Up-regulated humoral immune response in the soft tick, Ornithodoros moubata (Acari: Argasidae)

Ticks have an efficient defense system for preventing microbial infection. The antimicrobial peptide defensin is one effective molecule in this system. Here we investigated immune competence and the involvement of defensin in the humoral defense of the soft tick, Ornithodoros moubata. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) revealed that gene expression of all four defensin isoforms was up-regulated by bacteria or bacterial components. Defensin gene up-regulation by hemocoelic inoculation of bacteria involves the midgut and granulocytes. In immunodetection analysis, immunization by bacterial injection increases the relative concentration of defensin-like material in the hemolymph plasma. Furthermore, elevated antibacterial activity against Gram-positive bacteria but not against Gram-negative bacteria was observed after immunization by a liquid growth inhibition assay. Therefore, enhanced anti-Gram-positive bacterial activity appears to be partially dependent on the release of defensin into the hemolymph. These findings demonstrate that defensin plays an important role in the up-regulated humoral response of O. moubata.