Protective immune response to Plasmodium chabaudi, developed by mice after drug controlled infection or vaccination with parasite extracts: analysis of stage specific antigens from the asexual blood cycle.

The protective immune response to asexual blood infection by Plasmodium chabaudi was studied in mice immunized either by drug controlled infection or by vaccination with preparations of merozoïtes or free parasites at different stages of development. Animals immunized by the first method developed a sterile immunity. The passive transfer of their serum protected naïve recipients from the lethal development of the infection, but affected only moderately the initial course of the parasitaemia. Animals immunized with either ring, schizont or merozoïte preparations exhibited a limited but significant resistance to infection: when challenged with 10(6) parasites of the homologous strain they exhibited a reduced parasitaemia as compared to control mice, and in addition, 50% of them recovered from the infection. Immunochemical analysis of parasite antigens showed that a family of high molecular weight proteins synthesized essentially at the schizont stage and conserved in the merozoites are important immunogens. Quantitative rather than qualitative differences were observed in the pattern of parasite proteins immunoprecipitated by serum of animals exhibiting sterile immunity or moderate protective immunity. A schizont specific polypeptide of mol. wt 82 Kd which is found in the surface of the merozoite is preferentially immunoprecipited by serum from animals exhibiting sterile immunity.