Interactions between Wolbachia and the Mosquito Vector of Dengue Virus Aedes aegypti

Abstract

Dengue fever is a severely debilitating disease caused by the dengue virus (DENV) and transmitted by Aedes aegypti mosquitoes. There is no specific treatment for the disease and the current vaccine does not give complete protection against DENV. A novel control approach involves the use of the maternally transmitted endosymbiotic bacterium Wolbachia as a bio-control agent. Wolbachia is present in 40% of insects but not Ae. aegypti. However Wolbachia has been artificially introduced into Ae. aegypti where it forms stably inherited infections. Wolbachia is able to manipulate female reproduction leading to inviable eggs when an infected male mates with an uninfected female or females with a different strain of Wolbachia. This characteristic, known as cytoplasmic incompatibility enables Wolbachia infection to spread through wild populations. Wolbachia also has the ability to limit the replication of co-infecting pathogens in mosquitoes. Wolbachia infected mosquitoes are currently being released into the field to trial whether they will interrupt DENV transmission to humans. The mechanistic basis of Wolbachia-DENV blocking is not well understood. There is evidence that Wolbachia activates the host’s immune response thereby making it able to resist subsequent DENV infection in a process known as ‘immune priming’. Competition between Wolbachia and the virus for limited host resources has also been linked with Wolbachia-DENV blocking. Immune priming and resource competition, however, do not fully explain Wolbachia-DENV inhibition. The strength of blocking appears to correlate with Wolbachia density, with highly infected mosquitoes and cell lines exhibiting almost complete DENV inhibition. This thesis therefore focussed on understanding the effect Wolbachia density has on DENV blocking in various mosquito tissues by increasing nutritional resources through feeding Ae. aegypti mosquitoes multiple human blood meals. We found that multiple blood meals do not increase Wolbachia density or DENV blocking, indicating that the Wolbachia-mediated DENV inhibition should be stable throughout the lifespan of Ae. aegypti. This thesis also investigated whether particular mosquito tissues were important for DENV blocking by virtue of their Wolbachia density or functional roles in infection and immunity. We found that no particular tissue type was important for Wolbachia-DENV blocking, suggesting that the mechanisms underpinning blocking should be systemic or fundamental to diverse cell types. Mosquitoes are naturally infected with viruses, including flaviruses known as insect-specific flaviviruses (ISF). Although ISF are incapable of infecting vertebrates they may alter susceptibility of mosquitoes to medically important flaviviruses. The effect Wolbachia has on ISF is currently unknown. This thesis therefore examined whether the anti-virus effect demonstrated for flaviviruses including DENV is observed for ISF. Surprisingly, Wolbachia enhanced ISF infection rates and loads indicating that Wolbachia-mediated anti-virus effect is not universal to all flaviviruses. Further study is therefore required of Wolbachia-ISF interactions and the effect this has on arboviruses such as DENV. This thesis has clearly demonstrated that feeding mosquitoes repeatedly on human blood does not influence Wolbachia-DENV inhibition and that blocking is not dependent on a particular tissue type further adding to the emerging understanding of Wolbachia-DENV interactions. In addition it has demonstrated that the anti-virus effect of Wolbachia is not common to all flaviviruses.