Antigenic diversity and size diversity of Plasmodium falciparum antigens in isolates from Gambian patients. I. S-antigens

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Abstract
Ring-stage asexual parasites of P. falciparum were collected from six Gambian children and the S-antigens radiolabelled by 3H-glycine uptake during in vitro culture up to rupture of infected cells and merozoite release. Ouchterlony double diffusion of boiled culture supernatants against a panel of adult Gambian sera identified one S-antigen precipitin arc for five isolates and two precipitin arcs for one isolate. Five of the six isolates were serologically distinct. Analysis of S-antigens by comparison of SDS-polyacrylamide gel electrophoresis patterns of heat-treated soluble proteins revealed a more complex pattern of 3H-labelled S-antigens that was different for each isolate. There were between two and six different 3H-labelled bands for each isolate in the size range of molecular weight 137,000 to 285,000. This result confirms the large size range of S-antigens identified with culture adapted P. falciparum. Several bands were relatively weakly labelled with 3H-glycine, suggesting the natural isolates contain one or two predominant S-antigen phenotypes and several other S-antigen phenotypes expressed by minor parasite subpopulations. Immunoprecipitation was performed using a panel of sera from Gambian adults, or, acute and 3 week convalescent sera from the same patients used for S-antigen radiolabelling, Adult sera generally immunoprecipitated some of the S-antigens in each isolate, including antigens that must represent extremely minor parasite subpopulations since they could not be seen in the patterns of non-immunoprecipitated heat-stable proteins. Sera from convalescent children were generally negative on immunoprecipitation, even with the homologous isolate. In one case we observed the acquisition of specific immunoprecipitating antibody to one of the homologous S-antigens during the convalescent period. The antigenic and structural complexity of S-antigens in natural isolates that have not been submitted to the selection pressure of adaptation for in vitro culture is clearly greater than for culture adapted P. falciparum.