Antibodies against West Nile Virus nonstructural protein NS1 prevent lethal infection through Fc gamma receptor-dependent and -independent mechanisms

Structural biology of flavivirus NS1 protein and its antibody complexes

The genus of flavivirus includes many mosquito-borne human pathogens, such as Zika (ZIKV) and the four serotypes of dengue (DENV1-4) viruses, that affect billions of people as evidenced by epidemics and endemicity in many countries and regions in the world. Among the 10 viral proteins encoded by the viral genome, the nonstructural protein 1 (NS1) is the only secreted protein and has been used as a diagnostic biomarker. NS1 has also been an attractive target for its biotherapeutic potential as a vaccine antigen. This review focuses on the recent advances in the structural landscape of the secreted NS1 (sNS1) and its complex with monoclonal antibodies (mAbs). NS1 forms an obligatory dimer, and upon secretion, it has been reported to be hexametric (trimeric dimers) that could dissociate and bind to the epithelial cell membrane. However, high-resolution structural information has been missing about the high-order oligomeric states of sNS1. Several cryoEM studies have since shown that DENV and ZIKV recombinant sNS1 (rsNS1) are in dynamic equilibrium of dimer-tetramer-hexamer states, with tetramer being the predominant form. It was recently revealed that infection-derived sNS1 (isNS1) forms a complex of the NS1 dimer partially embedded in a High-Density Lipoprotein (HDL) particle. Structures of NS1 in complexes with mAbs have also been reported which shed light on their protective roles during infection. The biological significance of the diversity of NS1 oligomeric states remains to be further studied, to inform future research on flaviviral pathogenesis and the development of therapeutics and vaccines. Given the polymorphism of flavivirus NS1 across sample types with variations in antigenicity, we propose a nomenclature to accurately define NS1 based on the localization and origin.

Therapeutic efficacy of a potent anti-Venezuelan equine encephalitis virus antibody is contingent on Fc effector function

The development of specific, safe, and potent monoclonal antibodies (Abs) has led to novel therapeutic options for infectious disease. In addition to preventing viral infection through neutralization, Abs can clear infected cells and induce immunomodulatory functions through engagement of their crystallizable fragment (Fc) with complement proteins and Fc receptors on immune cells. Little is known about the role of Fc effector functions of neutralizing Abs in the context of encephalitic alphavirus infection. To determine the role of Fc effector function in therapeutic efficacy against Venezuelan equine encephalitis virus (VEEV), we compared the potently neutralizing anti-VEEV human IgG F5 (hF5) Ab with intact Fc function (hF5-WT) or containing the loss of function Fc mutations L234A and L235A (hF5-LALA) in the context of VEEV infection. We observed significantly reduced binding to complement and Fc receptors, as well as differential in vitro kinetics of Fc-mediated cytotoxicity for hF5-LALA compared to hF5-WT. The in vivo efficacy of hF5-LALA was comparable to hF5-WT at -24 and + 24 h post infection, with both Abs providing high levels of protection. However, when hF5-WT and hF5-LALA were administered + 48 h post infection, there was a significant decrease in the therapeutic efficacy of hF5-LALA. Together these results demonstrate that optimal therapeutic Ab treatment of VEEV, and possibly other encephalitic alphaviruses, requires neutralization paired with engagement of immune effectors via the Fc region.

Protective mechanisms of nonneutralizing antiviral antibodies

Antibodies that can bind to viruses but are unable to block infection in cell culture are known as "nonneutralizing antibodies." Such antibodies are nearly universally elicited following viral infection and have been characterized in viral infections such as influenza, rotavirus, cytomegalovirus, HIV, and SARS-CoV-2. It has been widely assumed that these nonneutralizing antibodies do not function in a protective way in vivo and therefore are not desirable targets of antiviral interventions; however, increasing evidence now shows this not to be true. Several virus-specific nonneutralizing antibody responses have been correlated with protection in human studies and also shown to significantly reduce virus replication in animal models. The mechanisms by which many of these antibodies function is only now coming to light. While nonneutralizing antibodies cannot prevent viruses entering their host cell, nonneutralizing antibodies work in the extracellular space to recruit effector proteins or cells that can destroy the antibody-virus complex. Other nonneutralizing antibodies exert their effects inside cells, either by blocking the virus life cycle directly or by recruiting the intracellular Fc receptor TRIM21. In this review, we will discuss the multitude of ways in which nonneutralizing antibodies function against a range of viral infections.

Linear epitope identification of monoclonal antibodies against the duck Tembusu virus NS1

Since 2010, the duck Tembusu virus (DTMUV) has caused a severe outbreak of egg drop syndrome in laying ducks in China, which has resulted in substantial financial losses in the poultry industry. DTMUV nonstructural protein 1 (NS1), as the only secreted protein, could aid in the development of therapeutic antibodies and diagnostic techniques; however, there are few studies on the preparation and epitope identification of monoclonal antibodies (mAbs) against DTMUV NS1. In this study, by indirect enzyme-linked immunosorbent assay (ELISA), Western blotting, and indirect immunofluorescence assay, we screened 6 mAbs (8A4, 8E6, 10F12, 1H11, 3D5, 5C11) that could specifically recognize DTMUV NS1. For epitope mapping of mAbs, a series of GST-tagged truncated fusion proteins of DTMUV NS1 were constructed by prokaryotic expression. Finally, the 4 shortest linear epitopes were identified by indirect ELISA and Western blotting. The epitope 133FVIDGPK139 was recognized by 8A4, the epitope 243IPKTLGGP250 was recognized by 8E6, the epitope 267PWDEK271 was recognized by 10F12, and 156EDFGFGVL163 was recognized by 1H11, 3D5, and 5C11. By sequence alignment and cross-reaction tests, we found that 8A4 and 8E6 had high specificity for DTMUV NS1 compared with that of other mAbs, but 10F12, 1H11, 3D5, and 5C11 exhibited a clear degree of cross-reaction with dengue virus (DENV), Japanese encephalitis virus (JEV), West Nile virus (WNV), and Zika virus (ZIKV) NS1. Finally, the predicted crystal structure analysis showed the approximate spatial positions of the 4 epitopes on the NS1 dimer. In summary, our study revealed 2 specific mAbs for DTMUV NS1 recognition and 4 multiflavivirus mAbs for DENV, JEV, WNV, and ZIKV NS1 recognition.

Secreted NS1 proteins of tick-borne encephalitis virus and West Nile virus block dendritic cell activation and effector functions

The flavivirus non-structural protein 1 (NS1) is secreted from infected cells into the circulation and the serum levels correlate with disease severity. The effect of secreted NS1 (sNS1) on non-infected mammalian immune cells is largely unknown. Here, we expressed recombinant sNS1 proteins of tick-borne encephalitis virus (TBEV) and West Nile virus (WNV) and investigated their effects on dendritic cell (DC) effector functions. Murine bone marrow-derived DCs (BMDCs) showed reduced surface expression of co-stimulatory molecules and decreased release of pro-inflammatory cytokines when treated with sNS1 of TBEV or WNV prior to poly(I:C) stimulation. Transcriptional profiles of BMDCs that were sNS1-exposed prior to poly(I:C) stimulation showed two gene clusters that were downregulated by TBEV or WNV sNS1 and that were associated with innate and adaptive immune responses. Functionally, both sNS1 proteins modulated the capacity for BMDCs to induce specific T-cell responses as indicated by reduced IFN-γ levels in both CD4+ and CD8+ T cells after BMDC co-cultivation. In human monocyte-derived DCs, poly(I:C)-induced upregulation of co-stimulatory molecules and cytokine responses were even more strongly impaired by TBEV sNS1 or WNV sNS1 pretreatment than in the murine system. Our findings indicate that exogenous flaviviral sNS1 proteins interfere with DC-mediated stimulation of T cells, which is crucial for the initiation of cell-mediated adaptive immune responses in human flavivirus infections. Collectively, our data determine soluble flaviviral NS1 as a virulence factor responsible for a dampened immune response to flavivirus infections. IMPORTANCE The effective initiation of protective host immune responses controls the outcome of infection, and dysfunctional T-cell responses have previously been associated with symptomatic human flavivirus infections. We demonstrate that secreted flavivirus NS1 proteins modulate innate immune responses of uninfected bystander cells. In particular, sNS1 markedly reduced the capacity of dendritic cells to stimulate T-cell responses upon activation. Hence, by modulating cellular host responses that are required for effective antigen presentation and initiation of adaptive immunity, sNS1 proteins may contribute to severe outcomes of flavivirus disease.

Secretory pathways and multiple functions of nonstructural protein 1 in flavivirus infection

The genus Flavivirus contains a wide variety of viruses that cause severe disease in humans, including dengue virus, yellow fever virus, Zika virus, West Nile virus, Japanese encephalitis virus and tick-borne encephalitis virus. Nonstructural protein 1 (NS1) is a glycoprotein that encodes a 352-amino-acid polypeptide and has a molecular weight of 46-55 kDa depending on its glycosylation status. NS1 is highly conserved among multiple flaviviruses and occurs in distinct forms, including a dimeric form within the endoplasmic reticulum, a cell-associated form on the plasma membrane, or a secreted hexameric form (sNS1) trafficked to the extracellular matrix. Intracellular dimeric NS1 interacts with other NSs to participate in viral replication and virion maturation, while extracellular sNS1 plays a critical role in immune evasion, flavivirus pathogenesis and interactions with natural vectors. In this review, we provide an overview of recent research progress on flavivirus NS1, including research on the structural details, the secretory pathways in mammalian and mosquito cells and the multiple functions in viral replication, immune evasion, pathogenesis and interaction with natural hosts, drawing together the previous data to determine the properties of this protein.

Structural and immunological basis of cross-reactivity between dengue and Zika infections: Implications in serosurveillance in endemic regions

Dengue and Zika are arthropod-borne viral diseases present in more than 100 countries around the world. In the past decade, Zika emerged causing widespread outbreaks in new regions, where dengue has been endemic-epidemic for a long period. The wide and extensive dissemination of the mosquito vectors, Aedes aegypti, and Ae. albopictus, favor the co-existence of both infections in the same regions. Together with an important proportion of asymptomatic infections, similar clinical manifestations, and a short time window for acute infection confirmatory tests, it is difficult to differentially estimate both dengue and Zika incidence and prevalence. DENV and ZIKV flavivirus share high structural similarity, inducing a cross-reactive immune response that leads to false positives in serological tests particularly in secondary infections. This results in overestimation of recent Zika outbreaks seroprevalence in dengue endemic regions. In this review, we address the biological basis underlying DENV and ZIKV structural homology; the structural and cellular basis of immunological cross reactivity; and the resulting difficulties in measuring dengue and Zika seroprevalence. Finally, we offer a perspective about the need for more research to improve serological tests performance.

Potential Protective Effect of Dengue NS1 Human Monoclonal Antibodies against Dengue and Zika Virus Infections

Due to the lack of an effective therapeutic treatment to flavivirus, dengue virus (DENV) nonstructural protein 1 (NS1) has been considered to develop a vaccine owing to its lack of a role in antibody-dependent enhancement (ADE). However, both NS1 and its antibody have shown cross-reactivity to host molecules and have stimulated anti-DENV NS1 antibody-mediated endothelial damage and platelet dysfunction. To overcome the pathogenic events and reactogenicity, human monoclonal antibodies (HuMAbs) against DENV NS1 were generated from DENV-infected patients. Herein, the four DENV NS1-specific HuMAbs revealed the therapeutic effects in viral neutralization, reduction of viral replication, and enhancement of cell cytolysis of DENV and zika virus (ZIKV) via complement pathway. Furthermore, we demonstrate that DENV and ZIKV NS1 trigger endothelial dysfunction, leading to vascular permeability in vitro. Nevertheless, the pathogenic effects from NS1 were impeded by 2 HuMAbs (D25-4D4C3 and D25-2B11E7) and also protected the massive cytokines stimulation (interleukin [IL-]-1b, IL-1ra, IL-2, IL-4, IL-5, IL-6, IL-8, IL-9, IL-13, IL-17, eotaxin, granulocyte colony-stimulating factor, granulocyte-macrophage colony-stimulating factor, Inducible protein-10, monocyte chemoattractant protein-1, macrophage inflammatory protein [MIP]-1 α, MIP-1β, tumor necrosis factor-α, platelet-derived growth factor, and RANTES). Collectively, our findings suggest that the novel protective NS1 monoclonal antibodies generated from humans has multiple therapeutic benefits against DENV and ZIKV infections.

Predicting Antigenic Peptides from Rocio Virus NS1 Protein for Immunodiagnostic Testing Using Immunoinformatics and Molecular Dynamics Simulation

The mosquito-borne disease caused by the Rocio virus is a neglected threat, and new immune inputs for serological testing are urgently required for diagnosis in low-resource settings and epidemiological surveillance. We used in silico approaches to identify a specific antigenic peptide (p_ROCV2) in the NS1 protein of the Rocio virus that was theoretically predicted to be stable and exposed on its surface, where it demonstrated key properties allowing it to interact with antibodies. These findings related to the molecular dynamics of this peptide provide important insights for advancing diagnostic platforms and investigating therapeutic alternatives.

Cross-reactive antibodies targeting surface-exposed non-structural protein 1 (NS1) of dengue virus-infected cells recognize epitopes on the spaghetti loop of the β-ladder domain

Non-structural protein 1 (NS1) is a glycoprotein component of dengue virus (DENV) that is essential for viral replication, infection and immune evasion. Immunization with NS1 has been shown to elicit antibody-mediated immune responses which protect mice against DENV infections. Here, we obtained peripheral blood mononuclear cells from human subjects with secondary dengue infections, which were used to construct a dengue immune phage library displaying single-chain variable fragments. Phage selective for DENV NS1 were obtained by biopanning. Twenty-one monoclonal antibodies (mAbs) against DENV NS1 were generated from the selected phage and characterized in detail. We found most anti-NS1 mAbs used IGHV1 heavy chain antibody genes. The mAbs were classified into strongly and weakly-reactive groups based on their binding to NS1 expressed in dengue virus 2 (DENV2)-infected cells. Antibody binding experiments with recombinant NS1 proteins revealed that the mAbs recognize conformational epitopes on the β-ladder domain (amino acid residues 178–273) of DENV NS1. Epitope mapping studies on alanine-substituted NS1 proteins identified distinct but overlapping epitopes. Protruding amino acids distributed around the spaghetti loop are required for the binding of the strongly-reactive mAbs, whereas the recognition residues of the weakly-reactive mAbs are likely to be located in inaccessible sites facing toward the cell membrane. This information could guide the design of an NS1 epitope-based vaccine that targets cross-reactive conserved epitopes on cell surface-associated DENV NS1.

Targeting tight junctions to fight against viral neuroinvasion

The clinical impact of viral neuroinvasion on the central nervous system (CNS) ranges from barely detectable to deadly, including acute and chronic outcomes. Developing innovative therapeutic strategies is important to mitigate virus-induced neurological and psychiatric disorders. A key gatekeeper to the CNS is the neurovascular unit (NVU), a major obstacle to viral neuroinvasion and antiviral therapies. The NVU isolates the brain from the blood through firm sealing operated by the tight junctions (TJs) of endothelial cells. Here, we make the thought-provoking assumption that TJs can be targets to prevent or treat viral neuroinvasion and resulting disorders. This review aims at defining the conceptual diverse mode of actions of such approaches, evaluates their feasibility, and discusses future challenges in the field.

Human Monoclonal Antibodies against NS1 Protein Protect against Lethal West Nile Virus Infection

Envelope protein-targeted vaccines for flaviviruses are limited by concerns of antibody-dependent enhancement (ADE) of infections. Nonstructural protein 1 (NS1) provides an alternative vaccine target that avoids this risk since this protein is absent from the virion. Beyond its intracellular role in virus replication, extracellular forms of NS1 function in immune modulation and are recognized by host-derived antibodies. The rational design of NS1-based vaccines requires an extensive understanding of the antigenic sites on NS1, especially those targeted by protective antibodies. Here, we isolated human monoclonal antibodies (MAbs) from individuals previously naturally infected with WNV, mapped their epitopes using structure-guided mutagenesis, and evaluated their efficacy in vivo against lethal WNV challenge. The most protective epitopes clustered at three antigenic sites that are exposed on cell surface forms of NS1: (i) the wing flexible loop, (ii) the outer, electrostatic surface of the wing, and (iii) the spaghetti loop face of the β-ladder. One additional MAb mapped to the distal tip of the β-ladder and conferred a lower level of protection against WNV despite not binding to NS1 on the surface of infected cells. Our study defines the epitopes and modes of binding of protective anti-NS1 MAb antibodies following WNV infection, which may inform the development of NS1-based countermeasures against flaviviruses. IMPORTANCE Therapeutic antibodies against flaviviruses often promote neutralization by targeting the envelope protein of the virion. However, this approach is hindered by a possible concern for antibody-dependent enhancement of infection and paradoxical worsening of disease. As an alternative strategy, antibodies targeting flavivirus nonstructural protein 1 (NS1), which is absent from the virion, can protect against disease and do not cause enhanced infection. Here, we evaluate the structure-function relationships and protective activity of West Nile virus (WNV) NS1-specific monoclonal antibodies (MAbs) isolated from the memory B cells of a naturally infected human donor. We identify several anti-NS1 MAbs that protect mice against lethal WNV challenge and map their epitopes using charge reversal mutagenesis. Antibodies targeting specific regions in the NS1 structure could serve as the basis for countermeasures that control WNV infection in humans.

A quasispecies in a BHK-21 cell-derived virulent Tembusu virus strain contains three groups of variants with distinct virulence phenotypes

Previous studies resulted in the isolation of a low-virulence plaque-purified variant from the third passage (P3) in BHK-21 cells of a Tembusu virus (TMUV) isolate, suggesting the presence of viral quasispecies in the P3 culture. To confirm this notion, the fourth passage virus (P4) was prepared by infecting BHK-21 cells with P3 for isolation of more variants. We isolated 10 plaque-purified viruses. Comparative genome sequence analysis identified six of the 10 viruses as genetically different variants, which harbored a total of eight amino acid differences in the envelope, NS1, NS3, and NS5 proteins. When tested in a 2-day-old Pekin duck model, P4 caused 80 % mortality, belonging to a high-virulence TMUV strain. Out of the six genetically different variants, two presented high-virulence, one exhibited moderate-virulence, and three displayed low-virulence, causing 60 %-70 %, 40 %, and 10 % mortalities, respectively. These results demonstrate that P4 contains at least three groups of variants with distinct virulence phenotypes. Analysis of links between the eight residues and virulence of the six variants identified NS1 protein residue 183 and NS5 protein residues 275 and/or 287 as novel determinants of TMUV virulence. The analysis also provided a new clue for future studies on the molecular basis of TMUV virulence in terms of genetic interaction of different proteins. Overall, our study provides direct evidence to suggest that TMUV exists in in vitro culture of a virulent isolate as a quasispecies, which may enhance our understanding of molecular mechanism of TMUV virulence.

Roles of Non-Structural Protein 4A in Flavivirus Infection

Infections with viruses in the genus Flavivirus are a worldwide public health problem. These enveloped, positive sense single stranded RNA viruses use a small complement of only 10 encoded proteins and the RNA genome itself to remodel host cells to achieve conditions favoring viral replication. A consequence of the limited viral armamentarium is that each protein exerts multiple cellular effects, in addition to any direct role in viral replication. The viruses encode four non-structural (NS) small transmembrane proteins (NS2A, NS2B, NS4A and NS4B) which collectively remain rather poorly characterized. NS4A is a 16kDa membrane associated protein and recent studies have shown that this protein plays multiple roles, including in membrane remodeling, antagonism of the host cell interferon response, and in the induction of autophagy, in addition to playing a role in viral replication. Perhaps most importantly, NS4A has been implicated as playing a critical role in fetal developmental defects seen as a consequence of Zika virus infection during pregnancy. This review provides a comprehensive overview of the multiple roles of this small but pivotal protein in mediating the pathobiology of flaviviral infections.

Levels of Circulating NS1 Impact West Nile Virus Spread to the Brain

Dengue virus (DENV) and West Nile virus (WNV) are arthropod-transmitted flaviviruses that cause systemic vascular leakage and encephalitis syndromes, respectively, in humans. However, the viral factors contributing to these specific clinical disorders are not completely understood. Flavivirus nonstructural protein 1 (NS1) is required for replication, expressed on the cell surface, and secreted as a soluble glycoprotein, reaching high levels in the blood of infected individuals. Extracellular DENV NS1 and WNV NS1 interact with host proteins and cells, have immune evasion functions, and promote endothelial dysfunction in a tissue-specific manner. To characterize how differences in DENV NS1 and WNV NS1 might function in pathogenesis, we generated WNV NS1 variants with substitutions corresponding to residues found in DENV NS1. We discovered that the substitution NS1-P101K led to reduced WNV infectivity in the brain and attenuated lethality in infected mice, although the virus replicated efficiently in cell culture and peripheral organs and bound at wild-type levels to brain endothelial cells and complement components. The P101K substitution resulted in reduced NS1 antigenemia in mice, and this was associated with reduced WNV spread to the brain. Because exogenous administration of NS1 protein rescued WNV brain infectivity in mice, we conclude that circulating WNV NS1 facilitates viral dissemination into the central nervous system and impacts disease outcomes. IMPORTANCE Flavivirus NS1 serves as an essential scaffolding molecule during virus replication but also is expressed on the cell surface and is secreted as a soluble glycoprotein that circulates in the blood of infected individuals. Although extracellular forms of NS1 are implicated in immune modulation and in promoting endothelial dysfunction at blood-tissue barriers, it has been challenging to study specific effects of NS1 on pathogenesis without disrupting its key role in virus replication. Here, we assessed WNV NS1 variants that do not affect virus replication and evaluated their effects on pathogenesis in mice. Our characterization of WNV NS1-P101K suggests that the levels of NS1 in the circulation facilitate WNV dissemination to the brain and affect disease outcomes. Our findings facilitate understanding of the role of NS1 during flavivirus infection and support antiviral strategies for targeting circulating forms of NS1.

The central role of Italy in the spatial spread of USUTU virus in Europe

USUTU virus (USUV) is an arbovirus maintained in the environment through a bird-mosquito enzootic cycle. Previous surveillance plans highlighted the endemicity of USUV in North-eastern Italy. In this work, we sequenced 138 new USUV full genomes from mosquito pools (Culex pipiens) and wild birds collected in North-eastern Italy and we investigated the evolutionary processes (phylogenetic analysis, selection pressure and evolutionary time-scale analysis) and spatial spread of USUV strains circulating in the European context and in Italy, with a particular focus on North-eastern Italy. Our results confirmed the circulation of viruses belonging to four different lineages in Italy (EU1, EU2, EU3 and EU4), with the newly sequenced viruses from the North-eastern regions, Veneto and Friuli Venezia Giulia, belonging to the EU2 lineage and clustering into two different sub-lineages, EU2-A and EU2-B. Specific mutations characterize each European lineage and geographic location seem to have shaped their phylogenetic structure. By investigating the spatial spread in Europe, we were able to show that Italy acted mainly as donor of USUV to neighbouring countries. At a national level, we identified two geographical clusters mainly circulating in Northern and North-western Italy, spreading both northward and southward. Our analyses provide important information on the spatial and evolutionary dynamics of USUTU virus that can help to improve surveillance plans and control strategies for this virus of increasing concern for human health.

The Input of Structural Vaccinology in the Search for Vaccines against Bunyaviruses

A significant increase in the number of viruses causing unexpected illnesses and epidemics among humans, wildlife and livestock has been observed in recent years. These new or re-emerging viruses have often caught the scientific community off-guard, without sufficient knowledge to combat them, as shown by the current coronavirus pandemic. The bunyaviruses, together with the flaviviruses and filoviruses, are the major etiological agents of viral hemorrhagic fever, and several of them have been listed as priority pathogens by the World Health Organization for which insufficient countermeasures exist. Based on new techniques allowing rapid analysis of the repertoire of protective antibodies induced during infection, combined with atomic-level structural information on viral surface proteins, structural vaccinology is now instrumental in the combat against newly emerging threats, as it allows rapid rational design of novel vaccine antigens. Here, we discuss the contribution of structural vaccinology and the current challenges that remain in the search for an efficient vaccine against some of the deadliest bunyaviruses.

Dengue virus: epidemiology, biology, and disease aetiology

Dengue is a vector-borne viral disease caused by the flavivirus dengue virus (DENV). Approximately 400 million cases and 22 000 deaths occur due to dengue worldwide each year. It has been reported in more than 100 countries in tropical and subtropical regions. A positive-stranded enveloped RNA virus (DENV) is principally transmitted by Aedes mosquitoes. It has four antigenically distinct serotypes, DENV-1 to DENV-4, with different genotypes and three structural proteins and seven non-structural proteins. Clinical symptoms of dengue range from mild fever to severe dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS), with thrombocytopenia, leucopenia, and increased vascular permeability. Although primary infection causes activation of immune responses against DENV serotypes, the severity of the disease is enhanced via heterotypic infection by various serotypes as well as antibody-dependent enhancement (ADE). The first licensed DENV vaccine was tetravalent CYD Denvaxia, but it has not been approved in all countries. The lack of a suitable animal model, a proper mechanistic study in pathogenesis, and ADE are the main hindrances in vaccine development. This review summarizes the current knowledge on DENV epidemiology, biology, and disease aetiology in the context of prevention and protection from dengue virus disease.

Flavivirus NS1 and Its Potential in Vaccine Development

The Flavivirus genus contains many important human pathogens, including dengue, Japanese encephalitis (JE), tick-borne encephalitis (TBE), West Nile (WN), yellow fever (YF) and Zika (ZIK) viruses. While there are effective vaccines for a few flavivirus diseases (JE, TBE and YF), the majority do not have vaccines, including WN and ZIK. The flavivirus nonstructural 1 (NS1) protein has an unusual structure-function because it is glycosylated and forms different structures to facilitate different roles intracellularly and extracellularly, including roles in the replication complex, assisting in virus assembly, and complement antagonism. It also plays a role in protective immunity through antibody-mediated cellular cytotoxicity, and anti-NS1 antibodies elicit passive protection in animal models against a virus challenge. Historically, NS1 has been used as a diagnostic marker for the flavivirus infection due to its complement fixing properties and specificity. Its role in disease pathogenesis, and the strong humoral immune response resulting from infection, makes NS1 an excellent target for inclusion in candidate flavivirus vaccines.

Requirement of Fc-Fc Gamma Receptor Interaction for Antibody-Based Protection against Emerging Virus Infections

Identification of therapeutics against emerging and re-emerging viruses remains a continued priority that is only reinforced by the recent SARS-CoV-2 pandemic. Advances in monoclonal antibody (mAb) isolation, characterization, and production make it a viable option for rapid treatment development. While mAbs are traditionally screened and selected based on potency of neutralization in vitro, it is clear that additional factors contribute to the in vivo efficacy of a mAb beyond viral neutralization. These factors include interactions with Fc receptors (FcRs) and complement that can enhance neutralization, clearance of infected cells, opsonization of virions, and modulation of the innate and adaptive immune response. In this review, we discuss recent studies, primarily using mouse models, that identified a role for Fc-FcγR interactions for optimal antibody-based protection against emerging and re-emerging virus infections.

Temperature-dependent secretion of Zika virus envelope and non-structural protein 1 in mammalian cells for clinical applications

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus associated with congenital Zika syndrome and Guillain-Barré syndrome in adults. The recombinant ZIKV envelope (E) antigen can be useful for serodiagnosis of ZIKV infection and for monitoring immune responses during preclinical and clinical ZIKV vaccine development. In this study, we describe production of ZIKV E using the modified polyethyleneimine (PEI) transfection in HEK293 cells to improve cost-effective large-scale production. We show that the secretion of ZIKV E in HEK293 cells is dependent on cell culture incubation temperatures where incubation at a low temperature of 28 °C improved protein secretion of both, E-CD4 and E, whereas a substantial decrease in secretion was observed at 37 °C. The resulting E-CD4 produced at low temperature yielded similar binding profiles in ELISAs in comparison with a commercially available E protein using human seropositive sera to ZIKV. We also show that ZIKV NS1 and NS1 β-ladder antigens produced in HEK293 cells, have similar binding profiles in ELISA which suggests that both NS1 or NS1 β-ladder can be used for serodiagnosis of ZIKV. In conclusion, we propose a cost-effective production of the ZIKV E and NS1, suitable for both, clinical and research applications in endemic countries.

Novel Japanese encephalitis virus NS1-based vaccine: Truncated NS1 fused with E. coli heat labile enterotoxin B subunit

BACKGROUND

Current vaccines against Japanese encephalitis virus (JEV) of flaviviruses have some disadvantages, such as the risk of virulent reversion. Non-structural protein NS1 is conserved among flaviviruses and confers immune protection without the risk of antibody-dependent enhancement (ADE). Therefore, NS1 has become a promising vaccine candidate against flaviviruses.

METHODS

A NS1-based vaccine (LTB-NS1_∆63) with a truncated NS1 protein (NS1_∆63) fused to E. coli heat-labile enterotoxin B subunit (LTB) was expressed in E.coli and explored for its ability to induce immune responses. Safety of LTB-NS1_∆63 was assessed by determining its toxicity in vitro and in vivo. Protective capability of LTB-NS1_∆63 and its-induced antisera was evaluated in the mice challenged with JEV by analyzing mortality and morbidity.

FINDINGS

LTB-NS1_∆63 induced immune responses to a similar level as LTB-NS1, but more robust than NS1_∆63 alone, particularly in the context of oral immunization of mice. Oral vaccination of LTB-NS1_∆63 led to a higher survival rate than that of NS1_∆63 or live-attenuated JEV vaccine SA14-14-2 in the mice receiving lethal JEV challenge. LTB-NS1_∆63 protein also significantly decreases the morbidity of JEV-infected mice. In addition, passive transfer of LTB-NS1_∆63-induced antisera provides a protection against JEV infection in mice.

INTERPRETATION

NS1_∆63 bears JEV NS1 antigenicity. Besides, LTB-NS1_∆63 could serve as a novel protein-based mucosa vaccine targeting JEV and other flaviviruses.

FUNDING

This work was supported by the National Natural Science Foundation, Jiangxi Province Science and Technology Committee, Education Department of Jiangxi Province.

How NS1 Antibodies Prevent Severe Flavivirus Disease

The flavivirus genus consists of several major human pathogens including dengue (DENV) and Zika viruses. The flavivirus nonstructural protein 1 (NS1) plays an important role in disease progression, for example, in the development of severe dengue disease. Anti-NS1 antibodies have been shown to confer protection, and two new studies by Biering et al. and Modhiran et al. on the structure of NS1:antibody complexes reveal their mechanism of neutralization.

Structural and biochemical insights into flavivirus proteins

Flaviviruses are the fastest spreading arthropod-borne viruses that cause severe symptoms such as hepatitis, hemorrhagic fever, encephalitis, and congenital deformities. Nearly 40 % of the entire human population is at risk of flavivirus epidemics. Yet, effective vaccination is restricted only to a few flaviviruses such as yellow fever and Japanese encephalitis viruses, and most recently for select cases of dengue virus infections. Despite the global spread of dengue virus, and emergence of new threats such as Zika virus and a new genotype of Japanese encephalitis virus, insights into flavivirus targets for potentially broad-spectrum vaccination are limited. In this review article, we highlight biochemical and structural differences in flavivirus proteins critical for virus assembly and host interactions. A comparative sequence analysis of pH-responsive properties of viral structural proteins identifies trends in conservation of complementary acidic-basic character between interacting viral structural proteins. This is highly relevant to the understanding of pH-sensitive differences in virus assembly in organelles such as neutral ER and acidic Golgi. Surface residues in viral interfaces identified by structural approaches are shown to demonstrate partial conservation, further reinforcing virus-specificity in assembly and interactions with host proteins. A comparative analysis of epitope conservation in emerging flaviviruses identifies therapeutic antibody candidates that have potential as broad spectrum anti-virals, thus providing a path towards development of vaccines.

A broadly protective antibody that targets the flavivirus NS1 protein

There are no approved flaviviral therapies and the development of vaccines against flaviruses has the potential of being undermined by antibody-dependent enhancement (ADE). The flavivirus nonstructural protein 1 (NS1) is a promising vaccine antigen with low ADE risk but has yet to be explored as a broad-spectrum therapeutic antibody target. Here, we provide the structural basis of NS1 antibody cross-reactivity through cocrystallization of the antibody 1G5.3 with NS1 proteins from dengue and Zika viruses. The 1G5.3 antibody blocks multi-flavivirus NS1-mediated cell permeability in disease-relevant cell lines, and therapeutic application of 1G5.3 reduces viremia and improves survival in dengue, Zika, and West Nile virus murine models. Finally, we demonstrate that 1G5.3 protection is independent of effector function, identifying the 1G5.3 epitope as a key site for broad-spectrum antiviral development.

Monoclonal Antibodies against Zika Virus NS1 Protein Confer Protection via Fc Receptor-Dependent and -Independent Pathways

Zika virus (ZIKV) is a mosquito-borne flavivirus that has been linked to congenital microcephaly during recent epidemics. No licensed antiviral drug or vaccine is available.

Zika virus (ZIKV) infection during pregnancy causes congenital defects such as fetal microcephaly. Monoclonal antibodies (MAbs) against the nonstructural protein 1 (NS1) have the potential to suppress ZIKV pathogenicity without enhancement of disease, but the pathways through which they confer protection remain obscure. Here, we report two types of NS1-targeted human MAbs that inhibit ZIKV infection through distinct mechanisms. MAbs 3G2 and 4B8 show a better efficacy than MAb 4F10 in suppressing ZIKV infection in C57BL/6 neonatal mice. Unlike MAb 4F10 that mainly triggers antibody-dependent cell-mediated cytotoxicity (ADCC), MAbs 3G2 and 4B8 not only trigger ADCC but inhibit ZIKV infection without Fcγ receptor-bearing effector cells, possibly at postentry stages. Destroying the Fc-mediated effector function of MAbs 3G2 and 4B8 reduces but does not abolish their protective effects, whereas destroying the effector function of MAb 4F10 eliminates the protective effects, suggesting that MAbs 3G2 and 4B8 engage both Fcγ receptor-dependent and -independent pathways. Further analysis reveals that MAbs 3G2 and 4B8 target the N-terminal region of NS1 protein, whereas MAb 4F10 targets the C-terminal region, implying that the protective efficacy of an NS1-targeted MAb may be associated with its epitope recognition. Our results illustrate that NS1-targeted MAbs have multifaceted protective effects and provide insights for the development of NS1-based vaccines and therapeutics.

Antibodies targeting epitopes on the cell-surface form of NS1 protect against Zika virus infection during pregnancy

There are no licensed therapeutics or vaccines available against Zika virus (ZIKV) to counteract its potential for congenital disease. Antibody-based countermeasures targeting the ZIKV envelope protein have been hampered by concerns for cross-reactive responses that induce antibody-dependent enhancement (ADE) of heterologous flavivirus infection. Nonstructural protein 1 (NS1) is a membrane-associated and secreted glycoprotein that functions in flavivirus replication and immune evasion but is absent from the virion. Although some studies suggest that antibodies against ZIKV NS1 are protective, their activity during congenital infection is unknown. Here we develop mouse and human anti-NS1 monoclonal antibodies that protect against ZIKV in both non-pregnant and pregnant mice. Avidity of antibody binding to cell-surface NS1 along with Fc effector functions engagement correlate with protection in vivo. Protective mAbs map to exposed epitopes in the wing domain and loop face of the β-platform. Anti-NS1 antibodies provide an alternative strategy for protection against congenital ZIKV infection without causing ADE.

Identification of a protective epitope in Japanese encephalitis virus NS1 protein

Japanese encephalitis virus (JEV) is one of the most important culex transmitted-flaviviruses, which can cause encephalitis in humans. Although non-structural protein 1 (NS1) of JEV does not stimulate neutralizing antibodies, this protein can provide high immunoprotection in vivo. The protective epitopes and the protective mechanism of NS1 still remain unclear. In this study, we generated five different monoclonal antibodies (mAbs) targeting the NS1 protein of JEV. In vitro experiments revealed that none of these five antibodies neutralized the JEV infection. In mouse protection studies, one of these mAbs, designated 2B8, provided a therapeutic effect against JEV lethal challenge (70% survival rate). Using peptide mapping analysis, we found that mAb 2B8 reacted with the epitope ²²⁵PETHTLWGD²³³ in the NS1 protein, in which any mutations among amino acid residues T²²⁸, H²²⁹, L²³¹ or W²³² could cause binding failure of 2B8 to the NS1 protein. Furthermore, mice immunized with KLH-polypeptide (²²⁵PETHTLWGD²³³) showed reduced mortality following JEV challenge. Collectively, we found a new protective epitope in the JEV NS1 protein. These results may facilitate the development of therapeutic agent and subunit-based vaccines based on the NS1 protein.

Vesicular Stomatitis Virus and DNA Vaccines Expressing Zika Virus Nonstructural Protein 1 Induce Substantial but Not Sterilizing Protection against Zika Virus Infection

The nonstructural protein 1 (NS1) of several flaviviruses, including West Nile, dengue, and yellow fever viruses, is capable of inducing variable degrees of protection against flavivirus infection in animal models. However, the immunogenicity of NS1 protein of Zika virus (ZIKV) is less understood. Here, we determined the efficacy of ZIKV NS1-based vaccine candidates using two delivery platforms, methyltransferase-defective recombinant vesicular stomatitis virus (mtdVSV) and a DNA vaccine. We first show that expression of ZIKV NS1 could be significantly enhanced by optimizing the signal peptide. A single dose of mtdVSV-NS1-based vaccine or two doses of DNA vaccine induced high levels of NS1-specfic antibody and T cell immune responses but provided only partial protection against ZIKV viremia in BALB/c mice. In Ifnar1^-/- mice, neither NS1-based vaccine provided protection against a lethal high dose (10⁵ PFU) ZIKV challenge, but mtdVSV-NS1-based vaccine prevented deaths from a low dose (10³ PFU) challenge, though they experienced viremia and body weight loss. We conclude that ZIKV NS1 alone conferred substantial, but not complete, protection against ZIKV infection. Nevertheless, these results highlight the value of ZIKV NS1 for vaccine development.IMPORTANCE Most Zika virus (ZIKV) vaccine research has focused on the E or prM-E proteins and the induction of high levels of neutralizing antibodies. However, these ZIKV neutralizing antibodies cross-react with other flaviviruses, which may aggravate the disease via an antibody-dependent enhancement (ADE) mechanism. ZIKV NS1 protein may be an alternative antigen for vaccine development, since antibodies to NS1 do not bind to the virion, thereby eliminating the risk of ADE. Here, we show that recombinant VSV and DNA vaccines expressing NS1, alone, confer partial protection against ZIKV infection in both immunocompetent and immunodeficient mice, highlighting the value of NS1 as a potential vaccine candidate.

Tick-Borne Encephalitis Virus: A Quest for Better Vaccines against a Virus on the Rise

Tick-borne encephalitis virus (TBEV), a member of the family Flaviviridae, is one of the most important tick-transmitted viruses in Europe and Asia. Being a neurotropic virus, TBEV causes infection of the central nervous system, leading to various (permanent) neurological disorders summarized as tick-borne encephalitis (TBE). The incidence of TBE cases has increased due to the expansion of TBEV and its vectors. Since antiviral treatment is lacking, vaccination against TBEV is the most important protective measure. However, vaccination coverage is relatively low and immunogenicity of the currently available vaccines is limited, which may account for the vaccine failures that are observed. Understanding the TBEV-specific correlates of protection is of pivotal importance for developing novel and improved TBEV vaccines. For affording robust protection against infection and development of TBE, vaccines should induce both humoral and cellular immunity. In this review, the adaptive immunity induced upon TBEV infection and vaccination as well as novel approaches to produce improved TBEV vaccines are discussed.

A replication-defective Japanese encephalitis virus (JEV) vaccine candidate with NS1 deletion confers dual protection against JEV and West Nile virus in mice

In our previous study, we have demonstrated in the context of WNV-ΔNS1 vaccine (a replication-defective West Nile virus (WNV) lacking NS1) that the NS1 trans-complementation system may offer a promising platform for the development of safe and efficient flavivirus vaccines only requiring one dose. Here, we produced high titer (10⁷ IU/ml) replication-defective Japanese encephalitis virus (JEV) with NS1 deletion (JEV-ΔNS1) in the BHK-21 cell line stably expressing NS1 (BHK_NS1) using the same strategy. JEV-ΔNS1 appeared safe with a remarkable genetic stability and high degrees of attenuation of in vivo neuroinvasiveness and neurovirulence. Meanwhile, it was demonstrated to be highly immunogenic in mice after a single dose, providing similar degrees of protection to SA14-14-2 vaccine (a most widely used live attenuated JEV vaccine), with healthy condition, undetectable viremia and gradually rising body weight. Importantly, we also found JEV-ΔNS1 induced robust cross-protective immune responses against the challenge of heterologous West Nile virus (WNV), another important member in the same JEV serocomplex, accounting for up to 80% survival rate following a single dose of immunization relative to mock-vaccinated mice. These results not only support the identification of the NS1-deleted flavivirus vaccines with a satisfied balance between safety and efficacy, but also demonstrate the potential of the JEV-ΔNS1 as an alternative vaccine candidate against both JEV and WNV challenge.

Modified mRNA-LNP Vaccines Confer Protection against Experimental DENV-2 Infection in Mice

Dengue virus (DENV) infection is a major global public health concern, and there is no effective vaccine for it. In this study, we describe the design and characterization of three nucleotide-modified mRNA vaccines (prME-mRNA, E80-mRNA, and NS1-mRNA) for DENV-2. Our results showed that vaccination with E80-mRNA alone or a combination of E80-mRNA and NS1-mRNA can induce high levels of neutralizing antibodies and antigen-specific T cell responses; furthermore, these vaccines confer complete protection against DENV-2 challenge in immunocompetent mice. These data provide foundations for further development of a tetravalent DENV vaccine based on nucleotide-modified mRNA.

TRiC/CCT Complex, a Binding Partner of NS1 Protein, Supports the Replication of Zika Virus in Both Mammalians and Mosquitoes

Mosquito-borne Zika virus (ZIKV) can cause congenital microcephaly and Guillain–Barré syndrome, among other symptoms. Specific treatments and vaccines for ZIKV are not currently available. To further understand the host factors that support ZIKV replication, we used mass spectrometry to characterize mammalian proteins that associate with the ZIKV NS1 protein and identified the TRiC/CCT complex as an interacting partner. Furthermore, the suppression of CCT2, one of the critical components of the TRiC/CCT complex, inhibited ZIKV replication in both mammalian cells and mosquitoes. These results highlight an important role for the TRiC/CCT complex in ZIKV infection, suggesting that the TRiC/CCT complex may be a promising therapeutic target.

Optimized Expression of Duck Tembusu Virus E Gene Delivered by a Vectored Duck Enteritis Virus In Vitro

In our previous study, a recombinant duck enteritis virus (DEV) delivering codon-optimized E gene (named as E-ch) of duck Tembusu virus (DTMUV) optimized referring to chicken's codon bias has been obtained based on the infectious bacterial artificial chromosome (BAC) clone of duck enteritis virus vaccine strain pDEV-EF1, but the expression level of E-ch in recombinant virus rDEV-E-ch-infected cells was very low. To optimize DTMUV E gene expression delivered by the vectored DEV, different forms of E gene (collectively called EG) including origin E gene (E-ori), truncated E451-ori gene, codon-optimized E-dk gene optimized referring to duck's codon bias, as well as the truncated E451-ch and E451-dk, Etpa-ori and Etpa-451-ori, which contain prefixing chick TPA signal peptide genes, were cloned into transfer vector pEP-BGH-end, and several recombinant plasmids pEP-BGH-EG were constructed. Then the expression cassettes pCMV-EG-polyA_BGH amplified from pEP-BGH-EG by PCR were inserted into US7/US8 gene intergenic region of pDEV-EF1 by two-step Red/ET recombination, 7 strain recombinant mutated BAC clones pDEV-EG carrying different E genes were constructed. Next, the recombinant viruses rDEV-EG were reconstituted from chicken embryo fibroblasts (CEFs) by calcium phosphate precipitation. Western blot analysis showed that E or E451 protein is expressed in rDEV-E-ori, rDEV-E-ch, rDEV-Etpa-ori, rDEV-E451-ori, rDEV-E451-dk, and rDEV-E451-ch-infected CEFs, and protein expression level in rDEV-E451-dk-infected CEFs is the highest. These studies have laid a foundation for developing bivalent vaccine controlling DEV and DTMUV infection.

Novel short peptide tag from a bacterial toxin for versatile applications

The specific recognition between a monoclonal antibody (mAb) and its epitope can be used in a tag system that has proved valuable in a wide range of biological applications. Herein, we describe a novel tag called RA-tag that is composed of a seven amino acid sequence (DIDLSRI) and recognized by a highly specific mAb, 47RA, against the bacterial toxin Vibrio vulnificus RtxA1/MARTX_Vv. By using recombinant proteins with the RA-tag at the N-terminal, C-terminal, or an internal site, we demonstrated that the tag system could be an excellent biological system for both protein purification and protein detection in enzyme-linked immunosorbent, Western blot, flow cytometry, and immunofluorescence staining analyses in Escherichia coli, mammalian cell lines, yeast, and plant. In addition, our RA-tag/47RA mAb combination showed high sensitivity and reliable affinity (K_D = 5.90 × 10^-8 M) when compared with conventional tags. Overall, our results suggest that the RA-tag system could facilitate the development of a broadly applicable tag system for biological research.

NS1 DNA vaccination protects against Zika infection through T cell-mediated immunity in immunocompetent mice

The causal association of Zika virus (ZIKV) with microcephaly, congenital malformations in infants, and Guillain-Barré syndrome in adults highlights the need for effective vaccines. Thus far, efforts to develop ZIKV vaccines have focused on the viral envelope. ZIKV NS1 as a vaccine immunogen has not been fully explored, although it can circumvent the risk of antibody-dependent enhancement of ZIKV infection, associated with envelope antibodies. Here, we describe a novel DNA vaccine encoding a secreted ZIKV NS1, that confers rapid protection from systemic ZIKV infection in immunocompetent mice. We identify novel NS1 T cell epitopes in vivo and show that functional NS1-specific T cell responses are critical for protection against ZIKV infection. We demonstrate that vaccine-induced anti-NS1 antibodies fail to confer protection in the absence of a functional T cell response. This highlights the importance of using NS1 as a target for T cell-based ZIKV vaccines.

Development of horse neutralizing immunoglobulin and immunoglobulin fragments against Junín virus

Argentine haemorrhagic fever (AHF) is a rodent-borne disease with a lethality as high as ~30%, which is caused by the New World arenavirus, Junín virus (JUNV). It was once a major epidemic in South America and puts millions of people in Argentina at risk. Here, we aimed to develop horse antibodies or antibody fragments against JUNV. Before preparing the horse antibodies, a strategy to efficiently generate horse antisera was established based on comparisons among immunogens and immunization methods in both mice and horses. Antisera against JUNV were finally obtained by vaccinating horses with vesicular stomatitis virus pseudotypes bearing JUNV GP. The horse antibodies IgG and F(ab’)₂ were subsequently demonstrated to effectively neutralize vesicular stomatitis virus pseudotypes bearing JUNV GP and to show some cross-neutralization against pathogenic New World arenaviruses. Further research revealed that Asp123 on GP1 is an important site for the binding of antibodies targeting mainly JUNV GP1 for neutralization. Collectively, this study presents an efficient strategy to develop horse antisera against JUNV and provides GP1-specific horse antibodies as potential therapeutics for AHF.

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Junín pseudo-typed virus efficiently stimulates neutralizing antibodies in mice and horses.

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Horse immunoglobulin and immunoglobulin fragments potentially neutralize Junín virus.

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Horse antibodies show some cross-neutralization against the pathogenic New World arenaviruses.

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Asp123 is an important site on glycoprotein 1 for the binding of horse neutralizing antibodies.

Current Progress of Avian Vaccines Against West Nile Virus

Birds are the main natural host of West Nile virus (WNV), the worldwide most distributed mosquito-borne flavivirus, but humans and equids can also be sporadic hosts. Many avian species have been reported as susceptible to WNV, particularly corvids. In the case that clinical disease develops in birds, this is due to virus invasion of different organs: liver, spleen, kidney, heart, and mainly the central nervous system, which can lead to death 24–48 h later. Nowadays, vaccines have only been licensed for use in equids; thus, the availability of avian vaccines would benefit bird populations, both domestic and wild ones. Such vaccines could be used in endangered species housed in rehabilitation and wildlife reserves, and in animals located at zoos and other recreational installations, but also in farm birds, and in those that are grown for hunting and restocking activities. Even more, controlling WNV infection in birds can also be useful to prevent its spread and limit outbreaks. So far, different commercial and experimental vaccines (inactivated, attenuated, and recombinant viruses, and subunits and DNA-based candidates) have been evaluated, with various regimens, both in domestic and wild avian species. However, there are still disadvantages that must be overcome before avian vaccination can be implemented, such as its cost-effectiveness for domestic birds since in many species the pathogenicity is low or zero, or the viability of being able to achieve collective immunity in wild birds in freedom. Here, a comprehensive review of what has been done until now in the field of avian vaccines against WNV is presented and discussed.

Endocytosis of flavivirus NS1 is required for NS1-mediated endothelial hyperpermeability and is abolished by a single N-glycosylation site mutation

Arthropod-borne flaviviruses cause life-threatening diseases associated with endothelial hyperpermeability and vascular leak. We recently found that vascular leak can be triggered by dengue virus (DENV) non-structural protein 1 (NS1) via the disruption of the endothelial glycocalyx-like layer (EGL). However, the molecular determinants of NS1 required to trigger EGL disruption and the cellular pathway(s) involved remain unknown. Here we report that mutation of a single glycosylated residue of NS1 (N207Q) abolishes the ability of NS1 to trigger EGL disruption and induce endothelial hyperpermeability. Intriguingly, while this mutant bound to the surface of endothelial cells comparably to wild-type NS1, it was no longer internalized, suggesting that NS1 binding and internalization are distinct steps. Using endocytic pathway inhibitors and gene-specific siRNAs, we determined that NS1 was endocytosed into endothelial cells in a dynamin- and clathrin-dependent manner, which was required to trigger endothelial dysfunction in vitro and vascular leak in vivo. Finally, we found that the N207 glycosylation site is highly conserved among flaviviruses and is also essential for West Nile and Zika virus NS1 to trigger endothelial hyperpermeability via clathrin-mediated endocytosis. These data provide critical mechanistic insight into flavivirus NS1-induced pathogenesis, presenting novel therapeutic and vaccine targets for flaviviral diseases.

Vascular leak is a hallmark of severe dengue disease. Recently, our group revealed a critical role for NS1 in induction of endothelial hyperpermeability and vascular leakage in an endothelial cell-intrinsic manner. However, the upstream pathway triggered by NS1, as well as the molecular determinants of NS1 required for this phenomenon, remain obscure. Gaining insight into this endothelial cell-intrinsic pathway is critical for understanding dengue pathogenesis, developing novel antiviral therapies, and developing NS1-based vaccine approaches that pose a minimal risk of antibody-dependent enhancement. Our current study expands our knowledge of this novel pathway not only by identifying the requirement of internalization of secreted NS1 via clathrin-mediated endocytosis, but also by pinpointing the NS1 molecular determinant (N207) required to trigger vascular leak. Further, our work identifies N207 as a residue conserved among multiple flaviviruses (Zika virus and West Nile virus, in addition to DENV), which is critical for NS1-mediated vascular leak in biologically relevant human endothelial cells modeling interstitial compartments in the lung or the blood-brain barrier. Thus, our study identifies endocytosis and a single amino acid (N207) of the NS1 viral toxin as critical for pan-flavivirus pathogenesis, representing a novel target for anti-flaviviral therapy and vaccine development.

Role of NS1 and TLR3 in Pathogenesis and Immunity of WNV

West Nile Virus (WNV) is a mosquito-transmitted flavivirus which causes encephalitis especially in elderly and immunocompromised individuals. Previous studies have suggested the protective role of the Toll-like receptor 3 (TLR3) pathway against WNV entry into the brain, while the WNV non-structural protein 1 (NS1) interferes with the TLR3 signaling pathway, besides being a component of viral genome replication machinery. In this study, we investigated whether immunization with NS1 could protect against WNV neuroinvasion in the context of TLR3 deficiency. We immunized mice with either an intact or deleted TLR3 system (TLR3KO) with WNV envelope glycoprotein (gE) protein, NS1, or a combination of gE and NS1. Immunization with gE or gE/NS1, but not with NS1 alone, induced WNV neutralizing antibodies and protected against WNV brain invasion and inflammation. The presence of intact TLR3 signaling had no apparent effect on WNV brain invasion. However, mock-immunized TLR3KO mice had higher inflammatory cell invasion upon WNV brain infection than NS1-immunized TLR3KO mice and wild type mice. Thus, immunization against NS1 may reduce brain inflammation in a context of TLR3 signaling deficiency.

GP38-targeting monoclonal antibodies protect adult mice against lethal Crimean-Congo hemorrhagic fever virus infection

Crimean-Congo hemorrhagic fever virus (CCHFV) is an important human pathogen. Limited evidence suggests that antibodies can protect humans against lethal CCHFV disease but the protective efficacy of antibodies has never been evaluated in adult animal models. Here, we used adult mice to investigate the protection provided against CCHFV infection by glycoprotein-targeting neutralizing and non-neutralizing monoclonal antibodies (mAbs). We identified a single non-neutralizing antibody (mAb-13G8) that protected adult type I interferon-deficient mice >90% when treatment was initiated before virus exposure and >60% when administered after virus exposure. Neutralizing antibodies known to protect neonatal mice from lethal CCHFV infection failed to confer protection regardless of immunoglobulin G subclass. The target of mAb-13G8 was identified as GP38, one of multiple proteolytically cleaved glycoproteins derived from the CCHFV glycoprotein precursor polyprotein. This study reveals GP38 as an important antibody target for limiting CCHFV pathogenesis and lays the foundation to develop immunotherapeutics against CCHFV in humans.

Single-Domain Antibodies and Their Formatting to Combat Viral Infections

Since their discovery in the 1990s, single-domain antibodies (VHHs), also known as Nanobodies^®, have changed the landscape of affinity reagents. The outstanding solubility, stability, and specificity of VHHs, as well as their small size, ease of production and formatting flexibility favor VHHs over conventional antibody formats for many applications. The exceptional ease by which it is possible to fuse VHHs with different molecular modules has been particularly explored in the context of viral infections. In this review, we focus on VHH formats that have been developed to combat viruses including influenza viruses, human immunodeficiency virus-1 (HIV-1), and human respiratory syncytial virus (RSV). Such formats may significantly increase the affinity, half-life, breadth of protection of an antiviral VHH and reduce the risk of viral escape. In addition, VHHs can be equipped with effector functions, for example to guide components of the immune system with high precision to sites of viral infection.

Human antibodies targeting Zika virus NS1 provide protection against disease in a mouse model

Zika virus is a mosquito-borne flavivirus closely related to dengue virus that can cause severe disease in humans, including microcephaly in newborns and Guillain-Barré syndrome in adults. Specific treatments and vaccines for Zika virus are not currently available. Here, we isolate and characterize four monoclonal antibodies (mAbs) from an infected patient that target the non-structural protein NS1. We show that while these antibodies are non-neutralizing, NS1-specific mAbs can engage FcγR without inducing antibody dependent enhancement (ADE) of infection in vitro. Moreover, we demonstrate that mAb AA12 has protective efficacy against lethal challenges of African and Asian lineage strains of Zika virus in Stat2^–/– mice. Protection is Fc-dependent, as a mutated antibody unable to activate known Fc effector functions or complement is not protective in vivo. This study highlights the importance of the ZIKV NS1 protein as a potential vaccine antigen.

Zika virus infection can cause severe disease in humans and there are currently no specific treatments or vaccines. Here, Bailey et al. isolate antibodies recognizing non-structural protein NS1 and show that they protect mice from disease by an Fc-dependent, non-neutralizing mechanism.

The immune response against flaviviruses

Arthropod-borne flaviviruses are important human pathogens that cause a diverse range of clinical conditions, including severe hemorrhagic syndromes, neurological complications and congenital malformations. Consequently, there is an urgent need to develop safe and effective vaccines, a process requiring better understanding of the immunological mechanisms involved during infection. Decades of research suggest a paradoxical role of the immune response against flaviviruses: although the immune response is crucial for the control, clearance and prevention of infection, poor clinical outcomes are commonly associated with virus-specific immunity and immunopathogenesis. This relationship is further complicated by the high homology among viruses and the implication of cross-reactive immune responses in protection and pathogenesis. This Review examines the dual role of the adaptive immune response against flaviviruses, particularly emphasizing the most recent findings regarding cross-reactive T cell and antibody responses, and the effects that these concepts have on vaccine-development endeavors.

The complete genome sequence analysis of West Nile virus strains isolated in Slovakia (central Europe)

The present study reports the first complete genome sequence analysis of West Nile virus (WNV) strains isolated from brain samples from raptors. The results prove the circulation of closely related WNV lineage II strains in central Europe and genetic analysis revealed seven amino acid substitutions in structural (PrM₃, E₁₅₉ and E₂₃₁) and in non-structural (NS1₁₀₉, NS5₂₅₉, NS5₃₁₀ and NS5₆₀₀) proteins. Observed amino acid substitutions Phe₃ and Ser₂₃₁ were common only within the lineage VII Koutango strain isolated from Rhipicephalus guilhoni tick in Senegal. Further research could reveal whether these substitutions influence the biological properties of WNV, including virulence and neuroinvasiveness.

A Zika virus vaccine expressing premembrane-envelope-NS1 polyprotein

Current efforts to develop Zika virus (ZIKV) subunit vaccines have been focused on pre-membrane (prM) and envelope (E) proteins, but the role of NS1 in ZIKV-specific immune response and protection is poorly understood. Here, we develop an attenuated recombinant vesicular stomatitis virus (rVSV)-based vaccine expressing ZIKV prM-E-NS1 as a polyprotein. This vectored vaccine candidate is attenuated in mice, where a single immunization induces ZIKV-specific antibody and T cell immune responses that provide protection against ZIKV challenge. Co-expression of prM, E, and NS1 induces significantly higher levels of Th2 and Th17 cytokine responses than prM-E. In addition, NS1 alone is capable of conferring partial protection against ZIKV infection in mice even though it does not induce neutralizing antibodies. These results demonstrate that attenuated rVSV co-expressing prM, E, and NS1 is a promising vaccine candidate for protection against ZIKV infection and highlights an important role for NS1 in ZIKV-specific cellular immune responses.

Incorporation of NS1 and prM/M are important to confer effective protection of adenovirus-vectored Zika virus vaccine carrying E protein

Current design of Zika virus (ZIKV) vaccine mainly considered envelope (E) as the major target antigen. Non-structural protein NS1 was seldom considered. Herein, we generated three adenovirus-vectored vaccines carrying E (Ad2-E), or premembrane/membrane (prM/M) with E (Ad2-prME), or NS1 in addition to prM/M with E (Ad2-prME-NS1). Ad2-prME induced higher neutralizing antibody response to ZIKV than Ad2-E, suggesting prM/M is important for the folding of immunogenic E. Most intriguingly, Ad2-prME-NS1 elicited the best viral inhibition when the immune sera were added to ZIKV-infected cells. In ZIKV-challenged neonatal mice born to maternally immunized dams, Ad2-prME-NS1 conferred the best protection in preventing weight loss, neurological disorders, and viral replication. Ad2-prME also conferred significant protection but was less effective than Ad2-prME-NS1, whereas Ad2-E only alleviated neurological symptoms but did not inhibit viral replication. Our study suggested that NS1 should be considered in the design of ZIKV vaccine in addition to prM/M and E.