Analysis of a Rel transcription factor that regulates expression of defensin in the soft tick Ornithodoros moubata (Acari: Argasidae)

Ticks transmit numerous microorganisms that cause disease despite the presence of an effective immune system. Invertebrate immunity is unique in that there is no acquired immune system, so this immunity depends on the cellular and humoral responses of the innate immune system. Antimicrobial peptides play a key role in the humoral response to injury or bacterial infection. Previously, we identified 4 antimicrobial peptide defensins from Ornithodoros moubata, and showed up-regulated gene expression of these defensin genes in response to blood feeding. O. moubata defensins also exhibited higher activity against bacteria than other insect defensins. Moreover, these defensins are secreted into the midgut indicating they are an essential immune response against microorganisms after blood feeding. In Drosophila, transcription factor NF- κB/Rel induces the transcription of the antimicrobial peptide genes. Therefore, in this study we investigated the regulation of defensin gene expression in O. moubata. The upstream regions of all four O. moubata defensin genes contain core promoter elements including a sequence motif homologous to the response element for transcription factor NF- κB/Rel. In addition, O. moubata NF- κB/Rel (hereafter OmRel) was identified. Furthermore, OmRel was expressed at similar levels in O. moubata females from 1 to 20 days after blood feeding. Analysis of expression in separate tissues revealed that OmRel is expressed in the midgut and fat body where defensins are highly expressed. Therefore, OmRel in the midgut and fat body appears to control expression of the defensin genes. These results indicate the Toll pathway is important in the regulation of O. moubata defensin gene expression and this regulatory mechanism is highly conserved in arthropod innate immunity. Further studies are underway to confirm the role of OmRel in the regulation of defensins.