Towards improving Subolesin and MSP-based vaccines targeting Rhipicephalus microplus and Anaplasma phagocytophilum

Abstract

Ticks are ectoparasites that impact livestock and transmit pathogens that cause tick-borne diseases that severely affect livestock industries. Consequently, the control of ticks and their tick-borne diseases is imperative. However, the current chemical tick control paradigm is marred by the increasing development of acaricide resistance and necessitates supplementary control strategies, such as vaccines. Subolesin is a promising tick vaccine candidate, however the mechanism through which this antigen elicits protection is still unknown. Consequently, the expression and diversity of Subolesin (and its many variants) in Rhipicephalus microplus ticks from different tissues (ovary, gut and gland), life stages (larvae, nymph and adult) and several South African locations (Mpumalanga and a pathogen-free laboratory-strain) were elucidated for the first time on a RNA (ribonucleic acid) sequencing level. This study elucidates for the first time the gene structure of Subolesin in R. microplus. Furthermore, it was found that Subolesin has four potential mRNA (messenger ribonucleic acid) variants which could translate into three protein variants. Thus far, this finding is exclusive to R. microplus, as Subolesin in other tick species have been reported as a single mRNA and protein. Additionally, vaccines simultaneously targeting both the tick vector and pathogen with a single formulation is another ambitious goal, given the challenges with traditional subunit vaccines. Consequently, this study evaluated the immune profile elicited by vaccination with antigen coated Poly D-L-lactide-co-glycolide acid (PLGA) microparticles simultaneously targeting both the tick vector (Subolesin) and the tick-borne pathogen Anaplasma phagocytophilum (MSP4) in a murine model. Sera results indicated that Subolesin antigens (soluble and adsorbed onto particles) were not immunogenic after ELISA (enzyme linked immunosorbent assay) analysis and were consequently excluded from further analysis. Nevertheless, ELISA results demonstrated that all MSP4-receiving groups elicited a Th2 biased response in sera. Furthermore, this study offers the first insights into the T and B cell profile elicited by immunization with MSP4 in any animal model. Therefore, this study offers valuable insights into the design and application of future PLGA microparticle based vaccines for the control of ticks and A. phagocytophilum.