AbstractAchievement of malaria elimination requires development of novel strategies interfering with parasite transmission, including targeting the parasite sexual stages (gametocytes). The formation of Plasmodium falciparum gametocytes in the human host takes several days during which immature gametocyte-infected erythrocytes (GIEs) sequester in host tissues. Only mature stage GIEs circulate in the peripheral blood, available to uptake by the Anopheles vector. Mechanisms underlying GIE sequestration and release in circulation are virtually unknown. We show here that mature GIEs are more deformable than immature stages using ektacytometry and microsphiltration methods, and that a switch in cellular deformability in the transition from immature to mature gametocytes is accompanied by the deassociation of parasite-derived STEVOR proteins from the infected erythrocyte membrane. We hypothesize that mechanical retention contributes to sequestration of immature GIEs and that regained deformability of mature gametocytes is associated with their release in the bloodstream and ability to circulate. These processes are proposed to play a key role in P falciparum gametocyte development in the host and to represent novel and unconventional targets for interfering with parasite transmission.

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Membrane-bound pyrophosphatases are homodimeric integral membrane proteins that hydrolyze pyrophosphate into orthophosphates, coupled to the active transport of protons or sodium ions across membranes. They are important in the life cycle of bacteria, archaea, plants, and parasitic protists, but no homologous proteins exist in vertebrates, making them a promising drug target. Here, we report the first nonphosphorus allosteric inhibitor of the thermophilic bacterium Thermotoga maritima membrane-bound pyrophosphatase and its bound structure together with the substrate analog imidodiphosphate. The unit cell contains two protein homodimers, each binding a single inhibitor dimer near the exit channel, creating a hydrophobic clamp that inhibits the movement of β-strand 1–2 during pumping, and thus prevents the hydrophobic gate from opening. This asymmetry of inhibitor binding with respect to each homodimer provides the first clear structural demonstration of asymmetry in the catalytic cycle of membrane-bound pyrophosphatases.

Antibodies from individuals living in areas where malaria is endemic are known to react with parasitederived erythrocyte surface proteins. The major immunogenic and clonally variant surface antigen described to date is Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP-1), which is encoded by members of the multicopy var gene family. We report here that rifin proteins (RIF proteins), belonging to the largest known family of variable infected erythrocyte surface-expressed proteins, are also naturally immunogenic. Recombinant RIF proteins were used to analyze the antibody responses of individuals living in an area of intense malaria transmission. Elevated anti-rifin antibody levels were detected in the majority of the adult population tested, whereas the prevalence of such antibodies was much lower in malaria-exposed children. Despite the high degree of diversity between rif sequences and the high gene copy number, it appears that P. falciparum infections can induce antibodies that cross-react with several variant rifin molecules in many parasite isolates in a given community, and the immune response is most likely to be stable over time in a hyperendemic area. The protein was localized by fluorescence microscopy on the membrane of ring and young trophozoite-infected erythrocytes with antibodies from human immune sera with specificities for recombinant RIF protein.Malaria is a major public health problem in many parts of the world, especially in Africa. Each year 300 to 500 million people contract the disease, and between 1 and 2 million, mostly children under the age of 5 years, die (39). Immunity to Plasmodium falciparum malaria develops when an individual acquires a broad repertoire of specific protective antibodies to polymorphic antigens that are present on the surface of the infected erythrocyte (IE) and that undergo antigenic variation (4,5,12,14,22,27). Further studies show that antibodies to a number of other monomorphic epitopes may also contribute to protection against disease (1,2,8,15,25,28). Cell-mediated immune mechanisms are also known to play a role in the development of immunity to malaria (13).To date, P. falciparum erythrocyte membrane protein 1 (PfEMP-1), encoded by ca. 50 var genes, is the best-characterized surface antigen of IEs (3,31,33). The proteins expressed in knobs are highly divergent in their amino acid sequences and clonally variant. Antibodies to PfEMP-1 in sera of adults living in endemic areas recognize specific conserved, semivariable and hypervariable regions on the protein and associations between such antibodies and protection from severe disease have been shown (4,23,36).The recent analysis of the P. falciparum genome sequencing project has identified other multicopy gene families unique to Plasmodia, the largest of which belongs to the rif (repetitive interspersed family) gene family (7,11,37). This family of genes encoding 27-to 39-kDa membrane-associated proteins is estimated to be present in the order of ca. 200 copies. Rif genes have two exons, the first of wh...

SummaryIn malaria-endemic regions, pregnant women are more susceptible to malarial infections than nonpregnant women. The main objective of this study, which was conducted in the malaria hyperendemic town of Lambaréné (Gabon, Central Africa), was to characterize Plasmodium falciparum infections in peripheral, placental and cord blood from women of different gravidities with submicroscopic infections. Using the P. falciparum merozoite surface protein 2 (MSP 2)* gene as a polymorphic marker in polymerase chain reactions, we analysed genetic diversity and multiplicity of infection in isolates from all three kinds of samples of 184 pregnant women at delivery. We detected infection in 44% of the women who were originally negative by microscopy. Equally important was the finding that the placenta had the highest prevalence of infection (P < 0.001). There was no correlation with gravidity status or age of the patients. The multiplicities of infection in the peripheral and placental blood samples did not differ and single infection was observed in cord blood, independently of the gravidity. The FC27/MSP 2 was the predominant allelic family. The major FC27 alleles detected in the peripheral, placental and cord blood were sequenced and found to be closely related to the published K1 form sequence. Below microscopy level, the placenta remains the most infected organ and this submicroscopic carriage of parasites may contribute to the development and maintenance of immunity to malaria during pregnancy.keywords MSP 2 marker, polymerase chain reaction, submicroscopic infections, multiplicity of infections

Background: Plasmodium falciparum STEVOR proteins, encoded by the multicopy stevor gene family have no known biological functions. Their expression and unique locations in different parasite life cycle stages evoke multiple functionalities. Their abundance and hypervariability support a role in antigenic variation.

Plasmodium falciparum-infected erythrocytes adhere to syncytiotrophoblast cells lining the placenta via glycosaminoglycans, such as chondroitin sulfate A (CSA) and hyaluronic acid. Adherence of infected erythrocytes to host receptors is mediated by P. falciparum erythrocyte membrane protein-1 (PfEMP-1). A single PfEMP-1 domain (duffy binding-like [DBL]-3, of the gamma sequence class) from laboratory-adapted strains is thought to be responsible for binding to CSA. In this study, DBL-gamma domains expressed by placental P. falciparum isolates were shown to have an affinity to CSA. All parasite populations accumulating in infected placentas express only 1 variant of PfEMP-1, each of which contains a DBL-gamma domain with CSA binding capacities. Furthermore, sequence analysis data provide evidence for antigenic conservation among the DBL-gamma sequences expressed by different placental parasites. This study offers a close reflection of the process of parasite adhesion in the placenta and is crucial to the understanding of the pathogenesis of malaria during pregnancy.

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