Pf I 55/RESA is not a surface antigen of Plasmodium falciparum-infected erythrocytes

Abstract

In a recent paper, Foley et al. described the binding of the Plasmodium falciparum vaccine candidate antigen, Pf155/ RESA (ring-infected erythrocyte surface antigen), to the membrane skeleton of human erythrocytes via an association with spectrin r. These results, together with data of other recently published studies concerning Pf155/RESA 2-s, have made it necessary to reconsider the role of this antigen in the merozoite invasion process. Since the identification of Pf155/ RESA (about eight years ago) as an antigen associated with the membrane of erythrocytes infectecl with early stages of the parasite, it was evident that the antigen was not easily accessible on the erythrocyte surface 6. In order to detect the antigen at this location by immunofluorescence, the,. cells had to be modified by fixation and air drying. Furthermore, the membrane-associated antigen was not susceptible to proteolytic digestion from the outside of intact unmodified erythrocytes 6. Brown et al. showed that Pf155/RESA was not solubilized by Triton X-100 when associated with the erythrocyte membrane, indicating an interaction of the antigen with the cytoskeleton 7. Initially it was thought that Pf155/ RESA was present in the micronemes and rhoptries of the merozoites 7'8 and discharged from these organelles during the invasion process in order to somehow modify the erythroq~e membrane and facilitate invasion. However, recently it was shown that Pf155/RESA is located in the dense granules, which are distinct organelles of the merozoite apical complex 2'3. The function of the dense granules is not known, but they release their contents into the parasitophorous vacuole space shortly after merozoite entry into the erythrocyte. Pf155/RESA is then trans ocated to the inner aspect of the erythrocyte membrane by an unknown mechanism. Foley et al. have now shown that a soluble form of Pf155/RI:SA, which is present in spent medium from P. falciparum cultures, shows a specific binding