Midgut barriers prevent the replication and dissemination of the yellow fever vaccine in Aedes aegypti

BackgroundTo be transmitted to vertebrate hosts via the saliva of their vectors, arthropod-borne viruses have to cross several barriers in the mosquito body, including the midgut infection and escape barriers. Yellow fever virus (YFV) belongs to the genus Flavivirus, which includes human viruses transmitted by Aedes mosquitoes, such as Dengue and Zika viruses. The live-attenuated YFV-17D vaccine has been used safely and efficiently on a large scale since the end of World War II. Early studies have shown, using viral titration from salivary glands of infected mosquitoes, that YFV-17D can infect Aedes aegypti midgut, but does not disseminate to other tissues.\n\nMethodology/Principal FindingsHere, we re-visited this issue using a panel of techniques, such as RT-qPCR, Western blot, immunofluorescence and titration assays. We showed that YFV-17D replication was not efficient in Aedes aegypti midgut, as compared to the clinical isolate YFV-Dakar. Viruses that replicated in the midgut failed to disseminate to secondary organs. When injected into the thorax of mosquitoes, viruses succeeded in replicating into midgut-associated tissues, suggesting that, during natural infection, the block for YFV-17D replication occurs at the basal membrane of the midgut. Our NGS analysis revealed that YFV-Dakar genome exhibited a greater diversity than the vaccine strain; a trait that may contribute to its ability to infect and disseminate efficiently in Ae. aegypti.\n\nConclusions/SignificanceThe two barriers associated with Ae. aegypti midgut prevent YFV-17D replication. Our study contributes to our basic understanding of vector-pathogen interactions and may also aid in the development of non-transmissible live virus vaccines.\n\nAuthor summaryMost flaviviruses, including yellow fever virus (YFV), are transmitted between hosts by mosquito bites. The yellow fever vaccine (YFV-17D) is one of the safest and most effective live virus vaccine ever developed. It is also used as a platform for engineering vaccines against other health-threatening flaviviruses, such as Japanese encephalitis, West Nile, Dengue and Zika viruses. We studied here the replication and dissemination of YFV-17D in mosquitoes. Our data showed that YFV-17D replicates poorly in mosquito midgut and is unable to disseminate to secondary organs, as compare to a YFV clinical isolate. Our study contributes to our basic understanding of the interactions between viruses and their vectors, which is key for conceiving new approaches in inhibiting virus transmission and designing non-transmissible live virus vaccines.