Eicosanoids mediate nodulation reactions to bacterial infections in adultsof two 17-year periodical cicadas, Magicicada septendecim and M-cassini

Nodulation is the first and quantitatively most important cellular defense reaction to bacterial infections in insects. Treating adults of the 17-year periodical cicadas, Magicicada septendecim and M. cassini, with eicosanoid biosynthesis inhibitors immediately prior to intrahemocoelic injections of the bacterium, Serratia marcescens, sharply reduced the nodulation respons e to bacterial challenges. Separate treatments with specific inhibitors of phospholipase A,, cyclooxygenase, and lipoxygenase reduced nodulation, supp orting our view that nodule formation is a multi-step process in which indi vidual steps are separately mediated by lipoxygenase and cyclooxygenase pro ducts. The inhibitory influence of dexamethasone was apparent by 2 h after injection, and nodulation was significantly reduced, relative to control in sects, over the following 14 h. The dexamethasone effects were reversed by treating bacteria-challenged insects with the eicosanoid-precursor polyunsa turated fatty acid, arachidonic acid. Low levels of arachidonic acid were d etected in fat body phospholipids. These findings in adults of an exopteryg ote insect species with an unusual life history pattern broaden our hypothe sis that eicosanoids mediate cellular immune reactions to bacterial infecti ons in most, if not all, insects. (C) 1999 Elsevier Science Ltd. All rights reserved.