CXCR4 and B-cell chronic lymphocytic leukemia: We confirmed the involvement of STAT3 as a mediator of CXCL12–CXCR4-induced IL-10 production by both incubating CLL cells with cucurbitacin for 2 h before adding CXCL12 and by knocking down STAT3 by shRNA as described in the Section “Materials and Methods.” These modifications led to near complete abrogation of IL-10 production by B-CLL cells (Figures 2A,B) without any significant impact on CLL viability (Figures S2B–D in Supplementary Material).