Cure of experimental Trypanosoma vivax infection with a single dose of an unmodified antibody-based drug targeting the invariant flagellum cell surface protein IFX

Animal African trypanosomiasis (AAT) is an infectious wasting disease of economically important livestock caused by Trypanosoma spp. parasites. The disease is primarily caused by two species: T. congolense and T. vivax which are endemic in many African countries. AAT is managed by therapeutic and prophylactic drugs; however, resistance is now widely reported and the development of new drugs has been impeded due to a chronic lack of investment. Recently, we identified an invariant flagellar-associated cell surface protein (IFX) that could elicit protective immune responses when used as a vaccine against T. vivax. We showed that a complement-recruiting anti-IFX monoclonal antibody can prevent infection when used prophylactically. Here, we show that this same unmodified antibody can be used to cure T. vivax infections in a murine experimental model. Importantly, we show that infections can be treated with a single dose and demonstrate full cure by the lack of detectable parasites in peripheral tissues even after immunosuppression. Using structural modelling and site-directed mutagenesis, we localise the protective antibody epitope thereby identifying targetable regions on IFX to improve vaccine design. Together, these findings validate IFX as both a prophylactic and curative drug target that could be useful in the management of AAT. ImportanceTrypanosoma vivax is a parasite that causes animal African trypanosomiasis (AAT), a chronic wasting disease that infects economically-important livestock animals which is a particular problem in African countries south of the Sahara. The impact of this disease is significant: it is responsible for over 3 million cattle deaths and an estimated $4.5 billion of annual lost productivity. There is a desperate need to develop new control measures because resistance is now widely reported to the drugs commonly used to treat this infection. We show here that a single dose of an unmodified monoclonal antibody that recognises IFX - a parasite cell surface protein localised to the flagellum - is sufficient to cure an established T. vivax infection with no parasite reservoirs detectable in peripheral tissues. Our finding validates IFX as a new drug target and provides a rationale route to the development of new drugs to target AAT.