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Background  Clip domain serine proteases (CLIPs), a very diverse group of proteolytic enzymes, play a crucial role in the innate immunity of insects. Innate immune responses are the first line of defense in mosquitoes against the invasion of pathogenic microorganisms. The Toll pathway, immunodeficiency (IMD) pathway and melanization are the main processes of innate immunity in  Aedes aegypti . CLIPS are classified into five subfamilies—CLIPA, CLIPB, CLIPC, CLIPD, and CLIPE—based on their sequence specificity and phylogenetic relationships. We report the functional characterization of the genes that code for two CLIPs in  Ae. aegypti  ( Ae ):  Ae-CLIPB15  and  Ae-CLIPB22 .    Methods  Clustal Omega was used for multiple amino acid sequence alignment of  Ae-CLIPB15  and  Ae-CLIPB22  with different  CLIP  genes from other insect species. The spatiotemporal expression profiles of  Ae-CLIPB15  and  Ae-CLIPB22  were examined. We determined whether  Ae-CLIPB15  and  Ae-CLIPB22  respond to microbial challenge and tissue injury. RNA interference (RNAi) was used to explore the function of  Ae-CLIPB15  and  Ae-CLIPB22  in the defense of  Ae. aegypti  against bacterial and fungal infections. The expression levels of nuclear factor kappa B (NF-κB) transcription factors  REL1  and  REL2  in the Toll pathway and IMD pathway after bacterial infection were investigated. Finally, the change in phenoloxidase (PO) activity in  Ae-CLIPB15  and  Ae-CLIPB22  knockdown adults was investigated.    Results  We performed spatiotemporal gene expression profiling of  Ae-CLIPB15  and  Ae-CLIPB22  genes in  Ae .  aegypti  using quantitative real-time polymerase chain reaction. These genes were expressed in different stages and tissues. The messenger RNA (mRNA) levels for both genes were also up-regulated by Gram-negative bacteria  Escherichia coli , Gram-positive bacteria  Staphylococcus aureus  and fungal  Beauveria bassiana  infections, as well as in the tissue injury experiments. RNAi-mediated knockdown of  Ae-CLIPB15  led to a significant decrease of PO activity in the hemolymph of  Ae. aegypti , while other RNAi experiments revealed that both  Ae-CLIPB15  and  Ae-CLIPB22  were involved in immune defense against bacterial and fungal infections. The mRNA expression of NF-κB transcription factors  REL1  and  REL2  in the Toll pathway and IMD pathway differed between  Ae-CLIPB15  and  Ae-CLIPB22  knockdown mosquitoes infected with bacteria and wild type mosquitoes infected with bacteria.    Conclusions  Our findings suggest that  Ae-CLIPB15  and  Ae-CLIPB22  play a critical role in mosquito innate immunity, and that they are involved in immune responses to injury and infection. Their regulation of transcription factors and PO activity indicates that they also play a specific role in the regulation of innate immunity.    Graphical Abstract

Functional characterization of two clip domain serine proteases in innate immune responses of Aedes aegypti