CHARACTERIZATION OF THE EFFECTOR MECHANISMS OF A TRANSMISSION-BLOCKING ANTIBODY UPON DIFFERENTIATION OF PLASMODIUM-BERGHEI GAMETOCYTES INTOOOKINETES IN-VITRO

The transmission-blocking monoclonal antibody 13.1, which recognizes t he ookinete surface antigen Pbs21 of Plasmodium berghei, and an IgG2a isotype control antibody 26.37 were purified by caprylic acid and ammo nium sulphate precipitation. Fab fragments were prepared by papain dig estion. IgG but not Fab from antibody 13.1 reduced ookinete formation by P. berghei in culture by as much as 94% at a concentration of 100 m u g/ml. There was little difference in antibody efficacy in the range 6.25-400 mu g/ml in this assay. The parasite was most sensitive to ant ibody activity in the first 6-9 h of culture, i.e. the gamete/zygote a nd early retort stages. Peripheral blood leucocytes (PBL) were essenti al to achieve maximal inhibition by mAb 13.1 (activity was abrogated t otally if PBL were removed). Together the data suggest that one of the mechanisms of action of this antibody is antibody-mediated PBL killin g. Phagocytosis of parasites was noted in these experiments in all cul tures. We have not attempted in this study to distinguish between Fc-m ediated opsonization, as opposed to antibody-dependent cellular cytoto xicity.