Functional implications of the peptidoglycan recognition proteins in the immunity of the yellow fever mosquito, Aedes aegypti

Peptidoglycan recognition proteins (PGRPs) play essential roles in the immune systems of insects and higher animals against certain pathogens, including bacteria. In insects, most studies on the functions of PGRPs have been performed in Drosophila, with only limited studies in mosquitoes, which are important disease vectors. In the present study, we analysed the PGRP sequences of the yellow fever mosquito, Aedes aegypti, acquired from two genome databases, and identified a total of seven PGRP genes; namely, PGRP-S1, -SC2, -LA, -LB, -LC, -LD and -LE. Bacterial injection using the Gram-negative bacteria Escherichia coli and the Gram-positive bacteria Micrococcus luteus showed that three PGRPs responded directly to both bacterial stimuli. Subsequently, the transcriptional expression of six of these PGRPs was knocked down using double-stranded RNA-injection-based RNA interference (RNAi). RNAi of the PGRPs resulted in different impacts on the immune responses of Ae.aegypti to the two bacteria, as evidenced by the changes in mosquito survival rates after bacterial challenges as well as the differential regulation of several antimicrobial peptides and a number of other genes involved in mosquito immune pathways. Our data suggest that PGRP-LC is a significant factor in mediating immune responses to both E.coli and M.luteus, and the other PGRPs play only minor roles against these two bacteria, with PGRP-SC2 and -LB also serving as potential negative regulators for certain immune pathway(s) in Ae.aegypti.