Eicosanoid actions in insect immunity

Insects express 3 lines of protection from infections and invasions. Their cuticles and peritrophic membranes are physical barriers. Infections and invasions are quickly recognized within insect bodies, and recognition launches 2 lines of innate immune reactions. Humoral reactions involve induced synthesis of antimicrobial peptides, the bacteriolytic enzyme lysozyme and activation of the prophenoloxidase system. Cellular immune reactions include phagocytosis, nodulation and encapsulation. These reactions entail direct interactions between circulating hemocytes and the invaders. Cellular immune reactions begin immediately after an invasion is detected while antimicrobial peptides typically appear in the hemolymph some hours after infection. Microaggregation is a step in the nodulation process, which is responsible for clearing the bulk of bacterial infections from circulation. Coordinated cellular actions lead to encapsulation of invaders, such as parasitoid eggs, that are very much larger than individual hemocytes. In this paper, we review the roles of eicosanoids as central mediators of insect immune reactions, particularly cellular reactions. We briefly describe insect immune functions, outline eicosanoid biosynthesis and treat eicosanoid actions in cellular immunity of insects. Eicosanoids act in several cellular defense functions, including phagocytosis, microaggregation, nodulation, encapsulation, cell spreading and hemocyte migration toward a source of a bacterial peptide. We also describe our most recent work on the influence of one group of eicosanoids, prostaglandins, on gene expression in an established insect cell line.