Characterisation of the immune modulatory effect of wild type Rift Valley fever virus strains

Rift Valley fever Phlebovirus (RVFV) belongs to the Bunyaviridae family. It is transmitted by mosquitoes and infects a wide range of vertebrate hosts. In humans, infection can lead to fatal hepatitis with hemorrhagic fever and encephalitis. In cattle, infection generally causes death of young animals, or abortion and teratogenesis in pregnant females. The disease is endemic in many countries of sub-Saharan Africa and in Egypt where it repeatedly provokes serious epizootics and concomitant epidemics. The differences in virulence among RVFV isolates may be due to the effectiveness of interference with either the innate and/or the adaptive immunity and may explain the wide range of clinical outcome in susceptible vertebrate hosts. Interferon-β (IFN-β) is a key molecule of the innate immune response Therefore, the IFN-β antagonistic function of NSs of RVFV isolates from different sources (animals, humans, and insects) was assessed after cloning and sequencing the non-structural S segment gene (NSs). The NSs clones were monitored for their immune modulatory effects by analysing their ability to suppress the activation of the IFN-β promoter using a reporter assay system. Additionally, expression of NSs in Vero E6 cells was monitored by immuno-fluorescence staining. Two RVFV NSs proteins (derived from isolates R7 and R18) failed to inhibit IFN-β promoter activation whereas the remaining 24 showed efficient suppression of IFN-β promoter activity. Additionally R7-NSs and R18-Nss were unable to form nuclear filaments which are a typical feature of wild-type RVFV-NSs. Sequencing of R18-NSs revealed a large internal in-frame deletion identical to the mutation described for the naturally occurring RVFV mutant clone 13, which leads to a non-functional NSs-protein. Indeed, R18 was later identified as a RVFV clone 13 isolate. In contrast, R7-NSs contains a point mutation in the NSs gene, which results in the replacement of a leucine by proline at amino acid position 115. Interestingly, this unique point mutation has effects comparable to the large in-frame deletion of clone 13 NSs. R10-NSs was the only NSs variant which efficiently suppressed IFN-β promoter activation but failed to from nuclear filaments. This can be attributed to replacement of leucine by proline at amino acid position 107. These results lead to the conclusion, that the domain containing amino acids 107 to 115 is essential for NSs functions. Dendritic cells (DCs) are professional antigen-presenting cells and represent a link between innate and adaptive immunity. Therefore the effect of RVFV infections on DC function was investigated Experiments with primary human myeloid and plasmacytoid dendritic cells (mDCs / pDCs) revealed for the first time that the wtRVFV ZH548 only replicates in mDCs, however incomplete activation of both pDCs and mDCs after RVFV infection was observed. High amounts of the proinflammatory cytokine interleukin-6 combined with the complete lack of type I IFN responses in both pDCs and mDCs might be responsible for the severe outcome of RVFV wt infections. Interestingly, even the RVFV strain clone 13 which is an efficient inducer of type I IFN in non-immune cells did not induce IFN-α in DCs The combination of incomplete DC activation and absence of IFN-α after infection with both wild-type RVFV and clone 13 indicates that RVFV interfere with the first steps of the adaptive immune response independent of NSs function and that RVFV may use mDCs for systemic dissemination in the infected host.