Spatiotemporal analysis of sporozoite maturation and infectivity

Abstract

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 to track maturation, we found that fluorescence increased over time and correlated with infectivity within each compartment, yet maximal infectivity was reached only in salivary gland sporozoites. 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, in which pre-salivary gland forms retain limited infectivity but do not reach the competence of mature salivary gland-associated parasites.