Modulators of the insect cellular immune response

Abstract

This thesis is intended to add to the growing body of knowledge pertaining to the immune systems of insects and to explore the hypothesis that the invertebrate immune system serves as a template of the vertebrate innate response. Insects are capable of mounting an effective defense reaction consisting of both cellular and humoral components. The focus of this thesis is on aspects of the insects' cellular immune response, the main effector of which is the insect blood cell or hemocyte.

Biogenic amines, such as octopamine, have previously been shown to enhance both phagocytosis and nodule formation in insects. Physiological levels of octopamine elicit increases in inositol trisphosphate, cAMP, and intracellular calcium in hemocytes. Since these second messengers are putative regulators of cytoskeletal organization in a variety of cell types, the effects of octopamine on locomotory behaviour, polarity, and the cytoskeleton of hemocytes from the Greater Wax Moth, Galleria mellonella, were analyzed. Morphological changes were reflected in alterations of the hemocyte cytoskeleton and were consistent with changes in the relative concentration of F-actin. These data suggest that one mechanism by which octopamine modulates hemocyte activity is via reorganization of the actin cytoskeleton.

Eicosanoids are biologically active, oxygenated metabolites derived from C20 polyunsaturated fatty acids, and have been implicated as regulators of bacterial clearance from the insect hemocoel (Stanley-Samuelson et al, 1991). I further refined our understanding of the action of eicosanoids by demonstrating their regulatory roles in key aspects of the cellular immune responses: phagocytosis, prophenoloxidase activity, nodulation and cell spreading.

Nitric oxide (NO) is a gaseous signalling molecule which has a variety of physiological roles in vertebrates, including regulation of the innate vertebrate immune response. In this thesis I have shown the first biochemical, molecular, and physiological evidence for the presence and role of nitric oxide synthase (NOS) in insect hemocytes. Furthermore, I demonstrate possible cross-talk between the eicosanoid and NOS pathways, a feature in common with vertebrate immunocytes. These findings suggest a conservation of immunomodulatory strategies between vertebrates and invertebrates, and have both theoretical and practical implications as discussed herein.