Sixty-six kilodalton-related antigens of Plasmodium knowlesi are merozoite surface antigens associated with the apical prominence.

We have previously reported that Fab fragments of a monoclonal antibody to the 66 kD-related antigens of Plasmodium knowlesi inhibit merozoite invasion of erythrocytes, strongly suggesting a role for these antigens as merozoite receptors for red cells. In this paper we have examined the distribution of these antigens on the surface of free merozoites. Rapid immunofluorescence microscopy on free, unfixed cells demonstrated a preferential association of antigen with a polar region of merozoites. This was confirmed, and localized essentially to the apical region, by immunoelectron microscopy under a variety of fixation regimes. Metabolic inhibitors did not affect the distribution, suggesting that apical localization is not due to capping induced by cross-linking antibodies. These observations suggest that 66 kD-related antigens may play a role in the orientation of the apical prominence of merozoites towards the red cell, and/or in the junction formation that occurs subsequent to orientation.