Reciprocal regulation of IRF4 and CD30 in PTCL cells. (A) CD30 ligand (100 ng/mL) activated the NF-κB pathway in Karpas 299 cells, as evidence by depletion of IκBα, increased RelB expression (156% of control), and cleavage of p100 into p52. Karpas 299 and the other cell lines studied do not express endogenous CD30 ligand (supplemental Figure 6B). (B) Stimulation with CD30 ligand for 24 hours increased IRF4 gene expression (P = .033). (C) IRF4 ChIP demonstrated interaction of IRF4 with the TNFRSF8 promoter in Karpas 299 cells (see also supplemental Figure 6A). (D) Overexpression of IRF4 increased TNFRSF8 promoter activity (luciferase assay, P = .0029). (E) Knockdown of IRF4 decreased surface CD30 protein expression, as detected by flow cytometry. (F) IRF4-positive PTCLs had significantly higher percentages of cells staining for p52 (nuclear; P < .0001), CD30 (P < .0001), and KI-67 (P < .0001). (G) ALCL, ALK negative. IHC showed strong nuclear expression of IRF4 and p52, uniform staining for CD30, and a high proliferative rate by KI-67 (all images, ×400). (H) Angioimmunoblastic T-cell lymphoma (AITL). The tumor cells were negative for IRF4, p52, and CD30 and showed a low proliferative rate by KI-67 (×400).