Human Herpesvirus 6B Downregulates Expression of Activating Ligands during Lytic Infection To Escape Elimination by Natural Killer Cells

The HHV-6B U20 glycoprotein binds ULBP1, masking it from recognition by NKG2D and interfering with natural killer cell activation

Introduction

Human Herpesvirus 6B (HHV-6B) impedes host immune responses by downregulating class I MHC molecules (MHC-I), hindering antigen presentation to CD8+ T cells. Downregulation of MHC-I disengages inhibitory receptors on natural killer (NK) cells, resulting in activation and killing of the target cell if NK cell activating receptors such as NKG2D have engaged stress ligands upregulated on the target cells. Previous work has shown that HHV-6B downregulates three MHC-like stress ligands MICB, ULBP1, and ULBP3, which are recognized by NKG2D. The U20 glycoprotein of the related virus HHV-6A has been implicated in the downregulation of ULBP1, but the precise mechanism remains undetermined.

Methods

We set out to investigate the role of HHV-6B U20 in modulating NK cell activity. We used HHV-6B U20 expressed as a recombinant protein or transduced into target cells, as well as HHV-6B infection, to investigate binding interactions with NK cell ligands and receptors and to assess effects on NK cell activation. Small-angle X-ray scattering was used to align molecular models derived from machine-learning approaches.

Results

We demonstrate that U20 binds directly to ULBP1 with sub-micromolar affinity. Transduction of U20 decreases NKG2D binding to ULBP1 at the cell surface but does not decrease ULBP1 protein levels, either at the cell surface or in toto. HHV-6B infection and soluble U20 have the same effect. Transduction of U20 blocks NK cell activation in response to cell-surface ULBP1. Structural modeling of the U20 - ULBP1 complex indicates some similarities to the m152-RAE1γ complex.

Evasion of NKG2D-mediated cytotoxic immunity by sarbecoviruses

SARS-CoV-2 and other sarbecoviruses continue to threaten humanity, highlighting the need to characterize common mechanisms of viral immune evasion for pandemic preparedness. Cytotoxic lymphocytes are vital for antiviral immunity and express NKG2D, an activating receptor conserved among mammals that recognizes infection-induced stress ligands (e.g., MIC-A/B). We found that SARS-CoV-2 evades NKG2D recognition by surface downregulation of MIC-A/B via shedding, observed in human lung tissue and COVID-19 patient serum. Systematic testing of SARS-CoV-2 proteins revealed that ORF6, an accessory protein uniquely conserved among sarbecoviruses, was responsible for MIC-A/B downregulation via shedding. Further investigation demonstrated that natural killer (NK) cells efficiently killed SARS-CoV-2-infected cells and limited viral spread. However, inhibition of MIC-A/B shedding with a monoclonal antibody, 7C6, further enhanced NK-cell activity toward SARS-CoV-2-infected cells. Our findings unveil a strategy employed by SARS-CoV-2 to evade cytotoxic immunity, identify the culprit immunevasin shared among sarbecoviruses, and suggest a potential novel antiviral immunotherapy.

Barcode clonal tracking of tissue-resident immune cells in rhesus macaque highlights distinct clonal distribution pattern of tissue NK cells

Tissue resident (TR) immune cells play important roles in facilitating tissue homeostasis, coordinating immune responses against infections and tumors, and maintaining immunological memory. While studies have shown these cells are distinct phenotypically and functionally from cells found in the peripheral blood (PB), the clonal relationship between these populations across tissues has not been comprehensively studied in primates or humans. We utilized autologous transplantation of rhesus macaque hematopoietic stem and progenitor cells containing high diversity barcodes to track the clonal distribution of T, B, myeloid and natural killer (NK) cell populations across tissues, including liver, spleen, lung, and gastrointestinal (GI) tract, in comparison with PB longitudinally post-transplantation, in particular we focused on NK cells which do not contain endogenous clonal markers and have not been previously studied in this context. T cells demonstrated tissue-specific clonal expansions as expected, both overlapping and distinct from blood T cells. In contrast, B and myeloid cells showed a much more homogeneous clonal pattern across various tissues and the blood. The clonal distribution of TR NK was more heterogenous between individual animals. In some animals, as we have previously reported, we observed large PB clonal expansions in mature CD56-CD16+ NK cells. Notably, we found a separate set of highly expanded PB clones in CD16-CD56- (DN) NK subset that were also contributing to TR NK cells in all tissues examined, both in TR CD56-CD16+ and DN populations but absent in CD56+16- TR NK across all tissues analyzed. Additionally, we observed sets of TR NK clones specific to individual tissues such as lung or GI tract and sets of TR NK clones shared across liver and spleen, distinct from other tissues. Combined with prior functional data that suggests NK memory is restricted to liver or other TR NK cells, these clonally expanded TR NK cells may be of interest for future investigation into NK cell tissue immunological memory, with implications for development of NK based immunotherapies and an understanding of NK memory.

Studying the Interactions of U24 from HHV-6 in Order to Further Elucidate Its Potential Role in MS

A number of studies have suggested that human herpesvirus 6A (HHV-6A) may play a role in multiple sclerosis (MS). Three possible hypotheses have been investigated: (1) U24 from HHV-6A (U24-6A) mimics myelin basic protein (MBP) through analogous phosphorylation and interaction with Fyn-SH3; (2) U24-6A affects endocytic recycling by binding human neural precursor cell (NPC) expressed developmentally down-regulated protein 4-like WW3* domain (hNedd4L-WW3*); and (3) MS patients who express Killer Cell Immunoglobulin Like Receptor 2DL2 (KIR2DL2) on natural killer (NK) cells are more susceptible to HHV-6 infection. In this contribution, we examined the validity of these propositions by investigating the interactions of U24 from HHV-6B (U24-6B), a variant less commonly linked to MS, with Fyn-SH3 and hNedd4L-WW3* using heteronuclear single quantum coherence (HSQC) nuclear magnetic resonance (NMR) titrations and isothermal titration calorimetry (ITC). In addition, the importance of phosphorylation and the specific role of U24 in NK cell activation in MS patients were examined. Overall, the findings allowed us to shed light into the models linking HHV-6 to MS and the involvement of U24.

Structural Models for Roseolovirus U20 And U21: Non-Classical MHC-I Like Proteins From HHV-6A, HHV-6B, and HHV-7

Human roseolovirus U20 and U21 are type I membrane glycoproteins that have been implicated in immune evasion by interfering with recognition of classical and non-classical MHC proteins. U20 and U21 are predicted to be type I glycoproteins with extracytosolic immunoglobulin-like domains, but detailed structural information is lacking. AlphaFold and RoseTTAfold are next generation machine-learning-based prediction engines that recently have revolutionized the field of computational three-dimensional protein structure prediction. Here, we review the structural biology of viral immunoevasins and the current status of computational structure prediction algorithms. We use these computational tools to generate structural models for U20 and U21 proteins, which are predicted to adopt MHC-Ia-like folds with closed MHC platforms and immunoglobulin-like domains. We evaluate these structural models and place them within current understanding of the structural basis for viral immune evasion of T cell and natural killer cell recognition.

The HHV-6A Proteins U20 and U21 Target NKG2D Ligands to Escape Immune Recognition

The coevolution of the human immune system and herpesviruses led to the emergence and diversification of both cellular danger molecules recognized by immune cells on the one hand and viral countermeasures that prevent the expression of these proteins on infected cells on the other. There are eight ligands for the activating receptor NKG2D in humans - MICA, MICB, ULBP1-6. Several of them are induced and surface-expressed on herpesvirus-infected cells to serve as danger signals to activate the immune system. Therefore, these ligands are frequently targeted for suppression by viral immune evasion mechanisms. Mechanisms to downregulate NKG2D ligands and thereby escape immune recognition have been identified in all other human herpesviruses (HHV), except for HHV-6A. In this study, we identify two HHV-6A encoded immunoevasins, U20 and U21, which suppress the expression of the NKG2D ligands ULBP1 and ULBP3, respectively, during infection. Additionally, MICB is targeted by a so far unexplored viral protein. Due to the diminished NKG2D ligand surface expression on infected cells, recognition of HHV-6A infected cells by innate immune cells is impaired. Importantly, our study indicates that immune escape mechanisms between the related herpesviruses HHV-6A and HHV-6B are evolutionary conserved as the same NKG2D ligands are targeted. Our data contribute an additional piece of evidence for the importance of the NKG2D receptor - NKG2D ligand axis during human herpesvirus infections and sheds light on immune evasion mechanisms of HHV-6A.

Molecular mechanisms of human herpes viruses inferring with host immune surveillance

Several human herpes viruses (HHVs) exert oncogenic potential leading to malignant transformation of infected cells and/or tissues. The molecular processes induced by viral-encoded molecules including microRNAs, peptides, and proteins contributing to immune evasion of the infected host cells are equal to the molecular processes of immune evasion mediated by tumor cells independently of viral infections. Such major immune evasion strategies include (1) the downregulation of proinflammatory cytokines/chemokines as well as the induction of anti-inflammatory cytokines/chemokines, (2) the downregulation of major histocompatibility complex (MHC) class Ia directly as well as indirectly by downregulation of the components involved in the antigen processing, and (3) the downregulation of stress-induced ligands for activating receptors on immune effector cells with NKG2D leading the way. Furthermore, (4) immune modulatory molecules like MHC class Ib molecules and programmed cell death1 ligand 1 can be upregulated on infections with certain herpes viruses. This review article focuses on the known molecular mechanisms of HHVs modulating the above-mentioned possibilities for immune surveillance and even postulates a temporal order linking regular tumor immunology with basic virology and offering putatively novel insights for targeting HHVs.

Evasion of the Host Immune Response by Betaherpesviruses

The human immune system boasts a diverse array of strategies for recognizing and eradicating invading pathogens. Human betaherpesviruses, a highly prevalent subfamily of viruses, include human cytomegalovirus (HCMV), human herpesvirus (HHV) 6A, HHV-6B, and HHV-7. These viruses have evolved numerous mechanisms for evading the host response. In this review, we will highlight the complex interplay between betaherpesviruses and the human immune response, focusing on protein function. We will explore methods by which the immune system first responds to betaherpesvirus infection as well as mechanisms by which viruses subvert normal cellular functions to evade the immune system and facilitate viral latency, persistence, and reactivation. Lastly, we will briefly discuss recent advances in vaccine technology targeting betaherpesviruses. This review aims to further elucidate the dynamic interactions between betaherpesviruses and the human immune system.

Pseudorabies Virus Infection Causes Downregulation of Ligands for the Activating NK Cell Receptor NKG2D

Herpesviruses display a complex and carefully balanced interaction with important players in the antiviral immune response of immunocompetent natural hosts, including natural killer (NK) cells. With regard to NK cells, this delicate balance is illustrated on the one hand by severe herpesvirus disease reported in individuals with NK cell deficiencies and on the other hand by several NK cell evasion strategies described for herpesviruses. In the current study, we report that porcine cells infected with the porcine alphaherpesvirus pseudorabies virus (PRV) display a rapid and progressive downregulation of ligands for the major activating NK cell receptor NKG2D. This downregulation consists both of a downregulation of NKG2D ligands that are already expressed on the cell surface of an infected cell and an inhibition of cell surface expression of newly expressed NKG2D ligands. Flow cytometry and RT-qPCR assays showed that PRV infection results in downregulation of the porcine NKG2D ligand pULBP1 from the cell surface and a very substantial suppression of mRNA expression of pULBP1 and of another potential NKG2D ligand, pMIC2. Furthermore, PRV-induced NKG2D ligand downregulation was found to be independent of late viral gene expression. In conclusion, we report that PRV infection of host cells results in a very pronounced downregulation of ligands for the activating NK cell receptor NKG2D, representing an additional NK evasion strategy of PRV.

Convergent Evolution by Cancer and Viruses in Evading the NKG2D Immune Response

The natural killer group 2 member D (NKG2D) receptor and its family of NKG2D ligands (NKG2DLs) are key components in the innate immune system, triggering NK, γδ and CD8+ T cell-mediated immune responses. While surface NKG2DL are rarely found on healthy cells, expression is significantly increased in response to various types of cellular stress, viral infection, and tumour cell transformation. In order to evade immune-mediated cytotoxicity, both pathogenic viruses and cancer cells have evolved various mechanisms of subverting immune defences and preventing NKG2DL expression. Comparisons of the mechanisms employed following virus infection or malignant transformation reveal a pattern of converging evolution at many of the key regulatory steps involved in NKG2DL expression and subsequent immune responses. Exploring ways to target these shared steps in virus- and cancer-mediated immune evasion may provide new mechanistic insights and therapeutic opportunities, for example, using oncolytic virotherapy to re-engage the innate immune system towards cancer cells.

Human Metapneumovirus Escapes NK Cell Recognition through the Downregulation of Stress-Induced Ligands for NKG2D

The Pneumoviridae family includes human metapneumovirus (HMPV) and human orthopneumovirus, which is also known as a respiratory syncytial virus (HRSV). These are large enveloped, negative single-strand RNA viruses. HMPV and HRSV are the human members, which commonly infect children. HMPV, which was discovered in 2001, infects most children until the age of five, which causes an influenza-like illness. The interaction of this virus with immune cells is poorly understood. In this study, we show that HMPV evades natural killer (NK) cell attack by downregulating stress-induced ligands for the activating receptor NKG2D including: Major histocompatibility complex (MHC) class I polypeptide-related sequences A and B (MICA, MICB), UL16 binding proteins ULBP2, and ULBP3, but not ULBP1. Mechanistically, we show that the viral protein G is involved in the downregulation of ULBP2 and that the viral protein M2.2 is required for MICA and MICB downregulation. These findings emphasize the importance of NK cells, in general, and NKG2D, in particular, in controlling HMPV infection, which opens new avenues for treating HMPV.

Human herpesvirus 6A promotes glycolysis in infected T cells by activation of mTOR signaling

Human herpesvirus 6 (HHV-6) is an important immunosuppressive and immunomodulatory virus worldwide. However, whether and how HHV-6 infection influences the metabolic machinery of the host cell to provide the energy and biosynthetic resources for virus propagation remains unknown. In this study, we identified that HHV-6A infection promotes glucose metabolism in infected T cells, resulting in elevated glycolytic activity with an increase of glucose uptake, glucose consumption and lactate secretion. Furthermore, we explored the mechanisms involved in HHV-6A-mediated glycolytic activation in the infected T cells. We found increased expressions of the key glucose transporters and glycolytic enzymes in HHV-6A-infected T cells. In addition, HHV-6A infection dramatically activated AKT-mTORC1 signaling in the infected T cells and pharmacological inhibition of mTORC1 blocked HHV-6A-mediated glycolytic activation. We also found that direct inhibition of glycolysis by 2-Deoxy-D-glucose (2-DG) or inhibition of mTORC1 activity in HHV-6A-infected T cells effectively reduced HHV-6 DNA replication, protein synthesis and virion production. These results not only reveal the mechanism of how HHV-6 infection affects host cell metabolism, but also suggest that targeting the metabolic pathway could be a new avenue for HHV-6 therapy.

DNA Sensors' Signaling in NK Cells During HHV-6A, HHV-6B and HHV-7 Infection

Objectives

The host DNA sensor proteins TLR9, STING, IFI16 are central signaling molecules that control the innate immune response to cytosolic nucleic acids. Here we propose to investigate how Natural killer (NK) cell infection by human herpesvirus (HHV)-6A, HHV-6B or HHV-7 is able to modify DNA sensor signaling in NK cells.

Methods

We infected the NK92 cell line and primary NK cells with cell-free inocula of HHV-6A, HHV-6B or HHV-7 and evaluated TLR9, STING, and IFI16 pathway expression by Real-Time PCR, Western Blot, immunofluorescence and flow cytometry for 1, 2, 3, and 6 days post-infection. We evaluated NK cell cytokine-producing by Real-Time PCR and enzyme immunosorbent assay.

Results

NK92 and primary NK cells were promptly infected by three viruses, as demonstrated by virus presence (DNA) and transcription (RNA) analysis. Our data show STING/STAT6 up-modulation in HHV-6A infected NK cells. NK cells infected with HHV-6B and HHV-7 up-regulated CCL3, IFN-alpha, TNF-alpha, IL-8 and IFN-gamma and slightly induced IL-4, and CCL4. HHV-6A infected NK cells up-regulated IL-4 and IL-13 and slightly induced IL-10, TNF-alpha, IFN-alpha, and IFN-gamma.

Conclusion

For the first time, we demonstrate that HHV-6A, HHV-6B, and HHV-7 infections have a differential impact on intracellular DNA sensors. HHV-6B and HHV-7 mainly lead to the active control of in vivo viral spreading by pro-inflammatory cytokine secretion via TLR9. HHV-6A infected NK cells conversely induced STING/STAT6 pathway, as a mechanism of anti-viral activation, but they were characterized by a Th2 type response and a non-cytotoxic profile, suggesting a potential novel mechanism of HHV-6A-mediated immunosuppression.

Betaherpesvirus assembly and egress: Recent advances illuminate the path

The human betaherpesviruses, human cytomegalovirus (HCMV; species Human betaherpesvirus 5) and human herpesviruses 6A, 6B, and 7 (HHV-6A, -6B, and -7; species Human betaherpesviruses 6A, 6B, and 7) are highly prevalent and can cause severe disease in immune-compromised and immune-naive populations in well- and under-developed communities. Herpesvirus virion assembly is an intricate process that requires viral orchestration of host systems. In this review, we describe recent advances in some of the many cellular events relevant to assembly and egress of betaherpesvirus virions. These include modifications of host metabolic, immune, and autophagic/recycling systems. In addition, we discuss unique aspects of betaherpesvirus virion structure, virion assembly, and the cellular pathways employed during virion egress.

The integrated stress response promotes B7H6 expression

The B7 family member, B7H6, is a ligand for the natural killer cell receptor NKp30. B7H6 is hardly expressed on normal tissues, but undergoes upregulation on different types of tumors, implicating it as an attractive target for cancer immunotherapy. The molecular mechanisms that control B7H6 expression are poorly understood. We report that in contrast to other NK cell ligands, endoplasmic reticulum (ER) stress upregulates B7H6 mRNA levels and surface expression. B7H6 induction by ER stress requires protein kinase R-like ER kinase (PERK), one of the three canonical sensors of the unfolded protein response. PERK phosphorylates eIF2α, which regulates protein synthesis and gene expression. Because eIF2α is phosphorylated by several kinases following different stress conditions, the program downstream to eIF2α phosphorylation is called the integrated stress response (ISR). Several drugs were reported to promote the ISR. Nelfinavir and lopinavir, two clinically approved HIV protease inhibitors, promote eIF2α phosphorylation by different mechanisms. We show that nelfinavir and lopinavir sustainably instigate B7H6 expression at their pharmacologically relevant concentrations. As such, ER stress and ISR conditions sensitize melanoma targets to CAR-T cells directed against B7H6. Our study highlights a novel mechanism to induce B7H6 expression and suggests a pharmacological approach to improve B7H6-directed immunotherapy. KEY MESSAGES: B7H6 is induced by ER stress in a PERK-dependent mechanism. Induction of B7H6 is obtained pharmacologically by HIV protease inhibitors. Exposure of tumor cells to the HIV protease inhibitor nelfinavir improves the recognition by B7H6-directed CAR-T.

Comparative Analysis of Roseoloviruses in Humans, Pigs, Mice, and Other Species

Viruses of the genus Roseolovirus belong to the subfamily Betaherpesvirinae, family Herpesviridae. Roseoloviruses have been studied in humans, mice and pigs, but they are likely also present in other species. This is the first comparative analysis of roseoloviruses in humans and animals. The human roseoloviruses human herpesvirus 6A (HHV-6A), 6B (HHV-6B), and 7 (HHV-7) are relatively well characterized. In contrast, little is known about the murine roseolovirus (MRV), also known as murine thymic virus (MTV) or murine thymic lymphotrophic virus (MTLV), and the porcine roseolovirus (PRV), initially incorrectly named porcine cytomegalovirus (PCMV). Human roseoloviruses have gained attention because they can cause severe diseases including encephalitis in immunocompromised transplant and AIDS patients and febrile seizures in infants. They have been linked to a number of neurological diseases in the immunocompetent including multiple sclerosis (MS) and Alzheimer's. However, to prove the causality in the latter disease associations is challenging due to the high prevalence of these viruses in the human population. PCMV/PRV has attracted attention because it may be transmitted and pose a risk in xenotransplantation, e.g., the transplantation of pig organs into humans. Most importantly, all roseoloviruses are immunosuppressive, the humoral and cellular immune responses against these viruses are not well studied and vaccines as well as effective antivirals are not available.

Human NK Cells and Herpesviruses: Mechanisms of Recognition, Response and Adaptation

NK cells contribute to early defenses against viruses through their inborn abilities that include sensing of PAMPs and inflammatory signals such as cytokines or chemokines, recognition, and killing of infected cells through activating surface receptors engagement. Moreover, they support adaptive responses via Ab-dependent mechanisms, triggered by CD16, and DC editing. Their fundamental role in anti-viral responses has been unveiled in patients with NK cell deficiencies suffering from severe Herpesvirus infections. Notably, these infections, often occurring as primary infections early in life, can be efficiently cleared by NK, T, and B cells in healthy hosts. Herpesviruses however, generate a complicated balance with the host immune system through their latency cycle moving between immune control and viral reactivation. This lifelong challenge has contributed to the development of numerous evasion mechanisms by Herpesviruses, many of which devoted to elude NK cell surveillance from viral reactivations rather than primary infections. This delicate equilibrium can be altered in proportions of healthy individuals promoting virus reactivation and, more often, in immunocompromised subjects. However, the constant stimulus provided by virus-host interplay has also favored NK-cell adaptation to Herpesviruses. During anti-HCMV responses, NK cells can reshape their receptor repertoire and function, through epigenetic remodeling, and acquire adaptive traits such as longevity and clonal expansion abilities. The major mechanisms of recognition and effector responses employed by NK cells against Herpesviruses, related to their genomic organization will be addressed, including those allowing NK cells to generate memory-like responses. In addition, the mechanisms underlying virus reactivation or control will be discussed.

The Natural Cytotoxicity Receptors in Health and Disease

The Natural Cytotoxicity Receptors (NCRs), NKp46, NKp44, and NKp30, were some of the first human activating Natural Killer (NK) cell receptors involved in the non-MHC-restricted recognition of tumor cells to be cloned over 20 years ago. Since this time many host- and pathogen-encoded ligands have been proposed to bind the NCRs and regulate the cytotoxic and cytokine-secreting functions of tissue NK cells. This diverse set of NCR ligands can manifest on the surface of tumor or virus-infected cells or can be secreted extracellularly, suggesting a remarkable NCR polyfunctionality that regulates the activity of NK cells in different tissue compartments during steady state or inflammation. Moreover, the NCRs can also be expressed by other innate and adaptive immune cell subsets under certain tissue conditions potentially conferring NK recognition programs to these cells. Here we review NCR biology in health and disease with particular reference to how this important class of receptors regulates the functions of tissue NK cells as well as confer NK cell recognition patterns to other innate and adaptive lymphocyte subsets. Finally, we highlight how NCR biology is being harnessed for novel therapeutic interventions particularly for enhanced tumor surveillance.

Human Gut-Associated Natural Killer Cells in Health and Disease

It is well established that natural killer (NK) cells are involved in both innate and adaptive immunity. Indeed, they can recognize molecules induced at the cell surface by stress signals and virus infections. The functions of NK cells in the gut are much more complex. Gut NK cells are not precisely organized in lymphoid aggregates but rather scattered in the epithelium or in the stroma, where they come in contact with a multitude of antigens derived from commensal or pathogenic microorganisms in addition to components of microbiota. Furthermore, NK cells in the bowel interact with several cell types, including epithelial cells, fibroblasts, macrophages, dendritic cells, and T lymphocytes, and contribute to the maintenance of immune homeostasis and development of efficient immune responses. NK cells have a key role in the response to intestinal bacterial infections, primarily through production of IFNγ, which can stimulate recruitment of additional NK cells from peripheral blood leading to amplification of the anti-bacterial immune response. Additionally, NK cells can have a role in the pathogenesis of gut autoimmune inflammatory bowel diseases (IBDs), such as Crohn's Disease and Ulcerative Colitis. These diseases are considered relevant to the generation of gastrointestinal malignancies. Indeed, the role of gut-associated NK cells in the immune response to bowel cancers is known. Thus, in the gut immune system, NK cells play a dual role, participating in both physiological and pathogenic processes. In this review, we will analyze the known functions of NK cells in the gut mucosa both in health and disease, focusing on the cross-talk among bowel microenvironment, epithelial barrier integrity, microbiota, and NK cells.

Human Natural Killer Receptors, Co-Receptors, and Their Ligands

In the last 20 years, the study of human natural killer (NK) cells has moved from the first molecular characterizations of very few receptor molecules to the identification of a plethora of receptors displaying surprisingly divergent functions. We have contributed to the description of inhibitory receptors and their signaling pathways, important in fine regulation in many cell types, but unknown until their discovery in the NK cells. Inhibitory function is central to regulating NK-mediated cytolysis, with different molecular structures evolving during speciation to assure its persistence. More recently, it has become possible to characterize the NK triggering receptors mediating natural cytotoxicity, unveiling the existence of a network of cellular interactions between effectors of both natural and adaptive immunity. This unit reviews the contemporary history of molecular studies of receptors and ligands involved in NK cell function, characterizing the ligands of the triggering receptor and the mechanisms for finely regulating their expression in pathogen-infected or tumor cells. © 2018 by John Wiley & Sons, Inc.

Deficient Natural Killer Cell NKp30-Mediated Function and Altered NCR3 Splice Variants in Hepatocellular Carcinoma

The activating natural cytotoxicity receptor NKp30 is critical for natural killer (NK) cell function and tumor immune surveillance. The natural cytotoxicity receptor-3 (NCR3) gene is transcribed into several splice variants whose physiological relevance is still incompletely understood. In this study, we investigated the role of NKp30 and its major ligand B7 homolog 6 (B7-H6) in patients with hepatocellular carcinoma (HCC). Peripheral blood NK cell phenotype was skewed toward a defective/exhausted immune profile with decreased frequencies of cells expressing NKp30 and natural killer group 2, member D and an increased proportion of cells expressing T-cell immunoglobulin and mucin-domain containing-3. Moreover, NKp30-positive NK cells had a reduced expression of NCR3 immunostimulatory splice variants and an increased expression of the inhibitory variant in patients with advanced tumor, resulting in deficient NKp30-mediated functionality. Tumor-infiltrating lymphocytes showed a prevalent inhibitory NKp30 isoform profile, consistent with decreased NKp30-mediated function. Of note, there were significant differences in the cytokine milieu between the neoplastic and the surrounding non-neoplastic tissue, which may have further influenced NKp30 function. Exposure of NK cells to B7-H6-expressing HCC cells significantly down-modulated NKp30, that was prevented by small interfering RNA-mediated knockdown, suggesting a role for this ligand in inhibiting NKp30-mediated responses. Interestingly, B7-H6 expression was reduced in HCC tissue and simultaneously augmented as a soluble form in HCC patients, particularly those with advanced staging or larger nodule size. Conclusion: These findings provide evidence in support of a role of NKp30 and its major ligand in HCC development and evolution.

B7H6 is a functional ligand for NKp30 in rat and cattle and determines NKp30 reactivity toward human cancer cell lines

NK cells kill cancer cells and infected cells upon activation by cell surface receptors. Human NKp30 is an activating receptor expressed by all mature NK cells. The B7 family member B7H6 has been identified as one ligand for NKp30. Several alternative ligands have also been reported, and the field remains unsettled. To this end, we have identified full-length functional B7H6 orthologs in rat and cattle, demonstrated by phylogenetic analysis and transfection experiments. In cell-cell contact-dependent assays, chimeric NKp30 reporter cells responded strongly to B7H6 in rat and cattle. Likewise, rat NKp30 expressing target cells induced strong activation of B7H6 reporter cells. Together, these observations demonstrate that B7H6 is conserved as a functional ligand for NKp30 in mammalian species separated by more than 100 million years of evolution. B7H6 and NKp30 are pseudogenes in laboratory mice. The rat thus represents an attractive experimental animal model to study the NKp30-B7H6 interaction in vivo. B7H6 was widely expressed among human cancer cell lines, and the expression level correlated strongly with the activation of human NKp30 reporter cells. Furthermore, siRNA knockdown of B7H6 abolished NKp30 reporter responses, suggesting that B7H6 is the major functionally relevant expressed ligand for NKp30 on these cancer cell lines.

Chronic viral infections in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS)

Background and main text

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex and controversial clinical condition without having established causative factors. Increasing numbers of cases during past decade have created awareness among patients as well as healthcare professionals. Chronic viral infection as a cause of ME/CFS has long been debated. However, lack of large studies involving well-designed patient groups and validated experimental set ups have hindered our knowledge about this disease. Moreover, recent developments regarding molecular mechanism of pathogenesis of various infectious agents cast doubts over validity of several of the past studies.

Conclusions

This review aims to compile all the studies done so far to investigate various viral agents that could be associated with ME/CFS. Furthermore, we suggest strategies to better design future studies on the role of viral infections in ME/CFS.

Herpesvirus Evasion of Natural Killer Cells

Natural killer (NK) cells play an important role in the host response against viral infections and cancer development. They are able to kill virus-infected and tumor cells, and they produce different important cytokines that stimulate the antiviral and antitumor adaptive immune response, particularly interferon gamma. NK cells are of particular importance in herpesvirus infections, which is illustrated by systemic and life-threatening herpesvirus disease symptoms in patients with deficiencies in NK cell activity and by the myriad of reports describing herpesvirus NK cell evasion strategies. The latter is particularly obvious for cytomegaloviruses, but increasing evidence indicates that most, if not all, members of the herpesvirus family suppress NK cell activity to some extent. This review discusses the different NK cell evasion strategies described for herpesviruses and how this knowledge may translate to clinical applications.

In vitro methods for testing antiviral drugs

Despite successful vaccination programs and effective treatments for some viral infections, humans are still losing the battle with viruses. Persisting human pandemics, emerging and re-emerging viruses, and evolution of drug-resistant strains impose continuous search for new antiviral drugs. A combination of detailed information about the molecular organization of viruses and progress in molecular biology and computer technologies has enabled rational antivirals design. Initial step in establishing efficacy of new antivirals is based on simple methods assessing inhibition of the intended target. We provide here an overview of biochemical and cell-based assays evaluating the activity of inhibitors of clinically important viruses.

Glycoproteins of HHV-6A and HHV-6B

Recently, human herpesvirus 6A and 6B (HHV-6A and HHV-6B) were classified into distinct species. Although these two viruses share many similarities, cell tropism is one of their striking differences, which is partially because of the difference in their entry machinery. Many glycoproteins of HHV-6A/B have been identified and analyzed in detail, especially in their functions during entry process into host cells. Some of these glycoproteins were unique to HHV-6A/B. The cellular factors associated with these viral glycoproteins (or glycoprotein complex) were also identified in recent years. Detailed interaction analyses were also conducted, which could partially prove the difference of entry machinery in these two viruses. Although there are still issues that should be addressed, all the knowledges that have been earned in recent years could not only help us to understand these viruses' entry mechanism well but also would contribute to the development of the therapy and/or prophylaxis methods for HHV-6A/B-associated diseases.

HHV-6A Infection of Endometrial Epithelial Cells Induces Increased Endometrial NK Cell-Mediated Cytotoxicity

Background: We have recently reported the presence of Human herpesvirus-6A (HHV-6A) DNA in the 43% of endometrial epithelial cells from primary idiopathic infertile women, with no positivity in fertile women. To investigate the possible effect of HHV-6A infection in endometrial (e)NK cells functions, we examined activating/inhibitory receptors expressed by eNK cells and the corresponding ligands on endometrial cells during HHV-6A infection.

Methods: Endometrial biopsies and uterine flushing samples during the secretory phase were obtained from 20 idiopathic infertile women and twenty fertile women. HHV-6A infection of endometrial epithelial cells was analyzed by Real-Time PCR, immunofluorescence and flow cytometry. eNKs receptors and endometrial ligands expression were evaluated by immunofluorescence and flow cytometry.

Results: We observed the presence of HHV-6A infection (DNA, protein) of endometrial epithelial cells in the 40% of idiopathic infertile women. The eNK from all the subgroups expressed high levels of NKG2D and NKG2A receptors. Functional studies showed that NKG2D activating receptor and FasL are involved in the acquired cytotoxic function of eNK cells during HHV-6A infection of endometrial epithelial cells. In the presence of HHV-6A infection, eNK cells increased expression of CCR2, CXCR3 and CX3CR1 chemokine receptors (p = 0.01) and endometrial epithelial cells up-modulated the corresponding ligands: MCP1 (Monocyte chemotactic protein 1, CCL2), IP-10 (Interferon gamma-induced protein 10, CXCL10) and Eotaxin-3 (CCL26).

Conclusion: Our results, for the first time, showed the implication of eNK cells in controlling HHV-6A endometrial infection and clarify the mechanisms that might be implicated in female idiopathic infertility.

The Interplay between Natural Killer Cells and Human Herpesvirus-6

Human Herpesvirus 6 (HHV-6) is a set of two closely related herpes viruses known as HHV-6A and HHV-6B. Both are lymphotropic viruses that establish latency in the host. The ability to evade the immune responses of effector cells is likely a major factor contributing to the development of a persistent HHV-6A/B (collectively termed HHV-6) infection. Natural killer (NK) cells are lymphocytes that, along with neutrophils and monocytes/macrophages, participate in the critical innate immune response during viral infections, but can also mediate the antigen-specific memory responses generally associated with adaptive immunity. NK cells compose the first barrier that viruses must break through to continue replication and dissemination, and a weak NK cell response may predispose an individual to chronic viral infections. Both HHV-6A and HHV-6B can interfere with NK cell-mediated anti-viral responses but the mechanisms by which each of these viruses affect NK cell activity differs. In this review, we will explore the nuanced relationships between the two viruses and NK cells, discussing, in addition, relevant disease associations.

HHV-6A/6B Infection of NK Cells Modulates the Expression of miRNAs and Transcription Factors Potentially Associated to Impaired NK Activity

Natural killer (NK) cells have a critical role in controlling virus infections, and viruses have evolved several mechanisms to escape NK cell functions. In particular, Human herpesvirus 6 (HHV-6) is associated with diseases characterized by immune dysregulation and has been reported to infect NK cells. We recently found that HHV-6 in vitro infection of human thyroid follicular epithelial cells and T-lymphocytes modulates several miRNAs associated with alterations in immune response. Since miRNAs are key regulators of many immune pathways, including NK cell functions, we aimed to study the impact of HHV-6A and -6B in vitro infection on the intracellular mediators correlated to NK cell function. To this purpose, a human NK cell line (NK-92) was infected in vitro with HHV-6A or 6B and analyzed for alterations in the expression of miRNAs and transcription factors. The results showed that both viruses establish lytic replication in NK-92 cells, as shown by the presence of viral DNA, expression of lytic transcripts and antigens, and by the induction of an evident cytopathic effect. Notably, both viruses, although with species-specific differences, induced significant modifications in miRNA expression of miRNAs known for their role in NK cell development, maturation and effector functions (miR-146, miR-155, miR-181, miR-223), and on at least 13 miRNAs with recognized role in inflammation and autoimmunity. Also the expression of transcription factors was significantly modified by HHV-6A/6B infection, with an early increase of ATF3, JUN and FOXA2 by both species, whereas HHV-6A specifically induced a 15-fold decrease of POU2AF1, and HHV-6B an increase of FOXO1 and a decrease of ESR1. Overall, our data show that HHV-6A and -6B infections have a remarkable effect on the expression of miRNAs and transcription factors, which might be important in the induction of NK cell function impairment, virus escape strategies and related pathologies.

Human cytomegalovirus escapes immune recognition by NK cells through the downregulation of B7-H6 by the viral genes US18 and US20

Human cytomegalovirus (HCMV) is a major human pathogen, causing serious diseases in immunocompromised populations and congenially infected neonates. One of the main immune cells acting against the virus are Natural Killer (NK) cells. Killing by NK cells is mediated by a small family of activating receptors such as NKp30 that interact with the cellular ligand B7-H6. The outcome of B7-H6-NKp30 interaction was, so far, mainly studied with regard to NK recognition and killing of tumors. Here, we demonstrated that the expression of B7-H6 is upregulated following HCMV infection and that HCMV uses two of its genes: US18 and US20, to interfere with B7-H6 surface expression, in a mechanism involving endosomal degradation, in order to evade NK cell recognition.

Natural Killer Cell-Based Cancer Immunotherapies: From Immune Evasion to Promising Targeted Cellular Therapies

Immunotherapies based on natural killer (NK) cells are among the most promising therapies under development for the treatment of so far incurable forms of leukemia and other types of cancer. The importance of NK cells for the control of viral infections and cancer is supported among others by the findings that viruses and tumors use a multitude of mechanisms to subvert and evade the NK cell system. Infections and malignant diseases can further lead to the shaping of NK cell populations with altered reactivity. Counter measures of potential therapeutic impact include the blocking of inhibitory interactions between NK cell receptors and their cellular ligands, the enhancement of activating receptor signals, and the infusion of large numbers of ex vivo generated and selected NK cells. Moreover, the specific cross-linking of NK cells to their target cells using chimeric antigen receptors or therapeutic bi-/trispecific antibody reagents is a promising approach. In this context, NK cells stand out by their positive effects and safety demonstrated in most clinical trials so far. Based in part on results of the recent EC-sponsored project “NATURIMMUN” and considering additional published work in the field, we discuss below new developments and future directions that have the potential to further advance and establish NK cell-based therapies at the clinics on a broader scale.

Disarming Cellular Alarm Systems-Manipulation of Stress-Induced NKG2D Ligands by Human Herpesviruses

The coevolution of viruses and their hosts led to the repeated emergence of cellular alert signals and viral strategies to counteract them. The herpesvirus family of viruses displays the most sophisticated repertoire of immune escape mechanisms enabling infected cells to evade immune recognition and thereby maintain infection. The herpesvirus family consists of nine viruses that are capable of infecting humans: herpes simplex virus 1 and 2 (HSV-1, HSV-2), varicella zoster virus (VZV), Epstein–Barr virus (EBV), human cytomegalovirus (HCMV), roseoloviruses (HHV-6A, HHV-6B, and HHV-7), and Kaposi’s-sarcoma-associated herpesvirus (KSHV). Most of these viruses are highly prevalent and infect a vast majority of the human population worldwide. Notably, research over the past 15 years has revealed that cellular ligands for the activating receptor natural-killer group 2, member D (NKG2D)—which is primarily expressed on natural killer (NK) cells—are common targets suppressed during viral infection, i.e., their surface expression is reduced in virtually all lytic herpesvirus infections by diverse mechanisms. Here, we review the viral mechanisms by which all herpesviruses known to date to downmodulate the expression of the NKG2D ligands. Also, in light of recent findings, we speculate about the importance of the emergence of eight different NKG2D ligands in humans and further allelic diversification during host and virus coevolution.

Herpesviruses dUTPases: A New Family of Pathogen-Associated Molecular Pattern (PAMP) Proteins with Implications for Human Disease

The human herpesviruses are ubiquitous viruses and have a prevalence of over 90% in the adult population. Following a primary infection they establish latency and can be reactivated over a person's lifetime. While it is well accepted that human herpesviruses are implicated in numerous diseases ranging from dermatological and autoimmune disease to cancer, the role of lytic proteins in the pathophysiology of herpesvirus-associated diseases remains largely understudies. Only recently have we begun to appreciate the importance of lytic proteins produced during reactivation of the virus, in particular the deoxyuridine triphosphate nucleotidohydrolases (dUTPase), as key modulators of the host innate and adaptive immune responses. In this review, we provide evidence from animal and human studies of the Epstein-Barr virus as a prototype, supporting the notion that herpesviruses dUTPases are a family of proteins with unique immunoregulatory functions that can alter the inflammatory microenvironment and thus exacerbate the immune pathology of herpesvirus-related diseases including myalgic encephalomyelitis/chronic fatigue syndrome, autoimmune diseases, and cancer.