SummaryCytoadhesion of infected red blood cells (iRBC) is mediated through parasite-encoded, clonally variant surface antigens (VSA) and is a central process in the pathogenesis of Plasmodium falciparum malaria. Pregnancy-associated malaria (PAM) has been linked to VSA-mediated adhesion of iRBC to the glycosaminoglycan chondroitin sulphate A (CSA) in the placental intervillous space. Several studies have pointed to members of the PfEMP1 VSA family as mediators of CSA-specific iRBC sequestration in the placenta. Here, we report marked upregulation of a single var gene in several P. falciparum parasite isolates after selection for adhesion to CSA in vitro . The gene belongs to a highly conserved and common var gene subfamily ( var2csa ). The var2csa genes are structurally distinct from all other var genes in the parasite genome in lacking both CIDR and DBL-g g g g domains. These domains have previously been implicated in PfEMP1-mediated adhesion to CD36 and CSA. We also show that var2csa was transcribed at higher levels in three placental parasite isolates compared with transcription in parasites from peripheral blood of two children with P. falciparum malaria. This var gene thus has the properties expected of a gene encoding the parasite adhesion molecule that initiates the pathology associated with PAM.
The var gene encoded hyper-variable Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) family mediates cytoadhesion of infected erythrocytes to human endothelium. Antibodies blocking cytoadhesion are important mediators of malaria immunity acquired by endemic populations. The development of a PfEMP1 based vaccine mimicking natural acquired immunity depends on a thorough understanding of the evolved PfEMP1 diversity, balancing antigenic variation against conserved receptor binding affinities. This study redefines and reclassifies the domains of PfEMP1 from seven genomes. Analysis of domains in 399 different PfEMP1 sequences allowed identification of several novel domain classes, and a high degree of PfEMP1 domain compositional order, including conserved domain cassettes not always associated with the established group A–E division of PfEMP1. A novel iterative homology block (HB) detection method was applied, allowing identification of 628 conserved minimal PfEMP1 building blocks, describing on average 83% of a PfEMP1 sequence. Using the HBs, similarities between domain classes were determined, and Duffy binding-like (DBL) domain subclasses were found in many cases to be hybrids of major domain classes. Related to this, a recombination hotspot was uncovered between DBL subdomains S2 and S3. The VarDom server is introduced, from which information on domain classes and homology blocks can be retrieved, and new sequences can be classified. Several conserved sequence elements were found, including: (1) residues conserved in all DBL domains predicted to interact and hold together the three DBL subdomains, (2) potential integrin binding sites in DBLα domains, (3) an acylation motif conserved in group A var genes suggesting N-terminal N-myristoylation, (4) PfEMP1 inter-domain regions proposed to be elastic disordered structures, and (5) several conserved predicted phosphorylation sites. Ideally, this comprehensive categorization of PfEMP1 will provide a platform for future studies on var/PfEMP1 expression and function.
Sequestration of Plasmodium falciparum-infected erythrocytes in host blood vessels is a key triggering event in the pathogenesis of severe childhood malaria, which is responsible for about one million deaths every year 1 . Sequestration is mediated by specific interactions between members of the P. falciparum erythrocyte membrane protein 1 (PfEMP1) family and receptors on the endothelial lining 2 . Severe malaria is associated with expression of specific PfEMP1 subtypes containing domain cassettes (DC) 8 and 13 3 , but the endothelial receptor for parasites expressing these proteins was unknown 4,5 . Here, we identify endothelial protein C receptor (EPCR), which mediates cytoprotective effects of activated protein C 6 , as the endothelial receptor for DC8 and DC13 PfEMP1. We show that EPCR binding is mediated through the N-terminal cysteine-rich interdomain region (CIDRα1) of DC8 and group A PfEMP1 subfamilies and that CIDRα1 interferes with protein C binding to EPCR. This PfEMP1 adhesive property links P. falciparum Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:http://www.nature.com/authors/editorial_policies/license.html#terms † Correspondence to: thomasl@sund.ku.dk and lturner@sund.ku.dk. * These authors contributed equally to the work.Supplementary Information is linked to the online version of the paper at www.nature.com/nature.Author Contributions: LT, TL, JDS and AJB produced recombinant proteins; JF performed the protein array experiments; SSB, CWW, JEVP, MAN, MA, JSJ and JDS performed the work with malaria parasites; PM, JL and TGT organized clinical work and processed clinical samples; MKK performed the surface plasmon resonance studies; LT performed the ELISA studies. The study was conceived and planned by LT, TL and TGT. The manuscript was written by TL, TGT, LT, JDS, and MH. All authors read and commented on the manuscript. LT and TL contributed equally to the work.Author Informaton: Reprints and permissions information is available at www.nature.com/reprints.The authors have no competing financial interests. To identify the DC8-PfEMP1 receptor, we produced a full-length DC8-containing PfEMP1 using the var gene IT4var20 from the FCR3/IT4 parasite. This 288 kDa His-tagged recombinant protein (rIT4VAR20) was screened against an array of 2505 full-length human plasma membrane proteins expressed on HEK293 cells (Table S1) S3) and all found to bind brain-derived endothelial cells via EPCR (Table S3). Previous work has shown that DC8-and DC13-variants selected on brain endothelial cells also bind to non-brain microvascular endothelial cells from heart and lung 4,5 . Binding of the FCR3 IT4VAR19b parasite line (described in 4 ) to brain, heart, lung and bone marrow endothelial cells was evaluated and found to be mediated by EPCR (Table S3). Altogether, these results demonstrate cytoadhesion of DC8 PfEMP1 expressing parasites via EPCR on endothelial cells of diverse tissu...
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