A high-throughput assay to measure antibodies that block adhesion of Plasmodium falciparum infected erythrocytes to chondroitin sulfate A.

Placental malaria due to Plasmodium falciparum (Pf) is associated with adverse pregnancy outcomes. Infected erythrocytes (IEs) bind to placental chondroitin sulfate A (CSA) and antibodies that inhibit this adhesion are a potential correlate of protection. We developed a simplified adhesion inhibition assay that uses diaminofluorene to measure haemoglobin release from IEs bound to CSA. Using hyperimmune plasma, the assay demonstrated concentration-dependent inhibition of CSA adhesion. Assay performance was consistent over time with strong reproducibility (r = 0.82), and results correlated with a published assay (r = 0.53). In 466 Malawian pregnant women (321 P. falciparum-infected and 145 uninfected at first antenatal visit), adhesion inhibitory antibodies were significantly higher in mid-pregnancy in infected multigravidae (46.3 % IQR 23.2 %, 74.8 %) compared to infected primigravidae (9.7 % IQR 0 %, 29.3 %, p < 0.001) and in uninfected multigravidae (29.9 % IQR 8.6 %, 54.5 %) than uninfected primigravidae (15.6 % IQR 0 %, 36.4 %, p = 0.04). Similar, significant gravidity-dependent differences were observed at delivery in both infected and uninfected women. Between enrolment and delivery, changes in antibodies were similar in infected and uninfected women. Adhesion inhibitory antibodies protected against placental malaria. Of 162 women with infection at mid-pregnancy, 88 with no placental malaria had more adhesion inhibitory antibody (33.8 %, IQR 1.9 %, 63.2 %) than 74 with past placental malaria (12.5 %, IQR 0 %, 37.8 %; p = 0.002). This low cost, reproducible, and rapid high-throughput adhesion inhibition assay shows promise as a correlate of protection against placental malaria.