Recognition of vaccinia virus-infected cells by human natural killer cells depends on natural cytotoxicity receptors

The alteration of NK cells phenotypes related to the functions and dengue disease outcomes

Natural killer cells (NK cells) are the front line of immune cells to combat pathogens and able to influence the subsequent adaptive immune responses. One of the factors contributing to pathogenesis in dengue hemorrhagic fever (DHF) disease is aberrant immune activation during early phase of infection. This study explored the profile of NK cells in dengue infected pediatric patients with different degrees of disease severity. DHF patients contained higher frequency of activated NK cells but lower ratio of CD56dim:CD56bright NK subsets. Activated NK cells exhibited alterations in several NK receptors. Interestingly, the frequencies of NKp30 expressing activated NK cells were more pronounced in dengue fever (DF) than in DHF pediatric patients. In vitro functional analysis indicated that degranulation of NK cells in responding to dengue infected dendritic cells (DCs) required cell-cell contact and type I IFNs. Meanwhile, Interferon gamma (IFN-γ) production initially required cell-cell contact and type I IFNs followed by Interleukin-12 (IL-12), Interleukin-15 (IL-15) and Interleukin-18 (IL-18) resulting in the amplification of IFN-γ producing NK cells over time. This study highlighted the complexity and the factors influencing NK cells responses to dengue virus. Degree of activation, phenotypes of activated cells and the crosstalk between NK cells and other immune cells, could modulate the outcome of NK cells function in the dengue disease.

Natural Killer Cells and Cytotoxic T Cells: Complementary Partners against Microorganisms and Cancer

Natural killer (NK) cells and cytotoxic T (CD8+) cells are two of the most important types of immune cells in our body, protecting it from deadly invaders. While the NK cell is part of the innate immune system, the CD8+ cell is one of the major components of adaptive immunity. Still, these two very different types of cells share the most important function of destroying pathogen-infected and tumorous cells by releasing cytotoxic granules that promote proteolytic cleavage of harmful cells, leading to apoptosis. In this review, we look not only at NK and CD8+ T cells but also pay particular attention to their different subpopulations, the immune defenders that include the CD56+CD16dim, CD56dimCD16+, CD57+, and CD57+CD16+ NK cells, the NKT, CD57+CD8+, and KIR+CD8+ T cells, and ILCs. We examine all these cells in relation to their role in the protection of the body against different microorganisms and cancer, with an emphasis on their mechanisms and their clinical importance. Overall, close collaboration between NK cells and CD8+ T cells may play an important role in immune function and disease pathogenesis. The knowledge of how these immune cells interact in defending the body against pathogens and cancers may help us find ways to optimize their defensive and healing capabilities with methods that can be clinically applied.

Recombinant Strains of Oncolytic Vaccinia Virus for Cancer Immunotherapy

Cancer virotherapy is an alternative therapeutic approach based on the viruses that selectively infect and kill tumor cells. Vaccinia virus (VV) is a member of the Poxviridae, a family of enveloped viruses with a large linear double-stranded DNA genome. The proven safety of the VV strains as well as considerable transgene capacity of the viral genome, make VV an excellent platform for creating recombinant oncolytic viruses for cancer therapy. Furthermore, various genetic modifications can increase tumor selectivity and therapeutic efficacy of VV by arming it with the immune-modulatory genes or proapoptotic molecules, boosting the host immune system, and increasing cross-priming recognition of the tumor cells by T-cells or NK cells. In this review, we summarized the data on bioengineering approaches to develop recombinant VV strains for enhanced cancer immunotherapy.

Transcriptional reprogramming of natural killer cells by vaccinia virus shows both distinct and conserved features with mCMV

Natural killer (NK) cells have an established role in controlling poxvirus infection and there is a growing interest to exploit their capabilities in the context of poxvirus-based oncolytic therapy and vaccination. How NK cells respond to poxvirus-infected cells to become activated is not well established. To address this knowledge gap, we studied the NK cell response to vaccinia virus (VACV) in vivo, using a systemic infection murine model. We found broad alterations in NK cells transcriptional activity in VACV-infected mice, consistent with both direct target cell recognition and cytokine exposure. There were also alterations in the expression levels of specific NK surface receptors (NKRs), including the Ly49 family and SLAM receptors, as well as upregulation of memory-associated NK markers. Despite the latter observation, adoptive transfer of VACV-expercienced NK populations did not confer protection from infection. Comparison with the NK cell response to murine cytomegalovirus (MCMV) infection highlighted common features, but also distinct NK transcriptional programmes initiated by VACV. Finally, there was a clear overlap between the NK transcriptional response in humans vaccinated with an attenuated VACV, modified vaccinia Ankara (MVA), demonstrating conservation between the NK response in these different host species. Overall, this study provides new data about NK cell activation, function, and homeostasis during VACV infection, and may have implication for the design of VACV-based therapeutics.

Selective modulation of cell surface proteins during vaccinia infection: A resource for identifying viral immune evasion strategies

The interaction between immune cells and virus-infected targets involves multiple plasma membrane (PM) proteins. A systematic study of PM protein modulation by vaccinia virus (VACV), the paradigm of host regulation, has the potential to reveal not only novel viral immune evasion mechanisms, but also novel factors critical in host immunity. Here, >1000 PM proteins were quantified throughout VACV infection, revealing selective downregulation of known T and NK cell ligands including HLA-C, downregulation of cytokine receptors including IFNAR2, IL-6ST and IL-10RB, and rapid inhibition of expression of certain protocadherins and ephrins, candidate activating immune ligands. Downregulation of most PM proteins occurred via a proteasome-independent mechanism. Upregulated proteins included a decoy receptor for TRAIL. Twenty VACV-encoded PM proteins were identified, of which five were not recognised previously as such. Collectively, this dataset constitutes a valuable resource for future studies on antiviral immunity, host-pathogen interaction, poxvirus biology, vector-based vaccine design and oncolytic therapy.

Vaccinia virus (VACV) is the vaccine used to eradicate smallpox and an excellent model for studying host-pathogen interactions. Many VACV-mediated immune evasion strategies are known, however how immune cells recognise VACV-infected cells is incompletely understood because of the complexity of surface proteins regulating such interactions. Here, a systematic study of proteins on the cell surface at different times during infection with VACV is presented. This shows not only the precise nature and kinetics of appearance of VACV proteins, but also the selective alteration of cellular surface proteins. The latter thereby identified potential novel immune evasion strategies and host proteins regulating immune activation. Comprehensive comparisons with published datasets provided further insight into mechanisms used to regulate surface protein expression. Such comparisons also identified proteins that are targeted by both VACV and human cytomegalovirus (HCMV), and which are therefore likely to represent host proteins regulating immune recognition and activation. Collectively, this work provides a valuable resource for studying viral immune evasion mechanisms and novel host proteins critical in host immunity.

Conditions That Simulate the Environment of Atopic Dermatitis Enhance Susceptibility of Human Keratinocytes to Vaccinia Virus

Individuals with underlying chronic skin conditions, notably atopic dermatitis (AD), are disproportionately affected by infections from members of the herpesviridae, papovaviridae, and poxviridae families. Many patients with AD experience recurrent, widespread cutaneous viral infections that can lead to viremia, serious organ complications, and even death. Little is known about how the type 2 inflammatory environment observed in the skin of AD patients impacts the susceptibility of epidermal cells (keratinocytes) to viral pathogens. Herein, we studied the susceptibility of keratinocytes to the prototypical poxvirus, vaccinia virus (VV)-the causative agent of eczema vaccinatum-under conditions that simulate the epidermal environment observed in AD. Treatment of keratinocytes with type 2 cytokines (IL-4 and -13) to simulate the inflammatory environment or a tight junction disrupting peptide to mirror the barrier disruption observed in AD patients, resulted in a differentiation-dependent increase in susceptibility to VV. Furthermore, pan JAK inhibition was able to diminish the VV susceptibility occurring in keratinocytes exposed to type 2 cytokines. We propose that in AD, the increased viral susceptibility of keratinocytes leads to enhanced virus production in the skin, which contributes to the rampant dissemination and pathology seen within patients.

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

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

Influenza A Virus Hemagglutinin and Other Pathogen Glycoprotein Interactions with NK Cell Natural Cytotoxicity Receptors NKp46, NKp44, and NKp30

Natural killer (NK) cells are part of the innate immunity repertoire, and function in the recognition and destruction of tumorigenic and pathogen-infected cells. Engagement of NK cell activating receptors can lead to functional activation of NK cells, resulting in lysis of target cells. NK cell activating receptors specific for non-major histocompatibility complex ligands are NKp46, NKp44, NKp30, NKG2D, and CD16 (also known as FcγRIII). The natural cytotoxicity receptors (NCRs), NKp46, NKp44, and NKp30, have been implicated in functional activation of NK cells following influenza virus infection via binding with influenza virus hemagglutinin (HA). In this review we describe NK cell and influenza A virus biology, and the interactions of influenza A virus HA and other pathogen lectins with NK cell natural cytotoxicity receptors (NCRs). We review concepts which intersect viral immunology, traditional virology and glycobiology to provide insights into the interactions between influenza virus HA and the NCRs. Furthermore, we provide expert opinion on future directions that would provide insights into currently unanswered questions.

The role of NK cells in fighting the virus infection and sepsis

Natural killer cells, one of the important types of innate immune cells, play a pivotal role in the antiviral process in vivo. It has been shown that increasing NK cell activity may promote the alleviation of viral infections, even severe infection-induced sepsis. Given the current state of the novel coronavirus (SARS-CoV-2) global pandemic, clarifying the anti-viral function of NK cells would be helpful for revealing the mechanism of host immune responses and decipher the progression of COVID-19 and providing important clues for combating this pandemic. In this review, we summarize the roles of NK cells in viral infection and sepsis as well as the potential possibilities of NK cell-based immunotherapy for treating COVID-19.

Natural Killer Cell Activation Receptor NKp30 Oligomerization Depends on Its N-Glycosylation

NKp30 is one of the main human natural killer (NK) cell activating receptors used in directed immunotherapy. The oligomerization of the NKp30 ligand binding domain depends on the length of the C-terminal stalk region, but our structural knowledge of NKp30 oligomerization and its role in signal transduction remains limited. Moreover, ligand binding of NKp30 is affected by the presence and type of N-glycosylation. In this study, we assessed whether NKp30 oligomerization depends on its N-glycosylation. Our results show that NKp30 forms oligomers when expressed in HEK293S GnTI^- cell lines with simple N-glycans. However, NKp30 was detected only as monomers after enzymatic deglycosylation. Furthermore, we characterized the interaction between NKp30 and its best-studied cognate ligand, B7-H6, with respect to glycosylation and oligomerization, and we solved the crystal structure of this complex with glycosylated NKp30, revealing a new glycosylation-induced mode of NKp30 dimerization. Overall, this study provides new insights into the structural basis of NKp30 oligomerization and explains how the stalk region and glycosylation of NKp30 affect its ligand affinity. This furthers our understanding of the molecular mechanisms involved in NK cell activation, which is crucial for the successful design of novel NK cell-based targeted immunotherapeutics.

Mechanisms of Natural Killer Cell Evasion Through Viral Adaptation

The continuous interactions between host and pathogens during their coevolution have shaped both the immune system and the countermeasures used by pathogens. Natural killer (NK) cells are innate lymphocytes that are considered central players in the antiviral response. Not only do they express a variety of inhibitory and activating receptors to discriminate and eliminate target cells but they can also produce immunoregulatory cytokines to alert the immune system. Reciprocally, several unrelated viruses including cytomegalovirus, human immunodeficiency virus, influenza virus, and dengue virus have evolved a multitude of mechanisms to evade NK cell function, such as the targeting of pathways for NK cell receptors and their ligands, apoptosis, and cytokine-mediated signaling. The studies discussed in this article provide further insights into the antiviral function of NK cells and the pathways involved, their constituent proteins, and ways in which they could be manipulated for host benefit.

Molluscum contagiosum virus MC80 sabotages MHC-I antigen presentation by targeting tapasin for ER-associated degradation

The human specific poxvirus molluscum contagiosum virus (MCV) produces skin lesions that can persist with minimal inflammation, suggesting that the virus has developed robust immune evasion strategies. However, investigations into the underlying mechanisms of MCV pathogenesis have been hindered by the lack of a model system to propagate the virus. Herein we demonstrate that MCV-encoded MC80 can disrupt MHC-I antigen presentation in human and mouse cells. MC80 shares moderate sequence-similarity with MHC-I and we find that it associates with components of the peptide-loading complex. Expression of MC80 results in ER-retention of host MHC-I and thereby reduced cell surface presentation. MC80 accomplishes this by engaging tapasin via its luminal domain, targeting it for ubiquitination and ER-associated degradation in a process dependent on the MC80 transmembrane region and cytoplasmic tail. Tapasin degradation is accompanied by a loss of TAP, which limits MHC-I access to cytosolic peptides. Our findings reveal a unique mechanism by which MCV undermines adaptive immune surveillance.

NKp44-NKp44 Ligand Interactions in the Regulation of Natural Killer Cells and Other Innate Lymphoid Cells in Humans

Natural Killer (NK) cells are potent cytotoxic cells belonging to the family of Innate Lymphoid Cells (ILCs). Their most characterized effector functions are directed to the control of aberrant cells in the body, including both transformed and virus-infected cells. NK cell-mediated recognition of abnormal cells primarily occurs through receptor-ligand interactions, involving an array of inhibitory and activating NK receptors and different types of ligands expressed on target cells. While most of the receptors have become known over many years, their respective ligands were only defined later and their impressive complexity has only recently become evident. NKp44, a member of Natural Cytotoxicity Receptors (NCRs), is an activating receptor playing a crucial role in most functions exerted by activated NK cells and also by other NKp44⁺ immune cells. The large and heterogeneous panel of NKp44 ligands (NKp44L) now includes surface expressed glycoproteins and proteoglycans, nuclear proteins that can be exposed outside the cell, and molecules that can be either released in the extracellular space or carried in extracellular vesicles. Recent findings have extended our knowledge on the nature of NKp44L to soluble plasma glycoproteins, such as secreted growth factors or extracellular matrix (ECM)-derived glycoproteins. NKp44L are induced upon tumor transformation or viral infection but may also be expressed in normal cells and tissues. In addition, NKp44-NKp44L interactions are involved in the crosstalk between NK cells and different innate and adaptive immune cell types. NKp44 expression in different ILCs located in tissues further extends the potential role of NKp44-NKp44L interactions.

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.

Microbial killing by NK cells

It is now evident that NK cells kill bacteria, fungi, and parasites in addition to tumor and virus-infected cells. In addition to a number of recent publications that have identified the receptors and ligands, and mechanisms of cytotoxicity, new insights are reflected in the reports from researchers all over the world at the 17th Meeting of the Society for Natural Immunity held in San Antonio, TX, USA from May 28 through June 1, 2018. We will provide an overview of the field and discuss how the presentations at the meeting might shape our knowledge and future directions in the field.

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.

Transcriptomic signatures of NK cells suggest impaired responsiveness in HIV-1 infection and increased activity post-vaccination

Natural killer (NK) cells limit viral replication by direct recognition of infected cells, antibody-dependent cellular cytotoxicity (ADCC), and releasing cytokines. Although growing evidence supports NK cell antiviral immunity in HIV-1 infection, further knowledge of their response is necessary. Here we show that NK cells responding to models of direct cell recognition, ADCC, and cytokine activation have unique transcriptional fingerprints. Compared with healthy volunteers, individuals with chronic HIV-1 infection have higher expression of genes commonly associated with activation, and lower expression of genes associated with direct cell recognition and cytokine stimulation in their NK cells. By contrast, NK cell transcriptional profiles of individuals receiving a modified vaccinia Ankara (MVA) vectored HIV-1 vaccine show upregulation of genes associated with direct cell recognition. These findings demonstrate that targeted transcriptional profiling provides a sensitive assessment of NK cell activity, which helps understand how NK cells respond to viral infections and vaccination.

Natural killer (NK) cells are important for eliminating cells under stress or infected by virus, and may have a function in anti-HIV immunity. Here the authors show that different NK-activating stimuli induce distinct transcriptional fingerprints in human NK cells that are analogous to changes caused by HIV vaccination or chronic infection.

Innate immune recognition of double-stranded RNA triggers increased expression of NKG2D ligands after virus infection

Self/non-self-discrimination by the innate immune system relies on germline-encoded, non-rearranging receptors expressed by innate immune cells recognizing conserved pathogen-associated molecular patterns. The natural killer group 2D (NKG2D) receptor is a potent immune-activating receptor that binds human genome-encoded ligands, whose expression is negligible in normal tissues, but increased in stress and disease conditions for reasons that are incompletely understood. Here it is not clear how the immune system reconciles receptor binding of self-proteins with self/non-self-discrimination to avoid autoreactivity. We now report that increased expression of NKG2D ligands after virus infection depends on interferon response factors activated by the detection of viral double-stranded RNA by pattern-recognition receptors (RIG-I/MDA-5) and that NKG2D ligand up-regulation can be blocked by the expression of viral dsRNA-binding proteins. Thus, innate immunity-mediated recognition of viral nucleic acids triggers the infected cell to release interferon for NK cell recruitment and to express NKG2D ligands to become more visible to the immune system. Finally, the observation that NKG2D-ligand induction is a consequence of signaling by pattern-recognition receptors that have been selected over evolutionary time to be highly pathogen-specific explains how the risks of autoreactivity in this system are minimized.

Role of NK cells in immunotherapy and virotherapy of solid tumors

Although natural killer (NK) cells are endowed with powerful cytolytic activity against cancer cells, their role in different therapies against solid tumors has not yet been fully elucidated. Their interactions with various elements of the tumor microenvironment as well as their possible effects in contributing to and/or limiting oncolytic virotherapy render this potential immunotherapeutic tool still difficult to exploit at the bedside. Here, we will review the current literature with the aim of providing new hints to manage this powerful cell type in future innovative therapies, such as the use of NK cells in combination with new cytokines, specific mAbs (inducing ADCC), Tyr-Kinase inhibitors, immunomodulatory drugs and/or the design of oncolytic viruses aimed at optimizing the effect of NK cells in virotherapy.

NKG2D is a Key Receptor for Recognition of Bladder Cancer Cells by IL-2-Activated NK Cells and BCG Promotes NK Cell Activation

Intravesical instillation of bacillus Calmette–Guérin (BCG) is used to treat superficial bladder cancer, either papillary tumors (after transurethral resection) or high-grade flat carcinomas (carcinoma in situ), reducing recurrence in about 70% of patients. Initially, BCG was proposed to work through an inflammatory response, mediated by phagocytic uptake of mycobacterial antigens and cytokine release. More recently, other immune effectors such as monocytes, natural killer (NK), and NKT cells have been suggested to play a role in this immune response. Here, we provide a comprehensive study of multiple bladder cancer cell lines as putative targets for immune cells and evaluated their recognition by NK cells in the presence and absence of BCG. We describe that different bladder cancer cells can express multiple activating and inhibitory ligands for NK cells. Recognition of bladder cancer cells depended mainly on NKG2D, with a contribution from NKp46. Surprisingly, exposure to BCG did not affect the immune phenotype of bladder cells nor increased NK cell recognition of purified IL-2-activated cell lines. However, NK cells were activated efficiently when BCG was included in mixed lymphocyte cultures, suggesting that NK activation after mycobacteria treatment requires the collaboration of various immune cells. We also analyzed the percentage of NK cells in peripheral blood of a cohort of bladder cancer patients treated with BCG. The total numbers of NK cells did not vary during treatment, indicating that a more detailed study of NK cell activation in the tumor site will be required to evaluate the response in each patient.

Lessons from NK Cell Deficiencies in the Mouse

Since their discovery in the late 1970s, in vivo studies on mouse natural killer (NK) cell almost entirely relied on the use of depleting antibodies and were associated with significant limitations. More recently, large-scale gene-expression analyses allowed the identification of NKp46 as one of the best markers of NK cells across mammalian species. Since then, NKp46 has been shown to be expressed on other subsets of innate lymphoid cells (ILCs) such as the closely related ILC1 and the mucosa-associated NCR(+) ILC3. Based on this marker, several mouse models specifically targeting NKp46-expressing cell have recently been produced. Here, we review recent advances in the generation of models of deficiency in NKp46-expressing cells and their use to address the role of NK cells in immunity, notably on the regulation of adaptive immune responses.

Profile of natural killer cells after a previous natural Vaccinia virus infection in an in vitro viral re-exposure

The present study compares the profile of NK cells in an in vitro re-exposure by Vaccinia virus (VACV), in groups that have had a previous vaccination or natural infection. Our data suggests that stimulation with VACV triggers a cytotoxic response by NK cells marked by an increase of NCRs: NKp30, NKp44, and NKp46 in infected (vaccinated and unvaccinated) subjects and in non-infected vaccinated patients, when compared with non-infected unvaccinated individuals. However, the degranulation and secretion processes are inhibited in infected (vaccinated and unvaccinated) subjects and in the non-infected vaccinated patients, when compared with non-infected unvaccinated individuals. We demonstrated that stimulation with VACV downregulates the percentage of expression of Perforin, Granzyme A, and CD107a, but upregulate CD94 in infected (vaccinated and unvaccinated) subjects and in non-infected vaccinated patients, when compared with non-infected unvaccinated individuals. Furthermore, the percentage of IFN-γ(+) NK cells was significantly lower in non-infected unvaccinated subjects, when compared with infected (vaccinated and unvaccinated) and non-infected vaccinated individuals. Our results also show that the percentage of TNF-α(+) NK cells was significantly higher in infected (vaccinated and unvaccinated) subjects and in non-infected vaccinated patients, when compared with non-infected unvaccinated individuals, after in vitro stimulation with UV-inactivated VACV. Our data suggest that the expression of NCRs NKp30, NKp44, NKp46 and cytokines by NK cells are important in the innate response against VACV.

Natural cytotoxicity receptors and their ligands

Natural killer (NK) cells are innate lymphoid cells (ILCs) that participate to the clearance of pathogen-infected cells and tumour cells. NK cells and subsets of ILCs express the natural cytotoxicity receptors (NCRs) NKp46, NKp44 and NKp30 at their surface. NCRs have been shown to recognize a broad spectrum of ligands ranging from viral-, parasite- and bacterial-derived ligands to cellular ligands; however, the full identification of NCR ligands remains to be performed and will undoubtedly contribute to a better understanding of NK cell and ILC biology.

Monkeypox virus infection of rhesus macaques induces massive expansion of natural killer cells but suppresses natural killer cell functions

Natural killer (NK) cells play critical roles in innate immunity and in bridging innate and adaptive immune responses against viral infection. However, the response of NK cells to monkeypox virus (MPXV) infection is not well characterized. In this intravenous challenge study of MPXV infection in rhesus macaques (Macaca mulatta), we analyzed blood and lymph node NK cell changes in absolute cell numbers, cell proliferation, chemokine receptor expression, and cellular functions. Our results showed that the absolute number of total NK cells in the blood increased in response to MPXV infection at a magnitude of 23-fold, manifested by increases in CD56+, CD16+, CD16-CD56- double negative, and CD16+CD56+ double positive NK cell subsets. Similarly, the frequency and NK cell numbers in the lymph nodes also largely increased with the total NK cell number increasing 46.1-fold. NK cells both in the blood and lymph nodes massively proliferated in response to MPXV infection as measured by Ki67 expression. Chemokine receptor analysis revealed reduced expression of CXCR3, CCR7, and CCR6 on NK cells at early time points (days 2 and 4 after virus inoculation), followed by an increased expression of CXCR3 and CCR5 at later time points (days 7-8) of infection. In addition, MPXV infection impaired NK cell degranulation and ablated secretion of interferon-γ and tumor necrosis factor-α. Our data suggest a dynamic model by which NK cells respond to MPXV infection of rhesus macaques. Upon virus infection, NK cells proliferated robustly, resulting in massive increases in NK cell numbers. However, the migrating capacity of NK cells to tissues at early time points might be reduced, and the functions of cytotoxicity and cytokine secretion were largely compromised. Collectively, the data may explain, at least partially, the pathogenesis of MPXV infection in rhesus macaques.

Vaccinia virus immune evasion: mechanisms, virulence and immunogenicity

Virus infection of mammalian cells is sensed by pattern recognition receptors and leads to an innate immune response that restricts virus replication and induces adaptive immunity. In response, viruses have evolved many countermeasures that enable them to replicate and be transmitted to new hosts, despite the host innate immune response. Poxviruses, such as vaccinia virus (VACV), have large DNA genomes and encode many proteins that are dedicated to host immune evasion. Some of these proteins are secreted from the infected cell, where they bind and neutralize complement factors, interferons, cytokines and chemokines. Other VACV proteins function inside cells to inhibit apoptosis or signalling pathways that lead to the production of interferons and pro-inflammatory cytokines and chemokines. In this review, these VACV immunomodulatory proteins are described and the potential to create more immunogenic VACV strains by manipulation of the gene encoding these proteins is discussed.

TLR3 impairment in human newborns

Newborns are highly susceptible to viral infections. We hypothesized that this susceptibility could be due to a dysregulated expression of innate virus-sensing receptors, i.e., TLR3, TLR7, TLR8, and TLR9 and the cytosolic receptors retinoic acid-inducible gene I, melanoma differentiation-associated protein 5, protein kinase R, and IFN-γ-inducible protein 16. Cord blood mononuclear cells (CBMCs) expressed mRNA for all these receptors except for TLR3. In peripheral blood mononuclear cells (PBMCs), TLR3 mRNA was preferentially expressed in cytotoxic cells, particularly CD56(dim) NK cells. Cord NK cells in contrast showed low TLR3 mRNA expression and lacked TLR3 protein expression. Cord NK cells did not produce IFN-γ in response to polyinosinic-polycytidylic acid [poly(I:C)], whereas strong IFN-γ production was observed in poly(I:C)-stimulated adult NK cells. Cord NK cells had poor cytotoxic function that was only marginally enhanced by exposure to the TLR3 ligand poly(I:C). Opposite to NK cells from adults, their cytotoxicity was not improved by herpes simplex virus (HSV) exposure and they were unable to kill HSV-infected cells. There were no differences in the TLR3 mRNA levels among men, women, and pregnant women, implying that TLR3 is not under sex hormone control. However, decidual NK cells expressed low levels of TLR3 mRNA, which was attributed to their CD56(bright) phenotype. Our data show that cord blood NK cells have deficient TLR3 expression associated with an inability to respond to poly(I:C) and HSV activation and to kill HSV-infected cells. This might explain why newborns are particularly sensitive to neonatal HSV infections.

Structural insights into activation of antiviral NK cell responses

Natural killer (NK) cells are key components of innate immune responses, providing surveillance against cells undergoing tumorigenesis or infection, by viruses or internal pathogens. NK cells can directly eliminate compromised cells and regulate downstream responses of the innate and acquired immune systems through the release of immune modulators (cytokines, interferons). The importance of the role NK cells play in immune defense was demonstrated originally in herpes viral infections, usually mild or localized, which become severe and life threatening in NK-deficient patients . NK cell effector functions are governed by balancing opposing signals from a diverse array of activating and inhibitory receptors. Many NK receptors occur in paired activating and inhibitory isoforms and recognize major histocompatibility complex (MHC) class I proteins with varying degrees of peptide specificity. Structural studies have made considerable inroads into understanding the molecular mechanisms employed to broadly recognize multiple MHC ligands or specific pathogen-associated antigens and the strategies employed by viruses to thwart these defenses. Although many details of NK development, signaling, and integration remain mysterious, it is clear that NK receptors are key components of a system exquisitely tuned to sense any dysregulation in MHC class I expression, or the expression of certain viral antigens, resulting in the elimination of affected cells.

Proposed mechanisms of action for prostate cancer vaccines

Prostate cancer is responsible for the deaths of more than 33,000 American men every year. Once this disease has become metastatic, there is no curative treatment. Alternative therapies to chemotherapy and radical prostatectomy are being increasingly explored. Prostate cancer vaccines--which trigger a tumour-specific cytotoxic-T-lymphocyte-mediated immune attack by the patient's immune system--have been investigated in clinical trials with modest, yet encouraging, results. When developing and administering prostate cancer vaccines, it is critical to consider how vital parameters, such as the stage of disease progression and the nature of adjuvant therapies, could influence treatment outcome. Of particular interest are current and future strategies for diminishing the activity of regulatory T lymphocytes.

NK cells and poxvirus infection

In recent years, our understanding of the role of natural killer (NK) cells in the response to viral infection has grown rapidly. Not only do we realize viruses have many immune-evasion strategies to escape NK cell responses, but that stimulation of NK cell subsets during an antiviral response occurs through receptors seemingly geared directly at viral products and that NK cells can provide a memory response to viral pathogens. Tremendous knowledge has been gained in this area through the study of herpes viruses, but appreciation for the significance of NK cells in the response to other types of viral infections is growing. The function of NK cells in defense against poxviruses has emerged over several decades beginning with the early seminal studies showing the role of NK cells and the NK gene complex in susceptibility of mouse strains to ectromelia, a poxvirus pathogen of mice. More recently, greater understanding has emerged of the molecular details of the response. Given that human diseases caused by poxviruses can be as lethal as smallpox or as benign as Molluscum contagiosum, and that vaccinia virus, the prototypic member of the pox family, persists as a mainstay of vaccine design and has potential as an oncolytic virus for tumor therapy, further research in this area remains important. This review focuses on recent advances in understanding the role of NK cells in the immune response to poxviruses, the receptors involved in activation of NK cells during poxvirus infection, and the viral evasion strategies poxviruses employ to avoid the NK response.

Virus-mediated inhibition of natural cytotoxicity receptor recognition

Natural killer (NK) cells are a part of the innate immune system that functions mainly to kill transformed and infected cells. Their activity is controlled by signals derived from a panel of activating and inhibitory receptors. The natural cytotoxicity receptors (NCRs): NKp30, NKp44, and NKp46 (NCR1 in mice) are prominent among the activating NK cell receptors and they are, notably, the only NK-activating receptors that are able to recognize pathogen-derived ligands. In addition, the NCRs also recognize cellular ligands, the identity of which remains largely unknown. In this review, we summarize the current knowledge regarding viruses that are recognized by the NCRs, focusing on the diverse immune-evasion mechanisms employed by viruses to escape this detection. We also discuss the unique role the NCRs have in regulating NK cell activity with particular emphasis on the in vivo function of NKp46/NCR1.

Natural killer cell responses during viral infections: flexibility and conditioning of innate immunity by experience

Natural killer (NK) cells mediate innate defense against viral infections, but the mechanisms in place to access their functions as needed during diverse challenges while limiting collateral damage are poorly understood. Recent molecular characterization of effects mediated through infection-induced inhibitory/activating NK receptor-ligand pairs and cytokines are providing new insights into pathways regulating their responses and revealing unexpected consequences for NK cell subset effects, maintenance, proliferation and function through times overlapping with adaptive and long-lived immunity. The observations define flexible pathways for experience-induced 'conditioning' and challenge narrowly defined roles for NK cells and innate immunity as first responders with prescribed functions. They suggest that individual experiences as well as genes influence the innate immune resources available to fight off an infection.

Human NK cell recognition of target cells in the prism of natural cytotoxicity receptors and their ligands

The matter of the pathogen- and cancer-associated ligands recognized by the Natural Cytotoxicity Receptors (NCRs) has been a subject of intense research ever since the identification of the NCRs more than 12 years ago by Alessandro and Lorenzo Moretta: NKp46 in 1997, NKp44 in 1998, and finally NKp30 in 1999. Expression patterns recognized by NCRs include pathogen-derived, pathogen-induced, and cancer-associated cellular 'self' ligands. Pathogen-exposed cells may exhibit both types of pathogen-associated ligands. Transformed cells, in contrast, exhibit only 'self' ligands which are derived from both the intracellular- and membrane-associated milieu of self molecules. These expression patterns allow for NCR-based NK cell discrimination between healthy and affected cells, in the realms of both pathogenic infection and potential tumorigenesis. The focus of this review is on the current knowledge regarding the identities of NCR ligands and the type of target cells expressing these ligands.

Innate immune recognition of poxviral vaccine vectors

The study of poxviruses pioneered the field of vaccinology after Jenner's remarkable discovery that 'vaccination' with the phylogenetically related cowpox virus conferred immunity to the devastating disease of smallpox. The study of poxviruses continues to enrich the field of virology because the global eradication of smallpox provides a unique example of the potency of effective immunization. Other poxviruses have since been developed as vaccine vectors for clinical and veterinary applications and include modified vaccinia virus strains such as modified vaccinia Ankara and NYVAC as well as the avipox viruses, fowlpox virus and canarypox virus. Despite the empirical development of poxvirus-based vectored vaccines, it is only now becoming apparent that we need to better understand how the innate arm of the immune system drives adaptive immunity to poxviruses, and how this information is relevant to vaccine design strategies, which are the topics addressed in this article.

Identification and characterization of monoclonal antibodies against the ORFV059 protein encoded by Orf virus

Recent outbreaks of orf in China have been attributed to a novel strain of Orf virus (ORFV) designated ORFV-Jilin. Currently, monoclonal antibodies (Mabs) have not yet been developed against this specific pathogen even though such entities could have potential applications regarding the diagnosis and characterization of ORFV-Jilin. Therefore, the current study was undertaken to generate Mab against the immunodominant ORFV059 protein of this virus. For this purpose, the ORFV-Jilin ORFV059 protein was expressed in Escherichia coli and subsequently used as an antigen to immunize mice and for the initial screening of hybridomas prepared from the mice for their ability to produce anti-ORFV059 protein Mabs via an indirect ELISA. Ten, positive hybridomas were identified in this manner and verified based on the ability of their released Mab to react specifically with both naturally and artificially expressed ORFV059 protein in Western blots. The two hybridomas with the greatest propensity to secrete Mab were subcloned three times before being introduced intraperitoneally into mice. Afterwards, both Mab were separately purified from the mice's ascetic fluids and found to successfully recognize the ORFV-Jilin ORFV059 protein in a variety of immunological assays. Thus, the widespread utility of these Mab as a diagnostic core reagent should prove invaluable for further investigations regarding the mechanisms of orf pathogenesis and the control of this disease. In this regard, it should be noted that Mab A3 was used to confirm the predicted late expression of the ORFV-Jilin ORFV059 protein during virus replication.

Activation of natural killer cells during microbial infections

Natural killer (NK) cells are large granular lymphocytes that express a diverse array of germline encoded inhibitory and activating receptors for MHC Class I and Class I-like molecules, classical co-stimulatory ligands, and cytokines. The ability of NK cells to be very rapidly activated by inflammatory cytokines, to secrete effector cytokines, and to kill infected or stressed host cells, suggests that they may be among the very early responders during infection. Recent studies have also identified a small number of pathogen-derived ligands that can bind to NK cell surface receptors and directly induce their activation. Here we review recent studies that have begun to elucidate the various pathways by which viral, bacterial, and parasite pathogens activate NK cells. We also consider two emerging themes of NK cell–pathogen interactions, namely their contribution to adaptive immune responses and their potential to take on regulatory and immunomodulatory functions.

Modulation of NKp30- and NKp46-mediated natural killer cell responses by poxviral hemagglutinin

Natural killer (NK) cells are an important element in the immune defense against the orthopox family members vaccinia virus (VV) and ectromelia virus (ECTV). NK cells are regulated through inhibitory and activating signaling receptors, the latter involving NKG2D and the natural cytotoxicity receptors (NCR), NKp46, NKp44 and NKp30. Here we report that VV infection results in an upregulation of ligand structures for NKp30 and NKp46 on infected cells, whereas the binding of NKp44 and NKG2D was not significantly affected. Likewise, infection with ectromelia virus (ECTV), the mousepox agent, enhanced binding of NKp30 and, to a lesser extent, NKp46. The hemagglutinin (HA) molecules from VV and ECTV, which are known virulence factors, were identified as novel ligands for NKp30 and NKp46. Using NK cells with selectively silenced NCR expression and NCR-CD3ζ reporter cells, we observed that HA present on the surface of VV-infected cells, or in the form of recombinant soluble protein, was able to block NKp30-triggered activation, whereas it stimulated the activation through NKp46. The net effect of this complex influence on NK cell activity resulted in a decreased NK lysis susceptibility of infected cells at late time points of VV infection when HA was expression was pronounced. We conclude that poxviral HA represents a conserved ligand of NCR, exerting a novel immune escape mechanism through its blocking effect on NKp30-mediated activation at a late stage of infection.

Involvement of activating NK cell receptors and their modulation in pathogen immunity

Natural Killer (NK) cells are endowed with cell-structure-sensing receptors providing inhibitory protection from self-destruction (inhibitory NK receptors, iNKRs, including killer inhibitory receptors and other molecules) and rapid triggering potential leading to functional cell activation by Toll-like receptors (TLRs), cytokine receptors, and activating NK cell receptors including natural cytotoxicity receptors (NCRs, i.e., NKp46, NKp46, and NKp44). NCR and NKG2D recognize ligands on infected cells which may be endogenous or may directly bind to some structures derived from invading pathogens. In this paper, we address the known direct or indirect interactions between activating receptors and pathogens and their expression during chronic HIV and HCV infections.

The structural basis of ligand recognition by natural killer cell receptors

Natural killer cells are a group of lymphocytes which function as tightly controlled surveillance operatives which identify transformed cells through a discrete balance of activating and inhibitory receptors ultimately leading to the destruction of incongruent cells. The understanding of this finely tuned balancing act has been aided by the high-resolution structure determination of activating and inhibitory receptors both alone and in complex with their ligands. This paper collates these structural studies detailing the aspects which directly relate to the natural killer cell function and serves to inform both the specialized structural biologist reader and a more general immunology audience.

Evasion of natural killer cells by influenza virus

NK cells are important innate immune effectors during influenza virus infection. However, the influenza virus seems able to use several tactics to counter NK cell recognition for immune evasion. In this review, we will summarize and discuss recent advances regarding the understanding of NK cell evasion mechanisms manipulated by the influenza virus to facilitate its rapid replication inside the respiratory epithelial cells.

Selective induction of host genes by MVA-B, a candidate vaccine against HIV/AIDS

The aim of this study was to define the effects on antigen-presenting cells of the expression of HIV antigens from an attenuated poxvirus vector. We have analyzed the transcriptional changes in gene expression following infection of human immature monocyte-derived dendritic cells (DC) with recombinant modified vaccinia virus Ankara (MVA) expressing the genes encoding the gp120 and Gag-Pol-Nef antigens of HIV type 1 clade B (referred to as MVA-B) versus parental MVA infection. Using microarray technology and real-time reverse transcription-PCR, we demonstrated that the HIV proteins induced the expression of cytokines, cytokine receptors, chemokines, chemokine receptors, and molecules involved in antigen uptake and processing, including major histocompatibility complex (MHC) genes. Levels of mRNAs for interleukin-1, beta interferon, CCR8, and SCYA20 were higher after HIV antigen production. MVA-B infection also modulated the expression of antigen processing and presentation genes: the gene for MICA was upregulated, whereas those for HLA-DRA and HSPA5 were downregulated. Indeed, the increased expression of the gene for MICA, a glycoprotein related to major histocompatibility complex class I molecules, was shown to enhance the interaction between MVA-B-infected target cells and cytotoxic lymphocytes. The expression profiles of the genes for protein kinases such as JAK1 and IRAK2 were activated after HIV antigen expression. Several genes included in the JAK-STAT and mitogen-activated protein kinase signaling pathways were regulated after HIV antigen expression. Our findings provide the first gene signatures in DC of a candidate MVA-B vaccine expressing four HIV antigens and identified the biological roles of some of the regulatory genes, like that for MICA, which will help in the design of more effective MVA-derived vaccines.

Here today--not gone tomorrow: roles for activating receptors in sustaining NK cells during viral infections

The conclusive evidence supporting a role for NK cells in defense against viruses has been obtained under conditions of NK cell deficiencies prior to infections. NK cell proliferation can be induced during infections, but the advantages of resulting expansion have been unclear because NK cell basal frequency is already high. However, NK cell decreases are also observed during certain conditions of viral infection. Given the range of potent antiviral and immunoregulatory functions of NK cells, such "disappearance" dramatically changes the resources available to the host. New studies demonstrate that proliferation dependent on activating receptors for virus-induced ligands is key for NK cell maintenance, and allows their continued availability for control of adaptive immune responses and immunopathology. This pathway for sustaining NK cells may represent a system used generally to select subsets for rescue during homeostatic purging. In the case of NK cells, though, nonselection limits continued access to the many beneficial functions of NK cells. The observations resolve the long-standing conundrum of reported NK cell increases and decreases during viral infections. Moreover, they demonstrate a previously unappreciated role for activating receptors, i.e. to keep NK cells here today and also tomorrow.

Direct TLR2 signaling is critical for NK cell activation and function in response to vaccinia viral infection

Natural killer (NK) cells play an essential role in innate immune control of poxviral infections in vivo. However, the mechanism(s) underlying NK cell activation and function in response to poxviruses remains poorly understood. In a mouse model of infection with vaccinia virus (VV), the most studied member of the poxvirus family, we identified that the Toll-like receptor (TLR) 2-myeloid differentiating factor 88 (MyD88) pathway was critical for the activation of NK cells and the control of VV infection in vivo. We further showed that TLR2 signaling on NK cells, but not on accessory cells such as dendritic cells (DCs), was necessary for NK cell activation and that this intrinsic TLR2-MyD88 signaling pathway was required for NK cell activation and played a critical role in the control of VV infection in vivo. In addition, we showed that the activating receptor NKG2D was also important for efficient NK activation and function, as well as recognition of VV-infected targets. We further demonstrated that VV could directly activate NK cells via TLR2 in the presence of cytokines in vitro and TLR2-MyD88-dependent activation of NK cells by VV was mediated through the phosphatidylinositol 3-kinase (PI3K)-extracellular signal-regulated kinase (ERK) pathway. Taken together, these results represent the first evidence that intrinsic TLR signaling is critical for NK cell activation and function in the control of a viral infection in vivo, indicate that multiple pathways are required for efficient NK cell activation and function in response to VV infection, and may provide important insights into the design of effective strategies to combat poxviral infections.

NK cells are an important component of innate immunity in fighting against poxviral infections in vivo. However, how NK cells are activated and exert their function in controlling poxviruses remains poorly understood. In this paper, we found that VV, the most studied member of the poxvirus family, could directly activate TLR2 on NK cells and that the direct TLR2 stimulation was critical for NK cell activation and function in the control of VV infection in vivo. We further showed that TLR2-dependent NK cell activation by VV was mediated through the PI3K-ERK pathway. In addition, we demonstrated that the activating receptor NKG2D was also required for efficient NK cell activation and function. Collectively, these results represent the first evidence that direct TLR signaling is crucial to NK cell activation and function in the control of a viral infection in vivo, indicate that multiple pathways are required for efficient NK cell activation, and may provide important insights into the design of effective strategies to combat poxviral infections.

A novel inhibitor of the NF-{kappa}B signaling pathway encoded by the parapoxvirus orf virus

The parapoxvirus orf virus (ORFV) is a pathogen of sheep and goats that has been used as a preventive and therapeutic immunomodulatory agent in several animal species. However, the functions (genes, proteins, and mechanisms of action) evolved by ORFV to modulate and manipulate immune responses are poorly understood. Here, the novel ORFV protein ORFV024 was shown to inhibit activation of the NF-kappaB signaling pathway, an important modulator of early immune responses against viral infections. Infection of primary ovine cells with an ORFV024 deletion mutant virus resulted in a marked increase in expression of NF-kappaB-regulated chemokines and other proinflammatory host genes. Expression of ORFV024 in cell cultures significantly decreased lipopolysaccharide (LPS)- and tumor necrosis factor alpha (TNF-alpha)-induced NF-kappaB-responsive reporter gene expression. Further, ORFV024 expression decreased TNF-alpha-induced phosphorylation and nuclear translocation of NF-kappaB-p65, phosphorylation, and degradation of IkappaBalpha, and phosphorylation of IkappaB kinase (IKK) subunits IKKalpha and IKKbeta, indicating that ORFV024 functions by inhibiting activation of IKKs, the bottleneck for most NF-kappaB activating stimuli. Although ORFV024 interferes with activation of the NF-kappaB signaling pathway, its deletion from the OV-IA82 genome had no significant effect on disease severity, progression, and time to resolution in sheep, indicating that ORFV024 is not essential for virus virulence in the natural host. This represents the first description of a NF-kappaB inhibitor encoded by a parapoxvirus.

Crosstalk between components of the innate immune system: promoting anti-microbial defenses and avoiding immunopathologies

Because it reaches full functional efficacy rapidly upon encounter with a pathogen, the innate immune system is considered as the first line of defense against infections. The sensing of microbes or of transformed or infected cells, through innate immune recognition receptors (referred to as activating I2R2), initiates pro-inflammatory responses and innate immune effector functions. Other I2R2 with inhibitory properties bind self-ligands constitutively expressed in host. However, this dichotomy in the recognition of foreign or induced self versus constitutive self by I2R2 is not always respected in certain non-infectious conditions reminiscent of immunopathologies. In this review, we discuss that immune mechanisms have evolved to avoid inappropriate inflammatory disorders in individuals. Molecular crossregulation exists between components of I2R2 signaling pathways, and intricate interactions between cells from both innate and adaptive immune systems set the bases of controlled immune responses. We also pinpoint that, like T or B cells, some cells of the innate immune system must go through education processes to prevent autoreactivity. In addition, we illustrate how gene expression profiling of immune cell types is a useful tool to find functional homologies between cell subsets of different species and to speculate about unidentified functions of these cells in the responses to pathogen infections.

Diminished intracellular invariant chain expression after vaccinia virus infection

Vaccinia virus (VV) has been used as a vaccine to eradicate smallpox and as a vaccine for HIV and tumors. However, the immunoevasive properties of VV have raised safety concerns. VV infection of APCs perturbs MHC class II-mediated Ag presentation. Exposure of human B cell lines to VV induced a substantial reduction in cellular expression of the class II chaperone, invariant chain (Ii), during the late stages (i.e., 8-10 h) of infection. Yet, cell viability and surface expression of MHC class II molecules were maintained up to 24 h after exposure to virus. Reductions in Ii and class II mRNA levels were detected as early as 6 h after VV infection of APCs. To examine whether VV was acting solely to disrupt host protein synthesis, B cells were treated with an inhibitor of translation, cycloheximide (CHX). Within 1 h of B cell CHX treatment, Ii protein expression decreased coupled with a loss of class II presentation. Analysis of Ii degradation in VV- or CHX-treated cells, revealed ongoing Ii proteolysis contributing to reduced steady-state Ii levels in these APC. Yet in contrast with CHX, VV infection of APCs altered lysosomal protease expression and Ii degradation. Virus infection induced cellular cathepsin L expression while reducing the levels of other lysosomal proteases. These results demonstrate that at late stages of VV infection, reductions in cellular Ii levels coupled with changes in lysosomal protease activity, contribute in part to defects in class II presentation.

Activation of natural killer cells by newcastle disease virus hemagglutinin-neuraminidase

The avian paramyxovirus Newcastle disease virus (NDV) selectively replicates in tumor cells and is known to stimulate T-cell-, macrophage-, and NK cell-mediated responses. The mechanisms of NK cell activation by NDV are poorly understood so far. We studied the expression of ligand structures for activating NK cell receptors on NDV-infected tumor cells. Upon infection with the nonlytic NDV strain Ulster and the lytic strain MTH-68/H, human carcinoma and melanoma cells showed enhanced expression of ligands for the natural cytotoxicity receptors NKp44 and NKp46, but not NKp30. Ligands for the activating receptor NKG2D were partially downregulated. Soluble NKp44-Fc and NKp46-Fc, but not NKp30-Fc, chimeric proteins bound specifically to NDV-infected tumor cells and to NDV particle-coated plates. Hemagglutinin-neuraminidase (HN) of the virus serves as a ligand structure for NKp44 and NKp46, as indicated by the blockade of binding to NDV-infected cells and viral particles in the presence of anti-HN antibodies and by binding to cells transfected with HN cDNA. Consistent with the recognition of sialic acid moieties by the viral lectin HN, the binding of NKp44-Fc and NKp46-Fc was lost after desialylation. NKp44- and NKp46-CD3zeta lacZ-inducible reporter cells were activated by NDV-infected cells. NDV-infected tumor cells stimulated NK cells to produce increased amounts of the effector lymphokines gamma interferon and tumor necrosis factor alpha. Primary NK cells and the NK line NK-92 lysed NDV-infected tumor cells with enhanced efficiency, an effect that was eliminated by the treatment of target cells with the neuraminidase inhibitor Neu5Ac2en. These results suggest that direct activation of NK cells contributes to the antitumor effects of NDV.