Protein synthesis in Rift Valley fever virus-infected cells

Host entry factors of Rift Valley Fever Virus infection

Rift Valley Fever Virus (RVFV) is a negative sense segmented RNA virus that can cause severe hemorrhagic fever. The tri-segmented virus genome encodes for six (6) multifunctional proteins that engage host factors at a variety of different stages in the replication cycle. The S segment encodes nucleoprotein (N) and nonstructural protein S (NSs), the M segment encodes viral glycoproteins Gn and Gc as well as nonstructural protein M (NSm) and the L segment encodes the viral polymerase (L). Viral glycoproteins Gn and Gc are responsible for entry by binding to a number of host factors. Our recent studies identified a scavenger receptor, LDL receptor related protein 1 (Lrp1), as a potential pro-viral host factor for RVFV and related viruses, including Oropouche virus (OROV) infection. Coincidentally, several recent studies identified other LDL family proteins as viral entry factors and receptors for other viral families. Collectively, these observations suggest that highly conserved LDL family proteins may play a significant role in facilitating entry of viruses from several distinct families. Given the significant roles of viral and host factors during infection, characterization of these interactions is critical for therapeutic targeting with neutralizing antibodies and vaccines.

Bacterium-Like Particles Displaying the Rift Valley Fever Virus Gn Head Protein Induces Efficacious Immune Responses in Immunized Mice

Rift Valley fever virus (RVFV), a mosquito-borne zoonotic phlebovirus, causes serious disease in humans and ruminants. According to the World Health Organization, Rift Valley fever is classified as a priority disease, and as such, vaccine development is of high priority due to the lack of licensed vaccines. In this study, a bacterium-like particle vaccine (BLP), RVFV-BLPs, is constructed. A novel display system is described, which is based on non-living and non-genetically modified Gram-positive bacterial cells, designated as Gram-positive enhancer matrix (GEM). The RVFV Gn head protein was displayed on the surface of GEM by co-expression with the peptidoglycan-binding domain (protein anchor) at the C-terminus. We determined that the RVFV Gn head-PA fusion protein was successfully displayed on the GEM. Mice immunized with RVFV-BLPs produced humoral and cellular immunity. Interestingly, comparing the production of RVFV Gn head-specific IgG and its subtype by vaccinating with different antigen doses of the RVFV-BLPs determined that the RVFV-BLPs (50 μg) group showed a greater effect than the other two groups. More importantly, antibodies produced by mice immunized with RVFV-BLPs (50 μg) exhibited potent neutralizing activity against RVFV pseudovirus. RVFV-BLPs (50 μg) also could induce IFN-γ and IL-4 in immunized mice; these mice generated memory cells among the proliferating T cell population after immunization with RVFV-BLPs with effector memory T cells as the major population, which means that RVFV-BLPs is an effective vaccine to establish a long-lived population of memory T cells. The findings suggest that the novel RVFV-BLPs subunit vaccine has the potential to be considered a safe and effective candidate vaccine against RVFV infection.

Insights into the Pathogenesis of Viral Haemorrhagic Fever Based on Virus Tropism and Tissue Lesions of Natural Rift Valley Fever

Rift Valley fever phlebovirus (RVFV) infects humans and a wide range of ungulates and historically has caused devastating epidemics in Africa and the Arabian Peninsula. Lesions of naturally infected cases of Rift Valley fever (RVF) have only been described in detail in sheep with a few reports concerning cattle and humans. The most frequently observed lesion in both ruminants and humans is randomly distributed necrosis, particularly in the liver. Lesions supportive of vascular endothelial injury are also present and include mild hydropericardium, hydrothorax and ascites; marked pulmonary congestion and oedema; lymph node congestion and oedema; and haemorrhages in many tissues. Although a complete understanding of RVF pathogenesis is still lacking, antigen-presenting cells in the skin are likely the early targets of the virus. Following suppression of type I IFN production and necrosis of dermal cells, RVFV spreads systemically, resulting in infection and necrosis of other cells in a variety of organs. Failure of both the innate and adaptive immune responses to control infection is exacerbated by apoptosis of lymphocytes. An excessive pro-inflammatory cytokine and chemokine response leads to microcirculatory dysfunction. Additionally, impairment of the coagulation system results in widespread haemorrhages. Fatal outcomes result from multiorgan failure, oedema in many organs (including the lungs and brain), hypotension, and circulatory shock. Here, we summarize current understanding of RVF cellular tropism as informed by lesions caused by natural infections. We specifically examine how extant knowledge informs current understanding regarding pathogenesis of the haemorrhagic fever form of RVF, identifying opportunities for future research.

Rift Valley Fever - epidemiological update and risk of introduction into Europe

Rift Valley fever (RVF) is a vector-borne disease transmitted by a broad spectrum of mosquito species, especially Aedes and Culex genus, to animals (domestic and wild ruminants and camels) and humans. Rift Valley fever is endemic in sub-Saharan Africa and in the Arabian Peninsula, with periodic epidemics characterised by 5-15 years of inter-epizootic periods. In the last two decades, RVF was notified in new African regions (e.g. Sahel), RVF epidemics occurred more frequently and low-level enzootic virus circulation has been demonstrated in livestock in various areas. Recent outbreaks in a French overseas department and some seropositive cases detected in Turkey, Tunisia and Libya raised the attention of the EU for a possible incursion into neighbouring countries. The movement of live animals is the most important pathway for RVF spread from the African endemic areas to North Africa and the Middle East. The movement of infected animals and infected vectors when shipped by flights, containers or road transport is considered as other plausible pathways of introduction into Europe. The overall risk of introduction of RVF into EU through the movement of infected animals is very low in all the EU regions and in all MSs (less than one epidemic every 500 years), given the strict EU animal import policy. The same level of risk of introduction in all the EU regions was estimated also considering the movement of infected vectors, with the highest level for Belgium, Greece, Malta, the Netherlands (one epidemic every 228-700 years), mainly linked to the number of connections by air and sea transports with African RVF infected countries. Although the EU territory does not seem to be directly exposed to an imminent risk of RVFV introduction, the risk of further spread into countries neighbouring the EU and the risks of possible introduction of infected vectors, suggest that EU authorities need to strengthen their surveillance and response capacities, as well as the collaboration with North African and Middle Eastern countries.

Seroprevalence of Rift Valley fever in cattle of smallholder farmers in Kwilu Province in the Democratic Republic of Congo

Rift Valley fever (RVF) is a zoonotic mosquito-borne disease caused by RVF virus (RVFV) that causes abortions and high mortalities in livestock and is also associated with acute and fatal disease in humans. In the Democratic Republic of Congo (DRC), information on the epidemiology of RVF is limited, particularly among cattle reared by smallholder farmers. This cross-sectional study was conducted to investigate the seroprevalence of RVF in cattle raised by smallholder farmers in Kwilu Province of DRC, which has not yet reported an RVF epidemic. A total of 677 cattle sera were collected from four territories and tested for anti-RVFV antibodies using immunofluorescent assay and enzyme-linked immunosorbent assay. The overall seroprevalence of anti-RVFV IgG was 6.5% (44/677) (95% CI 4.81-8.7). There was a statistically significant difference in the seroprevalence among the territories (χ² = 28.79, p < 0.001). Territory seroprevalences were as follows: Idiofa 14.08% (95% CI 9.78-19.76), Bulungu 4.14% (95% CI 1.83-8.68), Gungu 3.21% (95% CI 1.41-6.78), and Masi-Manimba 1.19% (95% CI 0.06-7.37). Seroprevalence differed significantly among age categories (p = 0.0017) and ecosystem (p < 0.001). The seroprevalence of animals aged between 1 and 2 years was 20.0% (95% CI 8.4-39.13) and was higher than group aged <1 year, between 2 and 3 years, and > 3 years. Forest area (18.92% (95% CI 12.35-27.7)) had higher seropositivity than savannah area (4.06% (95% CI 2.65-6.12)). Sex difference was not significant (χ² = 0.14, p = 0.704). These findings indicate that cattle in Kwilu Province had been exposed to RVFV, which represents a significant risk for both livestock and human health.

Rift Valley fever virus NSS gene expression correlates with a defect in nuclear mRNA export

We investigated the localization of host mRNA during Rift Valley fever virus (RVFV) infection. Fluorescence in situ hybridization revealed that infection with RVFV altered the localization of host mRNA. mRNA accumulated in the nuclei of RVFV-infected but not mock-infected cells. Further, overexpression of the NSS gene, but not the N, GN or NSM genes correlated with mRNA nuclear accumulation. Nuclear accumulation of host mRNA was not observed in cells infected with a strain of RVFV lacking the gene encoding NSS, confirming that expression of NSS is likely responsible for this phenomenon.

A ΩXaV motif in the Rift Valley fever virus NSs protein is essential for degrading p62, forming nuclear filaments and virulence

Rift Valley fever virus (RVFV) is a single-stranded RNA virus capable of inducing fatal hemorrhagic fever in humans. A key component of RVFV virulence is its ability to form nuclear filaments through interactions between the viral nonstructural protein NSs and the host general transcription factor TFIIH. Here, we identify an interaction between a ΩXaV motif in NSs and the p62 subunit of TFIIH. This motif in NSs is similar to ΩXaV motifs found in nucleotide excision repair (NER) factors and transcription factors known to interact with p62. Structural and biophysical studies demonstrate that NSs binds to p62 in a similar manner as these other factors. Functional studies in RVFV-infected cells show that the ΩXaV motif is required for both nuclear filament formation and degradation of p62. Consistent with the fact that the RVFV can be distinguished from other Bunyaviridae-family viruses due to its ability to form nuclear filaments in infected cells, the motif is absent in the NSs proteins of other Bunyaviridae-family viruses. Taken together, our studies demonstrate that p62 binding to NSs through the ΩXaV motif is essential for degrading p62, forming nuclear filaments and enhancing RVFV virulence. In addition, these results show how the RVFV incorporates a simple motif into the NSs protein that enables it to functionally mimic host cell proteins that bind the p62 subunit of TFIIH.

High-content image-based screening of a signal transduction pathway inhibitor small-molecule library against highly pathogenic RNA viruses

High-content image-based screening was developed as an approach to test a small-molecule library of compounds targeting signal transduction pathways for antiviral activity against multiple highly pathogenic RNA viruses. Of the 2843 compounds screened, 120 compounds exhibited ≥60% antiviral activity. Four compounds (E225-0969, E528-0039, G118-0778, and G544-0735), which were most active against Rift Valley fever virus (RVFV) and showed broad-spectrum antiviral activity, were selected for further evaluation for their concentration-response profile and cytotoxicity. These compounds did not show any visible cytotoxicity at the highest concentration of compound tested (200 µM). All four of these compounds were more active than ribavirin against several viruses. One compound, E225-0969, had the lowest effective concentration (EC50 = 1.9-8.92 µM) for all the viruses tested. This compound was 13- and 43-fold more inhibitory against RVFV and Chikungunya virus (CHIKV), respectively, than ribavirin. The highest selectivity index (>106.2) was for E225-0969 against CHIKV. Time-of-addition assays suggested that all four lead compounds targeted early steps in the viral life cycle (entry and/or replication) but not virus egress. Overall, this work demonstrates that high-content image analysis can be used to screen chemical libraries for new antivirals against highly pathogenic viruses.

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, G_N and G_C, and at least two accessory proteins, NSm, a 14-kDa cytosolic protein, and P78/NSm-G_N, 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.

High content image-based screening of a protease inhibitor library reveals compounds broadly active against Rift Valley fever virus and other highly pathogenic RNA viruses

High content image-based screening was developed as an approach to test a protease inhibitor small molecule library for antiviral activity against Rift Valley fever virus (RVFV) and to determine their mechanism of action. RVFV is the causative agent of severe disease of humans and animals throughout Africa and the Arabian Peninsula. Of the 849 compounds screened, 34 compounds exhibited ≥50% inhibition against RVFV. All of the hit compounds could be classified into 4 distinct groups based on their unique chemical backbone. Some of the compounds also showed broad antiviral activity against several highly pathogenic RNA viruses including Ebola, Marburg, Venezuela equine encephalitis, and Lassa viruses. Four hit compounds (C795-0925, D011-2120, F694-1532 and G202-0362), which were most active against RVFV and showed broad-spectrum antiviral activity, were selected for further evaluation for their cytotoxicity, dose response profile, and mode of action using classical virological methods and high-content imaging analysis. Time-of-addition assays in RVFV infections suggested that D011-2120 and G202-0362 targeted virus egress, while C795-0925 and F694-1532 inhibited virus replication. We showed that D011-2120 exhibited its antiviral effects by blocking microtubule polymerization, thereby disrupting the Golgi complex and inhibiting viral trafficking to the plasma membrane during virus egress. While G202-0362 also affected virus egress, it appears to do so by a different mechanism, namely by blocking virus budding from the trans Golgi. F694-1532 inhibited viral replication, but also appeared to inhibit overall cellular gene expression. However, G202-0362 and C795-0925 did not alter any of the morphological features that we examined and thus may prove to be good candidates for antiviral drug development. Overall this work demonstrates that high-content image analysis can be used to screen chemical libraries for new antivirals and to determine their mechanism of action and any possible deleterious effects on host cellular biology.

Rift Valley fever (RVF) is an arthropod-borne viral zoonosis that occurs in large parts of sub-Saharan and North Africa and in 2000 emerged outside the African continent for the first time, raising concerns that it could further expand its geographical range. The disease in humans can result in encephalitis or hemorrhagic fever and in ruminants often results in abortion in pregnant females. Due to the lack of a licensed and commercially available vaccine, efforts to discover effective antiviral drugs are underway. Drug discovery using high content image-based screening is an effective tool that has been successfully used to identify new drugs. In this study, we developed an image-based assay to identify compounds active against RVF virus and other highly pathogenic human viruses. We demonstrated the usefulness of our image-based high content assay in identifying potential RVF antivirals by screening a small subset of chemical compounds for inhibition of RVF virus in a human cell line (HeLa) and partially characterized their mechanism of action within infected cells. The methods we developed in this study will be useful in discovering new effective drugs to combat Rift Valley fever.

Evaluation of nonspreading Rift Valley fever virus as a vaccine vector using influenza virus hemagglutinin as a model antigen

Virus replicon particles are capable of infection, genome replication and gene expression, but are unable to produce progeny virions, rendering their use inherently safe. By virtue of this unique combination of features, replicon particles hold great promise for vaccine applications. We previously developed replicon particles of Rift Valley fever virus (RVFV) and demonstrated their high efficacy as a RVFV vaccine in the natural target species. We have now investigated the feasibility of using this nonspreading RVFV (NSR) as a vaccine vector using influenza virus hemagglutinin as a model antigen. NSR particles were designed to express either the full-length hemagglutinin of influenza A virus H1N1 (NSR-HA) or the respective soluble ectodomain (NSR-sHA). The efficacies of the two NSR vector vaccines, applied via either the intramuscular or the intranasal route, were evaluated. A single vaccination with NSR-HA protected all mice from a lethal challenge dose, while vaccination with NSR-sHA was not protective. Interestingly, whereas intramuscular vaccination elicited superior systemic immune responses, intranasal vaccination provided optimal clinical protection.

The consequences of reconfiguring the ambisense S genome segment of Rift Valley fever virus on viral replication in mammalian and mosquito cells and for genome packaging

Rift Valley fever virus (RVFV, family Bunyaviridae) is a mosquito-borne pathogen of both livestock and humans, found primarily in Sub-Saharan Africa and the Arabian Peninsula. The viral genome comprises two negative-sense (L and M segments) and one ambisense (S segment) RNAs that encode seven proteins. The S segment encodes the nucleocapsid (N) protein in the negative-sense and a nonstructural (NSs) protein in the positive-sense, though NSs cannot be translated directly from the S segment but rather from a specific subgenomic mRNA. Using reverse genetics we generated a virus, designated rMP12:S-Swap, in which the N protein is expressed from the NSs locus and NSs from the N locus within the genomic S RNA. In cells infected with rMP12:S-Swap NSs is expressed at higher levels with respect to N than in cells infected with the parental rMP12 virus. Despite NSs being the main interferon antagonist and determinant of virulence, growth of rMP12:S-Swap was attenuated in mammalian cells and gave a small plaque phenotype. The increased abundance of the NSs protein did not lead to faster inhibition of host cell protein synthesis or host cell transcription in infected mammalian cells. In cultured mosquito cells, however, infection with rMP12:S-Swap resulted in cell death rather than establishment of persistence as seen with rMP12. Finally, altering the composition of the S segment led to a differential packaging ratio of genomic to antigenomic RNA into rMP12:S-Swap virions. Our results highlight the plasticity of the RVFV genome and provide a useful experimental tool to investigate further the packaging mechanism of the segmented genome.

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus found primarily in sub-Saharan Africa that can infect both domestic animals and humans. RVFV has a tripartite RNA genome that encodes seven proteins. The smallest (S) segment has an unusual ambisense coding strategy whereby two genes (for the nucleocapsid N and nonstructural NSs proteins) are encoded in opposite orientations on the genomic RNA, and are translated from specific subgenomic mRNAs. N is the major structural protein of the virus while NSs is the major virulence factor. To investigate the biological significance of this coding arrangement, we used reverse genetics to create a recombinant virus in which the N and NSs coding sequences were swapped on the S segment. The recombinant virus grew less well in tissue culture cells compared to the parental virus, and rather than maintain persistence in insect cells, infection resulted in their death. In addition, packaging of the modified S genome segment into new virus particles was altered. We also showed that a foreign protein could be expressed to high levels when cloned in place of the NSs gene in the recombinant virus. These studies have implications for vaccine development and vector control strategies.

Rift Valley fever virus incorporates the 78 kDa glycoprotein into virions matured in mosquito C6/36 cells

Rift Valley fever virus (RVFV), genus Phlebovirus, family Bunyaviridae is a zoonotic arthropod-borne virus able to transition between distant host species, causing potentially severe disease in humans and ruminants. Viral proteins are encoded by three genomic segments, with the medium M segment coding for four proteins: nonstructural NSm protein, two glycoproteins Gn and Gc and large 78 kDa glycoprotein (LGp) of unknown function. Goat anti-RVFV polyclonal antibody and mouse monoclonal antibody, generated against a polypeptide unique to the LGp within the RVFV proteome, detected this protein in gradient purified RVFV ZH501 virions harvested from mosquito C6/36 cells but not in virions harvested from the mammalian Vero E6 cells. The incorporation of LGp into the mosquito cell line - matured virions was confirmed by immune-electron microscopy. The LGp was incorporated into the virions immediately during the first passage in C6/36 cells of Vero E6 derived virus. Our data indicate that LGp is a structural protein in C6/36 mosquito cell generated virions. The protein may aid the transmission from the mosquitoes to the ruminant host, with a possible role in replication of RVFV in the mosquito host. To our knowledge, this is a first report of different protein composition between virions formed in insect C6/36 versus mammalian Vero E6 cells.

Countermeasure development for Rift Valley fever: deletion, modification or targeting of major virulence factor NSs

Rift Valley fever (RVF) is a mosquito-borne zoonotic disease characterized by a high rate of abortion in ruminants, and febrile illness, hemorrhagic fever, retinitis and encephalitis in humans. RVF is caused by the RVF virus (RVFV), belonging to the genus Phlebovirus of the family Bunyaviridae. RVFV encodes a major virulence factor, NSs, which is dispensable for viral replication, yet required for evasion of host innate immune responses. RVFV NSs inhibits host gene upregulation at the transcriptional level, while promoting viral translation in the cytoplasm. In this article, we summarize the virology and pathology of RVF, and countermeasure development for RVF, with emphasis on NSs function and applications.

Nuclear relocalization of polyadenylate binding protein during rift valley fever virus infection involves expression of the NSs gene

Rift Valley fever virus (RVFV), an ambisense member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever, an important zoonotic infection in Africa and the Middle East. Phlebovirus proteins are translated from virally transcribed mRNAs that, like host mRNA, are capped but, unlike host mRNAs, are not polyadenylated. Here, we investigated the role of PABP1 during RVFV infection of HeLa cells. Immunofluorescence studies of infected cells demonstrated a gross relocalization of PABP1 to the nucleus late in infection. Immunofluorescence microscopy studies of nuclear proteins revealed costaining between PABP1 and markers of nuclear speckles. PABP1 relocalization was sharply decreased in cells infected with a strain of RVFV lacking the gene encoding the RVFV nonstructural protein S (NSs). To determine whether PABP1 was required for RVFV infection, we measured the production of nucleocapsid protein (N) in cells transfected with small interfering RNAs (siRNAs) targeting PABP1. We found that the overall percentage of RVFV N-positive cells was not changed by siRNA treatment, indicating that PABP1 was not required for RVFV infection. However, when we analyzed populations of cells producing high versus low levels of PABP1, we found that the percentage of RVFV N-positive cells was decreased in cell populations producing physiologic levels of PABP1 and increased in cells with reduced levels of PABP1. Together, these results suggest that production of the NSs protein during RVFV infection leads to sequestration of PABP1 in the nuclear speckles, creating a state within the cell that favors viral protein production.

The C-terminal region of Rift Valley fever virus NSm protein targets the protein to the mitochondrial outer membrane and exerts antiapoptotic function

The NSm nonstructural protein of Rift Valley fever virus (family Bunyaviridae, genus Phlebovirus) has an antiapoptotic function and affects viral pathogenesis. We found that NSm integrates into the mitochondrial outer membrane and that the protein's N terminus is exposed to the cytoplasm. The C-terminal region of NSm, which contains a basic amino acid cluster and a putative transmembrane domain, targeted the protein to the mitochondrial outer membrane and exerted antiapoptotic function.

Acid-activated structural reorganization of the Rift Valley fever virus Gc fusion protein

The entry of the enveloped Rift Valley fever virus (RVFV) into its host cell is mediated by the viral glycoproteins Gn and Gc. We investigated the RVFV entry process and, in particular, its pH-dependent activation mechanism using our recently developed nonspreading-RVFV-particle system. Entry of the virus into the host cell was efficiently inhibited by lysosomotropic agents that prevent endosomal acidification and by compounds that interfere with dynamin- and clathrin-dependent endocytosis. Exposure of plasma membrane-bound virions to an acidic pH (<pH 6) equivalent to the pH of late endolysosomal compartments allowed the virus to bypass the endosomal route of infection. Acid exposure of virions in the absence of target membranes triggered the class II-like Gc fusion protein to form extremely stable oligomers that were resistant to SDS and temperature dissociation and concomitantly compromised virus infectivity. By targeted mutagenesis of conserved histidines in Gn and Gc, we demonstrated that mutation of a single histidine (H857) in Gc completely abrogated virus entry, as well as acid-induced Gc oligomerization. In conclusion, our data suggest that after endocytic uptake, RVFV traffics to the acidic late endolysosomal compartments, where histidine protonation drives the reorganization of the Gc fusion protein that leads to membrane fusion.

Heparan sulfate facilitates Rift Valley fever virus entry into the cell

Rift Valley fever virus (RVFV), an emerging arthropod-borne pathogen, has a broad host and cell tropism. Here we report that the glycosaminoglycan heparan sulfate, abundantly present on the surface of most animal cells, is required for efficient entry of RVFV. Entry was significantly reduced by preincubating the virus inoculum with highly sulfated heparin, by enzymatic removal of heparan sulfate from cells and in cells genetically deficient in heparan sulfate synthesis.

Nuclear trafficking of proteins from RNA viruses: potential target for antivirals?

A key aspect of the infectious cycle of many viruses is the transport of specific viral proteins into the host cell nucleus to perturb the antiviral response. Examples include a number of RNA viruses that are significant human pathogens, such as human immunodeficiency virus (HIV)-1, influenza A, dengue, respiratory syncytial virus and rabies, as well agents that predominantly infect livestock, such as Rift valley fever virus and Venezuelan equine encephalitis virus. Inhibiting the nuclear trafficking of viral proteins as a therapeutic strategy offers an attractive possibility, with important recent progress having been made with respect to HIV-1 and dengue. The results validate nuclear protein import as an antiviral target, and suggest the identification and development of nuclear transport inhibitors as a viable therapeutic approach for a range of human and zoonotic pathogenic viruses.

Molecular biology and genetic diversity of Rift Valley fever virus

Rift Valley fever virus (RVFV), a member of the family Bunyaviridae, genus Phlebovirus, is the causative agent of Rift Valley fever (RVF), a mosquito-borne disease of ruminant animals and humans. The generation of a large sequence database has facilitated studies of the evolution and spread of the virus. Bayesian analyses indicate that currently circulating strains of RVFV are descended from an ancestral species that emerged from a natural reservoir in Africa when large-scale cattle and sheep farming were introduced during the 19th century. Viruses descended from multiple lineages persist in that region, through infection of reservoir animals and vertical transmission in mosquitoes, emerging in years of heavy rainfall to cause epizootics and epidemics. On a number of occasions, viruses from these lineages have been transported outside the enzootic region through the movement of infected animals or mosquitoes, triggering outbreaks in countries such as Egypt, Saudi Arabia, Mauritania and Madagascar, where RVF had not previously been seen. Such viruses could potentially become established in their new environments through infection of wild and domestic ruminants and other animals and vertical transmission in local mosquito species. Despite their extensive geographic dispersion, all strains of RVFV remain closely related at the nucleotide and amino acid level. The high degree of conservation of genes encoding the virion surface glycoproteins suggests that a single vaccine should protect against all currently circulating RVFV strains. Similarly, preservation of the sequence of the RNA-dependent RNA polymerase across viral lineages implies that antiviral drugs targeting the enzyme should be effective against all strains. Researchers should be encouraged to collect additional RVFV isolates and perform whole-genome sequencing and phylogenetic analysis, so as to enhance our understanding of the continuing evolution of this important virus. This review forms part of a series of invited papers in Antiviral Research on the genetic diversity of emerging viruses.

The pathogenesis of Rift Valley fever

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.

NSs protein of rift valley fever virus promotes posttranslational downregulation of the TFIIH subunit p62

Rift Valley fever virus (RVFV; family Bunyaviridae, genus Phlebovirus) is an important emerging pathogen of humans and ruminants. Its NSs protein has previously been identified as a major virulence factor that suppresses host defense through three distinct mechanisms: it directly inhibits beta interferon (IFN-β) promoter activity, it promotes the degradation of double-stranded RNA-dependent protein kinase (PKR), and it suppresses host transcription by disrupting the assembly of the basal transcription factor TFIIH through sequestration of its p44 subunit. Here, we report that in addition to PKR, NSs also promotes the degradation of the TFIIH subunit p62. Infection of cells with the RVFV MP-12 vaccine strain reduced p62 protein levels to below the detection limit early in the course of infection. This NSs-mediated downregulation of p62 was posttranslational, as it was unaffected by pharmacological inhibition of transcription or translation and MP-12 infection had no effect on p62 mRNA levels. Treatment of cells with proteasome inhibitors but not inhibition of lysosomal acidification or nuclear export resulted in a stabilization of p62 in the presence of NSs. Furthermore, p62 could be coprecipitated with NSs from lysates of infected cells. These data suggest that the RVFV NSs protein is able to interact with the TFIIH subunit p62 inside infected cells and promotes its degradation, which can occur directly in the nucleus.

Molecular biology of rift valley Fever virus

Rift Valley fever virus (RVFV) causes large outbreaks of acute febrile and often fatal illness among humans and domesticated animals in sub-saharan Africa and the Arabian peninsula. RVFV is a member of the family Bunyaviridae, genus Phlebovirus. Like all members of this large virus family, it contains a three-segmented genome of negative/ambisense strand RNA, packaged into viral nucleocapsid protein, and enveloped by a lipid bilayer containing two viral glycoproteins. During the past years, there was an increased interest in RVFV epidemiology, molecular biology, and virulence mechanisms. Here, we will try to provide an overview over the basic features of this significant pathogen, and review the latest developments in this highly active research field.

NSs protein of rift valley fever virus induces the specific degradation of the double-stranded RNA-dependent protein kinase

Rift Valley fever virus (RVFV) continues to cause large outbreaks of acute febrile and often fatal illness among humans and domesticated animals in Africa, Saudi Arabia, and Yemen. The high pathogenicity of this bunyavirus is mainly due to the viral protein NSs, which was shown to prevent transcriptional induction of the antivirally active type I interferons (alpha/beta interferon [IFN-alpha/beta]). Viruses lacking the NSs gene induce synthesis of IFNs and are therefore attenuated, whereas the noninducing wild-type RVFV strains can only be inhibited by pretreatment with IFN. We demonstrate here in vitro and in vivo that a substantial part of the antiviral activity of IFN against RVFV is due to a double-stranded RNA-dependent protein kinase (PKR). PKR-mediated virus inhibition, however, was much more pronounced for the strain Clone 13 with NSs deleted than for the NSs-expressing strain ZH548. In vivo, Clone 13 was nonpathogenic for wild-type (wt) mice but could regain pathogenicity if mice lacked the PKR gene. ZH548, in contrast, killed both wt and PKR knockout mice indiscriminately. ZH548 was largely resistant to the antiviral properties of PKR because RVFV NSs triggered the specific degradation of PKR via the proteasome. The NSs proteins of the related but less virulent sandfly fever Sicilian virus and La Crosse virus, in contrast, had no such anti-PKR activity despite being efficient suppressors of IFN induction. Our data suggest that RVFV NSs has gained an additional anti-IFN function that may explain the extraordinary pathogenicity of this virus.

Efficient production of Rift Valley fever virus-like particles: The antiviral protein MxA can inhibit primary transcription of bunyaviruses

Rift Valley fever virus (RVFV) is a highly pathogenic member of the family Bunyaviridae that needs to be handled under biosafety level (BSL) 3 conditions. Here, we describe reverse genetics systems to measure RVFV polymerase activity in mammalian cells and to generate virus-like particles (VLPs). Recombinant polymerase (L) and nucleocapsid protein (N), expressed together with a minireplicon RNA, formed transcriptionally active nucleocapsids. These could be packaged into VLPs by additional expression of viral glycoproteins. The VLPs resembled authentic virus particles and were able to infect new cells. After infection, VLP-associated nucleocapsids autonomously performed primary transcription, and co-expression of L and N in VLP-infected cells allowed subsequent replication and secondary transcription. Bunyaviruses are potently inhibited by a human interferon-induced protein, MxA. However, the affected step in the infection cycle is not entirely characterized. Using the VLP system, we demonstrate that MxA inhibits both primary and secondary transcriptions of RVFV. A set of infection assays distinguishing between virus attachment, entry, and subsequent RNA synthesis confirmed that MxA is able to target immediate early RNA synthesis of incoming RVFV particles. Thus, our reverse genetics systems are useful for dissecting individual steps of RVFV infection under non-BSL3 conditions.

T7 RNA polymerase-dependent and -independent systems for cDNA-based rescue of Rift Valley fever virus

Rift Valley fever virus (RVFV) is responsible for large and recurrent outbreaks of acute febrile illness among humans and domesticated animals in Africa. It belongs to the family Bunyaviridae, genus Phlebovirus, and its negative-stranded RNA genome consists of three segments. Here, we report the establishment and characterization of two different systems to rescue the RVFV wild-type strain ZH548. The first system is based on the BHK-21 cell clone BSR-T7/5, which stably expresses T7 RNA polymerase (T7 pol). Rescue of wild-type RVFV was achieved with three T7 pol-driven cDNA plasmids representing the viral RNA segments in the antigenomic sense. The second system involves 293T cells transfected with three RNA pol I-driven plasmids for the viral segments and two RNA pol II-driven support plasmids to express the viral polymerase components L and N. It is known that the 5' triphosphate group of T7 pol transcripts strongly activates the antiviral interferon system via the intracellular RNA receptor RIG-I. Nonetheless, both the T7 pol and the pol I/II system were of similar efficiency. This was even true for the rescue of a RVFV mutant lacking the interferon antagonist nonstructural proteins. Further experiments demonstrated that the unresponsiveness of BHK-21 and BSR-T7/5 cells to T7 pol transcripts is most probably due to a deficiency in the RIG-I pathway. Our reverse genetics systems now enable us to manipulate the genome of RVFV and study its virulence mechanisms. Moreover, the finding that BHK-derived cell lines have a compromised RIG-I pathway may explain their suitability for propagating and rescuing a wide variety of viruses.

Three-dimensional organization of Rift Valley fever virus revealed by cryoelectron tomography

Rift Valley fever virus (RVFV) is a member of the Bunyaviridae virus family (genus Phlebovirus) and is considered to be one of the most important pathogens in Africa, causing viral zoonoses in livestock and humans. Here, we report the characterization of the three-dimensional structural organization of RVFV vaccine strain MP-12 by cryoelectron tomography. Vitrified-hydrated virions were found to be spherical, with an average diameter of 100 nm. The virus glycoproteins formed cylindrical hollow spikes that clustered into distinct capsomeres. In contrast to previous assertions that RVFV is pleomorphic, the structure of RVFV MP-12 was found to be highly ordered. The three-dimensional map was resolved to a resolution of 6.1 nm, and capsomeres were observed to be arranged on the virus surface in an icosahedral lattice with clear T=12 quasisymmetry. All icosahedral symmetry axes were visible in self-rotation functions calculated using the Fourier transform of the RVFV MP-12 tomogram. To the best of our knowledge, a triangulation number of 12 had previously been reported only for Uukuniemi virus, a bunyavirus also within the Phlebovirus genus. The results presented in this study demonstrate that RVFV MP-12 possesses T=12 icosahedral symmetry and suggest that other members of the Phlebovirus genus, as well as of the Bunyaviridae family, may adopt icosahedral symmetry. Knowledge of the virus architecture may provide a structural template to develop vaccines and diagnostics, since no effective anti-RVFV treatments are available for human use.

NSm and 78-kilodalton proteins of Rift Valley fever virus are nonessential for viral replication in cell culture

Rift Valley fever viruses carrying mutations of the M gene preglycoprotein region, one lacking NSm protein expression, one lacking 78-kDa protein expression, and one lacking expression of both proteins, were compared in cell culture. All of the mutants and their parent virus produced plaques with similar sizes and morphologies in Vero E6 cells and had similar growth kinetics in Vero, C6/36, and MRC5 cells, demonstrating that the NSm and 78-kDa proteins were not needed for the virus to replicate efficiently in cell culture. A competition-propagation assay revealed that the parental virus was slightly more fit than the mutant virus lacking expression of both proteins.

The S segment of rift valley fever phlebovirus (Bunyaviridae) carries determinants for attenuation and virulence in mice

Unlike all the other Rift Valley fever virus strains (Bunyaviridae, Phlebovirus) studied so far, clone 13, a naturally attenuated virus, does not form the filaments composed of the NSs nonstructural protein in the nuclei of infected cells (R. Muller, J. F. Saluzzo, N. Lopez, T. Drier, M. Turell, J. Smith, and M. Bouloy, Am. J. Trop. Med. Hyg. 53:405-411, 1995). This defect is correlated with a large in-frame deletion in the NSs coding region of the S segment of the tripartite genome. Here, we show that the truncated NSs protein of clone 13 is expressed and remains in the cytoplasm, where it is degraded rapidly by the proteasome. Through the analysis of reassortants between clone 13 and a virulent strain, we localized the marker(s) of attenuation in the S segment of this attenuated virus. This result raises questions regarding the role of NSs in pathogenesis and highlights, for the first time in the Bunyaviridae family, a major role of the S segment in virulence and attenuation, possibly associated with a defect in the nonstructural protein.

The Rift Valley fever virus nonstructural protein NSs is phosphorylated at serine residues located in casein kinase II consensus motifs in the carboxy-terminus

The S segment of Rift Valley fever virus (Bunyaviridae, Phlebovirus) codes for two proteins, the nucleoprotein N and the nonstructural protein NSs. The NSs protein is a phosphoprotein of unknown function that is localized in the cytoplasm and the nuclei of infected cells where it forms filamentous structures. To characterize further the protein expressed in VC10 cells infected with the MP12 strain, we analyzed its phosphorylation states and showed that phosphorylated forms were found in both compartments. Cytoplasmic and nuclear NSs were phosphorylated only at serine residues. Phosphopeptide mapping and molecular analysis of mutants obtained by site-directed mutagenesis allowed us to map the major phosphorylation sites of nuclear and cytoplasmic forms of NSs to serine residues 252 and 256, located at the carboxy-terminus in consensus sequences for casein kinase II. A similar map was obtained when the protein was purified from mosquito cells infected with MP12. In addition, we showed that the purified unphosphorylated NSs protein expressed from pET-NSs plasmid in a coupled transcription-translation reaction containing Escherichia coli S30 extracts did not possess autophosphorylation activity but was phosphorylated in vitro after incubation with recombinant casein kinase II.

The carboxy-terminal acidic domain of Rift Valley Fever virus NSs protein is essential for the formation of filamentous structures but not for the nuclear localization of the protein

The ambisense S segment of Rift Valley fever (RVF) virus (a phlebovirus in the Bunyaviridae family) codes for two proteins: the viral complementary-sense RNA for the N nucleoprotein and the genomic-sense RNA for the nonstructural protein NSs. Except for the fact that the NSs protein is phosphorylated and forms filamentous structures in the nuclei of infected cells (R. Swanepoel and N. K. Blackburn, J. Gen. Virol. 34:557-561, 1977), its role is poorly understood, especially since the replication cycle of all these viruses takes place in the cytoplasm. To investigate the mechanisms involved in filament formation, we expressed NSs in mammalian cells via a recombinant Semliki Forest virus and demonstrated that the protein alone was able to form structures similar to those observed in RVF virus-infected cells, indicating that the presence of other RVF virus proteins is not required for filament formation. The yeast two-hybrid system was used to show that the protein interacts with itself and to map the interacting domains. Various deletion and substitution mutants were constructed, and the mutant proteins were analyzed by immunoprecipitation, Western blotting and immunofluorescence. These experiments indicated that the 10 to 17 amino acids of the carboxy-terminal domain were involved in self-association of the protein and that deletion of this acidic carboxy-terminal domain prevents the protein from forming filaments but does not affect its nuclear localization. The role of two phosphorylation sites present in this domain was also investigated, but they were not found to have a major influence on the formation of the nuclear filament.

Mapping of the mutations present in the genome of the Rift Valley fever virus attenuated MP12 strain and their putative role in attenuation

The MP12 attenuated strain of Rift Valley fever virus was obtained by 12 serial passages of a virulent isolate ZH548 in the presence of 5-fluorouracil (Caplen et al., 1985. Mutagen-directed attenuation of Rift Valley fever virus as a method for vaccine development. J. Gen. Virol., 66, 2271-2277). The comparison of the M segment of the two strains has already been reported by Takehara et al. (Takehara et al., 1989. Identification of mutations in the M RNA of a candidate vaccine strain of Rift Valley fever virus. Virology 169, 452-457). We have completed the comparison and found that altogether a total of nine, 12 and four nucleotides were changed in the L, M and S segments of the two strains, respectively. Three mutations induced amino acid changes in the L protein but none of them was located in the recognized motifs conserved among RNA dependent polymerases. In the S segment, a single change modified an amino acid in the NSs protein and in the M segment, seven of the mutations resulted in amino acid changes in each of the four encoded G1, G2, 14 kDa and 78 kDa proteins. Characterization of the MP12 virus indicated that determinants for attenuation were present in each segment and that they were introduced progressively during the 12 passages in the presence of the mutagen (Saluzzo and Smith, 1990. Use of reassortant viruses to map attenuating and temperature-sensitive mutations of the Rift Valley fever virus MP-12 vaccine. Vaccine 8, 369-375). Passages 4 and 7-9 were found to be essential for introduction of temperature-sensitive lesions and attenuation. In an attempt to correlate some of the mutations with the attenuated or temperature-sensitive phenotypes, we determined by sequencing the passage level at which the different mutations appeared. This work should help to address the question of the role of the viral gene products in Rift Valley fever pathogenesis.

Identification of mayaro virus nucleocapsid protein in nucleus of Aedes albopictus cells

The modifications in the pattern of nuclear proteins of Aedes albopictus cells in response to Mayaro virus infection were analysed early and late after infection. The viral capsid (C) protein of 34 kDa (p34) could be detected in the nuclear compartment 4 h after infection, soon after its synthesis in the cytoplasm. In addition to p34, a group of high molecular weight proteins was also present in this compartment late after infection. The exposition of infected cells to supra optimal temperature of growth modifies significantly the pattern of nuclear proteins. However, the stress condition does not inhibit the transport of p34 to the nucleus. The transport of proteins into nuclei was also followed under "in vitro' conditions by incubating radiolabeled post-mitochondrial extract of infected cells with unlabeled nuclei. Under these conditions, as observed "in vivo', a specific transport of viral C protein and of a group of proteins of high molecular weight to the nuclei was also detected. These results indicate that Mayaro virus infection modifies the nuclear protein pattern in invertebrate cells.

Association of the nonstructural protein NSs of Uukuniemi virus with the 40S ribosomal subunit

The small RNA segment (S segment) of Uukuniemi (UUK) virus encodes two proteins, the nucleocapsid protein (N) and a nonstructural protein (NSs), by an ambisense strategy. The function of NSs has not been elucidated for any of the bunyaviruses expressing this protein. We have now expressed the N and NSs proteins in Sf9 insect cells by using the baculovirus expression system. High yields of both proteins were obtained. A monospecific antibody was raised against gel-purified NSs and used to study the synthesis and localization of the protein in UUK virus-infected BHK21 cells. While the N protein was detected as early as 4 h postinfection (p.i.), NSs was identified only after 8 h p.i. Both proteins were still synthesized at high levels at 24 h p.i. The half-life of NSs was about 1.5 h, while that of the N protein was several hours. Sucrose gradient fractionation of [35S]methionine-labeled detergent-solubilized extracts of infected BHK21 cells indicated that NSs was firmly associated with the 40S ribosomal subunit. This association took place shortly after translation and was partially resistant to 1 M NaCl. NSs expressed by using the T7 vaccinia virus expression system, as well as in vitro-translated NSs, was also associated with the 40S subunit. In contrast, in vitro-translated N protein was found on top of the gradient. Immunolocalization of NSs, in UUK virus-infected cells, by using an affinity-purified antibody showed a granular cytoplasmic staining. A very similar pattern was seen for cells expressing NSs from a cDNA copy by using a vaccinia virus expression system. No staining was observed in the nuclei in either case. Furthermore, NSs was found neither in virions nor in nucleocapsids isolated from infected cells. In vivo labeling with 32Pi indicated that NSs is not phosphorylated. The possible function of NSs is discussed in light of these results.

Nuclear localization of Semliki Forest virus-specific nonstructural protein nsP2

About 50% of Semliki Forest virus-specific nonstructural protein nsP2 is associated with the nuclear fraction in virus-infected BHK cells. Transport into the nucleus must be specific, since only trace amounts of nsP3 and nsP4 and about 13% of nsP1, all derived from the same polyprotein, were found in the nucleus. Subfractionation of [35S]methionine-labeled Semliki Forest virus-infected cells showed that 80 to 90% of the nuclear nsP2 was associated with the nuclear matrix. Indirect immunofluorescence, with anti-nsP2 antiserum, showed the most intensive staining of structures which by Nomarski optics appeared to be nucleoli. In the presence of 1 to 5 micrograms of dactinomycin per ml the nuclei were stained evenly and no nucleoli could be found. Transport of nsP2 into the nucleus occurred early in infection and was fairly rapid. A cDNA encoding the complete nsP2 was isolated by the polymerase chain reaction technique and ligated into a simian virus 40 expression vector derivative. When BHK cells were transfected with this pSV-NS2 vector by the lipofection procedure, nsP2 was expressed in about 1 to 5% of the cells, as shown by indirect immunofluorescence. In positively transfected cells the immunofluorescence stain was most intensive in the nucleoli. Thus, Semliki Forest virus-specific nsP2 must have information which directs it into the nuclear matrix and, more specifically, into the nucleoli.

Isolation of Rift Valley fever virus from human peripheral blood mononuclear cells: Mauritanian epidemic

During the Mauritanian Rift Valley fever (RVF) epidemic of 1987, peripheral blood mononuclear cells (PBMC) were studied from 78 sick patients. RVF virus (RVFV) was isolated in 5 cases, on Aedes pseudoscutellaris AP61, from both PBMC and serum. Among the 78 cases studied, RVF was proven in 19 cases (24.3%) by specific IgM detection, and in 12 cases (15.3%) by virus isolation from serum, of which 3 also exhibited anti-RVF IgM. Among the 5 PBMC-positive RVFV cases, 2 strains were isolated in the presence of specific IgM from patients presenting with neurologic signs. These observations raised the question as to the role of specific IgM in cellular infection, and suggest that, in certain cases, mononuclear cells may promote RVFV dissemination into brain cells. Further investigations need to be undertaken to determine the RVFV receptor expressed on PBMC membranes.

Rift Valley fever virus M segment: use of recombinant vaccinia viruses to study Phlebovirus gene expression

Recombinant vaccinia viruses were constructed and used in conjunction with site-specific antisera to study the coding capacity and detailed expression strategy of the M segment of the Phlebovirus Rift Valley fever virus (RVFV). The M segment could be completely and faithfully expressed in recombinant RVFV-vaccinia virus-infected cells, the gene products apparently being correctly processed and modified in the absence of the RVFV L and S genomic segments. The proteins encoded by the RVFV M segment included, in addition to the viral glycoproteins G2 and G1, two previously uncharacterized polypeptides of 78 and 14 kilodaltons (kDa). By manipulation of RVFV sequences present in the recombinant vaccinia viruses and use of specific antibody reagents, it was found that the 78-kDa protein initiated at the first initiation codon of the open reading frame and encompassed the entire preglycoprotein and glycoprotein G2 coding sequences. The 14-kDa protein appeared to begin from the second in-phase ATG and was composed of only the preglycoprotein sequences. Both viral glycoproteins G2 and G1 could be synthesized and correctly processed in the absence of the 78- and 14-kDa proteins, as well as a large portion of the preglycoprotein sequences. However, the hydrophobic amino acid sequence immediately preceding the mature glycoprotein coding sequences was required for authentic glycoprotein production. The M-segment expression strategy involving aspects of translational initiation and protein processing are discussed. The functional roles of the 78- and 14-kDa proteins remain unclear.

Preparation and use of monoclonal antibodies for identifying Crimean-Congo hemorrhagic fever virus

Seven monoclonal antibodies were prepared against a South African strain of Crimean-Congo hemorrhagic fever (CCHF) virus and were found to be directed against viral nucleocapsid protein. Five of the monoclonal antibodies reacted to high titer in indirect immunofluorescence (IF) tests and enzyme-linked immunosorbent assays (ELISA) with 22 strains of CCHF virus and failed to cross-react with the closest antigenic relative of CCHF, Hazara virus, or with 4 other nairoviruses which need to be distinguished from CCHF virus in Africa. These antibodies, used in the IF technique, readily detected antigens induced by all strains of CCHF virus included in the study in cell culture monolayers and mouse brain tissue, which represent the systems commonly used for isolation of CCHF virus. The IF technique with monoclonal antibodies constitutes a rapid and specific means of identifying newly isolated strains of CCHF virus.

Comparison of techniques for demonstrating antibodies to Rift Valley fever virus

Nine serological techniques were compared by monitoring the response to infection with Rift Valley fever (RVF) virus in three sheep. Antibodies were monitored daily for the first 14 days after infection, then weekly and later fortnightly up to week 24. The earliest antibody response was detected in one sheep on day 3 by a plaque reduction neutralization test, and by day 6 antibodies were demonstrable in all three sheep by haemagglutination-inhibition, reversed passive haemagglutination-inhibition, immunodiffusion, indirect immunofluorescence (IF), enzyme-linked immunosorbent assay and neutralization of cytopathic effect in cell cultures. Antibodies were demonstrable by complement fixation on day 8 at the earliest. IF and the two neutralization techniques produced the highest titres, but all tests could be used satisfactorily for the serological diagnosis of RVF. Inactivated antigen could be used for all except the neutralization tests. A radioimmunoassay technique using 125I-labelled staphylococcal protein A detected antibodies on day 8 at the earliest and produced lower mean titres than some of the other techniques. This was probably because sheep immunoglobulins bind protein A poorly.