Infectious diseases disproportionately affect indigent regions and are the greatest cause of childhood mortality in developing countries. Practical, low-cost
vaccines for use in these countries are paramount to reducing disease burdens and concomitant poverty. Algae are a promising low-cost system for producing
vaccines that can be orally delivered, thereby avoiding expensive purification and
injectable delivery. We engineered the chloroplast of the eukaryotic alga Chlamydomonas reinhardtii to produce a chimeric
protein consisting of the 25-kDa Plasmodium falciparum
surface protein (Pfs25) fused to the β subunit of the
cholera toxin (CtxB) to investigate an alga-based whole-cell oral
vaccine. Pfs25 is a promising
malaria transmission-blocking
vaccine candidate that has been difficult to produce in traditional recombinant systems due to its structurally complex tandem repeats of
epidermal growth factor-like domains. The noncatalytic CtxB domain of the
cholera holotoxin assembles into a pentameric structure and acts as a mucosal adjuvant by binding
GM1 ganglioside receptors on gut epithelial cells. We demonstrate that CtxB-Pfs25 accumulates as a soluble, properly folded and functional
protein within algal chloroplasts, and it is stable in freeze-dried alga cells at ambient temperatures. In mice, oral vaccination using freeze-dried algae that produce CtxB-Pfs25 elicited CtxB-specific serum
IgG antibodies and both CtxB- and Pfs25-specific
secretory IgA antibodies. These data suggest that algae are a promising system for production and oral delivery of
vaccine antigens, but as an orally delivered adjuvant, CtxB is best suited for eliciting
secretory IgA antibodies for
vaccine antigens against pathogens that invade mucosal surfaces using this strategy.