Sequestration of Plasmodium falciparum–infected erythrocytes to chondroitin sulfate A, a receptor for maternal malaria: monoclonal antibodies against the native parasite ligand reveal pan-reactive epitopes in placental isolates

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Abstract
Plasmodium falciparum parasites express variant adhesion molecules on the surface of infected erythrocytes (IEs), which act as targets for natural protection. Recently it was shown that IE sequestration in the placenta is mediated by binding to chondroitin sulfate A via the duffy binding-like (DBL)–γ3 domain ofP falciparum erythrocyte membrane protein 1 (PfEMP1CSA). Conventional immunization procedures rarely result in the successful production of monoclonal antibodies (mAbs) against such conformational vaccine candidates. Here, we show that this difficulty can be overcome by rendering Balb/c mice B cells tolerant to the surface of human erythrocytes or Chinese hamster ovary (CHO) cells before injecting P falciparum IEs or transfected CHO cells expressing the chondroitin sulfate A (CSA)–binding domain (DBL-γ3) of the FCR3varCSA gene. We fused spleen cells with P3U1 cells and obtained between 20% and 60% mAbs that specifically label the surface of mature infected erythrocytes of the CSA phenotype (mIECSA) but not of other adhesive phenotypes. Surprisingly, 70.8% of the 43 mAbs analyzed in this work were IgM. All mAbs immunoprecipitated PfEMP1CSA from extracts of125I surface-labeled IECSA. Several mAbs bound efficiently to the surface of CSA-binding parasites from different geographic areas and to placental isolates from West Africa. The cross-reactive mAbs are directed against the DBL-γ3CSA, demonstrating that this domain, which mediates CSA binding, is able to induce a pan-reactive immune response. This work is an important step toward the development of a DBL-γ3–based vaccine that could protect pregnant women from pathogenesis.