Bm86 homologues and novel ATAQ proteins with multiple epidermal growth factor (EGF)-like domains from hard and soft ticks

Abstract

Tick control on livestock relies principally on the use of acaricides but the development of acaricide resistance and concerns for environmental pollution underscore the need for alternative control methods, for instance through the use of anti-tick vaccines. Two commercial vaccines based on the recombinant Bm86 protein from Rhipicephalus (Boophilus) microplus ticks were developed. Partial protection of the Bm86 vaccine against other Rhipicephalus (Boophilus) and Hyalomma tick species suggests that the efficacy of a Bm86-based vaccine may be enhanced when based on the orthologous recombinant Bm86 antigen. We therefore identified and analysed the Bm86 homologues from species representing the main argasid and ixodid tick genera, including two from the prostriate Ixodes ricinus tick species. A novel protein from metastriate ticks with multiple epidermal growth factor (EGF)-like domains which is structurally related to Bm86 was identified by using a 3′ rapid amplification of cDNA ends (3′-RACE) method with a degenerate primer based on a highly conserved region of Bm86 and its orthologues. This second protein was named ATAQ after a part of its signature peptide. Quantitative reverse transcriptase-PCR showed that ATAQ proteins are expressed in both midguts and Malpighian tubules, in contrast to Bm86 orthologues which are expressed exclusively in tick midguts. Furthermore, expression of this protein over the life stages of R. microplus and Rhipicephalus appendiculatus was more continuous compared with Bm86. Although a highly effective vaccine antigen, gene silencing of Bm86 by RNA interference (RNAi) produced only a weak phenotype. Similarly the RNAi phenotype of Rhipicephalus evertsi evertsi females in which the expression of Ree86, ReeATAQ or a combination of both genes was silenced by RNAi did not differ from a mock-injected control group. The vaccine potential of ATAQ proteins against tick infestations is yet to be evaluated.