Antigenic variation of Plasmodium knowlesi malaria: identification of the variant antigen on infected erythrocytes

Erythrocytes infected with mature asexual stages of Plasmodium knowlesi express a new surface antigen such that rhesus monkey antisera specifically agglutinate these cells. Cloned parasites can express different antigenic variants of this antigen. The variant antigen has been identified by comparison of the surface membrane antigens of a clone and of an antigenic variant of that clone of different agglutination phenotype. After lactoperoxidase labeling, 125I-labeled proteins of Mrs 210,000 and 190,000 with one clone and of Mrs 205,000 with the antigenic variant were only immunoprecipitated by antisera containing homologous anti-variant antibody. Antigens of the same Mr from each clone were labeled by [35S]methionine incorporation during parasite growth and also specifically immunoprecipitated only by agglutinating antisera. Therefore, the variant antigens are malarial proteins rather than modified host proteins.

Splenic requirement for antigenic variation and expression of the variant antigen on the erythrocyte membrane in cloned Plasmodium knowlesi malaria

Variant antigens appear on the surface of Plasmodium knowlesi-infected erythrocytes as the asexual parasite matures and are detected by antibody-mediated schizont-infected cell agglutination (SICA). We now show that cloned parasites can undergo antigenic variation in nonsplenectomized monkeys. In addition, we previously described a new P. knowlesi phenotype in which uncloned parasites passaged in splenectomized monkeys were no longer agglutinable by immune sera. We have designated this new phenotype SICA[-] and the one expressing the variant antigen SICA[+]. Cloned parasites can also switch from SICA[+] to SICA[-] in splenectomized monkeys. The switch from SICA[+] to SICA[-] is a gradual process that requires sequential subpassage in several monkeys. After passage in one monkey, the agglutination titer decreased 4- to 16-fold. Decreased agglutination was associated with decreased antibody binding on all infected erythrocytes as measured by fluorescein-conjugated anti-rhesus monkey immunoglobulin. The asexual malaria parasite can therefore alter its expression of variant antigen in response to the host environment (antivariant antibody or splenectomy). When cloned SICA[-] parasites were inoculated into intact monkeys, two courses of parasitemia were observed: fulminant parasitemia (greater than 20%) and parasitemia that was controlled. Fulminant infections were associated with conversion of the parasite from SICA[-] to SICA[+], i.e., from nonexpression to expression of the variant antigen on the erythrocyte surface. Parasitized erythrocytes remained SICA[-] in those infections that were controlled. It appears, therefore, that the expression of the variant antigen on the erythrocyte surface may influence parasite virulence.

Altered expression of Plasmodium knowlesi variant antigen on the erythrocyte membrane in splenectomized rhesus monkeys

Variant antigens are present on the surface of Plasmodium knowlesi malaria-infected erythrocytes as detected by the schizont-infected cell agglutination (SICA) assay. We found that parasitized erythrocytes passaged in splenectomized monkeys did not agglutinate with immune sera. On the first passage from intact to splenectomized monkeys, the SICA titers decreased 4- to 16-fold; after the second and subsequent passages in splenectomized monkeys, the infected cells became nonagglutinable to all sera tested, including sera from animals infected with the nonagglutinating parasites. This loss of agglutinability could have resulted from selection of a genetically distinct subpopulation of the original parasites or the ability of the original parasites to alter their phenotypic expression. We have designated the new nonagglutinable phenotype, SICA [-], and the agglutinable phenotype, SICA [+]. The loss of agglutinability indicates that the variant antigen normally expressed on the erythrocyte membrane of infected cells is altered or absent. Because SICA [-] parasites developed in the absence of the spleen, the major organ of host defense against malaria, then this organ may in some manner influence or modulate antigenic expression in P. knowlesi and possibly other malaria parasites.

Altered expression of Plasmodium knowlesi variant antigen on the erythrocyte membrane in splenectomized rhesus monkeys

Variant antigens are present on the surface of Plasmodium knowlesi malaria-infected erythrocytes as detected by the schizont-infected cell agglutination (SICA) assay. We found that parasitized erythrocytes passaged in splenectomized monkeys did not agglutinate with immune sera. On the first passage from intact to splenectomized monkeys, the SICA titers decreased 4- to 16-fold; after the second and subsequent passages in splenectomized monkeys, the infected cells became nonagglutinable to all sera tested, including sera from animals infected with the nonagglutinating parasites. This loss of agglutinability could have resulted from selection of a genetically distinct subpopulation of the original parasites or the ability of the original parasites to alter their phenotypic expression. We have designated the new nonagglutinable phenotype, SICA [-], and the agglutinable phenotype, SICA [+]. The loss of agglutinability indicates that the variant antigen normally expressed on the erythrocyte membrane of infected cells is altered or absent. Because SICA [-] parasites developed in the absence of the spleen, the major organ of host defense against malaria, then this organ may in some manner influence or modulate antigenic expression in P. knowlesi and possibly other malaria parasites.

Antibodies to canine helminth parasites in asthmatic and nonasthmatic children

We studied the prevalence of IgE and IgG antibodies to Toxocara canis and Dirofilaria immitis, helminth parasites of dogs, in asthmatic and nonasthmatic children born and raised in Hawaii. The serologic pattern of the two groups was found to differ significantly. Of the 80 asthmatic subjects, 45.0% had IgE antibody and 17.5% IgG-precipitating antibody to one or both parasites. In contrast, of the 96 nonasthmatic children studied 7.5% were positive for IgE antibody and 31.3% positive for IgG antibody.

Effect of selenium and dimethyl dioctadecyl ammonium bromide on the vaccine-induced immunity of Swiss-Webster mice against malaria (Plasmodium berghei)

The results of the study described in this paper demonstrate that selenium, administered in drinking water, potentiates the protective effect of a killed Plasmodium berghei vaccine for Swiss-Webster mice. We also report that a vaccine consisting of P. berghei antigen combined with the adjuvant dimethyl dioctadecyl ammonium bromide conferred a significantly high level of protective immunity. An additive effect was shown in that the greatest degree of protection was afforded to the group of mice maintained on selenium and vaccinated with antigen-dimethyl dioctadecyl ammonium bromide. Almost all of the animals treated in this manner survived the challenging infection, the course of which was typically of a transitory parasitemia not exceeding 10% at the peak.

Rapid decrease of precipitating and reaginic antibodies in Dirofilaria immitis-infected dogs which develop severe adverse reactions following treatment with diethylcarbamazine

Dirofilaria immitis-infected dogs were treated with diethylcarbamazine. Measurement of the level of IgG precipitating antibody was carried out by counterimmunoelectrophoresis and that of IgE by passive cutaneous anaphylaxis in pre-treatment and post-treatment serum samples. In dogs showing severe to moderate adverse reactions to the drug there were pronounced decreases of both antibodies to a "metabolic" antigen prepared from the medium in which adult female worms had been incubated. These antibody levels did not decline in non-reacting dogs. Precipitating antibody levels to somatic antigens extracted from adult worms and microfilariae remained unaltered in both reactive and non-reactive animals.

Studies on parasitic crisis in malaria: I. Signs of impending crisis in Plasmodium berghei infections of the white rat

White rats infected with Plasmodiam berghei were observed throughout the course of infection for indicators of impending crisis (penultimate stage). Two ratios, percentage young compact forms:percentage trophozoites and percentage reticulocytes:percentage parasitaemia, were reliable markers of the penultimate period and impending crisis. When the percentage young compact form:percentage trophozoite ratio exceeded 1.0 and was coincident with an increase of uninfected reticulocytes in the circulation (as given by a percentage reticulocyte:percentage parasitaemia ratio greater than 1.0) the parasitaemic decline of crisis consistently occurred 24-48 hours later.