Protective Plasmodium knowlesi Mr 74,000 antigen in membranes of schizont-infected rhesus erythrocytes.

Abstract

The immunogenic Plasmodium knowlesi (H strain) Mr 74,000 protein in membranes of schizont-infected rhesus erythrocytes was purified on a large scale, free of other polypeptides as monitored by dodecyl sulfate polyacrylamide gel electrophoresis and isoelectric focusing. In a limited vaccination trial, four rhesus monkeys were immunized four consecutive times with the Mr 74,000 protein and Freund's complete and incomplete adjuvants. Two monkeys were injected with adjuvant only. Upon challenge with 10(4) viable P. knowlesi schizonts of the heterologous W strain, the control monkeys developed fatal parasitemias after 7 d. In contrast, the vaccinated monkeys exhibited a delayed onset of patent parasitemias and underwent self-cure on days 14 to 16 after peak parasitemias of between 7 and 11%. The protective immunity that was induced crossed different strains of P. knowlesi. Blood smears at the time of cure demonstrated limited reinfection, as indicated by the presence of normally appearing ring and trophozoite stages. The absence of schizont stages in the peripheral blood suggested a specific interruption of the erythrocytic schizogony at that stage. Immunochemical analyses of the rhesus sera revealed antibody only against the Mr 74,000 protein after the first two immunizations. Upon repeated antigen injection, antibodies reacted with components of Mr of approximately 102,000, 140,000, and 230,000 in addition to the Mr 74,000 protein. Besides immunological cross-reactivity, relatedness between all four immune-precipitated proteins was indicated by a greater than 50% tryptic peptide homology, suggesting that the Mr 230,000 component represents a precursor protein that is cleaved within the infected erythrocyte into proteins with Mr of approximately 140,000, 102,000, and 74,000.

Untitled

“…In search of a mechanistic explanation for these observations, we initially focused on analyzing the effect that parasite proteins expressed on the erythrocyte membrane might have on DC function. It is known that the development of the parasites within erythrocytes is coupled with changes in the host cells, including the host-cell plasma membranes [ 67 ], and it is well established that parasites express 'neo-proteins' on the host-cell surface, some of which are reported to induce protective immunity [ 68 - 70 ]. Erythrocytes infected with the rodent-specific strain P. chabaudi adhere to specific cell types by interacting with molecules such as CD36 [ 71 ]; this is known as cytoadherence.…”

Antibodies in malarial sera to parasite antigens in the membrane of erythrocytes infected with early asexual stages of Plasmodium falciparum.

“…The occurrence of parasite antigens on the surface of infected E in malaria is well established (16,17,(30)(31)(32)(36)(37)(38)(39). In most instances, these antigens appear to be synthesized during relatively late phases of parasite maturation and are assumed to be inserted into the E surface by the parasites from within the infected host cell (35).…”

Human-human hybridomas secreting monoclonal antibodies to the Mr 195,000 Plasmodium falciparum blood stage antigen.

“…The biological activity of our human mAb and of IgG to the Pf Mr 195,000 protein in one other study (16), together with the demonstration that this category of proteins is protective in three host-parasite systems (7,10,17) makes the Mr 195,000 component a potential candidate antigen for use as a vaccine against Pf malaria. One can expect that human mAb against this protein will become valuable in three respects: (a) the identification, characterization, and isolation of antigenic peptides of high-Mr schizont/merozoite antigens of human Plasmodium, (b) identification of portions of the Pf genome encoding these peptides, and (c) their use as therapeutic reagents for severe cases of drugresistant malaria.…”

“…[35S]methionine-labeled Pf antigens reacting with human mAb were identified by immune precipitation as previously described (7). Preabsorption of the antigen, the specific immune deposition with mAb, and fractionation of precipitated Pf antigens by SDS-PAGE was also as previously described (7).…”