Partial maturation: an immune-evasion strategy of dengue virus?

TLR2 axis on peripheral blood mononuclear cells regulates inflammatory responses to non-infectious immature dengue virus particles

Severe dengue virus (DENV) infection is characterized by exacerbated inflammatory responses that lead to endothelial dysfunction and plasma leakage. We have recently demonstrated that Toll-like receptor 2 (TLR2) on blood monocytes senses DENV infection leading to endothelial activation. Here, we report that non-infectious immature DENV particles, which are released in large numbers by DENV-infected cells, drive endothelial activation via the TLR2 axis. We show that fully immature DENV particles induce a rapid, within 6 hours post-infection, inflammatory response in PBMCs. Furthermore, pharmacological blocking of TLR2/TLR6/CD14 and/or NF-kB prior to exposure of PBMCs to immature DENV reduces the initial production of inter alia TNF-α and IL-1β by monocytes and prevents endothelial activation. However, prolonged TLR2 block induces TNF-α production and leads to exacerbated endothelial activation, indicating that TLR2-mediated responses play an important role not only in the initiation but also the resolution of inflammation. Altogether, these data indicate that the maturation status of the virus has the potential to influence the kinetics and extent of inflammatory responses during DENV infection.

Innate and adaptive immune evasion by dengue virus

Dengue is a mosquito-borne disease which causes significant public health concerns in tropical and subtropical countries. Dengue virus (DENV) has evolved various strategies to manipulate the innate immune responses of the host such as ‘hiding’ in the ultrastructure of the host, interfering with the signaling pathway through RNA modifications, inhibiting type 1 IFN production, as well as inhibiting STAT1 phosphorylation. DENV is also able to evade the adaptive immune responses of the host through antigenic variation, antigen-dependent enhancement (ADE), partial maturation of prM proteins, and inhibition of antigen presentation. miRNAs are important regulators of both innate and adaptive immunity and they have been shown to play important roles in DENV replication and pathogenesis. This makes them suitable candidates for the development of anti-dengue therapeutics. This review discusses the various strategies employed by DENV to evade innate and adaptive immunity. The role of miRNAs and DENV non-structural proteins (NS) are promising targets for the development of anti-dengue therapeutics.

Zika virus-like particle vaccine fusion loop mutation increases production yield but fails to protect AG129 mice against Zika virus challenge

Zika virus (ZIKV) is a mosquito-borne flavivirus with maternal infection associated with preterm birth, congenital malformations, and fetal death, and adult infection associated with Guillain-Barré syndrome. Recent widespread endemic transmission of ZIKV and the potential for future outbreaks necessitate the development of an effective vaccine. We developed a ZIKV vaccine candidate based on virus-like-particles (VLPs) generated following transfection of mammalian HEK293T cells using a plasmid encoding the pre-membrane/membrane (prM/M) and envelope (E) structural protein genes. VLPs were collected from cell culture supernatant and purified by column chromatography with yields of approximately 1-2mg/L. To promote increased particle yields, a single amino acid change of phenylalanine to alanine was made in the E fusion loop at position 108 (F108A) of the lead VLP vaccine candidate. This mutation resulted in a modest 2-fold increase in F108A VLP production with no detectable prM processing by furin to a mature particle, in contrast to the lead candidate (parent). To evaluate immunogenicity and efficacy, AG129 mice were immunized with a dose titration of either the immature F108A or lead VLP (each alum adjuvanted). The resulting VLP-specific binding antibody (Ab) levels were comparable. However, geometric mean neutralizing Ab (nAb) titers using a recombinant ZIKV reporter were significantly lower with F108A immunization compared to lead. After virus challenge, all lead VLP-immunized groups showed a significant 3- to 4-Log₁₀ reduction in mean ZIKV RNAemia levels compared with control mice immunized only with alum, but the RNAemia reduction of 0.5 Log₁₀ for F108A groups was statistically similar to the control. Successful viral control by the lead VLP candidate following challenge supports further vaccine development for this candidate. Notably, nAb titer levels in the lead, but not F108A, VLP-immunized mice inversely correlated with RNAemia. Further evaluation of sera by an in vitro Ab-dependent enhancement assay demonstrated that the F108A VLP-induced immune sera had a significantly higher capacity to promote ZIKV infection in FcγR-expressing cells. These data indicate that a single amino acid change in the fusion loop resulted in increased VLP yields but that the immature F108A particles were significantly diminished in their capacity to induce nAbs and provide protection against ZIKV challenge.

Zika virus (ZIKV) is transmitted by mosquitoes and is a serious health threat due to potential epidemic spread. Infection in adults may lead to Guillain-Barré syndrome, a neurological disorder, or may cause harm to a developing fetus resulting in preterm birth, fetal death, or devastating congenital malformations. There are currently no approved vaccines against ZIKV. We previously developed a lead candidate vaccine based on a virus-like particle (VLP) that was generated in tissue culture. This ZIKV shell is devoid of any viral genetic material. In previous studies, this lead VLP candidate generated neutralizing antibodies (nAbs) that recognized wild-type ZIKV and prevented viral replication in both mice and non-human primates. To increase production of the lead VLP candidate and decrease cost-of-goods, we introduced a single amino acid change, phenylalanine to alanine, in the envelope glycoprotein. This change resulted in a modest increase in VLP yield. However, this single amino acid change resulted in reduced induction of nAbs following immunization and no significant reduction of RNAemia following challenge compared to the lead candidate. The results of this study suggest this investigational vaccine candidate is not suitable for further vaccine development and that ZIKV VLP maturation may have an important role in protection.

Spectrum of antiviral activity of 4-aminopyrimidine N-oxides against a broad panel of tick-borne encephalitis virus strains

Tick-borne encephalitis is an important human arbovirus neuroinfection spread across the Northern Eurasia. Inhibitors of tick-borne encephalitis virus (TBEV) strain Absettarov, presumably targeting E protein n-octyl-β-d-glucoside (β-OG) pocket, were reported earlier. In this work, these inhibitors were tested in vitro against seven strains representing three main TBEV subtypes. The most potent compound, 2-[(2-methyl-1-oxido-5,6,7,8-tetrahydroquinazolin-4-yl)amino]-phenol, showed EC₅₀ values lower than 22 µM against all the tested strains. Nevertheless, EC₅₀ values for virus samples of certain strains demonstrated a substantial variation, which appeared to be consistent with the presence of E protein not only in infectious virions, but also in non-infectious and immature virus particles, protein aggregates, and membrane complexes.

A Human Antibody Neutralizes Different Flaviviruses by Using Different Mechanisms

Human antibody SIgN-3C neutralizes dengue virus (DENV) and Zika virus (ZIKV) differently. DENV:SIgN-3C Fab and ZIKV:SIgN-3C Fab cryoelectron microscopy (cryo-EM) complex structures show Fabs crosslink E protein dimers at extracellular pH 8.0 condition and also when further incubated at acidic endosomal conditions (pH 8.0-6.5). We observe Fab binding to DENV (pH 8.0-5.0) prevents virus fusion, and the number of bound Fabs increase (from 120 to 180). For ZIKV, although there are already 180 copies of Fab at pH 8.0, virus structural changes at pH 5.0 are not inhibited. The immunoglobulin G (IgG):DENV structure at pH 8.0 shows both Fab arms bind to epitopes around the 2-fold vertex. On ZIKV, an additional Fab around the 5-fold vertex at pH 8.0 suggests one IgG arm would engage with an epitope, although the other may bind to other viruses, causing aggregation. For DENV2 at pH 5.0, a similar scenario would occur, suggesting DENV2:IgG complex would aggregate in the endosome. Hence, a single antibody employs different neutralization mechanisms against different flaviviruses.

Zika virus-like particle vaccine protects AG129 mice and rhesus macaques against Zika virus

Background

Zika virus (ZIKV), a mosquito-borne flavivirus, is a re-emerging virus that constitutes a public health threat due to its recent global spread, recurrent outbreaks, and infections that are associated with neurological abnormalities in developing fetuses and Guillain-Barré syndrome in adults. To date, there are no approved vaccines against ZIKV infection. Various preclinical and clinical development programs are currently ongoing in an effort to bring forward a vaccine for ZIKV.

Methodology/Principle findings

We have developed a ZIKV vaccine candidate based on Virus-Like-Particles (VLPs) produced in HEK293 mammalian cells using the prM (a precursor to M protein) and envelope (E) structural protein genes from ZIKV. Transient transfection of cells via plasmid and electroporation produced VLPs which were subsequently purified by column chromatography yielding approximately 2mg/L. Initially, immunogenicity and efficacy were evaluated in AG129 mice using a dose titration of VLP with and without Alhydrogel 2% (alum) adjuvant. We found that VLP with and without alum elicited ZIKV-specific serum neutralizing antibodies (nAbs) and that titers correlated with protection. A follow-up immunogenicity and efficacy study in rhesus macaques was performed using VLP formulated with alum. Multiple neutralization assay methods were performed on immune sera including a plaque reduction neutralization test, a microneutralization assay, and a Zika virus Renilla luciferase neutralization assay. All of these assays indicate that following immunization, VLP induces high titer nAbs which correlate with protection against ZIKV challenge.

Conclusions/Significance

These studies confirm that ZIKV VLPs could be efficiently generated and purified. Upon VLP immunization, in both mice and NHPs, nAb was induced that correlate with protection against ZIKV challenge. These studies support translational efforts in developing a ZIKV VLP vaccine for evaluation in human clinical trials.

Zika virus (ZIKV) is a significant global health threat particularly due to the speed in which epidemics can occur. The resulting infections have been demonstrated to harm a developing fetus and, in some adults, be a co-factor for the development of Guillain-Barré syndrome. ZIKV is typically spread by the Aedes mosquito, but sexual transmission is also possible. We sought to develop a ZIKV prophylactic vaccine based on surface glycoproteins of the virus that would be devoid of any viral genetic material. This Virus-Like-Particle (VLP) was generated in vitro following introduction of plasmid DNA encoding Zika structural protein (prM-E) genes into mammalian cells. The aluminum-adjuvanted VLP induced nAbs in mice and nonhuman primates and protected against ZIKV challenge in vivo. These studies support the evaluation of this VLP candidate vaccine in human clinical trials.

Generation and preliminary characterization of vertebrate-specific replication-defective Zika virus

Zika virus (ZIKV) is a mosquito-borne flavivirus that replicates in both vertebrate and insect cells, whereas insect-specific flaviviruses (ISF) replicate only in insect cells. We sought to convert ZIKV, from a dual-tropic flavivirus, into an insect-specific virus for the eventual development of a safe ZIKV vaccine. Reverse genetics was used to introduce specific mutations into the furin cleavage motif within the ZIKV pre-membrane protein (prM). Mutant clones were selected, which replicated well in C6/36 insect cells but exhibited reduced replication in non-human primate (Vero) cells. Further characterization of the furin cleavage site mutants indicated they replicated poorly in both human (HeLa, U251), and baby hamster kidney (BHK-21) cells. One clone with the induced mutation in the prM protein and at positions 291and 452 within the NS3 protein was totally and stably replication-defective in vertebrate cells (VSRD-ZIKV). Preliminary studies in ZIKV sensitive, immunodeficient mice demonstrated that VSRD-ZIKV-infected mice survived and were virus-negative. Our study indicates that a reverse genetic approach targeting the furin cleavage site in prM can be used to select an insect-specific ZIKV with the potential utility as a vaccine strain.

Adaptive Immunity to Dengue Virus: Slippery Slope or Solid Ground for Rational Vaccine Design?

The four serotypes of dengue virus are the most widespread causes of arboviral disease, currently placing half of the human population at risk of infection. Pre-existing immunity to one dengue virus serotype can predispose to severe disease following secondary infection with a different serotype. The phenomenon of immune enhancement has complicated vaccine development and likely explains the poor long-term safety profile of a recently licenced dengue vaccine. Therefore, alternative vaccine strategies should be considered. This review summarises studies dissecting the adaptive immune responses to dengue virus infection and (experimental) vaccination. In particular, we discuss the roles of (i) neutralising antibodies, (ii) antibodies to non-structural protein 1, and (iii) T cells in protection and pathogenesis. We also address how these findings could translate into next-generation vaccine approaches that mitigate the risk of enhanced dengue disease. Finally, we argue that the development of a safe and efficacious dengue vaccine is an attainable goal.

Cryo-EM Studies of Virus-Antibody Immune Complexes

Antibodies play critical roles in neutralizing viral infections and are increasingly used as therapeutic drugs and diagnostic tools. Structural studies on virus-antibody immune complexes are important for better understanding the molecular mechanisms of antibody-mediated neutralization and also provide valuable information for structure-based vaccine design. Cryo-electron microscopy (cryo-EM) has recently matured as a powerful structural technique for studying bio-macromolecular complexes. When combined with X-ray crystallography, cryo-EM provides a routine approach for structurally characterizing the immune complexes formed between icosahedral viruses and their antibodies. In this review, recent advances in the structural understanding of virus-antibody interactions are outlined for whole virions with icosahedral T = pseudo 3 (picornaviruses) and T = 3 (flaviviruses) architectures, focusing on the dynamic nature of viral shells in different functional states. Glycoprotein complexes from pleomorphic enveloped viruses are also discussed as immune complex antigens. Improving our understanding of viral epitope structures using virus-based platforms would provide a fundamental road map for future vaccine development.

Modelling the Host Immune Response to Mature and Immature Dengue Viruses

Immature dengue virions contained in patient blood samples are essentially not infectious because the uncleaved surface protein prM renders them incompetent for membrane fusion. However, the immature virions regain full infectivity when they interact with anti-prM antibodies, and once opsonised virion fusion into Fc receptor-expressing cells is facilitated. We propose a within-host mathematical model for the immune response which takes into account the dichotomy between mature infectious and immature noninfectious dengue virions. The model accounts for experimental observations on the different interactions of plasmacytoid dendritic cells with infected cells producing virions with different infectivity. We compute the basic reproduction number as a function of the proportion of infected cells producing noninfectious virions and use numerical simulations to compare the host's immune response in a primary and a secondary dengue infections. The results can be placed in the immunoregulatory framework with plasmacytoid dendritic cells serving as a bridge between the innate and adaptive immune response, and pose questions for potential experimental work to validate hypothesis about the evolutionary context whereby the virus strives to maximise its chance for transmission from the human host to the mosquito vector.

Structural disorder in the proteome and interactome of Alkhurma virus (ALKV)

Infection by the Alkhurma virus (ALKV) leading to the Alkhurma hemorrhagic fever is a common thread in Saudi Arabia, with no efficient treatment or prevention available as of yet. Although the rational drug design traditionally uses information on known 3D structures of viral proteins, intrinsically disordered proteins (i.e., functional proteins that do not possess unique 3D structures), with their multitude of disorder-dependent functions, are crucial for the biology of viruses. Here, viruses utilize disordered regions in their invasion of the host organisms and in hijacking and repurposing of different host systems. Furthermore, the ability of viruses to efficiently adjust and accommodate to their hostile habitats is also intrinsic disorder-dependent. However, little is currently known on the level of penetrance and functional utilization of intrinsic disorder in the ALKV proteome. To fill this gap, we used here multiple computational tools to evaluate the abundance of intrinsic disorder in the ALKV genome polyprotein. We also analyzed the peculiarities of intrinsic disorder predisposition of the individual viral proteins, as well as human proteins known to be engaged in interaction with the ALKV proteins. Special attention was paid to finding a correlation between protein functionality and structural disorder. To the best of our knowledge, this work represents the first systematic study of the intrinsic disorder status of ALKV proteome and interactome.

Dengue type 1 viruses circulating in humans are highly infectious and poorly neutralized by human antibodies

The four dengue virus (DENV) serotypes are mosquito-borne flaviviruses of humans. The interactions between DENVs and the human host that lead to asymptomatic, mild, or severe disease are poorly understood, in part, because laboratory models are poor surrogates for human DENV disease. Virologists are interested in how the properties of DENVs replicating in people compare with virions propagated on laboratory cell lines, which are widely used for research and vaccine development. Using clinical samples from a DENV type 1 epidemic in Sri Lanka and new ultrasensitive assays, we compared the properties of DENVs in human plasma and after one passage on laboratory cell lines. DENVs in plasma were 50- to 700-fold more infectious than cell culture-grown viruses. DENVs produced by laboratory cell lines were structurally immature and hypersensitive to neutralization by human antibodies compared with DENVs circulating in people. Human plasma and cell culture-derived virions had identical genome sequences, indicating that these phenotypic differences were due to the mature state of plasma virions. Several dengue vaccines are under development. Recent studies indicate that vaccine-induced antibodies that neutralized DENVs in cell culture assays were not sufficient for protecting people from DENV infections. Our results about structural differences between DENVs produced in humans versus cell lines may be key to understanding vaccine failure and developing better models for vaccine evaluation.

Variable selection and parameter estimation of viral amplification in vero cell cultures dedicated to the production of a dengue vaccine

In this study, a dynamic model of a Vero cell culture-based dengue vaccine production process is developed. The approach consists in describing the process dynamics as functions of the whole living (uninfected and infected) biomass whereas previous works are based on population balance approaches. Based on the assumption that infected biomass evolves faster than other variable, the model can be simplified using a slow-fast approximation. The structural identifiability of the model is analysed using differential algebra as implemented in the software DAISY. The model parameters are inferred from experimental datasets collected from an actual vaccine production process and the model predictive capability is confirmed both in direct and cross-validation. The model prediction shows the impact of the metabolism on virus yield and confirms observations reported in previous studies. Multi-modality and sensitivity analysis complement the parameter estimation, and allow to obtain confidence intervals on both parameters and state estimates. Finally, the model is used to compute the maximum infectious virus yield that can be obtained for different combinations of multiplicity of infection (MOI) and time of infection (TOI). © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2687, 2019.

The involvement of plasmacytoid cells in HIV infection and pathogenesis

Plasmacytoid dendritic cells (pDCs) are a unique dendritic cell subset that are specialized in type I interferon (IFN) production. pDCs are key players in the antiviral immune response and serve as bridge between innate and adaptive immunity. Although pDCs do not represent the main reservoir of the Human Immunodeficiency Virus (HIV), they are a crucial subset in HIV infection as they influence viral transmission, target cell infection and antigen presentation. pDCs act as inflammatory and immunosuppressive cells, thus contributing to HIV disease progression. This review provides a state of art analysis of the interactions between HIV and pDCs and their potential roles in HIV transmission, chronic immune activation and immunosuppression. A thorough understanding of the roles of pDCs in HIV infection will help to improve therapeutic strategies to fight HIV infection, and will further increase our knowledge on this important immune cell subset.

Dynamics of Dengue Virus (DENV)-Specific B Cells in the Response to DENV Serotype 1 Infections, Using Flow Cytometry With Labeled Virions

BACKGROUND

The development of reagents to identify and characterize antigen-specific B cells has been challenging.

METHODS

We recently developed Alexa Fluor-labeled dengue viruses (AF DENVs) to characterize antigen-specific B cells in the peripheral blood of DENV-immune individuals.

RESULTS

In this study, we used AF DENV serotype 1 (AF DENV-1) together with AF DENV-2 on peripheral blood mononuclear cells (PBMCs) from children in Thailand with acute primary or secondary DENV-1 infections to analyze the phenotypes of antigen-specific B cells that reflected their exposure or clinical diagnosis. DENV serotype-specific and cross-reactive B cells were identified in PBMCs from all subjects. Frequencies of AF DENV(+) class-switched memory B cells (IgD(-)CD27(+) CD19(+) cells) reached up to 8% during acute infection and early convalescence. AF DENV-labeled B cells expressed high levels of CD27 and CD38 during acute infection, characteristic of plasmablasts, and transitioned into memory B cells (CD38(-)CD27(+)) at the early convalescent time point. There was higher activation of memory B cells early during acute secondary infection, suggesting reactivation from a previous DENV infection.

CONCLUSIONS

AF DENVs reveal changes in the phenotype of DENV serotype-specific and cross-reactive B cells during and after natural DENV infection and could be useful in analysis of the response to DENV vaccination.

Dengue Antibody-Dependent Enhancement: Knowns and Unknowns

Dengue provides the most abundant example in human medicine and the greatest human illness burden caused by the phenomenon of intrinsic antibody-dependent infection enhancement (iADE). In this immunopathological phenomenon infection of monocytes or macrophages using infectious immune complexes suppresses innate antiviral systems, permitting logarithmic intracellular growth of dengue virus. The four dengue viruses evolved from a common ancestor yet retain similar ecology and pathogenicity, but although infection with one virus provides short-term cross-protection against infection with a different type, millions of secondary dengue infections occur worldwide each year. When individuals are infected in the virtual absence of cross-protective dengue antibodies, the dengue vascular permeability syndrome (DVPS) may ensue. This occurs in around 2 to 4% of second heterotypic dengue infections. A complete understanding of the biologic mechanism of iADE, dengue biology, and the mechanism of host responses to dengue infection should lead to a comprehensive and complete understanding of the pathogenesis of DVPS. A crucial emphasis must be placed on understanding ADE. Clinical and epidemiological observations of DVPS define the research questions and provide research parameters. This article will review knowledge related to dengue ADE and point to areas where there has been little research progress. These observations relate to the two stages of dengue illnesses: afferent phenomena are those that promote the success of the microorganism to infect and survive; efferent phenomena are those mounted by the host to inhibit infection and replication and to eliminate the infectious agent and infected tissues. Data will be discussed as "knowns" and "unknowns."

Generation and preclinical immunogenicity study of dengue type 2 virus-like particles derived from stably transfected mosquito cells

Recent phase IIb/III trials of a tetravalent live attenuated vaccine candidate revealed a need for improvement in the stimulation of protective immunity against diseases caused by dengue type 2 virus (DENV-2). Our attempts to develop particulate antigens for possibly supplementing live attenuated virus preparation involve generation and purification of recombinant DENV-2 virus-like particles (VLPs) derived from stably (prM+E)-expressing mosquito cells. Two VLP preparations generated with either negligible or enhanced prM cleavage exhibited different proportions of spherical particles and tubular particles of variable lengths. In BALB/c mice, VLPs were moderately immunogenic, requiring adjuvants for the induction of strong virus neutralizing antibody responses. VLPs with enhanced prM cleavage induced higher levels of neutralizing antibody than those without, but the stimulatory activity of both VLPs was similar in the presence of adjuvants. Comparison of EDIII-binding antibodies in mice following two adjuvanted doses of these VLPs revealed subtle differences in the stimulation of anti-EDIII binding antibodies. In cynomolgus macaques, VLPs with enhanced prM cleavage augmented strongly neutralizing antibody and EDIII-binding antibody responses in live attenuated virus-primed recipients, suggesting that these DENV-2 VLPs may be useful as the boosting antigen in prime-boost immunization. As the levels of neutralizing antibody induced in macaques with the prime-boost immunization were comparable to those infected with wild type virus, this virus-prime VLP-boost regimen may provide an immunization platform in which a need for robust neutralizing antibody response in the protection against DENV-2-associated illnesses could be tested.

Regulation of the Host Antiviral State by Intercellular Communications

Viruses usually induce a profound remodeling of host cells, including the usurpation of host machinery to support their replication and production of virions to invade new cells. Nonetheless, recognition of viruses by the host often triggers innate immune signaling, preventing viral spread and modulating the function of immune cells. It conventionally occurs through production of antiviral factors and cytokines by infected cells. Virtually all viruses have evolved mechanisms to blunt such responses. Importantly, it is becoming increasingly recognized that infected cells also transmit signals to regulate innate immunity in uninfected neighboring cells. These alternative pathways are notably mediated by vesicular secretion of various virus- and host-derived products (miRNAs, RNAs, and proteins) and non-infectious viral particles. In this review, we focus on these newly-described modes of cell-to-cell communications and their impact on neighboring cell functions. The reception of these signals can have anti- and pro-viral impacts, as well as more complex effects in the host such as oncogenesis and inflammation. Therefore, these “broadcasting” functions, which might be tuned by an arms race involving selective evolution driven by either the host or the virus, constitute novel and original regulations of viral infection, either highly localized or systemic.

Replacement of pr gene with Japanese encephalitis virus pr using reverse genetics reduces antibody-dependent enhancement of dengue virus 2 infection

Severe dengue is more likely found during secondary heterologous dengue virus (DENV) infection or primary infection of infants born to dengue-immune mothers and led to the hypothesis of antibody-dependent enhancement (ADE). It has been reported that pre-membrane (prM)-reactive antibodies do not efficiently neutralize DENV infection but instead potently promote ADE infection. Meanwhile, these enhancing anti-prM antibodies mainly react with the precursor (pr) peptide. To evaluate the effect of pr gene substitution on neutralization and ADE of DENV infection, a novel chimeric dengue virus (JEVpr/DENV2) was rationally constructed by replacing the DENV pr gene with Japanese encephalitis virus (JEV) pr gene, based on the full-length infectious complementary DNA (cDNA) clone of DENV2 ZS01/01. We found that chimeric JEVpr/DENV2 showed reduced virulence and good immunogenicity. In addition, anti-JEVpr/DENV2 sera showed broad cross-reactivity and efficient neutralizing activity with all four DENV serotypes and immature DENV2 (ImDENV2). Most importantly, compared with anti-DENV2 sera, anti-JEVpr/DENV2 sera showed significantly reduced enhancing activity of DENV infection in K562 cells. These results suggest that the ADE activities could be reduced by replacing the DENV pr gene with JEV pr gene. These findings may help us better understand the pathogenesis of DENV infection and provide a reference for the development of a vaccine against DENV.

Comprehensive mapping infection-enhancing epitopes of dengue pr protein using polyclonal antibody against prM

Dengue vaccine development is considered a global public health priority, but the antibody-dependent enhancement (ADE) issues have critically restricted vaccine development. Recent findings have demonstrated that pre-membrane (prM) protein was involved in dengue virus (DENV) infection enhancement. Although the importance of prM antibodies have been well characterized, only a few epitopes in DENV prM protein have ever been identified. In this study, we screened five potential linear epitopes located at positions pr1 (1-16aa), pr3 (13-28aa), pr4 (19-34aa), pr9 (49-64aa), and pr10 (55-70aa) in pr protein using peptide scanning and comprehensive bioinformatics analysis. Then, we found that only pr4 (19-34aa) could elicit high-titer antibodies in Balb/c mice, and this epitope could react with sera from DENV2-infected patients, suggesting that specific antibodies against epitope peptide pr4 were elicited in both DENV-infected mice and human. In addition, our data demonstrated that anti-pr4 sera showed limited neutralizing activity but significant ADE activity toward standard DENV serotypes and imDENV. Hence, it seems responsible to hypothesize that anti-pr4 serum was infection-enhancing antibody and pr4 was infection-enhancing epitope. In conclusion, we characterized a novel infection-enhancing epitope on dengue pr protein, a finding that may provide new insight into the pathogenesis of DENV infection and contribute to dengue vaccine design.

Antibodies against immature virions are not a discriminating factor for dengue disease severity

Humoral immunity plays an important role in controlling dengue virus (DENV) infection. Antibodies (Abs) developed during primary infection protect against subsequent infection with the same dengue serotype, but can enhance disease following secondary infection with a heterologous serotype. A DENV virion has two surface proteins, envelope protein E and (pre)-membrane protein (pr)M, and inefficient cleavage of the prM protein during maturation of progeny virions leads to the secretion of immature and partially immature particles. Interestingly, we and others found that historically regarded non-infectious prM-containing DENV particles can become highly infectious in the presence of E- and prM-Abs. Accordingly, we hypothesized that these virions contribute to the exacerbation of disease during secondary infection. Here, we tested this hypothesis and investigated the ability of acute sera of 30 DENV2-infected patients with different grades of disease severity, to bind, neutralize and/or enhance immature DENV2. We found that a significant fraction of serum Abs bind to the prM protein and to immature virions, but we observed no significant difference between the disease severity groups. Furthermore, functional analysis of the Abs did not underscore any specific correlation between the neutralizing/enhancing activity towards immature DENV2 and the development of more severe disease. Based on our analysis of acute sera, we conclude that Abs binding to immature virions are not a discriminating factor in dengue pathogenesis.

The four serotypes of the mosquito-borne dengue virus (DENV) cause an estimated 390 million human infections per annum. Symptomatic infection can manifest itself as a self-limiting febrile illness, dengue fever (DF), or as more severe and potentially life-threatening dengue hemorrhagic fever (DHF), and dengue shock syndrome (DSS). Severe disease development is usually associated with the presence of pre-existing Abs that enhance DENV infection rather than neutralize it. Antibody-dependent enhancement of infection is believed to contribute to high viral loads that prelude the development of severe disease. Indeed, Abs binding to the DENV surface glycoproteins E and prM are known to enhance infection. Here, we studied the role of prM Abs and prM-containing immature virions in the pathogenesis of severe disease. We analyzed the ability of acute sera of DF, DHF and DSS patients to bind, neutralize and/or enhance immature DENV infection. We found that a significant fraction of Abs bind to prM protein of DENV2; however, there was no difference between the disease severity groups. Moreover, we did not observed any specific correlation between the neutralizing/enhancing activity towards immature DENV2 and disease presentation. Based on these data we inferred that prM Abs and immature virions are not a discriminating factor in dengue pathogenesis. These findings are important for the understanding of dengue pathogenesis and the development of new vaccines.

A Multiplexed Cell-Based Assay for the Identification of Modulators of Pre-Membrane Processing as a Target against Dengue Virus

The DenV pre-membrane protein (prM) is a crucial chaperone for the viral envelope protein, preventing premature fusion with vesicles during viral export. prM molecules in immature particles are cleaved by host proteases, leading to mature fusogenic virions. Blockade of prM cleavage would restrict fusion and represents a novel druggable opportunity against DenV. We have thus established a cell-based platform to monitor prM processing that relies on an engineered two-tag scaffold that travels to the cell surface through the secretory pathway. The assay discriminates between a single cell-surface tag when prM is cleaved and two tags when it is not, as detected through fluorescent-coupled antibodies by flow cytometry. The assay, miniaturized into a 96-well plate format, was multiplexed with the HIV-1 envelope boundary, also cleaved in the same pathway. A pilot screen against 1280 compounds was executed, leading to the identification of a potential active and corroborating the robustness of our assay for large-scale screening. We describe for the first time a cell-based assay that monitors DenV prM processing within the classical secretory pathway, which was exploited to identify a potential novel drug against DenV.

Post-translational regulation and modifications of flavivirus structural proteins

Flaviviruses are a group of single-stranded, positive-sense RNA viruses that generally circulate between arthropod vectors and susceptible vertebrate hosts, producing significant human and veterinary disease burdens. Intensive research efforts have broadened our scientific understanding of the replication cycles of these viruses and have revealed several elegant and tightly co-ordinated post-translational modifications that regulate the activity of viral proteins. The three structural proteins in particular - capsid (C), pre-membrane (prM) and envelope (E) - are subjected to strict regulatory modifications as they progress from translation through virus particle assembly and egress. The timing of proteolytic cleavage events at the C-prM junction directly influences the degree of genomic RNA packaging into nascent virions. Proteolytic maturation of prM by host furin during Golgi transit facilitates rearrangement of the E proteins at the virion surface, exposing the fusion loop and thus increasing particle infectivity. Specific interactions between the prM and E proteins are also important for particle assembly, as prM acts as a chaperone, facilitating correct conformational folding of E. It is only once prM/E heterodimers form that these proteins can be secreted efficiently. The addition of branched glycans to the prM and E proteins during virion transit also plays a key role in modulating the rate of secretion, pH sensitivity and infectivity of flavivirus particles. The insights gained from research into post-translational regulation of structural proteins are beginning to be applied in the rational design of improved flavivirus vaccine candidates and make attractive targets for the development of novel therapeutics.

Therapeutic uses of furin and its inhibitors: a patent review

INTRODUCTION

Since the discovery of furin, numerous reports have studied its role in health and diseases, including cancer, inflammatory and infectious diseases. This interest has led to the development of both large protein- and peptide-based inhibitors aiming to control furin activity to treat these disorders. The most recent advances include the development of potent peptidomimetic furin inhibitors, considerably expanding the field of therapeutic applications.

AREA COVERED

In this review, the use of furin or its inhibitors for therapeutic conditions is described through the patent literature since 1994. Only compounds with biological efficacy or augmented properties demonstrated within the patent literature or the associated publications concerning their claimed uses are discussed.

EXPERT OPINION

Considering the diseases that may benefit from furin inhibition, several patents detail the use of the restricted number of furin inhibitors. However, there have been recent reports of new scaffolds, and even the use of furin itself, as a therapeutic agent. Despite considerable evidence of in vivo efficacy, limited confirmation from clinical trials supports or refutes the further use of these compounds in a therapeutic context. The most advanced application is the use of furin knockdown in the generation of an autologous cancer vaccine, which has initiated clinical trials.

Receptors and routes of dengue virus entry into the host cells

Dengue is the most prevalent arthropod-borne viral disease, caused by dengue virus, a member of the Flaviviridae family. Its worldwide incidence is now a major health problem, with 2.5 billion people living in risk areas. In this review, we integrate the structural rearrangements of each viral protein and their functions in all the steps of virus entry into the host cells. We describe in detail the putative receptors and attachment factors in mammalian and mosquito cells, and the recognition of viral immunocomplexes via Fcγ receptor in immune cells. We also discuss that virus internalization might occur through distinct entry pathways, including clathrin-mediated or non-classical clathrin-independent endocytosis, depending on the host cell and virus serotype or strain. The implications of viral maturation in virus entry are also explored. Finally, we discuss the mechanisms of viral genome access to the cytoplasm. This includes the role of low pH-induced conformational changes in the envelope protein that mediate membrane fusion, and original insights raised by our recent work that supports the hypothesis that capsid protein would also be an active player in this process, acting on viral genome translocation into the cytoplasm.

Sensing of immature particles produced by dengue virus infected cells induces an antiviral response by plasmacytoid dendritic cells

Dengue virus (DENV) is the leading cause of mosquito-borne viral illness and death in humans. Like many viruses, DENV has evolved potent mechanisms that abolish the antiviral response within infected cells. Nevertheless, several in vivo studies have demonstrated a key role of the innate immune response in controlling DENV infection and disease progression. Here, we report that sensing of DENV infected cells by plasmacytoid dendritic cells (pDCs) triggers a robust TLR7-dependent production of IFNα, concomitant with additional antiviral responses, including inflammatory cytokine secretion and pDC maturation. We demonstrate that unlike the efficient cell-free transmission of viral infectivity, pDC activation depends on cell-to-cell contact, a feature observed for various cell types and primary cells infected by DENV, as well as West Nile virus, another member of the Flavivirus genus. We show that the sensing of DENV infected cells by pDCs requires viral envelope protein-dependent secretion and transmission of viral RNA. Consistently with the cell-to-cell sensing-dependent pDC activation, we found that DENV structural components are clustered at the interface between pDCs and infected cells. The actin cytoskeleton is pivotal for both this clustering at the contacts and pDC activation, suggesting that this structural network likely contributes to the transmission of viral components to the pDCs. Due to an evolutionarily conserved suboptimal cleavage of the precursor membrane protein (prM), DENV infected cells release uncleaved prM containing-immature particles, which are deficient for membrane fusion function. We demonstrate that cells releasing immature particles trigger pDC IFN response more potently than cells producing fusion-competent mature virus. Altogether, our results imply that immature particles, as a carrier to endolysosome-localized TLR7 sensor, may contribute to regulate the progression of dengue disease by eliciting a strong innate response.

Coupling of replication and assembly in flaviviruses

Flaviviruses affect hundreds of millions of people each year causing tremendous morbidity and mortality worldwide. This genus includes significant human pathogens such as dengue, West Nile, yellow fever, tick-borne encephalitis and Japanese encephalitis virus among many others. The disease caused by these viruses can range from febrile illness to hemorrhagic fever and encephalitis. A deeper understanding of the virus life cycle is required to foster development of antivirals and vaccines, which are an urgent need for many flaviviruses, especially dengue. The focus of this review is to summarize our current knowledge of flaviviral replication and assembly, the proteins and lipids involved therein, and how these processes are coordinated for efficient virus production.

Structural differences observed in arboviruses of the alphavirus and flavivirus genera

Arthropod borne viruses have developed a complex life cycle adapted to alternate between insect and vertebrate hosts. These arthropod-borne viruses belong mainly to the families Togaviridae, Flaviviridae, and Bunyaviridae. This group of viruses contains many pathogens that cause febrile, hemorrhagic, and encephalitic disease or arthritic symptoms which can be persistent. It has been appreciated for many years that these viruses were evolutionarily adapted to function in the highly divergent cellular environments of both insect and mammalian phyla. These viruses are hybrid in nature, containing viral-encoded RNA and proteins which are glycosylated by the host and encapsulate viral nucleocapsids in the context of a host-derived membrane. From a structural perspective, these virus particles are macromolecular machines adapted in design to assemble into a packaging and delivery system for the virus genome and, only when associated with the conditions appropriate for a productive infection, to disassemble and deliver the RNA cargo. It was initially assumed that the structures of the virus from both hosts were equivalent. New evidence that alphaviruses and flaviviruses can exist in more than one conformation postenvelopment will be discussed in this review. The data are limited but should refocus the field of structural biology on the metastable nature of these viruses.

Evaluation of tetravalent and conserved synthetic peptides vaccines derived from Dengue virus Envelope domain I and II

Dengue is a major worldwide public health problem, especially in the tropical and subtropical regions of the world. Primary infection with a single Dengue virus (DENV) serotype causes a mild, self-limiting febrile illness called dengue fever. However, a subset of patients experiencing a secondary infection with a different serotype progress to the severe form of the disease, called dengue hemorrhagic fever. In this study, the vaccine potential of three tetravalent and conserved synthetic peptides derived from DENV envelope domain I (named Pep01) and II (named Pep02 and Pep03) was evaluated. Human dengue IgM/IgG positive serum (n=16) showed reactivity against Pep01, Pep02 and Pep03 in different degrees. Mice immunization experiments showed that these peptides were able to induce a humoral response characterized by antibodies with low neutralizing activity. The spleen cells derived from mice immunized with the peptides showed a significant cytotoxic activity (only for Pep02 and Pep03), a high expression of IL-10 (P<0.01) and a reduced expression of TNF-α and IFN-gamma (P<0.001) compared to DENV-1 infected splenocytes. Thus these peptides, and specially the Pep03, can induce a humoral response characterized by antibodies with low neutralizing activities and probably a T cell response that could be beneficial to induce an effective immune response against all DENV serotypes and do not contributed to the immunopathogenesis. However, further studies in peptide sequence will be required to induce the production of neutralizing antibodies against all four DENV serotypes and also to improve immunogenicity of these peptides.

Identification of a novel infection-enhancing epitope on dengue prM using a dengue cross-reacting monoclonal antibody

BACKGROUND

Dengue virus (DENV) infection is the most important arthropod- borne viral disease in human, but antiviral therapy and approved vaccines remain unavailable due to antibody-dependent enhancement (ADE) phenomenon. Many studies showed that pre-membrane (prM)-specific antibodies do not efficiently neutralize DENV infection but potently promote ADE infection. However, most of the binding epitopes of these antibodies remain unknown.

RESULTS

In the present study, we characterized a DENV cross-reactive monoclonal antibody (mAb), 4D10, that neutralized poorly but potently enhanced infection of four standard DENV serotypes and immature DENV (imDENV) over a broad range of concentration. In addition, the epitope of 4D10 was successfully mapped to amino acid residues 14 to18 of DENV1-4 prM protein using a phage-displayed peptide library and comprehensive bioinformatics analysis. We found that the epitope was DENV serocomplex cross-reactive and showed to be highly immunogenic in Balb/c mice. Furthermore, antibody against epitope peptide PL10, like 4D10, showed broad cross-reactivity and weak neutralizing activtity with four standard DENV serotypes and imDENV but significantly promoted ADE infection. These results suggested 4D10 and anti-PL10 sera were infection-enhancing antibodies and PL10 was infection-enhancing epitope.

CONCLUSIONS

We mapped the epitope of 4D10 to amino acid residues 14 to18 of DENV1-4 prM and found that this epitope was infection-enhancing. These findings may provide significant implications for future vaccine design and facilitate understanding the pathogenesis of DENV infection.

Secondary infection as a risk factor for dengue hemorrhagic fever/dengue shock syndrome: an historical perspective and role of antibody-dependent enhancement of infection

Today, dengue viruses are the most prevalent arthropod-borne viruses in the world. Since the 1960s, numerous reports have identified a second heterologous dengue virus (DENV) infection as a principal risk factor for severe dengue disease (dengue hemorrhagic fever/dengue shock syndrome, DHF/DSS). Modifiers of dengue disease response include the specific sequence of two DENV infections, the interval between infections, and contributions from the human host, such as age, ethnicity, chronic illnesses and genetic background. Antibody-dependent enhancement (ADE) of dengue virus infection has been proposed as the early mechanism underlying DHF/DSS. Dengue cross-reactive antibodies raised following a first dengue infection combine with a second infecting virus to form infectious immune complexes that enter Fc-receptor-bearing cells. This results in an increased number of infected cells and increased viral output per cell. At the late illness stage, high levels of cytokines, possibly the result of T cell elimination of infected cells, result in vascular permeability, leading to shock and death. This review is focused on the etiological role of secondary infections (SI) and mechanisms of ADE.

Molecular mechanisms involved in antibody-dependent enhancement of dengue virus infection in humans

Dengue is the most common arthropod-borne viral infection in humans with ∼50 million cases annually worldwide. In recent decades, a steady increase in the number of severe dengue cases has been seen. Severe dengue disease is most often observed in individuals that have pre-existing immunity against heterotypic dengue subtypes and in infants with low levels of maternal dengue antibodies. The generally accepted hypothesis explaining the immunopathogenesis of severe dengue is called antibody-dependent enhancement of dengue infection. Here, circulating antibodies bind to the newly infecting virus but do not neutralize infection. Rather, these antibodies increase the infected cell mass and virus production. Additionally, antiviral responses are diminished allowing massive virus particle production early in infection. The large infected cell mass and the high viral load are prelude for severe disease development. In this review, we discuss what is known about the trafficking of dengue virus in its human host cells, and the signalling pathways activated after virus detection, both in the absence and presence of antibodies against the virus. This review summarizes work that aims to better understand the complex immunopathogenesis of severe dengue disease.

Flaviviruses and flavivirus vaccines

Several human-pathogenic flaviviruses (including yellow fever, dengue, Japanese encephalitis, West Nile and tick-borne encephalitis viruses) have a significant public health impact in different parts of the world and the potential of emerging in previously non-endemic regions. For some viruses, the structure of the most important immunogen, the envelope protein E, has been determined to atomic resolution by X-ray crystallography, and the architecture of virus particles has been resolved by cryo-electron microscopy. Through the combination of structural and immunological investigations, we now have a detailed understanding of the mechanisms of virus neutralization and antibody-dependent enhancement (ADE) of infectivity at a molecular level. The latter phenomenon has been proposed to play an important role in the immunopathology of severe forms of dengue virus infections (hemorrhagic dengue fever and dengue shock syndrome) and is therefore of special relevance in the context of dengue vaccines. Effective human vaccines are in use for the prophylaxis of yellow fever (live attenuated), Japanese encephalitis (live attenuated and inactivated whole virus), and tick-borne encephalitis (inactivated whole virus). Although dengue is the most important flavivirus with respect to global disease incidence, the development and use of vaccines has been hampered so far by the theoretical risk of vaccine-related adverse events such as immune enhancement of infection and the requirement to induce a long-lasting protective immune response against all four dengue serotypes simultaneously. Currently, several kinds of dengue vaccines are in development, but only one of these candidates (a chimeric dengue-yellow fever live attenuated vaccine) has reached the stage of phase 3 clinical trials.

In vitro quantification of the relative packaging efficiencies of single-stranded RNA molecules by viral capsid protein

While most T=3 single-stranded RNA (ssRNA) viruses package in vivo about 3,000 nucleotides (nt), in vitro experiments have demonstrated that a broad range of RNA lengths can be packaged. Under the right solution conditions, for example, cowpea chlorotic mottle virus (CCMV) capsid protein (CP) has been shown to package RNA molecules whose lengths range from 100 to 10,000 nt. Furthermore, in each case it can package the RNA completely, as long as the mass ratio of CP to nucleic acid in the assembly mixture is 6:1 or higher. Yet the packaging efficiencies of the RNAs can differ widely, as we demonstrate by measurements in which two RNAs compete head-to-head for a limited amount of CP. We show that the relative efficiency depends nonmonotonically on the RNA length, with 3,200 nt being optimum for packaging by the T=3 capsids preferred by CCMV CP. When two RNAs of the same length-and hence the same charge-compete for CP, differences in packaging efficiency are necessarily due to differences in their secondary structures and/or three-dimensional (3D) sizes. For example, the heterologous RNA1 of brome mosaic virus (BMV) is packaged three times more efficiently by CCMV CP than is RNA1 of CCMV, even though the two RNAs have virtually identical lengths. Finally, we show that in an assembly mixture at neutral pH, CP binds reversibly to the RNA and there is a reversible equilibrium between all the various RNA/CP complexes. At acidic pH, excess protein unbinds from RNA/CP complexes and nucleocapsids form irreversibly.

IFITM proteins restrict antibody-dependent enhancement of dengue virus infection

Interferon-inducible transmembrane (IFITM) proteins restrict the entry processes of several pathogenic viruses, including the flaviviruses West Nile virus and dengue virus (DENV). DENV infects cells directly or via antibody-dependent enhancement (ADE) in Fc-receptor-bearing cells, a process thought to contribute to severe disease in a secondary infection. Here we investigated whether ADE-mediated DENV infection bypasses IFITM-mediated restriction or whether IFITM proteins can be protective in a secondary infection. We observed that IFITM proteins restricted ADE-mediated and direct infection with comparable efficiencies in a myelogenous leukemia cell line. Our data suggest that IFITM proteins can contribute to control of secondary DENV infections.

Degrees of maturity: the complex structure and biology of flaviviruses

Flaviviruses are small enveloped virions that enter target cells in a pH-dependent fashion. Virus attachment, entry, and membrane fusion are orchestrated by the envelope (E) and pre-membrane (prM) proteins, the two structural proteins displayed on the surface of virions. Flaviviruses assemble as an immature non-infectious form onto which prM and E form trimeric spikes. During egress from infected cells, flaviviruses undergo dramatic structural changes characterized by the formation of a herringbone arrangement of E proteins that lie flat against the surface of the virion and cleavage of the prM protein by the cellular protease furin. The result is a relatively smooth, infectious mature virion. This dynamic process is now understood in structural detail at the atomic level. However, recent studies indicate that many of the virions released from cells share structural features of both immature and mature virus particles. These mosaic partially mature virions are infectious and interact uniquely with target cells and the host immune response. Here, we will discuss recent advances in our understanding of the biology and significance of partially mature flaviviruses.

Persistence of circulating memory B cell clones with potential for dengue virus disease enhancement for decades following infection

Symptomatic dengue virus infection ranges in disease severity from an influenza-like illness to life-threatening shock. One model of the mechanism underlying severe disease proposes that weakly neutralizing, dengue serotype cross-reactive antibodies induced during a primary infection facilitate virus entry into Fc receptor-bearing cells during a subsequent secondary infection, increasing viral replication and the release of cytokines and vasoactive mediators, culminating in shock. This process has been termed antibody-dependent enhancement of infection and has significantly hindered vaccine development. Much of our understanding of this process has come from studies using mouse monoclonal antibodies (MAbs); however, antibody responses in mice typically exhibit less complexity than those in humans. A better understanding of the humoral immune response to natural dengue virus infection in humans is sorely needed. Using a high-efficiency human hybridoma technology, we isolated 37 hybridomas secreting human MAbs to dengue viruses from 12 subjects years or even decades following primary or secondary infection. The majority of the human antibodies recovered were broadly cross-reactive, directed against either envelope or premembrane proteins, and capable of enhancement of infection in vitro; few exhibited serotype-specific binding or potent neutralizing activity. Memory B cells encoding enhancing antibodies predominated in the circulation, even two or more decades following infection. Mapping the epitopes and activity of naturally occurring dengue antibodies should prove valuable in determining whether the enhancing and neutralizing activity of antibodies can be separated. Such principles could be used in the rational design of vaccines that enhance the induction of neutralizing antibodies, while lowering the risk of dengue shock syndrome.

A cross-protective mAb recognizes a novel epitope within the flavivirus NS1 protein

Despite a resurgence of flavivirus infections worldwide, no approved therapeutic agent exists for any member of the genus. While cross-reactive antibodies with therapeutic potential against flaviviruses have been generated, the majority of them are anti-E antibodies with the potential to cause antibody-dependent enhancement of flavivirus infection and disease. We described previously mAbs against the non-structural NS1 protein of the West Nile virus (WNV) that were protective in mice when administered pre- or post-infection of WNV. Here, we demonstrate that one of these mAbs (16NS1) cross-reacted with Japanese encephalitis virus (JEV) and exhibited protective activity against a lethal JEV infection. Overlapping peptide mapping analysis combined with site-specific mutations identified a novel epitope ¹¹⁶KAWGKSILFA¹²⁵ and critical amino acid residues (¹¹⁸W and ¹²²I) for 16NS1 mAb binding. These results may facilitate the development of a broadly therapeutic mAb that lacks enhancing potential and/or subunit-based vaccine against flaviviruses that target the NS1 protein.