Quantitative intravital imaging of Plasmodium falciparum sporozoites: A novel platform to test malaria intervention strategies

Malaria infection starts with the injection of motile Plasmodium sporozoites into the hosts skin during a mosquito bite. Previous studies using the rodent malaria model indicate that the dermal inoculation site may be where sporozoites are most vulnerable to antibodies, yet, functional in vivo assays with human malaria parasites are lacking. Here, we present the first characterization of P. falciparum sporozoites in the skin, comparing their motility to two rodent malaria species and investigating whether the environment of its natural host influences P. falciparum sporozoite motility using a human skin xenograft model. The combined data suggest that in contrast to the liver and blood stages, the skin is not a species-specific barrier for Plasmodium. We observe that P. falciparum sporozoites inoculated into mouse skin move with similar speed, displacement and duration, and enter blood vessels in similar numbers as the rodent parasites. Thus, interventions targeting P. falciparum sporozoite migration can be tested in the murine dermis. Importantly, to streamline quantification of sporozoite motility, we developed a toolbox allowing for automated detection and tracking of sporozoites in intravital microscopy videos. This establishes a platform to test vaccine candidates, immunization protocols, monoclonal antibodies and drug candidates for their impact on human malaria sporozoites in vivo. Screening of intervention strategies for in vivo efficacy against Pf sporozoites using this new platform will have the potential to validate targets prior to expensive clinical trials.