Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2007
Publication Date: 9/19/2007
Citation: Merchant, D., Ertl, R.L., Rennard, S.I., Stanley, D.W., Miller, J.S. 2007. Eicosanoids mediate insect hemocyte migration. Journal of Insect Physiology. 54:215-221. Interpretive Summary: Application of classical insecticides has introduced severe problems in agricultural sustainability. The concept of biological control of insects is a potentially powerful alternative to classical insecticides. Biological control is based on the idea that direct application of insect-specific 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. With this goal, we are investigating how insect immune reactions to infection are signaled. In this paper we report on identification of biological signals that are responsible for stimulating insect defenses to infection. This new research 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 supporting the long-term sustainability of agriculture.
Technical Abstract: Hemocyte chemotaxis toward infection and wound sites is an essential component of insect defense reactions, although the biochemical signal mechanisms responsible for mediating chemotaxis in insect cells are not well understood. Here we report on the outcomes of experiments designed to test the hypotheses that one, insect hemocytes are able to detect and migrate toward a source of N-formyl-Met-Leu-Phe (fMLP), the major chemotactic peptide from Escherichia coli and two, that pharmaceutical modulation of eicosanoid biosynthesis inhibits hemocyte chemotaxis. We used primary hemocyte cultures prepared from fifth-instar tobacco hornworms, Manduca sexta in Boyden chambers to assess hemocyte chemotaxis toward buffer (negative control) and toward buffer amended with fMLP (positive control). Approximately 42% of negative control hemocytes migrated toward buffer and about 64% of positive control hemocytes migrated toward fMLP. Hemocyte chemotaxis was inhibited (by >40%) by treating hornworms with pharmaceutical modulators of cycloxygenase (COX), lipoxygenase and phospholipase A2 (PLA2) before preparing primary hemocyte cultures. The influence of the COX inhibitor, indomethacin, and the glucocorticoid, dexamethasone, which leads to inhibition of PLA2, was expressed in a dose-dependent way. The influence of dexamethasone was reversed by injecting arachidonic acid (precursor to eicosanoid biosynthesis) into hornworms before preparing primary hemocyte cultures. The saturated fatty acid, palmitic acid, did not reverse the inhibitor effect. These findings support both of our hypotheses, first that insect hemocytes can detect and respond to fMLP, and second, that insect hemocyte chemotaxis is mediated by eicosanoids.