ICOS cross-linking induced increase in proliferation of ALK⁺ TCL cells. (A) Lack of impact of siRNA-mediated ICOS depletion (left panel) on growth of SUDHL-1 cells as detected by MTT enzymatic conversion assay (middle panel) and IL-10 synthesis as determined by enzyme immunoassay (right panel). (B) Effect of beads-coupled ICOS antibody on BrDU uptake-detected proliferative rate of ALK⁺ TCL cells (left panel) and enzyme immunoassay–detected IL-10 synthesis (right panel) with uncoated beads serving as negative control in both assays. (C) Expression of ICOS-L protein as determined by flow cytometry in the transformed T- and B-cell lines and control cell populations. (Top panel) Cell lines derived from CTCL (Sez-4, SeAx, MyLa2059, and MyLa3675) and cutaneous CD30⁺ lymphoproliferative disorder (PB-1 and 2B). (Middle panel) ALK⁺ TCL cell lines. (Bottom panel) B-cell lines from the germinal center–derived diffuse large B-cell lymphoma (Ly18 and VAL). (Bottom panel) Cell lines from the Burkitt lymphoma (Ramos), EBV-mediated transformation (MM and HH), and Kaposi sarcoma virus-associated body cavity lymphoma (JSC-1). (D) Effect of coculture of ALK⁺ TCL SUDHL-1 cells with irradiated ICOS-L⁺ Ramos B cells on growth of SUDHL-1 cells as determined by the MTT enzymatic conversion assay. (E) Effect of coculture of CFSE-labeled SUDHL-1 cells with ICOS-L⁺ HH or ICOS-L⁻ JSC B cells on proliferation of SUDHL-1 cells as determined by FACS for the cell CSFE labeling pattern. (F) Impact of the blocking anti-ICOS antibody on proliferation of the CFSE-labeled SUDHL-1 cells cocultured with ICOS-L⁺ HH.