Eicosanoids mediate microaggregation reactions to bacterial challenge in isolated insect hemocyte preparations

Nodule formation is the quantitatively predominant insect cellular defense reaction to bacterial challenges, responsible for clearing the largest proportion of infecting bacteria from circulation. It has been suggested that eicosanoids mediate several steps in the nodulation process, including formation of hemocyte microaggregates, an early step in the process. While fat body and hemocytes are competent to biosynthesize eicosanoids, the source of the nodulation-mediating eicosanoids remains unclear. To investigate this issue, we studied hemocyte microaggregation reactions to bacterial challenge in vitro. Hemocyte suspensions from the tobacco hornworm, Manduca sexta, were treated with the phospholipase A2 inhibitor, dexamethasone, then challenged with the bacterium Serratia marcescens. Preparations treated with dexamethasone yielded fewer hemocyte microaggregations than untreated, control preparations. Furthermore, the influence of dexamethasone was reversed by amending experimental (dexamethasone-treated) preparations with the eicosanoid biosynthesis precursor, arachidonic acid. Palmitic acid, which is not a substrate for eicosanoid biosynthesis, did not reverse the influence of dexamethasone on the microaggregation reaction. The influence of dexamethasone was also reversed by adding filtered media from challenged hemocyte preparations to dexamethasone-treated preparations. Finally, most hemocyte preparations treated with selected eicosanoid biosynthesis inhibitors formed fewer hemocyte microaggregations than control preparations. The 5- and 12-lipoxygenase inhibitor, esculetin, did not influence the formation of hemocyte microaggregations in this system. These results are consistent with similar investigations performed in vivo, and we infer that hemocytes are responsible for forming and secreting eicosanoids, which subsequently initiate nodulation by mediating hemocyte microaggregation.