Spatiotemporal analysis of sporozoite maturation and infectivity

Plasmodium sporozoites must undergo tightly regulated developmental transitions to become infectious and be successfully transmitted from the mosquito vector to a mammalian host. While transcriptomic studies have revealed stage-specific changes across sporozoite populations, the functional consequences of these transitions remain unclear. Here, using P. berghei, we characterised over time the infectivity of sporozoite forms collected from the midgut, haemolymph, salivary glands and saliva. We show that salivary gland invasion is required but not sufficient for sporozoite optimal infectivity, with the acquisition of hepatocyte cell traversal and invasion progressively increasing until a plateau from 18 days post-infection onwards. Using a stage-specific fluorescent reporter as maturation marker, we correlated its high expression with time and infectivity for each compartment but only salivary gland sporozoites acquired maximal infectivity. Notably, our data suggest that salivated sporozoites--the natural transmission form--exhibit enhanced infectivity relative to gland-resident forms both in vitro and in vivo early after salivary gland invasion. This difference decreases following optimal maturation inside the glands over time. These observations show a crescent gradient of sporozoite maturation and infectiveness from the midgut to the saliva when isolated at the same time of infection, which is mainly regulated by the sporozoite invasion of salivary glands.