Identification of a Plasmodium chabaudi antigen present in the membrane of ring stage infected erythrocytes.

A novel antigen of asexual blood stages of the rodent malaria parasite Plasmodium chabaudi, was detected by means of a modified indirect immunofluorescence assay (IFA), using glutaraldehyde fixed and air dried monolayers of P. chabaudi infected erythrocytes. P. chabaudi hyperimmune sera gave a distinct surface immunofluorescence of erythrocytes infected with early stages of the parasite. Fixation and drying of the erythrocytes was necessary for the antigenic activity to be exposed. The antigens were species specific as P. chabaudi hyperimmune serum only stained P. chabaudi but not P. yoelii or P. falciparum infected erythrocytes. The antigenic activity involved in the IFA was resistant to trypsin, phospholipases and neuraminidase but not to pronase, suggesting that the antigens were polypeptides. The surface immunofluorescence was inhibited by an extract of parasitized erythrocytes, but not by similar extracts of normal erythrocytes. The inhibitory antigens were soluble and heat stable (100 degrees C, 5 min). For identification and characterization of the antigens, antibodies were isolated by acid elution from monolayers of infected erythrocytes and monoclonal antibodies were produced. Probing in immunoblotting of extracts of parasitized erythrocytes separated by sodium dodecyl sulfate polyacrylamide gel electrophoresis with the eluted antibodies, showed that they reacted consistently with a polypeptide of Mr 105 000 (Pch105). The Pch105 antigen shares many characteristics with Pf155, a P. falciparum antigen considered as a candidate for a vaccine against that parasite.