Abstract 3552: Regulation of immune homeostasis by direct activator BH3-only proteins

The BH3-only proteins BIM, BID, and PUMA have emerged as both inhibitors of anti-apoptotic BCL-2 family proteins and direct activators of the pro-apoptotic executioner proteins BAX and BAK, and have been implicated in regulating immune cell homeostasis. For example, BIM is a master regulator of B and T cells and its deletion in mice causes splenomegaly and lymphocytosis. Although deletion of Bid or Puma do not grossly impact murine hematopoietic organs, loss of each can protect lymphocytes from certain forms of cell death. Combined deletion of Puma and Bim causes increased lymphocytosis and spontaneous development of B-cell lymphoma, while loss of BID and BIM leads to delayed T cell contraction following antigen-mediated proliferation. To investigate the role of direct activator BH3-only proteins in regulating the adaptive immune response over time, we performed peripheral lymphocyte analyses on single and combinatorial knock-outs of Bim, Puma, and Bid. Select genotypes, such as Bim −/− , Bim −/− Puma −/− , and Bim −/− Puma −/− Bid −/− mice, show lymphocytosis early in life but, unexpectedly, counts were observed to gradually normalize by 40 weeks of age. Given the dominant role of BIM in immune cell homeostasis, we investigated whether this phenotype was dictated by global deletion of Bim or intrinsic to T or B cell-specific Bim deletion. In generating and analyzing Lck-cre and CD19-cre Bim floxed animals, we found that T cell-specific Bim deletion recapitulated early lymphocyte expansion followed by contraction, whereas the lymphocytosis of mice with B cell-specific Bim deletion showed no age-related changes. Further characterization of aged Lck-cre Bim floxed mice revealed spleen weights similar to wild type mice, and thymocytes with increased sensitivity to a range of apoptotic stimuli; these mice also have an increased percentage of regulatory T cells compared to similarly aged wild-type mice, as has been reported for globally-deleted Bim −/− animals. To determine the mechanism underlying the observed lymphocyte contraction, we performed qRT-PCR analysis for a panel of BCL-2 family proteins. We find that aged Lck-cre Bim floxed mice manifest upregulation of a series of BH3-only transcripts, including Puma and Bid, reflective of an intriguing compensatory mechanism to remedy, over time, the lymphocytosis that derives from Bim deletion in T cells. These studies provide new insight into the complexity and dynamism underlying BH3-only regulation of immune cell homeostasis during health, disease, and aging. Citation Format: Lindsey M. Ludwig, Lauren E. Roach, Jill K. Fisher, Loren D. Walensky, James L. LaBelle. Regulation of immune homeostasis by direct activator BH3-only proteins. [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 3552.