1530 Biosynthesis and Processing of a Malaria Schizont

Protective immunizat ion of the vertebrate host against malar ia can operate against either of two distinct stages of the parasite's life cycle. The sporozoite forms, which develop in the salivary glands of the infected mosquito, are immunogenic in the vertebrate host (1) and possess a major surface antigen against which a protective immune response can be elicited (2, 3). The asexual blood stage, which is responsible for the clinical symptoms of malar ia as parasites invade and destroy the host's erythrocytes, is also susceptible to immune intervention. In man, acquired immuni ty against the blood stage of Plasmodiumfalciparum infection is mediated at least part ly by ant ibody (4). Immuniza t ion studies using killed parasites have confirmed that protective antigens are associated with the asexual blood forms (5), and immunochemica l analyses have shown that these forms are antigenically complex (6, 7). Monoclonal antibodies that agglutinate merozoites (8) or inhibit the growth of malar ia parasites in vitro (9) or in vivo (10) have been used to define specific antigens against which the host may make a protective immune response. We have recently shown (11) that two protein antigens, of 235,000 and 230,000 mol wt, associated with schizonts and merozoites of the rodent malar ia parasite, Plasmodiurnyoelii, can be purified using monoclonal antibodies and used successfully to immunize mice against challenge infection. In the present study we have characterized a 195,000-mol wt protein antigen associated with schizonts and merozoites of P. falciparum, which is one of the major antigens recognized by h u m a n immune serum. Using a monoclonal ant ibody specific for this protein, we have investigated its biosynthesis during synchronous development of the parasite in vitro. This has revealed a specific processing of the protein that coincides with schizont maturat ion. On the basis of its localization, size, and specific processing, we suggest that this protein may be analogous to one of the protective antigens of P. yoelii.