The Quantity, Quality and Two Major Effector Functions of Antibodies to VAR2CSA and their Association with Pregnancy Outcomes in a Low Malaria Transmission Area

Abstract

Background: Women produce antibodies to VAR2CSA when infected with Plasmodium falciparum during pregnancy that reduce disease severity in the current and subsequent pregnancies. In addition to antibody quantity, antibody quality (e.g., avidity) and function (e.g., inhibition of binding and opsonic phagocytosis) are immunologically important. Studies comparing the quantity, avidity and effector mechanisms of antibodies to VAR2CSA in the same group of women with pregnancy outcomes, especially in low transmission areas, are limited. Aims: The purpose of this study was to characterize antibodies to VAR2CSA using four assays, determine the correlation among the assays, and relate this to pregnancy outcome. Methods: A panel of 310 plasma samples from women in Yaoundé (a city with low malaria transmission) who had antibodies to full-length VAR2CSA were screened in assays that measured i) level of antibodies to VAR2CSA, ii) antibody avidity, iii) reduction in binding (RiB) of fluorescent VAR2CSA-coupled beads to fluorescent-CSA-coupled beads, and iv) opsonic phagocytosis using VAR2CSA-coupled beads and human THP1 cells. Results from the assays were compared with clinical information from 614 women who were Ab-negative. Results: A modest association was found among the 4 assays, i.e., as the amount of antibodies increased, a small increase in avidity, RiB and phagocytosis was observed; however, the association between RiB and avidity was poor. When results were dichotomized to above and below the median, antibody avidity, but not antibodies in the other assays, was associated with a significant reduction in prevalence of placental malaria and lower placental parasitemia. However, women who had antibody values above the median in amount (p=0.03), avidity (p=0.006), reduction in binding (p=0.018) and probably phagocytosis (p=0.065) had significantly lower placental parasitemia than women who lacked Abs to VAR2CSA. Conclusions: In this urban setting, women with the highest (above the median) antibody levels, in the four assays had a lower prevalence of placental malaria and placental parasitemia than women who lacked anti-VAR2CSA antibodies. Thus, VAR2CA-based vaccine trials in low transmission areas should consider using all four assays before and after vaccination.