Vaccination by single dose sporozoite injection of blood stage attenuated malaria parasites

An efficient malaria vaccine remains elusive. As an alternative to malaria subunit vaccines, vaccination approaches are currently explored using live Plasmodium parasites, either attenuated mosquito-derived sporozoites or attenuated blood stage parasites. Both approaches would profit from the availability of attenuated and avirulent parasites with a reduced blood stage multiplication rate. Ideally, such slow growing parasites would proceed normally through the mosquito but cause a self-limiting infection upon transmission. Here we screened gene-deletion mutants of the rodent parasite P. berghei and the human parasite P. falciparum for slow growth. In addition, we tested the P. berghei mutants for avirulence in mice and self-resolving blood stage infections, while preserving sporozoite formation and liver infection. Targeting fifty genes yielded seventeen P. berghei gene-deletion mutants with two mutants causing self-clearing infections in mice while retaining full transmissibility through mosquitoes. For those, infection of mice by a low number of blood stages, infected-mosquito bites or by single injection of sporozoites led to protection from disease after challenge with wild type sporozoites. Two of six generated P. falciparum gene-deletion mutants showed a slow growth rate. Slow growing, avirulent P. falciparum mutants will constitute valuable tools to inform on the induction of immune responses and aid in developing new as well as safeguarding existing attenuated parasite vaccines.