Quantification of wild-type and radiation attenuated Plasmodium falciparum sporozoite motility in human skin

Abstract

Given the number of global malaria cases and deaths, the need for a vaccine against Plasmodium falciparum (Pf) remains pressing. Administration of live, radiation-attenuated Pf sporozoites can fully protect malaria-naïve individuals. Despite the fact that motility of these attenuated parasites is key to their infectivity and ultimately protective efficacy, sporozoite motility in human tissue (e.g. skin) remains wholly uncharacterized to date. We show that the ability to quantitatively address the complexity of sporozoite motility in human tissue provides an additional tool in the development of attenuated sporozoite vaccines. We imaged Pf movement in the skin of its natural host and compared wild-type and radiation-attenuated GFP-expressing Pf sporozoites. Using custom image analysis software and human skin explants we were able to quantitatively study their key motility features. This head-to-head comparison revealed that radiation attenuation impaired the capacity of sporozoites to vary their movement angle, velocity and direction, promoting less refined movement patterns. Understanding and overcoming these changes in motility will contribute to the development of an efficacious attenuated parasite malaria vaccine.

Clustering and erratic movement patterns of syringe-injected versus mosquito-inoculated malaria sporozoites underlie decreased infectivity

“…Sporozoite velocity fluctuated along tracks (visualized by color-coding in Fig. 5A), also in line with our earlier findings (14,17). Plotting the average track-velocities revealed a distribution that could be described with a mixture of two normal distributions (Fig.…”

“…Conversely, the slower erratic movement of syr SPZ is most likely caused by the altered physical space as a consequence of liquid injection pressure. Sporozoite movement is strongly guided by their three-dimensional environment; without any confinement, sporozoites in vitro display a continuous, preferential counter-clockwise movement pattern (14,25), while in vivo, skin structure redirects sporozoites to display much more complex patterns (15,17). The role of physical confinement has been further supported by experiments whereby micropatterned in vitro environments were created that could induce specific movement patterns of sporozoites (26).…”

“…Directly after sporozoite delivery by 32 (IQR: 32-33) mosquito bites of which 18 (IQR: [17][18][19] contained blood, the skin of 4 exposed mice was cut out, snap-frozen and stored at -20 °C until further use. Parasite burden was measured by quantitative real time reverse transcription polymerase chain reactions (qRT-PCR).…”

“…The motility of the sporozoites was analysed using SMOOTmouse skin, an in-house developed software program, written in the MATLAB programming environment (MathWorks). This tool is an adapted version of the SMOOThuman skin (12,17) and SMOOTIn vitro (14) tools previously described. Via…”

“…The subsequent development of fluorophore-expressing sporozoites and a sporozoite fluorescent labelling approach has allowed for more detailed microscopic studies on the motility of individual sporozoites, both in vitro and in skin (11,12). Moreover, automated analysis of sporozoite motility now provides a platform to quantitatively study sporozoite motility under different conditions (13)(14)(15)(16)(17).…”