Serological Reactivity of in Vitro Cultured Exoerythrocytic Stages of Plasmodium Berghei in Indirect Immunofluorescent or Immunoperoxidase Antibody Tests

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Abstract
Exoerythrocytic (EE) stages of Plasmodium berghei were cultivated in vitro in WI38 cells inoculated with sporozoites, and examined for serological reactivity by indirect immunofluorescent or immunoperoxidase tests. At 24 hours post-inoculation, sporozoite and red blood cell (RBC) stage antigens were equally distributed, but by 48 hours, RBC stage antigens predominated. Merozoites, produced by 72 hours, reacted strongly with anti-RBC stage sera, but were weakly reactive with anti-sporozoite sera. Hybridoma-produced monoclonal antibodies to the surface protective antigen of P. berghei sporozoites also reacted with 24- and 48-hour EE parasites, and with EE merozoites, suggesting that the sporozoite-protective antigen may also be synthesized by the EE stage. Extra-EE parasite antigens associated with the host cell nucleus were detected as early as 24 hours using anti-sporozoite and anti-RBC stage sera, but did not contain the sporozoite-protective antigen.