BAFF Accelerates Development of Chronic Lymphocytic Leukemia in TCL1 Transgenic Mice.
Authors: Enzler Thomas , Widhopf George F. , Lee Jason , Zhang Weizhou , Croce Carlo M. , Karin Michael , Kipps Thomas J. 
Published: October 2019

Abstract
The B cell- activating factor of the tumor necrosis factor family (BAFF) is a potent regulator of normal B cells. We recently showed that BAFF supports chronic lymphocytic leukemia (CLL) B cell survival in vitro through activation of the canonical NF-kB pathway. To study the influence of BAFF on CLL development, we crossed BAFF transgenic (Tg) mice with mice that express human TCL1 under a B cell specific promoter/enhancer, and that are known to develop a lymphoproliferative disease resembling human B-CLL. BAFF/TCL1-Tg mice had a shorter mean survival than either TCL1-Tg or BAFF-Tg mice (12 mice each; BAFF/TCL1-Tg mice 9.6±3.4 months; TCL1-Tg 17.2±3.9; BAFF-Tg 17.9±3.6; B6 wildtype (wt) >19.2). To monitor for the development of CLL, mice were bled at 6-week intervals starting at 3 months of age, and blood mononuclear cells (PBMC) were analyzed via flow cytometry using fluorochrome-conjugated antibodies for murine CD5, CD3, CD45R, and human TCL1. Whereas all BAFF/TCL1-Tg mice began to develop a pathological CD5+CD3−CD45Rlo cell population at 3 months of age, such a population was not observed in TCL1-Tg mice before 6 months of age. BAFF-Tg or wt mice did not develop CD5+CD3−CD45Rlo cells over the entire observation period (26 months). CD5+CD3−CD45Rlo B cells expressed the TCL1 transgene. Over time, the CD5+CD3−CD45Rlo population increased in BAFF/TCL1-Tg mice, coming to represent >99% of the total PBMC of 9-month-old animals. To examine the capacity of these cells to propagate, 1x106 CD5+CD3−CD45Rlo B cells were transferred i.v. into either BAFF-Tg or wt mice that previously were irradiated with 600 rad. Ten days after transfer, CD5+TCL1+ cells were detected in BAFF-Tg, but not in wt recipients. Most CLL cells were located in the liver and spleen, as assessed by bioluminescent-based imaging of mice that received luciferase expressing CLL cells. Subsequent examination upon autopsy at 6 months of age, however, revealed that the majority of CLL cells populated the spleens of the recipient mice, which were massively enlarged. At this age, CLL cells also were found in wt recipient mice, although tumor burden was less than 20% of that of BAFF-Tg recipients (n=3 per group). We found that BAFF did not promote CLL cell proliferation in vitro or in vivo using assays to measure BrdU incorporation and flow cytometry to evaluate for enhanced intracellular expression of Ki67. However, BAFF induced CLL cells to express high levels of several anti-apoptotic proteins (e.g. Bcl-XL, Bcl-2, Bim, and A1/Bfl1). Also, while death-associated protein kinase 1 was repressed in CLL cells of TCL1-Tg mice, CLL cells of BAFF/TCL1-Tg mice expressed high-levels. Because of this, we examined whether treatment with BAFF-neutralizing BR3-Fc could influence the survival of CLL cells that were adoptively transferred into BAFF-Tg mice. We found that i.p. injection of 200 ug BR3-Fc into the recipient animals reduced the numbers of circulating CLL cells by nearly 20% (18.2%±5.3%; n=3) within 6 days. These data indicate that BAFF can accelerate the development of CLL cells in TCL1-Tg mice by promoting their survival. Because BAFF can similarly promote survival of human CLL cells, BAFF, and the signaling pathways it activates in neoplastic B cells, could be targeted for the development of novel therapies for this disease.