FIG. 1.
Infection of host cells is parasite dose dependent. Tachyzoites were added to monocytes, neutrophils, and lymphocytes (5 × 105/500 μl in 24-well tissue culture plates) or in vitro-cultured dendritic cells (27) (1.4 × 105/140 μl in 96-well tissue culture plates) at various cell-to-tachyzoite ratios. Triplicate cytospin preparations (100,000 cells centrifuged for 5 min at 700 rpm using a Shandon Cytospin 3) of each infected cell type were made 2 and 24 h postinfection. Cells were fixed and stained with Diff-Quik, and 200 to 300 cells were counted per slide. For each cell the number of tachyzoites per vacuole was determined. Cells with multiple vacuoles were scored for the highest number of tachyzoites per single vacuole (i.e., as if they contained a single vacuole), and the cell-to-tachyzoite ratios where multiple vacuoles occurred were recorded. Results from 24 h postinfection are reported as the mean + standard deviations of triplicate samples for lymphocytes, neutrophils, and monocytes and as the mean of duplicate samples for dendritic cells. Results are representative of two donors.