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 B2 (TXB2) 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 A2 (TXA2) is an unstable intermediate that is non-enzymatically hydrolyzed into the stable TXB2. 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 TXA2 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 TXA2 or TXB2 injections. Unlike other PGs, TXA2 or TXB2 did not influence oocyte development in adult females. We infer that thromboxanes are present in insect tissues, where they mediate innate immune responses.