Identification of four secretory phospholipase As in a lepidopteran insect, , and their functional association with cellular immune responses.

BACKGROUND

Eicosanoids are a group of the oxygenated C20 polyunsaturated fatty acids and play crucial roles in mediating various insect physiological processes. Catalytic activity of phospholipase A (PLA) provides an initial substrate, arachidonic acid (AA), for subsequent eicosanoid biosynthesis.

RESULTS

This study identified four different secretory PLA (-) genes encoded in the Asian onion moth, . A phylogenetic analysis indicated that and are clustered with Group III PLAs while and are clustered with Group XII and Group X PLAs, respectively. Expression levels of these PLA genes increased along with larval development, especially in the fat body. A bacterial immune challenge upregulated the basal expression levels of the four PLA genes, which resulted in significant increases of the PLA enzyme activity. The enzyme activity was susceptible to a calcium chelator or reducing agent, suggesting Ca dependency and disulfide linkage required for the catalytic activities of the secretory type of PLAs. In addition, the PLA activity was also susceptible to bromophenacyl bromide (BPB), a specific inhibitor to sPLA, but not to intracellular PLA inhibitors. An addition of BPB to the immune challenge significantly prevented hemocyte-spreading behavior of . 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 PLA(s) responsible for the immunity, individual RNA interference (RNAi) treatments specific to each of the four PLAs were performed. Injection of gene-specific double-stranded RNAs caused significant reductions in the transcript level in all four PLAs. In all four PLAs, the RNAi treatments prevented the cellular immune response even after the immune challenge.

CONCLUSION

This study reports four secretory PLAs encoded in and their function in mediating cellular immunity.