Plasmodium falciparum: Selection of parasite subpopulations with decreased sensitivity for antibody-mediated growth inhibition in vitro

The chronic, recrudescent nature of malaria has been linked to antigenic diversity of the parasite in which protective immunity against Plasmodium falciparum may be obtained after repeated exposure to infection during a long time. In this study we show that parasite populations with decreased sensitivity to antibody-mediated growth inhibition are readily generated in vitro. A laboratory strain, F32, was cultured for long periods (10-12 weeks) in the presence of suboptimal inhibitory antibody concentrations. The antibodies used were the human monoclonal antibody 33G2 reacting with a linear epitope of the P. falciparum blood-stage antigen 332 and rabbit antibodies to repeat sequences of the blood-stage antigen Pf155/RESA. Our data indicate that the P. falciparum parasites adapt to antibody pressure as reflected by their specifically decreased sensitivity to growth inhibition. A relative resistance of the parasite to growth inhibition mediated by the antibodies used in the culture developed successively, while the parasite remained sensitive to growth inhibition by other antibodies. When the antibody pressure was removed a successive return of sensitivity to growth inhibition developed. Immunofluorescence did not reveal any significant difference in antigen expression between the parasite populations. However, PCR analysis showed that a new population appeared in the parasites grown in the presence of mAb 33G2, while no such change was detected in those grown in the presence of the rabbit antibodies. Our data suggest that the specific decrease in sensitivity to growth inhibition may either be due to down-regulation of antigen synthesis or expression by antibody pressure or, that antibody pressure selects for parasites with low expression of a specific antigen from a heterogeneous parasite population.