Cure of experimental <i>Trypanosoma vivax</i> 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 <i>Trypanosoma</i> spp. parasites. The disease is primarily caused by two species: <i>Trypanosoma congolense</i> and <i>Trypanosoma vivax,</i> 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 <i>T. vivax</i>. 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 <i>T. vivax</i> 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 modeling and site-directed mutagenesis, we localize 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.IMPORTANCE<i>Trypanosoma vivax</i> 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 recognizes IFX-a parasite cell surface protein localized to the flagellum-is sufficient to cure an established <i>T. vivax</i> 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.