Abstract 294: BCL-2 family compensation regulates T cell homeostasis and reveals a BIM:BCL-W axis in T-ALL

Abstract

Abstract BIM is the master BH3-only BCL-2 family regulator of lymphocyte survival. To understand how long-term loss of BIM affects apoptotic resistance in T and B cells we examined peripheral blood counts in mice with T or B cell-specific deletion of Bim. In contrast to CD19CRE Bimfl/fl animals, LCKCRE Bimfl/fl mice had pronounced early lymphocytosis followed by lymphocyte count normalization as they aged. To explore BCL-2 family specific mechanism(s) leading to this normalization we compared cell death sensitivity and BCL-2 expression patterns of young versus old LCKCRE Bimfl/fl T cells. Young (6-12 weeks) and old (>40 weeks) LCKCRE Bimfl/fl mice had consistently abnormal thymocyte development hallmarked by a decrease in CD4+8+ double positive (DP) cells and accumulation of double negative (DN) and single positive (SP) cells as previously described in global Bim knockout mice. Thymocytes maintained cell death resistance to a number of apoptotic stimuli throughout all ages, reflecting a consistent loss of BIM. In contrast to thymocytes, mature splenic-derived LCKCRE Bimfl/fl T cells from young mice were equally sensitive to apoptotic stimuli as WT controls, indicating BCL-2 family compensation. Expression profiling in these cells revealed upregulation of other BH3-only proteins, including Noxa = Puma << Bmf, suggesting that these proteins functionally compensated for the loss of BIM. To determine if this was a T cell-intrinsic process and if the presence of other BH3-only proteins with pan-BCL-2 family binding affinities were required for lymphocyte number and apoptotic normalization, we analyzed peripheral blood from mice globally deleted in Bim, Bid, and Puma (TKO). TKO, like LCKCRE Bimfl/fl mice, developed profound early lymphocytosis followed by normalization as they aged. A subset of TKO mice developed T-ALL hallmarked by upregulation of Bmf and Bcl-w, an under characterized anti-apoptotic BCL-2 family protein, suggesting that BCL-W allowed for malignant T cell survival despite the presence of BMF. To determine if such a relationship exists in human disease, expression profiling of pediatric T-ALL patient samples revealed a significant negative correlation between BIM and BCL-W expression in developmentally immature and mature T-ALL. Our findings suggest a model in which the loss of BIM in T cells can be compensated for through upregulation of BMF, a BH3-only protein mechanistically similar to BIM. Loss of other ‘direct activating' BH3-only proteins increases the chances of malignant cell escape despite BMF upregulation, particularly when BCL-W is overexpressed. This work provides a developmental framework between BIM and BMF in T cells and for a BIM:BCL-W axis in T-ALL. Overexpression of BCL-W in BIMlow-expressing T-ALL may be an additional target responsible for early immature T-ALL therapeutic resistance. Citation Format: Lindsey M. Ludwig, Lauren E. Roach, Samuel G. Katz, Jill K. Fisher, Melissa Burns, Matthew R. Schnorenberg, Riyue Bao, Makda Zewde, Yusuke Nakamura, Alejandro Gutierrez, Loren D. Walensky, James L. LaBelle. BCL-2 family compensation regulates T cell homeostasis and reveals a BIM:BCL-W axis in T-ALL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 294.