Structural and immunological correlations between the variable blocks of the VAR2CSA domain DBL6ε from two Plasmodium
Stéphane Gangnard1, Cyril Badaut, Stéphanie Ramboarina
1Unité d'Immunologie Structurale, Département de Biologie Structurale et Chimie, Institut Pasteur, 25 rue du Docteur Roux, 75724 Paris, France.
Abstract:
Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), a family of adhesins of the falciparum species of the malaria parasite, is exposed on the surface of the infected erythrocyte. In general, only one PfEMP1 variant is expressed at a time but switching between variants occurs, changing both host-cell receptor specificity and serotype. The PfEMP1 variant VAR2CSA causes sequestration of infected erythrocytes in the intervillous spaces of the placenta via the glycosaminoglycan chondroitin sulfate A. This leads to pregnancy-associated malaria, which has severe consequences for the fetus and mother. The extracellular region of VAR2CSA comprises six DBL (Duffy-binding-like) domains and a single CIDR (cysteine-rich inter-domain region) domain. The C-terminal domain DBL6ε, the most polymorphic domain of VAR2CSA, has seven regions of high variability termed variable blocks (VBs). Here we have determined the crystal structure of DBL6ε from the FCR3 parasite line and have compared it with the previously determined structure of that from the 3D7 line. We found significant differences particularly in the N-terminal region, which contains the first VB (VB1). Although DBL6ε is the most variable VAR2CSA domain, DBL6ε-FCR3 and DBL6ε-3D7 react with IgG purified from immune sera of pregnant women. Furthermore, IgG purified on one domain cross-reacts with the other, confirming the presence of cross-reactive epitopes. We also examined reactivity of immune sera to the four least variable VB (VB1, VB2, VB4 and VB5) using peptides with the consensus sequence closest, in turn, to the FCR3 or 3D7 domain. These results provide new molecular insights into immune escape by parasites expressing the VAR2CSA variant.
Insights
Malaria parasite PfEMP1 variant VAR2CSA causes pregnancy-associated malaria. Structural differences in its DBL6ε domain between parasite lines suggest immune evasion, yet cross-reactive antibodies are present in immune pregnant women.
Area of Science:
- Malaria Parasitology
- Structural Biology
- Immunology
Background:
- Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1) mediates infected erythrocyte adhesion.
- VAR2CSA, a PfEMP1 variant, binds chondroitin sulfate A in the placenta, causing pregnancy-associated malaria.
- The DBL6ε domain of VAR2CSA is highly polymorphic and contributes to immune evasion.
Purpose of the Study:
- To determine the crystal structure of the VAR2CSA DBL6ε domain from the FCR3 parasite line.
- To compare the FCR3 DBL6ε structure with the previously determined 3D7 DBL6ε structure.
- To investigate the immunogenicity and cross-reactivity of VAR2CSA DBL6ε and its variable blocks (VBs) with antibodies from pregnant women.
Main Methods:
- X-ray crystallography was used to determine the DBL6ε structure from the FCR3 parasite line.
- Structural comparison between FCR3 and 3D7 DBL6ε domains.
- Immunoassays (ELISA) were performed using IgG purified from immune pregnant women's sera against DBL6ε domains and synthetic peptides representing variable blocks.
Main Results:
- Significant structural differences were observed between FCR3 and 3D7 DBL6ε domains, particularly in the N-terminal region containing VB1.
- Both FCR3 and 3D7 DBL6ε domains reacted with IgG from immune pregnant women.
- Cross-reactivity was observed between IgG purified on one domain and the other, indicating shared epitopes. Reactivity to specific variable blocks was also assessed.
Conclusions:
- Structural variations in the VAR2CSA DBL6ε domain may contribute to parasite immune evasion strategies.
- Despite structural differences, conserved epitopes exist within DBL6ε, eliciting cross-reactive antibodies in women exposed to pregnancy-associated malaria.
- These findings provide molecular insights into the immune response against VAR2CSA and potential targets for malaria control.
