Bioinformatic and Biochemical Characterization of Amblyomma americanum (Acari: Ixodidae) Serine Protease Inhibitors (Serpins)

Abstract

Ticks bites are a source of morbidity and mortality for humans and animals. Chemicals have been insufficient for tick control whereas an anti-tick vaccine may be an effective alternative. Hosts use serpins to control their immune pathways therefore ticks may inject serpins into the host for the same purpose. Amblyomma americanum transcriptome data were analyzed to determine extent and diversity of serpins. Candidate serpins were assayed for function against 17 proteses and for ability to affect blood coagulation and platelet aggregation. Spliced serpin constructs consisting of putatively immunogenic regions of selected serpins were designed for expression and vaccine trials.

A. americanum expresses 122 unique serpins. The greatest diversity of serpins were in males and in feeding ticks. Forty percent were conserved in other ticks. A. americanum serpins 4 and 8 were selected for characterization as representatives of similar serpin clusters. These serpins bound host antibodies to tick saliva indicating they are immunogenic and used in feeding. Serpin 8 inhibited inflammatory proteases cathepsin G and proteinase 3. Serpin 4 inhibited inflammatory proteases cathepsin G and chymase, and the cysteine protease papain. Neither serpin affected blood coagulation however, serpin 4 delayed platelet aggregation. A spliced serpin construct was cloned in bacteria and yeast cells, however recombinant proteins failed to express. Results indicate serpins 4 and 8 function in the vertebrate host as counter-defense serpins and thus may be useful as anti-tick vaccine candidates, however their relative importance in tick feeding physiology will need to be addressed using an alternative protocol.