Localization of CS and non-CS antigens in the sporogonic stages of Plasmodium yoelii.

Monoclonal antibodies (MAbs) and colloidal gold probes were used to localize circumsporozoite (CS) protein and two unrelated polypeptides in developing oocysts and salivary gland sporozoites of the 17X (NL) strain of Plasmodium yoelii. MAbs NYS1, NYS2, and NYS3 recognized different epitopes of the P. yoelii CS protein and produced similar patterns of immunolabelling on developing oocysts and sporozoites. A small percentage of oocysts contained developing sporoblasts and sporozoites that did not exhibit surface reactivity to MAbs NYS1, NYS2 or NYS3, although internal labelling was associated with endoplasmic reticulum (ER). These sporozoites were still capable of completing development and invading salivary glands where they could be found adjacent to sporozoites with densely labelled surface coats. If these sporozoites are infective, their presence may explain in part the failure of CS vaccines to completely protect immunized animals against challenge. The non-CS antigen recognized by MAbs NYS4 did not become abundant until late in sporogony. Some gold labelling was associated with the surface of budding and mature sporozoites, but the antigen was most abundant within the cytoplasm and micronemes. A second non-CS antigen identified by NYS5 first appeared in 7-day-old oocysts, although labelling was sparse. Small quantities of antigen appeared on the sporoblast membrane, cytoplasmic clefts and ER of oocysts and was associated with micronemes and the surface of budding and mature sporozoites. As the role played by non-CS antigens in the biology of the parasite is not yet known, further characterization of their function is needed before their potential as vaccine candidates can be determined.

Freeze-fracture studies on the sporoblast and sporozoite development in the early oocyst.

Freeze-fracturing has been used to study the formation of the triple layer pellicular complex of budding sporozoites of Plasmodium falciparum in the early oocyst. Sporozoites are formed from sporoblasts within the oocyst. The outer membrane of the sporozoites is derived from the single plasma membrane of the sporoblast while the inner 2 membranes are formed anew at the base of the differentiating sporozoites. A dense collar of intramembranous particles located on the P face of the outer membrane encircles the base of each budding sporozoite. The fact that this collar of intramembranous particles is located in the same region where the inner membranes of the sporozoites first make their appearance strongly suggests that the 2 are related, and that the collar may be related to either membrane synthesis or to membrane organization and assembly.