Vaccination with virosomally formulated recombinant CyRPA elicits protective antibodies against parasites in preclinical in vitro and in vivo models.

The () cysteine-rich protective antigen (CyRPA) has emerged as a promising blood-stage candidate antigen for inclusion into a broadly cross-reactive malaria vaccine. This highly conserved protein among various geographical strains plays a key role in the red blood cell invasion process by merozoites, and antibodies against CyRPA can efficiently prevent the entry of the malaria parasites into red blood cells. The aim of the present study was to develop a human-compatible formulation of the CyRPA vaccine candidate and confirming its activity in preclinical studies. Recombinant CyRPA expressed in HEK 293 cells was chemically coupled to phosphoethanolamine and then incorporated into the membrane of unadjuvanted influenza virosomes approved as antigen delivery system for humans. Laboratory animals were immunised with the virosome-based CyRPA vaccine to determine its immunogenic properties and in particular, its capacity to elicit parasite binding and growth-inhibitory antibodies. The vaccine elicited in mice and rabbits high titers of CyRPA-specific antibodies that bound to the blood-stage parasites. At a concentration of 10 mg/mL, purified total serum IgG from immunised rabbits inhibited parasite growth in vitro by about 80%. Furthermore, in a infection mouse model, passive transfer of 10 mg of purified total IgG from CyRPA vaccinated rabbits reduced the in vivo parasite load by 77%. Influenza virosomes thus represent a suitable antigen delivery system for the induction of protective antibodies against the recombinant CyRPA, designating it as a highly suitable component for inclusion into a multivalent and multi-stage virosomal malaria vaccine.