CD49d^hi CLL cells are superior stimulators of T-cell activation. Paired CD49d^hi and CD49d^lo CLL cells (irradiated for thymidine-incorporation assays) from 5 patients were mixed at a 1:1 or 1:10 ratio with allogeneic T cells in triplicate. (A) Compared with CD49d^lo cells, CD49d^hi cells have an increased ability to stimulate T-cell activation and proliferation, as shown by the increased expression of Ki67 on gated CD4⁺ (Ai) and CD8⁺ (Aii) cells after 48 hours and increased thymidine-incorporation in a 5-day MLR (Aiii). (B) Irradiated CLL cells from 6 patients known to express CD49d were mixed at a 1:1 ratio with allogeneic T cells in triplicate and in the absence or presence of natalizumab, and proliferation measured by thymidine-incorporation in a 5-day MLR. Blocking of CD49d had no effect on the ability of CLL cells to induce T-cell proliferation. The solid circles, squares, and bars represent the mean result for each individual patient MLR.