Plasmodium knowlesi variant antigens are found on schizont-infected erythrocytes but not on merozoites

In this report, we demonstrate that the variable antigens present on the surface of Plasmodium knowlesi-infected erythrocytes could not be found on the surface of merozoites. A number of technical problems had to be solved to make such a comparative study possible, including the purification of merozoites by affinity columns and gradient centrifugation, the use of hyperimmune rabbit sera instead of monkey sera, and the use of immunocytological methods (indirect immunofluorescence antibody test and electron microscopy with ferritin-labeled antibodies) instead of the schizont-infected cell agglutination test. To ensure that these new techniques were valid for variant-specific serotyping, we compared two well-characterized variant populations with both the standard and the new methods. The removal of variant antigens from the surface of infected erythrocytes by proteolytic enzymes provides further information on the biochemical nature of these antigens.