Inhibition of eicosanoid formation in larvae of the tobacco hornworm Manduca sexta, using specific inhibitors of phospholipase A2, cyclooxygenase, and lipoxygenase, severely weakened the ability of larvae to clear the bacterium Serratia marcescens from their hemolymph. The reduced capability to remove bacteria is associated with increased mortality due to these bacteria. There is a dose-dependent relationship between the phospholipase A2 inhibitor dexamethasone and both the reduced bacterial clearance and increased larval mortality. The dexamethasone effects on larval survival were reversed by treatment with arachidonic acid. Maleic acid, a nonspecific antioxidant, did not interfere with the insects' ability to remove bacterial cells from hemolymph. The larvae were shown to contain all of the C20 polyunsaturated fatty acids necessary for eicosanoid biosynthesis and to be capable of converting radioactive arachidonic acid into several primary prostaglandins. These results strongly suggest that eicosanoids mediate transduction of bacterial infection signals into the complex of cellular and humoral responses that comprise invertebrate immunity.Invertebrate immunity is an amalgam of several distinct systems, both cellular and humoral, that operate in a more or less coordinated way to provide protection from invading pathogens (1). Cellular responses include phagocytosis, encapsulation, and various cytotoxic reactions such as release of lysozyme (1, 2) and the prophenoloxidase activating system (3). These hemocyte-mediated responses are complemented by antibacterial activity, such as lectins, which occur constitutively in invertebrate hemolymph (4). In addition to normally occurring factors, several antibacterial peptides are induced by bacterial infections [namely, the cecropins (4 kDa), attacins (21 to 23 kDa), diptericins (8 kDa), and insect defensins (4 kDa)] (3,5).An important area of invertebrate immunity that has not been critically addressed concerns how the early responses to infection are mediated-that is, what is the sequence of biochemical events operative between microbial invasion and the appearance of defense mechanisms? To date, only two early steps in the immune response have been investigated. In one, immune-specific RNA was shown to be synthesized 2-4 hr after bacterial infection of the moths Hyalophora cecropia and Galleria mellonella (3). In the other, inducible lectins were synthesized in the flesh fly Sarcophaga peregrina almost immediately after their immune system was challenged (6). However, these genespecific events follow from earlier steps in transduction of bacterial invasion signals into physiological defense responses.Since many aspects of immunological defense in mammals are mediated by eicosanoid metabolites of arachidonic acid (7), we hypothesized that biosynthesis of eicosanoids is also an essential early step in mediation of the immune response in invertebrates. We tested this hypothesis by using an insect, the tobacco hornworm Manduca sexta. We report here that selective ...
