An Arthropod Defensin Expressed by the Hemocytes of the American Dog Tick, Dermacentor variabilis (Acari: Ixodidae)

Both soluble and cell-mediated components are involved in the innate immune response of arthropods. Injection of Borrelia burgdorferi, the Lyme disease agent, results in the secretion of defensin into the hemolymph of the ixodid tick, Dermacentor variabilis. The presence of the peptide is observed as early as 15 min post-challenge and remains present through 18 h post-challenge. As observed in insects and soft ticks, the transcript for defensin is detected as early as 1 h post-challenge in D. variabilis. RT-PCR resulted in an amplicon of 624 bp with a 225 bp region that translates to a 74 amino acid preprodefensin. The defensin encoding region was amplified, cloned and sequenced from the hemocytes. It appears as though defensin is stored in the granulocytes of the hemolymph and secreted into the hemolymph upon bacterial insult. The role of defensin as a contributing factor in determining vector competency is discussed.