Wolbachia is a maternal transmitted endosymbiotic bacterium that is estimated to infect up to 65% of insect species. The ability of Wolbachia to both induce viral interference and spread into mosquito vector population makes it possible to develop Wolbachia as a biological control agent for dengue control. While Wolbachia induces resistance to dengue virus in the transinfected Aedes aegypti mosquitoes, a similar effect was not observed in Aedes albopictus, which naturally carries Wolbachia infection but still serves as a dengue vector. In order to understand the mechanism of this lack of Wolbachia-mediated viral interference, we used both Ae. albopictus cell line (Aa23) and mosquitoes to characterize the impact of Wolbachia on dengue infection. A serial of sub-lethal doses of antibiotic treatment was used to partially remove Wolbachia in Aa23 cells and generate cell cultures with Wolbachia at different densities. We show that there is a strong negative linear correlation between the genome copy of Wolbachia and dengue virus with a dengue infection completely removed when Wolbacha density reaches a certain level. We then compared Wolbachia density between transinfected Ae. aegypti and naturally infected Ae. albopictus. The results show that Wolbachia density in midgut, fatbody and salivary gland of Ae. albopictus is 80-, 18-, and 24-fold less than that of Ae. aegypti, respectively. We provide evidence that Wolbachia density in somatic tissues of Ae. albopictus is too low to induce resistance to dengue virus. Our results will aid in understanding the mechanism of Wolbachia-mediated pathogen interference and developing novel methods to block disease transmission by mosquitoes carrying native Wolbachia infections.
Transmitted by mosquitoes, dengue virus causes the most important arbovirus disease in humans. Increasing problems in insecticide resistance and the lack of drugs and vaccines make it urgent to develop novel strategies for dengue control. Wolbachia is a maternally transmitted Gram-negative endosymbiotic bacterium that infects approximately 28% of mosquito species. It can not only spread within mosquito populations through its unique ability to manipulate mosquito reproduction but can also induce resistance to dengue virus in mosquito vectors. This leads to a genetic control strategy in which mosquito hosts are made to be inhospitable to dengue virus through population replacement. However, it is challenged by the fact that Ae. albopictus naturally carries Wolbachia infections but still services as a dengue vector. In this study we show the native Wolbachia induces a resistance to dengue virus in Wolbachia density-dependent manner in Ae. albopictus. With a decrease in Wolbachia density within the host cells, dengue infection increases dramatically. We provide evidence that a very low Wolbachia density in mosquito tissues where dengue virus will reside and travel could contribute to absence of Wolbachia-mediated resistance to dengue virus in Ae. albopictus.