Various eicosanoids modulate the cellular and humoral immune responses of the beet armyworm, Spodoptera exigua

Cyclooxygenase (COX) and lipoxygenase (LOX) can catalyze the oxidation of C20 fatty acids to produce certain eicosanoids, which play roles in mediating immune responses in insects. Despite their critical role in insect immunity, there have been few studies of the unique effects of different eicosanoids on immune responses. This study analyzed cellular and humoral immune responses of the beet armyworm, Spodoptera exigua, using seven eicosanoids selected from two major eicosanoid subgroups: prostaglandin (PG) and leukotriene (LT), derived from catalytic activities of COX and LOX respectively. Upon bacterial challenge, all seven eicosanoids (PGA(1), PGB(2), PGD(2), PGE(1), PGE(2), PGF(1alpha), and LTB(4)) significantly induced hemocyte nodulation and phagocytosis in the presence of dexamethasone, an eicosanoid biosynthesis inhibitor. However, only PGs induced cell lysis of oenocytoids to release prophenoloxidase, which resulted in an increase in phenoloxidase activity. These seven eicosanoids also induced expression of humoral immune-associated genes, including prophenoloxidase, serpin, dopa decarboxylase, cecropin, and lysozyme, in which PGB(2) and PGE(1) did not induce gene expression of prophenoloxidase. To understand the interactions between different eicosanoids, mixture effects of these eicosanoids were compared with their individual eicosanoid effects on mediating nodule formation in response to bacterial challenge. All six single PGs showed increases in nodule formation in a dose-dependent manner without significant difference among the different types. LTB(4) was more potent than the tested PGs in mediating the cellular immune response. At low doses, all combinations of two eicosanoids showed significant additive effects on nodule formation. These results indicate that immune target cells, such as hemocyte and fat body, of S. exigua can respond to different COX and LOX products to express cellular and humoral immune responses, and their overlapping, additive effects on nodulation suggest that in target cells, these eicosanoids share a hypothetical common eicosanoid signal pathway.