Abstract 3554: Functional disparity among BCL-2 members in tonsillar and leukemic B cell subsets probed by next generation BH3 mimetic profiling
Authors: Peperzak Victor , Thijssen Rachel , ter Burg Hanneke , Slinger Erik , Kater Arnon P. , Eldering Eric 
Published: July 2016

Abstract
Introduction and aim. Long-lived humoral immunity, in the form of plasma cells (PC) and memory B cells, as well as most B cell malignancies, originate in the germinal center (GC). For successful therapies against malignant B cells it is crucial to understand their survival requirements in relation to their normal counterparts. It was previously shown that murine GC B cells depend on pro-survival protein MCL-1, but not BCL-XL, although both are expressed. In contrast, naïve and memory B cells express BCL-2, and as a consequence, they are sensitive to the BH3 mimetic ABT-737, which blocks both BCL-2 and BCL-XL but not MCL-1. The divergent roles of MCL-1 and BCL-XL in GC B cells still remain unexplained, and it is unknown if this also holds for human B cells. We here dissect these aspects in human tonsillar B subsets and in chronic lymphocytic leukemia (CLL) cells, a B cell cancer where expression of BCL-2 members is influenced by signals that also occur in the normal GC.
Experimental procedures. We used FACS sorting of tonsil B subsets, mRNA and protein profiling, co-culture of CLL cells, co-IP of BCL-2 members, cell death assays with next generation BH3 mimetics specific for BCL-2, BCL-XL or MCL-1 (ABT-199, WEHI-539, A-1210477).
Results. Naive and memory B cells are mainly sensitive to specific inhibition of BCL-2 by ABT-199. In contrast, GC B cells and tonsil PC are insensitive to ABT-199, but undergo apoptosis when MCL-1 is inhibited. Tonsil PC display yet another profile and are remarkably sensitive to inhibition of BCL-XL. MCL-1 expression in GC B cells is regulated post-translationally and its physiological importance is highlighted by exclusive binding to pro-apoptotic BIM. In contrast, BCL-XL is transcriptionally induced in the GC-light zone, and is solely bound to the weak BH3-only sensitizer BIK, which may explain why BCL-XL is not required for GC B cell survival.
These approaches were extended to primary CLL cells either resting or stimulated via CD40, as model for circulating and lymph node resident cells. We showed recently that CD40 stimulation strongly induces MCL-1, BCL-XL and BFL-1, similar to the pattern in healthy GC B cells, and confers complete resistance to ABT-199, whereas untreated CLL cells are highly sensitive. We now demonstrate that dual or triple BH3 mimetic combinations are effective in killing CD40-stimulated CLL. In contrast to GC B cells, BIK is absent in CLL cells while NOXA is highly expressed and, like BIM, is bound to MCL-1. This may account for differential sensitivity to (combinations of) BH3 mimetics.
Conclusion. Using novel BH3 mimetics and interaction profiles, we were able to probe the contribution of individual BCL-2 members in survival of normal and malignant B cells. Our findings provide clues to determine therapeutic windows in novel treatment strategies in B cell and other malignancies.
Citation Format: Victor Peperzak, Rachel Thijssen, Hanneke ter Burg, Erik Slinger, Arnon P. Kater, Eric Eldering. Functional disparity among BCL-2 members in tonsillar and leukemic B cell subsets probed by next generation BH3 mimetic profiling. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3554.