Expansion of CD94/NKG2C+ NK cells in response to human cytomegalovirus-infected fibroblasts

Regulation of Adaptive NK Cells and CD8 T Cells by HLA-C Correlates with Allogeneic Hematopoietic Cell Transplantation and with Cytomegalovirus Reactivation

Mass cytometry was used to investigate the effect of CMV reactivation on lymphocyte reconstitution in hematopoietic cell transplant patients. For eight transplant recipients (four CMV negative and four CMV positive), we studied PBMCs obtained 6 mo after unrelated donor hematopoietic cell transplantation (HCT). Forty cell-surface markers, distinguishing all major leukocyte populations in PBMC, were analyzed with mass cytometry. This group included 34 NK cell markers. Compared with healthy controls, transplant recipients had higher HLA-C expression on CD56(-)CD16(+) NK cells, B cells, CD33(bright) myeloid cells, and CD4CD8 T cells. The increase in HLA-C expression was greater for CMV-positive HCT recipients than for CMV negative recipients. Present in CMV-positive HCT recipients, but not in CMV-negative HCT recipients or controls, is a population of killer cell Ig-like receptor (KIR)-expressing CD8 T cells not previously described. These CD8 T cells coexpress CD56, CD57, and NKG2C. The HCT recipients also have a population of CD57(+)NKG2A(+) NK cells that preferentially express KIR2DL1. An inverse correlation was observed between the frequencies of CD57(+)NKG2C(+) NK cells and CD57(+)NKG2A(+) NK cells. Although CD57(+)NKG2A(+) NK cells are less abundant in CMV-positive recipients, their phenotype is of a more activated cell than the CD57(+)NKG2A(+) NK cells of controls and CMV-negative HCT recipients. These data demonstrate that HCT and CMV reactivation are associated with an increased expression of HLA-C. This could influence NK cell education during lymphocyte reconstitution. The increased inhibitory KIR expression by proliferating CMV-specific CD8 T cells suggests regulatory interactions between HLA-C and KIR might promote Graft-versus-Leukemia effects following transplantation.

Adaptive Natural Killer Cell and Killer Cell Immunoglobulin-Like Receptor-Expressing T Cell Responses are Induced by Cytomegalovirus and Are Associated with Protection against Cytomegalovirus Reactivation after Allogeneic Donor Hematopoietic Cell Transplantation

Cytomegalovirus (CMV) reactivates in >30% of CMV-seropositive patients after allogeneic hematopoietic cell transplantation (HCT). Previously, we reported an increase of natural killer (NK) cells expressing NKG2C, CD57, and inhibitory killer cell immunoglobulin-like receptors (KIRs) in response to CMV reactivation after HCT. These NK cells persist after the resolution of infection and display "adaptive" or memory properties. Despite these findings, the differential impact of persistent/inactive versus reactivated CMV on NK versus T cell maturation after HCT from different graft sources has not been defined. We compared the phenotype of NK and T cells from 292 recipients of allogeneic sibling (n = 118) or umbilical cord blood (UCB; n = 174) grafts based on recipient pretransplantation CMV serostatus and post-HCT CMV reactivation. This cohort was utilized to evaluate CMV-dependent increases in KIR-expressing NK cells exhibiting an adaptive phenotype (NKG2C(+)CD57(+)). Compared with CMV-seronegative recipients, those who reactivated CMV had the highest adaptive cell frequencies, whereas intermediate frequencies were observed in CMV-seropositive recipients harboring persistent/nonreplicating CMV. The same effect was observed in T cells and CD56(+) T cells. These adaptive lymphocyte subsets were increased in CMV-seropositive recipients of sibling but not UCB grafts and were correlated with lower rates of CMV reactivation (sibling 33% versus UCB 51%; P < .01). These data suggest that persistent/nonreplicating recipient CMV induces rapid production of adaptive NK and T cells from mature cells from sibling but not UCB grafts. These adaptive lymphocytes are associated with protection from CMV reactivation.

Molecular characterization of HCMV-specific immune responses: Parallels between CD8(+) T cells, CD4(+) T cells, and NK cells

CD8(+) T cells are important for immunity against human cytomegalovirus (HCMV). The HCMV-specific CD8(+) T-cell response is characterized by the accumulation of terminally differentiated effector cells that have downregulated the costimulatory molecules CD27 and CD28. These HCMV-specific CD8(+) T cells maintain high levels of cytotoxic molecules such as granzyme B and rapidly produce the inflammatory cytokine IFN-γ upon activation. Remarkably, HCMV-specific CD8(+) T cells are able to persist long term as fully functional effector cells, suggesting a unique differentiation pathway that is distinct from the formation of memory CD8(+) T cells after infection with acute viruses. In this review, we aim to highlight the most recent developments in HCMV-specific CD8(+) T-cell differentiation, maintenance, tissue distribution, metabolism and function. HCMV also induces the differentiation of effector CD4(+) T cells and NK cells, which share characteristics with HCMV-specific CD8(+) T cells. We propose that the overlap in differentiation of NK cells, CD4(+) and CD8(+) T cells after HCMV infection may be regulated by a shared transcriptional machinery. A better understanding of the molecular framework of HCMV-specific CD8(+) T-cell responses may benefit vaccine design, as these cells uniquely combine the capacity to rapidly respond to infection with long-term survival.

HLA-E regulates NKG2C+ natural killer cell function through presentation of a restricted peptide repertoire

NK cells interact with the HLA-E molecule via the inhibitory receptor NKG2A and the activating receptor NKG2C. Hence, HLA-E can have a dual role in the immune response. In the present study, we aim to investigate the functional consequences of HLA-E for NKG2A and NKG2C expressing NK cell subsets by using a panel of HLA-E binding peptides derived from CMV, Hsp60 and HLA class I. PBMC derived from healthy subjects were used as targets for isolated NK cells and NK cell activation was examined by analysis of the expression of the degranulation marker CD107a. Peptide induced HLA-E expression inhibited degranulation of NKG2A+ NK cell subsets with almost all peptides, whereas NKG2A- NKG2C+ NK cell responses were enhanced only after incubation with four peptides; 1.3-fold with CMV(I), A80 and B13 and 3.2-fold with HLA-G derived peptide. In addition, the HLA-E:G peptide complex triggered NKG2C receptor internalization, as evidenced by reduction in the percentage of NKG2C+ NK cells when incubated with the peptide, which could be restored by addition of Bafilomycin.

IN CONCLUSION

in contrast to NKG2A, NKG2C is regulated by HLA-E only when HLA-E is in complex with a restricted peptide repertoire, especially in combination with the HLA-G leader peptide.

Comparison of FcRγ-Deficient and CD57+ Natural Killer Cells Between Cord Blood and Adult Blood in the Cytomegalovirus-Endemic Korean Population

Background

FcRγ-deficient natural killer (NK) cells (g^-NK cells) have been associated with cytomegalovirus (CMV) infection. However, the frequency of g^-NK cells in a CMV-endemic area (i.e., Korea) has not yet been studied. We examined the frequency of g^-NK cells and expression of CD57 on NK cells in cord blood (CB) and adult blood (AB).

Methods

Of the 24 AB samples collected, 95.8% (23/24) were CMV IgG⁺/IgM^-, while 100% of the 13 healthy CB samples were CMV IgG⁺/IgM^-. We performed whole-blood flow cytometry assays to analyze intracellular FcRγ and CD3ζ expression of CD3^-/CD56^dim NK cells from 13 CB and 24 AB samples, and surface CD57 expression on CD3^-/CD56^dim/CD16⁺ NK cells from 13 CB and 19 AB samples.

Results

All CMV seropositive AB samples contained g^-NK cells (23/23), and the median proportion of g^-NK cells in the CD3^-/CD56^dim NK cell pool was 35.0% (range: 11-77%). CD57⁺ NK cells in the CD3^-/CD56^dim/CD16⁺ NK cell population were detected in all 19 AB samples tested, but not in any CB samples.

Conclusions

Our data suggest that g^-NK cells and CD57⁺ NK cells are present at a very high frequency in CMV-seropositive AB, but rare in CMV-naïve CB.

Epigenetic modification and antibody-dependent expansion of memory-like NK cells in human cytomegalovirus-infected individuals

Long-lived "memory-like" NK cells have been identified in individuals infected by human cytomegalovirus (HCMV), but little is known about how the memory-like NK cell pool is formed. Here, we have shown that HCMV-infected individuals have several distinct subsets of memory-like NK cells that are often deficient for multiple transcription factors and signaling proteins, including tyrosine kinase SYK, for which the reduced expression was stable over time and correlated with epigenetic modification of the gene promoter. Deficient expression of these proteins was largely confined to the recently discovered FcRγ-deficient NK cells that display enhanced antibody-dependent functional activity. Importantly, FcRγ-deficient NK cells exhibited robust preferential expansion in response to virus-infected cells (both HCMV and influenza) in an antibody-dependent manner. These findings suggest that the memory-like NK cell pool is shaped and maintained by a mechanism that involves both epigenetic modification of gene expression and antibody-dependent expansion.

Human Cytokine-Induced Memory-Like Natural Killer Cells

Natural killer (NK) cells are innate lymphoid cells that are important for host defense against infection and mediate antitumor responses. Recent reports from several laboratories have identified that NK cells can remember a prior activation event and consequently respond more robustly when restimulated, a property termed innate NK cell memory. NK cell memory has now been identified following hapten exposure, viral infection, and combined cytokine preactivation with IL-12, IL-15, and IL-18. Many questions in the field remain regarding the cellular and molecular mechanisms regulating memory NK cells and their responses, as well as their formation and function in mice and humans. Here we review our current understanding of cytokine-induced memory-like (CIML) NK cells that are generated by combined preactivation with IL-12, IL-15, and IL-18. These cells exhibit enhanced NK cell effector functions weeks after the initial cytokine preactivation. Further, we highlight the preclinical rationale and ongoing therapeutic application of CIML NK cells for adoptive immunotherapy in patients with hematologic malignancies.

Therapeutic potential and challenges of natural killer cells in treatment of solid tumors

Natural killer (NK) cells are innate lymphoid cells that hold tremendous potential for effective immunotherapy for a broad range of cancers. Due to the mode of NK cell killing, requiring one-to-one target engagement and site-directed release of cytolytic granules, the therapeutic potential of NK cells has been most extensively explored in hematological malignancies. However, their ability to precisely kill antibody coated cells, cancer stem cells, and genotoxically altered cells, while maintaining tolerance to healthy cells makes them appealing therapeutic effectors for all cancer forms, including metastases. Due to their release of pro-inflammatory cytokines, NK cells may potently reverse the anti-inflammatory tumor microenvironment (TME) and augment adaptive immune responses by promoting differentiation, activation, and/or recruitment of accessory immune cells to sites of malignancy. Nevertheless, integrated and coordinated mechanisms of subversion of NK cell activity against the tumor and its microenvironment exist. Although our understanding of the receptor ligand interactions that regulate NK cell functionality has evolved remarkably, the diversity of ligands and receptors is complex, as is their mechanistic foundations in regulating NK cell function. In this article, we review the literature and highlight how the TME manipulates the NK cell phenotypes, genotypes, and tropism to evade tumor recognition and elimination. We discuss counter strategies that may be adopted to augment the efficacy of NK cell anti-tumor surveillance, the clinical trials that have been undertaken so far in solid malignancies, critically weighing the challenges and opportunities with this approach.

Infections and autoimmunity: role of human cytomegalovirus in autoimmune endothelial cell damage

Molecular mimicry between infectious agents and normal human host cell proteins represents one of the possible mechanisms responsible for autoimmunity. Among infectious agents, human cytomegalovirus (HCMV) is an ideal candidate for involvement in autoimmune disorders because of its lifelong persistence through periods of reactivation and latency and because of the extensive manipulation of innate and adaptive immunity. HCMV has been implicated in the pathogenesis of vascular damage in systemic sclerosis (SSc) and atherosclerosis. Based on our data, which demonstrate a cause-and-effect relationship between HCMV and endothelial cell aggression in SSc and atherosclerosis, we propose that immune responses to particular HCMV proteins may result in autoaggression through a mechanism of molecular mimicry of normally expressed endothelial cell surface molecules.

Generation of Natural Killer Cell Memory during Viral Infection

Immunological memory is classically regarded as an attribute of antigen-specific T and B lymphocytes of the adaptive immune system. Cells of the innate immune system, including natural killer (NK) cells, have been considered short-lived cytolytic cells that can rapidly respond against pathogens in an antigen-independent manner and then die off. However, NK cells have recently been described to possess traits of adaptive immunity, such as clonal expansion after viral antigen exposure to generate long-lived memory cells. In this review, we will discuss the current evidence for viral-induced NK cell memory in both mice and humans.

Balancing natural killer cell activation through paired receptors

Natural killer (NK) cells are innate lymphocytes that are crucial for the control of infections and malignancies. NK cells express a variety of inhibitory and activating receptors that facilitate fine discrimination between damaged and healthy cells. Among them, a family of molecules that bind nectin and nectin-like proteins has recently emerged and has been shown to function as an important regulator of NK cell functions. These molecules include CD226, T cell immunoreceptor with immunoglobulin and ITIM domains (TIGIT), CD96, and cytotoxic and regulatory T cell molecule (CRTAM). In this Review, we focus on the recent advances in our understanding of how these receptors regulate NK cell biology and of their roles in pathologies such as cancer, infection and autoimmunity.

DAP12-based activating chimeric antigen receptor for NK cell tumor immunotherapy

NK cells are emerging as new effectors for immunotherapy of cancer. In particular, the genetic engraftment of chimeric Ag receptors (CARs) in NK cells is a promising strategy to redirect NK cells to otherwise NK cell-resistant tumor cells. On the basis of DNAX-activation protein 12 (DAP12), a signaling adaptor molecule involved in signal transduction of activating NK cell receptors, we generated a new type of CAR targeting the prostate stem cell Ag (PSCA). We demonstrate in this article that this CAR, designated anti-PSCA-DAP12, consisting of DAP12 fused to the anti-PSCA single-chain Ab fragment scFv(AM1) confers improved cytotoxicity to the NK cell line YTS against PSCA-positive tumor cells when compared with a CAR containing the CD3ζ signaling chain. Further analyses revealed phosphorylation of the DAP12-associated ZAP-70 kinase and IFN-γ release of CAR-engineered cells after contact with PSCA-positive target cells. YTS cells modified with DAP12 alone or with a CAR bearing a phosphorylation-defective ITAM were not activated. Notably, infused YTS cells armed with anti-PSCA-DAP12 caused delayed tumor xenograft growth and resulted in complete tumor eradication in a significant fraction of treated mice. The feasibility of the DAP12-based CAR was further tested in human primary NK cells and confers specific cytotoxicity against KIR/HLA-matched PSCA-positive tumor cells, which was further enhanced by KIR-HLA mismatches. We conclude that NK cells engineered with DAP12-based CARs are a promising tool for adoptive tumor immunotherapy.

Mechanisms regulating NK cell activation during viral infection

ABSTRACT 

NK cells constitute a population of lymphocytes involved in innate immune functions. They play a critical role in antiviral immune surveillance. Viruses have evolved with their host species for millions of years, each exerting a selective pressure upon the other. As a corollary, the pathways used by the immune system that are critical to control viral infection can be revealed by defining the role of viral gene products that are nonessential for virus replication. We relate here the battery of resources available to NK cells to recognize and eliminate viruses and reciprocally the immune evasion mechanisms developed by viruses to prevent NK cell activation.

The Past, Present, and Future of NK Cells in Hematopoietic Cell Transplantation and Adoptive Transfer

Hematopoietic cell transplantation (HCT) has been used as a part of cancer therapy for over half a decade. Beyond the necessity for donor-derived cells to reconstitute hematopoiesis after radiation and chemotherapy, immunologic reconstitution from allogeneic cells is important for the elimination of residual tumor cells. Natural killer (NK) cells are first among lymphocytes to reconstitute post-transplant and protect against cancer relapse. In this review, we provide a historical perspective on the role of NK cells in cancer control in the transplant setting and focus on current research aimed at improving NK cell responses for therapeutic benefit.

Haploidentical Haematopoietic Stem Cell Transplantation: Role of NK Cells and Effect of Cytomegalovirus Infections

Natural killer cells play an important role in the immune responses against cancer and viral infections. In addition, NK cells have been shown to exert a key role in haploidentical hematopoietic stem cell (HSC) transplantation for the therapy of high-risk leukemias. The anti-leukemia effect is mostly related to the presence of "alloreactive" NK cells, i.e., mature KIR(+) NK cells that express inhibitory KIR mismatched with HLA class I (KIR-L) of the patient. In addition, an important role is played by certain activating KIR (primarily, but not only, KIR2DS1) upon interaction with their HLA class I ligand (C2 alleles). In general, the presence of activating KIR correlates with a better prognosis. Beside the infusion of "pure" CD34(+) cells, a novel protocol has been recently developed in which depletion of αβ T cells and CD19(+) B cells makes it possible to infuse into the patient, together with donor CD34(+) HSCs, important effector cells including mature PB NK cells and γδ T cells. Recent studies revealed that cytomegalovirus (CMV) infection/reactivation may induce rapid NK cell maturation and greatly influence the NK receptor repertoire. The remarkable expansion of a subset expressing the activating receptor NKG2C, together with a more efficient virus-specific effector response after rechallenge with CMV (i.e., antigen specificity), and the longevity of the expanded population are all features consistent with an adaptive type of response and support the notion of a memory-like activity of NK cells.

Molecular Programming of Immunological Memory in Natural Killer Cells

Immunological memory is a hallmark of the adaptive immune system. Although natural killer (NK) cells have traditionally been classified as a component of the innate immune system, they have recently been shown in mice and humans to exhibit certain features of immunological memory, including an ability to undergo a clonal-like expansion during virus infection, generate long-lived progeny (i.e. memory cells), and mediate recall responses against previously encountered pathogens--all characteristics previously ascribed only to adaptive immune responses by B and T cells in mammals. To date, the molecular events that govern the generation of NK cell memory are not completely understood. Using a mouse model of cytomegalovirus infection, we demonstrate that individual pro-inflammatory IL-12, IL-18, and type I-IFN signaling pathways are indispensible and play non-redundant roles in the generation of virus-specific NK cell memory. Furthermore, we discovered that antigen-specific proliferation and protection by NK cells is mediated by the transcription factor Zbtb32, which is induced by pro-inflammatory cytokines and promotes a cell cycle program in activated NK cells. A greater understanding of the molecular mechanisms controlling NK cell responses will provide novel strategies for tailoring vaccines to target infectious disease.

IL-12-producing monocytes and HLA-E control HCMV-driven NKG2C+ NK cell expansion

Human cytomegalovirus (HCMV) infection is the most common cause of congenital viral infections and a major source of morbidity and mortality after organ transplantation. NK cells are pivotal effector cells in the innate defense against CMV. Recently, hallmarks of adaptive responses, such as memory-like features, have been recognized in NK cells. HCMV infection elicits the expansion of an NK cell subset carrying an activating receptor heterodimer, comprising CD94 and NKG2C (CD94/NKG2C), a response that resembles the clonal expansion of adaptive immune cells. Here, we determined that expansion of this NKG2C(+) subset and general NK cell recovery rely on signals derived from CD14(+) monocytes. In a coculture system, a subset of CD14(+) cells with inflammatory monocyte features produced IL-12 in response to HCMV-infected fibroblasts, and neutralization of IL-12 in this model substantially reduced CD25 upregulation and NKG2C(+) subset expansion. Finally, blockade of CD94/NKG2C on NK cells or silencing of the cognate ligand HLA-E in infected fibroblasts greatly impaired expansion of NKG2C(+) NK cells. Together, our results reveal that IL-12, CD14(+) cells, and the CD94/NKG2C/HLA-E axis are critical for the expansion of NKG2C(+) NK cells in response to HCMV infection. Moreover, strategies targeting the NKG2C(+) NK cell subset have the potential to be exploited in NK cell-based intervention strategies against viral infections and cancer.

Clinical and immunological significance of HLA-E in stem cell transplantation and cancer

Human leukocyte antigen-E (HLA-E) is a nonclassical HLA class I molecule that canonically binds peptides derived from the leader sequence of classical HLA class I. HLA-E can also bind peptides from stress protein [e.g. heat shock protein 60 (Hsp60)] and pathogens, illustrating the importance of HLA-E for anti-viral and anti-tumor immunity. Like classical HLA class I molecules, HLA-E is ubiquitously expressed, however, it is characterized by only a very limited sequence variability and two dominant protein forms have been described (HLA-E*01:01 and HLA-E*01:03). HLA-E influences both the innate and the adaptive arms of the immune system by the engagement of inhibitory (e.g. NKG2A) and activating receptors [e.g. αβ T cell receptor (αβTCR) or NKG2C] on NK cells and CD8 T cells. The effects of HLA-E on the cellular immune response are therefore complex and not completely understood yet. Here, we aim to provide an overview of the immunological and clinical relevance of HLA-E and HLA-E polymorphism in stem cell transplantation and in cancer. We review novel insights in the mechanism via which HLA-E expression levels are controlled and how the cellular immune response in transplantation and cancer is influenced by HLA-E.

Classical and non-classical MHC I molecule manipulation by human cytomegalovirus: so many targets—but how many arrows in the quiver?

Major mechanisms for the recognition of pathogens by immune cells have evolved to employ classical and non-classical major histocompatibility complex class I (MHC I) molecules. Classical MHC I molecules present antigenic peptide ligands on infected cells to CD8⁺ T cells, whereas a key function for non-classical MHC I molecules is to mediate inhibitory or activating stimuli in natural killer (NK) cells. The structural diversity of MHC I puts immense pressure on persisting viruses, including cytomegaloviruses. The very large coding capacity of the human cytomegalovirus allows it to express a whole arsenal of immunoevasive factors assigned to individual MHC class I targets. This review summarizes achievements from more than two decades of intense research on how human cytomegalovirus manipulates MHC I molecules and escapes elimination by the immune system.

NK cells and interferons

The role of Natural Killer cells in host defense against infections as well as in tumour surveillance has been widely appreciated for a number of years. Upon recognition of "altered" cells, NK cells release the content of cytolytic granules, leading to the death of target cells. Moreover, NK cells are powerful producers of chemokines and cytokines, particularly Interferon-γ (IFN-γ), of which they are the earliest source upon a variety of infections. Despite being armed to fight against pathogens, NK cells become fully functional upon an initial phase of activation that requires the action of several cytokines, including type I IFNs. Type I IFNs are now recognized as key players in antiviral defense and immune regulation, and evidences from both mouse models of disease and in vitro studies support the existence of an alliance between type I IFNs and NK cells to ensure effective protection against viral infections. This review will focus on the role of type I IFNs in regulating NK cell functions to elicit antiviral response and on NK cell-produced IFN-γ beneficial and pathological effects.

Human cytomegalovirus drives epigenetic imprinting of the IFNG locus in NKG2Chi natural killer cells

Memory type 1 T helper (T(H)1) cells are characterized by the stable expression of interferon (IFN)-γ as well as by the epigenetic imprinting of the IFNG locus. Among innate cells, NK cells play a crucial role in the defense against cytomegalovirus (CMV) and represent the main source of IFN-γ. Recently, it was shown that memory-like features can be observed in NK cell subsets after CMV infection. However, the molecular mechanisms underlying NK cell adaptive properties have not been completely defined. In the present study, we demonstrated that only NKG2Chi NK cells expanded in human CMV (HCMV) seropositive individuals underwent epigenetic remodeling of the IFNG conserved non-coding sequence (CNS) 1, similar to memory CD8(+) T cells or T(H)1 cells. The accessibility of the CNS1 was required to enhance IFN-γ transcriptional activity in response to NKG2C and 2B4 engagement, which led to consistent IFN-γ production in NKG2C(hi) NK cells. Thus, our data identify epigenetic imprinting of the IFNG locus as selective hallmark and crucial mechanism driving strong and stable IFN-γ expression in HCMV-specific NK cell expansions, providing a molecular basis for the regulation of adaptive features in innate cells.

CMV-encoded Fcγ receptors: modulators at the interface of innate and adaptive immunity

The constant region of IgG antibodies mediates antiviral activities upon engaging host Fcγ receptors (FcγRs) expressed by a variety of immune cells, such as antibody-dependent cellullar cytotoxcity (ADCC) executed by natural killer (NK)cells. Human cytomegalovirus (HCMV) is unique among viruses by encoding also an array of several Fcγ-binding glycoproteins with cell surface disposition and concomitant incorporation into the virion. Evidence is increasing that the virus-encoded Fcγ receptors differ in their Fcγ binding mode but effectively operate as adversaries of host FcγRs since they are able to prevent IgG-mediated triggering of activating host FcγRs, i.e., FcγRI, FcγRIIA, and FcγRIIIA. Here we discuss virus-encoded FcγRs as the first known HCMV inhibitors of IgG-mediated immunity which could account for the limited efficacy of HCMV hyperimmune globulin in clinical settings. A better understanding of their molecular mode of action opens up new perspectives for improving IgG therapies against HCMV disease.

Natural killer cell regulation - beyond the receptors

Natural killer (NK) cells are lymphocytes that are important for early and effective immune responses against infections and cancer. In the last 40 years, many receptors, their corresponding ligands and signaling pathways that regulate NK cell functions have been identified. However, we now know that additional processes, such as NK cell education, differentiation and also the formation of NK cell memory, have a great impact on the reactivity of these cells. Here, we summarize the current knowledge about these modulatory processes.

Rapid NK cell differentiation in a population with near-universal human cytomegalovirus infection is attenuated by NKG2C deletions

Natural killer (NK) cells differentiate and mature during the human life course; human cytomegalovirus (HCMV) infection is a known driver of this process. We have explored human NK cell phenotypic and functional maturation in a rural African (Gambian) population with a high prevalence of HCMV. The effect of age on the frequency, absolute number, phenotype, and functional capacity of NK cells was monitored in 191 individuals aged from 1 to 49 years. Increasing frequencies of NK cells with age were associated with increased proportions of CD56dim cells expressing the differentiation marker CD57 and expansion of the NKG2C+ subset. Frequencies of NK cells responding to exogenous cytokines declined with age in line with a decreased proportion of CD57- cells. These changes coincided with a highly significant drop in anti-HCMV IgG titers by the age of 10 years, suggesting that HCMV infection is brought under control as NK cells differentiate (or vice versa). Deletion at the NKG2C locus was associated with a gene dose-dependent reduction in proportions of CD94+ and CD57+ NK cells. Importantly, anti-HCMV IgG titers were significantly elevated in NKG2C-/- children, suggesting that lack of expression of NKG2C may be associated with altered control of HCMV in childhood.

Favorable impact of natural killer cell reconstitution on chronic graft-versus-host disease and cytomegalovirus reactivation after allogeneic hematopoietic stem cell transplantation

Natural killer cells are the first lymphocyte subset to reconstitute, and play a major role in early immunity after allogeneic hematopoietic stem cell transplantation. Cells expressing the activating receptor NKG2C seem crucial in the resolution of cytomegalovirus episodes, even in the absence of T cells. We prospectively investigated natural killer-cell reconstitution in a cohort of 439 adult recipients who underwent non-T-cell-depleted allogeneic hematopoietic stem cell transplantation between 2005 and 2012. Freshly collected blood samples were analyzed 3, 6, 12 and 24 months after transplantation. Data were studied with respect to conditioning regimen, source of stem cells, underlying disease, occurrence of graft-versus-host disease, and profiles of cytomegalovirus reactivation. In multivariate analysis we found that the absolute numbers of CD56(bright) natural killer cells at month 3 were significantly higher after myeloablative conditioning than after reduced intensity conditioning. Acute graft-versus-host disease impaired reconstitution of total and CD56(dim) natural killer cells at month 3. In contrast, high natural killer cell count at month 3 was associated with a lower incidence of chronic graft-versus-host disease, independently of a previous episode of acute graft-versus-host disease and stem cell source. NKG2C(+)CD56(dim) and total natural killer cell counts at month 3 were lower in patients with reactivation of cytomegalovirus between month 0 and month 3, but expanded greatly afterwards. These cells were also less numerous in patients who experienced later cytomegalovirus reactivation between month 3 and month 6. Our results advocate a direct role of NKG2C-expressing natural killer cells in the early control of cytomegalovirus reactivation after allogeneic hematopoietic stem cell transplantation.

Increased numbers and functional activity of CD56⁺ T cells in healthy cytomegalovirus positive subjects

Human T cells expressing CD56 are capable of tumour cell lysis following activation with interleukin-2 but their role in viral immunity has been less well studied. Proportions of CD56(+) T cells were found to be highly significantly increased in cytomegalovirus-seropositive (CMV(+) ) compared with seronegative (CMV(-) ) healthy subjects (9.1 ± 1.5% versus 3.7 ± 1.0%; P < 0.0001). Proportions of CD56(+) T cells expressing CD28, CD62L, CD127, CD161 and CCR7 were significantly lower in CMV(+) than CMV(-) subjects but those expressing CD4, CD8, CD45RO, CD57, CD58, CD94 and NKG2C were significantly increased (P < 0.05), some having the phenotype of T effector memory cells. Levels of pro-inflammatory cytokines and CD107a were significantly higher in CD56(+) T cells from CMV(+) than CMV(-) subjects following stimulation with CMV antigens. This also resulted in higher levels of proliferation in CD56(+) T cells from CMV(+) than CMV(-) subjects. Using Class I HLA pentamers, it was found that CD56(+) T cells from CMV(+) subjects contained similar proportions of antigen-specific CD8(+) T cells to CD56(-) T cells in donors of several different HLA types. These differences may reflect the expansion and enhanced functional activity of CMV-specific CD56(+) memory T cells. In view of the link between CD56 expression and T-cell cytotoxic function, this strongly implicates CD56(+) T cells as being an important component of the cytotoxic T-cell response to CMV in healthy carriers.

Shaping of NK cell subsets by aging

NK cells are key players in the innate immune response against virus infection and tumors. Here we describe the current knowledge on age-associated changes in NK cells and the role of persistent CMV infection in configuring NK cell compartment in the elderly. Aging but not CMV causes a redistribution of NK cell subsets as shown by a decrease of CD56bright cells and an increase of CD56-CD16+ NK cells. On the contrary the changes in CD56dimCD16+ NK cells are compatible with the accumulation of CD57+ long-lived NK cells that can also be observed in young CMV-seropositive individuals. NK cell function and dynamics in the elderly will be related not only with age but also with exposure to pathogens, especially CMV.

The up side of decidual natural killer cells: new developments in immunology of pregnancy

Early phases of human pregnancy are associated with the accumulation of a unique subset of natural killer (NK) cells in the maternal decidua. Decidual NK (dNK) cells that are devoid of cytotoxicity play a pivotal role in successful pregnancy. By secreting large amounts of cytokines/chemokines and angiogenic factors, dNK cells participate in all steps of placentation including trophoblast invasion into the maternal endometrium and vascular remodelling. In this review, we summarize some of dNK cell features and discuss more recent exciting data that challenge the conventional view of these cells. Our new data demonstrate that dNK cells undergo fine tuning or even subvert their classical inhibitory machinery and turn into a real defence force in order to prevent the spread of viruses to fetal tissue. Today it is not clear how these phenotypic and functional adaptations impact cellular cross-talk at the fetal-maternal interface and tissue homeostasis. Ultimately, precise understanding of the molecular mechanisms that govern dNK cell plasticity during congenital human cytomegalovirus infection should lead to the design of more robust strategies to reverse immune escape during viral infection and cancer.

Human cytomegalovirus and autoimmune disease

Human cytomegalovirus (HCMV) represents a prototypic pathogenic member of the β-subgroup of the herpesvirus family. A range of HCMV features like its lytic replication in multiple tissues, the lifelong persistence through periods of latency and intermitting reactivation, the extraordinary large proteome, and extensive manipulation of adaptive and innate immunity make HCMV a high profile candidate for involvement in autoimmune disorders. We surveyed the available literature for reports on HCMV association with onset or exacerbation of autoimmune disease. A causative linkage between HCMV and systemic lupus erythematosus (SLE), systemic sclerosis (SSc), diabetes mellitus type 1, and rheumatoid arthritis (RA) is suggested by the literature. However, a clear association of HCMV seroprevalence and disease could not be established, leaving the question open whether HCMV could play a coresponsible role for onset of disease. For convincing conclusions population-based prospective studies must be performed in the future. Specific immunopathogenic mechanisms by which HCMV could contribute to the course of autoimmune disease have been suggested, for example, molecular mimicry by UL94 in SSc and UL83/pp65 in SLE patients, as well as aggravation of joint inflammation by induction and expansion of CD4+/CD28− T-cells in RA patients. Further studies are needed to validate these findings and to lay the grounds for targeted therapeutic intervention.

Independent skewing of the T cell and NK cell compartments associated with cytomegalovirus infection suggests division of labor between innate and adaptive immunity

Cytomegalovirus (CMV) infection induces profound changes in different subsets of the cellular immune system. We have previously identified an immune risk profile (IRP) where CMV-associated changes in the T cell compartment, defined as a CD4/CD8 ratio < 1, are associated with increased mortality in elderly people. Since natural killer (NK) cells have an important role in the defense against viral infections, we examined whether the expansion of CD8 + T cells seen in individuals with CD4/CD8 ratio < 1 is coupled to a parallel skewing of the NK cell compartment. A number of 151 subjects were examined with CMV serology and a flow cytometry panel for assessment of T cell and NK cell subsets. CMV-seropositive individuals had higher frequencies of CD57 + and NKG2C + NK cells and lower frequencies of NKG2A + NK cells, in line with a more differentiated NK cell compartment. Intriguingly, however, there was no correlation between CD4/CD8 ratio and NK cell repertoires among CMV-seropositive donors, despite the profound skewing of the T cell compartment in the group with CD4/CD8 ratio < 1. Conversely, donors with profound expansion of NK cells, defined as NKG2C + NK cells with high expression of CD57 and ILT-2, did not display more common changes in their T cell repertoire, suggesting that NK cell expansion is independent of the T cell-defined IRP. Altogether, these results indicate that the effect of CMV on CD8 T cells and NK cells is largely nonoverlapping and independent.

Immunological memory within the innate immune system

Immune memory has traditionally been the domain of the adaptive immune system, present only in antigen-specific T and B cells. The purpose of this review is to summarize the evidence for immunological memory in lower organisms (which are not thought to possess adaptive immunity) and within specific cell subsets of the innate immune system. A special focus will be given to recent findings in both mouse and humans for specificity and memory in natural killer (NK) cells, which have resided under the umbrella of innate immunity for decades. The surprising longevity and enhanced responses of previously primed NK cells will be discussed in the context of several immunization settings.

Clinical utility of natural killer cells in cancer therapy and transplantation

Natural killer (NK) cells recognize deranged cells that display stress receptors or loss of major histocompatibility complex (MHC) class I. During development, NK cells become "licensed" only after they encounter cognate human leukocyte antigen (HLA) class I, leading to the acquisition of effector function. NK cells can be exploited for cancer therapy in several ways. These include targeting with monoclonal antibodies alone or combined with ex vivo and in vivo NK cell activation to facilitate adoptive immunotherapy using donor-derived NK cell products to induce graft-vs-tumor effects. In the adoptive transfer setting, persistence and in vivo expansion requires lymphodepleting chemotherapy to prevent rejection and provide homeostatic cytokines (such as IL-15) that activate NK cells. IL-15 has the advantage of avoiding regulatory T-cell expansion. Clinical applications are currently being tested. To enhance in vivo expansion, IL-2 has been used at low doses. However, low dose administration also leads to the stimulation of regulatory T cells. Monoclonal antibodies and bispecific killer engagers (BiKEs) may enhance specificity by targeting CD16 on NK cells to tumor antigens. Inhibition of CD16 shedding may also promote enhanced cytotoxicity. Future strategies include exploiting favorable donor immunogenetics or ex vivo expansion of NK cells from blood, progenitors, or pluripotent cells. Comparative clinical trials are needed to test these approaches.

Human NK cell response to pathogens

NK cells represent important effectors of the innate immunity in the protection of an individual from microbes. During an NK-mediated anti-microbial response, the final fate (survival or death) of a potential infected target cell depends primarily on the type and the number of receptor/ligand interactions occurring at the effector/target immune synapse. The identification of an array of receptors involved in NK cell triggering has been crucial for a better understanding of the NK cell biology. In this context, NCR play a predominant role in NK cell activation during the process of natural cytotoxicity. Regarding the NK-mediated pathogen recognition and NK cell activation, an emerging concept is represented by the involvement of TLRs and activating KIRs. NK cells express certain TLRs in common with other innate cell types. This would mean that specific TLR ligands are able to promote the simultaneous and synergistic stimulation of these innate cells, providing a coordinated mechanism for regulating the initiation and amplification of immune responses. Evidences have been accumulated indicating that viral infections may have a significant impact on NK cell maturation, promoting the expansion of phenotypically and functionally aberrant NK cell subpopulations. For example, during chronic HIV-infection, an abnormal expansion of a dysfunctional CD56neg NK cell subset has been detected that may explain, at least in part, the defective NK cell-mediated antiviral activity. An analogous imbalance of NK cell subsets has been detected in patients receiving HSCT to cure high risk leukemias and experiencing HCMV infection/reactivation. Remarkably, NK cells developing after CMV reactivation may contain "memory-like" or "long-lived" NK cells that could exert a potent anti-leukemia effect.

Activating KIRs in Chronic Lymphoproliferative Disorder of NK Cells: Protection from Viruses and Disease Induction?

Human natural killer (NK) cells are functionally regulated by killer cell immunoglobulin-like receptors (KIRs) and their interactions with HLA class I molecules. As KIR expression in a given NK cell is stochastically established, KIR repertoire perturbations reflect a dominance of discrete NK-cell subsets as the consequence of adaptation of the NK-cell compartment to exogenous agents, more often represented by virus infection. Although inhibitory interactions between KIR and their cognate HLA class I ligands abrogate effector responses of NK cells, they are also required for the functional education of NK cell. The biology and molecular specificities of the activating KIRs are less well defined, and most interactions with presumed HLA class I ligands are weak. Interestingly, epidemiologic studies link activating KIR genes to resistance against numerous virus infections. Chronic lymphoproliferative disorder of NK cells (CLPD-NK) is an indolent NK cell disease characterized by a persistent increase of circulating NK cells (usually exceeding 500 NK cells/mm(3)). The mechanism through which NK cells are induced to proliferate during CLPD-NK pathogenesis is still a matter of debate. Accumulating data suggest that exogenous agents, in particular viruses, might play a role. The etiology of CLPD-NK, however, is largely unknown. This is likely due to the fact that not a single, specific agent is responsible for the NK cells proliferation, which perhaps represents the expression of an abnormal processing of different foreign antigens, sharing a chronic inflammatory background. Interestingly, proliferating NK cells are typically characterized by expression of a restricted pattern of KIR, which have been demonstrated to be mostly represented by the activating form. This finding indicates that these receptors may be directly involved in the priming of NK cells proliferation.

How to trigger a killer: modulation of natural killer cell reactivity on many levels

The functions of Natural Killer (NK) cells are regulated by a highly redundant set of germline-encoded surface receptors that can inhibit or activate NK cell activities. NK cells can be activated by cytokines or through the interaction with transformed or infected cells. This typically results in the production of cytokines, chemokines, and the induction of cellular cytotoxicity. However, the reactivity of NK cells is modulated on various levels and shaped by processes such as development, education, priming, exposure to antigens and cytokines, and the formation of memory-like phenotypes. Here, I will summarize our current understanding of these processes and describe how they influence NK cell reactivity on a molecular level.

Baseline and Dynamic Expression of Activating NK Cell Receptors in the Control of Chronic Viral Infections: The Paradigm of HIV-1 and HCV

Natural killer (NK) cell function is regulated by a balance between the triggering of activating and inhibitory receptors expressed on their surface. A relevant effort has been focused so far on the study of KIR carriage/expression setting the basis for NK cell education and self-tolerance. Focus on the evolution and regulation of activating NK receptors has lagged behind so far. Our understanding of activating receptor expression and regulation has recently improved by evidences derived from in vitro and in vivo studies. Virus infection - either acute or chronic - determines preferential expansion of NK cells with specific phenotype, activating receptors, and with recall-like functional activity. Studies on patients with viral infection (HIV and HCV) and specific diverging clinical courses confirm that inter-individual differences may exist in baseline expression of natural cytotoxicity receptors (NKp46 and NKp30). The findings that patients with divergent clinical courses have different kinetics of activating receptor density expression upon NK cell activation in vitro provide an additional, time-dependent, functional parameter. Kinetic changes in receptor expression thus represent an additional parameter to basal receptor density expression. Different expression and inducibilities of activating receptors on NK cells contribute to the high diversity of NK cell populations and may help our understanding of the inter-individual differences in innate responses that underlie divergent disease courses.

HTLV-1/-2 and HIV-1 co-infections: retroviral interference on host immune status

The human retroviruses HIV-1 and HTLV-1/HTLV-2 share similar routes of transmission but cause significantly different diseases. In this review we have outlined the immune mediated mechanisms by which HTLVs affect HIV-1 disease in co-infected hosts. During co-infection with HIV-1, HTLV-2 modulates the cellular microenvironment favoring its own viability and inhibiting HIV-1 progression. This is achieved when the HTLV-2 proviral load is higher than that of HIV-1, and thanks to the ability of HTLV-2 to: (i) up-regulate viral suppressive CCL3L1 chemokine expression; (ii) overcome HIV-1 capacity to activate the JAK/STAT pathway; (iii) reduce the activation of T and NK cells; (iv) modulate the host miRNA profiles. These alterations of immune functions have been mainly attributed to the effects of the HTLV-2 regulatory protein Tax and suggest that HTLV-2 exerts a protective role against HIV-1 infection. Contrary to HIV-1/HTLV-2, the effect of HIV-1/HTLV-1 co-infection on immunological and pathological conditions is still controversial. There is evidence that indicates a worsening of HIV-1 infection, while other evidence does not show clinically relevant effects in HIV-positive people. Possible differences on innate immune mechanisms and a particularly impact on NK cells are becoming evident. The differences between the two HIV-1/HTLV-1 and HIV-1/HTLV-2 co-infections are highlighted and further discussed.

Human NK cells and NK receptors

In early seventies "natural killer (NK) cells", a third lymphocyte subset was discovered that revealed an unexpected ability to kill syngeneic and allogeneic tumor targets, thus emerging as the most potent non-specific cytotoxic cells in both human and mouse. Decades of research revealed the multifaceted nature of these cells. Now we know that NK cells are highly specific cells able to discriminate between self (which is spared) and non-self (which is attacked). Most of the specific and non HLA-specific surface receptors involved in NK cell recognition and function have been identified and, to date, only few of them still remain orphans. We also know that NK cells contribute to both innate and adaptive immune responses, interact with other immune cell types and release type 1 cytokines and chemokines. Moreover, fundamental data are accumulating on NK cell development and migration under both physiological and pathological conditions. The time is arrived to exploit these cells in the cure of cancer patients. While encouraging results emerged in hematological malignances, the road to treat solid tumors using NK cells is still covered by obstacles that hamper their function and that just begin to be unveiled.

Impact of HCMV Infection on NK Cell Development and Function after HSCT

Natural Killer (NK) cell function is regulated by an array of inhibitory and activating surface receptors that during NK cell differentiation, at variance with T and B cells, do not require genetic rearrangement. Importantly, NK cells are the first lymphocyte population recovering after hematopoietic stem cell transplantation (HSCT). Thus, their role in early immunity after HSCT is considered crucial, as they can importantly contribute to protect the host from tumor recurrence and viral infections before T-cell immunity is fully recovered. In order to acquire effector functions and regulatory receptors, NK cell precursors undergo a maturation process that can be analyzed during immune reconstitution after HSCT. In this context, the occurrence of human cytomegalovirus (HCMV) infection/reactivation was shown to accelerate NK cell maturation by promoting the differentiation of high frequencies of NK cells characterized by a KIR⁺NKG2A⁻ and NKG2C⁺ mature phenotype. Thus, it appears that the development of NK cells and the distribution of NK cell receptors can be deeply influenced by HCMV infection. Moreover, in HCMV-infected subjects the emergence of so called “memory-like” or “long-lived” NK cells has been documented. These cells could play an important role in protecting from infections and maybe from relapse in patients transplanted for leukemia. All the aspects regarding the influence of HCMV infection on NK cell development will be discussed.

NKG2C zygosity influences CD94/NKG2C receptor function and the NK-cell compartment redistribution in response to human cytomegalovirus

Human cytomegalovirus (HCMV) infection promotes a persistent expansion of a functionally competent NK-cell subset expressing the activating CD94/NKG2C receptor. Factors underlying the wide variability of this effect observed in HCMV-seropositive healthy individuals and exacerbated in immunocompromized patients are uncertain. A deletion of the NKG2C gene has been reported, and an apparent relation of NKG2C genotype with circulating NKG2C(+) NK-cell numbers was observed in HCMV(+) children. We have assessed the influence of NKG2C gene dose on the NK-cell repertoire in a cohort of young healthy adults (N = 130, median age 19 years). Our results revealed a relation of NKG2C copy number with surface receptor levels and with NKG2C(+) NK-cell numbers in HCMV(+) subjects, independently of HLA-E dimorphism. Functional studies showed quantitative differences in signaling (i.e. iCa(2+) influx), degranulation, and IL-15-dependent proliferation, in response to NKG2C engagement, between NK cells from NKG2C(+/+) and hemizygous subjects. These observations provide a mechanistic interpretation on the way the NKG2C genotype influences steady-state NKG2C(+) NK-cell numbers, further supporting an active involvement of the receptor in the HCMV-induced reconfiguration of the NK-cell compartment. The putative implications of NKG2C zygosity over viral control and other clinical variables deserve attention.

Genetic and environmental determinants of human NK cell diversity revealed by mass cytometry

Natural killer (NK) cells play critical roles in immune defense and reproduction, yet remain the most poorly understood major lymphocyte population. Because their activation is controlled by a variety of combinatorially expressed activating and inhibitory receptors, NK cell diversity and function are closely linked. To provide an unprecedented understanding of NK cell repertoire diversity, we used mass cytometry to simultaneously analyze 37 parameters, including 28 NK cell receptors, on peripheral blood NK cells from 5 sets of monozygotic twins and 12 unrelated donors of defined human leukocyte antigen (HLA) and killer cell immunoglobulin-like receptor (KIR) genotype. This analysis revealed a remarkable degree of NK cell diversity, with an estimated 6000 to 30,000 phenotypic populations within an individual and >100,000 phenotypes in the donor panel. Genetics largely determined inhibitory receptor expression, whereas activation receptor expression was heavily environmentally influenced. Therefore, NK cells may maintain self-tolerance through strictly regulated expression of inhibitory receptors while using adaptable expression patterns of activating and costimulatory receptors to respond to pathogens and tumors. These findings further suggest the possibility that discrete NK cell subpopulations could be harnessed for immunotherapeutic strategies in the settings of infection, reproduction, and transplantation.

A distinct subpopulation of human NK cells restricts B cell transformation by EBV

NK cells constitute the first line of defense against pathogens and transformed cells. They mature in secondary lymphoid organs, including tonsils, where common pathogens, such as EBV, enter the host and potentially imprint differentiating cells, which then patrol the body via the blood stream. Therefore, we set out to characterize a distinct human NK cell population in tonsils that produces high amounts of the immunomodulatory and antiviral cytokine IFN-γ. We found that the tonsilar IFN-γ(high) NK cell subset is CD56(bright)NKG2A(+)CD94(+)CD54(+)CD62L(-), is present in tonsils ex vivo and is more mature than other CD56(bright) NK cells in tonsils and less mature than other NK cells in blood, shows very low plasticity even after prolonged cytokine stimulation, accumulates in tonsils of EBV carriers, and is able to potently restrict EBV-induced transformation of B cells. Thus, we characterized a distinct and stable IFN-γ(high) NK cell subpopulation that can specifically restrict malignant transformation of EBV-infected B cells. This subset should be exploited for future development of cell-based therapeutic approaches in EBV-associated malignancies.

Epstein-Barr virus coinfection in children boosts cytomegalovirus-induced differentiation of natural killer cells

During childhood, infections with cytomegalovirus (CMV) and Epstein-Barr virus (EBV) can occur in close temporal proximity. Active, as well as latent, CMV infection is associated with enlarged subsets of differentiated natural killer (NK) and cytotoxic T cells. How EBV infection may influence CMV-driven immune differentiation is not known. We found that EBV coinfection selectively influenced the NK cell compartment of CMV-seropositive (CMV(+)) children. Coinfected children had significantly higher proportions of peripheral-blood NKG2C(+) NK cells than CMV(+) EBV(-) children. Ex vivo NK cell degranulation after target cell stimulation and plasma IL-15 levels were significantly higher in CMV(+) children. EBV coinfection was related to the highest levels of plasma interleukin-15 (IL-15) and IL-12p70. Remarkably, in vitro EBV infection of peripheral blood mononuclear cells (PBMC) from EBV(-) CMV(+) children increased NKG2C(+) NK cell proportions. A similar tendency was seen in cocultures of PBMC with EBV(+) lymphoblastoid B-cell lines (LCL) and IL-15. After K562 challenge, NKG2C(+) NK cells excelled in regard to degranulation and production of gamma interferon, regardless of whether there was previous coculture with LCL. Taken together, our data suggest that dual latency with these herpesviruses during childhood could contribute to an in vivo environment supporting differentiation and maintenance of distinct NK cell populations. This viral imprint may affect subsequent immune responses through altered distributions of effector cells.