High arginase activity promoted parasite proliferation. (A) RAW264.7 cells were either pre-stimulated with 100 ng/ml IFN-γ and 100 ng/ml LPS, or 20 ng/ml IL-4 plus 5 ng/ml IL-13, or left unstimulated for 24 h, then left uninfected (white bars) or infected with TgCtwh3 (grey bars) or TgCtWh6 (black bars) for an additional 24 h (2:1 ratio of tachyzoites to cells), and assayed for arginase activity. (B) As in (A), the cells were stained with Wright-Giemsa at 24 h after infection and parasite replication was determined by counting the number of parasites from 100 parasitophorous vacuoles by microscopy. The bars depict the number of cells containing 1–3 parasites (white bars) or ≥4 parasites (dark bars) (Fisher’s exact test was used, *p<0.05, **p<0.01, IFN-γ plus LPS vs control or IL-4 plus IL-13). (C, D, and E) The proliferation of TgCtwh3 tachyzoites in RAW264.7 cells: unstimulated (C), pre-stimulated with IFN-γ plus LPS (D), and pre-stimulated with IL-4 and IL-13 (E). Arrow represents tachyzoites. NS: no stimulation.