Ability of recombinant or native proteins to protect monkeys against heterologous challenge with Plasmodium falciparum.

To circumvent problems associated with polymorphic vaccine candidates for Plasmodium falciparum malaria, we evaluated recombinant proteins representing sequences from relatively high conserved regions of the precursor to the major merozoite surface proteins, gp190, for their ability to protect Saimiri monkeys against malaria challenge. Recombinant proteins represented amino acid residues 147 to 321 (p190-1) or 147 to 321 and 1060 to 1195 (p190-3), and their efficacy was compared with that of native gp190 and its processed products. All antigens were derived from P. falciparum K1, a Thai isolate, while the challenge strain was Palo Alto (from Uganda, Africa), which contains, with the exception of the N-terminal 375 amino acids, which are almost identical to the K1 sequence, essentially the MAD-20 allelic form of gp190. By 12 days following challenge, each control monkey required drug treatment. Three monkeys injected with p190-3 required therapy, while one cleared the parasites without therapy. Two monkeys injected with p190-1 received therapy on day 14, while the remaining two cleared the parasites without therapy. Of four animals injected with native gp190, because of health reasons unrelated to malaria, one was not challenged with parasites and one was removed from the study 8 days after challenge when its parasitemia was 1.1% (parasitemias in control animals ranged from 4.3 to 9%); the remaining two cleared the parasites after maximum parasitemias of 0.45 and 0.53%. The highest levels of antiparasite antibody were produced by animals immunized with native gp190. There was a significant correlation between monkeys which did not require drug treatment and antiparasite antibody. These results may suggest that native gp190 and/or its processed products can provide excellent protection against heterologous challenge and that antibody is important for protection. The challenge for vaccine development is to identify the protective sequence(s).