Localization of circumsporozoite protein of Plasmodium ovale in midgut oocysts.

Circumsporozoite (CS) proteins are the major proteins found on the surface of salivary gland sporozoites and are the protective antigens of several species of malaria parasites. Little is known about the distribution of CS proteins in developing oocysts, however. Immunoelectron microscopy with protein A-gold and a monoclonal antibody specific for the CS protein of Plasmodium ovale was performed to investigate the distribution of CS protein within developing P. ovale oocysts. There was an almost complete absence of label in immature oocysts prior to the development of sporoblasts. In contrast, sporoblasts and budding and free sporozoites in mature oocysts were labeled uniformly on the outer surfaces of their plasma membranes, indicating a uniform distribution of CS protein on these membranes. Gold particles were frequently associated with the cytoplasm of sporoblasts and sporozoites, as well as with the inner surface of the oocyst capsule. This is the first evidence that CS protein is present in oocyst sporozoites and sporoblasts of P. ovale.