Redefining Memory: Building the Case for Adaptive NK Cells

Characterization and Potential Applications of Dog Natural Killer Cells in Cancer Immunotherapy

Natural killer (NK) cells of the innate immune system are a key focus of research within the field of immuno-oncology based on their ability to recognize and eliminate malignant cells without prior sensitization or priming. However, barriers have arisen in the effective translation of NK cells to the clinic, in part because of critical species differences between mice and humans. Companion animals, especially dogs, are valuable species for overcoming many of these barriers, as dogs develop spontaneous tumors in the setting of an intact immune system, and the genetic and epigenetic factors that underlie oncogenesis appear to be similar between dogs and humans. Here, we summarize the current state of knowledge for dog NK cells, including cell surface marker phenotype, key NK genes and genetic regulation, similarities and differences of dog NK cells to other mammals, especially human and mouse, expression of canonical inhibitory and activating receptors, ex vivo expansion techniques, and current and future clinical applications. While dog NK cells are not as well described as those in humans and mice, the knowledge of the field is increasing and clinical applications in dogs can potentially advance the field of human NK biology and therapy. Better characterization is needed to truly understand the similarities and differences of dog NK cells with mouse and human. This will allow for the canine model to speed clinical translation of NK immunotherapy studies and overcome key barriers in the optimization of NK cancer immunotherapy, including trafficking, longevity, and maximal in vivo support.

The Good and the Bad of Natural Killer Cells in Virus Control: Perspective for Anti-HBV Therapy

Immune modulatory therapies are widely believed to represent potential therapeutic strategies for chronic hepatitis B infection (CHB). Among the cellular targets for immune interventions, Natural Killer (NK) cells represent possible candidates because they have a key role in anti-viral control by producing cytokines and by exerting cytotoxic functions against virus-infected cells. However, in patients with chronic hepatitis B, NK cells have been described to be more pathogenic than protective with preserved cytolytic activity but with a poor capacity to produce anti-viral cytokines. In addition, NK cells can exert a regulatory activity and possibly suppress adaptive immune responses in the setting of persistent viral infections. Consequently, a potential drawback of NK-cell targeted modulatory interventions is that they can potentiate the suppressive NK cell effect on virus-specific T cells, which further causes impairment of exhausted anti-viral T cell functions. Thus, clinically useful NK-cell modulatory strategies should be not only suited to improve positive anti-viral NK cell functions but also to abrogate T cell suppression by NK cell-mediated T cell killing. This review outlines the main NK cell features with a particular focus on CHB infection. It describes different mechanisms involved in NK-T cell interplay as well as how NK cells can have positive anti-viral effector functions and negative suppressive effects on T cells activity. This review discusses how modulation of their balance can have potential therapeutic implications.

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.

Simian Immunodeficiency Virus Infection Modulates CD94+ (KLRD1+) NK Cells in Rhesus Macaques

Recently, we and others have shown that natural killer (NK) cells exhibit memory-like recall responses against cytomegalovirus (CMV) and human immunodeficiency/virus simian immunodeficiency virus (HIV/SIV) infections. Although the mechanism(s) have not been fully delineated, several groups have shown that the activating receptor NKG2C is elevated on NK cells in the context of rhesus CMV (rhCMV) or human CMV (hCMV) infections. CD94, which heterodimerizes with NKG2C is also linked to adaptive NK cell responses. Because nonhuman primates (NHP) play a crucial role in modeling HIV (SIV) infections, it is crucial to be able to assess and characterize the NKG2 family in NHP. Unfortunately, it is not possible to detect CD94 using commercially available antibodies in NHP. Our work, a first for NHP, has focused on developing RNA flow cytometry using mRNA transcripts as proxies distinguishing NKG2C from NKG2A. We have expanded the application of this technology and here we show the first characterization of CD94⁺ (KLRD1⁺) NK cells in NHP using multiparametric RNA flow cytometry. Peripheral blood mononuclear cells from naive and matched acutely (n = 4) or chronically (n = 12) SIV-infected rhesus macaques were analyzed by flow cytometry using commercially available antibodies, determining expression of transcripts for NKG2A, NKG2C, and CD94 (KLRC1, KLRC2, and KLRD1, respectively) on NK cells using RNA flow cytometry. Our data show that KLRC1^+/- KLRC2⁺ KLRD1⁺ NK cells decrease following chronic, but not acute, infection with SIV. This approach will allow us to investigate the kinetics of infection and NK memory formation and will further improve our understanding of basic NK cell biology, especially in the context of SIV infection.IMPORTANCE Nonhuman primates play a crucial role in approximating human biology and many diseases that are difficult, if not impossible, to achieve in other animal models, notably HIV. Current advances in adaptive NK cell research positions us to address fundamental deficiencies in our fight against infection and disease at the earliest moments after infection or substantially earlier in disease progression. We show here that we can identify specific NK cell subpopulations that are modulated following chronic, but not acute, SIV infection. The ability to identify these subsets more precisely will inform therapeutic and vaccine strategies targeting an optimized NK cell response.

Battle between Host Immune Cellular Responses and HCMV Immune Evasion

Human cytomegalovirus (HCMV) is ubiquitously prevalent. HCMV infection is typically asymptomatic and controlled by the immune system in healthy individuals, yet HCMV can be severely pathogenic for the fetus during pregnancy and in immunocompromised persons, such as transplant recipients or HIV infected patients. HCMV has co-evolved with the hosts, developed strategies to hide from immune effector cells and to successfully survive in the human organism. One strategy for evading or delaying the immune response is maintenance of the viral genome to establish the phase of latency. Furthermore, HCMV immune evasion involves the downregulation of human leukocyte antigens (HLA)-Ia molecules to hide infected cells from T-cell recognition. HCMV expresses several proteins that are described for downregulation of the HLA class I pathway via various mechanisms. Here, we review the wide range of immune evasion mechanisms of HCMV. Understanding the mechanisms of HCMV immune evasion will contribute to the development of new customized therapeutic strategies against the virus.

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.

Semaphorin 7A modulates cytokine-induced memory-like responses by human natural killer cells

Cytokine-induced memory-like (CIML) NK cells are endowed with the capacity to mediate enhanced effector functions upon cytokine or activating receptor restimulation for several weeks following short-term preactivation with IL-12, IL-15, and IL-18. Promising results from a first-in-human clinical trial highlighted the clinical potential of CIML NK cells as adoptive immunotherapy for patients with hematologic malignancies. However, the mechanisms underlying CIML NK cell differentiation and increased functionality remain incompletely understood. Semaphorin 7A (SEMA7A) is a potent immunomodulator expressed in activated lymphocytes and myeloid cells. In this study, we show that SEMA7A is substantially upregulated on NK cells stimulated with cytokines, and specifically marks activated NK cells with a strong potential to release IFN-γ. In particular, preactivation of NK cells with IL-12+IL-15+IL-18 resulted in greater than tenfold upregulation of SEMA7A and enhanced expression of the ligand for SEMA7A, integrin-β1, on CIML NK cells. Strikingly, preactivation in the presence of antibodies targeting SEMA7A lead to significantly decreased IFN-γ production following restimulation. These results imply a novel mechanism by which cytokine-enhanced SEMA7A/integrin-β1 interaction promotes CIML NK cell differentiation and maintenance of increased functionality. Our data suggest that targeting SEMA7A/integrin-β1 signaling might provide a novel immunotherapeutic approach to potentiate antitumor activity of CIML NK cells.

The Role of the Immune System in Cutaneous Squamous Cell Carcinoma

Cutaneous squamous cell carcinoma (cSCC) is the second most common skin cancer. In immunosuppressed populations it is a source of considerable morbidity and mortality due to its enhanced recurrence and metastatic potential. In common with many malignancies, leucocyte populations are both protective against cancer development and also play a role in 'sculpting' the nascent tumor, leading to loss of immunogenicity and tumor progression. UV radiation and chronic viral carriage may represent unique risk factors for cSCC development, and the immune system plays a key role in modulating the response to both. In this review, we discuss the lessons learned from animal and ex vivo human studies of the role of individual leucocyte subpopulations in the development of cutaneous SCC. We then discuss the insights into cSCC immunity gleaned from studies in humans, particularly in populations receiving pharmacological immunosuppression such as transplant recipients. Similar insights in other malignancies have led to exciting and novel immune therapies, which are beginning to emerge into the cSCC clinical arena.

Setting traps for NKG2A gives NK cell immunotherapy a fighting chance

The equilibrium of signaling through activating and inhibitory receptors dictates whether a given NK cell will execute cellular cytotoxicity. In this issue of the JCI, Kamiya et al. describe a novel approach to efficiently inhibiting surface expression of the inhibitory receptor CD94/NK group 2 member A (NKG2A) through retention of the protein in the endoplasmic reticulum. In adoptive transfer experiments into tumor-bearing immunodeficient mice, NKG2Anull NK cells were significantly more effective at eliminating HLA-E-expressing tumor cells than NKG2A+ NK cells. This study provides proof of concept for a new immunotherapeutic approach using NKG2Anull NK cells.

CXCR6+ NK Cells in Human Fetal Liver and Spleen Possess Unique Phenotypic and Functional Capabilities

Tissue-resident Natural Killer (NK) cells vary in phenotype according to tissue origin, but are typically CD56^bright, CXCR6⁺, and CD69⁺. NK cells appear very early in fetal development, but little is known about when markers of tissue residency appear during gestation and whether the expression of these markers, most notably the chemokine receptor CXCR6, are associated with differences in functional capability. Using multi-parametric flow cytometry, we interrogated fetal liver and spleen NK cells for the expression of a multitude of extracellular markers associated with NK cell maturation, differentiation, and migration. We analyzed total NK cells from fetal liver and spleen and compared them to their adult liver and spleen counterparts, and peripheral blood (PB) NK. We found that fetal NK cells resemble each other and their adult counterparts more than PB NK. Maturity markers including CD16, CD57, and KIR are lower in fetal NK cells than PB, and markers associated with an immature phenotype are higher in fetal liver and spleen NK cells (NKG2A, CD94, and CD27). However, T-bet/EOMES transcription factor profiles are similar amongst fetal and adult liver and spleen NK cells (T-bet⁻/EOMES⁺) but differ from PB NK cells (T-bet⁺EOMES⁻). Further, donor-matched fetal liver and spleen NK cells share similar patterns of expression for most markers as a function of gestational age. We also performed functional studies including degranulation, cytotoxicity, and antibody-dependent cellular cytotoxicity (ADCC) assays. Fetal liver and spleen NK cells displayed limited cytotoxic effector function in chromium release assays but produced copious amounts of TNFα and IFNγ, and degranulated efficiently in response to stimulation with PMA/ionomycin. Further, CXCR6⁺ NK cells in fetal liver and spleen produce more cytokines and degranulate more robustly than their CXCR6⁻ counterparts, even though CXCR6⁺ NK cells in fetal liver and spleen possess an immature phenotype. Major differences between CXCR6⁻ and + NK cell subsets appear to occur later in development, as a distinct CXCR6⁺ NK cell phenotype is much more clearly defined in PB. In conclusion, fetal liver and spleen NK cells share similar phenotypes, resemble their adult counterparts, and already possess a distinct CXCR6⁺ NK cell population with discrete functional capabilities.

Mutually assured destruction: the cold war between viruses and natural killer cells

Natural killer (NK) cells play a multitude of antiviral roles that are significant enough to provoke viral counterefforts to subvert their activity. As innate lymphocytes, NK cells provide a rapid source of pro-inflammatory antiviral cytokines and bring to bear cytolytic activities that are collectively meant to constrain viral replication and dissemination. Additionally, NK cells participate in adaptive immunity both by shaping virus-specific T-cell responses and by developing adaptive features themselves, including enhanced antibody-dependent effector functions. The relative importance of different functional activities of NK cells are poorly understood, thereby obfuscating clinical use of these cells. Here we focus on opposing efforts of NK cells and viruses to gain tactical superiority during infection.

NK cell immune responses differ after prime and boost vaccination

A better understanding of innate responses induced by vaccination is critical for designing optimal vaccines. Here, we studied the diversity and dynamics of the NK cell compartment after prime-boost immunization with the modified vaccinia virus Ankara using cynomolgus macaques as a model. Mass cytometry was used to deeply characterize blood NK cells. The NK cell subphenotype composition was modified by the prime. Certain phenotypic changes induced by the prime were maintained over time and, as a result, the NK cell composition prior to boost differed from that before prime. The key phenotypic signature that distinguished NK cells responding to the boost from those responding to the prime included stronger expression of several cytotoxic, homing, and adhesion molecules, suggesting that NK cells at recall were functionally distinct. Our data reveal potential priming or imprinting of NK cells after the first vaccine injection. This study provides novel insights into prime-boost vaccination protocols that could be used to optimize future vaccines.

Adaptive NK cell responses in HIV/SIV infections: A roadmap to cell-based therapeutics?

NK cells play a critical role in antiviral and antitumor responses. Although current NK cell immune therapies have focused primarily on cancer biology, many of these advances can be readily applied to target HIV/simian immunodeficiency virus (SIV)-infected cells. Promising developments include recent reports that CAR NK cells are capable of targeted responses while producing less off-target and toxic side effects than are associated with CAR T cell therapies. Further, CAR NK cells derived from inducible pluripotent stem cells or cell lines may allow for more rapid "off-the-shelf" access. Other work investigating the IL-15 superagonist ALT-803 (now N803) may also provide a recourse for enhancing NK cell responses in the context of the immunosuppressive and inflammatory environment of chronic HIV/SIV infections, leading to enhanced control of viremia. With a broader acceptance of research supporting adaptive functions in NK cells it is likely that novel immunotherapeutics and vaccine modalities will aim to generate virus-specific memory NK cells. In doing so, better targeted NK cell responses against virus-infected cells may usher in a new era of NK cell-tuned immune therapy.

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.

Off-the-shelf cell therapy with induced pluripotent stem cell-derived natural killer cells

Cell therapy is emerging as a very promising therapeutic modality against cancer, spearheaded by the clinical success of chimeric antigen receptor (CAR) modified T cells for B cell malignancies. Currently, FDA-approved CAR-T cell products are based on engineering of autologous T cells harvested from the patient, typically using a central manufacturing facility for gene editing before the product can be delivered to the clinic and infused to the patients. For a broader implementation of advanced cell therapy and to reduce costs, it would be advantageous to use allogeneic "universal" cell therapy products that can be stored in cell banks and provided upon request, in a manner analogous to biopharmaceutical drug products. In this review, we outline a roadmap for development of off-the-shelf cell therapy based on natural killer (NK) cells derived from induced pluripotent stem cells (iPSCs). We discuss strategies to engineer iPSC-derived NK (iPSC-NK) cells for enhanced functional potential, persistence, and homing.

Tracking KLRC2 (NKG2C)+ memory-like NK cells in SIV+ and rhCMV+ rhesus macaques

Natural killer (NK) cells classically typify the nonspecific effector arm of the innate immune system, but have recently been shown to possess memory-like properties against multiple viral infections, most notably CMV. Expression of the activating receptor NKG2C is elevated on human NK cells in response to infection with CMV as well as HIV, and may delineate cells with memory and memory-like functions. A better understanding of how NKG2C+ NK cells specifically respond to these pathogens could be significantly advanced using nonhuman primate (NHP) models but, to date, it has not been possible to distinguish NKG2C from its inhibitory counterpart, NKG2A, in NHP because of unfaithful antibody cross-reactivity. Using novel RNA-based flow cytometry, we identify for the first time true memory NKG2C+ NK cells in NHP by gene expression (KLRC2), and show that these cells have elevated frequencies and diversify their functional repertoire specifically in response to rhCMV and SIV infections.

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.