Rhesus monkeys protected against Plasmodium knowlesi malaria produce antibodies against a 65,000-MrP. knowlesi glycoprotein at the surface of infected erythrocytes

Plasmodium knowlesi: a malaria parasite of monkeys and humans

Plasmodium knowlesi is a malaria parasite of monkeys of Southeast Asia that is transmitted by mosquitoes of the Anopheles leucosphyrus group. Humans are frequently infected with this parasite and misdiagnosed as being infected with Plasmodium malariae. The parasite was a major monkey animal model for developing antimalarial vaccines and investigations of the biology of parasite invasion. P. knowlesi is the first monkey malaria parasite genome to be sequenced and annotated.

Cryptic disposition of antigenic parasite proteins in plasma membranes of erythrocytes infected with Plasmodium chabaudi

Plasma membranes of Plasmodium chabaudi-infected erythrocytes contain seven major neoproteins with apparent molecular masses of 154, 145, 90, 72, 67, 52, and 33 kDa, respectively. These neoproteins, with the exception of the two larger ones, can be metabolically labelled with [14C]isoleucine. The seven neoproteins are antigenic as revealed by Western blotting using hyperimmune sera obtained from two different mouse strains. None of the parasite proteins is accessible from the outside in intact P. chabaudi-infected erythrocytes as determined by lactoperoxidase-mediated radioiodination, indirect immune fluorescence microscopy, or post-embedding immunoelectron microscopy. These methods, however, identify parasite proteins in host cell plasma membranes when the latter are artificially changed either during isolation or by methanol fixation. We conclude therefore that parasitic proteins are cryptically arranged in intact host cell plasma membranes of P. chaubaudi-infected erythrocytes.

Membrane orientation and antigenic peptides of an immunoprotective 74 kDa Plasmodium knowlesi glycoprotein

Vaccination trials have shown that a purified, 74 kDa glycoprotein, GP74, isolated from the host cell membrane of Plasmodium knowlesi-infected rhesus erythrocytes, can provide protective immunity against P. knowlesi malaria. We have extended this work by a tryptic peptide analysis of the disposition of GP74 in the host cell membrane. Of the 18 peptides characterized by high-performance liquid chromatography only four were accessible to lactoperoxidase-catalyzed radioiodination of non-leaky, schizont-infected host cells from the extracellular space. Metabolic labeling with radioactive glucosamine indicates that two of the surface exposed peptides are glycopeptides, and one of these, peptide 12 appears to carry a dominant antigenic site, according to its reactivity with immunoglobulin from sera of monkeys protected against P. knowlesi malaria.

Membrane structure and function of malaria parasites and the infected erythrocyte

According to the World Health Organization the global estimate of malaria is over 200 million infections, the majority of which are caused by the most life-threatening species,Plasmodium falciparum(Report of the Steering Committees of the Scientific Working Groups on Malaria, World Health Organization, June 1983). The causative agent of the disease, the malarial parasite, requires two hosts: a blood-sucking mosquito and a blood-containing vertebrate. Commonly, infection of the vertebrate begins when an infected mosquito bites a suitable vertebrate and injects minute sporozoites into the bloodstream. Within 30 mm the introduced sporozoites leave the bloodstream and enter parenchymal cells of the liver (mammals) or endothelial cells (birds). In these sites the parasite undergoes asexual multiplication (= exo-erythrocytic schizogony) producing daughter progeny called merozoites. The exo-erythrocytic merozoites are released from the tissues into the circulation where they invade red blood cells. Within an erythrocyte the merozoite undergoes asexual multiplication (= erythrocytic schizogony) producing a substantial number of merozoites. The erythrocyte lyses, merozoites are released, and invasion of another erythrocyte may then take place. The synchronous rupture of the red cell and merozoite release is marked by the periodic fever–chill cycles so characteristic of the malarial infection. Some merozoites continue to reinvade other erythrocytes and multiply by asexual means, whereas others enter erythrocytes and differentiate into sexual stages, male or female gametocytes. When a suitable mosquito feeds on an infected vertebrate gametocytes are ingested and the sexual cycle of development is initiated. In the mosquito stomach the gametocytes transform into gametes, fertilization takes place, the resultant worm-like zygote penetrates the cells of the mosquito gut and comes to lie on the outer surface of the stomach. Here each zygote forms a cyst-like body, the oocyst, within which thousands of sporozoites are produced by asexual multiplication. When the swollen oocysts burst, sporozoites are freed and these make their way to the salivary gland. At the next blood feeding the mosquito injects the infective sporozoites and the life-cycle is completed.

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

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.

Alkali-extraction of membranes from mouse erythrocytes infected with Plasmodium berghei

Alkali-extracted membrane material from hypotonically lysed Plasmodium berghei-infected mouse erythrocytes has been analyzed by dodecyl sulphate polyacrylamide gel electrophoresis and fractionated by discontinuous sucrose gradient centrifugation. Components characteristic for the protein pattern associated with P. berghei infection have been recovered in the alkali-extracted material. Parasite components, free of host cell membrane contamination, have been obtained as a high-density fraction from sucrose gradients. This fraction is characterized by polypeptides with apparent molecular weight of 71,000 and 52,000.

Maturation of the intracellular parasite and antigenicity

The protein antigens synthesized by the malarial parasite change as the parasite matures, with a number of proteins showing strict stage-specificity. A detailed correlation between the stage-specificity of protein synthesis and parasite structure has yet to be established, but a number of proteins synthesized in the cycle are lost selectively during merozoite escape and reinvasion. These antigens are presumably associated with structures utilized and ultimately lost during this process. Particular interest has focused on some greater than 200K proteins identified as being on the surface of infected erythrocytes and internally and on the surface of merozoites. Smaller parasite proteins have also been identified in the erythrocyte membrane. The erythrocyte itself, including its membrane, is much modified by parasite growth. Changes include the presence of new cytoplasmic structures and differences in the surface labelling and isoantigenic characteristics of the surface membrane. An appreciation of the variability and specificity of exposed parasite antigens, and their relationship to newly exposed isoantigens, is central to our understanding of protective immunity to malaria.