Focused dengue vaccine development: outwitting nature's design

Stabilized dengue virus 2 envelope subunit vaccine redirects the neutralizing antibody response to all E-domains

The four dengue virus (DENV) serotypes cause several hundred million infections annually. Several live-attenuated tetravalent dengue vaccines (LAVs) are at different stages of clinical testing and regulatory approval. A major hurdle faced by the two leading LAVs is uneven replication of vaccine serotypes stimulating a dominant response to one serotype at the expense of the other three, leading to the potential for vaccine antibody (Ab)-enhanced, more severe infections by wild-type (WT) DENV serotypes that fail to replicate in the vaccine. Protein subunit vaccines are a promising alternative since antigen dosing can be precisely controlled. However, DENV envelope (E) protein subunit vaccines have not performed well to date, possibly due to differences between the monomeric structure of soluble E and the E homodimer of the viral surface. Previously, we have combined structure-guided computational and experimental approaches to design and produce DENV2 E antigens that are stable homodimers at 37℃ and stimulate higher levels of neutralizing Abs (NAbs) than the WT E antigen in mice. The goal of this study was to evaluate if DENV2 E homodimers stimulate NAbs that target different epitopes on E protein compared to the WT E monomer. Using DENV4/2 chimeric viruses and Ab depletion methods, we mapped the WT E-elicited NAbs to simple epitopes on domain III of E. In contrast, the stable E homodimer stimulated a more complex response toward all three surface-exposed domains of the E protein. Our findings highlight the impact of DENV2 E oligomeric state on the quality and specificity of DENV NAbs and the promise of DENV E homodimers as subunit vaccines.IMPORTANCEThe ideal dengue virus (DENV) vaccine should elicit a balanced and highly protective immune response against all four DENV serotypes. Current tetravalent live-attenuated DENV vaccines have faced challenges due to uneven replication of vaccine virus strains stimulating a strong immune response to one serotype and weak responses to the other three. Protein subunit vaccines provide novel opportunities to stimulate a balanced response because dosing can be precisely controlled and independent of vaccine virus replication. Here, we compare immune responses elicited by a new DENV serotype 2 protein vaccine designed to match the structure of proteins on the viral surface. We find that proteins designed to match the viral surface stimulate better immune responses targeting multiple sites on the viral surface compared to previous protein vaccines. Our results justify further testing and development of these second-generation DENV protein subunit vaccines.

Impact of structural dynamics on biological functions of flaviviruses

Flaviviruses comprise a number of mosquito- or tick-transmitted human pathogens of global public health importance. Advances in structural biology techniques have contributed substantially to our current understanding of the life cycle of these small enveloped RNA viruses and led to deep insights into details of virus assembly, maturation and cell entry. In addition to large-scale conformational changes and oligomeric rearrangements of envelope proteins during these processes, there is increasing evidence that smaller-scale protein dynamics (referred to as virus "breathing") can confer extra flexibility to these viruses for the fine-tuning of their interactions with the immune system and possibly with cellular factors they encounter in their complex ecological cycles in arthropod and vertebrate hosts. In this review, we discuss how work with tick-borne encephalitis virus has extended our view on flavivirus breathing, leading to the identification of a novel mechanism of antibody-mediated infection enhancement and demonstrating breathing intermediates of the envelope protein in the process of membrane fusion. These data are discussed in the context of other flaviviruses and the perspective of a potential role of virus breathing to cope with the requirements of adaptation and replication in evolutionarily very different hosts.

Designed, highly expressing, thermostable dengue virus 2 envelope protein dimers elicit quaternary epitope antibodies

Dengue virus (DENV) is a worldwide health burden, and a safe vaccine is needed. Neutralizing antibodies bind to quaternary epitopes on DENV envelope (E) protein homodimers. However, recombinantly expressed soluble E proteins are monomers under vaccination conditions and do not present these quaternary epitopes, partly explaining their limited success as vaccine antigens. Using molecular modeling, we found DENV2 E protein mutations that induce dimerization at low concentrations (<100 pM) and enhance production yield by more than 50-fold. Cross-dimer epitope antibodies bind to the stabilized dimers, and a crystal structure resembles the wild-type (WT) E protein bound to a dimer epitope antibody. Mice immunized with the stabilized dimers developed antibodies that bind to E dimers and not monomers and elicited higher levels of DENV2-neutralizing antibodies compared to mice immunized with WT E antigen. Our findings demonstrate the feasibility of using structure-based design to produce subunit vaccines for dengue and other flaviviruses.

Dengue Vaccines: The Promise and Pitfalls of Antibody-Mediated Protection

More than 390 million human dengue virus (DENV) infections occur each year, worldwide. Dengvaxia, a live-virus tetravalent vaccine from Sanofi Pasteur, was recently approved for human clinical use, although vaccine performance against the four DENV serotypes is highly variable. Other dengue vaccines in advanced clinical testing also demonstrate variability in efficacy. In this review, we outline the benefits and challenges of developing a safe, effective, and balanced DENV vaccine that can provide uniform protection against all four serotypes. Even though T cell biology plays an important role in establishing protective immunity, this review focuses on B cell responses. We discuss the leading dengue vaccine candidates and review the specificity of antibody responses and the known immune correlates of protection against DENV infection. A better understanding of immune correlates of protection against DENV infection will inform the development of a vaccine that can provide long-term, uniform protection.

Dimerization of Dengue Virus E Subunits Impacts Antibody Function and Domain Focus

Dengue virus (DENV) is responsible for the most prevalent and significant arthropod-borne viral infection of humans. The leading DENV vaccines are based on tetravalent live-attenuated virus platforms. In practice, it has been challenging to induce balanced and effective responses to each of the four DENV serotypes because of differences in the replication efficiency and immunogenicity of individual vaccine components. Unlike live vaccines, tetravalent DENV envelope (E) protein subunit vaccines are likely to stimulate balanced immune responses, because immunogenicity is replication independent. However, E protein subunit vaccines have historically performed poorly, in part because the antigens utilized were mainly monomers that did not display quaternary-structure epitopes found on E dimers and higher-order structures that form the viral envelope. In this study, we compared the immunogenicity of DENV2 E homodimers and DENV2 E monomers. The stabilized DENV2 homodimers, but not monomers, were efficiently recognized by virus-specific and flavivirus cross-reactive potently neutralizing antibodies that have been mapped to quaternary-structure epitopes displayed on the viral surface. In mice, the dimers stimulated 3-fold-higher levels of virus-specific neutralizing IgG that recognized epitopes different from those recognized by lower-level neutralizing antibodies induced by monomers. The dimer induced a stronger E domain I (EDI)- and EDII-targeted response, while the monomer antigens stimulated an EDIII epitope response and induced fusion loop epitope antibodies that are known to facilitate antibody-dependent enhancement (ADE). This study shows that DENV E subunit antigens that have been designed to mimic the structural organization of the viral surface are better vaccine antigens than E protein monomers.IMPORTANCE Dengue virus vaccine development is particularly challenging because vaccines have to provide protection against four different dengue virus stereotypes. The leading dengue virus vaccine candidates in clinical testing are all based on live-virus vaccine platforms and struggle to induce balanced immunity. Envelope subunit antigens have the potential to overcome these limitations but have historically performed poorly as vaccine antigens, because the versions tested previously were presented as monomers and not in their natural dimer configuration. This study shows that the authentic presentation of DENV2 E-based subunits has a strong impact on antibody responses, underscoring the importance of mimicking the complex protein structures that are found on DENV particle surfaces when designing subunit vaccines.