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

Abstract

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...

Secreted Antibody Is Required for Immunity toPlasmodium berghei

“…Variant antigen families in Plasmodium species have been shown to be immunogenic (1,2,14,16), and antibodies produced against them are variant specific and thought to play a role in protection (31,36). One example of a known variant antigen family in P. berghei is bir, a homologue of the major variant antigen family in malaria.…”

Immunoglobulin G Isotype Responses to Variant Surface Antigens ofPlasmodium falciparumin Healthy Gabonese Adults and Children during and after Successive Malaria Attacks

“…Such associations have been reported in the context of responses to VSA expressed by both autologous and heterologous parasite isolates, which may contribute to the putatively rapid acquisition of immunity to malaria (24). P. falciparum erythrocyte membrane protein-1 (PfEMP-1) is considered to be the principal target of anti-VSA antibodies, although rifin proteins, a second, polymorphic, parasite-derived family of antigens that are inserted into the infected erythrocyte membrane, also induce antibody responses in exposed populations (2,7,31,32,42). Studies incorporating longitudinal components have shown that, in most but not all cases, antibodies with specificity for the VSA expressed by the autologous parasites causing a given malaria attack are infrequent or absent prior to the attack but are enhanced and sustained posttreatment (10,13,21,26,37,46).…”

Antibodies to Plasmodium falciparum Rifin Proteins Are Associated with Rapid Parasite Clearance and Asymptomatic Infections

“…The recombinant rifins used in this study were purified on affinity columns to 95% homogeneity, and plasma immunoglobulin G antibody responses to the purified His 6 -Rif proteins were measured by a standard enzyme-linked immunosorbent assay by using previously established procedures and conditions (1). Blank values were determined in parallel with each individual test serum with no antigen in the well.…”

“…A large cross-sectional survey of individuals exposed to natural P. falciparum infections was carried out recently to evaluate the presence in their sera of specific antirifin antibodies capable of recognizing recombinant rifin proteins. The most significant finding was the high frequency of recognition by malaria-exposed immune adults whereas semi-immune children were less reactive (1). In this study, serum samples available from a longitudinal study were analyzed for specific antirifin antibodies to find whether rifin antibodies play a role in natural immunity to malaria.…”