Community recommendations on cryoEM data archiving and validation.

In January 2020, a workshop was held at EMBL-EBI (Hinxton, UK) to discuss data requirements for the deposition and validation of cryoEM structures, with a focus on single-particle analysis. The meeting was attended by 47 experts in data processing, model building and refinement, validation, and archiving of such structures. This report describes the workshop's motivation and history, the topics discussed, and the resulting consensus recommendations. Some challenges for future methods-development efforts in this area are also highlighted, as is the implementation to date of some of the recommendations.

Community recommendations on cryoEM data archiving and validation: Outcomes of a wwPDB/EMDB workshop on cryoEM data management, deposition and validation.

In January 2020, a workshop was held at EMBL-EBI (Hinxton, UK) to discuss data requirements for deposition and validation of cryoEM structures, with a focus on single-particle analysis. The meeting was attended by 47 experts in data processing, model building and refinement, validation, and archiving of such structures. This report describes the workshop's motivation and history, the topics discussed, and consensus recommendations resulting from the workshop. Some challenges for future methods-development efforts in this area are also highlighted, as is the implementation to date of some of the recommendations.

Structure and neutralization mechanism of a human antibody targeting a complex Epitope on Zika virus.

We currently have an incomplete understanding of why only a fraction of human antibodies that bind to flaviviruses block infection of cells. Here we define the footprint of a strongly neutralizing human monoclonal antibody (mAb G9E) with Zika virus (ZIKV) by both X-ray crystallography and cryo-electron microscopy. Flavivirus envelope (E) glycoproteins are present as homodimers on the virion surface, and G9E bound to a quaternary structure epitope spanning both E protomers forming a homodimer. As G9E mainly neutralized ZIKV by blocking a step after viral attachment to cells, we tested if the neutralization mechanism of G9E was dependent on the mAb cross-linking E molecules and blocking low-pH triggered conformational changes required for viral membrane fusion. We introduced targeted mutations to the G9E paratope to create recombinant antibodies that bound to the ZIKV envelope without cross-linking E protomers. The G9E paratope mutants that bound to a restricted epitope on one protomer poorly neutralized ZIKV compared to the wild-type mAb, demonstrating that the neutralization mechanism depended on the ability of G9E to cross-link E proteins. In cell-free low pH triggered viral fusion assay, both wild-type G9E, and epitope restricted paratope mutant G9E bound to ZIKV but only the wild-type G9E blocked fusion. We propose that, beyond antibody binding strength, the ability of human antibodies to cross-link E-proteins is a critical determinant of flavivirus neutralization potency.

CryoEM structures of the multimeric secreted NS1, a major factor for dengue hemorrhagic fever.

Dengue virus infection can cause dengue hemorrhagic fever (DHF). Dengue NS1 is multifunctional. The intracellular dimeric NS1 (iNS1) forms part of the viral replication complex. Previous studies suggest the extracellular secreted NS1 (sNS1), which is a major factor contributing to DHF, exists as hexamers. The structure of the iNS1 is well-characterised but not that of sNS1. Here we show by cryoEM that the recombinant sNS1 exists in multiple oligomeric states: the tetrameric (stable and loose conformation) and hexameric structures. Stability of the stable and loose tetramers is determined by the conformation of their N-terminal domain - elongated ß-sheet or ß-roll. Binding of an anti-NS1 Fab breaks the loose tetrameric and hexameric sNS1 into dimers, whereas the stable tetramer remains largely unbound. Our results show detailed quaternary organization of different oligomeric states of sNS1 and will contribute towards the design of dengue therapeutics.

Human antibody C10 neutralizes by diminishing Zika but enhancing dengue virus dynamics.

The human monoclonal antibody (HmAb) C10 potently cross-neutralizes Zika virus (ZIKV) and dengue virus. Analysis of antibody fragment (Fab) C10 interactions with ZIKV and dengue virus serotype 2 (DENV2) particles by cryoelectron microscopy (cryo-EM) and amide hydrogen/deuterium exchange mass spectrometry (HDXMS) shows that Fab C10 binding decreases overall ZIKV particle dynamics, whereas with DENV2, the same Fab causes increased dynamics. Testing of different Fab C10:DENV2 E protein molar ratios revealed that, at higher Fab ratios, especially at saturated concentrations, the Fab enhanced viral dynamics (detected by HDXMS), and observation under cryo-EM showed increased numbers of distorted particles. Our results suggest that Fab C10 stabilizes ZIKV but that with DENV2 particles, high Fab C10 occupancy promotes E protein dimer conformational changes leading to overall increased particle dynamics and distortion of the viral surface. This is the first instance of a broadly neutralizing antibody eliciting virus-specific increases in whole virus particle dynamics.

Morphological Diversity and Dynamics of Dengue Virus Affecting Antigenicity.

The four serotypes of the mature dengue virus can display different morphologies, including the compact spherical, the bumpy spherical and the non-spherical clubshape morphologies. In addition, the maturation process of dengue virus is inefficient and therefore some partially immature dengue virus particles have been observed and they are infectious. All these viral particles have different antigenicity profiles and thus may affect the type of the elicited antibodies during an immune response. Understanding the molecular determinants and environmental conditions (e.g., temperature) in inducing morphological changes in the virus and how potent antibodies interact with these particles is important for designing effective therapeutics or vaccines. Several techniques, including cryoEM, site-directed mutagenesis, hydrogen-deuterium exchange mass spectrometry, time-resolve fluorescence resonance energy transfer, and molecular dynamic simulation, have been performed to investigate the structural changes. This review describes all known morphological variants of DENV discovered thus far, their surface protein dynamics and the key residues or interactions that play important roles in the structural changes.

An NTD supersite of attack.

To date, most antibodies recognizing the SARS-CoV-2 spike (S) protein target the receptor binding domain (RBD). Three recent studies (Cerutti et al., 2021; McCallum et al., 2021; and Suryadevara et al., 2021) report on highly protective antibodies that are specific to the N-terminal domain (NTD) and target a conserved supersite.

How NS1 Antibodies Prevent Severe Flavivirus Disease.

The flavivirus genus consists of several major human pathogens including dengue (DENV) and Zika viruses. The flavivirus nonstructural protein 1 (NS1) plays an important role in disease progression, for example, in the development of severe dengue disease. Anti-NS1 antibodies have been shown to confer protection, and two new studies by Biering et al. and Modhiran et al. on the structure of NS1:antibody complexes reveal their mechanism of neutralization.

Antibody affinity versus dengue morphology influences neutralization.

Different strains within a dengue serotype (DENV1-4) can have smooth, or "bumpy" surface morphologies with different antigenic characteristics at average body temperature (37°C). We determined the neutralizing properties of a serotype cross-reactive human monoclonal antibody (HMAb) 1C19 for strains with differing morphologies within the DENV1 and DENV2 serotypes. We mapped the 1C19 epitope to E protein domain II by hydrogen deuterium exchange mass spectrometry, cryoEM and molecular dynamics simulations, revealing that this epitope is likely partially hidden on the virus surface. We showed the antibody has high affinity for binding to recombinant DENV1 E proteins compared to those of DENV2, consistent with its strong neutralizing activities for all DENV1 strains tested regardless of their morphologies. This finding suggests that the antibody could out-compete E-to-E interaction for binding to its epitope. In contrast, for DENV2, HMAb 1C19 can only neutralize when the epitope becomes exposed on the bumpy-surfaced particle. Although HMAb 1C19 is not a suitable therapeutic candidate, this study with HMAb 1C19 shows the importance of choosing a high-affinity antibody that could neutralize diverse dengue virus morphologies for therapeutic purposes.

Structural basis of Chikungunya virus inhibition by monoclonal antibodies.

Chikungunya virus (CHIKV) is an emerging viral pathogen that causes both acute and chronic debilitating arthritis. Here, we describe the functional and structural basis as to how two anti-CHIKV monoclonal antibodies, CHK-124 and CHK-263, potently inhibit CHIKV infection in vitro and in vivo. Our in vitro studies show that CHK-124 and CHK-263 block CHIKV at multiple stages of viral infection. CHK-124 aggregates virus particles and blocks attachment. Also, due to antibody-induced virus aggregation, fusion with endosomes and egress are inhibited. CHK-263 neutralizes CHIKV infection mainly by blocking virus attachment and fusion. To determine the structural basis of neutralization, we generated cryogenic electron microscopy reconstructions of Fab:CHIKV complexes at 4- to 5-Å resolution. CHK-124 binds to the E2 domain B and overlaps with the Mxra8 receptor-binding site. CHK-263 blocks fusion by binding an epitope that spans across E1 and E2 and locks the heterodimer together, likely preventing structural rearrangements required for fusion. These results provide structural insight as to how neutralizing antibody engagement of CHIKV inhibits different stages of the viral life cycle, which could inform vaccine and therapeutic design.

High flavivirus structural plasticity demonstrated by a non-spherical morphological variant.

Previous flavivirus (dengue and Zika viruses) studies showed largely spherical particles either with smooth or bumpy surfaces. Here, we demonstrate flavivirus particles have high structural plasticity by the induction of a non-spherical morphology at elevated temperatures: the club-shaped particle (clubSP), which contains a cylindrical tail and a disc-like head. Complex formation of DENV and ZIKV with Fab C10 stabilize the viruses allowing cryoEM structural determination to ~10 Å resolution. The caterpillar-shaped (catSP) Fab C10:ZIKV complex shows Fabs locking the E protein raft structure containing three E dimers. However, compared to the original spherical structure, the rafts have rotated relative to each other. The helical tail structure of Fab C10:DENV3 clubSP showed although the Fab locked an E protein dimer, the dimers have shifted laterally. Morphological diversity, including clubSP and the previously identified bumpy and smooth-surfaced spherical particles, may help flavivirus survival and immune evasion.

A Human Antibody Neutralizes Different Flaviviruses by Using Different Mechanisms.

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

Capsid protein structure in Zika virus reveals the flavivirus assembly process.

Structures of flavivirus (dengue virus and Zika virus) particles are known to near-atomic resolution and show detailed structure and arrangement of their surface proteins (E and prM in immature virus or M in mature virus). By contrast, the arrangement of the capsid proteins:RNA complex, which forms the core of the particle, is poorly understood, likely due to inherent dynamics. Here, we stabilize immature Zika virus via an antibody that binds across the E and prM proteins, resulting in a subnanometer resolution structure of capsid proteins within the virus particle. Fitting of the capsid protein into densities shows the presence of a helix previously thought to be removed via proteolysis. This structure illuminates capsid protein quaternary organization, including its orientation relative to the lipid membrane and the genomic RNA, and its interactions with the transmembrane regions of the surface proteins. Results show the capsid protein plays a central role in the flavivirus assembly process.

Molecular basis of dengue virus serotype 2 morphological switch from 29°C to 37°C.

The ability of DENV2 to display different morphologies (hence different antigenic properties) complicates vaccine and therapeutics development. Previous studies showed most strains of laboratory adapted DENV2 particles changed from smooth to "bumpy" surfaced morphology when the temperature is switched from 29°C at 37°C. Here we identified five envelope (E) protein residues different between two alternative passage history DENV2 NGC strains exhibiting smooth or bumpy surface morphologies. Several mutations performed on the smooth DENV2 infectious clone destabilized the surface, as observed by cryoEM. Molecular dynamics simulations demonstrated how chemically subtle substitution at various positions destabilized dimeric interactions between E proteins. In contrast, three out of four DENV2 clinical isolates showed a smooth surface morphology at 37°C, and only at high fever temperature (40°C) did they become "bumpy". These results imply vaccines should contain particles representing both morphologies. For prophylactic and therapeutic treatments, this study also informs on which types of antibodies should be used at different stages of an infection, i.e., those that bind to monomeric E proteins on the bumpy surface or across multiple E proteins on the smooth surfaced virus.

Structural perspectives of antibody-dependent enhancement of infection of dengue virus.

Dengue virus (DENV) consists of four serotypes. Sequential serotype infections can cause increased disease severity, likely due to antibody-dependent enhancement (ADE) of infection. Here, we review two recent papers showing major advancements in the understanding of the ADE mechanism for both mature and immature DENV. The surface of both mature and immature DENV contains E and another protein - M in mature and prM in immature virus. On mature DENV, the orientation of anti-E antibody with respect to the virus surface determines the antibody enhancement properties. On the immature virus, binding of anti-prM antibody aids the dissociation of pr from the fusion loop of E protein allowing virus-endosomal membrane interaction, thus overcoming the hurdle in the early step of fusion.

Mechanism of Enhanced Immature Dengue Virus Attachment to Endosomal Membrane Induced by prM Antibody.

Dengue virus (DENV) particles are released from cells in different maturation states. Fully immature DENV (immDENV) is generally non-infectious, but can become infectious when complexed with anti-precursor membrane (prM) protein antibodies. It is unknown how anti-prM antibody-coated particles can undergo membrane fusion since the prM caps the envelope (E) protein fusion loop. Here, we determined cryoelectron microscopy (cryo-EM) maps of the immDENV:anti-prM complex at different pH values, mimicking the extracellular (pH 8.0) or endosomal (pH 5.0) environments. At pH 5.0, there are two structural classes with fewer antibodies bound than at pH 8.0. These classes may represent different maturation states. Molecular simulations, together with the measured high-affinity pr:antibody interaction (versus the weak pr:E interaction) and also the low pH cryo-EM structures, suggest how antibody:pr complex can dislodge from the E protein at low pH. This exposes the E protein fusion loop enhancing virus interaction with endosomes.

Inhibiting EBV: The Key to Carefree Smooching.

There are no vaccines or therapies to treat Epstein-Barr virus (EBV). Snijder et al. (2018) isolated a potent human antibody against EBV that blocks infection of both B cells and epithelial cells. Structural analysis of the antibody complexed with a viral surface protein complex identified a site that may be useful in vaccine development.