Immunogenicity of Plasmodium falciparum antigenic determinants produced by Escherichia coli recombinant clones

The immunogenicity of two parasite antigens produced by Escherichia coli as proteins fused to beta-galactosidase was investigated in three animal species: mice, rabbits and squirrel monkeys. 2L protein carries 71 amino acids of a parasite antigen and 11.1 protein carries 23 repeats of a 9-amino-acid repetitive unit. The humoral response was studied using indirect immunofluorescence and immunoprecipitation. The results indicate that immunization of mice, rabbits and squirrel monkeys using SDS-denatured 2L fusion protein induced antibodies able to bind to parasite antigen 2L in the IFA or in the immunoprecipitation assays. Immunization using the native fusion protein did not induce antibodies able to immunoprecipitate the 2L parasite antigen. The same observation was made for the animals immunized with 11.1 recombinant protein. In this case, the antibody response was also measured by ELISA using synthetic dimers of the repeat as antigen. In mice and rabbits, high titres of anti-11.1 antibodies were found by ELISA. However, when the antigen produced by the parasite itself was used to evaluate the response, low titres were found. This indicates that the animals produced high levels of antibodies to a structure which is not exposed in the parasite. In squirrel monkeys, the same observation was made, but the overall levels of the response to 11.1 antigen were considerably lower than those observed in mice or rabbits.