1648 Surface Antigens of Malaria Merozoites

Immunity against blood stage Plasmodium falciparum malaria can be passively transferred with immune gamma globulin (1), and protective antibodies appear to act against the merozoite form of the parasite (2). For vaccine development, therefore, it is important to identify and characterize the surface antigens of merozoites against which protective antibodies may be directed. We have reported the identification of a 195,000 mol wt protein antigen of P. falciparum that is synthesized in schizonts and processed through a series of discrete fragments during the last few hours of the parasite's intraerythrocytic growth cycle (3). The major processing product detected after merozoite differentiation was an 83,000 mol wt fragment, and it was suggested that this fragment of the •95,000 tool wt precursor may be expressed on the merozoite surface. More recently, we have demonstrated a serological cross-reaction between the 195,000 mol wt P. falciparum antigen and a 230,000 mol wt antigen ofP . yoelii that can be used in purified form to immunize mice against P. yoelii malaria (4). The 195,000 tool wt P. falciparum protein, and its form on the merozoite surface, are therefore of considerable interest for malaria vaccine development. We have now analyzed the surface antigens of naturally released merozoites harvested from P. falciparum cultures. The results show that the 83,000 mol wt fragment of the 195,000 mol wt precursor is expressed on the surface of merozoites, and that it is a major merozoite surface antigen immunoprecipitated by human immune serum.