Defensin in the Ticks Dermacentor variabilis and Ixodes scapularis

Date of Award

Spring 2005

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biomedical Sciences

Committee Director

Daniel E. Sonenshine

Committee Member

Wayne L. Hynes

Committee Member

Christopher J. Osgood

Committee Member

Robert E. Ratzlaff


This dissertation presents the identification, molecular structure and expression of defensin, an antimicrobial peptide, in the ticks, Dermacentor variabilis and Ixodes scapularis. A 624 bp cDNA molecule for defensin was cloned and sequenced from hemocytes, fat body and midgut of unstimulated D. variabilis ticks. Defensin peptide was detected only in the hemocyte lysates. These results suggest that the defensin from D. variabilis is synthesized and stored in the hemocytes, although the role of other tissues is not excluded.

The ability of the juvenile stage ticks to express the defensin gene was confirmed. Detection of the defensin transcript in all juvenile stages of D. variabilis ticks except early stage eggs proved that a potential for peptide expression exists. These data indicate that a developmental threshold is required before the expression of defensin can occur.

Defensin transcript in hemocytes and defensin peptide in the plasma is observed to be constitutively present in D. variabilis ticks that are challenged with the Lyme disease spirochete, Borrelia burgdorferi . Due to the presence of defensin peptide in hemocytes from unstimulated ticks and the lack of a significant increase in defensin transcript abundance when compared to the sham control, it is hypothesized that ticks rely on secretion of stored defensin and not the immediate transcription/translation of defensin at the challenge timepoint.

A 397 bp defensin-like cDNA from whole tissue of fed-only I. scapularis ticks was also cloned and sequenced. In contrast to D. variabilis, no defensin peptide was observed in the plasma, hemocyte lysates or whole tissue lysates from unstimulated I. scapularis ticks. The absence of stored defensin peptide in I. scapularis hemocytes is a significant factor that may contribute to the immunotolerance of I. scapularis for B. burgdorferi. Characterizing the role of defensin in these two important vectors is integral to elucidating arthropod innate immunity. These studies implicate defensin as an intrinsic factor that may affect, on some level, vector competency.





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