Identification of human NK cells that are deficient for signaling adaptor FcRγ and specialized for antibody-dependent immune functions

Rhesus Macaque Activating Killer Immunoglobulin-Like Receptors Associate With Fc Receptor Gamma (FCER1G) and Not With DAP12 Adaptor Proteins Resulting in Stabilized Expression and Enabling Signal Transduction

Activating killer cell immunoglobulin-like receptors (KIR) in macaques are thought to be derived by genetic recombination of the region encoding the transmembrane and intracellular part of KIR2DL4 and a KIR3D gene. As a result, all macaque activating KIR possess a positively charged arginine residue in the transmembrane region. As human KIR2DL4 associates with the FCER1G (also called Fc receptor-gamma, FcRγ) adaptor, we hypothesized that in contrast to human and great ape the activating KIRs of macaques associate with FcRγ instead of DAP12. By applying co-immunoprecipitation of transfected as well as primary cells, we demonstrate that rhesus macaque KIR3DS05 indeed associates with FcRγ and not with DAP12. This association with FcRγ results in increased and substantially stabilized surface expression of KIR3DS05. In addition, we demonstrate that binding of specific ligands of KIR3DS05, Mamu-A1*001 and A1*011, resulted in signal transduction in the presence of FcRγ in contrast to DAP12.

Human NK cells: From development to effector functions

NK cells are the major lymphocyte subset of the innate immune system that mediates antiviral and anti-tumor responses. It is well established that they develop mechanisms to distinguish self from non-self during the process of NK cell education. Unlike T and B cells, natural killer cells lack clonotypic receptors and are activated after recognizing their target via germline-encoded receptors through natural cytotoxicity, cytokine stimulation, and Ab-dependent cellular cytotoxicity. Subsequently, they utilize cytotoxic granules, death receptor ligands, and cytokines to perform their effector functions. In this review, we provide a general overview of human NK cells, as opposed to murine NK cells, discussing their ontogeny, maturation, receptor diversity, types of responses, and effector functions. Furthermore, we also describe recent advances in human NK cell biology, including tissue-resident NK cell populations, NK cell memory, and novel approaches used to target NK cells in cancer immunotherapy.

Virus-specific NK cell memory

NK cells express a limited number of germline-encoded receptors that identify infected or transformed cells, eliciting cytotoxicity, effector cytokine production, and in some circumstances clonal proliferation and memory. To maximize the functional diversity of NK cells, the array and expression level of surface receptors vary between individual NK cell “clones” in mice and humans. Cytomegalovirus infection in both species can expand a population of NK cells expressing receptors critical to the clearance of infected cells and generate a long-lived memory pool capable of targeting future infection with greater efficacy. Here, we discuss the pathways and factors that regulate the generation and maintenance of effector and memory NK cells and propose how this understanding may be harnessed therapeutically.

TRIGGERED: could refocused cell signaling be key to natural killer cell-based HIV immunotherapeutics?

Natural killer (NK) cells are one of the critical innate immune effector cells that directly kill tumors and virus-infected cells, and modulate other immune cells including dendritic cells, CD4+ and CD8+ T cells. Signals from activating and inhibitory surface receptors orchestrate the regulatory and cytotoxic functions of NK cells. Although a number of surface receptors are involved, multiple signaling molecules are shared so that NK cell responses are synergistically regulated. Many pathogens and tumors evade NK cell responses by targeting NK cell signaling. Particularly in HIV/simian immunodeficiency virus (SIV) infection, the NK cell repertoire is diminished by changes in subsets of NK cells, expression of activating and inhibitory receptors, and intracellular signaling molecules. However, in-depth studies on intracellular signaling in NK cells in HIV/SIV infections remain limited. Checkpoint blockade and chimeric antigen receptor (CAR)-NK cells have demonstrated enhanced NK cell activities against tumors and viral infections. In addition, targeting intracellular signaling molecules by small molecules could also improve NK cell responses towards HIV/SIV infection in vivo. Therefore, further understanding of NK cell signaling including identification of key signaling molecules is crucial to maximize the efficacy of NK cell-based treatments. Herein, we review the current state of the literature and outline potential future avenues where optimized NK cells could be utilized in HIV-1 cure strategies and other immunotherapeutics in PLWH.

Adaptive Natural Killer Cell Functional Recovery in Hepatitis C Virus Cured Patients

BACKGROUND AND AIMS

Current evidence suggests that dysfunctional natural killer (NK) cell responses during hepatitis C virus (HCV) infection can be restored after viral eradication with direct acting antivirals (DAAs). However, the fate of the recently described adaptive NK cell population, endowed with increased ability to mediate antibody-dependent cell-mediated cytotoxicity (ADCC), during HCV infection is poorly defined, while no study has explored the effects of DAAs on this NK subset.

APPROACH AND RESULTS

We performed multicolor flow cytometry to investigate CD57⁺ FcεRIγ^neg adaptive and FcεRIγ^pos conventional NK cell phenotype and function before and after DAA treatment in 59 patients chronically infected with HCV, 39 with advanced liver fibrosis, and 20 with mild-moderate liver fibrosis. Moreover, bulk NK cell phenotype and function were analyzed after cytokine activation following contact with K562 target cells. The proportion of FcεRIγ^neg NK cells in patients with HCV was associated with increased HCV load at baseline, and it was significantly reduced after treatment. Patients with an advanced fibrosis stage displayed increased NK cell activation and exhaustion markers that normalized after therapy. Of note, adaptive NK cells from patients with HCV were characterized by increased programmed death receptor 1 expression and reduced ADCC activity at baseline. DAA treatment restored ADCC ability and reduced programmed death receptor 1 expression.

CONCLUSIONS

HCV profoundly affects the frequency, phenotype, and function of adaptive NK cells. DAA therapy restores a normal adaptive NK phenotype and enhances interferon-gamma production by this cell subset.

IL-18 Responsiveness Defines Limitations in Immune Help for Specialized FcRγ- NK Cells

Despite being prolific innate killers, NK cells are also key helper cells in antiviral defense, influencing adaptive immune responses via interactions with dendritic cells (DCs). In addition to causing NK cell dysfunction, HIV-1 infection contributes to the expansion of a rare population of NK cells deficient in FcRγ (FcRγ^-), an intracellular adaptor protein that associates with CD16. The implications of this inflated NK cell subset in treated HIV-1 infection remain unclear. In this study, we explored the helper function of human NK cells in chronic HIV-1 infection, with a particular focus on characterizing FcRγ^- NK cells. Exposure of NK cells to innate DC-derived costimulatory factors triggered their helper activity, defined by their ability to produce IFN-γ and to drive the maturation of high IL-12-producing DCs. In this setting, however, FcRγ^- NK cells were defective at producing the dominant DC-polarizing agent IFN-γ. The reduced responsiveness of FcRγ^- NK cells to IL-18 in particular, which was attributable to impaired inducible expression of IL-18Rα, extended beyond an inability to produce IFN-γ, as FcRγ^- NK cells showed limited potential to differentiate into CD16^-/CD25⁺/CD83⁺ helper cells. Notwithstanding their deficiencies in responsiveness to innate environmental cues, FcRγ^- NK cells responded robustly to adaptive Ab-mediated signaling through CD16. The presence of an expanded population of FcRγ^- NK cells with a diminished capacity to respond to IL-18 and to effectively modulate DC function may contribute to disturbances in proper immune homeostasis associated with HIV-1 infection and to defects in the initiation of optimal adaptive antiviral responses.

FcRγ Gene Editing Reprograms Conventional NK Cells to Display Key Features of Adaptive Human NK Cells

Adaptive human natural killer (NK) cells display significantly enhanced responsiveness to a broad-range of antibody-bound targets through the engagement of CD16 compared to conventional NK cells, yet direct reactivity against tumor targets is generally reduced. Adaptive NK cells also display a distinct phenotype and differential expression of numerous genes, including reduced expression of signaling adapter FcRγ and transcription factor PLZF. However, it is unclear whether differential expression of specific genes is responsible for the characteristics of adaptive NK cells. Using CRISPR-Cas9, we show deletion of FcRγ in conventional NK cells led to enhanced CD16 responsiveness, abolished cell surface expression of natural cytotoxicity receptors, NKp46 and NKp30, and dramatically reduced responsiveness to K562 and Raji tumor cells. However, deletion of PLZF had no notable effects. These results suggest multiple roles for FcRγ and identify its deficiency as an important factor responsible for the functional and phenotypic characteristics exhibited by adaptive NK cells.

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FcRγ deletion leads to increased cytokine production in response to CD16 stimulation

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FcRγ deletion abolishes cell surface expression of NKp46 and NKp30

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FcRγ deletion results in reduced responsiveness to K562 and Raji cells

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PLZF deletion does not change responsiveness to CD16 and cytokine stimulation

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.

Harnessing Memory NK Cell to Protect Against COVID-19

The worldwide struggle against the coronavirus disease 2019 (COVID-19) as a public health crisis continues to sweep across the globe. Up to now, effective antiviral treatment against COVID-19 is not available. Therefore, throughout virus infections, a thorough clarification of the virus-host immune system interactions will be most probably helpful to encounter these challenges. Emerging evidence suggests that just like SARS and MERS, COVID-19 primarily suppresses the innate immune system, enabling its stable propagation during the early stage of infection. Consequently, proinflammatory cytokines and chemokines have been increasing during infection progression associated with severe lung pathology. It is imperative to consider hyper inflammation in vaccine designing, as vaccine-induced immune responses must have a protective role against infection without leading to immunopathology. Among the front-line responders to viral infections, Natural Killer (NK) cells have immense therapeutic potential, forming a bridge between innate and adaptive responses. A subset of NK cells exhibits putatively increased effector functions against viruses following pathogen-specific and immunization. Memory NK cells have higher cytotoxicity and effector activity, compared with the conventional NK cells. As a pioneering strategy, prompt accumulation and long-term maintenance of these memory NK cells could be an efficacious viral treatment. According to the high prevalence of human cytomegalovirus (HCMV) infection in the world, it remains to be determined whether HCMV adaptive NK cells could play a protective role against this new emerging virus. In addition, the new adaptive-like KIR+NKG2C+ NK cell subset (the adaptive-like lung tissue residue [tr]NK cell) in the context of the respiratory infection at this site could specifically exhibit the expansion upon COVID-19. Another aspect of NK cells we should note, utilizing modified NK cells such as allogeneic off-the-shelf CAR-NK cells as a state-of-the-art strategy for the treatment of COVID-19. In this line, we speculate introducing NKG2C into chimeric antigen receptors in NK cells might be a potential approach in future viral immunotherapy for emerging viruses. In this contribution, we will briefly discuss the current status and future perspective of NK cells, which provide to successfully exploit NK cell-mediated antiviral activity that may offer important new tools in COVID-19 treatment.

Harnessing Natural Killer Cell Innate and Adaptive Traits in HIV Infection

Despite efficient virological suppression on antiretroviral therapy (ART), people living with HIV (PLWH), experience an increased burden of premature co-morbidities, such as cancer and end-organ disease. With remaining challenges in terms of access to therapy, adherence and potential long-term drug toxicity, improving their long-term healthcare outcome, including new strategies for HIV clearance, remains a global priority. There is, therefore, an ongoing need to better characterize and harness the immune response in order to develop new strategies and supplement current therapeutic approaches for a “functional” cure. Current efforts toward HIV eradication to enhance immune recognition and elimination of persistently infected cells have highlighted the need for an optimized “kill” approach. Natural killer (NK) cells play an important role in antiviral defense and by virtue of their innate and adaptive features hold great promise as a focus of “kill” efforts. Galvanized by advances in the cancer field, NK cell exploitation, represents a transformative approach to augment HIV therapeutic modalities, circumventing many of the limitations inherent to T cell approaches. In this review we will discuss recent advances in our understanding of the development of NK cell adaptive/memory responses in HIV infection and highlight new and exciting opportunities to exploit the beneficial attributes of NK cells for HIV immunotherapy.

Delayed NK Cell Reconstitution and Reduced NK Activity Increased the Risks of CMV Disease in Allogeneic-Hematopoietic Stem Cell Transplantation

Cytomegalovirus (CMV) infection has a significant impact in patients after allogeneic hematopoietic stem cell transplantation (HSCT). We investigated natural killer (NK) cell reconstitution and cytotoxic/cytokine production in controlling CMV infection, especially severe CMV disease in HSCT patients. Fifty-eight patients with acute myeloid leukemia (AML) who received allo-HSCT were included. We monitored NK reconstitution and NK function at baseline, 30, 60, 90, 120, 150, and 180 days after HSCT, and compared the results in recipients stratified on post-HSCT CMV reactivation (n = 23), non-reactivation (n = 24) versus CMV disease (n = 11) groups. The CMV disease group had a significantly delayed recovery of CD56dim NK cells and expansion of FcRγ-CD3ζ+NK cells started post-HSCT 150 days. Sequential results of NK cytotoxicity, NK cell-mediated antibody-dependent cellular cytotoxicity (NK-ADCC), and NK-Interferon-gamma (NK-IFNγ) production for 180 days demonstrated delayed recovery and decreased levels in the CMV disease group compared with the other groups. The results within 1 month after CMV viremia also showed a significant decrease in NK function in the CMV disease group compared to the CMV reactivation group. It suggests that NK cells' maturation and cytotoxic/IFNγ production contributes to CMV protection, thereby revealing the NK phenotype and functional NK monitoring as a biomarker for CMV risk prediction, especially CMV disease.

OMIP-064: A 27-Color Flow Cytometry Panel to Detect and Characterize Human NK Cells and Other Innate Lymphoid Cell Subsets, MAIT Cells, and γδ T Cells

This 27-color flow cytometry panel was developed in order to assess immunological changes over the course of an immunization and challenge regimen in two experimental malaria vaccine trials. The aim of the study was to find correlates of vaccine-induced protection. Several studies have indicated that protection against malaria appears to involve immune responses at various immunological sites, with liver-resident responses playing an essential role. As it is not feasible to monitor the immune responses within the liver in humans, this panel is developed with the aim to thoroughly characterize the immune responses over time in blood in addition to detecting changes that might reflect what happens in other immunological sites like the liver. The focus of this panel is to detect several innate lymphoid cell populations, including NK cells and their activation status. Moreover, unconventional T cells like mucosal associated invariant T cells and γδ T cells are assessed in the panel. © 2020 International Society for Advancement of Cytometry.

A New Hope for CD56negCD16pos NK Cells as Unconventional Cytotoxic Mediators: An Adaptation to Chronic Diseases

Natural Killer (NK) cells play an essential role in antiviral and anti-tumoral immune responses. In peripheral blood, NK cells are commonly classified into two major subsets: CD56^brightCD16^neg and CD56^dimCD16^pos despite the characterization of a CD56^negCD16^pos subset 25 years ago. Since then, several studies have described the prevalence of an CD56^negCD16^pos NK cell subset in viral non-controllers as the basis for their NK cell dysfunction. However, the mechanistic basis for their cytotoxic impairment is unclear. Recently, using a strict flow cytometry gating strategy to exclude monocytes, we reported an accumulation of CD56^negCD16^pos NK cells in Plasmodium falciparum malaria-exposed children and pediatric cancer patients diagnosed with endemic Burkitt lymphoma (eBL). Here, we use live-sorted cells, histological staining, bulk RNA-sequencing and flow cytometry to confirm that this CD56^negCD16^pos NK cell subset has the same morphological features as the other NK cell subsets and a similar transcriptional profile compared to CD56^dimCD16^pos NK cells with only 120 genes differentially expressed (fold change of 1.5, p < 0.01 and FDR<0.05) out of 9235 transcripts. CD56^negCD16^pos NK cells have a distinct profile with significantly higher expression of MPEG1 (perforin 2), FCGR3B (CD16b), FCGR2A, and FCGR2B (CD32A and B) as well as CD6, CD84, HLA-DR, LILRB1/2, and PDCD1 (PD-1), whereas Interleukin 18 (IL18) receptor genes (IL18RAP and IL18R1), cytotoxic genes such as KLRF1 (NKp80) and NCR1 (NKp46), and inhibitory HAVCR2 (TIM-3) are significantly down-regulated compared to CD56^dimCD16^pos NK cells. Together, these data confirm that CD56^negCD16^pos cells are legitimate NK cells, yet their transcriptional and protein expression profiles suggest their cytotoxic potential is mediated by pathways reliant on antibodies such as antibody-dependent cell cytotoxicity (ADCC), antibody-dependent respiratory burst (ADRB), and enhanced by complement receptor 3 (CR3) and FAS/FASL interaction. Our findings support the premise that chronic diseases induce NK cell modifications that circumvent proinflammatory mediators involved in direct cytotoxicity. Therefore, individuals with such altered NK cell profiles may respond differently to NK-mediated immunotherapies, infections or vaccines depending on which cytotoxic mechanisms are being engaged.

Memory and Memory-Like NK Cell Responses to Microbial Pathogens

NK cells are cytotoxic lymphocytes that provide systemic defense against pathogens and malignancy. Although historically considered cells of the innate immune system, NK cells are now known to be capable of memory or memory-like immune responses in certain settings. Memory NK responses were initially reported over a decade ago in studies involving mouse models of cytomegalovirus infection and delayed-type hypersensitivity reactions to chemical haptens and viral antigens. Since then, a growing body of literature suggests that memory or memory-like NK cell responses may occur in a broader range of immunological settings, including in response to various viral and bacterial infections, and some immunization protocols. Memory-like NK cell responses have also now been reported in humans and non-human primates. Here, we summarize recent studies demonstrating memory or memory-like responses by NK cells in settings of infection and immunization against infectious agents.

Distinct Human NK Cell Phenotypes and Functional Responses to Mycobacterium tuberculosis in Adults From TB Endemic and Non-endemic Regions

Mycobacterium tuberculosis (Mtb) is the causative agent of tuberculosis (TB), which leads to an estimated 1. 5 million deaths worldwide each year. Although the immune correlates of protection against Mtb infection and TB disease have not been well-defined, natural killer (NK) cells are increasingly recognized as a key component of the innate immune response to Mtb and as a link between innate and adaptive immunity. In this study, we evaluated NK cell phenotypic and functional profiles in QuantiFERON-TB (QFT)⁺ and QFT⁻ adults in a TB endemic setting in Kisumu, Kenya, and compared their NK cell responses to those of Mtb-naïve healthy adult controls in the U.S. We used flow cytometry to define the phenotypic profile of NK cells and identified distinct CD56^dim NK cell phenotypes that differentiated the Kenyan and U.S. groups. Additionally, among Kenyan participants, NK cells from QFT⁺ individuals with latent Mtb infection (LTBI) were characterized by significant downregulation of the natural cytotoxicity receptor NKp46 and the inhibitory receptor TIGIT, compared with QFT⁻ individuals. Moreover, the distinct CD56^dim phenotypic profiles in Kenyan individuals correlated with dampened NK cell responses to tumor cells and diminished activation, degranulation, and cytokine production following stimulation with Mtb antigens, compared with Mtb-naïve U.S. healthy adult controls. Taken together, these data provide evidence that the phenotypic and functional profiles of NK cells are modified in TB endemic settings and will inform future studies aimed at defining NK cell-mediated immune correlates that may be protective against acquisition of Mtb infection and progression to TB disease.

Natural killer cells in multiple sclerosis: A review

As the most common non-traumatic disabling disease among adolescents, multiple sclerosis (MS) is a devastating neurological inflammatory disease of the central nervous system. Research has not yet fully elucidated its pathogenesis, but it has shown MS to be a complex, multifactorial disease with many interplaying factors. One of these factors, natural killer (NK) cells, lymphocytes of the innate immune system, have recently gained attention due to the effects of daclizumab therapy, causing an expansion of the immunoregulatory subset of NK cells. Since then, NK cells and their relation to MS have been the focus of research, with many new findings being published in the last decade. In this review, NK cells are pictured as potent cytotoxic killers, as well as unique immune-regulators. Additionally, an overview of our current knowledge regarding NK cells in MS is given. The role of NK cells in MS is reviewed in the context of well-established environmental factors and current disease modifying therapies to gain further understanding of the pathogenesis and treatment options in MS.

Blood natural killer cell deficiency reveals an immunotherapy strategy for atopic dermatitis

Atopic dermatitis (AD) is a widespread, chronic skin disease associated with aberrant allergic inflammation. Current treatments involve either broad or targeted immunosuppression strategies. However, enhancing the immune system to control disease remains untested. We demonstrate that patients with AD harbor a blood natural killer (NK) cell deficiency that both has diagnostic value and improves with therapy. Multidimensional protein and RNA profiling revealed subset-level changes associated with enhanced NK cell death. Murine NK cell deficiency was associated with enhanced type 2 inflammation in the skin, suggesting that NK cells play a critical immunoregulatory role in this context. On the basis of these findings, we used an NK cell-boosting interleukin-15 (IL-15) superagonist and observed marked improvement in AD-like disease in mice. These findings reveal a previously unrecognized application of IL-15 superagonism, currently in development for cancer immunotherapy, as an immunotherapeutic strategy for AD.

Phenotypic and Functional Analysis of Human NK Cell Subpopulations According to the Expression of FcεRIγ and NKG2C

Human memory-like NK cells are commonly defined by either a lack of FcεRIγ or gain of NKG2C expression. Here, we investigated the heterogeneity of human CD56^dim NK cell subpopulations according to the expression of FcεRIγ and NKG2C in a large cohort (n = 127). Although the frequency of FcεRIγ⁻ and NKG2C⁺ NK cells positively correlated, the FcεRIγ⁻ and NKG2C⁺ NK cell populations did not exactly overlap. The FcεRIγ⁺NKG2C⁺, FcεRIγ⁻NKG2C⁺, and FcεRIγ⁻NKG2C⁻ NK cell populations were only evident after HCMV infection, but each had distinct characteristics. Among the subpopulations, FcεRIγ⁻NKG2C⁺ NK cells exhibited the most restricted killer immunoglobulin-like receptor repertoire, suggesting clonal expansion. Moreover, FcεRIγ⁻NKG2C⁺ NK cells exhibited the lowest Ki-67 and highest Bcl-2 expression, indicating the long-lived quiescent memory-like property. Functionally, FcεRIγ⁻NKG2C⁺ NK cells had weak natural effector function against K562 but strong effector functions by CD16 engagement, whereas FcεRIγ⁺NKG2C⁺ NK cells had strong effector functions in both settings. Anatomically, the FcεRIγ⁺NKG2C⁺, FcεRIγ⁻NKG2C⁺, and FcεRIγ⁻NKG2C⁻ NK cell populations were present in multiple human peripheral organs. In conclusion, we demonstrate the heterogeneity of memory-like NK cells stratified by FcεRIγ and NKG2C and suggest both markers be utilized to better define these cells.

CMV Primes Functional Alternative Signaling in Adaptive Δg NK Cells but Is Subverted by Lentivirus Infection in Rhesus Macaques

Despite burgeoning evidence demonstrating the adaptive properties of natural killer (NK) cells, mechanistic data explaining these phenomena are lacking. Following antibody sensitization, NK cells lacking the Fc receptor (FcR) signaling chain (Δg) acquire adaptive features, including robust proliferation, multi-functionality, rapid killing, and mobilization to sites of virus exposure. Using the rhesus macaque model, we demonstrate the systemic distribution of Δg NK cells expressing memory features, including downregulated Helios and Eomes. Furthermore, we find that Δg NK cells abandon typical γ-chain/Syk in lieu of CD3ζ-Zap70 signaling. FCγRIIIa (CD16) density, mucosal homing, and function are all coupled to this alternate signaling, which in itself requires priming by rhesus cytomegalovirus (rhCMV). Simian immunodeficiency virus (SIV) infections further expand gut-homing adaptive NK cells but result in pathogenic suppression of CD3ζ-Zap70 signaling and function. Herein, we provide a mechanism of virus-dependent alternative signaling that may explain the acquisition of adaptive features by primate NK cells and could be targeted for future vaccine or curative therapies.

Gamma-chain-deficient adaptive NK cells are robust mediators of antiviral immunity via ADCC. Shah et al. demonstrate using macaque models that acquisition of these features requires previous priming with CMV infection and involves alternative signaling via CD3zeta but is actively suppressed by lentivirus infection.

MicroRNA regulation of natural killer cell development and function in leukemia

MicroRNAs (miRNAs) are now recognized as important regulators of all cellular processes, including immune function and cancer survival. These evolutionary preserved, single-stranded, non-coding RNA molecules mediate important functional effects primarily through post-transcriptional regulation of protein expression. MiRNAs are known to mediate multiple oncogenic pathways in tumor cells, both tumor promoting and tumor suppressing. In addition to a direct tumor cell effect, miRNAs have also been shown to play a critical role in immune cell development, function and survival. Here we expand on previous reports to evaluate miRNA regulation in natural killer (NK) cells primarily in humans and focus on their influence on NK cell development and function in the setting of hematologic malignancies. In addition, we highlight the most recent miRNA discoveries in hematologic malignancies and discuss areas of future exploration relevant to the translational field of innate immunology and miRNA-based therapeutic intervention.

The Evolutionary Arms Race between Virus and NK Cells: Diversity Enables Population-Level Virus Control

Viruses and natural killer (NK) cells have a long co-evolutionary history, evidenced by patterns of specific NK gene frequencies in those susceptible or resistant to infections. The killer immunoglobulin-like receptors (KIR) and their human leukocyte antigen (HLA) ligands together form the most polymorphic receptor-ligand partnership in the human genome and govern the process of NK cell education. The KIR and HLA genes segregate independently, thus creating an array of reactive potentials within and between the NK cell repertoires of individuals. In this review, we discuss the interplay between NK cell education and adaptation with virus infection, with a special focus on three viruses for which the NK cell response is often studied: human immunodeficiency virus (HIV), hepatitis C virus (HCV) and human cytomegalovirus (HCMV). Through this lens, we highlight the complex co-evolution of viruses and NK cells, and their impact on viral control.

Adaptive Features of Natural Killer Cells in Multiple Sclerosis

Human cytomegalovirus (HCMV) has been recently related with a lower susceptibility to multiple sclerosis (MS). HCMV promotes an adaptive development of NK cells bearing the CD94/NKG2C receptor with a characteristic phenotypic and functional profile. NK cells are proposed to play an immunoregulatory role in MS, and expansion of the NKG2C(+) subset was recently associated with reduced disability progression. To further explore this issue, additional adaptive NK cell markers, i.e., downregulation of FcεRIγ chain (FcRγ) and PLZF transcription factor, as well as antibody-dependent NK cell activation were assessed in controls and MS patients considering HCMV serology and clinical features. In line with previous reports, increased proportions of NKG2C(+), FcRγ(-), and PLZF(-) CD56^dim NK cells were found in HCMV(+) cases. However, PLZF(-) NK cells were detected uncoupled from other adaptive markers within the CD56^bright subset from HCMV(+) cases and among CD56^dim NK cells from HCMV(-) MS patients, suggesting an additional effect of HCMV-independent factors in PLZF downregulation. Interferon-β therapy was associated with lower proportions of FcRγ(-) CD56^dim NK cells in HCMV(+) and increased PLZF(-) CD56^bright NK cells in HCMV(-) patients, pointing out to an influence of the cytokine on the expression of adaptive NK cell-associated markers. In addition, proportions of NKG2C(+) and FcRγ(-) NK cells differed in progressive MS patients as compared to controls and other clinical forms. Remarkably, an adaptive NK cell phenotype did not directly correlate with enhanced antibody-triggered degranulation and TNFα production in MS in contrast to controls. Altogether, our results provide novel insights into the putative influence of HCMV and adaptive NK cells in MS.

Differential Effect of Mucosal NKp44+ Innate Lymphoid Cells and Δγ Cells on Simian Immunodeficiency Virus Infection Outcome in Rhesus Macaques

NK cells are essential for controlling viral infections. We investigated NK cell and innate lymphoid cell (ILC) dynamics and function in rhesus macaque rectal tissue and blood following mucosal priming with replicating adenovirus (Ad)-SIV recombinants, systemic boosting with SIV envelope protein, and subsequent repeated low-dose intravaginal SIV exposures. Mucosal memory-like NK and ILC subsets in rectal and vaginal tissues of chronically infected macaques were also evaluated. No differences in NK cell or ILC frequencies or cytokine production were seen between vaccinated and Ad-empty/alum controls, suggesting responses were due to the Ad-vector and alum vaccine components. Mucosal NKp44⁺ ILCs increased postvaccination and returned to prelevels postinfection. The vaccine regimen induced mucosal SIV-specific Ab, which mediated Ab-dependent cellular cytotoxicity and was correlated with mucosal NKp44⁺CD16⁺ ILCs. Postvaccination NKp44⁺ and NKp44⁺IL-17⁺ ILC frequencies were associated with delayed SIV acquisition and decreased viremia. In chronically SIV-infected animals, NKp44⁺ ILCs negatively correlated with viral load, further suggesting a protective effect, whereas, NKG2A^- NKp44^- double-negative ILCs positively correlated with viral load, indicating a pathogenic role. No such associations of circulating NK cells were seen. Δγ NK cells in mucosal tissues of chronically infected animals exhibited impaired cytokine production compared with non-Δγ NK cells but responded to anti-gp120 Ab and Gag peptides, whereas non-Δγ NK cells did not. Mucosal Δγ NKp44⁺ and Δγ DN cells were similarly associated with protection and disease progression, respectively. Thus, the data suggest NKp44⁺ ILCs and Δγ cells contribute to SIV infection outcomes. Vaccines that promote mucosal NKp44⁺ and suppress double-negative ILCs are likely desirable.

A Natural Impact: NK Cells at the Intersection of Cancer and HIV Disease

Despite efficient suppression of plasma viremia in people living with HIV (PLWH) on cART, evidence of HIV-induced immunosuppression remains, and normally benign and opportunistic pathogens become major sources of co-morbidities, including virus-induced cancers. In fact, cancer remains a primary cause of death even in virally suppressed PLWH. Natural killer (NK) cells provide rapid early responses to HIV infection, contribute substantially to disease modulation and vaccine protection, and are also major therapeutic targets for cancer immunotherapy. However, much like other lymphocyte populations, recent burgeoning evidence suggests that in chronic conditions like HIV, NK cells can become functionally exhausted with impaired cytotoxic function, altered cytokine production and impaired antibody-dependent cell-mediated cytotoxicity. Recent work suggests functional anergy is likely due to low-level ongoing virus replication, increased inflammatory cytokines, or increased presence of MHC^low target cells. Indeed, HIV-induced loss of NK cell-mediated control of lytic EBV infection has been specifically shown to cause lymphoma and also increases replication of CMV. In this review, we will discuss current understanding of NK cell modulation of HIV disease, reciprocal exhaustion of NK cells, and how this may impact increased cancer incidences and prospects for NK cell-targeted immunotherapies. Finally, we will review the most recent evidence supporting adaptive functions of NK cells and highlight the potential of adaptive NK cells for cancer immunotherapy.

KLRG1+ natural killer cells exert a novel antifibrotic function in chronic hepatitis B

BACKGROUND & AIMS

Natural killer (NK) cells are known to exert strong antiviral activity. Killer cell lectin-like receptor subfamily G member 1 (KLRG1) is expressed by terminally differentiated NK cells and KLRG1-expressing lymphocytes are known to expand following chronic viral infections. We aimed to elucidate the previously unknown role of KLRG1 in the pathogenesis of chronic hepatitis B (CHB).

METHODS

KLRG1+ NK cells were taken from the blood and liver of healthy individuals and patients with CHB. The phenotype and function of these cells was assessed using flow cytometry and in vitro stimulation.

RESULTS

Patients with CHB had a higher frequency of KLRG1+ NK cells compared to healthy controls (blood 13.4 vs. 2.3%, p <0.0001 and liver 23.4 vs. 2.6%, p <0.01). KLRG1+ NK cells were less responsive to K562 and cytokine stimulation, but demonstrated enhanced cytotoxicity (9.0 vs. 4.8%, p <0.05) and IFN-γ release (8.0 vs. 1.5%, p <0.05) via antibody dependent cellular cytotoxicity compared to their KLRG1- counterparts. KLRG1+ NK cells possessed a mature phenotype, demonstrating stronger cytolytic activity and IFN-γ secretion against hepatic stellate cells (HSCs) than KLRG1- NK cells. Moreover, KLRG1+ NK cells more effectively induced primary HSC apoptosis in a TRAIL-dependent manner. Increased KLRG1+ NK cell frequency in the liver and blood was associated with lower fibrosis stage (F0/F1) in patients with CHB. Finally, the expression of CD44, degranulation and IFN-γ production were all increased in KLRG1+ NK cells following stimulation with osteopontin, the CD44 ligand, suggesting that HSC-derived osteopontin may cause KLRG1+ NK cell activation.

CONCLUSIONS

KLRG1+ NK cells likely play an antifibrotic role during the natural course of CHB infection. Harnessing this antifibrotic function may provide a novel therapeutic approach to treat liver fibrosis in patients with CHB.

LAY SUMMARY

Individuals that are chronically infected with hepatitis B virus (HBV) possess an increased number of immune cells, called natural killer (NK) cells expressing the surface marker KLRG1 in the blood and liver. Here, we demonstrate that these specific NK cells are able to kill activated stellate cells in the liver. Because activated stellate cells contribute to liver scarring, i.e. fibrosis, and subsequent liver dysfunction in individuals with chronic HBV infection, KLRG1+ NK cells are a novel immune cell type that can limit liver scarring.

Short-term clinical and immunologic effects of poly-gamma-glutamic acid (γ-PGA) in women with cervical intraepithelial neoplasia 1 (CIN 1): A multicenter, randomized, double blind, phase II trial

OBJECTIVE

The aim of this study was to investigate the short-term efficacy and safety of Poly-gamma-glutamic acid (γ-PGA) and the immunologic changes in patients with CIN 1.

METHODS

Participants were randomly assigned to one of two groups and orally treated with placebo or 1,500 mg of γ-PGA for 4 weeks. The primary endpoint of the study was histologic regression rate of CIN 1 at 12 weeks between γ-PGA and control groups. The secondary endpoints were HPV clearance and change in immune responses.

RESULT

From April 2013 to December 2015, 195 patients participated in the study. In the intention-to-treat analysis, 42 (42.4%) of the women who received γ-PGA experienced histologic remission versus 26 (27.1%) in the control group, with a statistically significant difference (p = 0.018). In the γ-PGA group, HPV clearance was found in 37 (43.5%) of 85 patients infected with high-risk HPV, showing a significant difference compared to the control group, in which 20 (26.7%) of 75 patients exhibited HPV clearance (p = 0.026). However, there was no significant difference between the two groups in the change of NK cell activity, major histocompatibility complex (MHC) class II CD8 count, and CD56 count.

CONCLUSION

γ-PGA showed a short-term therapeutic effect on CIN 1 and high-risk HPV infection. It is a non-invasive, promising oral medication for women with these conditions.

TRIAL REGISTRATION

Clinical Trials NCT01826045.

NK cell-intrinsic FcεRIγ limits CD8+ T-cell expansion and thereby turns an acute into a chronic viral infection

During viral infection, tight regulation of CD8⁺ T-cell functions determines the outcome of the disease. Recently, others and we determined that the natural killer (NK) cells kill hyperproliferative CD8⁺ T cells in the context of viral infection, but molecules that are involved in shaping the regulatory capability of NK cells remain virtually unknown. Here we used mice lacking the Fc-receptor common gamma chain (FcRγ, FcεRIγ, Fcer1g^–/– mice) to determine the role of Fc-receptor and NK-receptor signaling in the process of CD8⁺ T-cell regulation. We found that the lack of FcRγ on NK cells limits their ability to restrain virus-specific CD8⁺ T cells and that the lack of FcRγ in Fcer1g^–/– mice leads to enhanced CD8⁺ T-cell responses and rapid control of the chronic docile strain of the lymphocytic choriomeningitis virus (LCMV). Mechanistically, FcRγ stabilized the expression of NKp46 but not that of other killer cell–activating receptors on NK cells. Although FcRγ did not influence the development or activation of NK cell during LCMV infection, it specifically limited their ability to modulate CD8⁺ T-cell functions. In conclusion, we determined that FcRγ plays an important role in regulating CD8⁺ T-cell functions during chronic LCMV infection.

FcRγ is a signaling molecule for Fc receptors and NK cell killer activating receptor (KAR) complex. FcRγ is highly expressed by NK cells and involved in NK cell activity. NK cells are widely defined to regulate the expansion of T cells. Here using chronic LCMV model, we described the role of FcRγ in NK cell mediated shaping of CD8⁺ T cell response and viral control. We observed that FcRγ does not affect the early activity of NK cells which is mainly innate immune cytokines driven, but rather the specific activation due to NKp46 inadequacy. We detected that FcRγ stabilizes NKp46 protein by preventing it from proteasomal degradation. Due to lack of NKp46 expression in absence of FcRγ, we observed strong CD8⁺ T cell response and faster viral clearance during chronic LCMV infection. These data demonstrate that FcRγ is crucial for specific activity of NK cells for regulation of CD8⁺ T cell response during viral infection.

Monkeying Around: Using Non-human Primate Models to Study NK Cell Biology in HIV Infections

Natural killer (NK) cells are the major innate effectors primed to eliminate virus-infected and tumor or neoplastic cells. Recent studies also suggest nuances in phenotypic and functional characteristics among NK cell subsets may further permit execution of regulatory and adaptive roles. Animal models, particularly non-human primate (NHP) models, are critical for characterizing NK cell biology in disease and under homeostatic conditions. In HIV infection, NK cells mediate multiple antiviral functions via upregulation of activating receptors, inflammatory cytokine secretion, and antibody dependent cell cytotoxicity through antibody Fc-FcR interaction and others. However, HIV infection can also reciprocally modulate NK cells directly or indirectly, leading to impaired/ineffective NK cell responses. In this review, we will describe multiple aspects of NK cell biology in HIV/SIV infections and their association with viral control and disease progression, and how NHP models were critical in detailing each finding. Further, we will discuss the effect of NK cell depletion in SIV-infected NHP and the characteristics of newly described memory NK cells in NHP models and different mouse strains. Overall, we propose that the role of NK cells in controlling viral infections remains incompletely understood and that NHP models are indispensable in order to efficiently address these deficits.

Knowns and Unknowns of Assaying Antibody-Dependent Cell-Mediated Cytotoxicity Against HIV-1

It is now well-accepted that Fc-mediated effector functions, including antibody-dependent cellular cytotoxicity (ADCC), can contribute to vaccine-elicited protection as well as post-infection control of HIV viremia. This picture was derived using a wide array of ADCC assays, no two of which are strictly comparable, and none of which is qualified at the clinical laboratory level. An earlier comparative study of assay protocols showed that while data from different ADCC assay formats were often correlated, they remained distinct in terms of target cells and the epitopes and antigen(s) available for recognition by antibodies, the effector cells, and the readout of cytotoxicity. This initial study warrants expanded analyses of the relationships among all current assay formats to determine where they detect overlapping activities and where they do not. Here we summarize knowns and unknowns of assaying ADCC against HIV-1.

Human cytomegalovirus IL-10 augments NK cell cytotoxicity

Human cytomegalovirus (HCMV) persistently infects most of the adult population with periods of productive and latent infection differentially orchestrated by multiple HCMV-encoded gene products. One HCMV gene (UL111a) encodes cmvIL-10, a virokine homologous to human IL (hIL)-10. Although the effects of cmvIL-10 on most human lymphocyte subsets have been extensively studied, its impact on NK cell function was unreported prior to this study. We investigated effects of short-term cmvIL-10 exposure on human NK cells and found it substantially enhanced NK cell cytotoxicity through natural cytotoxicity receptors NKp30 and NKp46 as well as through C-type lectin-like receptors NKG2C and NKG2D. Antibody-dependent cell-mediated cytotoxicity triggered through CD16 also increased significantly with short-term cmvIL-10 exposure. These effects of cmvIL-10 on NK cell cytotoxicity were rapid, dose dependent, neutralized by polyclonal anti-cmvIL-10 or monoclonal anti-IL-10 receptor (IL-10R) antibodies and independent of increased perforin synthesis or up-regulation of activating receptors. A low percentage (0.5-5.4%; n = 12) of NK cells expressed IL-10R and the impact of cmvIL-10 on NK cells degranulation following CD16 stimulation directly correlated with this percentage (P = 0.0218). Short-term exposure of human NK cells to cmvIL-10 did not introduce phenotypic changes reminiscent of NK adaptation to HCMV infection in vivo. Determining how expression of a viral protein that activates NK cells contributes to their function in vivo will increase understanding of HCMV infection and NK cell biology.

Optimizing intracellular signaling domains for CAR NK cells in HIV immunotherapy: a comprehensive review

Natural killer (NK) cells are innate immune lymphocytes with a key role in host defense against HIV infection. Recent advances in chimeric antigen receptors (CARs) have made NK cells a prime target for expressing recombinant receptors capable of redirecting NK cytotoxic functions towards HIV-infected cells. In this review, we discuss the role of NK cells in HIV and the mechanisms of actions of HIV-targeting CAR strategies. Furthermore, we also review NK cells signal transduction and its application to CAR NK cell strategies to develop new combinations of CAR intracellular domains and to improve CAR NK signaling and cytotoxic functions.

Cytomegalovirus-Driven Adaption of Natural Killer Cells in NKG2Cnull Human Immunodeficiency Virus-Infected Individuals

Expansion of natural killer (NK) cells expressing NKG2C occurs following human cytomegalovirus (HCMV) infection and is amplified by human immunodeficiency virus (HIV) co-infection. These NKG2C-expressing NK cells demonstrate enhanced CD16-dependent cytokine production and downregulate FcεRIγ and promyelocytic leukemia zinc finger protein (PLZF). Lacking NKG2C diminishes resistance to HIV infection, but whether this affects NK cell acquisition of superior antibody-dependent function is unclear. Therefore, our objective was to investigate whether HCMV-driven NK cell differentiation is impaired in NKG2C^null HIV-infected individuals. Phenotypic (CD2, CD16, CD57, NKG2A, FcεRIγ, and PLZF expression) and functional (cytokine induction and cytotoxicity) properties were compared between HIV⁻infected NKG2C^null and NKG2C-expressing groups. Cytokine production was compared following stimulation through natural cytotoxicity receptors or through CD16. Cytotoxicity was measured by anti-CD16-redirected lysis and by classical antibody-dependent cell-mediated cytotoxicity (ADCC) against anti-class I human leukocyte antigen (HLA) antibody-coated cells. Our data indicate highly similar HCMV-driven NK cell differentiation in HIV infection with or without NKG2C. While the fraction of mature (CD57^pos) NK cells expressing CD2 (p = 0.009) or co-expressing CD2 and CD16 (p = 0.03) was significantly higher in NKG2C^null HIV-infected individuals, there were no significant differences in NKG2A, FcεRIγ, or PLZF expression. The general phenotypic and functional equivalency observed suggests NKG2C-independent routes of HCMV-driven NK cell differentiation, which may involve increased CD2 expression.

From the "missing self" hypothesis to adaptive NK cells: Insights of NK cell-mediated effector functions in immune surveillance

The original discovery of NK cells approximately 40 yr ago was based on their unique capability to kill tumor cells without prior sensitization or priming, a process named natural cytotoxicity. Since then, several studies have documented that NK cells can kill hematopoietic and nonhematopoietic cancer cells. NK cells also recognize and kill cells that have undergone viral infections. Besides natural cytotoxicity, NK cells are also major effectors of antibody-dependent cell cytotoxicity (ADCC). Therefore, NK cells are well "armed" to recognize and mount immune responses against "insults" that result from cell transformation and viral infections. Because of these attributes, an essential role of NK cells in tumor surveillance was noted. Indeed, several studies have shown a correlation between impaired NK cell cytotoxicity and a higher risk of developing cancer. This evidence led to the idea that cancer initiation and progress is intimately related to an abnormal or misdirected immune response. Whereas all these ideas remain current, it is also true that NK cells represent a heterogeneous population with different abilities to secrete cytokines and to mediate cytotoxic functions. In addition, recent data has shown that NK cells are prone to suffer epigenetic modifications resulting in the acquisition of previously unrecognized attributes such as memory and long-term survival. Such NK cells, referred as "adaptive" or "memory-like," also display effector functions that are not necessarily equal to those observed in conventional NK cells. Given the new evidence available, it is essential to discuss the conceptual reasoning and misconceptions regarding the role of NK cells in immune surveillance and immunotherapy.

NK-cell responses are biased towards CD16-mediated effector functions in chronic hepatitis B virus infection

BACKGROUND & AIMS

Phenotypic and functional natural killer (NK)-cell alterations are well described in chronic hepatitis B virus (cHBV) infection. However, it is largely unknown whether these alterations result from general effects on the overall NK-cell population or the emergence of distinct NK-cell subsets. Human cytomegalovirus (HCMV) is common in cHBV and is associated with the emergence of memory-like NK cells. We aimed to assess the impact of these cells on cHBV infection.

METHODS

To assess the impact of memory-like NK cells on phenotypic and functional alterations in cHBV infection, we performed in-depth analyses of circulating NK cells in 52 patients with cHBV, 45 with chronic hepatitis C virus infection and 50 healthy donors, with respect to their HCMV serostatus.

RESULTS

In patients with cHBV/HCMV+, FcεRIγ- memory-like NK cells were present in higher frequencies and with higher prevalence than in healthy donors with HCMV+. This pronounced HCMV-associated memory-like NK-cell expansion could be identified as key determinant of the NK-cell response in cHBV infection. Furthermore, we observed that memory-like NK cells consist of epigenetically distinct subsets and exhibit key metabolic characteristics of long-living cells. Despite ongoing chronic infection, the phenotype of memory-like NK cells was conserved in patients with cHBV/HCMV+. Functional characteristics of memory-like NK cells also remained largely unaffected by cHBV infection with the exception of an increased degranulation capacity in response to CD16 stimulation that was, however, detectable in both memory-like and conventional NK cells.

CONCLUSIONS

The emergence of HCMV-associated memory-like NK cells shapes the overall NK-cell response in cHBV infection and contributes to a general shift towards CD16-mediated effector functions. Therefore, HCMV coinfection needs to be considered in the design of immunotherapeutic approaches that target NK cells in cHBV.

LAY SUMMARY

In chronic hepatitis B virus infection, natural killer (NK)-cell phenotype and function is altered. In this study, we demonstrate that these changes are linked to the emergence of a distinct NK-cell subset, namely memory-like NK cells. The emergence of these memory-like NK cells is associated with coinfection of human cytomegalovirus that affects the majority of patients with chronic hepatitis B.

Memory NK Cell Features Exploitable in Anticancer Immunotherapy

Besides their innate ability to rapidly produce effector cytokines and kill virus-infected or transformed cells, natural killer (NK) cells display a strong capability to adapt to environmental modifications and to differentiate into long-lived, hyperfunctional populations, dubbed memory or memory-like NK cells. Despite significant progress in the field of NK cell-based immunotherapies, some factors including their short life span and the occurrence of a tumor-dependent functional exhaustion have limited their clinical efficacy so that strategies aimed at overcoming these limitations represent one of the main current challenges in the field. In this scenario, the exploitation of NK cell memory may have a considerable potential. This article summarizes recent evidence in the literature on the peculiar features that render memory NK cells an attractive tool for antitumor immunotherapy, including their long-term survival and in vivo persistence, the resistance to tumor-dependent immunosuppressive microenvironment, the amplified functional responses to IgG-opsonized tumor cells, and in vitro expansion capability. Along with highlighting these issues, we speculate that memory NK cell-based adoptive immunotherapy settings would greatly take advantage from the combination with tumor-targeting therapeutic antibodies (mAbs), as a strategy to fully unleash their clinical efficacy.

Trained Memory of Human Uterine NK Cells Enhances Their Function in Subsequent Pregnancies

Natural killer cells (NKs) are abundant in the human decidua, regulating trophoblast invasion and angiogenesis. Several diseases of poor placental development are associated with first pregnancies, so we thus looked to characterize differences in decidual NKs (dNKs) in first versus repeated pregnancies. We discovered a population found in repeated pregnancies, which has a unique transcriptome and epigenetic signature, and is characterized by high expression of the receptors NKG2C and LILRB1. We named these cells Pregnancy Trained decidual NK cells (PTdNKs). PTdNKs have open chromatin around the enhancers of IFNG and VEGFA. Activation of PTdNKs led to increased production and secretion of IFN-γ and VEGFα, with the latter supporting vascular sprouting and tumor growth. The precursors of PTdNKs seem to be found in the endometrium. Because repeated pregnancies are associated with improved placentation, we propose that PTdNKs, which are present primarily in repeated pregnancies, might be involved in proper placentation.

Natural Killer Cells Adapt to Cytomegalovirus Along a Functionally Static Phenotypic Spectrum in Human Immunodeficiency Virus Infection

Events related to HCMV infection drive accumulation of functionally enhanced CD57^posNKG2C^pos adapted NK cells. We investigated NK cell adaptation to HCMV along a proposed continuum progressing from acute activation through maturation and memory formation towards functional exhaustion. Acute exposure to conditioned medium collected 24 h after HCMV infection (HCMVsn) increased NK cell cytotoxicity for all HCMV-seronegative and seropositive donors tested, with mean 38 and 29% boosts in natural and antibody-dependent cell-mediated cytotoxicity (ADCC), respectively. Increases in NK cell cytotoxicity were completely abrogated by blocking type I interferon (IFN) receptors and equivalent responses occurred with exposure to IFN-α2 alone at the same concentration present in HCMVsn. To study longer term effects of HCMV infection, we focused on three groups of human immunodeficiency virus (HIV)-infected subjects distinguished as HCMV-seronegative or HCMV-seropositive with either high (>20%) or low (<6%) fractions of their NK cells expressing NKG2C. The NK cells of all three HIV-infected groups responded to HCMVsn and IFN-α2 in a manner similar to the NK cells of either HCMV-seronegative or seropositive controls. Neither HCMV status, nor the extent of phenotypic evidence of adaptation to HCMV infection significantly affected mean levels of ADCC or CD16-mediated NK cell degranulation and IFN-γ production compared between the HIV-infected groups. Levels of IFN-γ production correlated significantly with the fraction of NK cells lacking FcεRIγ (FcRγ), but not with the fraction of NK cells expressing NKG2C. There was negligible expression of exhaustion markers Lag-3 and PD-1 on NK cells in any of the groups and no significant difference between groups in the fraction of NK cells expressing Tim-3. The fraction of NK cells expressing Tim-3 was unaffected by CD16 stimulation. Relative to the total NK cell population, responses of Tim-3-expressing cells to CD16 stimulation were variably compromised in HCMV seronegative and seropositive groups. In general, NK cell function in response to signaling through CD16 was well preserved in HIV infection and although HCMV had a clear effect on NK cell FcRγ and NKG2C expression, there was little evidence that the level of adaptation to HCMV infection affected CD16-dependent NK cell signaling in HIV infection.

HLA-E Peptide Repertoire and Dimorphism-Centerpieces in the Adaptive NK Cell Puzzle?

Adaptive Natural Killer (NK) cells, a heterogenous subpopulation of human NK cells with a unique phenotypic and functional signature, became arguably one of the central areas of interest in the field. While their existence seems closely associated with prior exposure to human cytomegalovirus (HCMV), many questions regarding their origin and regulation remain unanswered. However, a common denominator for the majority of adaptive NK cells is the expression of the activating heterodimeric receptor CD94/NKG2C that binds to HLA-E, a non-classical HLA molecule, that displays a comparably restricted expression pattern, very limited polymorphism and presents a distinct set of peptides. Recent studies suggest that-in analogy to T cell responses-peptides presented on HLA-E could play an unexpectedly decisive role for the biology of adaptive NK cells. Here, we discuss how this perspective on the CD94/NKG2C-HLA-E axis aligns with the existing literature and speculate about possible translational implication.

Is There Natural Killer Cell Memory and Can It Be Harnessed by Vaccination? Vaccination Strategies Based on NK Cell and ILC Memory

Studies over the last decade have decisively shown that innate immune natural killer (NK) cells exhibit enhanced long-lasting functional responses following a single activation event. With the increased recognition of memory and memory-like properties of NK cells, questions have arisen with regard to their ability to effectively mediate vaccination responses in humans. Moreover, recently discovered innate lymphoid cells (ILCs) could also potentially exhibit memory-like functions. Here, we review different forms of NK cell memory, and speculate about the ability of these cells and ILCs to meaningfully contribute to vaccination responses.

Altered NKp30, NKp46, NKG2D, and DNAM-1 Expression on Circulating NK Cells Is Associated with Tumor Progression in Human Gastric Cancer

Natural killer (NK) cell activity is influenced by a complex integration of signaling pathways activated downstream of both activating and inhibitory surface receptors. The tumor microenvironment can suppress NK cell activity, and there is a great clinical interest in understanding whether modulating tumor-mediated NK cell suppression and/or boosting preexisting NK cell numbers in cancer patients is therapeutically viable. To this light, we characterized the surface receptor phenotypes of peripheral blood NK cells and examined their clinical relevance to human gastric cancer (GC). We found that the proportion of peripheral blood NK cells which expressed the activating receptors NKp30, NKp46, NKG2D, and DNAM-1 was significantly decreased in GC patients compared to healthy donors, and that this decrease was positively associated with tumor progression. At the same time, plasma TGF-β1 concentrations were significantly increased in GC patients and negatively correlated with the proportion of NKp30, NKp46, NKG2D, and DNAM-1 expressing NK cells. Furthermore, TGF-β1 significantly downregulated the expression of NKp30, NKp46, NKG2D, and DNAM-1 on NK cells in vitro, and the addition of galunisertib, an inhibitor of the TGF-β receptor subunit I, reversed this downregulation. Altogether, our data suggest that the decreased expression of activating receptors NKp30, NKp46, NKG2D, and DNAM-1 on peripheral blood NK cells is positively associated with GC progression, and that TGF-β1-mediated NK cell suppression may be a therapeutically targetable characteristic of GC.

Distinct HLA-E Peptide Complexes Modify Antibody-Driven Effector Functions of Adaptive NK Cells

Adaptive NK cells are characterized by profound alterations in multiple signaling molecules, transcription factors, and epigenetic modifications compared with canonical NK cells. Although their existence is associated with prior exposure to human cytomegalovirus (HCMV), key questions regarding their regulation and function remain. A large proportion of adaptive NK cells express the activating receptor CD94/NKG2C, binding to human leukocyte antigen E (HLA-E), that presents a limited set of peptides. We show that adaptive NK cells discriminate differences between HLA-E-peptide complexes with exquisite specificity. Prolonged exposure to an environment displaying the HLA-E peptide ligand VMAPRTLFL, derived from the leader sequence of HLA-G, enriched adaptive NK cells with low FcεRγ expression, upregulated CD25 expression, increased proliferative activity, and resulted in elevated antibody-dependent cellular cytotoxicity and IFN-γ responses compared with other HLA-E peptide complexes. Our study demonstrates that recognition of alterations in the HLA-E ligandome via an activating receptor can influence heterologous effector mechanisms and proliferation in adaptive NK cells.

Late Development of FcεRγneg Adaptive Natural Killer Cells Upon Human Cytomegalovirus Reactivation in Umbilical Cord Blood Transplantation Recipients

In human natural killer (NK) cells, human cytomegalovirus (HCMV) has been shown to be a driving force capable of inducing the expansion of a highly differentiated NKG2C⁺CD57⁺ subset, persisting over time in both HCMV⁺ healthy subjects and umbilical cord blood transplantation (UCBT) recipients experiencing HCMV viral reactivation. In HCMV⁺ healthy subjects, such expanded NK-cells are characterized by epigenetic modifications that modulate their phenotypic and functional characteristics. In particular, an enhanced ADCC activity is detectable in NK cells lacking the signaling protein FcεRγ. Timing and mechanisms involved in the acquisition of HCMV-induced, adaptive-like features by NK cells are currently unknown. In this study, we investigated the de novo acquisition of several adaptive features in NK cells developing after UCBT by monitoring NK-cell differentiation for at least 2 years after transplant. In UCBT recipients experiencing HCMV reactivation, a rapid phenotypic reconfiguration occurred resulting in the expected expansion of CD56^dim NKG2C⁺CD57⁺ NK cells. However, while certain HCMV-driven adaptive hallmarks, including high KIR, LILRB1, CD2 and low/negative NKG2A, Siglec-7, and CD161 expression, were acquired early after UCBT (namely by month 6), downregulation of the signaling protein FcεRγ was detected at a later time interval (i.e., by month 12). This feature characterized only a minor fraction of the HCMV-imprinted NKG2C⁺CD57⁺ CD56^dim NK cell subset, while it was detectable in higher proportions of CD57⁺ NK cells lacking NKG2C. Interestingly, in patients developing a hyporesponsive CD56^-CD16^bright NK-cell subset, FcεRγ downregulation occurred in these cells earlier than in CD56^dim NK cells. Our data suggest that the acquisition of a fully "adaptive" profile requires signals that may lack in UCBT recipients and/or longer time is needed to obtain a stable epigenetic reprogramming. On the other hand, we found that both HCMV-induced FcεRγ^neg and FcεRγ⁺ NK cells from these patients, display similar CD107a degranulation and IFN-γ production capabilities in response to different stimuli, thus indicating that the acquisition of specialized effector functions can be achieved before the "adaptation" to HCMV is completed. Our study provides new insights in the process leading to the generation of different adaptive NK-cell subsets and may contribute to develop new approaches for their employment as novel immunotherapeutic tools.

Tumor-Targeting Anti-CD20 Antibodies Mediate In Vitro Expansion of Memory Natural Killer Cells: Impact of CD16 Affinity Ligation Conditions and In Vivo Priming

Natural killer (NK) cells represent a pivotal player of innate anti-tumor immune responses. The impact of environmental factors in shaping the representativity of different NK cell subsets is increasingly appreciated. Human cytomegalovirus (HCMV) infection profoundly affects NK cell compartment, as documented by the presence of a CD94/NKG2C⁺FcεRIγ^- long-lived “memory” NK cell subset, endowed with enhanced CD16-dependent functional capabilities, in a fraction of HCMV-seropositive subjects. However, the requirements for memory NK cell pool establishment/maintenance and activation have not been fully characterized yet. Here, we describe the capability of anti-CD20 tumor-targeting therapeutic monoclonal antibodies (mAbs) to drive the selective in vitro expansion of memory NK cells and we show the impact of donor’ HCMV serostatus and CD16 affinity ligation conditions on this event. In vitro expanded memory NK cells maintain the phenotypic and functional signature of their freshly isolated counterpart; furthermore, our data demonstrate that CD16 affinity ligation conditions differently affect memory NK cell proliferation and functional activation, as rituximab-mediated low-affinity ligation represents a superior proliferative stimulus, while high-affinity aggregation mediated by glycoengineered obinutuzumab results in improved multifunctional responses. Our work also expands the molecular and functional characterization of memory NK cells, and investigates the possible impact of CD16 functional allelic variants on their in vivo and in vitro expansions. These results reveal new insights in Ab-driven memory NK cell responses in a therapeutic setting and may ultimately inspire new NK cell-based intervention strategies against cancer, in which the enhanced responsiveness to mAb-bound target could significantly impact therapeutic efficacy.

Downregulation of CD3ζ in NK Cells from Systemic Lupus Erythematosus Patients Confers a Proinflammatory Phenotype

Cytotoxic function and cytokine profile of NK cells are compromised in patients with systemic lupus erythematosus (SLE). CD3ζ, an important molecule for NK cell activation, is downregulated in SLE T cells and contributes to their altered function. However, little is known about the role of CD3ζ in SLE NK cells. We studied CD3ζ levels and its contribution to cytotoxic, degranulation, and cytokine production capacity of NK cells from patients with SLE. Furthermore, we studied the human NK cell line, NKL, in which manipulation of CD3ζ levels was achieved using small interfering RNA and NK cells from Rag2 mice deficient in CD3ζ. We found reduced CD3ζ expression in NK cells from SLE patients independent of disease activity. Downregulation of CD3ζ expression in NK cells is mediated, at least in part, by Caspase 3, the activity of which is higher in NK cells from patients with SLE compared with NK cells from healthy donors. CD3ζ levels correlated inversely with natural cytotoxicity and the percentage of cells capable of producing the proinflammatory cytokines IFN-γ and TNF. In contrast, CD3ζ levels showed a direct correlation with levels of Ab-dependent cellular cytotoxicity. Experiments performed in CD3ζ-silenced NKL and CD3ζ-deficient NK cells from Rag2 mice confirmed the dependence of NK cell function on CD3ζ levels. Our results demonstrate a differential role for CD3ζ in natural cytotoxicity and Ab-dependent cellular cytotoxicity. We conclude that downregulated CD3ζ confers a proinflammatory phenotype to SLE NK cells and contributes to their altered function in patients with SLE.

Origins of natural killer cell memory: special creation or adaptive evolution

The few initial formative studies describing non-specific and apparently spontaneous activity of natural killer (NK) cells have since multiplied into thousands of scientific reports defining their unique capacities and means of regulation. Characterization of the array of receptors that govern NK cell education and activation revealed an unexpected relationship with the major histocompatibility molecules that NK cells originally became well known for ignoring. Proceeding true to form, NK cells continue to up-end archetypal understanding of their ever-expanding capabilities. Discovery that the NK cell repertoire is extremely diverse and can be reshaped by particular viruses into unique subsets of adaptive NK cells challenges, or at least broadens, the definition of immunological memory. This review provides an overview of studies identifying adaptive NK cells, addressing the origins of NK cell memory and introducing the heretical concept of NK cells with extensive antigenic specificity. Whether these newly apparent properties reflect adaptive utilization of known NK cell attributes and receptors or a specially creative allocation from an undefined receptor array remains to be fully determined.

The Broad Spectrum of Human Natural Killer Cell Diversity

Natural killer (NK) cells provide protection against infectious pathogens and cancer. For decades it has been appreciated that two major NK cell subsets (CD56^bright and CD56^dim) exist in humans and have distinct anatomical localization patterns, phenotypes, and functions in immunity. In light of this traditional NK cell dichotomy, it is now clear that the spectrum of human NK cell diversity is much broader than originally appreciated as a result of variegated surface receptor, intracellular signaling molecule, and transcription factor expression; tissue-specific imprinting; and foreign antigen exposure. The recent discoveries of tissue-resident NK cell developmental intermediates, non-NK innate lymphoid cells, and the capacity for NK cells to adapt and differentiate into long-lived memory cells has added further complexity to this field. Here we review our current understanding of the breadth and generation of human NK cell diversity.

Natural Killer Cells in Human Immunodeficiency Virus-1 Infection: Spotlight on the Impact of Human Cytomegalovirus

Human cytomegalovirus (HCMV) has been closely associated with the human race across evolutionary time. HCMV co-infection is nearly universal in human immunodeficiency virus-1 (HIV-1)-infected individuals and remains an important cofactor in HIV-1 disease progression even in the era of effective antiretroviral treatment. HCMV infection has been shown to have a broad and potent influence on the human immune system and has been linked with the discovery and characterization of adaptive natural killer (NK) cells. Distinct NK-cell subsets, predominately expressing the activating receptor NKG2C and the marker of terminal differentiation CD57, expand in response to HCMV. These NK-cell populations engaged in the long-lasting interaction with HCMV, in addition to characteristic but variable expression of surface receptors, exhibit reduced expression of signaling proteins and transcription factors expressed by canonical NK cells. Broad epigenetic modifications drive the emergence and persistence of HCMV-adapted NK cells that have distinct functional characteristics. NKG2C⁺ NK-cell expansions have been observed in HIV-1 infected patients and other acute and chronic viral infections being systematically associated with HCMV seropositivity. The latter is potentially an important confounding variable in studies focused on the cellular NK-cell receptor repertoire and functional capacity. Here, focusing on HIV-1 infection we review the evidence in favor of “adaptive” changes likely induced by HCMV co-infection in NK-cell subsets. We highlight a number of key questions and how insights into the adaptive behavior of NK cells will inform new strategies exploiting their unique properties in the fight against HIV-1.

Phenotypic and Functional Characterization of Circulatory, Splenic, and Hepatic NK Cells in Simian Immunodeficiency Virus-Controlling Macaques

NK cells are key components of the immune system because of their rapid response potential and their ability to mediate cytotoxic and immunomodulatory functions. Additionally, NK cells have recently been shown to persist for long periods in vivo and to have the capacity to establish immunologic memory. In the current study, we assessed the phenotype and function of circulatory and tissue-resident NK cells in a unique cohort of SIV-controlling rhesus macaques that maintained low to undetectable levels of viremia in the chronic phase of infection. By contrasting NK responses of these macaques with those observed in SIV-noncontrolling and uninfected macaques, we aimed to identify markers and activities of NK subpopulations associated with disease control. We show in this article that most differences among NK cells of the three groups of macaques were observed in tissue-resident cells. Although SIV infection resulted in NK cell dysfunction, double-negative NK cells and those expressing CXCR3, NKG2D, and IL-18Rα were associated with viremia control, as was Ab-dependent cytotoxic function. Our results suggest several novel targets for therapeutic intervention.