Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/30/2006
Publication Date: 3/1/2006
Citation: Stanley, D.W., Miller, J.S. 2006. Eicosanoid actions in insect cellular immune functions. Entomologia Experimentalis et Applicata. 119:1-13. Interpretive Summary: 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. The concept of biological control of insects is based on the idea that direct application of some of these pathogens and parasites can reduce pest insect populations and the economic damage due to pest insects. The problem, however, is the efficiency of these organisms in biological control programs is limited by insect immune defense reactions to challenge. One approach to improving the efficiency of biocontrol agents would be to somehow disable insect immune reactions to viral, bacterial, fungal and parasitic infections. We have discovered one group of molecules that mediate insect cellular immune reactions. More recent work indicates some pathogens have usurped these mediators. They disable insect immunity by inhibiting their biosynthesis. Insects infected with these pathogens quickly succumb to infection because they lack defense mechanisms. In this paper we identify the significance immune-disabling pathogens with respect to the biological control of insects. In doing so, we join two disparate concepts in insect science: one is research designed to understand how chemical mediators act in cellular immunity. The other is research designed to understand how insect pathogens disable insect immunity. This new conjunction will be directly useful to scientists who are working to improve the efficacy of biological control methods. The ensuing improved biological control methods will benefit a wide range of agricultural producers by enhancing long-term sustainability.
Technical Abstract: Insects are constantly challenged with an 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 an arsenal of efficacious immune defense mechanisms. Two general categories of defenses are recognized, humoral defenses and hemocytic or cellular defenses. The former involves induced synthesis of various antibacterial proteins, 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 that mediate and coordinate the cellular actions. We now know eicosanoids mediate phagocytosis, microaggregation and nodulation reactions to challenge, as well as cell spreading, a specific step in nodulation. Here, 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 review these findings and forge a previously unappreciated link between our eicosanoid-mediated immunity and biological control of insects.