Opsonized but not unopsonized CS2 IE are ingested by IFN-γ primed and unprimed MDM. (A) MDM were cultured for seven days in 96-well plates at 50,000 per well then exposed to human erythrocytes (●), human erythrocytes opsonized with rabbit anti-erythrocyte antibody (○), purified CS2 IE (▲) or purified CS2 IE opsonized with immune serum (■) at the indicated target to cell ratios. After 2 hr, the number of ingested erythrocytes was determined using phagocytosis assay as described in Methods and expressed as a phagocytic index (ingested erythrocytes per 100 MDM). Values represent mean ± SEM of triplicate determinations from a single donor monocyte preparation. (B) MDM were cultured as in (A) but incubated with (+) or without (−) 100 ng/ml IFN-γ between day 5 and day 7. MDM were then exposed to targets (20:1 target to cell ratio) for 1 hr and phagocytic indices determined. E: human erythrocytes, IE: CS2 trophozoites, IE-PPS: CS2 trophozoites opsonized with PPS, IE-IgG: CS2 trophozoites opsonized with rabbit anti-human erythrocyte IgG. Data represent mean ± SEM of quadruplicate determinations from a single donor monocyte preparation. Experiment is representative of two independent experiments.