Variability in parasite protein antigen structure and protective immunity to malaria

Cloned lines of the rodent malaria parasite Plasmodium chabaudi (denoted AS and CB) have been used to investigate the strain specificity of immunity to malaria. One defined difference between these lines is their expression of serologically and structurally distinct forms of an Mr 250Kd parasite-encoded antigen. This antigen is a member of a family of schizont/merozoite-associated polypeptides which have been implicated in the induction of protective immunity to rodent, simian and human malaria parasites. CBA/Ca mice were immunized by either (a) purified P. chabaudi AS-250Kd antigen, (b) chronic AS infection or (c) irradiated nonreplicating AS-parasitized erythrocytes. Post-immunization sera were examined by immunoprecipitation of 35S-methionine-labelled parasites, and the mice challenged with either AS or CB parasites. On challenge, mice developed a parasitaemia, the level of which was determined in part by isolate specificity, but only mice in groups (b) and (c) later developed a response which transcended AS/CB differences. The implications of these findings for the nature of exposed parasite antigens and the induction of protective immunity to malaria is discussed.