A Robust Phagocytosis Assay to Evaluate the Opsonic Activity of Antibodies against Plasmodium falciparum-Infected Erythrocytes.

Infection with Plasmodium falciparum parasites causes the majority of malaria-related morbidity and mortality. Constant exposure to the pathogen leads to the acquisition of antibodies and high levels of antibodies have been associated with clinical protection against malaria. A possible protective mechanism is the opsonization of parasites, or malaria-infected erythrocytes (IEs), for phagocytic clearance. Current assays use adherent or chemically differentiated THP-1 cells to evaluate opsonic antibodies in patients' samples, but these assays are often time consuming and damage the effector cells. We have developed a high throughput flow cytometry-based phagocytosis assay using undifferentiated THP-1 cells to quantify the opsonic activity against late stage P. falciparum-IEs. Opsonic antibodies bound to IEs promote their phagocytic uptake through Fcγ receptors found on THP-1 cells. Moreover, undifferentiated THP-1 cells do not express CD36, a surface scavenger receptor that promotes non-opsonic phagocytosis. This technical advance allows quantification of opsonic antibodies and is an important tool for the performance of large, population-based studies of malaria immunity, and to provide a significant increase in the statistical power for such studies.