Eicosanoids: progress towards manipulating insect immunity

Abstract

Insect immunity is exclusively innate, lacking the antibody-based adaptive immunity of vertebrates. Innate immunity is a naturally occurring, non-specific system that does not require previous infectious experience. In this paper, I describe insect immunity and review the roles of prostaglandins and other eicosanoids as crucial signals that mediate insect immune reactions to challenge. Despite physical barriers (the integument) and robust epithelial immune functions in salivary glands, tracheal systems and midgut, many microbes and parasitoids invade insect haemocoels. Once invaders are detected within the body, insects unleash potent immune effectors, traditionally assorted into cellular (haemocytic) and humoral immunity. Humoral immunity refers to induced expression of genes encoding antimicrobial peptides and the enzymes lysozyme and prophenoloxidase. These proteins appear in haemolymph of infected insects 6–12 h post-infection. Haemocytic immunity is characterized by direct interactions between invaders and circulating haemocytes. These reactions begin immediately when an infection is detected and, in the case of bacterial infections, are the predominant responses, responsible for clearing the vast majority of infecting bacterial cells within the first 2 h after infection. As a measure of cellular immunity, a recent field study shows that virtually all insects become infected and many fully recover from the infections. Insect immune functions are of such biological significance that they may limit the effectiveness of some biological control programmes. I highlight the efficacy of insect immunity and the importance of eicosanoids as immune signals by reviewing two cases in which microbes have evolved the ability to suppress host insect immunity by inhibiting eicosanoid biosynthesis. I complete the paper with a description of a major research goal in the Biological Control of Insects Research Laboratory, now focused on developing molecular tools to cripple pest insect immunity at the field level.