Complexity of Human Antibody Response to Dengue Virus: Implication for Vaccine Development

Detection of anti-premembrane antibody as a specific marker of four flavivirus serocomplexes and its application to serosurveillance in endemic regions

In the past few decades, several emerging/re-emerging mosquito-borne flaviviruses have resulted in disease outbreaks of public health concern in the tropics and subtropics. Due to cross-reactivities of antibodies recognizing the envelope protein of different flaviviruses, serosurveillance remains a challenge. Previously we reported that anti-premembrane (prM) antibody can discriminate between three flavivirus infections by Western blot analysis. In this study, we aimed to develop a serological assay that can discriminate infection or exposure with flaviviruses from four serocomplexes, including dengue (DENV), Zika (ZIKV), West Nile (WNV) and yellow fever (YFV) viruses, and explore its application for serosurveillance in flavivirus-endemic countries. We employed Western blot analysis including antigens of six flaviviruses (DENV1, 2 and 4, WNV, ZIKV and YFV) from four serocomplexes. We tested serum samples from YF-17D vaccinees, and from DENV, ZIKV and WNV panels that had been confirmed by RT-PCR or by neutralization assays. The overall sensitivity/specificity of anti-prM antibodies for DENV, ZIKV, WNV, and YFV infections/exposure were 91.7%/96.4%, 91.7%/99.2%, 88.9%/98.3%, and 91.3%/92.5%, respectively. When testing 48 samples from Brazil, we identified multiple flavivirus infections/exposure including DENV and ZIKV, DENV and YFV, and DENV, ZIKV and YFV. When testing 50 samples from the Philippines, we detected DENV, ZIKV, and DENV and ZIKV infections with a ZIKV seroprevalence rate of 10%, which was consistent with reports of low-level circulation of ZIKV in Asia. Together, these findings suggest that anti-prM antibody is a flavivirus serocomplex-specific marker and can be employed to delineate four flavivirus infections/exposure in regions where multiple flaviviruses co-circulate.

Dominant epitopes of cross-reactive anti-domain III human antibody response change from early to late convalescence of infection with dengue virus

Cross-neutralizing activity of human antibody response against Dengue virus complex (DENV) changes importantly over time. Domain III (DIII) of the envelope protein of DENV elicits a potently neutralizing and mostly type-specific IgG response. We used sera from 24 individuals from early- or late convalescence of DENV1 infection to investigate the evolution of anti-DIII human IgG with the time lapse since the infection. We evaluated the correlation between the serotype-specific reactivity against recombinant DIII proteins and the neutralization capacity against the four serotypes, and examined its behavior with the time of convalescence. Also, we use a library of 71 alanine mutants of surface-exposed amino acid residues to investigate the dominant epitopes. In early convalescence anti-DIII titers and potency of virus neutralization were positively associated with correlation coefficients from 0.82 to 1.0 for the four serotypes. For late convalescence, a positive correlation (r = 0.69) was found only for DENV1. The dominant epitope of the type-specific response is centered in the FG-loop (G383, E384, and K385) and includes most of the lateral ridge. The dominant epitope of the anti-DIII cross-reactive IgG in secondary infections shifts from the A-strand during early convalescence to a site centered in residues E314-H317 of the AB-loop and I352-E368 of the DI/DIII interface, in late convalescence. An immunoassay based on the detection of IgG anti-DIII response can be implemented for detection of infecting serotype in diagnosis of DENV infection, either primary or secondary. Human dominant epitopes of the cross-reactive circulating antibodies change with time of convalescence.

Detection of anti-premembrane antibody as a specific marker of four flavivirus serocomplexes and its application to serosurveillance in endemic regions

In the past few decades, several emerging/re-emerging mosquito-borne flaviviruses have resulted in disease outbreaks of public health concern in the tropics and subtropics. Due to cross-reactivities of antibodies recognizing the envelope protein of different flaviviruses, serosurveillance remains a challenge. Previously we reported that anti-premembrane (prM) antibody can discriminate between three flavivirus infections by Western blot analysis. In this study, we aimed to develop a serological assay that can discriminate infection or exposure with flaviviruses from four serocomplexes, including dengue (DENV), Zika (ZIKV), West Nile (WNV) and yellow fever (YFV) viruses, and explore its application for serosurveillance in flavivirus-endemic countries. We employed Western blot analysis including antigens of six flaviviruses (DENV1, 2 and 4, WNV, ZIKV and YFV) from four serocomplexes. We tested serum samples from YF-17D vaccinees, and from DENV, ZIKV and WNV panels that had been confirmed by RT-PCR or by neutralization assays. The overall sensitivity/specificity of anti-prM antibodies for DENV, ZIKV, WNV, and YFV infections/exposure were 91.7%/96.4%, 91.7%/99.2%, 88.9%/98.3%, and 91.3%/92.5%, respectively. When testing 48 samples from Brazil, we identified multiple flavivirus infections/exposure including DENV and ZIKV, DENV and YFV, and DENV, ZIKV and YFV. When testing 50 samples from the Philippines, we detected DENV, ZIKV, and DENV and ZIKV infections with a ZIKV seroprevalence rate of 10%, which was consistent with reports of low-level circulation of ZIKV in Asia. Together, these findings suggest that anti-prM antibody is a flavivirus serocomplex-specific marker and can be employed to delineate four flavivirus infections/exposure in regions where multiple flaviviruses co-circulate.

Blockade-of-Binding Activities toward Envelope-Associated, Type-Specific Epitopes as a Correlative Marker for Dengue Virus-Neutralizing Antibody

Humans infected with dengue virus (DENV) acquire long-term protection against the infecting serotype, whereas cross-protection against other serotypes is short-lived. Long-term protection induced by low levels of type-specific neutralizing antibodies can be assessed using the virus-neutralizing antibody test. However, this test is laborious and time-consuming. In this study, a blockade-of-binding enzyme-linked immunoassay was developed to assess antibody activity by using a set of neutralizing anti-E monoclonal antibodies and blood samples from dengue virus-infected or -immunized macaques. Diluted blood samples were incubated with plate-bound dengue virus particles before the addition of an enzyme-conjugated antibody specific to the epitope of interest. Based on blocking reference curves constructed using autologous purified antibodies, sample blocking activity was determined as the relative concentration of unconjugated antibody that resulted in the same percent signal reduction. In separate DENV-1-, -2-, -3-, and -4-related sets of samples, moderate to strong correlations of the blocking activity with neutralizing antibody titers were found with the four type-specific antibodies 1F4, 3H5, 8A1, and 5H2, respectively. Significant correlations were observed for single samples taken 1 month after infection as well as samples drawn before and at various time points after infection/immunization. Similar testing using a cross-reactive EDE-1 antibody revealed a moderate correlation between the blocking activity and the neutralizing antibody titer only for the DENV-2-related set. The potential usefulness of the blockade-of-binding activity as a correlative marker of neutralizing antibodies against dengue viruses needs to be validated in humans. IMPORTANCE This study describes a blockade-of-binding assay for the determination of antibodies that recognize a selected set of serotype-specific or group-reactive epitopes in the envelope of dengue virus. By employing blood samples collected from dengue virus-infected or -immunized macaques, moderate to strong correlations of the epitope-blocking activities with the virus-neutralizing antibody titers were observed with serotype-specific blocking activities for each of the four dengue serotypes. This simple, rapid, and less laborious method should be useful for the evaluation of antibody responses to dengue virus infection and may serve as, or be a component of, an in vitro correlate of protection against dengue in the future.

Comparing the performance of dengue virus IgG and IgG-capture enzyme-linked immunosorbent assays in seroprevalence study

BACKGROUND

Dengue virus (DENV) is the leading cause of arboviral diseases in humans worldwide. Currently Dengvaxia, the first dengue vaccine licensed in 20 countries, was recommended for DENV seropositive individuals aged 9-45 years. Studying dengue seroprevalence can improve our understanding of the epidemiology and transmission dynamics of DENV, and facilitate future intervention strategies and assessment of vaccine efficacy. Several DENV envelope protein-based serological tests including IgG and IgG-capture enzyme-linked immunosorbent assays (ELISAs) have been employed in seroprevalence studies. Previously DENV IgG-capture ELISA was reported to distinguish primary and secondary DENV infections during early convalescence, however, its performance over time and in seroprevalence study remains understudied.

METHODS

In this study, we used well-documented neutralization test- or reverse-transcription-polymerase-chain reaction-confirmed serum/plasma samples including DENV-naïve, primary and secondary DENV, primary West Nile virus, primary Zika virus, and Zika with previous DENV infection panels to compare the performance of three ELISAs.

RESULTS

The sensitivity of the InBios IgG ELISA was higher than that of InBios IgG-capture and SD IgG-capture ELISAs. The sensitivity of IgG-capture ELISAs was higher for secondary than primary DENV infection panel. Within the secondary DENV infection panel, the sensitivity of InBios IgG-capture ELISA decreased from 77.8% at < 6 months to 41.7% at 1-1.5 years, 28.6% at 2-15 years and 0% at > 20 years (p < 0.001, Cochran-Armitage test for trend), whereas that of IgG ELISA remains 100%. A similar trend was observed for SD IgG-capture ELISA.

CONCLUSIONS

Our findings demonstrate higher sensitivity of DENV IgG ELISA than IgG-capture ELISA in seroprevalence study and interpretation of DENV IgG-capture ELISA should take sampling time and primary or secondary DENV infection into consideration.

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.

Clinical Safety Experience of TAK-003 for Dengue Fever: A New Tetravalent Live Attenuated Vaccine Candidate

BACKGROUND

An unmet medical need remains for an effective dengue tetravalent vaccine that can be administered irrespective of previous dengue exposure. TAK-003, a dengue tetravalent vaccine, has demonstrated efficacy in an ongoing phase 3 trial in children and adolescents living in dengue-endemic areas, with an acceptable safety profile in both dengue-naive and dengue-exposed individuals.

METHODS

Safety findings are presented herein from an integrated analysis of data for healthy 4-60-year-olds from two phase 2 and three phase 3 double-blind, placebo-controlled clinical trials of TAK-003 (TAK-003, n = 14 627; placebo, n = 7167). Safety evaluation included analyses of postinjection reactogenicity, unsolicited adverse events (AEs), serious AEs (SAEs), and deaths. Subgroup analyses were performed by age group, baseline serostatus, and gender.

RESULTS

The most common local and systemic AEs were injection site pain (43% for TAK-003 and 26% for placebo) and headache (34% and 30%, respectively). Injection site AEs were mostly mild and resolved within 1-3 days. Unsolicited AEs and AEs leading to discontinuation occurred with similar frequency across both groups, while SAEs were fewer for TAK-003 recipients (6% vs 8% for placebo). Four of the 5 vaccine-related SAEs (which included hypersensitivity, dengue fever, and dengue hemorrhagic fever) occurred in the placebo group. No deaths were considered vaccine-related. Subgroup analyses showed no differences in safety by baseline serostatus or by gender, albeit analysis by age indicated greater local reactogenicity rates for adolescents (46% for TAK-003 and 28% for placebo) and adults (56% and 19%, respectively) than for children (37% and 25%, respectively).

CONCLUSIONS

No important safety risks were identified, and TAK-003 was well tolerated irrespective of age, gender, or baseline dengue serostatus in recipients aged 4-60 years.

Beneath the surface: Amino acid variation underlying two decades of dengue virus antigenic dynamics in Bangkok, Thailand

Neutralizing antibodies are important correlates of protection against dengue. Yet, determinants of variation in neutralization across strains within the four dengue virus serotypes (DENV1-4) is imperfectly understood. Studies focus on structural DENV proteins, especially the envelope (E), the primary target of anti-DENV antibodies. Although changes in immune recognition (antigenicity) are often attributed to variation in epitope residues, viral processes influencing conformation and epitope accessibility also affect neutralizability, suggesting possible modulating roles of nonstructural proteins. We estimated effects of residue changes in all 10 DENV proteins on antigenic distances between 348 DENV collected from individuals living in Bangkok, Thailand (1994-2014). Antigenic distances were derived from response of each virus to a panel of twenty non-human primate antisera. Across 100 estimations, excluding 10% of virus pairs each time, 77 of 295 positions with residue variability in E consistently conferred antigenic effects; 52 were within ±3 sites of known binding sites of neutralizing human monoclonal antibodies, exceeding expectations from random assignments of effects to sites (p = 0.037). Effects were also identified for 16 sites on the stem/anchor of E which were only recently shown to become exposed under physiological conditions. For all proteins, except nonstructural protein 2A (NS2A), root-mean-squared-error (RMSE) in predicting distances between pairs held out in each estimation did not outperform sequences of equal length derived from all proteins or E, suggesting that antigenic signals present were likely through linkage with E. Adjusted for E, we identified 62/219 sites embedding the excess signals in NS2A. Concatenating these sites to E additionally explained 3.4% to 4.0% of observed variance in antigenic distances compared to E alone (50.5% to 50.8%); RMSE outperformed concatenating E with sites from any protein of the virus (ΔRMSE, 95%IQR: 0.01, 0.05). Our results support examining antigenic determinants beyond the DENV surface.

Dengue viruses, even of the same serotype, are differentially recognized by preexisting antibodies of individuals. With antibody levels being an important indicator of infection risk and pathogenicity, understanding mechanisms underlying these differences are crucial for vaccine design and development. Investigations have primarily targeted surface regions of the envelope protein (E) where virus-antibody interactions were thought to primarily occur. However, the roles of non-surface regions of the E protein as well as nonstructural proteins has been limited. We looked at the entire virus to identify associations between specific changes in the protein sequence and differences in how viruses were recognized by antibodies. In addition to recovering known determinants on the surface, we found signals in other areas on the structural building blocks of the virus. We also identified additional signals on specific areas of a protein that does not form structures of the virus but orchestrate virus formation. Our results point towards broadening the frame of investigation to gain a more comprehensive understanding of mechanisms giving rise to antibody recognition of dengue viruses, and may aid the design and evaluation of vaccines and/or assays to characterize dengue immunity.

Single-cell Temporal Analysis of Natural Dengue Infection Reveals Skin-Homing Lymphocyte Expansion One Day before Defervescence

Effective clinical management of acute dengue virus (DENV) infection relies on the timing of suitable treatments during the disease progression. We analyzed single-cell transcriptomic profiles of the peripheral blood mononuclear cell samples from two DENV patients, collected daily during acute phase and also at convalescence. Key immune cell types demonstrated different dynamic responses over the course of the infection. On the day before defervescence (Day −1), we observed the peak expression of several prominent genes in the adaptive immunological pathways. We also characterized unique effector T cell clusters that expressed skin-homing signature genes at Day −1, whereas upregulation of skin and gut homing genes was also observed in plasma cells and plasmablasts during the febrile period. This work provides an overview of unique molecular dynamics that signify the entry of the critical phase, and the findings could improve the patient management of DENV infection.

•

Time-course scRNA-seq reveals immune response dynamics during acute dengue infection

•

Rapid transcriptional switching was observed one day before fever subsided (Day -1)

•

Skin-homing signatures were observed in specific T cells during the febrile phase

•

Expansion of skin-homing CD69⁺ PD-1⁺ T cells at Day -1 was confirmed with FACS

Associations of human leukocyte antigen with neutralizing antibody titers in a tetravalent dengue vaccine phase 2 efficacy trial in Thailand

The recombinant, live, attenuated, tetravalent dengue vaccine CYD-TDV has shown efficacy against all four dengue serotypes. In this exploratory study (CYD59, NCT02827162), we evaluated potential associations of host human leukocyte antigen (HLA) alleles with dengue antibody responses, CYD-TDV vaccine efficacy, and virologically-confirmed dengue (VCD) cases. Children 4-11 years old, who previously completed a phase 2b efficacy study of CYD-TDV in a single center in Thailand, were included in the study. Genotyping of HLA class I and II loci was performed by next-generation sequencing from DNA obtained from 335 saliva samples. Dengue neutralizing antibody titers (NAb) were assessed as a correlate of risk and protection. Regression analyses were used to assess associations between HLA alleles and NAb responses, vaccine efficacy, and dengue outcomes. Month 13 NAb log geometric mean titers (GMTs) were associated with decreased risk of VCD. In the vaccine group, HLA-DRB1*11 was significantly associated with higher NAb log GMT levels (beta: 0.76; p = 0.002, q = 0.13). Additionally, in the absence of vaccination, HLA associations were observed between the presence of DPB1*03:01 and increased NAb log GMT levels (beta: 1.24; p = 0.005, q = 0.17), and between DPB1*05:01 and reduced NAb log GMT levels (beta: -1.1; p = 0.001, q = 0.07). This study suggests associations of HLA alleles with NAb titers in the context of dengue outcomes. This study was registered with clinicaltrials.gov: NCT02827162.

An Engineered Receptor-Binding Domain Improves the Immunogenicity of Multivalent SARS-CoV-2 Vaccines

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike (S) protein mediates viral entry into cells expressing angiotensin-converting enzyme 2 (ACE2). The S protein engages ACE2 through its receptor-binding domain (RBD), an independently folded 197-amino-acid fragment of the 1,273-amino-acid S-protein protomer. The RBD is the primary SARS-CoV-2 neutralizing epitope and a critical target of any SARS-CoV-2 vaccine. Here, we show that this RBD conjugated to each of two carrier proteins elicited more potent neutralizing responses in immunized rodents than did a similarly conjugated proline-stabilized S-protein ectodomain. Nonetheless, the native RBD is expressed inefficiently, limiting its usefulness as a vaccine antigen. However, we show that an RBD engineered with four novel glycosylation sites (gRBD) is expressed markedly more efficiently and generates a more potent neutralizing responses as a DNA vaccine antigen than the wild-type RBD or the full-length S protein, especially when fused to multivalent carriers, such as a Helicobacter pylori ferritin 24-mer. Further, gRBD is more immunogenic than the wild-type RBD when administered as a subunit protein vaccine. Our data suggest that multivalent gRBD antigens can reduce costs and doses, and improve the immunogenicity, of all major classes of SARS-CoV-2 vaccines.IMPORTANCE All available vaccines for coronavirus disease 2019 (COVID-19) express or deliver the full-length SARS-CoV-2 spike (S) protein. We show that this antigen is not optimal, consistent with observations that the vast majority of the neutralizing response to the virus is focused on the S-protein receptor-binding domain (RBD). However, this RBD is not expressed well as an independent domain, especially when expressed as a fusion protein with a multivalent scaffold. We therefore engineered a more highly expressed form of the SARS-CoV-2 RBD by introducing four glycosylation sites into a face of the RBD normally occluded in the full S protein. We show that this engineered protein, gRBD, is more immunogenic than the wild-type RBD or the full-length S protein in both genetic and protein-delivered vaccines.

Antigenic Variation of the Dengue Virus 2 Genotypes Impacts the Neutralization Activity of Human Antibodies in Vaccinees

Dengue virus (DENV) infects an estimated 390 million people each year worldwide. As tetravalent DENV vaccines have variable efficacy against DENV serotype 2 (DENV2), we evaluated the role of genetic diversity within the pre-membrane (prM) and envelope (E) proteins of DENV2 on vaccine performance. We generated a recombinant DENV2 genotype variant panel with contemporary prM and E isolates that are representative of global genetic diversity. The DENV2 genotype variants differ in growth kinetics, morphology, and virion stability. Importantly, the DENV2 genotypic variants are differentially neutralized by monoclonal antibodies, polyclonal serum neutralizing antibodies from DENV2-infected human subjects, and vaccine-elicited antibody responses from the TV003 NIH DENV2 monovalent and DENV tetravalent vaccines. We conclude that DENV2 prM and E genetic diversity significantly modulates antibody neutralization activity. These findings have important implications for dengue vaccines, which are being developed under the assumption that intraserotype variation has minimal impact on neutralizing antibodies.

Martinez et al. demonstrate that dengue virus serotype 2 (DENV2) genetic variation modulates neutralizing antibody activity from infection and vaccination. This observation underlines that genotypic variation impacts dengue virus 2 evasion from humoral immunity, suggesting that intraserotype genotypic variation should be considered in designing dengue vaccines.

Nanosensors based on LSPR are able to serologically differentiate dengue from Zika infections

The Flaviviridae virus family was named after the Yellow-fever virus, and the latin term flavi means “of golden color”. Dengue, caused by Dengue virus (DENV), is one of the most important infectious diseases worldwide. A sensitive and differential diagnosis is crucial for patient management, especially due to the occurrence of serological cross-reactivity to other co-circulating flaviviruses. This became particularly important with the emergence of Zika virus (ZIKV) in areas were DENV seroprevalence was already high. We developed a sensitive and specific diagnostic test based on gold nanorods (GNR) functionalized with DENV proteins as nanosensors. These were able to detect as little as one picogram of anti-DENV monoclonal antibodies and highly diluted DENV-positive human sera. The nanosensors could differentiate DENV-positive sera from other flavivirus-infected patients, including ZIKV, and were even able to distinguish which DENV serotype infected individual patients. Readouts are obtained in ELISA-plate spectrophotometers without the need of specific devices.

Human Heavy Chain Antibody Genes Elicited in Thai Dengue Patients during DENV2 Secondary Infection

Dengue is one of the most serious mosquito-borne viral diseases occurring in humans. To combat the complexity of 4 antigenically distinct serotypes, the ideal vaccine for dengue should be able to stimulate cross-neutralizing antibodies. Recently, genetics-based immune responses have been studied to guide vaccine design against several viral pathogens. Despite a recent approval of dengue vaccine, information on genetics-based immune responses against dengue virus (DENV) is still limited. Consequently, we aimed to determine the profiles of immunoglobulin heavy chain genes from DENV2 infected patients. The immunoglobulin heavy chain variable region genes (IGHV) were amplified from peripheral blood mononuclear cells of DENV2 secondary infected patients in the acute, convalescence, and recovery phases. Antibody heavy chain genes were sequenced using next-generation sequencing, and analyzed to identify correlations with neutralizing and enhancing activities of the serum samples. IGHV1-69, 3-23, and 3-30 were frequently discovered in our Thai DENV2 infected patients. Our findings provide new data on the human B cell response during secondary DENV2 infections in Thai dengue patients that offer supportive information for dengue vaccine design and therapeutics development.

The role of anti-flavivirus humoral immune response in protection and pathogenesis

Flavivirus infections are a public health threat in the world that requires the development of safe and effective vaccines. Therefore, the understanding of the anti-flavivirus humoral immune response is fundamental to future studies on flavivirus pathogenesis and the design of anti-flavivirus therapeutics. This review aims to provide an overview of the current understanding of the function and involvement of flavivirus proteins in the humoral immune response as well as the ability of the anti-envelope (anti-E) antibodies to interfere (neutralizing antibodies) or not (non-neutralizing antibodies) with viral infection, and how they can, in some circumstances enhance dengue virus infection on Fc gamma receptor (FcγR) bearing cells through a mechanism known as antibody-dependent enhancement (ADE). Thus, the dual role of the antibodies against E protein poses a formidable challenge for vaccine development. Also, we discuss the roles of antibody binding stoichiometry (the concentration, affinity, or epitope recognition) in the neutralization of flaviviruses and the "breathing" of flavivirus virions in the humoral immune response. Finally, the relevance of some specific antibodies in the design and improvement of effective vaccines is addressed.

Dengue: Status of current and under-development vaccines

Dengue is an emerging mosquito-borne viral infection with increasing reports of outbreaks. The clinical picture ranges from a benign febrile illness through to severe and potentially fatal manifestations. No specific anti-viral treatment exists, and therapy only consists of supportive care. During the last three decades, several attempts to develop an effective vaccine have been made. The first dengue vaccine to obtain licensure was Dengvaxia, which was authorized in 2015 and is currently available in over 20 countries. Its use has been approved with strict limitations regarding age and serostatus of the recipients, highlighting the necessity for a more safe and efficacious vaccine. At present several vaccine, candidates are undergoing clinical and pre-clinical trials. The most advanced candidates are TDV and TDV 003/005, two live-attenuated vaccines, but another 15 vaccines are under development, introducing novel immunization strategies to the traditional dengue vaccine scenario. This work reviews the current research status on dengue vaccines.

Zika Virus-Immune Plasmas from Symptomatic and Asymptomatic Individuals Enhance Zika Pathogenesis in Adult and Pregnant Mice

Preexisting immunity against dengue virus or West Nile virus was previously reported to mediate antibody-dependent enhancement (ADE) of Zika virus (ZIKV) infection in a mouse model. We show here that ZIKV-immune plasma samples from both symptomatic and asymptomatic individuals mediated ZIKV ADE of infection in vitro and in mice. In a lethal infection model with a viral inoculum 10 times higher, both ADE and protection were observed, depending on the amount of infused immune plasma. In a vertical-transmission model, ZIKV-immune plasma infused to timed pregnant mice increased fetal demise and decreased the body weight of surviving fetuses. Depletion of IgG from an immune plasma abolished ADE of infection, and the presence of purified IgG alone mediated ADE of infection. Higher viral loads and proinflammatory cytokines were detected in mice treated with ZIKV-immune plasma samples compared to those receiving control plasma. Together, these data show that passive immunization with homotypic ZIKV antibodies, depending on the concentration, could either worsen or limit a subsequent ZIKV infection.IMPORTANCE Antibody-dependent enhancement (ADE) of virus infection is common to many viruses and is problematic when plasma antibody levels decline to subneutralizing concentrations. ADE of infection is especially important among flaviviruses, many of which are the cause of global health problems. Recently, human plasma samples immune to heterologous flaviviruses were shown to promote Zika virus (ZIKV) infection. Here we showed in immunocompromised mouse models that homologous immune plasma samples protect mice from subsequent infection at high antibody concentrations but that they mediate ADE of infection and increase ZIKV pathogenesis in adult mice and fetal demise during pregnancy at low concentrations.

Key Amino Acid Substitution for Infection-Enhancing Activity-Free Designer Dengue Vaccines

Dengue is a globally important disease caused by four serotypes of dengue virus. Dengue vaccine development has been hampered by antigenic cross-reactivity among serotypes, which potentially causes antibody-dependent enhancement of infection and disease severity. Here we found that a single amino acid substitution in the envelope protein at position 87 from aspartic acid to asparagine or at position 107 from leucine to phenylalanine is critical for suppressing the induction of infection-enhancing antibody in a mouse model. The site and type of amino acid substitution were determined via neutralization escape using an enhancing-activity-only monoclonal antibody that was engineered to reveal neutralizing activity. Mutated dengue type 1 DNA vaccines containing either or both amino acid substitutions induced neutralizing antibodies devoid of enhancing activity against all serotypes. The effect of substitution was further demonstrated using other serotypes and a tetravalent formulation. This finding may contribute to the development of infection-enhancing-activity-free dengue vaccines.

•

Amino acids at E87 or E107 are critical for dengue-enhancing antibody induction

•

Neutralization escape is useful for identifying the key types or sites of amino acids

•

Each substitution can be applied to antigens of all four dengue serotypes

•

A modified tetravalent DNA vaccine suppresses enhancing antibody induction in mice

Potent Neutralizing Human Monoclonal Antibodies Preferentially Target Mature Dengue Virus Particles: Implication for Novel Strategy for Dengue Vaccine

The four serotypes of dengue virus (DENV) cause the most important mosquito-borne viral disease in humans. The envelope (E) protein is the major target of neutralizing antibodies and contains 3 domains (domain I [DI], DII, and DIII). Recent studies reported that human monoclonal antibodies (MAbs) recognizing DIII, the D1/DII hinge, the E-dimer epitope, or a quaternary epitope involving DI/DII/DIII are more potently neutralizing than those recognizing the fusion loop (FL) of DII. Due to inefficient cleavage of the premembrane protein, DENV suspensions consist of a mixture of mature, immature, and partially immature particles. We investigated the neutralization and binding of 22 human MAbs to DENV serotype 1 (DENV1) virions with differential maturation status. Compared with FL MAbs, DIII, DI/DII hinge, and E-dimer epitope MAbs showed higher maximum binding and avidity to mature particles relative to immature particles; this feature may contribute to the strong neutralizing potency of such MAbs. FL-specific MAbs required 57 to 87% occupancy on mature particles to achieve half-maximal neutralization (NT₅₀), whereas the potently neutralizing MAbs achieved NT₅₀ states at 20 to 38% occupancy. Analysis of the MAb repertoire and polyclonal sera from patients with primary DENV1 infection supports the immunodominance of cross-reactive anti-E antibodies over type-specific antibodies. After depletion with viral particles from a heterologous DENV serotype, the type-specific neutralizing antibodies remained and showed binding features shared by potent neutralizing MAbs. Taken together, these findings suggest that the use of homogeneous mature DENV particles as an immunogen may induce more potent neutralizing antibodies against DENV than the use of immature or mixed particles.IMPORTANCE With an estimated 390 million infections per year, the four serotypes of dengue virus (DENV) cause the most important mosquito-borne viral disease in humans. The dengue vaccine Dengvaxia was licensed; however, its low efficacy among dengue-naive individuals and increased risk of causing severe dengue in children highlight the need for a better understanding of the role of human antibodies in immunity against DENV. DENV suspensions contain mature, immature, and partially immature particles. We investigated the binding of 22 human monoclonal antibodies (MAbs) to the DENV envelope protein on particles with different maturation states. Potently neutralizing MAbs had higher relative maximum binding and avidity to mature particles than weakly neutralizing MAbs. This was supported by analysis of MAb repertoires and polyclonal sera from patients with primary DENV infection. Together, these findings suggest that mature particles may be the optimal form of presentation of the envelope protein to induce more potent neutralizing antibodies against DENV.

Position Paper on Road Map for RNA Virus Research in India

The Indian subcontinent with its population density, climatic conditions, means of subsistence, socioeconomic factors as well as travel and tourism presents a fertile ground for thriving of RNA viruses. Despite being pathogens of huge significance, there is very little focus on research into the biology and pathogenesis of RNA viruses in India. Studies on epidemiology and disease burden, risk factors, the immune response to RNA viruses, circulating virus strains and virus evolution, animal models of disease, antivirals and vaccines are strikingly absent. Emerging RNA viruses such as Zika virus, Nipah virus and Crimean-Congo haemorrhagic fever virus are a matter of grave concern to India. Here we summarize the outcome of the India|EMBO symposium on “RNA viruses: immunology, pathogenesis and translational opportunities” organized at Faridabad, National Capital Region, India, on March 28–30, 2018. The meeting focused on RNA viruses (non-HIV), and both national and international experts on RNA viruses covered topics ranging from epidemiology, immune response, virus evolution and vaccine trials concerning RNA viruses. The aim of the symposium was to create a road map for RNA virus research in India. Both concrete and tentative ideas pointing towards short-term and long-term goals were presented with recommendations for follow-up at government level.