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.
Small‐scale dynamics of the major envelope protein E of flaviviruses (“virus breathing”) together with binding of specific antibodies can promote fusion‐loop exposure and insertion into plasma membranes of target cells, providing the basis for a special mechanism of antibody‐mediated enhanced infection. Upon cellular uptake by endocytosis, dynamic “breathing” intermediates are functionally important for the structural conversions of E into a post‐fusion conformation to drive membrane fusion in the endosome.
Zika virus has recently spread to South- and Central America, causing congenital birth defects and neurological complications. Many people at risk are flavivirus pre-immune due to prior infections ...with other flaviviruses (e.g. dengue virus) or flavivirus vaccinations. Since pre-existing cross-reactive immunity can potentially modulate antibody responses to Zika virus infection and may affect the outcome of disease, we analyzed fine-specificity as well as virus-neutralizing and infection-enhancing activities of antibodies induced by a primary Zika virus infection in flavivirus-naïve as well as yellow fever- and/or tick-borne encephalitis-vaccinated individuals.
Antibodies in sera from convalescent Zika patients with and without vaccine-induced immunity were assessed by ELISA with respect to Zika virus-specificity and flavivirus cross-reactivity. Functional analyses included virus neutralization and infection-enhancement. The contribution of IgM and cross-reactive antibodies to these properties was determined by depletion experiments.
Pre-existing flavivirus immunity had a strong influence on the antibody response in primary Zika virus infections, resulting in higher titers of broadly flavivirus cross-reactive antibodies and slightly lower levels of Zika virus-specific IgM. Antibody-dependent enhancement (ADE) of Zika virus was mediated by sub-neutralizing concentrations of specific IgG but not by cross-reactive antibodies. This effect was potently counteracted by the presence of neutralizing IgM. Broadly cross-reactive antibodies were able to both neutralize and enhance infection of dengue virus but not Zika virus, indicating a different exposure of conserved sequence elements in the two viruses.
Our data point to an important role of flavivirus-specific IgM during the transient early stages of infection, by contributing substantially to neutralization and by counteracting ADE. In addition, our results highlight structural differences between strains of Zika and dengue viruses that are used for analyzing infection-enhancement by cross-reactive antibodies. These findings underscore the possible impact of specific antibody patterns on flavivirus disease and vaccination efficacy.
Arthropod‐borne flaviviruses include a number of medically relevant human pathogens such as the mosquito‐borne dengue (DEN), Zika, and yellow fever (YF) viruses as well as tick‐borne encephalitis ...virus (TBEV). All flaviviruses are antigenically related and anamnestic responses due to prior immunity can modulate antibody specificities in subsequent infections or vaccinations. In our study, we analyzed the induction of broadly flavivirus cross‐reactive antibodies in tick‐borne encephalitis (TBE) and DEN patients without or with prior flavivirus exposure through TBE and/or YF vaccination, and determined the contribution of these antibodies to TBE and dengue virus (DENV) neutralization. In addition, we investigated the formation of cross‐reactive antibodies in TBE‐vaccination breakthroughs (VBTs). A TBEV infection without prior YF or TBE vaccination induced predominantly type‐specific antibodies. In contrast, high levels of broadly cross‐reactive antibodies were found in samples from TBE patients prevaccinated against YF as well as in DEN patients prevaccinated against TBE and/or YF. While these cross‐reactive antibodies did not neutralize TBEV, they were effective in neutralizing DENV. This discrepancy points to structural differences between the two viruses and indicates that broadly cross‐reactive epitopes are less accessible in TBEV than in DENV. In TBE VBT infections, type‐specific antibodies dominated the antibody response, thus revealing no difference from that of unvaccinated TBE patients. Our results emphasize significant differences in the structural properties of different flaviviruses that have an impact on the induction of broadly cross‐reactive antibodies and their functional activities in virus neutralization.
PDF
