Upregulation of BCL-XL induced by microenvironmental factors belongs to key mediators of VEN resistance in vitro. (A) Western blot analysis of BCL-XL protein after 24-hour coculture of the tested lymphoma cell lines (HBL-2, MAVER-1, UPF1H, and MINO) with HS5 CD40L feeder cells with stable transgenic expression of human CD40 ligand (HS5 CD40L). (B) Bar charts showing the differences of normalized expression levels of BCL2L1/BCL-XL in the tested WT lymphoma cell lines (blue bars) and after 24-hour coculture with HS5 CD40L feeder cells (red bars). (C) Coculture of HBL-2 and MAVER-1 WT lymphoma cell lines with HS5 CD40L feeder cells for 24 hours resulted in VEN resistance as measured by numbers of apoptotic cells after exposure to VEN (10 and 100 ng/mL); in contrast, coculture of HBL-2 and MAVER-1 BCL-XL K/O clones with HS5 CD40L did not change their sensitivity to VEN. (D) Combined treatment of the WT lymphoma cell lines cocultured for 24 hours on HS5 CD40L feeder cells with the combination of VEN (10 and 100 nM) and A1155463 (1000 nM) overcame the microenvironment-induced VEN resistance.