Mosquitoes are vectors for epidemic transmission of viruses of public and veterinary health. The mosquito vector is generally infected for life although, unlike the vertebrate counterpart, does not suffer a high fitness cost. This is usually attributed to the mosquito immune response, especially the RNAi system, which cleaves viral genomes reducing the pathogen’s replication. In addition, another process occurring during viral infection of mosquitoes involves the reverse transcription of the viral RNA genome into viral DNA (vDNA). This genetic material is then integrated into the mosquito’s genome and used to produce dsRNA with antiviral properties. Viral DNA persists in infected mosquitoes in the form of endogenous viral elements (EVE) integrated in the mosquito genome, as well as in episomes.
In this study we infect Aedes aegypti mosquitoes with Semliki Forest virus (SFV), Ross river virus (RRV) and dengue virus (DENV) through a bloodmeal and we monitor the offspring for vertical transmission of the pathogen. We observe vertical transmission of virus in dengue infected derived mosquito offspring. However, we do not observe infectious virus in mosquitoes derived from SFV or RRV infected parents. We however find virus RNA. We demonstrate that this RNA is not from viral genomes but from integrated EVEs. In addition, we observed integrated and episomal vDNA sequences. More strikingly, offspring derived from SFV and RRV infected parents showed viral refraction to these viruses. This vertically transmitted immune response is probably also mediated thought the PIWI and RNAi pathways and lasts a couple of generations. The vDNA integrated however lasts longer, although it becomes inactive.
Mosquitoes immunity has always been considered innate, however this proves that these vectors have an adaptive inheritable immune response. To our knowledge this is the first description of vertical transmission of antiviral immunity in arthropods.