A DENV-2-type-specific monoclonal antibody binds to the DENV-complex-reactive antigenic site on envelope protein domain 3.

The Dengue virus (DENV) envelope (E) protein is the major component of the viral surface and is structurally subdivided into three domains, ED1, ED2 and ED3. ED3 elicits potent neutralizing antibodies and contains two major antigenic sites: the DENV-type-specific and DENV-complex-reactive antigenic sites. Each site is composed of a limited subset of residues that are required for monoclonal antibody (mAb) binding. Here we show that DENV-2-type-specific mAb 9A3D-8 utilizes the functionally critical residues K307, V308, K310, I312, P332, L387, L389 and N390 for ED3 binding. Surprisingly, this DENV-type-specific epitope is predicted to overlap with the ED3 DENV-complex-reactive antigenic site on the viral surface. Further, this unique binding site enables mAb 9A3D-8 to neutralize virus infectivity at relatively low occupancy of virions compared to other ED3 mAbs identified to date. Together, the data in this study indicate that this is a new DENV-2-type-specific antigenic site on ED3.

Locking and blocking the viral landscape of an alphavirus with neutralizing antibodies.

UNLABELLED: Alphaviruses are serious, sometimes lethal human pathogens that belong to the family Togaviridae. The structures of human Venezuelan equine encephalitis virus (VEEV), an alphavirus, in complex with two strongly neutralizing antibody Fab fragments (F5 and 3B4C-4) have been determined using a combination of cryo-electron microscopy and homology modeling. We characterize these monoclonal antibody Fab fragments, which are known to abrogate VEEV infectivity by binding to the E2 (envelope) surface glycoprotein. Both of these antibody Fab fragments cross-link the surface E2 glycoproteins and therefore probably inhibit infectivity by blocking the conformational changes that are required for making the virus fusogenic. The F5 Fab fragment cross-links E2 proteins within one trimeric spike, whereas the 3B4C-4 Fab fragment cross-links E2 proteins from neighboring spikes. Furthermore, F5 probably blocks the receptor-binding site, whereas 3B4C-4 sterically hinders the exposure of the fusion loop at the end of the E2 B-domain. IMPORTANCE: Alphaviral infections are transmitted mainly by mosquitoes. Venezuelan equine encephalitis virus (VEEV) is an alphavirus with a wide distribution across the globe. No effective vaccines exist for alphaviral infections. Therefore, a better understanding of VEEV and its associated neutralizing antibodies will help with the development of effective drugs and vaccines.

Genetic Adaptation by Dengue Virus Serotype 2 to Enhance Infection of Aedes aegypti Mosquito Midguts.

Dengue viruses (DENVs), serotypes 1-4, are arthropod-borne viruses transmitted to humans by mosquitoes, primarily Aedes aegypti. The transmission cycle begins when Ae. aegypti ingest blood from a viremic human and the virus infects midgut epithelial cells. In studying viruses derived from the DENV2 infectious clone 30P-NBX, we found that when the virus was delivered to female Ae. aegypti in an infectious blood meal, the midgut infection rate (MIR) was very low. To determine if adaptive mutations in the DENV2 envelope (E) glycoprotein could be induced to increase the MIR, we serially passed 30P-NBX in Ae. aegypti midguts. After four passages, a single, non-conservative mutation in E protein domain II (DII) nucleotide position 1300 became dominant, resulting in replacement of positively-charged amino acid lysine (K) at position 122 with negatively-charged glutamic acid (E; K122E) and a significantly-enhanced MIR. Site directed mutagenesis experiments showed that reducing the positive charge of this surface-exposed region of the E protein DII correlated with improved Ae. aegypti midgut infection.

Exposing cryptic epitopes on the Venezuelan equine encephalitis virus E1 glycoprotein prior to treatment with alphavirus cross-reactive monoclonal antibody allows blockage of replication early in infection.

Eastern equine encephalitis virus (EEEV), western equine encephalitis virus (WEEV) and Venezuelan equine encephalitis virus (VEEV) can cause fatal encephalitis in humans and equids. Some MAbs to the E1 glycoprotein are known to be cross-reactive, weakly neutralizing in vitro but can protect from disease in animal models. We investigated the mechanism of neutralization of VEEV infection by the broadly cross-reactive E1-specific MAb 1A4B-6. 1A4B-6 protected 3-week-old Swiss Webster mice prophylactically from lethal VEEV challenge. Likewise, 1A4B-6 inhibited virus growth in vitro at a pre-attachment step after virions were incubated at 37 °C and inhibited virus-mediated cell fusion. Amino acid residue N100 in the fusion loop of E1 protein was identified as critical for binding. The potential to elicit broadly cross-reactive MAbs with limited virus neutralizing activity in vitro but that can inhibit virus entry and protect animals from infection merits further exploration for vaccine and therapeutic developmental research.

A Monoclonal Antibody Specific for Japanese Encephalitis Virus with High Neutralizing Capability for Inclusion as a Positive Control in Diagnostic Neutralization Tests.

Japanese encephalitis virus (JEV) is the most common cause of viral encephalitis in Asia, and it is increasingly a global public health concern because of its recent geographic expansion. Although commercial vaccines are available and used in some endemic countries, JEV continues to cause illness, with more than 60,000 cases reported annually. To develop a reproducible positive control antibody useable in diagnosis of JEV infections, murine hybridomas were developed from mice inoculated with a combination of IXIARO JEV vaccine and JEV domain III of the envelope protein (E-DIII). Monoclonal antibodies (MAbs) were characterized for their ability to neutralize virus in vitro. Monoclonal antibody 17BD3-2 was found to be JEV specific and highly neutralizing, with a plaque reduction neutralization test (PRNT)90 endpoint titer of 1.25 µg/mL. The functional epitopes were mapped using virus neutralization escape variants to amino acid residues S309, K312, and G333 in E-DIII. This MAb may be substituted for human immune sera used as a positive control in PRNT for distribution to public health laboratories worldwide in potential future outbreaks of JEV.

Type- and subcomplex-specific neutralizing antibodies against domain III of dengue virus type 2 envelope protein recognize adjacent epitopes.

Neutralization of flaviviruses in vivo correlates with the development of an antibody response against the viral envelope (E) protein. Previous studies demonstrated that monoclonal antibodies (MAbs) against an epitope on the lateral ridge of domain III (DIII) of the West Nile virus (WNV) E protein strongly protect against infection in animals. Based on X-ray crystallography and sequence analysis, an analogous type-specific neutralizing epitope for individual serotypes of the related flavivirus dengue virus (DENV) was hypothesized. Using yeast surface display of DIII variants, we defined contact residues of a panel of type-specific, subcomplex-specific, and cross-reactive MAbs that recognize DIII of DENV type 2 (DENV-2) and have different neutralizing potentials. Type-specific MAbs with neutralizing activity against DENV-2 localized to a sequence-unique epitope on the lateral ridge of DIII, centered at the FG loop near residues E383 and P384, analogous in position to that observed with WNV-specific strongly neutralizing MAbs. Subcomplex-specific MAbs that bound some but not all DENV serotypes and neutralized DENV-2 infection recognized an adjacent epitope centered on the connecting A strand of DIII at residues K305, K307, and K310. In contrast, several MAbs that had poor neutralizing activity against DENV-2 and cross-reacted with all DENV serotypes and other flaviviruses recognized an epitope with residues in the AB loop of DIII, a conserved region that is predicted to have limited accessibility on the mature virion. Overall, our experiments define adjacent and structurally distinct epitopes on DIII of DENV-2 which elicit type-specific, subcomplex-specific, and cross-reactive antibodies with different neutralizing potentials.

Guidelines for Plaque-Reduction Neutralization Testing of Human Antibodies to Dengue Viruses.

Through the Advisory Committee on Dengue and other Flavivirus Vaccines, the World Health Organization(WHO) has had a long-standing commitment to facilitate and to guide research and development of vaccines for medically important flaviviruses. Recently, the Paediatric Dengue Vaccine Initiative (PDVI) was formed to accelerate the development, testing, and introduction of dengue (DEN)vaccines worldwide, partnering with WHO in this important public health effort. There are now a variety of DEN vaccines in various stages of the developmental pipeline. In an attempt to make interlaboratory information more directly comparable, WHO with the support of PDVI initiated a program to coordinate the procedures used for the plaque-reduction neutralization test (PRNT). ThePRNT is the most common assay used to measure neutralizing antibody. The presence of antibody is believed to be most relevant means of determining protective anti-DEN virus (DENV) immunity. While other neutralizing antibody assays are being considered for use in large-scale vaccine field trials, the PRNT is still considered to be the laboratory standard against which other neutralizing antibody assays should be compared. The need for PRNT coordination has been identified at several consultations between the WHO and PDVI. A more complete version of these guidelines is available on the WHO website: http://www.who.int/immunization/documents/date/en/index.html.

A humanized monoclonal antibody neutralizes yellow fever virus strain 17D-204 in vitro but does not protect a mouse model from disease.

The yellow fever virus (YFV) vaccine 17D-204 is considered safe and effective, yet rare severe adverse events (SAEs), some resulting in death, have been documented following vaccination. Individuals exhibiting post-vaccinal SAEs are ideal candidates for antiviral monoclonal antibody (MAb) therapy; the time until appearance of clinical signs post-exposure is usually short and patients are quickly hospitalized. We previously developed a murine-human chimeric monoclonal antibody (cMAb), 2C9-cIgG, reactive with both virulent YFV and 17D-204, and demonstrated its ability to prevent and treat YF disease in both AG129 mouse and hamster models of infection. To counteract possible selection of 17D-204 variants that escape neutralization by treatment with a single MAb (2C9-cIgG), we developed a second cMAb, 864-cIgG, for use in combination with 2C9-cIgG in post-vaccinal therapy. MAb 864-cIgG recognizes/neutralizes only YFV 17D-204 vaccine substrain and binds to domain III (DIII) of the viral envelope protein, which is different from the YFV type-specific binding site of 2C9-cIgG in DII. Although it neutralized 17D-204 in vitro, administration of 864-cIgG had no protective capacity in the interferon receptor-deficient AG129 mouse model of 17D-204 infection. The data presented here show that although DIII-specific 864-cIgG neutralizes virus infectivity in vitro, it does not have the ability to abrogate disease in vivo. Therefore, combination of 864-cIgG with 2C9-cIgG for treatment of YF vaccination SAEs does not appear to provide an improvement on 2C9-cIgG therapy alone.

Next generation dengue vaccines: A review of the preclinical development pipeline.

Dengue represents a significant and growing public health problem across the globe, with approximately half of the world's population at risk. The increasing and expanding burden of dengue has highlighted the need for new tools to prevent dengue, including development of dengue vaccines. Recently, the first dengue vaccine candidate was evaluated in Phase 3 clinical trials, and other vaccine candidates are under clinical evaluation. There are also a number of candidates in preclinical development, based on diverse technologies, with promising results in animal models and likely to move into clinical trials and could eventually be next-generation dengue vaccines. This review provides an overview of the various technological approaches to dengue vaccine development with specific focus on candidates in preclinical development and with evaluation in non-human primates.

Construction of yellow fever/St. Louis encephalitis chimeric virus and the use of chimeras as a diagnostic tool.

St. Louis encephalitis (SLE) and West Nile (WN) flaviviruses are genetically closely related and cocirculate in the United States. Virus neutralization tests provide the most specific means for serodiagnosis of infections with these viruses. However, use of wild-type SLE and WN viral strains for laboratory testing is constrained by the biocontainment requirements. We constructed two highly attenuated yellow fever (YF) virus chimeras that contain the premembrane-envelope (prM-E) protein genes from the virulent MSI-7 (isolated in the United States) or the naturally attenuated CorAn9124 (Argentina) SLE strains. The YF/SLE (CorAn version) virus and the previously constructed YF/WN chimera were shown to specifically distinguish between confirmed human SLE and WN cases in a virus neutralization test using patient sera. These chimeras have the potential for use as diagnostic reagents and vaccines against SLE and WN.

Development of a small animal peripheral challenge model of Japanese encephalitis virus using interferon deficient AG129 mice and the SA14-14-2 vaccine virus strain.

Japanese encephalitis virus (JEV) is the most common cause of viral encephalitis in Asia, and it is increasingly a global public health concern due to its recent geographic expansion. While commercial vaccines are available and used in some endemic countries, JEV continues to be a public health problem, with 50,000 cases reported annually. Research with virulent JEV in mouse models to develop new methods of prevention and treatment is restricted to BSL-3 containment facilities, confining these studies to investigators with access to these facilities. We have developed an adult small animal peripheral challenge model using interferon-deficient AG129 mice and the JEV live-attenuated vaccine SA14-14-2, thus requiring only BSL-2 containment. A low dose of virus (10PFU/0.1ml) induced 100% morbidity in infected mice. Increased body temperatures measured by implantable temperature transponders correlated with an increase in infectious virus and viral RNA in serum, spleen and brain as well as an increase in pro-inflammatory markers measured by a 58-biomarker multi-analyte profile (MAP) constructed during the course of infection. In the future, the MAP measurements can be used as a baseline for comparison in order to better assess the inhibition of disease progression by other prophylactic and therapeutic agents. The use of the AG129/JEV SA14-14-2 animal model makes vaccine and therapeutic studies feasible for laboratories with limited biocontainment facilities.

Humanized monoclonal antibody 2C9-cIgG has enhanced efficacy for yellow fever prophylaxis and therapy in an immunocompetent animal model.

Yellow fever virus (YFV) causes significant human disease and mortality in tropical regions of South and Central America and Africa, despite the availability of an effective vaccine. No specific therapy for YF is available. We previously showed that the humanized monoclonal antibody (MAb) 2C9-cIgG provided prophylactic and therapeutic protection from mortality in interferon receptor-deficient strain AG129 mice challenged with YF 17D-204 vaccine. In this study we tested the prophylactic and therapeutic efficacy of this MAb against virulent YFV infection in an immunocompetent hamster model. Intraperitoneal (ip) administration of a single dose of MAb 2C9-cIgG 24h prior to YFV challenge resulted in significantly improved survival rates in animals treated with 380 or 38 µg of MAb compared to untreated animals. Treatment with the higher dose also resulted in significantly improved weight gain and reductions in serum alanine aminotransferase (ALT) and virus titers in serum and liver. Prophylactic treatment with 2C9-cIgG 24h prior to virus challenge prevented the development of a virus-neutralizing antibody (vnAb) response in hamsters. Administration of a single ip dose of 380 µg of 2C9-cIgG as late as 72 h post-YFV challenge also resulted in significant improvement in survival rates. Hamsters treated at 4-72 h post-virus challenge developed a robust vnAb response. Enhanced survival and improvement of various disease parameters in the hamster model when MAb 2C9-cIgG is administered up to 3 days after virus challenge demonstrate the clinical potential of specific antibody therapy for YF.

Molecular determinants of dengue virus 2 envelope protein important for virus entry in FcγRIIA-mediated antibody-dependent enhancement of infection.

Antibody-dependent enhancement (ADE) of infection may cause severe illness in patients suffering a secondary infection by a heterologous dengue virus (DENV) serotype. During ADE of infection, cross-reactive non- or poorly-neutralizing antibodies form infectious virus-Ab complexes with the newly infecting serotype and enhance virus infection by binding to the Fcγ receptors (FcγR) on FcγR-bearing cells. In this study, we determined that molecular determinants of DENV2 envelope protein critical for virus entry during non-ADE infection are also required for ADE infection mediated by FcγRIIA, and binding of virus-Ab complexes with FcγRIIA alone is not sufficient for ADE of infection. The FcγRIIA mainly plays an auxiliary role in concentrating the virus-Ab complex to the cell surface, and other primary cellular receptors are required for virus entry. Understanding the viral entry pathway in ADE of DENV infection will greatly facilitate rational designs of anti-viral therapeutics against severe dengue disease associated with ADE.

West nile virus in the United States - a historical perspective.

Prior to 1999, West Nile virus (WNV) was a bit player in the screenplay of global vector-borne viral diseases. First discovered in the West Nile District of Uganda in 1937, this Culex sp.-transmitted virus was known for causing small human febrile outbreaks in Africa and the Middle East. Prior to 1995, the last major human WNV outbreak was in the 1950s in Israel. The epidemiology and ecology of WNV began to change in the mid-1990s when an epidemic of human encephalitis occurred in Romania. The introduction of WNV into Eastern Europe was readily explained by bird migration between Africa and Europe. The movement of WNV from Africa to Europe could not, however, predict its surprising jump across the Atlantic Ocean to New York City and the surrounding areas of the United States (U.S.). This movement of WNV from the Eastern to Western Hemisphere in 1999, and its subsequent dissemination throughout two continents in less than ten years is widely recognized as one of the most significant events in arbovirology during the last two centuries. This paper documents the early events of the introduction into and the spread of WNV in the Western Hemisphere.

Long-term safety assessment of live attenuated tetravalent dengue vaccines: deliberations from a WHO technical consultation.

Dengue is a rapidly growing public health threat with approximately 2.5 billion people estimated to be at risk. Several vaccine candidates are at various stages of pre-clinical and clinical development. Thus far, live dengue vaccine candidates have been administered to several thousands of volunteers and were well-tolerated, with minimal short-term safety effects reported in Phase I and Phase II clinical trials. Based on the natural history of dengue, a theoretical possibility of an increased risk of severe dengue as a consequence of vaccination has been hypothesized but not yet observed. In October 2011, the World Health Organization (WHO) convened a consultation of experts in dengue, vaccine regulation and vaccine safety to review the current scientific evidence regarding safety concerns associated with live attenuated dengue vaccines and, in particular, to consider methodological approaches for their long-term evaluation. In this paper we summarize the scientific background and methodological considerations relevant to the safety assessment of these vaccines. Careful planning and a coordinated approach to safety assessment are recommended to ensure adequate long-term evaluation of dengue vaccines that will support their introduction and continued use.

Mutation of the dengue virus type 2 envelope protein heparan sulfate binding sites or the domain III lateral ridge blocks replication in Vero cells prior to membrane fusion.

Using an infectious cDNA clone we engineered seven mutations in the putative heparan sulfate- and receptor-binding motifs of the envelope protein of dengue virus serotype 2, strain 16681. Four mutant viruses, KK122/123EE, E202K, G304K, and KKK305/307/310EEE, were recovered following transfection of C6/36 cells. A fifth mutant, KK291/295EE, was recovered from C6/36 cells with a compensatory E295V mutation. All mutants grew in and mediated fusion of virus-infected C6/36 cells, but three of the mutants, KK122/123EE, E202K, G304K, did not grow in Vero cells without further modification. Two Vero cell lethal mutants, KK291/295EV and KKK307/307/310EEE, failed to replicate in DC-SIGN-transformed Raji cells and did not react with monoclonal antibodies known to block DENV attachment to Vero cells. Additionally, both mutants were unable to initiate negative-strand vRNA synthesis in Vero cells by 72h post-infection, suggesting that the replication block occurred prior to virus-mediated membrane fusion.

Mutations in the West Nile prM protein affect VLP and virion secretion in vitro.

Mutation of the West Nile virus-like particle (WN VLP) prM protein (T20D, K31A, K31V, or K31T) results in undetectable VLP secretion from transformed COS-1 cells. K31 mutants formed intracellular prM-E heterodimers; however these proteins remained in the ER and ER-Golgi intermediary compartments of transfected cells. The T20D mutation affected glycosylation, heterodimer formation, and WN VLP secretion. When infectious viruses bearing the same mutations were used to infect COS-1 cells, K31 mutant viruses exhibited delayed growth and reduced infectivity compared to WT virus. Epitope maps of WN VLP and WNV prM were also different. These results suggest that while mutations in the prM protein can reduce or eliminate secretion of WN VLPs, they have less effect on virus. This difference may be due to the quantity of prM in WN VLPs compared to WNV or to differences in maturation, structure, and symmetry of these particles.

A small animal peripheral challenge model of yellow fever using interferon-receptor deficient mice and the 17D-204 vaccine strain.

Yellow fever virus (YFV), a member of the genus Flavivirus, is a mosquito-borne pathogen that requires wild-type (wt), virulent strains to be handled at biosafety level (BSL) 3, with HEPA-filtration of room air exhaust (BSL3+). YFV is found in tropical regions of Africa and South America and causes severe hepatic disease and death in humans. Despite the availability of effective vaccines (17D-204 or 17DD), YFV is still responsible for an estimated 200,000 cases of illness and 30,000 deaths annually. Besides vaccination, there are no other prophylactic or therapeutic strategies approved for use in human YF. Current small animal models of YF require either intra-cranial inoculation of YF vaccine to establish infection, or use of wt strains (e.g., Asibi) in order to achieve pathology. We have developed and characterized a BSL2, adult mouse peripheral challenge model for YFV infection in mice lacking receptors for interferons α, ß, and γ (strain AG129). Intraperitoneal challenge of AG129 mice with 17D-204 is a uniformly lethal in a dose-dependent manner, and 17D-204-infected AG129 mice exhibit high viral titers in both brain and liver suggesting this infection is both neurotropic and viscerotropic. Furthermore the use of a mouse model permitted the construction of a 59-biomarker multi-analyte profile (MAP) using samples of brain, liver, and serum taken at multiple time points over the course of infection. This MAP serves as a baseline for evaluating novel therapeutics and their effect on disease progression. Changes (4-fold or greater) in serum and tissue levels of pro- and anti-inflammatory mediators as well as other factors associated with tissue damage were noted in AG129 mice infected with 17D-204 as compared to mock-infected control animals.

A humanized IgG but not IgM antibody is effective in prophylaxis and therapy of yellow fever infection in an AG129/17D-204 peripheral challenge mouse model.

Yellow fever virus (YFV), a member of the genus Flavivirus, is a mosquito-borne virus found in tropical regions of Africa and South America that causes severe hepatic disease and death in humans. Despite the availability of effective vaccines, YFV is responsible for an estimated 200,000 cases and 30,000 deaths annually. There are currently no prophylactic or therapeutic strategies approved for use in human YFV infections. Furthermore, implementation of YFV 17D-204 vaccination campaigns has become problematic due to an increase in reported post-vaccinal adverse events. We have created human/murine chimeric MAbs of a YFV-reactive murine monoclonal antibody (mMAb), 2C9, that was previously shown to protect mice from lethal YFV infection and to have therapeutic activity. The new chimeric (cMAbs) were constructed by fusion of the m2C9 IgG gene variable regions with the constant regions of human IgG and IgM and expressed in Sp2 murine myelomas. The 2C9 cMAbs (2C9-cIgG and 2C9-cIgM) reacted with 17D-204 vaccine strain in an enzyme-linked immunosorbent assay and neutralized virus in vitro similarly to the parent m2C9. Both m2C9 and 2C9-cIgG when administered prophylactically 24h prior to infection protected AG129 mice from peripheral 17D-204 challenge at antibody concentrations ≥1.27 µg/mouse; however, the 2C9-cIgM did not protect even at a dose of 127 µg/mouse. The 17D-204 infection of AG129 mice is otherwise uniformly lethal. While the m2C9 was shown previously to be therapeutically effective in YFV-infected BALB/c mice at day 4 post-infection, the m2C9 and 2C9-cIgG demonstrated therapeutic activity only when administered 1 day post-infection in 17D-204-infected AG129 mice.