Anti-Tick Vaccines To Prevent Tick-Borne Diseases: An Overview And A Glance At The Future

Ticks are ectoparasites that transmit various pathogens with great impact on human and animal health throughout the world. In the USA, Europe and Asia, Ixodes ticks are the medically most important vectors for human tick-borne pathogens. After the tick hypostome penetrates the skin, ticks secrete saliva that facilitates feeding and paves the way for pathogen transmission. Tick saliva contains molecules interfering with host defence mechanisms, such as the coagulation system and the immune responses, in order to try to prevent rejection. Despite the tick's countermeasures, repeated tick infestations can lead to natural immunity that partially protects against tick feeding and pathogen transmission, a phenomenon known as 'tick immunity'. Two commercial veterinary anti-tick vaccines have been developed against another tick species, Boophilus microplus, showing the feasibility of mimicking acquired tick-immunity through vaccination. However, the protective veterinary antigen Bm86 does not seem to protect against Ixodes ticks, but other interesting Ixodes vaccine candidates, have been described. These antigens are able to partially protect against Ixodes challenge, and some also against pathogen transmission. The ideal vaccine candidate, one that is expressed in all tick stages, protects against multiple tick species, provides long lasting immunity and blocks pathogen transmission has not been identified yet. Also, the application of such antigens in vaccines for domesticated animals, wildlife, let alone humans, remains a challenge. Therefore, a better understanding of the molecular interactions between Ixodes ticks, hosts and pathogens will help in the identification of new antigens to prevent tick-borne diseases in both humans and animals. In this review we describe the known molecular mechanisms underlying the interaction between ticks and their hosts, the mechanisms by which pathogens exploit these interactions, and selected Ixodes proteins that have been investigated as vaccine candidates to prevent tick feeding and/or transmission of pathogens from the tick to host.