients. Patients in CR showed estimated 1.5-year OS and PFS of 50.1% and 42.7%, respectively. CR was induced or rather sustained in ten children, with two still being alive 9.6 and 9.3 years after ACI. Naïve, central and effector memory T-cells correlated with responses. However, the majority of patients relapsed. Cumulative incidence of relapse was 79.8% at 1.5 years. Acute graft versus host disease (aGVHD) occurred in nine of 18 patients (50%) with aGVHD grade I–II observed in six (33%) and aGVHD grade III seen in three (17%) patients, manageable in all cases.

Altogether, study results indicate that donor-derived ACI at its current state offers palliation but no clear curative benefit for refractory childhood cancers and warrants further improvement.

NK Cell-Based Immunotherapy for Hematological Malignancies

Natural killer (NK) lymphocytes are an integral component of the innate immune system and represent important effector cells in cancer immunotherapy, particularly in the control of hematological malignancies. Refined knowledge of NK cellular and molecular biology has fueled the interest in NK cell-based antitumor therapies, and recent efforts have been made to exploit the high potential of these cells in clinical practice. Infusion of high numbers of mature NK cells through the novel graft manipulation based on the selective depletion of T cells and CD19+ B cells has resulted into an improved outcome in children with acute leukemia given human leucocyte antigen (HLA)-haploidentical hematopoietic transplantation. Likewise, adoptive transfer of purified third-party NK cells showed promising results in patients with myeloid malignancies. Strategies based on the use of cytokines or monoclonal antibodies able to induce and optimize NK cell activation, persistence, and expansion also represent a novel field of investigation with remarkable perspectives of favorably impacting on outcome of patients with hematological neoplasia. In addition, preliminary results suggest that engineering of mature NK cells through chimeric antigen receptor (CAR) constructs deserve further investigation, with the goal of obtaining an "off-the-shelf" NK cell bank that may serve many different recipients for granting an efficient antileukemia activity.

Natural Killer Cell Therapy: A New Treatment Paradigm for Solid Tumors

In treatments of solid tumors, adoptive transfer of ex vivo expanded natural killer (NK) cells has dawned as a new paradigm. Compared with cytotoxic T lymphocytes, NK cells take a unique position targeting tumor cells that evade the host immune surveillance by down-regulating self-antigen presentation. Recent findings highlighted that NK cells can even target cancer stem cells. The efficacy of allogeneic NK cells has been widely investigated in the treatment of hematologic malignancies. In solid tumors, both autologous and allogeneic NK cells have demonstrated potential efficacy. In allogeneic NK cell therapy, the mismatch between the killer cell immunoglobulin-like receptor (KIR) and human leukocyte antigen (HLA) can be harnessed to increase the antitumor activity. However, the allogeneic NK cells cause more adverse events and can be rejected by the host immune system after repeated injections. In this regard, the autologous NK cell therapy is safer. This article reviews the published results of clinical trials and discusses strategies to enhance the efficacy of the NK cell therapy. The difference in immunophenotype of the ex vivo expanded NK cells resulted from different culture methods may affect the final efficacy. Furthermore, currently available standard anticancer therapy, molecularly targeted agents, and checkpoint inhibitors may directly or indirectly enhance the efficacy of NK cell therapy. A recent study discovered that NK cell specific genetic defects are closely associated with the tumor immune microenvironment that determines clinical outcomes. This finding warrants future investigations to find the implication of NK cell specific genetic defects in cancer development and treatment, and NK cell deficiency syndrome should be revisited to enhance our understanding. Overall, it is clear that NK cell therapy is safe and promises a new paradigm for the treatment of solid tumors.

The Role of Natural Killer Cells as a Platform for Immunotherapy in Pediatric Cancers

PURPOSE OF REVIEW

We aim to review the most recent findings in the use of NK cells in childhood cancers.

RECENT FINDINGS

Natural killer cells are cytotoxic to tumor cells. In pediatric leukemias, adoptive transfer of NK cells can bridge children not in remission to transplant. Interleukins (IL2, IL15) can enhance NK cell function. NK cell-CAR therapy has advantages of shorter life span that lessens chronic toxicities, lower risk of graft versus host disease when using allogeneic cells, ability of NK cells to recognize tumor cells that have downregulated MHC to escape T cells, and possibly less likelihood of cytokine storm. Cytotoxicity to solid tumors (rhabdomyosarcoma, Ewing's sarcoma, neuroblastoma) is seen with graft versus tumor effect in transplant and in combination with antibodies. Challenges lie in the microenvironment which is suppressive for NK cells. NK cell immunotherapy in childhood cancers is promising and recent works aim to overcome challenges.

Interferon β and Anti-PD-1/PD-L1 Checkpoint Blockade Cooperate in NK Cell-Mediated Killing of Nasopharyngeal Carcinoma Cells

Nasopharyngeal carcinoma (NPC) is a highly malignant epithelial cancer linked to EBV infection. Addition of interferon-β (IFNβ) to chemo- and radiochemotherapy has led to survival rates >90% in children and adolescents. As NPC cells are sensitive to apoptosis via tumor necrosis factor-related apoptosis inducing ligand (TRAIL), we explored the role of TRAIL and IFNβ in the killing of NPC cells by natural killer (NK) cells. NPC cells, including cells of a patient-derived xenograft were exposed to NK cells in the presence or absence of IFNβ. NK cells killed NPC- but not nasoepithelial cells and killing was predominately mediated via TRAIL. Incubation of NK cells with IFNβ increased cytotoxicity against NPC cells. Concomitant incubation of NK- and NPC cells with IFNβ before coculture reduced cytotoxicity and could be overcome by blocking the PD-1/PD-L1 axis leading to the release of intracellular TRAIL from NK cells. In conclusion, combination of IFNβ and anti-PD-1, augmenting cytotoxicity of NK cells against NPC cells, could be a strategy to improve NPC-directed therapy and warrants further evaluation in vivo.

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.

Improving Clinical Manufacturing of IL-15 Activated Cytokine-Induced Killer (CIK) Cells

Cytokine-induced killer (CIK) cells are an immunotherapeutic approach to combat relapse following allogeneic hematopoietic stem cell transplantation (HSCT) in acute leukemia or myelodysplastic syndrome (MDS) patients. Prompt and sequential administration of escalating cell doses improves the efficacy of CIK cell therapy without exacerbating graft vs. host disease (GVHD). This study addresses manufacturing-related issues and aimed to develop a time-, personal- and cost-saving good manufacturing process (GMP)-compliant protocol for the generation of ready-for-use therapeutic CIK cell doses starting from one unstimulated donor-derived peripheral blood (PB) or leukocytapheresis (LP) products. Culture medium with or without the addition of either AB serum, fresh frozen plasma (FFP) or platelet lysate (PL) was used for culture. Fresh and cryopreserved CIK cells were compared regarding expansion rate, viability, phenotype, and ability to inhibit leukemia growth. Cell numbers increased by a median factor of 10-fold in the presence of FFP, PL, or AB serum, whereas cultivation in FFP/PL-free or AB serum-free medium failed to promote adequate CIK cell proliferation (p < 0.01) needed to provide clinical doses of 1 × 10⁶ T cells/kG, 5 × 10⁶ T cells/kG, 1 × 10⁷ T cells/kG, and 1 × 10⁸ T cells/kG recipient body weight. CIK cells consisting of T cells, T- natural killer (T-NK) cells and a minor fraction of NK cells were not significantly modified by different medium supplements. Moreover, neither cytotoxic potential against leukemic THP-1 cells nor cell activation shown by CD25 expression were significantly influenced. Moreover, overnight and long-term cryopreservation had no significant effect on the composition of CIK cells, their phenotype or cytotoxic potential. A viability of almost 93% (range: 89–96) and 89.3% (range: 84–94) was obtained after freeze-thawing procedure and long-term storage, respectively, whereas viability was 96% (range: 90-97) in fresh CIK cells. Altogether, GMP-complaint CIK cell generation from an unstimulated donor-derived PB or LP products was feasible. Introducing FFP, which is easily accessible, into CIK cell cultures was time- and cost-saving without loss of viability and potency in a 10-12 day batch culture. The feasibility of cryopreservation enabled storage and delivery of sequential highly effective ready-for-use CIK cell doses and therefore reduced the number of manufacturing cycles.

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.

Human NK cells adapt their immune response towards increasing multiplicities of infection of Aspergillus fumigatus

BACKGROUND

The saprophytic fungus Aspergillus fumigatus reproduces by generation of conidia, which are spread by airflow throughout nature. Since humans are inhaling certain amounts of spores every day, the (innate) immune system is constantly challenged. Even though macrophages and neutrophils carry the main burden, also NK cells are regarded to contribute to the antifungal immune response. While NK cells reveal a low frequency, expression and release of immunomodulatory molecules seem to be a natural way of their involvement.

RESULTS

In this study we show, that NK cells secrete chemokines such as CCL3/MIP-1α, CCL4/MIP-1β and CCL5/RANTES early on after stimulation with Aspergillus fumigatus and, in addition, adjust the concentration of chemokines released to the multiplicity of infection of Aspergillus fumigatus.

CONCLUSIONS

These results further corroborate the relevance of NK cells within the antifungal immune response, which is regarded to be more and more important in the development and outcome of invasive aspergillosis in immunocompromised patients after hematopoietic stem cell transplantation. Additionally, the correlation between the multiplicity of infection and the expression and release of chemokines shown here may be useful in further studies for the quantification and/or surveillance of the NK cell involvement in antifungal immune responses.

Natural Killer Cell-Based Cancer Immunotherapy: A Review on 10 Years Completed Clinical Trials

NK cell cancer immunotherapy is an emerging anti-tumour therapeutic strategy that explores NK cell stimulation. In this review, we address strategies developed to circumvent limitations to clinical application of NK cell-based therapies, and comprehensively review the design and results of clinical trials conducted in the past 10 years (2008-2018) to test their therapeutic potential. NK cell-based immunotherapy of solid cancers remains controversial, but merit further detailed investigation.

Super-charged NK cells inhibit growth and progression of stem-like/poorly differentiated oral tumors in vivo in humanized BLT mice; effect on tumor differentiation and response to chemotherapeutic drugs

Therapeutic role of NK cells in solid tumors was challenged previously even though their role in hematological malignancies has clearly been established. Furthermore, functions and numbers of NK cells are greatly suppressed in oral cancer patients necessitating effective future NK immunotherapeutic strategies to aid in the control of disease. The humanized-BLT (hu-BLT) mice were used to implant stem-like/undifferentiated oral tumors to study the role of super-charged NK cells with and without feeding with AJ2 probiotic bacteria. Implanted CSC/undifferentiated tumors resected from NK-injected mice exhibited differentiated phenotype, grew slowly, and did not cause weight loss, whereas those from tumor-bearing mice without NK-injection remained relatively more stem-like/poorly-differentiated, grew faster, and caused significant weight loss. Moreover, in vitro NK-differentiated tumors were sensitive to chemotherapeutic drugs, and when implanted in the oral-cavity grew no or very small tumors in mice. When NK-mediated differentiation of tumors was blocked by IFN-γ and TNF-α antibodies before implantation, tumors grew rapidly, remained stem-like/poorly-differentiated and became resistant to chemotherapeutic drugs. Loss of NK cytotoxicity and decreased IFN-γ secretion in tumor-bearing mice in PBMCs, splenocytes, bone marrow derived immune cells and enriched NK cells was restored by the injection of super-charged NK cells with or without feeding with AJ2. Much greater infiltration of CD45⁺ and T cells were observed in tumors resected from the mice, along with the restored secretion of IFN-γ from purified T cells from splenocytes in NK-injected tumor-bearing mice fed with AJ2 probiotic bacteria. Thus, super-charged NK cells prevent tumor growth by restoring effector function resulting in differentiation of CSCs/undifferentiated-tumors in hu-BLT mice.

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.

KIR downregulation by IL-12/15/18 unleashes human NK cells from KIR/HLA-I inhibition and enhances killing of tumor cells

To exploit autologous NK cells for cancer immunotherapy, it is highly relevant to circumvent killer cell immunoglobulin-like receptor (KIR)-mediated self-inhibition of human NK cells by HLA-I-expressing tumor cells. Here, we show that stimulation of NK cells with IL-12/15/18 for two days led to downregulation of surface expression of the inhibitory KIR2DL2/L3, KIR2DL1 and KIR3DL1 receptors on peripheral blood NK cells. Downregulation of KIR expression was attributed to decreased KIR mRNA levels which could be re-induced already 3 days after re-culture in IL-2. Reduced KIR2DL2/L3 expression on IL-12/15/18-activated NK cells resulted in less inhibition upon antibody-mediated KIR engagement and increased CD16-dependent cytotoxicity in redirected lysis assays. Most importantly, downregulated KIR2DL2/L3 expression enabled enhanced cytotoxicity of IL-12/15/18-stimulated NK cells against tumor cells expressing cognate HLA-I molecules. NK cells pre-activated with IL-12/15/18 were previously shown to exert potent anti-tumor activity and memory-like long-lived functionality, mediating remission in a subset of acute myeloid leukemia (AML) patients in a clinical trial. Our study reveals a novel mechanism of IL-12/15/18 in improving the cytotoxicity of NK cells by reducing their sensitivity to inhibition by self-HLA-I due to decreased KIR expression, highlighting the potency of IL-12/15/18-activated NK cells for anti-tumor immunotherapy protocols.

Chimeric antigen receptor-engineered natural killer and natural killer T cells for cancer immunotherapy

Natural killer (NK) cells of the innate immune system and natural killer T (NKT) cells, which have roles in both the innate and adaptive responses, are unique lymphocyte subsets that have similarities in their functions and phenotypes. Both cell types can rapidly respond to the presence of tumor cells and participate in immune surveillance and antitumor immune responses. This has incited interest in the development of novel cancer therapeutics based on NK and NKT cell manipulation. Chimeric antigen receptors (CARs), generated through the fusion of an antigen-binding region of a monoclonal antibody or other ligand to intracellular signaling domains, can enhance lymphocyte targeting and activation toward diverse malignancies. Most of the CAR studies have focused on their expression in T cells; however, the functional heterogeneity of CAR T cells limits their therapeutic potential and is associated with toxicity. CAR-modified NK and NKT cells are becoming more prevalent because they provide a method to direct these cells more specifically to target cancer cells, with less risk of adverse effects. This review will outline current NK and NKT cell CAR constructs and how they compare to conventional CAR T cells, and discuss future modifications that can be explored to advance adoptive cell transfer of NK and NKT cells.

Recombinant Antibodies to Arm Cytotoxic Lymphocytes in Cancer Immunotherapy

Immunotherapy has the potential to support and expand the body's own armamentarium of immune effector functions, which have been circumvented during malignant transformation and establishment of cancer and is presently considered to be the most promising treatment option for cancer patients. Recombinant antibody technologies have led to a multitude of novel antibody formats, which are in clinical development and hold great promise for future therapies. Among these formats, bispecific antibodies are extremely versatile due to their high efficacy to recruit and activate anti-tumoral immune effector cells, their excellent safety profile, and the opportunity for use in combination with cellular therapies. This review article summarizes the latest developments in cancer immunotherapy using immuno-engagers for recruiting T cells and NK cells to the tumor site. In addition to antibody formats, malignant cell targets, and immune cell targets, opportunities for combination therapies, including check point inhibitors, cytokines and adoptive transfer of immune cells, will be summarized and discussed.

A Two-Phase Expansion Protocol Combining Interleukin (IL)-15 and IL-21 Improves Natural Killer Cell Proliferation and Cytotoxicity against Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue malignancy in children. Despite intensive research in recent decades the prognosis for patients with metastatic or relapsed diseases has hardly improved. New therapeutic concepts in anti-tumor therapy aim to modulate the patient’s immune system to increase its aggressiveness or targeted effects toward tumor cells. Besides surgery, radiotherapy and chemotherapy, immune activation by direct application of cytokines, antibodies or adoptive cell therapy are promising approaches. In the last years, adoptive transfer of natural killer (NK) cells came into the focus of translational medicine, because of their high cytotoxic potential against transformed malignant cells. A main challenge of NK cell therapy is that it requires a high amount of functional NK cells. Therefore, ex vivo NK cell expansion protocols are currently being developed. Many culturing strategies are based on the addition of feeder or accessory cells, which need to be removed prior to the clinical application of the final NK cell product. In this study, we addressed feeder cell-free expansion methods using common γ-chain cytokines, especially IL-15 and IL-21. Our results demonstrated high potential of IL-15 for NK cell expansion, while IL-21 triggered NK cell maturation and functionality. Hence, we established a two-phase expansion protocol with IL-15 to induce an early NK cell expansion, followed by short exposure to IL-21 that boosted the cytotoxic activity of NK cells against RMS cells. Further functional analyses revealed enhanced degranulation and secretion of pro-inflammatory cytokines such as interferon-γ and tumor necrosis factor-α. In a proof of concept in vivo study, we also observed a therapeutic effect of adoptively transferred IL-15 expanded and IL-21 boosted NK cells in combination with image guided high precision radiation therapy using a luciferase-transduced RMS xenograft model. In summary, this two-phased feeder cell-free ex vivo culturing protocol combined efficient expansion and high cytolytic functionality of NK cells for treatment of radiation-resistant RMS.

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.

Shaping of Natural Killer Cell Antitumor Activity by Ex Vivo Cultivation

Natural killer (NK) cells are a promising tool for the use in adoptive immunotherapy, since they efficiently recognize and kill tumor cells. In this context, ex vivo cultivation is an attractive option to increase NK cells in numbers and to improve their antitumor potential prior to clinical applications. Consequently, various strategies to generate NK cells for adoptive immunotherapy have been developed. Here, we give an overview of different NK cell cultivation approaches and their impact on shaping the NK cell antitumor activity. So far, the cytokines interleukin (IL)-2, IL-12, IL-15, IL-18, and IL-21 are used to culture and expand NK cells. The selection of the respective cytokine combination is an important factor that directly affects NK cell maturation, proliferation, survival, distribution of NK cell subpopulations, activation, and function in terms of cytokine production and cytotoxic potential. Importantly, cytokines can upregulate the expression of certain activating receptors on NK cells, thereby increasing their responsiveness against tumor cells that express the corresponding ligands. Apart from using cytokines, cocultivation with autologous accessory non-NK cells or addition of growth-inactivated feeder cells are approaches for NK cell cultivation with pronounced effects on NK cell activation and expansion. Furthermore, ex vivo cultivation was reported to prime NK cells for the killing of tumor cells that were previously resistant to NK cell attack. In general, NK cells become frequently dysfunctional in cancer patients, for instance, by downregulation of NK cell activating receptors, disabling them in their antitumor response. In such scenario, ex vivo cultivation can be helpful to arm NK cells with enhanced antitumor properties to overcome immunosuppression. In this review, we summarize the current knowledge on NK cell modulation by different ex vivo cultivation strategies focused on increasing NK cytotoxicity for clinical application in malignant diseases. Moreover, we critically discuss the technical and regulatory aspects and challenges underlying NK cell based therapeutic approaches in the clinics.

Role of Natural Killer Cells in HIV-Associated Malignancies

Now in its fourth decade, the burden of HIV disease still persists, despite significant milestone achievements in HIV prevention, diagnosis, treatment, care, and support. Even with long-term use of currently available antiretroviral therapies (ARTs), eradication of HIV remains elusive and now poses a unique set of challenges for the HIV-infected individual. The occurrence of HIV-associated non-AIDS-related comorbidities outside the scope of AIDS-defining illnesses, in particular non-AIDS-defining cancers, is much greater than the age-matched uninfected population. The underlying mechanism is now recognized in part to be related to the immune dysregulated and inflammatory status characteristic of HIV infection that persists despite ART. Natural killer (NK) cells are multifunctional effector immune cells that play a critical role in shaping the innate immune responses to viral infections and cancer. NK cells can modulate the adaptive immune response via their role in dendritic cell (DC) maturation, removal of immature tolerogenic DCs, and their ability to produce immunoregulatory cytokines. NK cells are therefore poised as attractive therapeutic targets that can be harnessed to control or clear both HIV and HIV-associated malignancies. To date, features of the tumor microenvironment and the evolution of NK-cell function among individuals with HIV-related malignancies remain unclear and may be distinct from malignancies observed in uninfected persons. This review intends to uncouple anti-HIV and antitumor NK-cell features that can be manipulated to halt the evolution of HIV disease and HIV-associated malignancies and serve as potential preventative and curative immunotherapeutic options.

Cytotoxic potential of IL-15-activated cytokine-induced killer cells against human neuroblastoma cells

BACKGROUND

Neuroblastoma (NB) is the most common solid extracranial tumor in childhood. Despite advances in therapy, the prognosis is poor and optimized therapies are urgently needed. Therefore, we investigated the antitumor potential of interleukin-15 (IL-15)-activated cytokine-induced killer (CIK) cells against different NB cell lines.

PROCEDURE

CIK cells were generated from peripheral blood mononuclear cells by the stimulation with interferon-γ (IFN-γ), IL-2, OKT-3 and IL-15 over a period of 10-12 days. The cytotoxic activity against NB cells was analyzed by nonradioactive Europium release assay before and after blocking of different receptor-ligand interactions relevant in CIK cell-mediated cytotoxicity.

RESULTS

The final CIK cell products consisted in median of 83% (range: 75.9-91.9%) CD3⁺ CD56^- T cells, 14% (range: 5.2-20.7%) CD3⁺ CD56⁺ NK-like T cells and 2% (range: 0.9-4.8%) CD3^- CD56⁺ NK cells. CIK cells expanded significantly upon ex vivo stimulation with median rates of 22.3-fold for T cells, 58.3-fold for NK-like T cells and 2.5-fold for NK cells. Interestingly, CD25 surface expression increased from less than equal to 1% up to median 79.7%. Cytotoxic activity of CIK cells against NB cells was in median 34.7, 25.9 and 34.8% against the cell lines UKF-NB-3, UKF-NB-4 and SK-N-SH, respectively. In comparison with IL-2-stimulated NK cells, CIK cells showed a significantly higher cytotoxicity. Antibody-mediated blocking of the receptors NKG2D, TRAIL, FasL, DNAM-1, NKp30 and lymphocyte function-associated antigen-1 (LFA-1) significantly reduced lytic activity, indicating that diverse cytotoxic mechanisms might be involved in CIK cell-mediated NB killing.

CONCLUSIONS

Unlike the mechanism reported in other malignancies, NKG2D-mediated cytotoxicity does not constitute the major killing mechanism of CIK cells against NB.

Immunological and Translational Aspects of NK Cell-Based Antitumor Immunotherapies

Natural killer (NK) cells play a pivotal role in the first line of defense against cancer. NK cells that are deficient in CD3 and a clonal T cell receptor (TCR) can be subdivided into two major subtypes, CD56^dimCD16⁺ cytotoxic and CD56^brightCD16⁻ immunoregulatory NK cells. Cytotoxic NK cells not only directly kill tumor cells without previous stimulation by cytotoxic effector molecules, such as perforin and granzymes or via death receptor interactions, but also act as regulatory cells for the immune system by secreting cytokines and chemokines. The aim of this review is to highlight therapeutic strategies utilizing autologous and allogenic NK cells, combinations of NK cells with monoclonal antibodies to induce antibody-dependent cellular cytotoxicity, or immune checkpoint inhibitors. Additionally, we discuss the use of chimeric antigen receptor-engineered NK cells in cancer immunotherapy.

Natural Killer Cells for Therapy of Leukemia

Clinical application of natural killer (NK) cells against leukemia is an area of intense investigation. In human leukocyte antigen-mismatched allogeneic hematopoietic stem cell transplantations (HSCT), alloreactive NK cells exert powerful anti-leukemic activity in preventing relapse in the absence of graft-versus-host disease, particularly in acute myeloid leukemia patients. Adoptive transfer of donor NK cells post-HSCT or in non-transplant scenarios may be superior to the currently widely used unmanipulated donor lymphocyte infusion. This concept could be further improved through transfusion of activated NK cells. Significant progress has been made in good manufacturing practice (GMP)-compliant large-scale production of stimulated effectors. However, inherent limitations remain. These include differing yields and compositions of the end-product due to donor variability and inefficient means for cryopreservation. Moreover, the impact of the various novel activation strategies on NK cell biology and in vivo behavior are barely understood. In contrast, reproduction of the third-party NK-92 drug from a cryostored GMP-compliant master cell bank is straightforward and efficient. Safety for the application of this highly cytotoxic cell line was demonstrated in first clinical trials. This novel 'off-the-shelf' product could become a treatment option for a broad patient population. For specific tumor targeting chimeric-antigen-receptor-engineered NK-92 cells have been designed.

Selection and expansion of natural killer cells for NK cell-based immunotherapy

Natural killer (NK) cells have been used in several clinical trials as adaptive immunotherapy. The low numbers of these cells in peripheral blood mononuclear cells (PBMC) have resulted in various approaches to preferentially expand primary NK cells from PBMC. While some clinical trials have used the addition of interleukin 2 (IL-2) to co-stimulate the expansion of purified NK cells from allogeneic donors, recent studies have shown promising results in achieving in vitro expansion of NK cells to large numbers for adoptive immunotherapy. NK cell expansion requires multiple cell signals for survival, proliferation and activation. Thus, expansion strategies have been focused either to substitute these factors using autologous feeder cells or to use genetically modified allogeneic feeder cells. Recent developments in the clinical use of genetically modified NK cell lines with chimeric antigen receptors, the development of expansion protocols for the clinical use of NK cell from human embryonic stem cells and induced pluripotent stem cells are challenging improvements for NK cell-based immunotherapy. Transfer of several of these protocols to clinical-grade production of NK cells necessitates adaptation of good manufacturing practice conditions, and the development of freezing conditions to establish NK cell stocks will require some effort and, however, should enhance the therapeutic options of NK cells in clinical medicine.

Natural Killer Cell-Based Therapies Targeting Cancer: Possible Strategies to Gain and Sustain Anti-Tumor Activity

Natural killer (NK) cells were discovered 40 years ago, by their ability to recognize and kill tumor cells without the requirement of prior antigen exposure. Since then, NK cells have been seen as promising agents for cell-based cancer therapies. However, NK cells represent only a minor fraction of the human lymphocyte population. Their skewed phenotype and impaired functionality during cancer progression necessitates the development of clinical protocols to activate and expand to high numbers ex vivo to be able to infuse sufficient numbers of functional NK cells to the cancer patients. Initial NK cell-based clinical trials suggested that NK cell-infusion is safe and feasible with almost no NK cell-related toxicity, including graft-versus-host disease. Complete remission and increased disease-free survival is shown in a small number of patients with hematological malignances. Furthermore, successful adoptive NK cell-based therapies from haploidentical donors have been demonstrated. Disappointingly, only limited anti-tumor effects have been demonstrated following NK cell infusion in patients with solid tumors. While NK cells have great potential in targeting tumor cells, the efficiency of NK cell functions in the tumor microenvironment is yet unclear. The failure of immune surveillance may in part be due to sustained immunological pressure on tumor cells resulting in the development of tumor escape variants that are invisible to the immune system. Alternatively, this could be due to the complex network of immune-suppressive compartments in the tumor microenvironment, including myeloid-derived suppressor cells, tumor-associated macrophages, and regulatory T cells. Although the negative effect of the tumor microenvironment on NK cells can be transiently reverted by ex vivo expansion and long-term activation, the aforementioned NK cell/tumor microenvironment interactions upon reinfusion are not fully elucidated. Within this context, genetic modification of NK cells may provide new possibilities for developing effective cancer immunotherapies by improving NK cell responses and making them less susceptible to the tumor microenvironment. Within this review, we will discuss clinical trials using NK cells with a specific reflection on novel potential strategies, such as genetic modification of NK cells and complementary therapies aimed at improving the clinical outcome of NK cell-based immune therapies.

Advances in clinical NK cell studies: Donor selection, manufacturing and quality control

Natural killer (NK) cells are increasingly used in clinical studies in order to treat patients with various malignancies. The following review summarizes platform lectures and 2013–2015 consortium meetings on manufacturing and clinical use of NK cells in Europe and United States. A broad overview of recent pre-clinical and clinical results in NK cell therapies is provided based on unstimulated, cytokine-activated, as well as genetically engineered NK cells using chimeric antigen receptors (CAR). Differences in donor selection, manufacturing and quality control of NK cells for cancer immunotherapies are described and basic recommendations are outlined for harmonization in future NK cell studies.

Adoptive Immunotherapies After Allogeneic Hematopoietic Stem Cell Transplantation in Patients With Hematologic Malignancies

Hematopoietic stem cell transplantation (HSCT) is the only curative therapy for patients with chemotherapy-resistant hematologic malignancies that are usually fatal in absence of treatment. Hematopoietic stem cell transplantation is associated with significant early and late morbidity and mortality. Graft-versus-host disease, infections, and relapse are the most important causes of mortality after HSCT. Until now, these complications have been managed mainly with pharmacological drugs, but in some situations, this approach clearly shows its limit. As such, there is a significant need for novel therapies for the treatment of complications after allogeneic HSCT. In this review, the currently available adoptive immunotherapies offering an alternative in case of treatment failure of HSCT complications will be described. The results of the main clinical trials based on immune cell infusion will be discussed and the strategies aiming at maximizing cytotoxic T-lymphocyte, regulatory T-cell, natural killer cell, cytokine-induced killer cell, and γδ T-cell efficacies in the context of immunotherapy approaches after allogeneic HSCT in patients with hematologic malignancies will be gathered.

Two-Stage Priming of Allogeneic Natural Killer Cells for the Treatment of Patients with Acute Myeloid Leukemia: A Phase I Trial

Human Natural Killer (NK) cells require at least two signals to trigger tumor cell lysis. Absence of ligands providing either signal 1 or 2 provides NK resistance. We manufactured a lysate of a tumour cell line which provides signal 1 to resting NK cells without signal 2. The tumor-primed NK cells (TpNK) lyse NK resistant Acute Myeloid Leukemia (AML) blasts expressing signal 2 ligands. We conducted a clinical trial to determine the toxicity of TpNK cell infusions from haploidentical donors. 15 patients with high risk AML were screened, 13 enrolled and 7 patients treated. The remaining 6 either failed to respond to re-induction chemotherapy or the donor refused to undergo peripheral blood apheresis. The conditioning consisted of fludarabine and total body irradiation. This was the first UK trial of a cell therapy regulated as a medicine. The complexity of Good Clinical Practice compliance was underestimated and led to failures requiring retrospective independent data review. The lessons learned are an important aspect of this report. There was no evidence of infusional toxicity. Profound myelosuppression was seen in the majority (median neutrophil recovery day 55). At six months follow-up, three patients treated in Complete Remission (CR) remained in remission, one patient infused in Partial Remission had achieved CR1, two had relapsed and one had died. One year post-treatment one patient remained in CR. Four patients remained in CR after treatment for longer than their most recent previous CR. During the 2 year follow-up six of seven patients died; median overall survival was 400 days post infusion (range 141–910). This is the first clinical trial of an NK therapy in the absence of IL-2 or other cytokine support. The HLA-mismatched NK cells survived and expanded in vivo without on-going host immunosuppression and appeared to exert an anti-leukemia effect in 4/7 patients treated.

Natural Killer Cell Immunotherapy: From Bench to Bedside

The potential of natural killer (NK) cells to target numerous malignancies in vitro has been well documented; however, only limited success has been seen in the clinic. Although NK cells prove non-toxic and safe regardless of the cell numbers injected, there is often little persistence and expansion observed in a patient, which is vital for mounting an effective cellular response. NK cells can be isolated directly from peripheral blood, umbilical cord blood, or bone marrow, expanded in vitro using cytokines or differentiated in vitro from hematopoietic stem cells. Drugs that support NK cell function such as lenalidomide and bortezomib have also been studied in the clinic, however, the optimum combination, which can vary among different malignancies, is yet to be identified. NK cell proliferation, persistence, and function can further be improved by various activation techniques such as priming and cytokine addition though whether stimulation pre- or post-injection is more favorable is another obstacle to be tackled. Here, we review the various methods of obtaining and activating NK cells for use in the clinic while considering the ideal product and drug complement for the most successful cellular therapy.