Bacterial immune priming and Wolbachia-mediated dengue blocking in Aedes albopictus mosquitoes

BACKGROUND: Wolbachia is a maternally-transmitted bacterial endosymbiont that is highly prevalent amongst arthropods. Many Wolbachia strains induce pathogen blocking, thereby protecting their hosts from pathogenic microbial infections. Blocking against pathogenic bacteria occurs in Aedes aegypti mosquitoes, where Wolbachia has been artificially established by microinjection. However, in Drosophila fruit flies, it occurs after oral bacterial challenge but not after systemic challenge via injection. Pathogen blocking has been linked to increased host immune activity associated with Wolbachia infection. This is similar in principle to immune priming, a memory immune response in insects, where sub-lethal exposure to a pathogen increases the ability to survive future infections. Aedes albopictus mosquitoes naturally harbor Wolbachia; however, the extent to which native Wolbachia-mediated pathogen blocking and immune priming influence host response to infection remains unclear. Accordingly, we sought to characterize the response of Wolbachia-infected Ae. albopictus to bacterial challenge, and to determine whether the combination of these microbes could hinder a subsequent infection with dengue virus. METHODOLOGY/PRINCIPAL FINDINGS: We challenged adult female Ae. albopictus with and without their native Wolbachia infection with a sub-lethal dose of four mosquito-associated bacteria: Micrococcus, Staphylococcus (gram positive), Brevundimonas, Enterobacter (gram negative). Assessing post-challenge mortality, we observed that Wolbachia infection reduced cumulative mortality and delayed the onset of senescence amongst mosquitoes challenged with each bacterium. However, this protective effect was stronger against gram positive bacteria. Bacteria-primed Ae. albopictus were then orally challenged with DENV. We observed that mosquitoes that experienced a pre-challenge with gram negative bacteria were associated with reduced DENV prevalence and viral load, whereas Wolbachia-infected mosquitoes exhibited further reductions in viral load. CONCLUSIONS/SIGNIFICANCE: We highlight novel characteristics of native Wolbachia infection in Ae. albopictus mosquitoes, with protection against bacterial-induced mortality, and interactions between pathogen blocking and bacterial priming that may depend on bacterial gram status. These findings provide insight into the immune interactions associated with native Wolbachia and build on our understanding of adaptive immunity in mosquitoes.