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

Authors: BUSCH, JOSEPH - Northern Arizona University; STONE, NATHAN - Northern Arizona University; PEMBERTON, GRANT - Northern Arizona University; ROBERTS, MACKENZIE - Northern Arizona University; TURNER, REBEKAH - Northern Arizona University; THORNTON, NATALIE - Northern Arizona University; SAHL, JASON - Northern Arizona University; LEMMER, DARRIN - Northern Arizona University; Buckmeier, Beverly; Davis, Sara; KARIM, SHAHID - Northern Arizona University; KLAFKE, GUILHERME - Desiderio Finamore Veterinary Research Institute (FEPAGRO); Thomas, Donald; Olafson, Pia; Ueti, Massaro; MOSQUEDA, JUAN - Autonomous University Of Queretaro; Scoles, Glen; WAGNER, DAVID - Northern Arizona University

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2025
Publication Date: 4/15/2025
Citation: Busch, J.D., Stone, N.E., Pemberton, G., Roberts, M.L., Turner, R.E., Thornton, N., Sahl, J.W., Lemmer, D., Buckmeier, B.G., Davis, S.K., Karim, S., Klafke, G., Thomas, D.B., Olafson, P.U., Ueti, M.W., Mosqueda, J., Scoles, G.A., Wagner, D.M. 2025. Fourteen anti-tick vaccine targets are variably conserved in cattle fever ticks. Parasites & Vectors. 18. Article e140. https://doi.org/10.1186/s13071-025-06683-5.
DOI: https://doi.org/10.1186/s13071-025-06683-5

Interpretive Summary: Cattle fever ticks parasitize agricultural animals and wildlife and transmit pathogens of cattle, including cattle tick fever (Bovine Babesiosis) and Bovine Anaplasmosis. Both the ticks and the diseases they transmit cause significant losses for cattle producers worldwide. Control of these ticks has relied primarily on treatment of cattle with chemical acaricides, but cattle fever ticks are rapidly developing acaricide resistance and development of new approaches for control of these ticks is critically important. Vaccinating cattle to protect them from ticks has been proposed as a more sustainable approach for tick control. Anti-tick vaccines, based on tick proteins, have previously been available, but due to variable efficacy and other economic concerns they are no longer on the market, and new vaccines are desperately needed. When vaccines target highly conserved antigenic epitopes in the tick they are more likely to be broadly effective and the risk of vaccine escape or vaccine failure due to mismatches with genetically different tick populations will be reduced. We examined variation in 14 different tick proteins that have been proposed as vaccine targets. To do this we used DNA sequences from a diverse collection of ticks to predict the protein sequence and then examined the variation in those sequences by comparing them to a tick genome sequence from Texas. The previously available anti-tick vaccine, which was based on a tick gut protein known as Bm86, was one of the most variable and the sequence of the original Australian Bm86 vaccine was very different from this protein in ticks from the Americas, which explains why anti-tick vaccines based on this protein were not very effective outside of Australia. Some of the newer vaccine candidates in development for the next generation of anti-tick vaccines were very highly conserved, suggesting that they would be more universally effective in producing a tick-protective immune response. This paper provides data on the conservation levels of 14 different candidate anti tick vaccines and emphasizes the importance of understanding the variability of the vaccine target before going further to develop a vaccine.

Technical Abstract: Cattle fever ticks, Rhipicephalus (Boophilus) annulatus and R. (B.) microplus, parasitize agricultural animals and wildlife and transmit pathogens of cattle, including Babesia bovis, B. bigemina and Anaplasma marginale, that cause significant production losses worldwide. Control of these ticks has relied primarily on treatment of cattle with chemical acaricides, but frequent use, exacerbated by the one-host life cycle of these ticks, has led to very high levels of acaricide resistance. Consequently, development of new approaches for control of these ticks is critically important. Anti-tick vaccines based on tick antigens have great potential for controlling these ticks and when highly conserved antigenic epitopes are targeted, they will be more broadly effective and the risk of vaccine escape or vaccine failure will be reduced. Therefore, we evaluated conservation in 14 putative anti-tick vaccine targets, including Bm86, the first protein target used as an anti-tick vaccine for cattle. We analyzed a diverse set of tick samples including 167 R. microplus and 12 R. annulatus from the Americas and Pakistan using PCR amplicon sequencing and in silico translation of exons to examine amino acid (AA) conservation. Variable AA positions in the 14 proteins were compared against the published genome from a Texas R. microplus population to calculate the proportion of identical AA positions for each protein. Only one target, VDAC, was fully conserved in all 167 R. microplus samples (AA similarity = 1.0). Conservation varied considerably across the other proteins in R. microplus, but other highly conserved proteins included AQP1 (0.989), VgR (0.985), S1 (0.985), and Sub (0.981). In contrast, Bm86 had a much lower level of protein similarity (0.872) and only one candidate, MP4 (0.819), ranked lower in terms of sequence conservation. The Bm86 sequence used in the original Australian TickGARD vaccine carried many AA replacements compared to the R. microplus in our study, supporting the hypothesis that this vaccine target is not optimal for use against tick populations in the Americas. We mapped the location of each AA replacement onto predicted 3D protein models of the most highly conserved proteins (VDAC, AQP1, VgR, S-1, and Sub) to identify potential locations for antibody binding on the exposed outer surfaces of these proteins. Some AA replacements occur within specific epitopes targeted by published small peptide vaccines. These findings emphasize the importance of a thorough analysis of protein conservation across tick populations when selecting vaccine candidates.