Evaluation of immunogenicity and protective efficacy of carrier-free Plasmodium falciparum R23 antigen in pre-exposed saimiri sciureus monkeys.

We have reported previously that the recombinant Glutathione S-transferase GTR23, induced protection after immunisation of naive or previously exposed Saimiri monkeys. We investigated the immunogenicity of carrier-free R23 repeats in pre-exposed animals in two adjuvant formulations. Three of five monkeys immunised with alum-formulated repeats and one of two animals immunised with the Polyalphaolefine formulation produced high titres of cytophilic antibodies with a primary type kinetics, indicating that the anti-P. falciparum antibodies present on the day of challenge were R23-specific. The four responders in R23-specific antibodies were protected against a challenge infection with the virulent FUP/SP strain. The other three animals failed to respond to immunisation and experienced an infection that required drug treatment. Unlike the other three animals that experienced an infection requiring drug treatment. These experiments support further development of the R23 repeats as a vaccine candidate.