The stromal cells did not protect CLL cells from HHT-induced apoptosis. (A) CLL cells were incubated with 100nM HHT or 10μM F-ara-A (fludarabine (9-β-D-arabinofuranosyl-2-fluoroadenine)) for 24 hours at the absence (□) or presence ■) of the (A) murine stromal cell line KUSA-H1 (n = 3) or the (B) human stromal cell line StromaNKtert (n = 4). Cell death was measured by annexin V–PI staining. Toxicity of HHT to the KUSA-H1 (C) or StromaNKtert (D) was measured by incubating the stromal cells with HHT or F-ara-A for 24 hours before the cells were trypsinized for cell death measurement by annexin V–PI staining. *Difference was significant, P ≤ .05. (E) Incubating of CLL cells with stromal cells induced Mcl-1 expression. CLL cells were incubated with StromaNKtert cells for 24 hours, and Mcl-1 level was measured by immunoblotting. Data (mean ± SEM) represent the average of 4 CLL samples. (F) Reducing Mcl-1 level by HHT was not affected by stromal cells. CLL cells were incubated at the presence or absence of StromaNKtert cells for 24 hours before exposure to 200 and 400nM HHT for 6 and 24 hours. Mcl-1 levels were analyzed by immunoblotting. Data showed 1 of 4 CLL samples with similar results.