Fig. 1

Specificity and functional activity of vaccine antibodies to the DBL5 and DBL3 domains of VAR2CSA. a The level of anti-DBL5 antibodies binding to the IE surface of different CSA-binding isolates (CS2, 7G8-CSA, 3D7-CSA, HCS3 and XIE-CSA) was measured by flow cytometry. Antibody reactivity varied depending on the recombinant proteins (derived from IT4, 7G8 or 3D7 DBL5 alleles) used for immunization. Bars represent mean and standard deviation of samples tested in duplicate; IgG binding levels are expressed as geometric mean fluorescence intensity (MFI). b Opsonic phagocytosis of P. falciparum-IEs by undifferentiated THP-1 cells using pre-immune (PI) and final bleed (FB) sera from immunized rabbits. Opsonic phagocytosis activity was significant for CS2 IEs (P = 0.024) but not for the isogenic control, non-CSA binding E8B parasite line (P = 0.74). Means and standard deviation of samples tested in duplicate are shown. Initial attachment and engulfment of CS2 IEs by THP-1 monocytes were visualized by scanning (c) and transmission electron microscopy (d). For scanning electron microscopy, IE were opsonized by a pool of immune human sera from pregnant Papua New Guinean women, or by polyclonal rabbit anti-DBL5 immune sera, while only the latter was used for transmission electron microscopy. e, f Opsonic phagocytosis of P. falciparum-IEs by undifferentiated THP-1 cells using final bleed sera from rabbits immunized with IT4 DBL5 (R7-R9). There was very little phagocytosis of 7G8-CSA (e) or HCS3 (f) parasites. A plasma pool from P. falciparum exposed pregnant women was used as a control in the phagocytosis assay. Means and standard deviation of samples tested in duplicate are shown. g, h Opsonic phagocytosis of P. falciparum-IEs by undifferentiated THP-1 cells using final bleed sera from rabbits immunized with IT4 DBL3 (R10-R12). Antibodies effectively promoted opsonic phagocytosis of CS2-IEs (g), but there was little phagocytosis of the heterologous isolate, HCS3 (h). A rabbit anti-human erythrocyte antibody was used as a positive control and the level of phagocytosis is expressed as a percentage of the positive control. Means and standard deviation of samples tested in duplicate are shown