Figure 1.

Human antibodies to Plasmodium vivax Duffy binding protein (PvDBP) recognize the Plasmodium falciparum surface antigen VAR2CSA. A, Sera from different populations in Colombia were tested for reactivity to full-length VAR2CSA (based on detection of the FCR3 allele) by enzyme-linked immunosorbent assay (ELISA; dilution, 1:1000). Values are expressed as mean AUs (±SD) relative to a positive control on each plate. A cutoff (stippled line) was determined for each antigen, based on the mean ODs (+2 SDs) for individual sera from the Canadian population. The percentage of samples with an AU above the cutoff is indicated for each population. AUs of the sera from the malaria-exposed groups in the clinic and community were significantly different from that for the unexposed group from Medellín, using a Dunn multiple comparisons test (P < .0001, for all comparisons between groups). B, VAR2CSA reactivity in sera positive against P. vivax MSP1 (PvMSP1) and negative against P. falciparum MSP1 (PfMSP1). Samples were collected from individuals in Colombia and Brazil and were analyzed against VAR2CSA as described in panel A. C, Colombian sera from panel B that were seropositive against VAR2CSA and with P. vivax exposure were pooled, and immunoglobulin G (IgG) was purified. Total IgG was tested in the inhibition of binding assay against a placental isolate that was adapted to in vitro culture. Total IgG was purified from the pooled sera of unexposed Colombians as control IgG. Results are expressed as the number of parasites bound to chondroitin sulfate A (CSA) from triplicates of a representative experiment. D, PvDBP antibodies underwent affinity purification from a pool of sera from Colombian men and children previously exposed to P. vivax and titrated against VAR2CSA by ELISA, with an initial IgG concentration of 12 μg/mL (1:10 dilution on the x-axis). IgG from unexposed Colombians served as the control. Data are mean ODs (±SD).