A Bacterially-Expressed Recombinant Envelope Protein from Usutu Virus Induces Neutralizing Antibodies in Rabbits

Sterile protection against relapsing malaria with a single-shot vaccine

Vaccine development for Plasmodium vivax, an important human relapsing malaria, is lagging behind. In the case of the most deadly human malaria P. falciparum, unprecedented high levels of protection have been obtained by immunization with live sporozoites under accompanying chemoprophylaxis, which prevents the onset of blood-stage malaria. Such an approach has not been fully evaluated for relapsing malaria. Here, in the P. cynomolgi-rhesus macaque model for relapsing malaria, we employ the parasites' natural relapsing phenotype to self-boost the immune response against liver-stage parasites, following a single-shot high-dose live sporozoite vaccination. This approach resulted in sterile protection against homologous sporozoite challenge in three out of four animals in the group that was also exposed for several days to blood stages during primary infection and relapses. One out of four animals in the group that received continuous chemoprophylaxis to abort blood-stage exposure was also protected from sporozoite challenge. Although obtained in a small number of animals as part of a Proof-of-Concept study, these results suggest that limited blood-stage parasite exposure may augment protection in this model. We anticipate our data are a starting point for further research into correlates of protection and extrapolation of the single-shot approach to develop efficacious malaria vaccines against relapsing human malaria.

The Japanese Encephalitis Antigenic Complex Viruses: From Structure to Immunity

In the last three decades, several flaviviruses of concern that belong to different antigenic groups have expanded geographically. This has resulted in the presence of often more than one virus from a single antigenic group in some areas, while in Europe, Africa and Australia, additionally, multiple viruses belonging to the Japanese encephalitis (JE) serogroup co-circulate. Morphological heterogeneity of flaviviruses dictates antibody recognition and affects virus neutralization, which influences infection control. The latter is further impacted by sequential infections involving diverse flaviviruses co-circulating within a region and their cross-reactivity. The ensuing complex molecular virus-host interplay leads to either cross-protection or disease enhancement; however, the molecular determinants and mechanisms driving these outcomes are unclear. In this review, we provide an overview of the epidemiology of four JE serocomplex viruses, parameters affecting flaviviral heterogeneity and antibody recognition, host immune responses and the current knowledge of the cross-reactivity involving JE serocomplex flaviviruses that leads to differential clinical outcomes, which may inform future preventative and therapeutic interventions.

Structure of Usutu virus SAAR-1776 displays fusion loop asymmetry

Usutu virus (USUV) is an emerging arbovirus in Europe that has been increasingly identified in asymptomatic humans and donated blood samples and is a cause of increased incidents of neuroinvasive human disease. Treatment or prevention options for USUV disease are currently nonexistent, the result of a lack of understanding of the fundamental elements of USUV pathogenesis. Here, we report two structures of the mature USUV virus, determined at a resolution of 2.4 Å, using single-particle cryogenic electron microscopy. Mature USUV is an icosahedral shell of 180 copies of envelope (E) and membrane (M) proteins arranged in the classic herringbone pattern. However, unlike previous reports of flavivirus structures, we observe virus subpopulations and differences in the fusion loop disulfide bond. Presence of a second, unique E glycosylation site could elucidate host interactions, contributing to the broad USUV tissue tropism. The structures provide a basis for exploring USUV interactions with glycosaminoglycans and lectins, the role of the RGD motif as a receptor, and the inability of West Nile virus therapeutic antibody E16 to neutralize the mature USUV strain SAAR-1776. Finally, we identify three lipid binding sites and predict key residues that likely participate in virus stability and flexibility during membrane fusion. Our findings provide a framework for the development of USUV therapeutics and expand the current knowledge base of flavivirus biology.