Lipoxin B4 and lipocalin play a crucial role in insect immune-priming induced by a gut microbial commensal

Host cabbage possesses an endophyte, Bacillus subtilis, which induced immune-priming of the diamondback moth, Plutella xylostella. In contrast, larvae raised under axenic conditions lost the chance to feed the bacteria and were highly susceptible to various pathogens. Addition of B. subtilis to axenic larvae significantly restored immune responses and enhanced survival rates following pathogen infections. The immune-priming factor(s) was determined among 18 apolipoprotein D (ApoD) genes identified as lipocalin candidates in P. xylostella, in which ApoD1 expression was functionally linked with B. subtilis-induced immune-priming. In addition, lipoxins were analyzed in immune-primed larvae via LC-MS/MS, in which LXB₄ was detected, but not LXA₄. The LXB₄ titer was significantly higher than that that in the larvae reared under axenic conditions. Notably, LXB₄ alone sufficiently induced significant immune responses. To support lipoxin biosynthesis in insects, this study identified a lipoxygenase-like peroxidase gene, HemP2. Its expression was induced in the immune-primed larvae. However, its suppression prevented LXB₄ production under the immune-priming conditions. To explain the up-regulations of lipocalin/lipoxin by the gut commensal, Toll and IMD immune signaling pathways were analyzed. The up-regulation of ApoD1 and HemP2 expressions was mediated through the IMD, but not the Toll, immune signaling pathway in the larval gut of P. xylostella under B. subtilis-induced immune-priming conditions. This study highlights the potential role of commensal gut microbes including B. subtilis in driving immune-priming via an insect lipoxin-lipocalin complex through the IMD immune signaling pathway.