Potent dengue virus neutralization by a therapeutic antibody with low monovalent affinity requires bivalent engagement

Antibodies from dengue patients with prior exposure to Japanese encephalitis virus are broadly neutralizing against Zika virus

Exposure to multiple mosquito-borne flaviviruses within a lifetime is not uncommon; however, how sequential exposures to different flaviviruses shape the cross-reactive humoral response against an antigen from a different serocomplex has yet to be explored. Here, we report that dengue-infected individuals initially primed with the Japanese encephalitis virus (JEV) showed broad, highly neutralizing potencies against Zika virus (ZIKV). We also identified a rare class of ZIKV-cross-reactive human monoclonal antibodies with increased somatic hypermutation and broad neutralization against multiple flaviviruses. One huMAb, K8b, binds quaternary epitopes with heavy and light chains separately interacting with overlapping envelope protein dimer units spanning domains I, II, and III through cryo-electron microscopy and structure-based mutagenesis. JEV virus-like particle immunization in mice further confirmed that such cross-reactive antibodies, mainly IgG3 isotype, can be induced and proliferate through heterologous dengue virus (DENV) serotype 2 virus-like particle stimulation. Our findings highlight the role of prior immunity in JEV and DENV in shaping the breadth of humoral response and provide insights for future vaccination strategies in flavivirus-endemic countries.

Dissecting the contributions of membrane affinity and bivalency of the spider venom protein DkTx to its sustained mode of TRPV1 activation

The spider venom protein, double-knot toxin (DkTx), partitions into the cellular membrane and binds bivalently to the pain-sensing ion channel, TRPV1, triggering long-lasting channel activation. In contrast, its monovalent single knots membrane partition poorly and invoke rapidly reversible TRPV1 activation. To discern the contributions of the bivalency and membrane affinity of DkTx to its sustained mode of action, here, we developed diverse toxin variants including those containing truncated linkers between individual knots, precluding bivalent binding. Additionally, by appending the single-knot domains to the Kv2.1 channel-targeting toxin, SGTx, we created monovalent double-knot proteins that demonstrated higher membrane affinity and more sustained TRPV1 activation than the single-knots. We also produced hyper-membrane affinity-possessing tetra-knot proteins, (DkTx)2 and DkTx-(SGTx)2, that demonstrated longer-lasting TRPV1 activation than DkTx, establishing the central role of the membrane affinity of DkTx in endowing it with its sustained TRPV1 activation properties. These results suggest that high membrane affinity-possessing TRPV1 agonists can potentially serve as long-acting analgesics.

Dengue virus neutralizing antibody: a review of targets, cross-reactivity, and antibody-dependent enhancement

Dengue is the most common viral infection spread by mosquitoes, prevalent in tropical countries. The acute dengue virus (DENV) infection is a benign and primarily febrile illness. However, secondary infection with alternative serotypes can worsen the condition, leading to severe and potentially fatal dengue. The antibody raised by the vaccine or the primary infections are frequently cross-reactive; however, weakly neutralizing, and during subsequent infection, they may increase the odds of antibody-dependent enhancement (ADE). Despite that, many neutralizing antibodies have been identified against the DENV, which are thought to be useful in reducing dengue severity. Indeed, an antibody must be free from ADE for therapeutic application, as it is pretty common in dengue infection and escalates disease severity. Therefore, this review has described the critical characteristics of DENV and the potential immune targets in general. The primary emphasis is given to the envelope protein of DENV, where potential epitopes targeted for generating serotype-specific and cross-reactive antibodies have critically been described. In addition, a novel class of highly neutralizing antibodies targeted to the quaternary structure, similar to viral particles, has also been described. Lastly, we have discussed different aspects of the pathogenesis and ADE, which would provide significant insights into developing safe and effective antibody therapeutics and equivalent protein subunit vaccines.

Speer S, VanBlargan LA, Chen RE, Mukherjee S, Whitener BM, Govero J, Aleshnick M, Larman B, Sukupolvi-Petty S, Sevvana M, Miller AS, Klose T, Zheng A, Koenig S, Kielian M, Kuhn RJ, Diamond MS, Pierson TC. prM-reactive antibodies reveal a role for partially mature virions in dengue virus pathogenesis

Cleavage of the flavivirus premembrane (prM) structural protein during maturation can be inefficient. The contribution of partially mature flavivirus virions that retain uncleaved prM to pathogenesis during primary infection is unknown. To investigate this question, we characterized the functional properties of newly-generated dengue virus (DENV) prM-reactive monoclonal antibodies (mAbs) in vitro and using a mouse model of DENV disease. Anti-prM mAbs neutralized DENV infection in a virion maturation state-dependent manner. Alanine scanning mutagenesis and cryoelectron microscopy of anti-prM mAbs in complex with immature DENV defined two modes of attachment to a single antigenic site. In vivo, passive transfer of intact anti-prM mAbs resulted in an antibody-dependent enhancement of disease. However, protection against DENV-induced lethality was observed when the transferred mAbs were genetically modified to inhibit their ability to interact with Fcγ receptors. These data establish that in addition to mature forms of the virus, partially mature infectious prM+ virions can also contribute to pathogenesis during primary DENV infections.

B-cell epitope discovery: The first protein flexibility-based algorithm-Zika virus conserved epitope demonstration

Antibody-antigen interaction-at antigenic local environments called B-cell epitopes-is a prominent mechanism for neutralization of infection. Effective mimicry, and display, of B-cell epitopes is key to vaccine design. Here, a physical approach is evaluated for the discovery of epitopes which evolve slowly over closely related pathogens (conserved epitopes). The approach is 1) protein flexibility-based and 2) demonstrated with clinically relevant enveloped viruses, simulated via molecular dynamics. The approach is validated against 1) seven structurally characterized enveloped virus epitopes which evolved the least (out of thirty-nine enveloped virus-antibody structures), 2) two structurally characterized non-enveloped virus epitopes which evolved slowly (out of eight non-enveloped virus-antibody structures), and 3) eight preexisting epitope and peptide discovery algorithms. Rationale for a new benchmarking scheme is presented. A data-driven epitope clustering algorithm is introduced. The prediction of five Zika virus epitopes (for future exploration on recombinant vaccine technologies) is demonstrated. For the first time, protein flexibility is shown to outperform solvent accessible surface area as an epitope discovery metric.

Prefusion-specific antibody-derived peptides trivalently presented on DNA-nanoscaffolds as an innovative strategy against RSV entry

Human respiratory syncytial virus (RSV) is the primary cause of acute lower respiratory tract infections in children and the elderly worldwide, for which neither a vaccine nor an effective therapy is approved. The entry of RSV into the host cell is mediated by stepwise structural changes in the surface RSV fusion (RSV-F) glycoprotein. Recent progress in structural and functional studies of RSV-F glycoprotein revealed conformation-dependent neutralizing epitopes which have become attractive targets for vaccine and therapeutic development. As RSV-F is present on viral surface in a trimeric form, a trivalent binding interaction between a candidate fusion inhibitor and the respective epitopes on each of the three monomers is expected to prevent viral infection at higher potency than a monovalent or bivalent inhibitor. Here we demonstrate a novel RSV entry inhibitory approach by implementing a trimeric DNA nanostructure as a template to display up to three linear peptide moieties that simultaneously target an epitope on the surface of the prefusion RSV-F protein. In order to design synthetic binding peptides that can be coupled to the DNA nanostructure, the prefusion RSV-F-specific monoclonal antibody (D25) was selected. Complementarity-determining region 3 (CDR3) derived peptides underwent truncation and alanine-scanning mutagenesis analysis, followed by systematic sequence modifications using non-canonical amino acids. The most effective peptide candidate was used as a binding moiety to functionalize the DNA nanostructure. The designed DNA-peptide construct was able to block RSV infection on cells more efficiently than the monomeric peptides, however a more moderate reduction of viral load was observed in the lungs of infected mice upon intranasal application, likely due to dissociation or absorption of the underlying DNA structure by cells in the lungs. Taken together, our results point towards the inhibitory potential of a novel trimeric DNA-peptide based approach against RSV and open the possibility to apply this platform to target other viral infections.

Structural and functional analysis of an inter-Spike bivalent neutralizing antibody against SARS-CoV-2 variants

The different variants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have attracted most public concern because they caused “wave and wave” COVID-19 pandemic. The initial step of viral infection is mediated by the SARS-CoV-2 Spike (S) protein, which mediates the receptor recognition and membrane fusion between virus and host cells. Neutralizing antibodies (nAbs) targeting the S protein of SARS-CoV-2 have become promising candidates for clinical intervention strategy, while multiple studies have shown that different variants have enhanced infectivity and antibody resistance. Here, we explore the structure and function of STS165, a broadly inter-Spike bivalent nAb against SARS-CoV-2 variants and even SARS-CoV, contributing to further understanding of the working mechanism of nAbs.

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STS165 broadly neutralizes different variants of SARS-CoV-2 and even SARS-CoV

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STS165 exhibits inter-Spike bivalent binding characteristics

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STS165 may work as an ideal partner to form therapeutic antibody cocktails

ITS1 amplicon sequencing of feline gut mycobiome of Malaysian local breeds using Nanopore Flongle

The gut mycobiome exhibits major influence on the gastrointestinal health and disease but received less attention due to low abundance. This study characterizes the fungal community and compares the microbial diversity between indoor and outdoor cats. Genomic DNA was extracted and sequenced by targeting the Internal Transcribed Spacer 1 (ITS1) region using Flongle flow cell on MinION™ sequencing platform. Results show the phylum Ascomycota and genus Peniophorella were numerous in indoor cats, whereas the Basidiomycota and Pichia were abundant in outdoor cats. Peniophorella formed the core mycobiome in both feline populations. Furthermore, alpha (p value = 0.0207) and beta diversities (p value = 0.009) results showed significant differences between the two groups. Overall, indoor cats have greater amounts of Peniophorella, whereas outdoor cats have higher Trichosporon and unclassified Sordariaceae. The study also suggests that keeping a cat indoors or left as a stray will affect their respective gut mycobiome.

Flavivirus Entry Inhibitors

Flaviviruses, including Dengue virus, Zika virus, Yellow fever virus, Japanese encephalitis virus, West Nile virus, cause thousands of deaths and millions of illnesses each year. The large outbreak of ZIKV in 2016 reminds us that flaviviruses can pose a serious threat to human safety and public health as emerging and re-emerging viruses. However, there are no specific drugs approved for the treatment of flavivirus infections. Due to no need to enter the cells, viral entry inhibitors have the unique advantage in suppressing viral infections. Flaviviruses bind to receptors and attach to the cell surface, then enter the endosome in a clathrin-dependent manner and finalizes the viral entry process after fusion with the cell membrane in a low pH environment. Small molecules, antibodies or peptides can inhibit flavivirus entry by targeting the above processes. Here, we focus on flavivirus entry inhibitors with well-defined target and antiviral activity. We hope that our review will provide a theoretical basis for flavivirus treatment and drug research and help to accelerate the clinical application of flavivirus entry inhibitors.

Charged Residue Implantation Improves the Affinity of a Cross-Reactive Dengue Virus Antibody

Dengue virus (DENV) has four serotypes that complicate vaccine development. Envelope protein domain III (EDIII) of DENV is a promising target for therapeutic antibody development. One EDIII-specific antibody, dubbed 1A1D-2, cross-reacts with DENV 1, 2, and 3 but not 4. To improve the affinity of 1A1D-2, in this study, we analyzed the previously solved structure of 1A1D-2-DENV2 EDIII complex. Mutations were designed, including A54E and Y105R in the heavy chain, with charges complementary to the epitope. Molecular dynamics simulation was then used to validate the formation of predicted salt bridges. Interestingly, a surface plasmon resonance experiment showed that both mutations increased affinities of 1A1D-2 toward EDIII of DENV1, 2, and 3 regardless of their sequence variation. Results also revealed that A54E improved affinities through both a faster association and slower dissociation, whereas Y105R improved affinities through a slower dissociation. Further simulation suggested that the same mutants interacted with different residues in different serotypes. Remarkably, combination of the two mutations additively improved 1A1D-2 affinity by 8, 36, and 13-fold toward DENV1, 2, and 3, respectively. In summary, this study demonstrated the utility of tweaking antibody-antigen charge complementarity for affinity maturation and emphasized the complexity of improving antibody affinity toward multiple antigens.

Stochastic modeling of antibody binding predicts programmable migration on antigen patterns

Viruses and bacteria commonly exhibit spatial repetition of surface molecules that directly interface with the host immune system. However the complex interaction of patterned surfaces with immune molecules containing multiple binding domains is poorly understood. We developed a pipeline for constructing mechanistic models of antibody interactions with patterned antigen substrates. Our framework relies on immobilized DNA origami nanostructures decorated with precisely placed antigens. The results revealed that antigen spacing is a spatial control parameter that can be tuned to influence antibody residence time and migration speed. The model predicts that gradients in antigen spacing can drive persistent, directed antibody migration in the direction of more stable spacing. These results depict antibody-antigen interactions as a computational system wherein antigen geometry constrains and potentially directs antibody movement. We propose that this form of molecular programmability could be exploited during co-evolution of pathogens and immune systems or in the design of molecular machines.

Discovery of B-cell epitopes for development of dengue vaccines and antibody therapeutics

Dengue is one of the most frequently transmitted viral infections globally which creates a serious burden to the healthcare system in many countries in the tropical and subtropical regions. To date, no vaccine has demonstrated balanced protection against the four dengue serotypes. Dengvaxia as the only vaccine that has been licensed for use in endemic areas has shown an increased risk in dengue-naïve vaccines to develop severe dengue. A crucial element in protection from dengue infection is the neutralizing antibody responses. Therefore, the identification of protective linear B-cell epitopes can guide vaccine design and facilitate the development of monoclonal antibodies as dengue therapeutics. This review summarizes the identification of dengue B-cell epitopes within the envelope (E) protein of dengue that can be incorporated into peptide vaccine constructs. These epitopes have been identified through approaches such as bioinformatics, three-dimensional structure analysis of antibody-dengue complexes, mutagenesis/alanine scanning and escape mutant studies. Additionally, the therapeutic potential of monoclonal antibodies targeting the E protein of dengue is reviewed. This can provide a basis for the design of future dengue therapies.

How a broadly neutralizing antibody grapples with antigenic and conformational diversity in dengue virus

In this issue of Cell, two studies apply powerful structural approaches to probe the modes of interaction between a broadly neutralizing antibody and a conserved epitope found on four dengue virus serotypes and Zika virus. These findings offer new insights into how a broadly neutralizing antibody surmounts antigenic and conformational variation.

Prevalence of arbovirus antibodies in young healthy adult population in Brazil

Background

The emergence and re-emergence of infectious diseases are a cause for worldwide concern. The introduction of Zika and Chikungunya diseases in the Americas has exposed unforeseen medical and logistical challenges for public health systems. Moreover, the lack of preventive measures and vaccination against known and emerging mosquito-transmitted pathogens, and the occurrence of unanticipated clinical complications, has had an enormous social and economic impact on the affected populations. In this study, we aimed to measure the seroprevalence of endemic and emerging viral pathogens in military personnel stationed in Manaus, Amazonas state.

Methods

We measured the seropositivity of antibodies against 19 endemic and emerging viruses in a healthy military personnel group using a hemagglutination inhibition assay (HIA).

Results

Overall, DENV positivity was 60.4%, and 30.9% of the individuals reacted against ZIKV. Also, 46.6%, 54.7%, 51.3% and 48.7% individuals reacted against West Nile virus (WNV), Saint Louis encephalitis virus (SLEV), Ilheus virus (ILHV) and Rocio virus (ROCV), respectively. Individuals with high DENV HIA titer reacted more frequently with ZIKV or WNV compared to those with low HIA titers. Observed cross-reactivity between Flaviviruses varied depending on the virus serogroup. Additionally, 0.6% and 0.3% individuals were seropositive for Oropouche virus (OROV) and Catu virus (CATUV) from the family Peribunyaviridae, respectively. All samples were negative for Eastern Equine Encephalitis virus (EEEV), Western Equine Encephalomyelitis virus (WEEV), Mayaro virus (MAYV), Mucambo virus (MUCV) and CHIKV from the family Togaviridae.

Conclusions

A high proportion of individuals in our high-risk population (~ 60%) lacked antibodies against major endemic and emerging viruses, which makes them susceptible for further infections. Military personnel serving in the Amazon region could serve as sentinels to strengthen global infectious disease surveillance, particularly in remote areas.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-021-04901-4.

Computational and Rational Design of Single-Chain Antibody against Tick-Borne Encephalitis Virus for Modifying Its Specificity

Tick-borne encephalitis virus (TBEV) causes 5−7 thousand cases of human meningitis and encephalitis annually. The neutralizing and protective antibody ch14D5 is a potential therapeutic agent. This antibody exhibits a high affinity for binding with the D3 domain of the glycoprotein E of the Far Eastern subtype of the virus, but a lower affinity for the D3 domains of the Siberian and European subtypes. In this study, a 2.2-fold increase in the affinity of single-chain antibody sc14D5 to D3 proteins of the Siberian and European subtypes of the virus was achieved using rational design and computational modeling. This improvement can be further enhanced in the case of the bivalent binding of the full-length chimeric antibody containing the identified mutation.

Broad and potent neutralizing human antibodies to tick-borne flaviviruses protect mice from disease

Tick-borne encephalitis virus (TBEV) is an emerging human pathogen that causes potentially fatal disease with no specific treatment. Mouse monoclonal antibodies are protective against TBEV, but little is known about the human antibody response to infection. Here, we report on the human neutralizing antibody response to TBEV in a cohort of infected and vaccinated individuals. Expanded clones of memory B cells expressed closely related anti-envelope domain III (EDIII) antibodies in both groups of volunteers. However, the most potent neutralizing antibodies, with IC50s below 1 ng/ml, were found only in individuals who recovered from natural infection. These antibodies also neutralized other tick-borne flaviviruses, including Langat, louping ill, Omsk hemorrhagic fever, Kyasanur forest disease, and Powassan viruses. Structural analysis revealed a conserved epitope near the lateral ridge of EDIII adjoining the EDI-EDIII hinge region. Prophylactic or early therapeutic antibody administration was effective at low doses in mice that were lethally infected with TBEV.

Structural basis for bivalent binding and inhibition of SARS-CoV-2 infection by human potent neutralizing antibodies

Neutralizing monoclonal antibodies (nAbs) to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) represent promising candidates for clinical intervention against coronavirus disease 2019 (COVID-19). We isolated a large number of nAbs from SARS-CoV-2-infected individuals capable of disrupting proper interaction between the receptor binding domain (RBD) of the viral spike (S) protein and the receptor angiotensin converting enzyme 2 (ACE2). However, the structural basis for their potent neutralizing activity remains unclear. Here, we report cryo-EM structures of the ten most potent nAbs in their native full-length IgG-form or in both IgG-form and Fab-form bound to the trimeric S protein of SARS-CoV-2. The bivalent binding of the full-length IgG is found to associate with more RBDs in the "up" conformation than the monovalent binding of Fab, perhaps contributing to the enhanced neutralizing activity of IgG and triggering more shedding of the S1 subunit from the S protein. Comparison of a large number of nAbs identified common and unique structural features associated with their potent neutralizing activities. This work provides a structural basis for further understanding the mechanism of nAbs, especially through revealing the bivalent binding and its correlation with more potent neutralization and the shedding of S1 subunit.

Diverse contributions of avidity to the broad neutralization of Dengue virus by antibodies targeting the E dimer epitope

Antibodies (Abs) recognizing the Dengue virus (DENV) E dimer epitope (EDE) that potently neutralize all DENV serotypes are promising templates for vaccine design. As an important feature for some Abs is their bivalency, we sought to define the role avidity plays in neutralization by EDE Abs. We compared neutralization activity between bivalent IgGs and monovalent Ab fragments (Fabs) for two EDE Abs, A11 and C10. IgG forms of both Abs exhibited more potent neutralization activity than their counterpart Fabs, yet only for C10 was this enhanced activity associated with bivalent binding. A11 and C10 also exhibited differential binding profiles to DENV virus-like particles under acidic conditions mimicking the environment that triggers viral membrane fusion, suggesting that EDE Abs employ diverse neutralization mechanisms despite sharing an epitope. Delineating the full range of Ab binding modes and neutralization mechanisms against a single epitope may inform therapeutic approaches and refine vaccine design.

Antigenicity, stability, and reproducibility of Zika reporter virus particles for long-term applications

The development of vaccines against flaviviruses, including Zika virus (ZIKV) and dengue virus (DENV), continues to be a major challenge, hindered by the lack of efficient and reliable methods for screening neutralizing activity of sera or antibodies. To address this need, we previously developed a plasmid-based, replication-incompetent DENV reporter virus particle (RVP) production system as an efficient and safe alternative to the Plaque Reduction Neutralization Test (PRNT). As part of the response to the 2015-2016 ZIKV outbreak, we developed pseudo-infectious ZIKV RVPs by modifying our DENV RVP system. The use of ZIKV RVPs as critical reagents in human clinical trials requires their further validation using stability and reproducibility metrics for large-scale applications. In the current study, we validated ZIKV RVPs using infectivity, neutralization, and enhancement assays with monoclonal antibodies (MAbs) and human ZIKV-positive patient serum. ZIKV RVPs are antigenically equivalent to live virus based on binding ELISA and neutralization results and are nonreplicating based on the results of live virus replication assays. We demonstrate reproducible neutralization titer data (NT50 values) across different RVP production lots, volumes, time frames, and laboratories. We also show RVP stability across experimentally relevant time intervals and temperatures. Our results demonstrate that ZIKV RVPs provide a safe, high-throughput, and reproducible reagent for large-scale, long-term studies of neutralizing antibodies and sera, which can facilitate large-scale screening and epidemiological studies to help expedite ZIKV vaccine development.

Prediction of the binding interface between monoclonal antibody m102.4 and Nipah attachment glycoprotein using structure-guided alanine scanning and computational docking

Nipah Virus (NiV) has been designated as a priority disease with an urgent need for therapeutic development by World Health Organization. The monoclonal antibody m102.4 binds to the immunodominant NiV receptor-binding glycoprotein (GP), and potently neutralizes NiV, indicating its potential as a therapeutic agent. Although the co-crystal structure of m102.3, an m102.4 derivative, in complex with the GP of the related Hendra Virus (HeV) has been solved, the structural interaction between m102.4 and NiV is uncharacterized. Herein, we used structure-guided alanine-scanning mutagenesis to map the functional epitope and paratope residues that govern the antigen-antibody interaction. Our results revealed that the binding of m102.4 is mediated predominantly by two residues in the HCDR3 region, which is unusually small for an antibody-antigen interaction. We performed computational docking to generate a structural model of m102.4-NiV interaction. Our model indicates that m102.4 targets the common hydrophobic central cavity and a hydrophilic rim on the GP, as observed for the m102.3-HeV co-crystal, albeit with Fv orientation differences. In summary, our study provides insight into the m102.4-NiV interaction, demonstrating that structure-guided alanine-scanning and computational modeling can serve as the starting point for additional antibody reengineering (e.g. affinity maturation) to generate potential therapeutic candidates.

Mechanism of differential Zika and dengue virus neutralization by a public antibody lineage targeting the DIII lateral ridge

Evaluation of the human antibody response to Zika virus has identified common germline-derived mAbs capable of cross flavivirus neutralization. Zhao et al. provide a detailed mechanistic understanding of how flavivirus infections are prevented in a strain-specific manner by a representative mAb.

We previously generated a panel of human monoclonal antibodies (mAbs) against Zika virus (ZIKV) and identified one, ZIKV-116, that shares germline usage with mAbs identified in multiple donors. Here we show that ZIKV-116 interferes with ZIKV infection at a post-cellular attachment step by blocking viral fusion with host membranes. ZIKV-116 recognizes the lateral ridge of envelope protein domain III, with one critical residue varying between the Asian and African strains responsible for differential binding affinity and neutralization potency (E³⁹³D). ZIKV-116 also binds to and cross-neutralizes some dengue virus serotype 1 (DENV1) strains, with genotype-dependent inhibition explained by variation in a domain II residue (R²⁰⁴K) that potentially modulates exposure of the distally located, partially cryptic epitope. The V-J reverted germline configuration of ZIKV-116 preferentially binds to and neutralizes an Asian ZIKV strain, suggesting that this epitope may optimally induce related B cell clonotypes. Overall, these studies provide a structural and molecular mechanism for a cross-reactive mAb that uniquely neutralizes ZIKV and DENV1.

Recognition of a highly conserved glycoprotein B epitope by a bivalent antibody neutralizing HCMV at a post-attachment step

Human cytomegalovirus (HCMV) is one of the main causative agents of congenital viral infection in neonates. HCMV infection also causes serious morbidity and mortality among organ transplant patients. Glycoprotein B (gB) is a major target for HCMV neutralizing antibodies, yet the underlying neutralization mechanisms remain largely unknown. Here we report that 3–25, a gB-specific monoclonal antibody previously isolated from a healthy HCMV-positive donor, efficiently neutralized 14 HCMV strains in both ARPE-19 cells and MRC-5 cells. The core epitope of 3–25 was mapped to a highly conserved linear epitope on antigenic domain 2 (AD-2) of gB. A 1.8 Å crystal structure of 3–25 Fab in complex with the peptide epitope revealed the molecular determinants of 3–25 binding to gB at atomic resolution. Negative-staining electron microscopy (EM) 3D reconstruction of 3–25 Fab in complex with de-glycosylated postfusion gB showed that 3–25 Fab fully occupied the gB trimer at the N-terminus with flexible binding angles. Functionally, 3–25 efficiently inhibited HCMV infection at a post-attachment step by interfering with viral membrane fusion, and restricted post-infection viral spreading in ARPE-19 cells. Interestingly, bivalency was required for HCMV neutralization by AD-2 specific antibody 3–25 but not the AD-4 specific antibody LJP538. In contrast, bivalency was not required for HCMV binding by both antibodies. Taken together, our results reveal the structural basis of gB recognition by 3–25 and demonstrate that inhibition of viral membrane fusion and a requirement of bivalency may be common for gB AD-2 specific neutralizing antibody.

HCMV infection is usually asymptomatic in healthy individuals. However, life-threatening diseases frequently accompany HCMV infection in individuals with under-developed or compromised immune systems. Glycoprotein B antigenic domain 2 (AD-2) is a major target for HCMV-neutralizing antibodies that potentially provide immune protection. We report the structure-based study of gB recognition by a potent neutralizing antibody named 3–25 that binds a highly conserved epitope on AD-2. Functionally, 3–25 efficiently inhibited HCMV infection at a post-attachment step by interfering with viral membrane fusion, and restricted post-infection viral spreading. Furthermore, bivalency of 3–25 is required for viral neutralization but not for binding. Our findings advance understanding of gB antibody-mediated HCMV neutralization and facilitate development of gB-targeted vaccines and antibody drugs against HCMV infection.

Structural Basis for Neutralization and Protection by a Zika Virus-Specific Human Antibody

We previously reported a human monoclonal antibody, ZK2B10, capable of protection against Zika virus (ZIKV) infection and microcephaly in developing mouse embryos. Here, we report the structural features and mechanism of action of ZK2B10. The crystal structure at a resolution of 2.32 Å revealed that the epitope is located on the lateral ridge of DIII of the envelope glycoprotein. Cryo-EM structure with mature ZIKV showed that the antibody binds to DIIIs around the icosahedral 2-fold, 3-fold, and 5-fold axes, a distinct feature compared to those reported for DIII-specific antibodies. The binding of ZK2B10 to ZIKV has no detectable effect on viral attachment to target cells or on conformational changes of the E glycoprotein in the acidic environment, suggesting that ZK2B10 functions at steps between the formation of the fusion intermediate and membrane fusion. These results provide structural and mechanistic insights into how ZK2B10 mediates protection against ZIKV infection.

Cryo-EM Structures of Eastern Equine Encephalitis Virus Reveal Mechanisms of Virus Disassembly and Antibody Neutralization

Alphaviruses are enveloped pathogens that cause arthritis and encephalitis. Here, we report a 4.4-Å cryoelectron microscopy (cryo-EM) structure of eastern equine encephalitis virus (EEEV), an alphavirus that causes fatal encephalitis in humans. Our analysis provides insights into viral entry into host cells. The envelope protein E2 showed a binding site for the cellular attachment factor heparan sulfate. The presence of a cryptic E2 glycan suggests how EEEV escapes surveillance by lectin-expressing myeloid lineage cells, which are sentinels of the immune system. A mechanism for nucleocapsid core release and disassembly upon viral entry was inferred based on pH changes and capsid dissociation from envelope proteins. The EEEV capsid structure showed a viral RNA genome binding site adjacent to a ribosome binding site for viral genome translation following genome release. Using five Fab-EEEV complexes derived from neutralizing antibodies, our investigation provides insights into EEEV host cell interactions and protective epitopes relevant to vaccine design.

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EEEV cryo-EM structure shows the basis of receptor binding and pH-triggered disassembly

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Cryptic envelope protein glycosylation interferes with immune detection

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EEEV RNA genome binding site on capsid protein has an extended conformation

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Antibody inhibition of EEEV entry involves cross-linking of viral envelope proteins

Potent neutralizing antibodies elicited by dengue vaccine in rhesus macaque target diverse epitopes

There is still no safe and effective vaccine against dengue virus infection. Epidemics of dengue virus infection are increasingly a threat to human health around the world. Antibodies generated in response to dengue infection have been shown to impact disease development and effectiveness of dengue vaccine. In this study, we investigated monoclonal antibody responses to an experimental dengue vaccine in rhesus macaques. Variable regions of both heavy chain (VH) and light chain (VL) were cloned from single antibody-secreting B cells. A total of 780 monoclonal antibodies (mAbs) composed of paired VH and VL were characterized. Results show that the vaccination induces mAbs with diverse germline sequences and a wide range of binding affinities. Six potent neutralizing mAbs were identified among 130 dengue envelope protein binders. Critical amino acids for each neutralizing antibody binding to the dengue envelope protein were identified by alanine scanning of mutant libraries. Diverse epitopes were identified, including epitopes on the lateral ridge of DIII, the I-III hinge, the bc loop adjacent to the fusion loop of DII, and the β-strands and loops of DI. Significantly, one of the neutralizing mAbs has a previously unknown epitope in DII at the interface of the envelope and membrane protein and is capable of neutralizing all four dengue serotypes. Taken together, the results of this study not only provide preclinical validation for the tested experimental vaccine, but also shed light on a potential application of the rhesus macaque model for better dengue vaccine evaluation and design of vaccines and immunization strategies.

Dengue virus (DENV) is a leading cause of human illness in the tropics and subtropics, with about 40% of the world’s population living in areas at risk for infection. There are four DENV serotypes. Patients who have previously been infected by one dengue serotype may develop more severe symptoms such as bleeding and endothelial leakage upon secondary infection with another dengue serotype. This study reports the extensive cloning and analysis of 780 monoclonal antibodies (mAbs) from single B cells of rhesus macaques after immunization with an experimental dengue vaccine. We identified a panel of potent neutralizing mAbs with diverse epitopes on the DENV envelope protein. Antibodies in this panel were found to bind to the lateral ridge of DIII, the I-III hinge, the bc loop adjacent to the fusion loop of DII, and the β-strands and the loops of DI. We also isolated one mAb (d448) that can neutralize all four dengue serotypes and binds to a novel epitope at the interface of the DENV envelope and membrane proteins. Further investigation of these neutralizing monoclonal antibodies is warranted for better vaccine efficacy evaluation and vaccine design.

Enhancement of binding avidity by bivalent binding enables PrPSc-specific detection by anti-PrP monoclonal antibody 132

Anti-prion protein (PrP) monoclonal antibody 132, which recognizes mouse PrP amino acids 119–127, enables us to reliably detect abnormal isoform prion protein (PrP^Sc) in cells or frozen tissue sections by immunofluorescence assay, although treatment with guanidinium salts is a prerequisite. Despite the benefit of this mAb, the mechanism of PrP^Sc-specific detection remains unclear. Therefore, to address this mechanism, we analyzed the reactivities of mono- and bivalent mAb 132 to recombinant mouse PrP (rMoPrP) by enzyme-linked immunosorbent assay (ELISA) and surface plasmon resonance (SPR). In ELISA, binding of the monovalent form was significantly weaker than that of the bivalent form, indicating that bivalent binding confers a higher binding stability to mAb 132. Compared with other anti-PrP mAbs tested, the reactivity of bivalent mAb 132 was easily affected by a decrease in antigen concentration. The binding kinetics of mAb 132 assessed by SPR were consistent with the results of ELISA. The dissociation constant of the monovalent form was approximately 260 times higher than that of the bivalent form, suggesting that monovalent binding is less stable than bivalent binding. Furthermore, the amount of mAb 132 that bound to rMoPrP decreased if the antigen density was too low to allow bivalent binding. If two cellular PrP (PrP^C) are close enough to allow bivalent binding, mAb 132 binds to PrP^C. These results indicate that weak monovalent binding to monomeric PrP^C diminishes PrP^C signals to background level, whereas after exposure of the epitope, mAb 132 binds stably to oligomeric PrP^Sc in a bivalent manner.

Molecular Basis of a Protective/Neutralizing Monoclonal Antibody Targeting Envelope Proteins of both Tick-Borne Encephalitis Virus and Louping Ill Virus

Tick-borne encephalitis virus (TBEV) and louping ill virus (LIV) are members of the tick-borne flaviviruses (TBFVs) in the family Flaviviridae which cause encephalomeningitis and encephalitis in humans and other animals. Although vaccines against TBEV and LIV are available, infection rates are rising due to the low vaccination coverage. To date, no specific therapeutics have been licensed. Several neutralizing monoclonal antibodies (MAbs) show promising effectiveness in the control of TBFVs, but the underlying molecular mechanisms are yet to be characterized. Here, we determined the crystal structures of the LIV envelope (E) protein and report the comparative structural analysis of a TBFV broadly neutralizing murine MAb (MAb 4.2) in complex with either the LIV or TBEV E protein. The structures reveal that MAb 4.2 binds to the lateral ridge of domain III of the E protein (EDIII) of LIV or TBEV, an epitope also reported for other potently neutralizing MAbs against mosquito-borne flaviviruses (MBFVs), but adopts a unique binding orientation. Further structural analysis suggested that MAb 4.2 may neutralize flavivirus infection by preventing the structural rearrangement required for membrane fusion during virus entry. These findings extend our understanding of the vulnerability of TBFVs and other flaviviruses (including MBFVs) and provide an avenue for antibody-based TBFV antiviral development.IMPORTANCE Understanding the mechanism of antibody neutralization/protection against a virus is crucial for antiviral countermeasure development. Tick-borne encephalitis virus (TBEV) and louping ill virus (LIV) are tick-borne flaviviruses (TBFVs) in the family Flaviviridae They cause encephalomeningitis and encephalitis in humans and other animals. Although vaccines for both viruses are available, infection rates are rising due to low vaccination coverage. In this study, we solved the crystal structures of the LIV envelope protein (E) and a broadly neutralizing/protective TBFV MAb, MAb 4.2, in complex with E from either TBEV or LIV. Key structural features shared by TBFV E proteins were analyzed. The structures of E-antibody complexes showed that MAb 4.2 targets the lateral ridge of both the TBEV and LIV E proteins, a vulnerable site in flaviviruses for other potent neutralizing MAbs. Thus, this site represents a promising target for TBFV antiviral development. Further, these structures provide important information for understanding TBFV antigenicity.

Binding to nanopatterned antigens is dominated by the spatial tolerance of antibodies

Although repetitive patterns of antigens are crucial for certain immune responses, an understanding of how antibodies bind and dynamically interact with various spatial arrangements of molecules is lacking. Hence, we introduced a new method in which molecularly precise nanoscale patterns of antigens are displayed using DNA origami and immobilized in a surface plasmon resonance set-up. Using antibodies with identical antigen-binding domains, we found that all the subclasses and isotypes studied bind bivalently to two antigens separated at distances that range from 3 to 17 nm. The binding affinities of these antibodies change with the antigen distances, with a distinct preference for antigens separated by approximately 16 nm, and considerable differences in spatial tolerance exist between IgM and IgG and between low- and high-affinity antibodies.

Characterization of a potent and highly unusual minimally enhancing antibody directed against dengue virus

Dengue virus is a major pathogen, and severe infections can lead to life-threatening dengue hemorrhagic fever. Dengue virus exists as four serotypes, and dengue hemorrhagic fever is often associated with secondary heterologous infections. Antibody-dependent enhancement (ADE) may drive higher viral loads in these secondary infections and is purported to result from antibodies that recognize dengue virus but fail to fully neutralize it. Here we characterize two antibodies, 2C8 and 3H5, that bind to the envelope protein. Antibody 3H5 is highly unusual as it not only is potently neutralizing but also promotes little if any ADE, whereas antibody 2C8 has strong capacity to promote ADE. We show that 3H5 shows resilient binding in endosomal pH conditions and neutralizes at low occupancy. Immunocomplexes of 3H5 and dengue virus do not efficiently interact with Fcγ receptors, which we propose is due to the binding mode of 3H5 and constitutes the primary mechanism of how ADE is avoided.

Small heat shock protein speciation: novel non-canonical 44 kDa HspB5-related protein species in rat and human tissues

When analyzing small stress proteins of rat and human tissues by electrophoretic methods followed by western blotting, and using the anti-HspB1/anti-HspB5 antibody clone 8A7, we unexpectedly found a protein with a molecular mass of ~44 kDa. On two-dimensional gels, this protein resolved into four distinct species. Electrophoretic and immunological evidence suggests that this 44 kDa protein is a derivative of HspB5, most likely a covalently linked HspB5 dimer. This HspB5-like 44 kDa protein (HspB5L-P44) is particularly abundant in rat heart, brain, and renal cortex and glomeruli. HspB5L-P44 was also found in human brains, including those from patients with Alexander disease, a condition distinguished by cerebral accumulation of HspB5. Gray matter of such a patient contained an elevated amount of HspB5L-P44. A spatial model of structurally ordered dimeric HspB5 α-crystallin domains reveals the exposed and adjacent position of the two peptide segments homologous to the HspB1-derived 8A7 antigen determinant peptide (epitope). This explains the observed extraordinary high avidity of the 8A7 antibody towards HspB5L-P44, as opposed to commonly used HspB5-specific antibodies which recognize other epitopes. This scenario also explains the remarkable fact that no previous study reported the existence of HspB5L-P44 species. Exposure of rat endothelial cells to UV light, an oxidative stress condition, temporarily increased HspB5L-P44, suggesting physiological regulation of the dimerization. The existence of HspB5L-P44 supports the protein speciation discourse and fits to the concept of the protein code, according to which the expression of a given gene is reflected only by the complete set of the derived protein species.

Mathematical Modeling of Bioassays

The high affinity and specificity of biological receptors determine the demand for and the intensive development of analytical systems based on use of these receptors. Therefore, theoretical concepts of the mechanisms of these systems, quantitative parameters of their reactions, and relationships between their characteristics and ligand-receptor interactions have become extremely important. Many mathematical models describing different bioassay formats have been proposed. However, there is almost no information on the comparative characteristics of these models, their assumptions, and predictive insights. In this review we suggested a set of criteria to classify various bioassays and reviewed classical and contemporary publications on these bioassays with special emphasis on immunochemical analysis systems as the most common and in-demand techniques. The possibilities of analytical and numerical modeling are discussed, as well as estimations of the minimum concentrations that may be detected in bioassays and recommendations for the choice of assay conditions.

Mouse and Human Monoclonal Antibodies Protect against Infection by Multiple Genotypes of Japanese Encephalitis Virus

Japanese encephalitis virus (JEV) remains a leading cause of viral encephalitis worldwide. Although JEV-specific antibodies have been described, an assessment of their ability to neutralize multiple genotypes of JEV has been limited. Here, we describe the development of a panel of mouse and human neutralizing monoclonal antibodies (MAbs) that inhibit infection in cell culture of four different JEV genotypes tested. Mechanism-of-action studies showed that many of these MAbs inhibited infection at a postattachment step, including blockade of virus fusion. Mapping studies using site-directed mutagenesis and hydrogen-deuterium exchange with mass spectrometry revealed that the lateral ridge on domain III of the envelope protein was a primary recognition epitope for our panel of strongly neutralizing MAbs. Therapeutic studies in mice demonstrated protection against lethality caused by genotype I and III strains when MAbs were administered as a single dose even 5 days after infection. This information may inform the development of vaccines and therapeutic antibodies as emerging strains and genotypic shifts become more prevalent.IMPORTANCE Although Japanese encephalitis virus (JEV) is a vaccine-preventable cause of viral encephalitis, the inactivated and live attenuated platforms available are derived from strains belonging to a single genotype (GIII) due to its historical prevalence in areas of JEV epidemics. Related to this, studies with vaccines and antibodies have focused on assessing the in vitro and in vivo protective responses to homologous or heterologous GIII strains. An epidemiological shift in JEV genotype distribution warrants the induction of broadly neutralizing antibody responses that inhibit infection of multiple JEV genotypes. Here, we generated a panel of mouse and human neutralizing monoclonal antibodies and evaluated their inhibitory activity, epitope location, and capacity for protection against multiple JEV genotypes in mice.

Neutralization of antibody-enhanced dengue infection by VIS513, a pan serotype reactive monoclonal antibody targeting domain III of the dengue E protein

Dengue virus (DENV) infection imposes enormous health and economic burden worldwide with no approved treatment. Several small molecules, including lovastatin, celgosivir, balapiravir and chloroquine have been tested for potential anti-dengue activity in clinical trials; none of these have demonstrated a protective effect. Recently, based on identification and characterization of cross-serotype neutralizing antibodies, there is increasing attention on the potential for dengue immunotherapy. Here, we tested the ability of VIS513, an engineered cross-neutralizing humanized antibody targeting the DENV E protein domain III, to overcome antibody-enhanced infection and high but brief viremia, which are commonly encountered in dengue patients, in various in vitro and in vivo models. We observed that VIS513 efficiently neutralizes DENV at clinically relevant viral loads or in the presence of enhancing levels of DENV immune sera. Single therapeutic administration of VIS513 in mouse models of primary infection or lethal secondary antibody-enhanced infection, reduces DENV titers and protects from lethal infection. Finally, VIS513 administration does not readily lead to resistance, either in cell culture systems or in animal models of dengue infection. The findings suggest that rapid viral reduction during acute DENV infection with a monoclonal antibody is feasible.

Immune Response to Dengue and Zika

Flaviviruses such as dengue (DENV), yellow fever (YFV), West Nile (WNV), and Zika (ZIKV) are human pathogens of global significance. In particular, DENV causes the most prevalent mosquito-borne viral diseases in humans, and ZIKV emerged from obscurity into the spotlight in 2016 as the etiologic agent of congenital Zika syndrome. Owing to the recent emergence of ZIKV as a global pandemic threat, the roles of the immune system during ZIKV infections are as yet unclear. In contrast, decades of DENV research implicate a dual role for the immune system in protection against and pathogenesis of DENV infection. As DENV and ZIKV are closely related, knowledge based on DENV studies has been used to prioritize investigation of ZIKV immunity and pathogenesis, and to accelerate ZIKV diagnostic, therapeutic, and vaccine design. This review discusses the following topics related to innate and adaptive immune responses to DENV and ZIKV: the interferon system as the key mechanism of host defense and viral target for immune evasion, antibody-mediated protection versus antibody-dependent enhancement, and T cell-mediated protection versus original T cell antigenic sin. Understanding the mechanisms that regulate the balance between immune-mediated protection and pathogenesis during DENV and ZIKV infections is critical toward development of safe and effective DENV and ZIKV therapeutics and vaccines.

Development of Antibody Therapeutics against Flaviviruses

Recent outbreaks of Zika virus (ZIKV) highlight the urgent need to develop efficacious interventions against flaviviruses, many of which cause devastating epidemics around the world. Monoclonal antibodies (mAb) have been at the forefront of treatment for cancer and a wide array of other diseases due to their specificity and potency. While mammalian cell-produced mAbs have shown promise as therapeutic candidates against several flaviviruses, their eventual approval for human application still faces several challenges including their potential risk of predisposing treated patients to more severe secondary infection by a heterologous flavivirus through antibody-dependent enhancement (ADE). The high cost associated with mAb production in mammalian cell cultures also poses a challenge for the feasible application of these drugs to the developing world where the majority of flavivirus infection occurs. Here, we review the current therapeutic mAb candidates against various flaviviruses including West Nile (WNV), Dengue virus (DENV), and ZIKV. The progress of using plants for developing safer and more economical mAb therapeutics against flaviviruses is discussed within the context of their expression, characterization, downstream processing, neutralization, and in vivo efficacy. The progress of using plant glycoengineering to address ADE, the major impediment of flavivirus therapeutic development, is highlighted. These advancements suggest that plant-based systems are excellent alternatives for addressing the remaining challenges of mAb therapeutic development against flavivirus and may facilitate the eventual commercialization of these drug candidates.

A Human Bi-specific Antibody against Zika Virus with High Therapeutic Potential

Zika virus (ZIKV), a mosquito-borne flavivirus, causes devastating congenital birth defects. We isolated a human monoclonal antibody (mAb), ZKA190, that potently cross-neutralizes multi-lineage ZIKV strains. ZKA190 is highly effective in vivo in preventing morbidity and mortality of ZIKV-infected mice. NMR and cryo-electron microscopy show its binding to an exposed epitope on DIII of the E protein. ZKA190 Fab binds all 180 E protein copies, altering the virus quaternary arrangement and surface curvature. However, ZIKV escape mutants emerged in vitro and in vivo in the presence of ZKA190, as well as of other neutralizing mAbs. To counter this problem, we developed a bispecific antibody (FIT-1) comprising ZKA190 and a second mAb specific for DII of E protein. In addition to retaining high in vitro and in vivo potencies, FIT-1 robustly prevented viral escape, warranting its development as a ZIKV immunotherapy.

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

The four serotypes of dengue virus (DENV) are the leading cause of arboviral diseases in humans. Decades of efforts have made remarkable progress in dengue vaccine development. Despite the first dengue vaccine (dengvaxia from Sanofi Pasteur), a live-attenuated tetravalent chimeric yellow fever-dengue vaccine, has been licensed by several countries since 2016, its overall moderate efficacy (56.5–60.8%) in the presence of neutralizing antibodies during the Phase 2b and 3 trials, lower efficacy among dengue naïve compared with dengue experienced individuals, and increased risk of hospitalization among young children during the follow-up highlight the need for a better understanding of humoral responses after natural DENV infection. Recent studies of more than 300 human monoclonal antibodies (mAbs) against DENV have led to the discovery of several novel epitopes on the envelope protein recognized by potent neutralizing mAbs. This information together with in-depth studies on polyclonal sera and B-cells following natural DENV infection has tremendous implications for better immunogen design for a safe and effective dengue vaccine. This review outlines the progress in our understanding of mouse mAbs, human mAbs, and polyclonal sera against DENV envelope and precursor membrane proteins, two surface proteins involved in vaccine development, following natural infection; analyses of these discoveries have provided valuable insight into new strategies involving molecular technology to induce more potent neutralizing antibodies and less enhancing antibodies for next-generation dengue vaccine development.

An Introduction to B-Cell Epitope Mapping and In Silico Epitope Prediction

Identification of B-cell epitopes is a fundamental step for development of epitope-based vaccines, therapeutic antibodies, and diagnostic tools. Epitope-based antibodies are currently the most promising class of biopharmaceuticals. In the last decade, in-depth in silico analysis and categorization of the experimentally identified epitopes stimulated development of algorithms for epitope prediction. Recently, various in silico tools are employed in attempts to predict B-cell epitopes based on sequence and/or structural data. The main objective of epitope identification is to replace an antigen in the immunization, antibody production, and serodiagnosis. The accurate identification of B-cell epitopes still presents major challenges for immunologists. Advances in B-cell epitope mapping and computational prediction have yielded molecular insights into the process of biorecognition and formation of antigen-antibody complex, which may help to localize B-cell epitopes more precisely. In this paper, we have comprehensively reviewed state-of-the-art experimental methods for B-cell epitope identification, existing databases for epitopes, and novel in silico resources and prediction tools available online. We have also elaborated new trends in the antibody-based epitope prediction. The aim of this review is to assist researchers in identification of B-cell epitopes.

Deconstructing the Antiviral Neutralizing-Antibody Response: Implications for Vaccine Development and Immunity

The antibody response plays a key role in protection against viral infections. While antiviral antibodies may reduce the viral burden via several mechanisms, the ability to directly inhibit (neutralize) infection of cells has been extensively studied. Eliciting a neutralizing-antibody response is a goal of many vaccine development programs and commonly correlates with protection from disease. Considerable insights into the mechanisms of neutralization have been gained from studies of monoclonal antibodies, yet the individual contributions and dynamics of the repertoire of circulating antibody specificities elicited by infection and vaccination are poorly understood on the functional and molecular levels. Neutralizing antibodies with the most protective functionalities may be a rare component of a polyclonal, pathogen-specific antibody response, further complicating efforts to identify the elements of a protective immune response. This review discusses advances in deconstructing polyclonal antibody responses to flavivirus infection or vaccination. Our discussions draw comparisons to HIV-1, a virus with a distinct structure and replication cycle for which the antibody response has been extensively investigated. Progress toward deconstructing and understanding the components of polyclonal antibody responses identifies new targets and challenges for vaccination strategies.

Modeling the Role of Epitope Arrangement on Antibody Binding Stoichiometry in Flaviviruses

Cryo-electron-microscopy (cryo-EM) structures of flaviviruses reveal significant variation in epitope occupancy across different monoclonal antibodies that have largely been attributed to epitope-level differences in conformation or accessibility that affect antibody binding. The consequences of these variations for macroscopic properties such as antibody binding and neutralization are the results of the law of mass action-a stochastic process of innumerable binding and unbinding events between antibodies and the multiple binding sites on the flavivirus in equilibrium-that cannot be directly imputed from structure alone. We carried out coarse-grained spatial stochastic binding simulations for nine flavivirus antibodies with epitopes defined by cryo-EM or x-ray crystallography to assess the role of epitope spatial arrangement on antibody-binding stoichiometry, occupancy, and neutralization. In our simulations, all epitopes were equally competent for binding, representing the upper limit of binding stoichiometry that results from epitope spatial arrangement alone. Surprisingly, our simulations closely reproduced the relative occupancy and binding stoichiometry observed in cryo-EM, without having to account for differences in epitope accessibility or conformation, suggesting that epitope spatial arrangement alone may be sufficient to explain differences in binding occupancy and stoichiometry between antibodies. Furthermore, we found that there was significant heterogeneity in binding configurations even at saturating antibody concentrations, and that bivalent antibody binding may be more common than previously thought. Finally, we propose a structure-based explanation for the stoichiometric threshold model of neutralization.

Structural Basis of Zika Virus-Specific Antibody Protection

Zika virus (ZIKV) infection during pregnancy has emerged as a global public health problem because of its ability to cause severe congenital disease. Here, we developed six mouse monoclonal antibodies (mAbs) against ZIKV including four (ZV-48, ZV-54, ZV-64, and ZV-67) that were ZIKV specific and neutralized infection of African, Asian, and American strains to varying degrees. X-ray crystallographic and competition binding analyses of Fab fragments and scFvs defined three spatially distinct epitopes in DIII of the envelope protein corresponding to the lateral ridge (ZV-54 and ZV-67), C-C' loop (ZV-48 and ZV-64), and ABDE sheet (ZV-2) regions. In vivo passive transfer studies revealed protective activity of DIII-lateral ridge specific neutralizing mAbs in a mouse model of ZIKV infection. Our results suggest that DIII is targeted by multiple type-specific antibodies with distinct neutralizing activity, which provides a path for developing prophylactic antibodies for use in pregnancy or designing epitope-specific vaccines against ZIKV.

Cryo-EM structures elucidate neutralizing mechanisms of anti-chikungunya human monoclonal antibodies with therapeutic activity

Chikungunya virus (CHIKV) is a mosquito-transmitted alphavirus that causes severe acute and chronic disease in humans. Although highly inhibitory murine and human monoclonal antibodies (mAbs) have been generated, the structural basis of their neutralizing activity remains poorly characterized. Here, we determined the cryo-EM structures of chikungunya virus-like particles complexed with antibody fragments (Fab) of two highly protective human mAbs, 4J21 and 5M16, that block virus fusion with host membranes. Both mAbs bind primarily to sites within the A and B domains, as well as to the B domain's β-ribbon connector of the viral glycoprotein E2. The footprints of these antibodies on the viral surface were consistent with results from loss-of-binding studies using an alanine scanning mutagenesis-based epitope mapping approach. The Fab fragments stabilized the position of the B domain relative to the virus, particularly for the complex with 5M16. This finding is consistent with a mechanism of neutralization in which anti-CHIKV mAbs that bridge the A and B domains impede movement of the B domain away from the underlying fusion loop on the E1 glycoprotein and therefore block the requisite pH-dependent fusion of viral and host membranes.

Mechanism of quinine-dependent monoclonal antibody binding to platelet glycoprotein IIb/IIIa

Drug-dependent antibodies (DDAbs) that cause acute thrombocytopenia upon drug exposure are nonreactive in the absence of the drug but bind tightly to a platelet membrane glycoprotein, usually α(IIb)/β3 integrin (GPIIb/IIIa) when the drug is present. How a drug promotes binding of antibody to its target is unknown and is difficult to study with human DDAbs, which are poly-specific and in limited supply. We addressed this question using quinine-dependent murine monoclonal antibodies (mAbs), which, in vitro and in vivo, closely mimic antibodies that cause thrombocytopenia in patients sensitive to quinine. Using surface plasmon resonance (SPR) analysis, we found that quinine binds with very high affinity (K(D) ≈ 10⁻⁹ mol/L) to these mAbs at a molar ratio of ≈ 2:1 but does not bind detectably to an irrelevant mAb. Also using SPR analysis, GPIIb/IIIa was found to bind monovalently to immobilized mAb with low affinity in the absence of quinine and with fivefold greater affinity (K(D) ≈ 2.2 × 10⁻⁶) when quinine was present. Measurements of quinine-dependent binding of intact mAb and fragment antigen-binding (Fab) fragments to platelets showed that affinity is increased 10 000- to 100 000-fold by bivalent interaction between antibody and its target. Together, the findings indicate that the first step in drug-dependent binding of a DDAb is the interaction of the drug with antibody, rather than with antigen, as has been widely thought, where it induces structural changes that enhance the affinity/specificity of antibody for its target epitope. Bivalent binding may be essential for a DDAb to cause thrombocytopenia.

Comprehensive mapping of functional epitopes on dengue virus glycoprotein E DIII for binding to broadly neutralizing antibodies 4E11 and 4E5A by phage display

Here we investigated the binding of Dengue virus envelope glycoprotein domain III (DIII) by two broadly neutralizing antibodies (bNAbs), 4E11 and 4E5A. There are four serotypes of Dengue virus (DENV-1 to -4), whose DIII sequences vary by up to 49%. We used combinatorial alanine scanning mutagenesis, a phage display approach, to map functional epitopes (those residues that contribute most significantly to the energetics of antibody-antigen interaction) on these four serotypes. Our results showed that 4E11, which binds strongly to DENV-1, -2, and -3, and moderately to DENV-4, recognized a common conserved core functional epitope involving DIII residues K310, L/I387, L389, and W391. There were also unique recognition features for each serotype, suggesting that 4E11 has flexible recognition requirements. Similar scanning studies for the related bNAb 4E5A, which binds more tightly to DENV-4, identified broader functional epitopes on DENV-1. These results provide useful information for immunogen and therapeutic antibody design.

A game of numbers: the stoichiometry of antibody-mediated neutralization of flavivirus infection

The humoral response contributes to the protection against viral pathogens. Although antibodies have the potential to inhibit viral infections via several mechanisms, an ability to neutralize viruses directly may be particularly important. Neutralizing antibody titers are commonly used as predictors of protection from infection, especially in the context of vaccine responses and immunity. Despite the simplicity of the concept, how antibody binding results in virus inactivation is incompletely understood despite decades of research. Flaviviruses have been an attractive system in which to seek a structural and quantitative understanding of how antibody interactions with virions modulate infection because of the contribution of antibodies to both protection and pathogenesis. This review will present a stoichiometric model of antibody-mediated neutralization of flaviviruses and discuss how these concepts can inform the development of vaccines and antibody-based therapeutics.

A high-throughput shotgun mutagenesis approach to mapping B-cell antibody epitopes

Characterizing the binding sites of monoclonal antibodies (mAbs) on protein targets, their 'epitopes', can aid in the discovery and development of new therapeutics, diagnostics and vaccines. However, the speed of epitope mapping techniques has not kept pace with the increasingly large numbers of mAbs being isolated. Obtaining detailed epitope maps for functionally relevant antibodies can be challenging, particularly for conformational epitopes on structurally complex proteins. To enable rapid epitope mapping, we developed a high-throughput strategy, shotgun mutagenesis, that enables the identification of both linear and conformational epitopes in a fraction of the time required by conventional approaches. Shotgun mutagenesis epitope mapping is based on large-scale mutagenesis and rapid cellular testing of natively folded proteins. Hundreds of mutant plasmids are individually cloned, arrayed in 384-well microplates, expressed within human cells, and tested for mAb reactivity. Residues are identified as a component of a mAb epitope if their mutation (e.g. to alanine) does not support candidate mAb binding but does support that of other conformational mAbs or allows full protein function. Shotgun mutagenesis is particularly suited for studying structurally complex proteins because targets are expressed in their native form directly within human cells. Shotgun mutagenesis has been used to delineate hundreds of epitopes on a variety of proteins, including G protein-coupled receptor and viral envelope proteins. The epitopes mapped on dengue virus prM/E represent one of the largest collections of epitope information for any viral protein, and results are being used to design better vaccines and drugs.