High-level STAT3 expression and activation are preferentially associated with the BCL6-low ABC-DLBCL. (A) IL-6 production, expression, and activation of STAT3 are inversely correlated with BCL6 in DLBCL cell lines. Protein expression of BCL6, MTA3, total and phosphorylated forms of STAT1, STAT3, STAT5, and STAT6 was analyzed by Western blot. GAPDH levels were used as loading control. The COO status of the cell lines are labeled on the top. The titers of secreted IL-6 as measured by ELISA are given at the bottom of the panels. (B,C) Representative IHC staining for BCL6 and CD20/PY-STAT3 in serial sections. (B) Staining of a GCB case that is positive for BCL6 but negative for PY-STAT3. (C) Staining of a non-GCB case that has very few BCL6-positive malignant cells but is strongly positive for PY-STAT3. Note strongly positive endothelial cell nuclei as endogenous positive controls for PY-STAT3 staining. Scale bar = 1 μm (40×). (D) Distribution of BCL6 and PY-STAT3 staining in GCB and non-GCB subgroups. The staining was scored as described in the Document S1. The association between BCL6 expression and DLBCL subgroups or PY-STAT3 and the subgroups was tested by chi-square analysis.