Fourteen anti-tick vaccine targets are variably conserved in cattle fever ticks

Abstract

Background: Rhipicephalus (Boophilus) microplus causes significant cattle production losses worldwide because it transmits Babesia bovis and B. bigemina, the causative agents of bovine babesiosis. Control of these ticks has primarily relied on treatment of cattle with chemical acaricides, but frequent use, exacerbated by the one-host lifecycle of these ticks, has led to high-level resistance to multiple classes of acaricides. Consequently, new approaches for control, such as anti-tick vaccines, are critically important. Key to this approach is targeting highly conserved antigenic epitopes to reduce the risk of vaccine escape in heterologous tick populations.

Methods: We evaluated amino acid conservation within 14 tick proteins across 167 R. microplus collected from geographically diverse locations in the Americas and Pakistan using polymerase chain reaction (PCR) amplicon sequencing and in silico translation of exons.

Results: We found that amino acid conservation varied considerably across these proteins. Only the voltage-dependent anion channel (VDAC) was fully conserved in all R. microplus samples (protein similarity 1.0). Four other proteins were highly conserved: the aquaporin RmAQP1 (0.989), vitellogenin receptor (0.985), serpin-1 (0.985), and subolesin (0.981). In contrast, the glycoprotein Bm86 was one of the least conserved (0.889). The Bm86 sequence used in the original Australian TickGARD vaccine carried many amino acid replacements compared with the R. microplus populations examined here, supporting the hypothesis that this vaccine target is not optimal for use in the Americas. By mapping amino acid replacements onto predicted three-dimensional (3D) protein models, we also identified amino acid changes within several small-peptide vaccines targeting portions of the aquaporin RmAQP2, chitinase, and Bm86.

Conclusions: These findings emphasize the importance of thoroughly analyzing protein variation within anti-tick vaccine targets across diverse tick populations before selecting candidate vaccine antigens. When considering protein conservation alone, RmAQP1, vitellogenin receptor, serpin-1, subolesin, and especially VDAC rank as high-priority anti-tick vaccine candidates for use in the Americas and perhaps globally.

Keywords: R. annulatus; Rhipicephalus microplus; Anti-tick vaccine; Conserved targets; Surface-exposed epitopes.

Fig. 1

Protein conservation in 14 anti-tick targets evaluated in 167 samples of Rhipicephalus (Boophilus) microplus from North America (n = 145), South America (n = 17), and Pakistan (n = 5). The y-axis shows protein similarity; the x-axis is set to cross the y-axis at 0.80. Proteins are ranked from most to least conserved; the Bm86 protein (red box) used in all first-generation cattle vaccines is one of the least conserved proteins in our study

Fig. 2

Locations of amino acid replacements in 10-aa windows of the 14 proteins analyzed in this study. Only replacements identified in the Rhipicephalus (Boophilus) microplus samples from our study (n = 167) are used. Amino acid positions are shown on the top scale. Outlined boxes indicate each full-length protein except for VgR; the VgR boxes outline two ligand binding domains (LBDs) that were assayed in this study. Shading key: blue = conserved positions; orange to red = 1–5 replacements per window of 10 amino acids; white = missing data

Fig. 3

Location of amino acid replacements (blue) mapped onto predicted 3D structural models of selected proteins: A voltage-dependent anion channel (RmVDAC); B vitellogenin receptor (VgR); C aquaporin-1 (RmAQP1); D serine protease inhibitor-1 (RmS-1); E subolesin (RmSub); F aquaporin-2 (RmAQP2); G chitinase (Chit); H glycoprotein Bm86 (Bm86). Published short-peptide vaccine targets (magenta) are highlighted for RmAQP2, Chit, RmSub, and Bm86; magenta is also used to highlight two lipid-binding domains (LBDs) that were assayed in VgR. Only those replacements identified in our Rhipicephalus (Boophilus) microplus dataset from the Americas and Pakistan are highlighted; additional replacements identified from previously published sequences of R. microplus and other Rhipicephalus species are documented in the amino acid alignments within Additional file 6. All 3D protein models were generated using the Alphafold website; specific URL addresses for each protein are provided

Fig. 3

Location of amino acid replacements (blue) mapped onto predicted 3D structural models of selected proteins: A voltage-dependent anion channel (RmVDAC); B vitellogenin receptor (VgR); C aquaporin-1 (RmAQP1); D serine protease inhibitor-1 (RmS-1); E subolesin (RmSub); F aquaporin-2 (RmAQP2); G chitinase (Chit); H glycoprotein Bm86 (Bm86). Published short-peptide vaccine targets (magenta) are highlighted for RmAQP2, Chit, RmSub, and Bm86; magenta is also used to highlight two lipid-binding domains (LBDs) that were assayed in VgR. Only those replacements identified in our Rhipicephalus (Boophilus) microplus dataset from the Americas and Pakistan are highlighted; additional replacements identified from previously published sequences of R. microplus and other Rhipicephalus species are documented in the amino acid alignments within Additional file 6. All 3D protein models were generated using the Alphafold website; specific URL addresses for each protein are provided

Fig. 3

Location of amino acid replacements (blue) mapped onto predicted 3D structural models of selected proteins: A voltage-dependent anion channel (RmVDAC); B vitellogenin receptor (VgR); C aquaporin-1 (RmAQP1); D serine protease inhibitor-1 (RmS-1); E subolesin (RmSub); F aquaporin-2 (RmAQP2); G chitinase (Chit); H glycoprotein Bm86 (Bm86). Published short-peptide vaccine targets (magenta) are highlighted for RmAQP2, Chit, RmSub, and Bm86; magenta is also used to highlight two lipid-binding domains (LBDs) that were assayed in VgR. Only those replacements identified in our Rhipicephalus (Boophilus) microplus dataset from the Americas and Pakistan are highlighted; additional replacements identified from previously published sequences of R. microplus and other Rhipicephalus species are documented in the amino acid alignments within Additional file 6. All 3D protein models were generated using the Alphafold website; specific URL addresses for each protein are provided