Lack of protective effect of PMNs on merozoite reinvasion. (A) Late-stage pRBCs were allowed to rupture and new infection was allowed to proceed in active human serum in the absence (i) or presence (ii) of PMNs. Twenty-four hours later, ring forms could be seen microscopically in both cases. Infected erythrocytes were detected by fluorescent staining of parasite DNA with hydroethidium-bromide (iii). Flow cytometric analyses revealed identical infection rates in the presence and absence of PMNs (right peaks: fluorescing infected cells; left peaks: nonfluorescing, noninfected cells). (B) Detection of merozoite phagocytosis mediated by Abs against P falciparum. The experiment shown in Figure 3B was repeated in the presence of 1 mg/mL of IgG isolated from a pool of 5 sera containing high-titered Abs against P falciparum. Stacked images (i-vi, top to bottom) now revealed that merozoites (blue) had been phagocytosed alongside with the DVs. Left rows: fluorescence; right rows: merged fluorescence and phase-contrast images. Arrows: phagocytosed merozoites colocalizing with DVs in the central plane (iii-iv) of a cell. Similar results were obtained with IgG from 5 individual sera with high-titered Abs against P falciparum. Scale bar indicates 20 μm. (C) Paucity of protective effects of PMNs and specific Abs upon parasite reinvasion. The ³H-hypoxanthine DNA-incorporation assay was used and values obtained in active serum alone were defined as 100%. Results obtained with 5 high-titered Abs are shown. ³H-hypoxanthine incorporation was assessed in active serum (AS) in the presence of either PMNs or IgG (Ab) or in the presence of PMNs and IgG. A small but significant reduction in ³H-hypoxanthine incorporation was observed in the presence of Abs plus PMNs compared with the control (n = 5; *P < .001).