Eicosanoid actions in insect cellular immune functions

Insects are more or less constantly challenged with a daunting array of pathogenic organisms, including viruses, bacteria, fungi, protozoans as well as various metazoan parasites and parasitoids. At the first level of defense, the pathogens are rebuffed by physical barriers, including the cuticle and peritrophic membrane. Upon breaching these barriers, pathogens meet with an arsenal of robust and efficacious immune defense mechanisms. Two general categories of defenses are typically recognized, humoral defenses and hemocytic or cellular defenses. The former involves induced synthesis of various antibacterial proteins and peptides, such as cecropins and lysozyme. Cellular defense mechanisms are characterized by direct interactions between circulating hemocytes and the invaders. These include phagocytosis, microaggregation, nodulation, and encapsulation. 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. While the defense mechanisms are broadly appreciated, less is known about the biochemical signals responsible for mediating and coordinating the cellular actions. We now know eicosanoids mediate phagocytosis, microaggregation, and nodulation reactions to immune challenge, as well as cell spreading, a specific step in nodulation. We have several goals in this mini review. We provide a brief background on cellular immunity, outline eicosanoid biosynthesis, and review eicosanoid actions in cellular immunity in insects. Recent work indicates some pathogens have usurped eicosanoid-mediated immunity; they disable insect immunity by inhibiting eicosanoid biosynthesis. We interpret these findings and their significance with respect to the biological control of insects. We also present preliminary work designed to test hypotheses on how eicosanoids exert their actions. We address shortcomings in our knowledge on eicosanoids in insect biology.