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International audience; 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 to 2% of RVFV infections result in severe disease (encephalitis, hepatitis, or retinitis) with a high rate of lethality when associated with hemorrhagic fever. The RVFV NSs protein, which is the main virulence factor, counteracts the host innate antiviral response to favor viral replication and spread. However, the mechanisms underlying RVFV-induced cytopathic effects and the role of NSs in these alterations remain for the most part unknown. In this work, we have analyzed the effects of NSs expression on the 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 upregulation of Abl2 (a major regulator of the 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 an increased mobility of ZH548-infected cells compared to ZH548ΔNSs-infected fibroblasts and to strong changes in cell morphology in nonmigrating hepatocytes, with reduction of lamellipodia, cell spreading, and dissolution of adherens junctions reminiscent of the ZH548-induced cytopathic effects observed in vivo. Finally, we show evidence of the presence of NSs within long actin-rich structures associated with NSs dissemination from NSs-expressing toward non-NSs-expressing cells.IMPORTANCE: Rift Valley fever virus (RVFV) is a dangerous human and animal pathogen that was ranked by the World Health Organization in 2018 as among the eight pathogens of most concern for being likely to cause wide epidemics in the near future and for which there are no, or insufficient, countermeasures. The focus of this work is to address the question of the mechanisms underlying RVFV-induced cytopathic effects that participate in RVFV pathogenicity. We demonstrate here that RVFV targets cell adhesion and the actin cytoskeleton at the transcriptional and cellular level, affecting cell mobility and inducing cell shape collapse, along with distortion of cell-cell adhesion. All these effects may participate in RVFV-induced pathogenicity, facilitate virulent RVFV dissemination, and thus constitute interesting potential targets for future development of antiviral therapeutic strategies that, in the case of RVFV, as with several other emerging arboviruses, are presently lacking.

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 whichRvfs2confers increased susceptibility to BALB/c mice that are congenic forRvfs2(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-Rvfs2mice between the third and fifth days after infection. However, C.MBT-Rvfs2mice 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-Rvfs2mice. Likewise, RVFV protein amounts were much lower in BALB/c liver compared to C.MBT-Rvfs2liver. Primary cultured hepatocytes showed higher viral replication rate in C.MBT-Rvfs2which could contribute to the susceptibility conferred byRvfs2. Using bone marrow chimera experiments, we uncovered that both hematopoietic and non-hematopoietic cells are required for the BALB/c allele ofRvfs2to exert its protective effects against the RVFV-induced acute liver disease. Taken together, we have established thatRvfs2acts 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 theRvfs2locus 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 virusin vivoin the blood and liver andin vitroin 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 establishRvfs2as a single locus that enables mice to survive RVF virus-induced liver disease.