The prostanoids, thromboxanes, mediate hemocytic immunity to bacterial infection in the lepidopteran Spodoptera exigua.

We report on a new insect prostanoid in a lepidopteran insect, Spodoptera exigua. Thromboxane B (TXB) was detected by LC-MS/MS in extracts of larval epidermis, midgut, fat body and hemocytes, with highest amounts in hemocytes (about 300 ng/g tissue with substantial variation). Thromboxane A (TXA) is an unstable intermediate that is non-enzymatically hydrolyzed into the stable TXB In S. exigua, both thromboxanes mediate at least two cellular immune responses to bacterial infection, hemocyte-spreading behavior and nodule formation. At the molecular level, a TXA synthase (SeTXAS) was identified from a group of 139 S. exigua cytochrome P450 monooxygenases. SeTXAS was highly similar to mammalian TXAS genes and is expressed in all developmental stages and four tested larval tissues. Immune challenge significantly enhanced SeTXAS expression, especially in hemocytes. RNA interference (RNAi) injections using gene-specific double stranded RNA led to reduced SeTXAS expression and suppressed the cellular immune responses, which were rescued following TXA or TXB injections. Unlike other PGs, TXA or TXB did not influence oocyte development in adult females. We infer that thromboxanes are present in insect tissues, where they mediate innate immune responses.