Toll/IMD signal pathways mediate cellular immune responses via induction of intracellular PLA 2 expression.

Phospholipase A2 (PLA2 ) hydrolyzes fatty acids from phospholipids at the sn-2 position. Two intracellular PLA2 s, iPLA2 A and iPLA2 B, have been found in Spodoptera exigua. Both are calcium-independent cellular PLA2 . Their orthologs have been found in other insects. These two iPLA2 s are different in ankyrin motif of N terminal region. The objective of this study was to determine whether Toll/immune deficiency (IMD) signal pathways could mediate cellular immune responses via induction of iPLA2 expression. Both iPLA 2 s were expressed in all developmental stages of S. exigua, showing the highest expression in the adult stage. During larval stage, hemocyte is the main tissue showing expression of these iPLA2 s. Both iPLA2 s exhibited similar expression patterns after immune challenge with different microbial pathogens such as virus, bacteria, and fungi. Promoter component analysis of orthologs encoded in S. frugiperda indicated nuclear factor-κB- and Relish-responsible elements on their promoters, suggesting their expression in S. exigua under Toll/IMD immune signaling pathways. RNA interference (RNAi) of MyD88 or Pelle under Toll pathway suppressed inducible expression levels of both iPLA2 s in response to Gram-positive bacteria containing Lys-type peptidoglycan or fungal infection. In contrast, RNAi against Relish under IMD pathway suppressed both iPLA2 s in response to infection with Gram-negative bacteria. Under RNAi conditions, hemocytes significantly lost cellular immune response measured by nodule formation. However, addition of arachidonic acid (a catalytic product of PLA2 ) rescued such immunosuppression. These results suggest that Toll/IMD signal pathways can mediate cellular immune responses via eicosanoid signaling by inducing iPLA2 expression.