The ring finger protein 213 gene (Rnf213) contributes to Rift Valley fever resistance in mice.

Rift Valley fever (RVF) is an emerging viral zoonosis that primarily affects ruminants and humans. We have previously shown that wild-derived MBT/Pas mice are highly susceptible to RVF virus and that part of this phenotype is controlled by a locus located on distal Chromosome 11. Using congenic strains, we narrowed down the critical interval to a 530 kb region containing five protein-coding genes among which Rnf213 emerged as a potential candidate. We generated Rnf213-deficient mice by CRISPR/CAS9 on the C57BL/6 J background and showed that they were significantly more susceptible to RVF than control mice, with an average survival time post-infection reduced from 7 to 4 days. The human RNF213 gene had been associated with the cerebrovascular Moyamoya disease (MMD or MYMY) but the inactivation of this gene in the mouse resulted only in mild anomalies of the neovascularization. This study provides the first evidence that the Rnf213 gene may also impact the resistance to infectious diseases such as RVF.

The NSs Protein Encoded by the Virulent Strain of Rift Valley Fever Virus Targets the Expression of Abl2 and the Actin Cytoskeleton of the Host, Affecting Cell Mobility, Cell Shape, and Cell-Cell Adhesion.

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.

Genetic dissection of Rift Valley fever pathogenesis: Rvfs2 locus on mouse chromosome 11 enables survival to early-onset hepatitis.

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.

Genetic Diversity of Collaborative Cross Mice Controls Viral Replication, Clinical Severity, and Brain Pathology Induced by Zika Virus Infection, Independently of Oas1b.

The explosive spread of Zika virus (ZIKV) has been associated with major variations in severe disease and congenital afflictions among infected populations, suggesting an influence of host genes. We investigated how genome-wide variants could impact susceptibility to ZIKV infection in mice. We first describe that the susceptibility of Ifnar1-knockout mice is largely influenced by their genetic background. We then show that Collaborative Cross (CC) mice, which exhibit a broad genetic diversity, in which the type I interferon receptor (IFNAR) was blocked by an anti-IFNAR antibody expressed phenotypes ranging from complete resistance to severe symptoms and death, with large variations in the peak and the rate of decrease in the plasma viral load, in the brain viral load, in brain histopathology, and in the viral replication rate in infected cells. The differences in susceptibility to ZIKV between CC strains correlated with the differences in susceptibility to dengue and West Nile viruses between the strains. We identified highly susceptible and resistant mouse strains as new models to investigate the mechanisms of human ZIKV disease and other flavivirus infections. Genetic analyses revealed that phenotypic variations are driven by multiple genes with small effects, reflecting the complexity of ZIKV disease susceptibility in the human population. Notably, our results rule out the possibility of a role of the Oas1b gene in the susceptibility to ZIKV. Altogether, the findings of this study emphasize the role of host genes in the pathogeny of ZIKV infection and lay the foundation for further genetic and mechanistic studies.IMPORTANCE In recent outbreaks, ZIKV has infected millions of people and induced rare but potentially severe complications, including Guillain-Barré syndrome and encephalitis in adults. While several viral sequence variants were proposed to enhance the pathogenicity of ZIKV, the influence of host genetic variants in mediating the clinical heterogeneity remains mostly unexplored. We addressed this question using a mouse panel which models the genetic diversity of the human population and a ZIKV strain from a recent clinical isolate. Through a combination of in vitro and in vivo approaches, we demonstrate that multiple host genetic variants determine viral replication in infected cells and the clinical severity, the kinetics of blood viral load, and brain pathology in mice. We describe new mouse models expressing high degrees of susceptibility or resistance to ZIKV and to other flaviviruses. These models will facilitate the identification and mechanistic characterization of host genes that influence ZIKV pathogenesis.

Identification of new loci involved in the host susceptibility to Salmonella Typhimurium in collaborative cross mice.

BACKGROUND: Salmonella is a Gram-negative bacterium causing a wide range of clinical syndromes ranging from typhoid fever to diarrheic disease. Non-typhoidal Salmonella (NTS) serovars infect humans and animals, causing important health burden in the world. Susceptibility to salmonellosis varies between individuals under the control of host genes, as demonstrated by the identification of over 20 genetic loci in various mouse crosses. We have investigated the host response to S. Typhimurium infection in 35 Collaborative Cross (CC) strains, a genetic population which involves wild-derived strains that had not been previously assessed. RESULTS: One hundred and forty-eight mice from 35 CC strains were challenged intravenously with 1000 colony-forming units (CFUs) of S. Typhimurium. Bacterial load was measured in spleen and liver at day 4 post-infection. CC strains differed significantly (P < 0.0001) in spleen and liver bacterial loads, while sex and age had no effect. Two significant quantitative trait loci (QTLs) on chromosomes 8 and 10 and one suggestive QTL on chromosome 1 were found for spleen bacterial load, while two suggestive QTLs on chromosomes 6 and 17 were found for liver bacterial load. These QTLs are caused by distinct allelic patterns, principally involving alleles originating from the wild-derived founders. Using sequence variations between the eight CC founder strains combined with database mining for expression in target organs and known immune phenotypes, we were able to refine the QTLs intervals and establish a list of the most promising candidate genes. Furthermore, we identified one strain, CC042/GeniUnc (CC042), as highly susceptible to S. Typhimurium infection. CONCLUSIONS: By exploring a broader genetic variation, the Collaborative Cross population has revealed novel loci of resistance to Salmonella Typhimurium. It also led to the identification of CC042 as an extremely susceptible strain.

Innate Immune Basis for Rift Valley Fever Susceptibility in Mouse Models.

Rift Valley fever virus (RVFV) leads to varied clinical manifestations in animals and in humans that range from moderate fever to fatal illness, suggesting that host immune responses are important determinants of the disease severity. We investigated the immune basis for the extreme susceptibility of MBT/Pas mice that die with mild to acute hepatitis by day 3 post-infection compared to more resistant BALB/cByJ mice that survive up to a week longer. Lower levels of neutrophils observed in the bone marrow and blood of infected MBT/Pas mice are unlikely to be causative of increased RVFV susceptibility as constitutive neutropenia in specific mutant mice did not change survival outcome. However, whereas MBT/Pas mice mounted an earlier inflammatory response accompanied by higher amounts of interferon (IFN)-α in the serum compared to BALB/cByJ mice, they failed to prevent high viral antigen load. Several immunological alterations were uncovered in infected MBT/Pas mice compared to BALB/cByJ mice, including low levels of leukocytes that expressed type I IFN receptor subunit 1 (IFNAR1) in the blood, spleen and liver, delayed leukocyte activation and decreased percentage of IFN-γ-producing leukocytes in the blood. These observations are consistent with the complex mode of inheritance of RVFV susceptibility in genetic studies.

Haplosufficiency of PAX3 for melanoma development in Tyr: NRASQ61K; Cdkn2a-/- mice allows identification and sorting of melanoma cells using a Pax3GFP reporter allele.

The role of the Pax3 gene in embryonic development of pigment cells is well characterized. By contrast, the function of Pax3 in melanoma development is controversial. Indeed, data obtained from cultured cells suggest that PAX3 may contribute to melanomagenesis. PAX3 is found to be overexpressed in melanomas and also in nevi compared with normal skin samples. Pax3 homozygous loss of function is embryonic lethal. To assess the role of Pax3 in melanoma development in vivo, we analyzed Pax3 haploinsufficiency in a mouse model of melanoma predisposition. The Pax3(GFP/+) knock-in reporter system was combined with the Tyr::NRAS(Q61K); Cdkn2a(-/-) mouse melanoma model. Melanoma development was followed over 18 months. Histopathological, immunohistochemical, and molecular analyses of lesions at different stages of melanoma progression were carried out. Fluorescence-activated cell sorting on GFP of cells from primary or metastatic melanoma was followed by ex-vivo transformation tests and in-vivo passaging. We report here that Tyr::NRAS(Q61K); Cdkn2a(-/-); Pax3(GFP/+) mice developed metastasizing melanoma as their Tyr::NRAS(Q61K); Cdkn2a(-/-); littermates. Histopathology showed no differences between the two genotypes, although Pax3 mRNA and PAX3 protein levels in Pax3(GFP/+) lesions were reduced by half. The Pax3(GFP) allele proved to be a convenient marker to identify and directly sort heterogeneous populations of melanoma cells within the tumor bulk at each stage of melanoma progression. This new mouse model represents an accurate and reproducible means for identifying melanoma cells in vivo to study the mechanisms of melanoma development.

ß-Catenin Upregulates the Constitutive and Virus-Induced Transcriptional Capacity of the Interferon Beta Promoter through T-Cell Factor Binding Sites.

Rapid upregulation of interferon beta (IFN-ß) expression following virus infection is essential to set up an efficient innate antiviral response. Biological roles related to the antiviral and immune response have also been associated with the constitutive production of IFN-ß in naive cells. However, the mechanisms capable of modulating constitutive IFN-ß expression in the absence of infection remain largely unknown. In this work, we demonstrate that inhibition of the kinase glycogen synthase kinase 3 (GSK-3) leads to the upregulation of the constitutive level of IFN-ß expression in noninfected cells, provided that GSK-3 inhibition is correlated with the binding of ß-catenin to the IFN-ß promoter. Under these conditions, IFN-ß expression occurred through the T-cell factor (TCF) binding sites present on the IFN-ß promoter independently of interferon regulatory factor 3 (IRF3). Enhancement of the constitutive level of IFN-ß per se was able to confer an efficient antiviral state to naive cells and acted in synergy with virus infection to stimulate virus-induced IFN-ß expression. Further emphasizing the role of ß-catenin in the innate antiviral response, we show here that highly pathogenic Rift Valley fever virus (RVFV) targets the Wnt/ß-catenin pathway and the formation of active TCF/ß-catenin complexes at the transcriptional and protein level in RVFV-infected cells and mice.

The Rift Valley fever accessory proteins NSm and P78/NSm-GN are distinct determinants of virus propagation in vertebrate and invertebrate hosts.

Rift Valley fever virus (RVFV) is an enzootic virus circulating in Africa that is transmitted to its vertebrate host by a mosquito vector and causes severe clinical manifestations in humans and ruminants. RVFV has a tripartite genome of negative or ambisense polarity. The M segment contains five in-frame AUG codons that are alternatively used for the synthesis of two major structural glycoproteins, GN and GC, and at least two accessory proteins, NSm, a 14-kDa cytosolic protein, and P78/NSm-GN, a 78-kDa glycoprotein. To determine the relative contribution of P78 and NSm to RVFV infectivity, AUG codons were knocked out to generate mutant viruses expressing various sets of the M-encoded proteins. We found that, in the absence of the second AUG codon used to express NSm, a 13-kDa protein corresponding to an N-terminally truncated form of NSm, named NSm', was synthesized from AUG 3. None of the individual accessory proteins had any significant impact on RVFV virulence in mice. However, a mutant virus lacking both NSm and NSm' was strongly attenuated in mice and grew to reduced titers in murine macrophages, a major target cell type of RVFV. In contrast, P78 was not associated with reduced viral virulence in mice, yet it appeared as a major determinant of virus dissemination in mosquitoes. This study demonstrates how related accessory proteins differentially contribute to RVFV propagation in mammalian and arthropod hosts.

IS-98-ST1 West Nile virus derived from an infectious cDNA clone retains neuroinvasiveness and neurovirulence properties of the original virus.

Infectious clones of West Nile virus (WNV) have previously been generated and used to decipher the role of viral proteins in WNV virulence. The majority of molecular clones obtained to date have been derived from North American, Australian, or African isolates. Here, we describe the construction of an infectious cDNA clone of a Mediterranean WNV strain, IS-98-ST1. We characterized the biological properties of the recovered recombinant virus in cell culture and in mice. The growth kinetics of recombinant and parental WNV were similar in Vero cells. Moreover, the phenotype of recombinant and parental WNV was indistinguishable as regards viremia, viral load in the brain, and mortality in susceptible and resistant mice. Finally, the pathobiology of the infectious clone was examined in embryonated chicken eggs. The capacity of different WNV strains to replicate in embryonated chicken eggs closely paralleled their ability to replicate in mice, suggesting that inoculation of embryonated chicken eggs could provide a practical in vivo model for the study of WNV pathogenesis. In conclusion, the IS-98-ST1 infectious clone will allow assessment of the impact of selected mutations and novel genomic changes appearing in emerging European strains pathogenicity and endemic or epidemic potential. This will be invaluable in the context of an increasing number of outbreaks and enhanced severity of infections in the Mediterranean basin and Eastern Europe.

Centronuclear myopathy in Labrador retrievers: a recent founder mutation in the PTPLA gene has rapidly disseminated worldwide.

Centronuclear myopathies (CNM) are inherited congenital disorders characterized by an excessive number of internalized nuclei. In humans, CNM results from ~70 mutations in three major genes from the myotubularin, dynamin and amphiphysin families. Analysis of animal models with altered expression of these genes revealed common defects in all forms of CNM, paving the way for unified pathogenic and therapeutic mechanisms. Despite these efforts, some CNM cases remain genetically unresolved. We previously identified an autosomal recessive form of CNM in French Labrador retrievers from an experimental pedigree, and showed that a loss-of-function mutation in the protein tyrosine phosphatase-like A (PTPLA) gene segregated with CNM. Around the world, client-owned Labrador retrievers with a similar clinical presentation and histopathological changes in muscle biopsies have been described. We hypothesized that these Labradors share the same PTPLA(cnm) mutation. Genotyping of an international panel of 7,426 Labradors led to the identification of PTPLA(cnm) carriers in 13 countries. Haplotype analysis demonstrated that the PTPLA(cnm) allele resulted from a single and recent mutational event that may have rapidly disseminated through the extensive use of popular sires. PTPLA-deficient Labradors will help define the integrated role of PTPLA in the existing CNM gene network. They will be valuable complementary large animal models to test innovative therapies in CNM.

RACK1, a clue to the diagnosis of cutaneous melanomas in horses.

BACKGROUND: Melanocytic proliferations are common in horses but the diagnosis of malignancy is not always straightforward. To improve diagnosis and prognosis, markers of malignancy are needed. Receptor for activated C kinase 1 (RACK1) protein may be such a marker. RACK1 was originally found to characterize malignant melanocytic lesions in the Melanoblastoma-bearing Libechov minipig (MeLiM) and, later, in human patients. Our purpose was to investigate the value of RACK1 in the classification of cutaneous melanocytic proliferations in horses. RESULTS: Using immunofluorescence, we report here that both MITF (Microphthalmia-associated transcription factor) and PAX3 (Paired box 3) allow the identification of melanocytic cells in horse skin samples. Importantly, RACK1 was detected in melanocytic lesions but not in healthy skin melanocytes. Finally, we found that RACK1 labeling can be used in horses to distinguish benign melanocytic tumors from melanomas. Indeed, RACK1 labeling appeared more informative to assess malignancy than individual histomorphological features. CONCLUSIONS: This study confirms that horses provide an interesting model for melanoma genesis studies. It establishes MITF and PAX3 as markers of horse melanocytic cells. RACK1 emerges as an important marker of malignancy which may contribute to progress in the diagnosis of melanomas in both human and veterinary medicine.

I-SceI-mediated double-strand break does not increase the frequency of homologous recombination at the Dct locus in mouse embryonic stem cells.

Targeted induction of double-strand breaks (DSBs) at natural endogenous loci was shown to increase the rate of gene replacement by homologous recombination in mouse embryonic stem cells. The gene encoding dopachrome tautomerase (Dct) is specifically expressed in melanocytes and their precursors. To construct a genetic tool allowing the replacement of Dct gene by any gene of interest, we generated an embryonic stem cell line carrying the recognition site for the yeast I-SceI meganuclease embedded in the Dct genomic segment. The embryonic stem cell line was electroporated with an I-SceI expression plasmid, and a template for the DSB-repair process that carried sequence homologies to the Dct target. The I-SceI meganuclease was indeed able to introduce a DSB at the Dct locus in live embryonic stem cells. However, the level of gene targeting was not improved by the DSB induction, indicating a limited capacity of I-SceI to mediate homologous recombination at the Dct locus. These data suggest that homologous recombination by meganuclease-induced DSB may be locus dependent in mammalian cells.

Pax3( GFP ) , a new reporter for the melanocyte lineage, highlights novel aspects of PAX3 expression in the skin.

The paired box gene 3 (Pax3) is expressed during pigment cell development. We tested whether the targeted allele Pax3(GFP) can be used as a reporter gene for pigment cells in the mouse. We found that enhanced green fluorescent protein (GFP) can be seen readily in every melanoblast and melanocyte in the epidermis and hair follicles of Pax3(GFP/+) heterozygotes. The GFP was detected at all differentiation stages, including melanocyte stem cells. In the dermis, Schwann cells and nestin-positive cells of the piloneural collars resembling the nestin-positive hair follicle multipotent stem cells exhibited a weaker GFP signal. Pigment cells could be purified by fluorescent activated cell sorting and grown in vitro without feeder cells, giving pure cultures of melanocytes. The Schwann cells and nestin-positive cells of the piloneural collars were FACS-isolated based on their weak expression of GFP. Thus Pax3(GFP) can discriminate distinct populations of cells in the skin.

p53 Activation following Rift Valley fever virus infection contributes to cell death and viral production.

Rift Valley fever virus (RVFV) is an emerging viral zoonosis that is responsible for devastating outbreaks among livestock and is capable of causing potentially fatal disease in humans. Studies have shown that upon infection, certain viruses have the capability of utilizing particular cellular signaling pathways to propagate viral infection. Activation of p53 is important for the DNA damage signaling cascade, initiation of apoptosis, cell cycle arrest and transcriptional regulation of multiple genes. The current study focuses on the role of p53 signaling in RVFV infection and viral replication. These results show an up-regulation of p53 phosphorylation at several serine sites after RVFV MP-12 infection that is highly dependent on the viral protein NSs. qRT-PCR data showed a transcriptional up-regulation of several p53 targeted genes involved in cell cycle and apoptosis regulation following RVFV infection. Cell viability assays demonstrate that loss of p53 results in less RVFV induced cell death. Furthermore, decreased viral titers in p53 null cells indicate that RVFV utilizes p53 to enhance viral production. Collectively, these experiments indicate that the p53 signaling pathway is utilized during RVFV infection to induce cell death and increase viral production.

MBT/Pas mouse: a relevant model for the evaluation of Rift Valley fever vaccines.

Currently, there are no worldwide licensed vaccines for Rift Valley fever (RVF) that are both safe and effective. Development and evaluation of vaccines, diagnostics and treatments depend on the availability of appropriate animal models. Animal models are also necessary to understand the basic pathobiology of infection. Here, we report the use of an inbred MBT/Pas mouse model that consistently reproduces RVF disease and serves our purpose for testing the efficacy of vaccine candidates; an attenuated Rift Valley fever virus (RVFV) and a recombinant RVFV-capripoxvirus. We show that this model is relevant for vaccine testing.

[The Collaborative Cross, a groundbreaking tool to tackle complex traits]. / Le Collaborative Cross: un outil révolutionnaire à l'assaut des caractères complexes.

Complex traits, like the susceptibility to common diseases, are controlled by numerous genomic regions which individual effect is generally weak. These observations led geneticists to develop an experimental system to dissect the genetic of complex traits in the mouse. The Collaborative Cross (CC) is a genetic reference population of over 300 inbred lines derived from eight inbred strains of three Mus musculus sub-species that captures 90% of the genetic variation known in the mouse genome. We present here the generation and the characteristics of the CC and we report the results of the first experiments with partially inbred CC lines.

Early systemic bacterial dissemination and a rapid innate immune response characterize genetic resistance to plague of SEG mice.

BACKGROUND: Although laboratory mice are usually highly susceptible to Yersinia pestis, we recently identified a mouse strain (SEG) that exhibited an exceptional capacity to resist bubonic plague and used it to identify immune mechanisms associated with resistance. METHODS: The kinetics of infection, circulating blood cells, granulopoiesis, lesions, and cellular populations in the spleen, and cytokine production in various tissues were compared in SEG and susceptible C57BL/6J mice after subcutaneous infection with the virulent Y. pestis CO92. RESULTS: Bacterial invasion occurred early (day 2) but was transient in SEG/Pas mice, whereas in C57BL/6J mice it was delayed but continuous until death. The bacterial load in all organs significantly correlated with the production of 5 cytokines (granulocyte colony-stimulating factor, keratinocyte-derived chemokine (KC), macrophage cationic peptide-1 (MCP-1), interleukin 1α, and interleukin 6) involved in monocyte and neutrophil recruitment. Indeed, higher proportions of these 2 cell types in blood and massive recruitment of F4/80(+)CD11b(-) macrophages in the spleen were observed in SEG/Pas mice at an early time point (day 2). Later times after infection (day 4) were characterized in C57BL/6J mice by destructive lesions of the spleen and impaired granulopoiesis. CONCLUSION: A fast and efficient Y. pestis dissemination in SEG mice may be critical for the triggering of an early and effective innate immune response necessary for surviving plague.

Tissue tropism and target cells of NSs-deleted rift valley fever virus in live immunodeficient mice.

BACKGROUND: Rift Valley fever virus (RVFV) causes disease in livestock and humans. It can be transmitted by mosquitoes, inhalation or physical contact with the body fluids of infected animals. Severe clinical cases are characterized by acute hepatitis with hemorrhage, meningoencephalitis and/or retinitis. The dynamics of RVFV infection and the cell types infected in vivo are poorly understood. METHODOLOGY/PRINCIPAL FINDINGS: RVFV strains expressing humanized Renilla luciferase (hRLuc) or green fluorescent protein (GFP) were generated and inoculated to susceptible Ifnar1-deficient mice. We investigated the tissue tropism in these mice and the nature of the target cells in vivo using whole-organ imaging and flow cytometry. After intraperitoneal inoculation, hRLuc signal was observed primarily in the thymus, spleen and liver. Macrophages infiltrating various tissues, in particular the adipose tissue surrounding the pancreas also expressed the virus. The liver rapidly turned into the major luminescent organ and the mice succumbed to severe hepatitis. The brain remained weakly luminescent throughout infection. FACS analysis in RVFV-GFP-infected mice showed that the macrophages, dendritic cells and granulocytes were main target cells for RVFV. The crucial role of cells of the monocyte/macrophage/dendritic lineage during RVFV infection was confirmed by the slower viral dissemination, decrease in RVFV titers in blood, and prolonged survival of macrophage- and dendritic cell-depleted mice following treatment with clodronate liposomes. Upon dermal and nasal inoculations, the viral dissemination was primarily observed in the lymph node draining the injected ear and in the lungs respectively, with a significant increase in survival time. CONCLUSIONS/SIGNIFICANCE: These findings reveal the high levels of phagocytic cells harboring RVFV during viral infection in Ifnar1-deficient mice. They demonstrate that bioluminescent and fluorescent viruses can shed new light into the pathogenesis of RVFV infection.