Rift Valley fever virus (RVFV) is a highly pathogenic zoonotic arbovirus endemic in many African countries and the Arabian Peninsula. Animal infections cause high rates of mortality and abortion among sheep, goats and cattle. In humans, an estimated 1-2% of RVFV infections result in severe disease (encephalitis, hepatitis, retinitis) with a high rate of lethality when associated to hemorrhagic fever. RVFV's NSs protein, which is RVFV's main factor of virulence, counteracts the host innate antiviral response favoring viral replication and spread. However, the mechanisms underlying RVFV-induced cytopathic effects and the role of NSs in these alterations remain for the most undeciphered. In this work we have analyzed the effects of NSs expression on actin cytoskeleton while conducting infections with the NSs expressing virulent (ZH548) and attenuated (MP12) strains of RVFV and the non-NSs expressing avirulent (ZH548ΔNSs) strain as well as after the ectopic expression of NSs. In macrophages, fibroblasts and hepatocytes NSs expression prevented the up-regulation of Abl2 (a major regulator of actin cytoskeleton) expression otherwise induced by avirulent infections and identified here as part of the antiviral response. The presence of NSs was also linked to and increased mobility of ZH548- as compared to ZH548ΔNSs-infected fibroblasts and to strong changes in cell morphology in non-migrating hepatocytes with reduction of lamellipodia, cell spreading and dissolution of adherens junctions reminiscent of ZH548-induced cytopathic effects observed in vivo. Finally, we show evidence of the presence of NSs within long actin-rich structures associated to NSs dissemination from NSs expressing towards non-NSs expressing cells. Importance. Rift Valley fever virus (RVFV) is a dangerous human and animal pathogen that was ranked in 2018 by the World Health Organization among the eight pathogens of most concern likely to cause wide epidemics in the near future for which there is no, or insufficient, countermeasures. The interest of this work resides in the fact that it addresses the question of the mechanisms underlying RVFV-induced cytopathic effects that participate in RVFV's pathogenicity. We demonstrate here that RVFV targets cell adhesion and actin cytoskeleton at the transcriptional and cellular level, affecting cell mobility and inducing cell shape collapse alongside with distortion of cell-cell adhesion. All these effects are susceptible to participate in RVFV-induced pathogenicity, facilitate virulent RVFV dissemination and thus constitute interesting potential targets in future development of antiviral therapeutic strategies that in the case of RVFV, as several other emerging arboviruses, are presently lacking.

Infection of mice with Rift Valley fever virus (RVFV) reproduces major pathological features of severe human disease, notably the early-onset hepatitis and delayed-onset encephalitis. We previously reported that the Rvfs2 locus from the susceptible MBT/Pas strain reduces survival time after RVFV infection. Here, we used BALB/cByJ (BALB) mice congenic for Rvfs2 (C.MBT-Rvfs2) to investigate the pathophysiological mechanisms impacted by Rvfs2. Clinical, biochemical and histopathological features indicated similar liver damage in BALB and C.MBT-Rvfs2 mice until day 5 after infection. However, while C.MBT-Rvfs2 mice succumbed from acute liver injury, most BALB mice recovered and died later of encephalitis. Hepatocytes of BALB infected liver proliferated actively on day 6, promoting organ regeneration and recovery from liver damage. By comparison with C.MBT-Rvfs2, BALB mice had up to 100-fold lower production of infectious virions in the peripheral blood and liver, strongly decreased RVFV protein in liver and reduced viral replication in primary cultured hepatocytes, suggesting that the BALB Rvfs2 haplotype limits RVFV pathogenicity through decreased virus replication. Moreover, bone marrow chimera experiments showed that both hematopoietic and non-hematopoietic cells are required for the protective effect of the BALB Rvfs2 haplotype. Altogether, these results indicate that Rvfs2 controls critical events which allow survival to RVFV-induced hepatitis.

The systemic inoculation of mice with Rift Valley fever virus (RVFV) reproduces major pathological features of severe human disease, notably the acute-onset hepatitis and delayed-onset encephalitis. We previously reported that a genomic interval (Rvfs2) derived from the susceptible MBT/Pas strain is associated with reduced survival time after RVFV infection. In this study, we investigated the pathophysiological mechanisms by which Rvfs2 confers increased susceptibility to BALB/c mice that are congenic for Rvfs2 (C.MBT-Rvfs2) after infection with virulent RVFV. Clinical traits, biochemical parameters, and histopathological features indicated similar liver damage in BALB/c and C.MBT-Rvfs2 mice between the third and fifth days after infection. However, C.MBT-Rvfs2 mice died at that point from acute liver injury while most BALB/c mice recovered from this condition but eventually died of encephalitis. We observed that hepatocytes proliferated actively within the infected liver of BALB/c mice on the sixth day after infection, promoting organ regeneration on the eighth day after infection and recovery from liver damage. We found that the production of infectious virions was up to 100-fold lower in the peripheral blood and liver of BALB/c compared to C.MBT-Rvfs2 mice. Likewise, RVFV protein amounts were much lower in BALB/c liver compared to C.MBT-Rvfs2 liver. Primary cultured hepatocytes showed higher viral replication rate in C.MBT-Rvfs2 which could contribute to the susceptibility conferred by Rvfs2. Using bone marrow chimera experiments, we uncovered that both hematopoietic and non-hematopoietic cells are required for the BALB/c allele of Rvfs2 to exert its protective effects against the RVFV-induced acute liver disease. Taken together, we have established that Rvfs2 acts as an important RVFV restriction factor by limiting virus multiplication in mice.Author SummaryRift Valley fever (RVF) is a mosquito-borne viral disease with potential to generate a public health emergency. The wide variation in RVF symptoms and severity observed within patient population suggests that natural host genetic determinants, among other factors, can influence the disease outcome. Infection of mice mimics several features of the pathology in humans, including acute-onset hepatitis and delayed-onset encephalitis. BALB/c inbred mice bearing the BALB/c haplotype at the Rvfs2 locus survive longer than those bearing the MBT haplotype. In this study, we investigated clinical traits, biochemical parameters, virological evidence, and histological features to characterize the pathogenesis of RVF in early and late susceptible mice. We show that animals of both groups develop acute liver disease shortly after infection. We demonstrate that, by comparison with early susceptible mice, BALB/c mice exhibit significantly reduced replication of RVF virus in vivo in the blood and liver and in vitro in primary cultured hepatocytes, and eventually self-recover from the liver damages. We use reciprocal transplantations of bone marrow cells between early and late susceptible mice to show that survival to severe liver disease requires both hematopoietic and non-hematopoietic cells. Taken together, we establish Rvfs2 as a single locus that enables mice to survive RVF virus-induced liver disease.