A Comparative Analysis of Serpin Genes and Recombinant Proteins from the Salivary Glands of Thai Cattle Ticks, Rhipicephalus microplus

he cattle tick, Rhipicephalus microplus, is the main ectoparasite of livestock in tropical countries, in Thailand particularly. This tick causes a range of problems, including anemia, widespread production losses, as well as lethal tick-borne diseases in animals. Control of tick infestations is based on the use of chemical acaricides, which have numerous undesirable side-effects, for example, environmental pollution and contamination of the food animals eat. This has led to the development of alternative, environmentally friendly methods of tick control, such as anti-tick vaccines. Tick salivary gland (TSG) proteins are one potential source of vaccine candidates. Molecules secreted from the TSG modulate the vertebrate host immune response, and are thus potential targets for novel tick-control measures. TSG serine protease inhibitor (serpin) is one such molecule, which may facilitate tick feeding, blood meal digestion, and pathogen transmission. In this study, we cloned serpin cDNA from the TSG of the cattle tick (R. microplus) by reverse transcriptase-PCR, and analyzed their nucleotides and deduced amino-acid sequences. The results demonstrated that 10 serpin cDNA 1,200 bp in length encoded a serpin protein with 399 amino acid residues, which were 96-98% identical to each other. Based on this result, recombinant serpin protein might be used as an antigen in anti-tick vaccines against R. microplus in numerous regions. TSG serpins of Thai R. microplus were clustered into groups of serpins belonging to each tick species. Phylogenic analysis of other serpins in the GenBank database indicated that Thai serpin sequences contained minor variations in their amino-acid residues, compared with other tick serpins. Greater numbers of variations have been shown for other arthropods.