An antibody to lock dengue virus out: Mosquito-borne dengue virus (DENV) is a growing public health threat. Nearly 400 million people are infected annually, and no vaccine is currently available. Fibriansah et al. report that a human antibody (2D22) specific for DENV serotype 2, when given therapeutically, can protect mice from a lethal form of this virus. Structural analysis revealed that 2D22 binds across multiple DENV envelope proteins, which probably blocks the ability of these proteins to assemble into the orientation necessary for viral entry. The epitope where 2D22 binds to the virus may therefore represent a potential vaccine target. Science , this issue p. 88

Dengue virus (DENV), which consists of four serotypes (DENV1‐4), infects over 400 million people annually. Previous studies have indicated most human monoclonal antibodies (HMAbs) from dengue patients are cross‐reactive and poorly neutralizing. Rare neutralizing HMAbs are usually serotype‐specific and bind to quaternary structure‐dependent epitopes. We determined the structure of DENV1 complexed with Fab fragments of a highly potent HMAb 1F4 to 6 Å resolution by cryo‐EM. Although HMAb 1F4 appeared to bind to virus and not E proteins in ELISAs in the previous study, our structure showed that the epitope is located within an envelope (E) protein monomer, and not across neighboring E proteins. The Fab molecules bind to domain I (DI), and DI‐DII hinge of the E protein. We also showed that HMAb 1F4 can neutralize DENV at different stages of viral entry in a cell type and receptor dependent manner. The structure reveals the mechanism by which this potent and specific antibody blocks viral infection.

Previous binding studies of antibodies that recognized a partially or fully hidden epitope suggest that insect cell-derived dengue virus undergoes structural changes at an elevated temperature. This was confirmed by our cryo-electron microscopy images of dengue virus incubated at 37°C, where viruses change their surface from smooth to rough. Here we present the cryo-electron microscopy structures of dengue virus at 37°C. Image analysis showed four classes of particles. The three-dimensional (3D) map of one of these classes, representing half of the imaged virus population, shows that the E protein shell has expanded and there is a hole at the 3-fold vertices. Fitting E protein structures into the map suggests that all of the interdimeric and some intradimeric E protein interactions are weakened. The accessibility of some previously found cryptic epitopes on this class of particles is discussed.

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.

Severe dengue virus (DENV)-associated diseases can occur in patients who have preexisting DENV antibodies (Abs) through antibody-dependent enhancement (ADE) of infection. It is well established that during ADE, DENV-antibody immune complexes (ICs) infect Fcγ receptor-bearing cells and increase the systemic viral burden that can be measured in the blood. For protection against infection with DENV serotypes 1 to 4, strongly neutralizing Abs must be elicited to overcome the effect of ADE. Clinical observations in infants who have maternal DENV Abs or recent phase II/III clinical trials with a leading tetravalent dengue vaccine suggested a lack of correlation between Ab neutralization and in vivo disease prevention. In addressing this gap in knowledge, we found that inoculation of ICs formed with serotype cross-reactive Abs that are more than 98% neutralized in vitro promotes high mortality in AG129 mice even though peak viremia was lower than that in direct virus infection. This suggests that the serum viremia level is not always correlated with disease severity. We further demonstrated that infection with the ICs resulted in increased vascular permeability, specifically in the small intestine, accompanied with increased tissue viral load and cytokine production, which can be suppressed by anti-tumor necrosis factor alpha (anti-TNF-α) Abs. Flow cytometric analysis identified increased infection in CD11b ^int CD11c ^int/hi CD103 ⁻ antigen-presenting cells by IC inoculation, suggesting that these infected cells may be responsible for the increase in TNF-α production and vascular permeability in the small intestine that lead to mortality in mice. Our findings may have important implications for the development of dengue therapeutics. IMPORTANCE We examined the relationship between the neutralizing level of Abs at the time of infection and subsequent disease progression in a mouse model in order to understand why patients who are shown to have a neutralizing quantity of Abs still allow sufficient DENV replication to induce severe dengue manifestations, which sometimes do not correlate with viremia level. Strikingly, we found that high mortality was induced in AG129 mice by the increase in TNF-α-induced vascular permeability accompanied by an increased viral load, specifically in the small intestine, even when the initial infection level is suppressed to less than 5% and the peak viremia level is not enhanced. This suggests that ADE overcomes the protective efficacy of Abs in a tissue-dependent manner that leads to severe small intestinal pathology. Our findings may serve to address the pathogenic role of Abs on severe dengue disease and also help to develop safe Ab-based therapeutic strategies.