Insect immune reactions to purified lipopolysaccharides are mediated by eicosanoids

Nodulation is the predominant cellular defense reaction to bacterial infections in insects. Gram-negative bacteria, and the lipopolysaccharide components (LPS) purified from them, stimulate defense reactions in mammals and insects. The cellular reactions to LPS formed the basis of three hypotheses, all tested in this dissertation. First, insect cellular reactions to LPS depend on eicosanoid biosynthesis. Second, insect hemocytes discriminate in their reactions to LPS from different biological sources. Third, cellular defense reactions to bacterial infections are mediated by eicosanoids in a social insect, the honeybee, Apis mellifera. On the first, purified LPS stimulated microaggregation reactions in a dose-dependent manner in larvae of the beetle, Zophobas atratus. Treatments with eicosanoid biosynthesis inhibitors prior to LPS injections reduced the responses. On the second hypothesis, I report on the influence of an LPS-like molecule (aLPS) from the pathogenic alga, Prototheca (Strain 289) on insect and murine innate immune reactions. I recorded nodulation reactions to aLPS in tobacco hornworms, Manduca sexta. Contrarily, murine macrophages reacted to challenge with Escherichia coli LPS, but did not react to aLPS. Moreover, hornworms treated with LPS purified from E. coli produced over 200 nodules/hornworm. Insects treated with LPS prepared from other bacterial species yielded significantly fewer nodules, about 85/hornworm for Providencia rettgeri, 30/hornworm for Klebsiella aerogenes, and 15/homworm for Halomonas elongata. The lipid A component of LPS is essential for the insect nodulation reaction. Pre-treating the LPS from each bacterial species with polymyxin-B, which specifically binds to the lipid A component of LPS, resulted in substantial reductions (\u3e75%) in nodulation, compared to positive controls injected with purified LPS. I infer that insect and mammalian immune surveillance systems differ in recognition of LPS molecular types, although both require lipid A for recognition. On the third hypothesis, treating newly-emerged bees with dexamethasone impaired nodulation reactions to bacteria, and influence was reversed by treatment with arachidonic acid. I infer that eicosanoids mediate cellular immune responses to bacterial infections in newly emerged bees, and more broadly, in all insect species