Molecular Analysis of Calreticulin Expressed in Salivary Glands of Rhipicephalus (Boophilus) microplus Indigenous to Thailand

Abstract

The tropical cattle tick, Rhipicephalus (Boophilus) microplus, is an important ectoparasite of livestock in Thailand that causes economic losses due to the direct effects of tick feeding and by the pathogens they transmit. Intensive acaricide use has several drawbacks, which spurred efforts to develop anti-tick vaccines. Vaccines targeting concealed antigens localized in the tick midgut result in reduced tick fecundity, but molecules localized in the tick salivary glands, which could play a role in pathogen transmission, remain largely unexplored for R. microplus. Calreticulin (CRT) is a protein found in tick salivary glands and saliva, and CRT might facilitate tick feeding and pathogen transmission through anti-thrombotic and complement-inhibition activities. This then suggests that CRT should be evaluated as a vaccine candidate antigen to control cattle ticks in Thailand. The objective of this work was to clone, sequence, and analyze cDNA encoding CRT from salivary glands of R. microplus indigenous to Thailand. Nucleotide sequence analysis showed an open reading frame of 1233 bp. Comparison of the amino acid sequence showed 65-99% identities to other known CRTs from Oryctolagus cuniculus, Rattus norvegicus, Homo sapiens, Bos taurus, R. sanguineus, and R. microplus. The N- and P-domains of CRT were the most conserved, whereas the C-domain was high acid and more variable. CRT primary sequences were most conserved among mammals. Further investigations are warranted to determine whether immunization with Thai B. microplus CRT can affect tick performances and experimental pathogen transmission.