CTLA4 and B-cell chronic lymphocytic leukemia: In contrast, in the high CTLA-4 expressors, CTLA-4 blockade led to a marked increase in proliferation activity after 24 and 72 h of culture (Figs. 7 and 8), and the median proportion of Ki67-positive cells after 72 h of blocking culture in this group of CLL patients was significantly higher than in healthy individuals (P = 0.03).