CD8⁺ T cells from patients with CLL show functional defects in proliferation and cytoxicity. The proliferative potential of CD8⁺ T cells from CLL patients and healthy controls was assessed using a CFSE based assay. Representative histograms are shown of (A) a healthy donor and (B) a CLL patient. (C) The proliferation index was significantly lower for CD8⁺ T cells from CLL patients compared with healthy controls. (D) There was also a reduction in the proportion of CD8⁺ T cells that had entered cell division (percentage divided) in CLL patients compared with healthy controls. The proliferative capacity of the various CD8⁺ lymphocyte subsets as defined by coexpression of CCR7 and CD45RA was assessed. (E-F) CD8⁺CCR7⁻ T cells (T_EM and T_EMRA) had reduced proliferative capacity compared with CD8⁺CCR7⁻ T cells (Naive and T_CM). The cytotoxic activity of CD8⁺ T cells from patients and controls was assessed. CD8⁺ T cells from CLL patients fail to lyse idiotype-pulsed target cells in contrast to CD8⁺ T cells from healthy controls: percentage specific lysis at all effector target ratios (G), and at a effector-target ratio of 30:1 (H).