The Road toward a Trojan Horse Malaria Vaccine: Chemical Augmentation of Live Sporozoites without Affecting Their Viability.

In our effort to design an effective ultralow-dose attenuated whole sporozoite (SPZ) vaccine, we explored the use of chemical adjuvants to increase the immunogenicity of sporozoites. Critical to the potency of attenuated sporozoites is their ability to migrate to and infect the liver. We thus used an in vivo mouse challenge model to compare the effects of external and internal chemical modifications to sporozoites. We loaded sporozoites with CL307 using two strategies: a supramolecular surface modification adjuvant system (SPZ-SAS(CL307)) and a mitochondrial membrane potential system (SPZ-MMP(CL307)). SPZ-SAS(CL307) contained 200 times more CL307 as compared to SPZ-MMP(CL307). However, SPZ-SAS(CL307) could not establish a liver infection after intravenous injection (25 000 sporozoites), suggesting loss of function of the sporozoites. On the contrary, SPZ-MMP(CL307) could establish an equivalent liver infection to the control already at a dose of 5000 sporozoites. When the immunized mice were challenged, SPZ-MMP(CL307) equaled the protective efficacy of control sporozoites. Our findings indicate that the MMP system can be used to chemically augment live cells, such as sporozoites, with adjuvants without negatively affecting their infectivity and protective efficacy in vivo. For malaria, further testing of different types of adjuvants at different concentrations is needed to ultimately develop vaccines with increased potency. However, in essence, this platform provides a proof-of-concept for a Trojan horse adjuvanting or drug loading concept for any whole-cell vaccine or therapy, within the infectious diseases space or beyond in oncological applications.