Eicosanoids mediate nodulation reactions to bacterial infections in adultsof the cricket, Gryllus assimilis

Nodulation is the temporally and quantitatively most important cellular def ense reaction to bacterial infections in insects. Inhibition of eicosanoid biosynthesis in adults of the cricket, Gryllus assimilis, immediately prior to intrahemocoelic injections of the bacterium, Serratia marcescens, sharp ly reduced the nodulation response. Separate treatments with specific inhib itors of phospholipase A,, cyclooxygenase, and lipoxygenase reduced nodulat ion, supporting our view that nodule formation is a complex process involvi ng lipoxygenase and cyclooxygenase products. The inhibitory influence of de xamethasone was apparent within 2 h of injection, and nodulation was signif icantly reduced, relative to control crickets, over 22 h. The dexamethasone effects were reversed by treating bacteria-injected insects with the eicos anoid-precursor polyunsaturated fatty acid, arachidonic acid. Low levels of arachidonic acid were detected in fat body phospholipids, and fat body pre parations were shown to be competent to biosynthesize eicosanoids from exog enous radioactive arachidonic acid. These findings in a hemimetabolous inse ct broaden our hypothesis that eicosanoids mediate cellular immune reaction s to bacterial infections in most, if not all, insects. (C) 1998 Elsevier S cience Ltd. All rights reserved.