FIG 4.

IFN-γ activation of macrophages kills intracellular bacteria. RAW 264.7 macrophages were preactivated with IFN-γ (10 ng/ml) for 18 h prior to infection with B. thailandensis. At the indicated times, lysates were plated to enumerate surviving intracellular bacteria. (A) Time course of intracellular killing due to preactivation with IFN-γ prior to infection. The difference in intracellular bacterial numbers at time t = 0 between untreated and preactivated macrophages was likely due to the time lapse between the initial start of infection and the end of the kanamycin treatment step. Significant differences compared to the untreated control were assessed at each time point by two-way ANOVA (***, P < 0.001). (B) Intracellular bacterial burden after 6 h of infection in macrophages pretreated with IFN-γ (as before) or with ceftazidime (10 μg/ml). Statistical differences were assessed by one-way ANOVA, a > b (P < 0.0001). (C) NAC (50 mM) or GSH (50 mM) were applied to macrophages for the last 3 h of the pretreatment regimen. Then all treatments were washed off and macrophages were infected with B. thailandensis. Intracellular bacterial burden was assessed 6 h after the end of the kanamycin step by plating lysates, a > b > c > d (P < 0.05). Data are representative of results from two independent experiments run in triplicate.