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

Dengue fever is a severely debilitating disease caused by the dengue virus (DENV) and transmitted by <i>Aedes aegypti</i> 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 <i>Wolbachia</i> as a bio-control agent. <i>Wolbachia</i> is present in 40% of insects but not <i>Ae. aegypti</i>. However <i>Wolbachia </i>has been artificially introduced into <i>Ae. aegypti</i> where it forms stably inherited infections. <i>Wolbachia</i> 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 <i>Wolbachia</i>. This characteristic, known as cytoplasmic incompatibility enables <i>Wolbachia</i> infection to spread through wild populations. <i>Wolbachia</i> also has the ability to limit the replication of co-infecting pathogens in mosquitoes. <i>Wolbachia</i> infected mosquitoes are currently being released into the field to trial whether they will interrupt DENV transmission to humans. <br> <br> The mechanistic basis of <i>Wolbachia</i>-DENV blocking is not well understood. There is evidence that <i>Wolbachia</i> activates the host’s immune response thereby making it able to resist subsequent DENV infection in a process known as ‘immune priming’. Competition between <i>Wolbachia</i> and the virus for limited host resources has also been linked with <i>Wolbachia</i>-DENV blocking. Immune priming and resource competition, however, do not fully explain <i>Wolbachia</i>-DENV inhibition. The strength of blocking appears to correlate with <i>Wolbachia</i> density, with highly infected mosquitoes and cell lines exhibiting almost complete DENV inhibition. <br> <br> This thesis therefore focussed on understanding the effect <i>Wolbachia</i> density has on DENV blocking in various mosquito tissues by increasing nutritional resources through feeding <i>Ae. aegypti</i> mosquitoes multiple human blood meals. We found that multiple blood meals do not increase <i>Wolbachia</i> density or DENV blocking, indicating that the <i>Wolbachia</i>-mediated DENV inhibition should be stable throughout the lifespan of <i>Ae. aegypti</i>. This thesis also investigated whether particular mosquito tissues were important for DENV blocking by virtue of their <i>Wolbachia</i> density or functional roles in infection and immunity. We found that no particular tissue type was important for <i>Wolbachia</i>-DENV blocking, suggesting that the mechanisms underpinning blocking should be systemic or fundamental to diverse cell types. <br> <br> 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 <i>Wolbachia</i> 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, <i>Wolbachia</i> enhanced ISF infection rates and loads indicating that <i>Wolbachia</i>-mediated anti-virus effect is not universal to all flaviviruses. Further study is therefore required of <i>Wolbachia</i>-ISF interactions and the effect this has on arboviruses such as DENV. <br> <br> This thesis has clearly demonstrated that feeding mosquitoes repeatedly on human blood does not influence <i>Wolbachia</i>-DENV inhibition and that blocking is not dependent on a particular tissue type further adding to the emerging understanding of <i>Wolbachia</i>-DENV interactions. In addition it has demonstrated that the anti-virus effect of <i>Wolbachia</i> is not common to all flaviviruses.