TRANSGENIC EXPRESSION OF A MOSQUITO-STAGE MALARIAL PROTEIN, PBS21, INBLOOD STAGES OF TRANSFORMED PLASMODIUM-BERGHEI AND INDUCTION OF AN IMMUNE-RESPONSE UPON INFECTION

Pbs21 is a surface protein of the ookinete of Plasmodium berghei, whic h can induce a potent transmission-blocking immune response. Pbs21 is normally expressed only by parasite stages in the mosquito, i.e., fema le gametes/zygotes, ookinetes, and oocysts, However, the Pbs21 gene is transcribed in female gametocytes which circulate in the bloodstream of the host, where translation of the resulting mRNA is totally repres sed. Episomal transfection has been used to investigate whether expres sion of Pbs21 protein could be achieved in blood stages of the parasit e. By using plasmid pMD221, the complete mRNA-encoding region of Pbs21 , flanked only by 218 nucleotides (nt) of its promoter region and 438 nt of its 3' region downstream from the polyadenylation site, was intr oduced into the blood stages of gametocyte-producing and non-gametocyt e-producing clones of P. berghei. In both of these transformed parasit e lines, Pbs21 protein was expressed in asexual trophozoites, schizont s, and, when present, in both male and female gametocytes, Hence, the flanking regions present are sufficient to allow transcription but lac k the elements that exert natural control of sex-and stage-specific tr anscription. The mRNA and the protein expressed by transformed blood s tages were indistinguishable from the wild-type forms by the criteria tested, and the protein was recognized by both conformation-dependent and conformation-independent monoclonal antibodies raised against nati ve Pbs21, In mice infected with transformed non-gametocyte producing p arasites, a Pbs21-specific immune response was induced and characteriz ed with respect to isotype (IgG2a/IgG2b) and quantity (11.5 +/- 10 pg/ ml) of antibody produced. However, as found in previous studies, these antibody levels were insufficient to inhibit development of the paras ites in the mosquito. The ability to express mosquito midgut-stage ant igens in blood-stage parasites will facilitate further investigations of molecular and immunological properties of these proteins.