Effective interference between Simbu serogroup orthobunyaviruses in mammalian cells

Immunization with a small fragment of the Schmallenberg virus nucleoprotein highly conserved across the Orthobunyaviruses of the Simbu serogroup reduces viremia in SBV challenged IFNAR-/- mice

Schmallenberg Virus (SBV), an arbovirus from the Peribunyaviridae family and Orthobunyavirus genus, was discovered in late 2011 in Germany and has been circulating in Europe, Asia and Africa ever since. The virus causes a disease associated with ruminants that includes fever, fetal malformation, drop in milk production, diarrhoea and stillbirths, becoming a burden for small and large farms. Building on previous studies on SBV nucleoprotein (SBV-N) as a promising vaccine candidate, we have investigated the possible protein regions responsible for protection. Based on selective truncation of domains designed from the available crystal structure of the SBV-N, we identified both the N-terminal domain (N-term; Met1 - Thr133) and a smaller fragment within (C4; Met1 - Ala58) as vaccine prototypes. Two injections of the N-term and C4 polypeptides protected mice knockout for type I interferon (IFN) receptors (IFNAR^-/-) challenged with virulent SBV, opposite to control groups that presented severe signs of morbidity and weight loss. Viremia analyses along with the presence of IFN-γ secreted from splenocytes re-stimulated with the N-terminal region of the protein corroborate that these two portions of SBV-N can be employed as subunit vaccines. Apart from both proteinaceous fragments being easily produced in bacterial cells, the C4 polypeptide shares a high sequence homology (∼87.1 %) with the corresponding region of nucleoproteins of several viruses of the Simbu serogroup, a group of Orthobunyaviruses that comprises SBV and veterinary pathogens like Akabane virus and human infecting viruses like Oropouche. Thus, we propose that this smaller fragment is better suited for vaccine nanoparticle formulation, and it paves the way to further research with other related Orthobunyaviruses.

Incomplete bunyavirus particles can cooperatively support virus infection and spread

Bunyaviruses lack a specific mechanism to ensure the incorporation of a complete set of genome segments into each virion, explaining the generation of incomplete virus particles lacking one or more genome segments. Such incomplete virus particles, which may represent the majority of particles produced, are generally considered to interfere with virus infection and spread. Using the three-segmented arthropod-borne Rift Valley fever virus as a model bunyavirus, we here show that two distinct incomplete virus particle populations unable to spread autonomously are able to efficiently complement each other in both mammalian and insect cells following co-infection. We further show that complementing incomplete virus particles can co-infect mosquitoes, resulting in the reconstitution of infectious virus that is able to disseminate to the mosquito salivary glands. Computational models of infection dynamics predict that incomplete virus particles can positively impact virus spread over a wide range of conditions, with the strongest effect at intermediate multiplicities of infection. Our findings suggest that incomplete particles may play a significant role in within-host spread and between-host transmission, reminiscent of the infection cycle of multipartite viruses.

Batai Orthobunyavirus: An Emerging Mosquito-Borne Virus in Europe

Batai virus (BATV) is a zoonotic orthobunyavirus transmitted by a wide range of mosquito vectors. The virus is distributed throughout Asia and parts of Africa and has been sporadically detected in several European countries. There is increasing evidence that BATV is emerging in Europe as a potential threat to both animal and human health, having been detected in mosquitoes, mammals, birds and humans. In recent years, serological surveillance in cattle, sheep and goats has suggested an antibody prevalence of up to 46% in European livestock, although human serological prevalence remains generally low. However, the recent and continued spread of invasive mosquito species into Europe may facilitate the establishment of competent populations of mosquitoes leading to increased BATV transmission. Migratory birds may also potentially facilitate the emergence of BATV in geographical locations where it was previously undetected. Although BATV has the potential to cause disease in humans and livestock, our understanding of the impact in wild animal populations is extremely limited. Therefore, there is a need for increased surveillance for BATV in mosquitoes, livestock, wild mammals and birds in Europe to understand the true impact of this virus.

Mammals Preferred: Reassortment of Batai and Bunyamwera orthobunyavirus Occurs in Mammalian but Not Insect Cells

Reassortment is a viral genome-segment recomposition known for many viruses, including the orthobunyaviruses. The co-infection of a host cell with two viruses of the same serogroup, such as the Bunyamwera orthobunyavirus and the Batai orthobunyavirus, can give rise to novel viruses. One example is the Ngari virus, which has caused major outbreaks of human infections in Central Africa. This study aimed to investigate the potential for reassortment of Bunyamwera orthobunyavirus and the Batai orthobunyavirus during co-infection studies and the replication properties of the reassortants in different mammalian and insect cell lines. In the co-infection studies, a Ngari-like virus reassortant and a novel reassortant virus, the Batunya virus, arose in BHK-21 cells (Mesocricetus auratus). In contrast, no reassortment was observed in the examined insect cells from Aedes aegypti (Aag2) and Aedes albopictus (U4.4 and C6/36). The growth kinetic experiments show that both reassortants are replicated to higher titers in some mammalian cell lines than the parental viruses but show impaired growth in insect cell lines.

Host-dependence of in vitro reassortment dynamics among the Sathuperi and Shamonda Simbuviruses

Orthobunyaviruses are arboviruses (Arthropod Borne Virus) and possess multipartite genomes made up of three negative RNAs corresponding to the small (S), medium (M) and large (L) segments. Reassortment and recombination are evolutionary driving forces of such segmented viruses and lead to the emergence of new strains and species. Retrospective studies based on phylogenetical analysis are able to evaluate these mechanisms at the end of the selection process but fail to address the dynamics of emergence. This issue was addressed using two Orthobunyaviruses infecting ruminants and belonging to the Simbu serogroup: the Sathuperi virus (SATV) and the Shamonda virus (SHAV). Both viruses were associated with abortion, stillbirth and congenital malformations occurring after transplacental transmission and were suspected to spread together in different ruminant and insect populations. This study showed that different viruses related to SHAV and SATV are spreading simultaneously in ruminants and equids of the Sub-Saharan region. Their reassortment and recombination potential was evaluated in mammalian and in insect contexts. A method was set up to determine the genomic background of any clonal progeny viruses isolated after in vitro coinfections assays. All the reassortment combinations were generated in both contexts while no recombinant virus was isolated. Progeny virus populations revealed a high level of reassortment in mammalian cells and a much lower level in insect cells. In vitro selection pressure that mimicked the host switching (insect-mammal) revealed that the best adapted reassortant virus was connected with an advantageous replicative fitness and with the presence of a specific segment.