BackgroundEicosanoids are a group of the oxygenated C20 polyunsaturated fatty acids and play crucial roles in mediating various insect physiological processes. Catalytic activity of phospholipase A2(PLA2) provides an initial substrate, arachidonic acid (AA), for subsequent eicosanoid biosynthesis.ResultsThis study identified four different secretory PLA2(As-PLA2A–As-PLA2D) genes encoded in the Asian onion moth,Acrolepiopsis sapporensis. A phylogenetic analysis indicated thatAs-PLA2AandAs-PLA2Dare clustered with Group III PLA2s whileAs-PLA2BandAs-PLA2Care clustered with Group XII and Group X PLA2s, respectively. Expression levels of these PLA2genes increased along with larval development, especially in the fat body. A bacterial immune challenge upregulated the basal expression levels of the four PLA2genes, which resulted in significant increases of the PLA2enzyme activity. The enzyme activity was susceptible to a calcium chelator or reducing agent, suggesting Ca2+dependency and disulfide linkage required for the catalytic activities of the secretory type of PLA2s. In addition, the PLA2activity was also susceptible to bromophenacyl bromide (BPB), a specific inhibitor to sPLA2, but not to intracellular PLA2inhibitors. An addition of BPB to the immune challenge significantly prevented hemocyte-spreading behavior ofA. sapporensis. BPB treatment also suppressed a cellular immune response measured by hemocyte nodule formation. However, the immunosuppression was significantly rescued by the AA addition. To determine the PLA2(s) responsible for the immunity, individual RNA interference (RNAi) treatments specific to each of the four PLA2s were performed. Injection of gene-specific double-stranded RNAs caused significant reductions in the transcript level in all four PLA2s. In all four PLA2s, the RNAi treatments prevented the cellular immune response even after the immune challenge.ConclusionThis study reports four secretory PLA2s encoded inA. sapporensisand their function in mediating cellular immunity.