Immunostaining of phospho-ERK5 in HL cell lines and lymph nodes. (A) Cytospins of HL cell lines (HDLM-2, L-428, L-540) and of NHL cell line (SC-1) were stained by immunofluorescence with phospho-ERK5 antibody (magnification, × 630). In contrast to SC-1, HL cells show intensive nuclear staining. DAPI counterstaining indicates the nuclei. (B) The HL cell line KM-H2 was treated with 10 μg/mL anti-FGF2 antibody or with 200 μM tyrosine kinase inhibitor genistein and subsequently stained with anti–phospho-ERK5 (magnification, × 630). In contrast to control cells, treated cells show weak staining of nuclei indicating that (1) FGF2 signaling is mediated by ERK5 and (2) genistein inhibits this pathway. (C) Lymph nodes obtained from 3 patients with HL (HL1, HL8, HL10) stained with anti–phospho-ERK5 (magnification, × 630) show positive giant cells, indicating a constitutively active ERK5 pathway in H/RS cells.