The Major Parasite Surface Antigen Associated with Human Resistance

Schistosomiasis due to Schistosoma mansoni is a snail-transmitted parasitic disease that affects millions of individuals and is amajor concern for a number of subtropical countries (1) . Control by chemotherapy and mollusciciding is costly, may lead to the emergenceof resistant strains of parasites, and requires well-trained personnel who are seldom available in endemic areas (2-6); for these reasons, additional control methods are sought . Immunological studies in laboratory animals have raised expectations that a vaccine against S. mansoni could be produced . However, a major difficulty in developing a vaccine against this parasite is that natural immunization schemes consisting of repeated infections interrupted by chemotherapy fail to protect a sizeable fraction of the population of endemic areas (7-10) . The most susceptible individuals often bear heavy infections andare at high risks ofdeveloping severe clinical disease (11-13); they are also the principal reservoir of parasites. It is, therefore, most important to identify the causes of the higher susceptibility of these individuals in order to define the properties that a vaccine should have in order to protect them . Since a number of studies indicated that antischistosomular IgG antibodies were central to animal protection against S, mansoni (reviewed in reference 14), we have evaluated the antilarval surface IgG antibody response of subjects with high or low susceptibilities to infection by S. mansont (10) . Subjects of both susceptibility hadcomparable levels of antilarval IgG; IgGfrom the sera ofthe majority (50-80%) of subjects with low susceptibility reacted with three major larval surface antigens of M, 165, 72, and37 kD. Sera from 50-80% ofthe most susceptible individuals also reacted with the 165 and 72 antigens (Ag) ; ' most of them, however, failed to react with the 37 Ag (P-37) . This indicated that the 37 Ag may represent a marker of resistance to S. mansoni and may be a target of protective immunity. We report here on the cloning ofthe cDNA for this antigen and on the homology it shares with a glycolytic