Increased IFN-mediated STAT3 phosphorylation and proliferation in p53^−/− B-cell lines. (A) The p53 axes of TK6 (left) and NH32 cells (right) were assessed by measuring p53 and p21 levels 18 hours after irradiation. Other cells were treated with IFN-α2b (1000 U/mL) for the indicated times, and whole cell lysates were then immunoblotted with phosphotyrosine 701-specific STAT1 or phosphotyrosine 705-specific STAT3 antibodies. The pSTAT3 level relative to pSTAT1 was quantified by densitometry and shown below the blot. (B) TK6 and NH32 (5 × 10⁴ cells/mL) were cultured with or without IFN-α (1000 U/mL) for 48 hours and then counted manually in a hemocytometer. The average (+ SE) of 3 separate measurements is shown. (C) Supernatants were also collected after 48 hours and the concentrations of IL-6, IL-10, IL-8, and TNF-α were determined as described in “Cytokine measurement.”