Recombinant Rhipicephalus appendiculatus gut (Ra86) and salivary gland cement (Trp64) proteins as candidate antigens for inclusion in tick vaccines: protective effects of Ra86 on infestation with adult R. appendiculatus

Margaret Saimo^1,2,*, David O Odongo^3,4,*, Stephen Mwaura³, Just M Vlak¹, Anthony J Musoke⁵, George W Lubega², Richard P Bishop³, Monique M van Oers^1
1Laboratory of Virology, Wageningen University, Wageningen, The Netherlands; ²School of Veterinary Medicine, Makerere University, Kampala, Uganda; ³International Livestock Research Institute, Nairobi, Kenya; ⁴School of Biological Sciences, University of Nairobi, Nairobi, Kenya; ⁵Onderstepoort Veterinary Institute, Onderstepoort, Pretoria, South Africa
^*These two authors made an equal contribution to this work

Abstract: Rhipicephalus appendiculatus gut protein Ra86 (variants Ra85A and Ra92A) and the salivary gland cement protein (Trp64) were expressed in the baculovirus-insect cell system. The recombinant gut proteins expressed as soluble proteins and the recombinant cement protein, as insoluble inclusion bodies, were used to immunize rabbits, which were then challenged with larval, nymphal, and adult stages of R. appendiculatus ticks. High tick mortality (23.3%) occurred on adult ticks that fed on rabbits vaccinated with the gut proteins, compared with 1.9% mortality in ticks that fed on unvaccinated naïve control rabbits. The mean weight of engorged female ticks was significantly reduced by 31.5% in rabbits vaccinated with the Ra86 recombinant protein compared with controls, as was egg production. Marked effects on these parameters were also observed in adult ticks as a result from vaccination using Trp64, but these were not statistically significant. For both antigens, there was no demonstrable effect on larval or nymphal ticks. This study demonstrates for the first time the protective efficacy of a homolog of Boophilus microplus Bm86 in reducing tick infestation by the adult stage of the three-host tick R. appendiculatus. The results demonstrate the potential of Ra86 for vaccine development against this tick and for the control of East Coast fever.

Keywords: baculovirus, Ra85A, Ra92A, Boophilus decoloratus, Boophilus microplus