Molecular characterization of Bm86 gene orthologs from Hyalomma excavatum, Hyalomma dromedarii and Hyalomma marginatum marginatum and comparison with a vaccine candidate from Hyalomma scupense

Abstract

The ixodid ticks from the Hyalomma genus are important pests of livestock, having major medical and veterinary significance in Northern Africa. Beside their direct pathogenic effects, these species are vectors of important diseases of livestock and in some instances of zoonoses. Anti-tick vaccines developed in Australia and Cuba based on the concealed antigen Bm86 have variable efficacy against H. anatolicum and H. dromedarii. This variation in vaccine efficacy could be explained by the variability in protein sequence between the recombinant Bm86 vaccine and Bm86 orthologs expressed in different Hyalomma species. Bm86 orthologs from three Hyalomma tick species were amplified in two overlapping fragments and sequenced. The rate of identity of the amino acid sequence of Hm86, He86 and Hdr86, the orthologs of Bm86, respectively, in H. marginatum marginatum, H. excavatum and H. dromedarii, with the Bm86 proteins from Rhipicephalus microplus (Australia, Argentina and Mozambique) ranged between 60 and 66%. The obtained amino-acid sequences of Hmm86, He86 and Hdr86 were compared with the Hd86-A1 sequence from H. scupense used as an experimental vaccine. The results showed an identity of 91, 88 and 87% for Hmm86, He86 and Hdr86, respectively. A specific program has been used to predict B cells epitopes sites. The comparison of antigenic sites between Hd86-A1 and Hm86/Hdr86/He86 revealed a diversity affecting 4, 8 and 12 antigenic peptides out of a total of 28 antigenic peptides, respectively. When the Bm86 orthologs amplification protocol adopted in this study was applied to H. excavatum, two alleles named He86p2a1 and He86p2a2 were detected in this species. This is the first time that two different alleles of Bm86 gene are recorded in the same tick specimen. He86p2a1 and He86p2a2 showed an amino acid identity of 92%. When He86p2a1 and He86p2a2 were compared to the corresponding sequence of Hd86-A1 protein, an identity of 86.4 and 91.0% was recorded, respectively. When compared to He86, Hdr86 and Hm86, Bm86 used in commercial and experimental vaccines, showed a greater extent of diversity than noted when the same Hyalomma orthologs were compared to Hd86-A1. Although significant, these variations were less extensive within the Hyalomma genus. Accordingly, thus suggesting that Hd86-A1 vaccine candidate might be more appropriate to target Hyalomma tick species than corresponding Bm86 commercial vaccines. However, vaccination trials with both antigens are required to validate this hypothesis.