Figure 4

Transferrin-mediated delivery of iron increases the labile iron pool in Francisella -infected cells more efficiently than in uninfected cells. RAW macrophages were infected with Francisella LVS for 2 h (A) or 24 h (B) or left uninfected (control) and then loaded with Calcein-AM. The cell suspension was maintained at 37°C in a fluorometer. After stabilization of the fluorescence signal, holo-transferrin was added to the solution (t = 0) and the fluorescence signal recorded at one-second intervals. A decrease in the fluorescence indicates chelation of incoming iron with calcein, the amount of which is proportional to the slope and amplitude of the fluorescence signal. Results of triplicate measurements from triplicate experiments (n = 9) as described in A and B were analyzed for total amount of iron acquired as measured by arbitrary fluorescence units (C) and velocity of iron acquisition as measured by the change of fluorescence over time (D). Total iron and rate of iron uptake was also analyzed for macrophages whose TfR1 expression was suppressed by siRNA (siRNA TfR1 in Figure 4C and 4D). Measurements were made 24 h after transfection of uninfected macrophages (RAW264.7) with siRNA. All Values are given as means +/- 1 standard error of mean (SEM).