Invasion-Inhibitory Antibodies Elicited by Immunization with Plasmodium vivax Apical Membrane Antigen-1 Expressed in Pichia pastoris Yeast

ABSTRACTIn a recent vaccine trial performed with African children, immunization with a recombinant protein based onPlasmodium falciparumapical membrane antigen 1 (AMA-1) conferred a significant degree of strain-specific resistance against malaria. To contribute to the efforts of generating a vaccine againstPlasmodium vivaxmalaria, we expressed the ectodomain ofP. vivaxAMA-1 (PvAMA-1) as a secreted soluble protein in the methylotrophic yeastPichia pastoris. Recognized by a high percentage of sera from individuals infected byP. vivax, this recombinant protein was found to have maintained its antigenicity. The immunogenicity of this protein was evaluated in mice using immunization protocols that included homologous and heterologous prime-boost strategies with plasmid DNA and recombinant protein. We used the following formulations containing different adjuvants: aluminum salts (Alum),Bordetella pertussismonophosphoryl lipid A (MPLA), flagellin FliC fromSalmonella entericaserovar Typhimurium, saponin Quil A, or incomplete Freund's adjuvant (IFA). The formulations containing the adjuvants Quil A or IFA elicited the highest IgG antibody titers. Significant antibody titers were also obtained using a formulation developed for human use containing MPLA or Alum plus MPLA. Recombinant PvAMA-1 produced under “conditions of good laboratory practice” provided a good yield, high purity, low endotoxin levels, and no microbial contaminants and reproduced the experimental immunizations. Most relevant for vaccine development was the fact that immunization with PvAMA-1 elicited invasion-inhibitory antibodies against different Asian isolates ofP. vivax. Our results show that AMA-1 expressed inP. pastorisis a promising antigen for use in future preclinical and clinical studies.