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

Abstract

The immunogenic P. knowlesi (H strain) MW 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, 4 rhesus monkeys were immunized 4 consecutive times with the MW 74,000 protein and Freund''s complete and incomplete adjuvants. Two monkeys were injected with adjuvant only. Upon challenge with 104 viable P. knowlesi schizonts of the heterologous W strain, the control monkeys developed fatal parasitemias after 7 days. The vaccinated monkeys exhibited a delayed onset of patent parasitemias and underwent self-cure on days 14-16 after peak parasitemias of between 7-11%. 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 interrupton of the erythrocytic schizogony at that stage. Immunochemical analyses of the rhesus sera revealed antibody only against the MW 74,000 protein after the first 2 immunizations. Upon repeated antigen injection, antibodies reacted with components of MW of .apprx. 102,000, .apprx. 140,000 and .apprx. 230,000 in addition to the MW 74,000 protein. Besides immunological cross-reactivity, relatedness between all 4 immune-precipitated proteins was indicated by a > 50% tryptic peptide homology, suggesting that the MW 230,000 component represents a precursor protein that is cleaved within the infected erythrocyte into proteins with MW of .apprx. 140,000, .apprx. 102,000 and .apprx. 74,000.