Analysis of NK cell clones obtained using interleukin-2 and gene-modified K562 cells revealed the ability of "senescent" NK cells to lose CD57 expression and start expressing NKG2A

Obtaining Gene-Modified HLA-E-Expressing Feeder Cells for Stimulation of Natural Killer Cells

Human cytomegalovirus (HCMV)-specific adaptive NK cells are capable of recognizing viral peptides presented by HLA-E on infected cells via the NKG2C receptor. Using retroviral transduction, we have generated a K562-cell-based line expressing HLA-E in the presence of the HLA-E-stabilizing peptide, which has previously shown the capacity to enhance adaptive NK cell response. The obtained K562-21E cell line was employed to investigate proliferative responses of the CD57- NK cell subset of HCMV-seropositive and seronegative donors. Stimulation of CD57- NK cells with K562-21E/peptide resulted in an increased cell expansion during the 12-day culturing period, regardless of the serological HCMV status of the donor. The enhanced proliferation in response to the peptide was associated with a greater proportion of CD56brightHLA-DR+ NK cells. In later stages of cultivation, the greatest proliferative response to K562-21E/peptide was shown for a highly HCMV-seropositive donor. These expanded NK cells were characterized by the accumulation of CD57-KIR2DL2/3+NKG2C+NKG2A- cells, which are hypothesized to represent adaptive NK cell progenitors. The K562-21E feeder cells can be applied both for the accumulation of NK cells as therapeutic effectors, and for the study of NK cell maturation into the adaptive state after the HLA-E peptide presentation.

[Interferon-α mediating the functional damage of CD56dimCD57+natural killer cells in peripheral blood of systemic lupus erythematosuss]

OBJECTIVE

To investigate the regulatory effect of interferon-α (IFN-α) on the apoptosis and killing function of CD56dimCD57+ natural killer (NK) cells in systemic lupus erythematosus (SLE) patients, and to explore the specific mechanism.

METHODS

A total of sixty-four newly treated SLE patients and sixteen healthy controls (HC) enrolled in the Second Hospital of Dalian Medical University were selected as the research subjects. And the gene expression levels of molecules related to NK cell-killing function were detected by real-time quantitative polymerase chain reaction. CD56dimCD57+ NK cells were co-cultured with the K562 cells, and the apoptotic K562 cells were labeled with Annexin-Ⅴ and 7-amino-actinomycin D. Peripheral blood mononuclear cells were treated with 20, 40, and 80 μmol/L hydrogen peroxide (H2O2), and treated without H2O2 as control, the expression level of perforin (PRF) was detected by flow cytometry. The concentration of IFN-α in serum was determined by enzyme linked immunosorbent assay. The expression levels of IFN-α receptors (IFNAR) on the surface of CD56dimCD57+ NK cells were detected by flow cytometry, and were represented by mean fluorescence intensity (MFI). CD56dimCD57+ NK cells were treated with 1 000 U/mL IFN-α for 24, 48 and 72 h, and no IFN-α treatment was used as the control, the apoptosis and the expression levels of mitochondrial reactive oxygen species (mtROS) were measured by flow cytometry and represented by MFI.

RESULTS

Compared with HC(n=3), the expression levels of PRF1 gene in peripheral blood NK cells of the SLE patients (n=3) were decreased (1.24±0.41 vs. 0.57±0.12, P=0.05). Compared with HC(n=5), the ability of peripheral blood CD56dimCD57+ NK cells in the SLE patients (n=5) to kill K562 cells was significantly decreased (58.61%±10.60% vs. 36.74%±6.27%, P < 0.01). Compared with the control (n=5, 97.51%±1.67%), different concentrations of H2O2 treatment significantly down-regulated the PRF expression levels of CD56dimCD57+ NK cells in a dose-dependent manner, the 20 μmol/L H2O2 PRF was 83.23%±8.48% (n=5, P < 0.05), the 40 μmol/L H2O2 PRF was 79.53%±8.56% (n=5, P < 0.01), the 80 μmol/L H2O2 PRF was 76.67%±7.16% (n=5, P < 0.01). Compared to HC (n=16), the serum IFN-α levels were significantly increased in the SLE patients (n=45) with moderate to high systemic lupus erythematosus disease activity index (SLEDAI≥10) [(55.07±50.36) ng/L vs. (328.2±276.3) ng/L, P < 0.001]. Meanwhile, compared with HC (n=6), IFNAR1 expression in peripheral blood CD56dimCD57+ NK cells of the SLE patients (n=6) were increased (MFI: 292.7±91.9 vs. 483.2±160.3, P < 0.05), and compared with HC (n=6), IFNAR2 expression in peripheral blood CD56dimCD57+ NK cells of the SLE patients (n=7) were increased (MFI: 643.5±113.7 vs. 919.0±246.9, P < 0.05). Compared with control (n=6), the stimulation of IFN-α (n=6) significantly promoted the apoptosis of CD56dimCD57+ NK cells (20.48%±7.01% vs. 37.82%±5.84%, P < 0.05). In addition, compared with the control (n=4, MFI: 1 049±174.5), stimulation of CD56dimCD57+ NK cells with IFN-α at different times significantly promoted the production of mtROS in a time-dependent manner, 48 h MFI was 3 437±1 472 (n=4, P < 0.05), 72 h MFI was 6 495±1 089 (n=4, P < 0.000 1), but there was no significant difference at 24 h of stimulation.

CONCLUSION

High serum IFN-α level in SLE patients may induce apoptosis by promoting mtROS production and inhibit perforin expression, which can down-regulate CD56dimCD57+ NK killing function.

End-Stage Idiopathic Pulmonary Fibrosis Lung Microenvironment Promotes Impaired NK Activity

Idiopathic pulmonary fibrosis (IPF) is a fibrotic age-related chronic lung disease characterized by the accumulation of senescent cells. Whether impaired immune response is responsible for the accumulation of senescent cells in the IPF lung remains unknown. In this study, we characterized the NK phenotype in IPF lungs via flow cytometry using 5-dodecanoylaminofluorescein di-β-d-galactopyranoside, markers of tissue residence, and chemokine receptors. The effect of the lung microenvironment was evaluated using lung fibroblast (LF) conditioned media (CM), and the bleomycin-induced pulmonary fibrosis mouse model was used to assess the in vivo relationship between NK cells and the accumulation of senescent cells. We found that NK cells from the lower lobe of IPF patients exhibited immune-senescent and impaired CD57-NKG2A+ phenotype. We also observed that culture of NK cells from healthy donors in CM from IPF lower lobe lung fibroblasts induced a senescent-like phenotype and impaired cytotoxic capacity. There is an impaired NK recruitment by LF, and NKs presented decreased migration toward their CM. In addition, NK cell-depleted mice treated with bleomycin showed increased collagen deposition and accumulation of different populations of senescent cells compared with controls. The IPF lung microenvironment induces a dysfunctional NK phenotype limiting the clearance of lung senescent cells and the resolution of lung fibrosis. We propose that impaired NK activity could be one of the mechanisms responsible for perpetuating the accumulation of senescent cells in IPF lungs.

Phenotypic and functional characterisation of locally produced natural killer cells ex vivo expanded with the K562-41BBL-mbIL21 cell line

We characterised the expansion, phenotype and functional activity of natural killer (NK) cells obtained for a clinical trial. Nineteen expansion procedures were performed to obtain NK cell products for 16 patients. NK cells were expanded ex vivo from haploidentical donor peripheral blood mononuclear cells in the presence of the locally generated feeder cell line K-562 with ectopic expression of 4-1BBL and mbIL-21. The median duration of expansion was 18 days (interquartile range 15-19). The median number of live cells yielded was 2.26 × 109 (range 1.6-3.4 × 109) with an NK content of 96.6% (range 95.1-97.9%). The median NK cell fold expansion was 171 (range 124-275). NK cell fold expansion depended on the number of seeded NK cells, the initial level of C-myc expression and the initial number of mature and immature NK cells. The majority of expanded NK cells had the phenotype of immature activated cells (NKG2A + , double bright CD56 +  + CD16 +  + , CD57-) expressing NKp30, NKp44, NKp46, NKG2D, CD69, HLA-DR and CD96. Despite the expression of exhaustion markers, expanded NK cells exhibited high cytolytic activity against leukaemia cell lines, high degranulation activity and cytokine production. There was a noted decrease in the functional activity of NK cells in tests against the patient's blasts.In conclusion, NK cells obtained by ex vivo expansion with locally generated K562-41BBL-mbIL21 cells had a relatively undifferentiated phenotype and enhanced cytolytic activity against cancer cell lines. Expansion of NK cells with feeder cells yielded a sufficient quantity of the NK cell product to reach high cell doses or increase the frequency of cell infusions for adoptive immunotherapy. Registered at clinicaltrials.gov as NCT04327037.

Surveying the Metabolic and Dysfunctional Profiles of T Cells and NK Cells in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Millions globally suffer from myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). The inflammatory symptoms, illness onset, recorded outbreak events, and physiological variations provide strong indications that ME/CFS, at least sometimes, has an infectious origin, possibly resulting in a chronic unidentified viral infection. Meanwhile, studies exposing generalized metabolic disruptions in ME/CFS have stimulated interest in isolated immune cells with an altered metabolic state. As the metabolism dictates the cellular function, dissecting the biomechanics of dysfunctional immune cells in ME/CFS can uncover states such as exhaustion, senescence, or anergy, providing insights into the consequences of these phenotypes in this disease. Despite the similarities that are seen metabolically between ME/CFS and other chronic viral infections that result in an exhausted immune cell state, immune cell exhaustion has not yet been verified in ME/CFS. This review explores the evidence for immunometabolic dysfunction in ME/CFS T cell and natural killer (NK) cell populations, comparing ME/CFS metabolic and functional features to dysfunctional immune cell states, and positing whether anergy, exhaustion, or senescence could be occurring in distinct immune cell populations in ME/CFS, which is consistent with the hypothesis that ME/CFS is a chronic viral disease. This comprehensive review of the ME/CFS immunometabolic literature identifies CD8+ T cell exhaustion as a probable contender, underscores the need for further investigation into the dysfunctional state of CD4+ T cells and NK cells, and explores the functional implications of molecular findings in these immune-cell types. Comprehending the cause and impact of ME/CFS immune cell dysfunction is critical to understanding the physiological mechanisms of ME/CFS, and developing effective treatments to alleviate the burden of this disabling condition.

Subpopulation Heterogeneity of NK Cells during the Genetic Modification for Subsequent Use in Targeted Therapy

Obtaining genetically engineered NK cells is a developing area of immunotherapy. In this work, we analyzed the subset heterogeneity of NK cells subjected to retroviral transduction, taking into account the content of adaptive NK cell progenitors. It was shown that subsets KIR2DL2/DL3⁺, as well as CD57^-KIR2DL2/DL3⁺NKG2C⁺, can be modified with greater efficiency than the corresponding subsets that do not carry the KIR2DL2/DL3 and NKG2C markers. After genetic modification, the CD57^-KIR2DL2/DL3⁺NKG2C⁺ cells began to express CD57 de novo, acquiring the adaptive NK cell phenotype.

Overcoming tumor resistance mechanisms in CAR-NK cell therapy

Despite the impressive results of autologous CAR-T cell therapy in refractory B lymphoproliferative diseases, CAR-NK immunotherapy emerges as a safer, faster, and cost-effective approach with no signs of severe toxicities as described for CAR-T cells. Permanently scrutinized for its efficacy, recent promising data in CAR-NK clinical trials point out the achievement of deep, high-quality responses, thus confirming its potential clinical use. Although CAR-NK cell therapy is not significantly affected by the loss or downregulation of its CAR tumor target, as in the case of CAR-T cell, a plethora of common additional tumor intrinsic or extrinsic mechanisms that could also disable NK cell function have been described. Therefore, considering lessons learned from CAR-T cell therapy, the emergence of CAR-NK cell therapy resistance can also be envisioned. In this review we highlight the processes that could be involved in its development, focusing on cytokine addiction and potential fratricide during manufacturing, poor tumor trafficking, exhaustion within the tumor microenvironment (TME), and NK cell short in vivo persistence on account of the limited expansion, replicative senescence, and rejection by patient’s immune system after lymphodepletion recovery. Finally, we outline new actively explored alternatives to overcome these resistance mechanisms, with a special emphasis on CRISPR/Cas9 mediated genetic engineering approaches, a promising platform to optimize CAR-NK cell function to eradicate refractory cancers.

Increased Susceptibility of the CD57- NK Cells Expressing KIR2DL2/3 and NKG2C to iCasp9 Gene Retroviral Transduction and the Relationships with Proliferative Potential, Activation Degree, and Death Induction Response

Nowadays, the use of genetically modified NK cells is a promising strategy for cancer immunotherapy. The additional insertion of genes capable of inducing cell suicide allows for the timely elimination of the modified NK cells. Different subsets of the heterogenic NK cell population may differ in proliferative potential, in susceptibility to genetic viral transduction, and to the subsequent induction of cell death. The CD57⁻NKG2C⁺ NK cells are of special interest as potential candidates for therapeutic usage due to their high proliferative potential and certain features of adaptive NK cells. In this study, CD57⁻ NK cell subsets differing in KIR2DL2/3 and NKG2C expression were transduced with the iCasp9 suicide gene. The highest transduction efficacy was observed in the KIR2DL2/3⁺NKG2C⁺ NK cell subset, which demonstrated an increased proliferative potential with prolonged cultivation. The increased transduction efficiency of the cell cultures was associated with the higher expression level of the HLA-DR activation marker. Among the iCasp9-transduced subsets, KIR2DL2/3⁺ cells had the weakest response to the apoptosis induction by the chemical inductor of dimerization (CID). Thus, KIR2DL2/3⁺NKG2C⁺ NK cells showed an increased susceptibility to the iCasp9 retroviral transduction, which was associated with higher proliferative potential and activation status. However, the complete elimination of these cells with CID is impeded.

Reduced Immunosenescence of Peripheral Blood T Cells in Parkinson's Disease with CMV Infection Background

Immunosenescence is a process of remodeling the immune system under the influence of chronic inflammation during aging. Parkinson’s disease (PD) is a common age-associated neurodegenerative disorder and is frequently accompanied by neuroinflammation. On the other hand, cytomegalovirus (CMV), one of the most spread infections in humans, may induce chronic inflammation which contributes to immunosenescence, differentiation and the inflation of T cells and NK cells. Currently, there is no clear understanding of immunosenescence severity in PD patients infected with CMV. In this study, we analyzed differentiation stages and immunosenescence characteristics of T cells and NK cells in 31 patients with mild and moderate PD severity, 33 age-matched and 30 young healthy donors. The PD patients were 100% CMV-seropositive compared to 76% age-matched and 73% young CMV-infected healthy donors. The proportion of effector memory T cells re-expressing CD45RA, CD57⁺CD56⁻ T cells and CD57⁺CD56⁺ T cells was significantly reduced in PD patients compared with CMV-seropositive age-matched healthy individuals. The CD57⁺CD56⁻ T cell proportion in PD patients was similar to that of CMV-seropositive young healthy donors. Thus, PD is characterized by reduced peripheral blood T cell immunosenescence, even against the background of CMV infection.

Making a Killer: Selecting the Optimal Natural Killer Cells for Improved Immunotherapies

Over the past 20 years natural killer (NK) cell-based immunotherapies have emerged as a safe and effective treatment option for patients with relapsed or refractory leukemia. Unlike T cell-based therapies, NK cells harbor an innate capacity to eliminate malignant cells without prior sensitization and can be adoptively transferred between individuals without the need for extensive HLA matching. A wide variety of therapeutic NK cell sources are currently being investigated clinically, including allogeneic donor-derived NK cells, stem cell-derived NK cells and NK cell lines. However, it is becoming increasingly clear that not all NK cells are endowed with the same antitumor potential. Despite advances in techniques to enhance NK cell cytotoxicity and persistence, the initial identification and utilization of highly functional NK cells remains essential to ensure the future success of adoptive NK cell therapies. Indeed, little consideration has been given to the identification and selection of donors who harbor NK cells with potent antitumor activity. In this regard, there is currently no standard donor selection criteria for adoptive NK cell therapy. Here, we review our current understanding of the factors which govern NK cell functional fate, and propose a paradigm shift away from traditional phenotypic characterization of NK cell subsets towards a functional profile based on molecular and metabolic characteristics. We also discuss previous selection models for NK cell-based immunotherapies and highlight important considerations for the selection of optimal NK cell donors for future adoptive cell therapies.

Adoptive immunotherapy with double-bright (CD56bright /CD16bright ) expanded natural killer cells in patients with relapsed or refractory acute myeloid leukaemia: a proof-of-concept study

Patients with acute myeloid leukaemia (AML) have a five-year survival rate of 28·7%. Natural killer (NK)-cell have anti-leukaemic activity. Here, we report on a series of 13 patients with high-risk R/R AML, treated with repeated infusions of double-bright (CD56^bright /CD16^bright ) expanded NK cells at an academic centre in Brazil. NK cells from HLA-haploidentical donors were expanded using K562 feeder cells, modified to express membrane-bound interleukin-21. Patients received FLAG, after which cryopreserved NK cells were thawed and infused thrice weekly for six infusions in three dose cohorts (10⁶ -10⁷ cells/kg/infusion). Primary objectives were safety and feasibility. Secondary endpoints included overall response (OR) and complete response (CR) rates at 28-30 days after the first infusion. Patients received a median of five prior lines of therapy, seven with intermediate or adverse cytogenetics, three with concurrent central nervous system (CNS) leukaemia, and one with concurrent CNS mycetoma. No dose-limiting toxicities, infusion-related fever, or cytokine release syndrome were observed. An OR of 78·6% and CR of 50·0% were observed, including responses in three patients with CNS disease and clearance of a CNS mycetoma. Multiple infusions of expanded, cryopreserved NK cells were safely administered after intensive chemotherapy in high-risk patients with R/R AML and demonstrated encouraging outcomes.

Telomerase Reverse Transcriptase Increases Proliferation and Lifespan of Human NK Cells without Immortalization

NK cells are the first line of defense against viruses and malignant cells, and their natural functionality makes these cells a promising candidate for cancer cell therapy. The genetic modifications of NK cells, allowing them to overcome some of their inherent limitations, such as low proliferative potential, can enable their use as a therapeutic product. We demonstrate that hTERT-engineered NK cell cultures maintain a high percentage of cells in the S/G2 phase for an extended time after transduction, while the life span of NK cells is measurably extended. Bulk and clonal NK cell cultures pre-activated in vitro with IL-2 and K562-mbIL21 feeder cells can be transduced with hTERT more efficiently compared with the cells activated with IL-2 alone. Overexpressed hTERT was functionally active in transduced NK cells, which displayed upregulated expression of the activation marker HLA-DR, and decreased expression of the maturation marker CD57 and activating receptor NKp46. Larger numbers of KIR2DL2/3+ cells in hTERT-engineered populations may indicate that NK cells with this phenotype are more susceptible to transduction. The hTERT-modified NK cells demonstrated a high natural cytotoxic response towards K562 cells and stably expressed Ki67, a proliferation marker. Overall, our data show that ectopic hTERT expression in NK cells enhances their activation and proliferation, extends in vitro life span, and can be a useful tool in developing NK-based cancer cell therapies.

Intra-lineage Plasticity and Functional Reprogramming Maintain Natural Killer Cell Repertoire Diversity

Natural killer (NK) cell repertoires are made up of phenotypically distinct subsets with different functional properties. The molecular programs involved in maintaining NK cell repertoire diversity under homeostatic conditions remain elusive. Here, we show that subset-specific NK cell proliferation kinetics correlate with mTOR activation, and global repertoire diversity is maintained through a high degree of intra-lineage subset plasticity during interleukin (IL)-15-driven homeostatic proliferation in vitro. Slowly cycling sorted KIR⁺CD56^dim NK cells with an induced CD57 phenotype display increased functional potential associated with increased transcription of genes involved in adhesion and immune synapse formation. Rapidly cycling cells upregulate NKG2A, display a general loss of functionality, and a transcriptional signature associated with increased apoptosis/cellular stress, actin-remodeling, and nuclear factor κB (NF-κB) activation. These results shed light on the role of intra-lineage plasticity during NK cell homeostasis and suggest that the functional fate of the cell is tightly linked to the acquired phenotype and transcriptional reprogramming.

CD56dim CD57- NKG2C+ NK cells retaining proliferative potential are possible precursors of CD57+ NKG2C+ memory-like NK cells

Formation of the adaptive-like NK cell subset in response to HCMV infection is associated with epigenetic rearrangements, accompanied by multiple changes in the protein expression. This includes a decrease in the expression level of the adapter chain FcεRIγ, NKp30, and NKG2A receptors and an increase in the expression of NKG2C receptor, some KIR family receptors, and co-stimulating molecule CD2. Besides, adaptive-like NK cells are characterized by surface expression of CD57, a marker of highly differentiated cells. Here, it is shown that CD57-negative CD56^dim NKG2C⁺ NK cells may undergo the same changes, as established by the similarity of the phenotypic expression pattern with that of the adaptive-like CD57⁺ NKG2C⁺ NK cells. Regardless of their differentiation stage, NKG2C-positive NK cells had increased HLA-DR expression indicating an activated state, both ex vivo and after cultivation in stimulating conditions. Additionally, CD57^- NKG2C⁺ NK cells exhibited better proliferative activity compared to CD57⁺ NKG2C⁺ and NKG2C^- NK cells, while retaining high level of natural cytotoxicity. Thus, CD57^- NKG2C⁺ NK cells may represent a less differentiated, but readily expanding stage of the adaptive-like CD57⁺ NKG2C⁺ NK cells. Moreover, it is shown that NK cells have certain phenotypic plasticity and may both lose NKG2C expression and acquire it de novo during proliferation, induced by IL-2 and K562-mbIL21 feeder cells.

Senescence in the Development and Response to Cancer with Immunotherapy: A Double-Edged Sword

Cellular senescence was first described as a physiological tumor cell suppressor mechanism that leads to cell growth arrest with production of the senescence-associated secretory phenotype known as SASP. The main role of SASP in physiological conditions is to attract immune cells to clear senescent cells avoiding tumor development. However, senescence can be damage-associated and, depending on the nature of these stimuli, additional types of senescence have been described. In the context of cancer, damage-associated senescence has been described as a consequence of chemotherapy treatments that were initially thought of as a tumor suppressor mechanism. However, in certain contexts, senescence after chemotherapy can promote cancer progression, especially when immune cells become senescent and cannot clear senescent tumor cells. Moreover, aging itself leads to continuous inflammaging and immunosenescence which are responsible for rewiring immune cells to become defective in their functionality. Here, we define different types of senescence, pathways that activate them, and functions of SASP in these events. Additionally, we describe the role of senescence in cancer and its treatments, including how aging and chemotherapy contribute to senescence in tumor cells, before focusing on immune cell senescence and its role in cancer. Finally, we discuss potential therapeutic interventions to reverse cell senescence.

Evaluation of serum-free media formulations in feeder cell-stimulated expansion of natural killer cells

BACKGROUND

Optimal expansion of therapeutic natural killer (NK) cell products has required media supplementation with human or fetal bovine serum, which raises safety and regulatory concerns for clinical manufacturing. Serum-free media (SFM) have been optimized for T-cell expansion, but few SFM systems have been developed for NK cells. Here, we compare six commercial clinical-grade SFM with our standard fetal bovine serum-containing medium for their ability to support NK cell expansion and function.

METHODS

Human peripheral blood NK cells were expanded in selected media by recursive weekly stimulation with K562-based feeder cells expressing membrane-bound interleukin-21 and CD137L. Expansion was the primary readout, and the best-performing SFM was then compared with standard medium for cytotoxicity, phenotype, degranulation and cytokine secretion. Multiple lots were compared for consistency, and media was analyzed throughout for nutrient consumption and metabolic byproducts.

RESULTS

TexMACS, OpTmizer, SCGM, ABS-001 and StemXVivo demonstrated equal or inferior NK cell expansion kinetics compared with standard medium, but expansion was markedly superior with AIM V + 5% Immune Cell Serum Replacement (ICSR; mean 5448 vs. 2621-fold expansion in 14 days). Surprisingly, NK cells expanded in AIM V + ICSR also showed increased cytotoxicity, tumor necrosis factor α secretion and DNAM-1, NKG2D, NKp30, FasL, granzyme B and perforin expression. Lot-to-lot variability was minimal. Glucose and glutamine consumption were inversely related to lactate and ammonia production.

DISCUSSION

The AIM V + ICSR SFM system supports excellent ex vivo expansion of clinical-grade NK cells with the phenotype and function needed for adoptive immunotherapy.

Characterizing the Dysfunctional NK Cell: Assessing the Clinical Relevance of Exhaustion, Anergy, and Senescence

There is a growing body of literature demonstrating the importance of T cell exhaustion in regulating and shaping immune responses to pathogens and cancer. Simultaneously, the parallel development of therapeutic antibodies targeting inhibitory molecules associated with immune exhaustion (such as PD-1, but also TIGIT, and LAG-3) has led to a revolution in oncology with dramatic benefits in a growing list of solid and hematologic malignancies. Given this success in reinvigorating exhausted T cells and the related anti-tumor effects, there are increasing efforts to apply immune checkpoint blockade to other exhausted immune cells beyond T cells. One approach involves the reinvigoration of “exhausted” NK cells, a non-T, non-B lymphoid cell of the innate immune system. However, in contrast to the more well-defined and established molecular, genetic, and immunophenotypic characteristics of T cell exhaustion, a consensus on the defining functional and phenotypic features of NK “exhaustion” is less clear. As is well-known from T cell biology, separate and distinct molecular and cellular processes including senescence, anergy and exhaustion can lead to diminished immune effector function with different implications for immune regulation and recovery. For NK cells, it is unclear if exhaustion, anergy, and senescence entail separate and distinct entities of dysfunction, though all are typically characterized by decreased effector function or proliferation. In this review, we seek to define these distinct spheres of NK cell dysfunction, analyzing how they have been shown to impact NK biology and clinical applications, and ultimately highlight key characteristics in NK cell function, particularly in relation to the role of “exhaustion.”

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.

What Defines NK Cell Functional Fate: Phenotype or Metabolism?

NK cells are capable of an array of functions that range widely from their classic anti-tumor and anti-viral cytotoxic effector functions, to their critical regulatory roles in controlling inflammatory immune responses and promoting tissue growth. However, the mechanisms that polarize NK cells to these distinct and opposing functions are incompletely understood. NK cell functional subsets are primarily identified and studied based on phenotype, which has served as an accessible means for profiling NK cells and does offer information on NK cell activation state. However, inconsistencies have emerged in using classic phenotypes to inform function, which raise the questions: Can phenotype in fact define NK cell functional fate? What factors do profile and drive NK cell fate? In other immune cells, cell metabolism has been shown to critically determine subset polarization. There is a growing body of evidence that cell metabolism is integral to NK cell effector functions. Glucose-driven glycolysis and oxidative metabolism have been shown to drive classic NK cell anti-tumor and anti-viral effector functions. Recent studies have uncovered a critical role for metabolism in NK cell development, education, and memory generation. In this review, we will draw on the evidence to date to investigate the relationship between NK cell phenotype, metabolism, and functional fate. We explore a paradigm in which the differential activity of metabolic pathways within NK cells produce distinct metabolic fingerprints that comprehensively distinguish and drive the range of NK cell functional abilities. We will discuss future areas of study that are needed to develop and test this paradigm and suggest strategies to efficiently profile NK cells based on metabolism. Given the emerging role of metabolism in driving NK cell fates, profiling and modulating NK cell metabolism holds profound therapeutic potential to tune inflammatory and regulatory NK cell responses to treat disease.