Two strains of Toscana virus show different virulence and replication capacity in mice and cell culture models

Toscana virus (TOSV), belonging to Phenuiviridae family, is circulating in most Mediterranean countries and is transmitted to humans by infected female sand flies. While most infections are asymptomatic, TOSV is considered as a leading cause of meningitis and encephalitis in humans during summer. Three TOSV genotypes (named A, B, and C) have been identified, although no virus strain belonging to lineage C has been isolated so far. To date, the relationship between TOSV genetic diversity and viral pathogenicity or replication capacity remains unknown. This study aimed to compare two TOSV strains from either lineage A (TOSV-A) or B (TOSV-B) in several cell culture and two mouse models. We showed that TOSV-A replicated more efficiently in BSR and A549 cells while TOSV-B had a replication advantage in human induced pluripotent stem cells differentiated in neural cells and LL-5 sand fly cells. In vivo, we were unable to detect any virus in the brains of immunocompetent C57BL/6JRj mice infected with either strain of TOSV. On the contrary, we showed that TOSV-A disseminated to the central nervous system of 129/Sv ifnar -/- mice unlike TOSV-B, despite higher viremia of TOSV-B and a greater dissemination of this strain in other organs. The reasons for these differences are not yet known, although we showed that the presence of TOSV neutralizing antibodies in serum was slightly delayed in TOSV-A-infected mice. Altogether, the data presented in this study provide new avenues to study TOSV-induced pathogenesis and ultimately unveil molecular viral determinants modulating TOSV replication capacity. Author SummaryToscana virus is a sand fly-borne phlebovirus with tropism for the human central nervous system (CNS) and the causal agent of aseptic meningitis and encephalitis during summer in countries surrounding the Mediterranean Sea. The pathogenesis induced by TOSV is still poorly understood as well as the correlation between TOSV genetic diversity and viral pathogenicity or replication capacity. In this study, we characterised two TOSV strains belonging to two different genetic lineages (named A and B) in several in vitro, ex vivo and in vivo models of infection. We showed that the biological properties of these two strains is strikingly different and, importantly, we identified 129/Sv ifnar -/- mice as a suitable model to study the mechanism of TOSV invasion of the CNS and viral pathogenesis. Our comparative study also highlights the importance of strengthening our efforts to identify molecular viral determinants modulating TOSV-induced pathogenesis and vector transmission, in order to better characterise the epidemiological risk posed by circulating strains.