FIG. 2.

Assessment of the CSPG binding strengths of IRBCs at different stages of parasite development. The strength of IRBC binding to human placental CSPG was determined by measuring the dose-dependent inhibition of binding by soluble C4S containing 36% 4-sulfate. 3D7-CSA and FCR3-CSA IRBCs with ∼25% parasitemia were analyzed. Inhibition assay was performed at 14, 18, 22, 26, 30, 34, 38, 42, and 46 h after parasite invasion. The results for IRBCs at 22, 30, and 38 h postinvasion are not presented in the figure. The number of IRBCs bound per mm² with control untreated samples was in the range of 2,500 to 6,500. At each developmental stage, the plotted values are percent IRBCs bound per mm² compared to samples not treated with C4S and are means from three independent experiments done in duplicate; vertical bars indicate standard errors. For both 3D7-CSA and FCR3-CSA parasites, the IC₅₀ (binding strength) gradually decreased during trophozoite maturation. Shown are the results for 3D7-CSA parasites; similar stage-specific differences in IRBC binding were also observed for FCR3-CSA parasites (not shown).