Analysis of IgG with specificity for variant surface antigens expressed by placental Plasmodium falciparum isolates

BACKGROUND: Pregnancy-associated malaria (PAM) is caused by Plasmodium falciparum-infected erythrocytes that can sequester in placental intervillous space by expressing particular variant surface antigens (VSA) that can mediate adhesion to chondroitin sulfate A (CSA) in vitro. IgG antibodies with specificity for the VSA expressed by these parasites (VSA(PAM)) are associated with protection from maternal anaemia, prematurity and low birth weight, which is the greatest risk factor for death in the first month of life. METHODS: In this study, the development of anti-VSA(PAM )antibodies in a group of 151 women who presented to the maternity ward of Albert Schweitzer Hospital in Lambaréné, Gabon for delivery was analysed using flow cytometry assays. Plasma samples from placenta infected primiparous women were also investigated for their capacity to inhibit parasite binding to CSA in vitro. RESULTS: In the study cohort, primiparous as well as secundiparous women had the greatest risk of infection at delivery as well as during pregnancy. Primiparous women with infected placentas at delivery showed higher levels of VSA(PAM)-specific IgG compared to women who had no malaria infections at delivery. Placental isolates of Gabonese and Senegalese origin tested on plasma samples from Gabon showed parity dependency and gender specificity patterns. There was a significant correlation of plasma reactivity as measured by flow cytometry between different placental isolates. In the plasma of infected primiparous women, VSA(PAM)-specific IgG measured by flow cytometry could be correlated with anti-adhesion antibodies measured by the inhibition of CSA binding. CONCLUSION: Recognition of placental parasites shows a parity- and sex- dependent pattern, like that previously observed in laboratory strains selected to bind to CSA. Placental infections at delivery in primiparous women appear to be sufficient to induce functional antibodies which can both recognize the surface of the infected erythrocytes as well as block their binding to CSA. The correlation between serum reactivities of placental field isolates from different geographic locations and collected at different times is indicative of the conserved nature of the antigen(s) mediating PAM

Plasmodium falciparum variant STEVOR antigens are expressed in merozoites and possibly associated with erythrocyte invasion

<p>Abstract</p> <p>Background</p> <p><it>Plasmodium falciparum </it>STEVOR proteins, encoded by the multicopy <it>stevor </it>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.</p> <p>Methods</p> <p>Immunoblotting of total parasite proteins with an anti-STEVOR antibody was used to identify variant antigens of this gene family and to follow changes in STEVOR expression in parasite populations panned on CSA or CD36 receptors. Immunofluorescence assays and immunoelectron microscopy were performed to study the subcellular localization of STEVOR proteins in different parasite stages. The capacity of the antibody to inhibit merozoite invasion of erythrocytes was assessed to determine whether STEVOR variants were involved in the invasion process.</p> <p>Results</p> <p>Antigenic variation of STEVORs at the protein level was observed in blood stage parasites. STEVOR variants were found to be present on the merozoite surface and in rhoptries. An insight into a participation in erythrocyte invasion was gained through an immunofluorescence analysis of a sequence of thin slides representing progressive steps in erythrocyte invasion. An interesting feature of the staining pattern was what appeared to be the release of STEVORs around the invading merozoites. Because the anti-STEVOR antibody did not inhibit invasion, the role of STEVORs in this process remains unknown.</p> <p>Conclusion</p> <p>The localization of STEVOR proteins to the merozoite surface and the rhoptries together with its prevalence as a released component in the invading merozoite suggest a role of these antigens in adhesion and/or immune evasion in the erythrocyte invasion process. These observations would also justify STEVORs for undergoing antigenic variation. Even though a role in erythrocyte invasion remains speculative, an association of members of the STEVOR protein family with invasion-related events has been shown.</p

Recognition of Variant Rifin Antigens by Human Antibodies Induced during Natural Plasmodium falciparum Infections

Antibodies from individuals living in areas where malaria is endemic are known to react with parasite-derived 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