Pfs230 yields higher malaria transmission-blocking vaccine activity than Pfs25 in humans but not mice

Background. Vaccines that block human-to-mosquito Plasmodium transmission are needed for malaria eradication and clinical trials have targeted zygote antigen Pfs25 for decades. We reported that a Pfs25 protein-protein conjugate vaccine formulated in alum adjuvant induced significant serum functional activity in both US and Malian adults. However, antibody titers declined rapidly, and transmission-reducing activity required four vaccine doses. Functional immunogenicity and durability must be improved before advancing TBV further in clinical development. We hypothesized that the pre-fertilization protein Pfs230 alone or in combination with Pfs25 would improve functional activity. Methods. Transmission-blocking vaccine candidates based on gamete antigen Pfs230 or Pfs25 were conjugated with Exoprotein A, formulated in Alhydrogel®, and administered to mice, rhesus macaques, and humans. Antibody titers were measured by ELISA and transmission-reducing activity was assess by the Standard Membrane Feeding Assay. Results. Pfs25-EPA/Alhydrogel® and Pfs230D1-EPA/Alhydrogel® induced similar serum functional activity in mice, but Pfs230D1-EPA induced significantly greater activity in rhesus monkeys that was enhanced by complement. In U.S. adults, two vaccine doses induced complement-dependent activity in 4 of 5 Pfs230D1-EPA/Alhydrogel® recipients but no significant activity in five Pfs25-EPA recipients, and combination with Pfs25-EPA did not increase activity over Pfs230D1-EPA alone. Conclusion. The complement-dependent functional immunogenicity of Pfs230D1-EPA represents a significant improvement over Pfs25-EPA. The rhesus model is more predictive of the functional human immune response to Pfs230D1 than is the mouse model. Trial Registration. ClinicalTrials.gov NCT02334462 Funding. This work was supported by the Intramural Research Program of the National Institute of Allergy and Infectious Diseases, National Institutes of Health.