Structural basis for antibody inhibition of flavivirus NS1-triggered endothelial dysfunction

Therapeutic efficacy of humanized monoclonal antibodies targeting dengue virus nonstructural protein 1 in the mouse model

Dengue virus (DENV) which infects about 390 million people per year in tropical and subtropical areas manifests various disease symptoms, ranging from fever to life-threatening hemorrhage and even shock. To date, there is still no effective treatment for DENV disease, but only supportive care. DENV nonstructural protein 1 (NS1) has been shown to play a key role in disease pathogenesis. Recent studies have shown that anti-DENV NS1 antibody can provide disease protection by blocking the DENV-induced disruption of endothelial integrity. We previously demonstrated that anti-NS1 monoclonal antibody (mAb) protected mice from all four serotypes of DENV challenge. Here, we generated humanized anti-NS1 mAbs and transferred them to mice after DENV infection. The results showed that DENV-induced prolonged bleeding time and skin hemorrhage were reduced, even several days after DENV challenge. Mechanistic studies showed the ability of humanized anti-NS1 mAbs to inhibit NS1-induced vascular hyperpermeability and to elicit Fcγ-dependent complement-mediated cytolysis as well as antibody-dependent cellular cytotoxicity of cells infected with four serotypes of DENV. These results highlight humanized anti-NS1 mAb as a potential therapeutic agent in DENV infection.

DENV comprising four serotypes has a complicated pathogenesis and remains an unresolved global health problem. To date, supportive therapy is the mainstay for treatment of dengue patients. Despite a licensed Sanofi vaccine and ongoing clinical trials, more effective vaccines and/or licensed therapeutic drugs are required. Therapeutic mAbs are a potential tool to treat many epidemic diseases because of their high target specificity. Humanized anti-NS1 mAbs can recognize the NS1 from all four serotypes of DENV without danger of inducing ADE. In the DENV infection mouse model, we demonstrate that humanized NS1 mAbs have therapeutic benefits such as reducing DENV-induced prolonged bleeding time and skin hemorrhage. In vitro mechanistic studies showed a reduction of NS1-induced vascular permeability and an increase in cytolysis of DENV-infected cells. Our results showed that humanized anti-NS1 mAbs show strong potential for development toward clinical use.

One antibody to treat them all

Conserved flavivirus protein holds potential as target for versatile vaccines and therapies.

Structural Biology Illuminates Molecular Determinants of Broad Ebolavirus Neutralization by Human Antibodies for Pan-Ebolavirus Therapeutic Development

Monoclonal antibodies (mAbs) have proven effective for the treatment of ebolavirus infection in humans, with two mAb-based drugs Inmazeb™ and Ebanga™ receiving FDA approval in 2020. While these drugs represent a major advance in the field of filoviral therapeutics, they are composed of antibodies with single-species specificity for Zaire ebolavirus. The Ebolavirus genus includes five additional species, two of which, Bundibugyo ebolavirus and Sudan ebolavirus, have caused severe disease and significant outbreaks in the past. There are several recently identified broadly neutralizing ebolavirus antibodies, including some in the clinical development pipeline, that have demonstrated broad protection in preclinical studies. In this review, we describe how structural biology has illuminated the molecular basis of broad ebolavirus neutralization, including details of common antigenic sites of vulnerability on the glycoprotein surface. We begin with a discussion outlining the history of monoclonal antibody therapeutics for ebolaviruses, with an emphasis on how structural biology has contributed to these efforts. Next, we highlight key structural studies that have advanced our understanding of ebolavirus glycoprotein structures and mechanisms of antibody-mediated neutralization. Finally, we offer examples of how structural biology has contributed to advances in anti-viral medicines and discuss what opportunities the future holds, including rationally designed next-generation therapeutics with increased potency, breadth, and specificity against ebolaviruses.

A perfect storm: acute portal vein thrombosis in a patient with severe dengue and hemorrhagic manifestations-a case report

Background

Dengue constitutes a public health problem in endemic regions. The clinical course can range from asymptomatic to severe expressions. Hemorrhagic manifestations are the most frequently reported complications; on the contrary, thrombotic complications are unusual.

Clinical case

We present the case of an adult patient who presented hemodynamic instability, severe thrombocytopenia, and positive serology for dengue, in whom acute portal vein thrombosis was documented. The possible pathophysiology of thrombocytopenia and thrombosis in dengue is discussed, as well as the dilemmas regarding the treatment of associated hemorrhagic and thrombotic manifestations.

Conclusions

The present case brings up the importance of considering the possibility of thrombotic events in patients with severe dengue. A high degree of suspicion, close assessment of hemostatic function, and quality supportive care are essential to improve outcomes. To our knowledge, this is the first report of dengue-associated portal vein thrombosis.

SARS-CoV-2 Spike triggers barrier dysfunction and vascular leak via integrins and TGF-β signaling

Severe COVID-19 is associated with epithelial and endothelial barrier dysfunction within the lung as well as in distal organs. While it is appreciated that an exaggerated inflammatory response is associated with barrier dysfunction, the triggers of this pathology are unclear. Here, we report that cell-intrinsic interactions between the Spike (S) glycoprotein of SARS-CoV-2 and epithelial/endothelial cells are sufficient to trigger barrier dysfunction in vitro and vascular leak in vivo , independently of viral replication and the ACE2 receptor. We identify an S-triggered transcriptional response associated with extracellular matrix reorganization and TGF-β signaling. Using genetic knockouts and specific inhibitors, we demonstrate that glycosaminoglycans, integrins, and the TGF-β signaling axis are required for S-mediated barrier dysfunction. Our findings suggest that S interactions with barrier cells are a contributing factor to COVID-19 disease severity and offer mechanistic insight into SARS-CoV-2 triggered vascular leak, providing a starting point for development of therapies targeting COVID-19 pathogenesis.

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.

Altered Moesin and Actin Cytoskeleton Protein Rearrangements Affect Transendothelial Permeability in Human Endothelial Cells upon Dengue Virus Infection and TNF-α Treatment

It has been hypothesized that the host, viral factors, and secreted cytokines (especially TNF-α) play roles in the pathogenesis of secondary dengue infections. Mass spectrometry-based proteomic screening of cytoskeleton fractions isolated from human endothelial (EA.hy926) cells upon dengue virus (DENV) infection and TNF-α treatment identified 450 differentially altered proteins. Among them, decreased levels of moesin, actin stress fiber rearrangements, and dot-like formations of vinculin were observed with western blot analyses and/or immunofluorescence staining (IFA). In vitro vascular permeability assays using EA.hy926 cells, seeded on collagen-coated transwell inserts, showed low levels of transendothelial electrical resistance in treated cells. The synergistic effects of DENV infection and TNF-α treatment caused cellular permeability changes in EA.hy926 cells, which coincided with decreasing moesin levels and the production of abnormal organizations of actin stress fibers and vinculin. Functional studies demonstrated moesin overexpression restored transendothelial permeability in DENV/TNF-α-treated EA.hy926 cells. The present study improves the understanding of the disruption mechanisms of cytoskeleton proteins in enhancing vascular permeability during DENV infection and TNF-α treatment. The study also suggests that these disruption mechanisms are major factors contributing to vascular leakage in severe dengue patients.

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 immunodominant antibody response to Zika virus NS1 protein is characterized by cross-reactivity to self

Besides antigen-specific responses to viral antigens, humoral immune response in virus infection can generate polyreactive and autoreactive antibodies. Dengue and Zika virus infections have been linked to antibody-mediated autoimmune disorders, including Guillain-Barré syndrome. A unique feature of flaviviruses is the secretion of nonstructural protein 1 (NS1) by infected cells. NS1 is highly immunogenic, and antibodies targeting NS1 can have both protective and pathogenic roles. In the present study, we investigated the humoral immune response to Zika virus NS1 and found NS1 to be an immunodominant viral antigen associated with the presence of autoreactive antibodies. Through single B cell cultures, we coupled binding assays and BCR sequencing, confirming the immunodominance of NS1. We demonstrate the presence of self-reactive clones in germinal centers after both infection and immunization, some of which present cross-reactivity with NS1. Sequence analysis of anti-NS1 B cell clones showed sequence features associated with pathogenic autoreactive antibodies. Our findings demonstrate NS1 immunodominance at the cellular level as well as a potential role for NS1 in ZIKV-associated autoimmune manifestations.

A dark side to NS1 antibodies?

The NS1 protein of flaviviruses is taking center stage. Recent work has made it an attractive target for development of vaccines and immunotherapeutics. Cavazzoni and colleagues (2021. J. Exp. Med.https://doi.org/10.1084/jem.20210580) now reveal a dark side to NS1, linking it to the development of self-reactive antibodies.

Continuous Automated Model EvaluatiOn (CAMEO)-Perspectives on the future of fully automated evaluation of structure prediction methods

The Continuous Automated Model EvaluatiOn (CAMEO) platform complements the biennial CASP experiment by conducting fully automated blind evaluations of three‐dimensional protein prediction servers based on the weekly prerelease of sequences of those structures, which are going to be published in the upcoming release of the Protein Data Bank. While in CASP14, significant success was observed in predicting the structures of individual protein chains with high accuracy, significant challenges remain in correctly predicting the structures of complexes. By implementing fully automated evaluation of predictions for protein–protein complexes, as well as for proteins in complex with ligands, peptides, nucleic acids, or proteins containing noncanonical amino acid residues, CAMEO will assist new developments in those challenging areas of active research.

Flavivirus: From Structure to Therapeutics Development

Flaviviruses are still a hidden threat to global human safety, as we are reminded by recent reports of dengue virus infections in Singapore and African-lineage-like Zika virus infections in Brazil. Therapeutic drugs or vaccines for flavivirus infections are in urgent need but are not well developed. The Flaviviridae family comprises a large group of enveloped viruses with a single-strand RNA genome of positive polarity. The genome of flavivirus encodes ten proteins, and each of them plays a different and important role in viral infection. In this review, we briefly summarized the major information of flavivirus and further introduced some strategies for the design and development of vaccines and anti-flavivirus compound drugs based on the structure of the viral proteins. There is no doubt that in the past few years, studies of antiviral drugs have achieved solid progress based on better understanding of the flavivirus biology. However, currently, there are no fully effective antiviral drugs or vaccines for most flaviviruses. We hope that this review may provide useful information for future development of anti-flavivirus drugs and vaccines.

Dengue and the Lectin Pathway of the Complement System

Dengue is a mosquito-borne viral disease causing significant health and economic burdens globally. The dengue virus (DENV) comprises four serotypes (DENV1-4). Usually, the primary infection is asymptomatic or causes mild dengue fever (DF), while secondary infections with a different serotype increase the risk of severe dengue disease (dengue hemorrhagic fever, DHF). Complement system activation induces inflammation and tissue injury, contributing to disease pathogenesis. However, in asymptomatic or primary infections, protective immunity largely results from the complement system’s lectin pathway (LP), which is activated through foreign glycan recognition. Differences in N-glycans displayed on the DENV envelope membrane influence the lectin pattern recognition receptor (PRR) binding efficiency. The important PRR, mannan binding lectin (MBL), mediates DENV neutralization through (1) a complement activation-independent mechanism via direct MBL glycan recognition, thereby inhibiting DENV attachment to host target cells, or (2) a complement activation-dependent mechanism following the attachment of complement opsonins C3b and C4b to virion surfaces. The serum concentrations of lectin PRRs and their polymorphisms influence these LP activities. Conversely, to escape the LP attack and enhance the infectivity, DENV utilizes the secreted form of nonstructural protein 1 (sNS1) to counteract the MBL effects, thereby increasing viral survival and dissemination.

A Live-Attenuated Zika Virus Vaccine with High Production Capacity Confers Effective Protection in Neonatal Mice

Zika virus (ZIKV) infection during pregnancy has been linked to congenital abnormalities, such as microcephaly in infants. An efficacious vaccine is desirable for preventing the potential recurrence of ZIKV epidemic. Here, we report the generation of an attenuated ZIKV (rGZ02a) that has sharply decreased virulence in mice but grows to high titers in Vero cells, a widely approved cell line for manufacturing human vaccines. Compared to the wild-type ZIKV (GZ02) and a plasmid-launched rGZ02p, rGZ02a has 3 unique amino acid alterations in the envelope (E, S304F), nonstructural protein 1 (NS1, R103K), and NS5 (W637R). rGZ02a is more sensitive to type I interferon than GZ02 and rGZ02p, and causes no severe neurological disorders in either wild-type neonatal C57BL/6 mice or type I interferon receptor knockout (Ifnar1^-/-) C57BL/6 mice. Immunization with rGZ02a elicits robust inhibitory antibody responses with a certain long-term durability. Neonates born to the immunized dams are effectively protected against ZIKV-caused neurological disorders and brain damage. rGZ02a as a booster vaccine greatly improves the protective immunity primed by Ad2-prME, an adenovirus-vectored vaccine expressing ZIKV prM and E proteins. Our results illustrate that rGZ02a-induced maternal immunity can be transferred to the neonates and confer effective protection. Hence, rGZ02a may be developed as an alternative live-attenuated vaccine and warrants further evaluation. IMPORTANCE Zika virus (ZIKV), a mosquito-borne flavivirus that has caused global outbreaks since 2013, is associated with severe neurological disorders, such as Guillian-Barré syndrome in adults and microcephaly in infants. The ZIKV epidemic has gradually subsided, but a safe and effective vaccine is still desirable to prevent its potential recurrence, especially in countries of endemicity with competent mosquito vectors. Here, we describe a novel live-attenuated ZIKV, rGZ02a, that carries 3 unique amino acid alterations compared to the wild-type GZ02 and a plasmid-launched rGZ02p. The growth capacity of rGZ02a is comparable to GZ02 in Vero cells, but the pathogenicity is significantly attenuated in two mice models. Immunization with rGZ02a elicits robust inhibitory antibody responses in the dams and effectively protects their offspring against ZIKV disease. Importantly, in a heterologous prime-boost regimen, rGZ02a effectively boosts the protective immunity primed by an adenovirus-vectored vaccine. Thus, rGZ02a is a promising candidate for a live-attenuated ZIKV vaccine.

Endothelial Dysfunction in Atherosclerotic Cardiovascular Diseases and Beyond: From Mechanism to Pharmacotherapies

The endothelium, a cellular monolayer lining the blood vessel wall, plays a critical role in maintaining multiorgan health and homeostasis. Endothelial functions in health include dynamic maintenance of vascular tone, angiogenesis, hemostasis, and the provision of an antioxidant, anti-inflammatory, and antithrombotic interface. Dysfunction of the vascular endothelium presents with impaired endothelium-dependent vasodilation, heightened oxidative stress, chronic inflammation, leukocyte adhesion and hyperpermeability, and endothelial cell senescence. Recent studies have implicated altered endothelial cell metabolism and endothelial-to-mesenchymal transition as new features of endothelial dysfunction. Endothelial dysfunction is regarded as a hallmark of many diverse human panvascular diseases, including atherosclerosis, hypertension, and diabetes. Endothelial dysfunction has also been implicated in severe coronavirus disease 2019. Many clinically used pharmacotherapies, ranging from traditional lipid-lowering drugs, antihypertensive drugs, and antidiabetic drugs to proprotein convertase subtilisin/kexin type 9 inhibitors and interleukin 1β monoclonal antibodies, counter endothelial dysfunction as part of their clinical benefits. The regulation of endothelial dysfunction by noncoding RNAs has provided novel insights into these newly described regulators of endothelial dysfunction, thus yielding potential new therapeutic approaches. Altogether, a better understanding of the versatile (dys)functions of endothelial cells will not only deepen our comprehension of human diseases but also accelerate effective therapeutic drug discovery. In this review, we provide a timely overview of the multiple layers of endothelial function, describe the consequences and mechanisms of endothelial dysfunction, and identify pathways to effective targeted therapies. SIGNIFICANCE STATEMENT: The endothelium was initially considered to be a semipermeable biomechanical barrier and gatekeeper of vascular health. In recent decades, a deepened understanding of the biological functions of the endothelium has led to its recognition as a ubiquitous tissue regulating vascular tone, cell behavior, innate immunity, cell-cell interactions, and cell metabolism in the vessel wall. Endothelial dysfunction is the hallmark of cardiovascular, metabolic, and emerging infectious diseases. Pharmacotherapies targeting endothelial dysfunction have potential for treatment of cardiovascular and many other diseases.

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.

Non-structural protein 1-specific antibodies directed against Zika virus in humans mediate antibody-dependent cellular cytotoxicity

There is growing interest in understanding antibody (Ab) function beyond neutralization. The non-structural protein 1 (NS1) of Zika virus (ZIKV) is an attractive candidate for an effective vaccine as Abs against NS1, unlike the envelope or premembrane, do not carry the risk of mediating antibody-dependent enhancement. Our aim was to evaluate whether ZIKV NS1 Abs elicited following natural infection in humans can mediate antibody-dependent cellular cytotoxicity (ADCC). We evaluated the isotype specificity of ZIKV-specific Abs in immune sera and supernatants from stimulated immune PBMC and found that Abs against ZIKV NS1 and virus-like particles were predominantly of the IgG1 isotype. Using a recently developed FluoroSpot assay, we found robust frequencies of NS1-specific Ab-secreting cells in PBMC of individuals who were naturally infected with ZIKV. We developed assays to measure both natural killer cell activation by flow cytometry and target cell lysis of ZIKV NS1-expressing cells using an image cytometry assay in the presence of ZIKV NS1 Abs. Our data indicate efficient opsonization of ZIKV NS1-expressing CEM-NK^R cell lines using ZIKV-immune but not ZIKV-naïve sera, a prerequisite of ADCC. Furthermore, sera from immune donors were able to induce both NK cell degranulation and lysis of ZIKV NS1 CEM-NK^R cells in vitro. Our data suggest that ADCC is a possible mechanism for ZIKV NS1 Abs to eliminate virally infected target cells.

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.

Molecular Insights into the Flavivirus Replication Complex

Flaviviruses are vector-borne RNA viruses, many of which are clinically relevant human viral pathogens, such as dengue, Zika, Japanese encephalitis, West Nile and yellow fever viruses. Millions of people are infected with these viruses around the world each year. Vaccines are only available for some members of this large virus family, and there are no effective antiviral drugs to treat flavivirus infections. The unmet need for vaccines and therapies against these flaviviral infections drives research towards a better understanding of the epidemiology, biology and immunology of flaviviruses. In this review, we discuss the basic biology of the flavivirus replication process and focus on the molecular aspects of viral genome replication. Within the virus-induced intracellular membranous compartments, flaviviral RNA genome replication takes place, starting from viral poly protein expression and processing to the assembly of the virus RNA replication complex, followed by the delivery of the progeny viral RNA to the viral particle assembly sites. We attempt to update the latest understanding of the key molecular events during this process and highlight knowledge gaps for future studies.

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.