Increased STAT3 activation and its role in proliferation ofBCL-6^-/-macrophages. (A) IKK kinase assay. KA indicates kinase assay; IKKα WB, IKKα Western blot for loading control. (B) Activation status of several cell-signaling pathways assessed by Western blotting using antibodies to phosphorylated/active forms of signal transducers. We did not detect any signal for activated STAT1 (phospho-Tyr701). (C) Growth behavior of BCL-6^-/- macrophages can be corrected by retroviral-mediated expression of either a wild-type BCL-6 or a DN STAT3 gene. 3H-thymidine incorporation assay of sorted GFP⁺ cells was performed 30 hours after retroviral infection. (D) Proliferation of BCL-6^-/- macrophages as measured by MTT assays was decreased in a dose-dependent manner by AG490. Numbers on top of the graph are the P values of 2-tailed Student t tests performed on the corresponding pair of triplicate tests. Error bars represent SD. Results are representative of 2 independent experiments.