Plasmodium falciparum: protein antigens identified by analysis of serum samples from vaccinated Aotus monkeys

Serum samples from Aotus trivirgatus subsp. griseimembra monkeys obtained at different stages of a vaccination experiment were analyzed for total antibody titer to Plasmodium falciparum and were used for identifying protective antigens of the human malaria parasite. Total malarial antibody titers were higher in serum samples from protected monkeys (vaccinated with antigen in an adjuvant) than in those from unprotected monkeys (vaccinated with either antigen or adjuvant only). Parasite proteins were labeled with [3H]isoleucine, solubilized with nonionic detergent, and reacted with immune Aotus sera. Immunoprecipitates obtained were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. Thirteen protein antigen bands in the molecular weight range 73,000 to 180,000 were resolved. Serum samples obtained from protected Aotus monkeys reacted more intensely with these proteins than samples from unprotected monkeys did. Evidence is presented that the protective antigen is not a single, normally nonimmunogenic, protein that is recognized only in protected monkeys. Rather, the present data indicate that a heightened immune response to multiple proteins correlated with in vivo protection to P. falciparum in Aotus monkeys. This finding may have a significant bearing on strategies for the development of a human P. falciparum vaccine.