B Cells with BCL2/IGH Translocation Compose a Distinctive Cell Population That May Serve as a Reservoir of Lymphoma of Germinal Center B-Cell Type.
Authors: Sakai Tomomi , Nishikori Momoko , Tashima Masaharu , Yamamoto Ryo , Kitawaki Toshio , Takaori-Kondo Akifumi , Tsuzuki Shinobu , Uchiyama Takashi 
Published: April 2020

Abstract
BCL2/IGH translocation is a hallmark of follicular lymphoma and diffuse large B-cell lymphoma of germinal center B-cell type. Although being a strong determinant of these histological subtypes, this translocation is considered to be insufficient by itself and further gene alterations are necessary for cellular transformation. In Eμ-BCL2 transgenic (Tg) mice, B-lineage cells are increased by several-fold compared to wild-type (WT) mice, but only 5–15 % of them develop disease in the first year of life. To clarify how the BCL2 translocation contributes to the development of specific lymphoma subtypes, we created two types of chimeric mouse models to characterize the biological features of BCL2-overexpressing B cells in normal individuals. First, we introduced CD19 promoter-driven BCL2 and its mutant genes to a minor population of murine bone marrow cells by using a lentiviral vector system and transplanted into irradiated mice. BCL2-overexpressing B cells showed increased follicular and reduced marginal zone populations. The same phenotypic shift was observed in B cells introducing BCL2-Y28F mutant that retained anti-apoptotic function, but a defective mutant BCL2-G142A and a mock vector did not affect B-cell phenotype. Additionally, BCL2-introduced B cells showed decreased cell size compared to those introduced BCL2-G142A and mock vectors. To assess the functional alteration of BCL2-overexpressing B cells, TNP-Ficoll binding experiment was performed. The result showed diminished T-cell independent response in parallel with decreased marginal zone B cells. The low transformation frequency of B cells in Eμ-BCL2 Tg mice has been partly explained by their propensity to reside in the G0 phase of the cell cycle (reviewed in Oncogene, 18:5268,1999). We hypothesized that the microenvironment of B cells in Eμ-BCL2 Tg mice might be altered by abnormal B cells themselves. To evaluate the influence of the different microenvironments on BCL2-overexpressing B cells, we next made Eμ-BCL2/CAG-GFP double Tg mice and transferred their bone marrow mononuclear cells into WT or Eμ-BCL2 Tg mice. Blastic cell population of BCL2+GFP+ B cells was larger in those transferred to WT mice compared to those transferred to Eμ-BCL2 Tg mice, regardless of the same phenotypic preference toward follicular B cells. BrdU uptake experiments demonstrated continuous cell cycle progression of the BCL2+GFP+ B cells in WT mice but repressed cell cycle of those in Eμ-BCL2 Tg mice. In immunohistochemical analysis, splenic follicles were disorganized with reduced follicular dendritic cells and inadequate T cell accumulation in Eμ-BCL2 Tg mice. Functional impairment of splenic follicles in Eμ-BCL2 Tg mice might be caused by decreased marginal zone B cell subset, as the antigen capture and delivery by marginal zone B cells was reported to play an important role in the development of follicular dendritic cells. To understand the fate of BCL2-overexpressing B cells after stimulation, we finally assessed their terminal differentiation capacity in vitro. Plasma cell differentiation was suppressed in B cells derived from Eμ-BCL2 Tg mice under either LPS or anti-IgM antibody stimulation. BCL2 is reported to impede the activity of transcription factor NF-AT (Proc Natl Acad Sci93:9545,1996; Nature386:728,1997), and we found that calcineurin inhibitor FK506 suppressed plasma cell differentiation of WT B cells. Gene regulation patterns of the Eμ-BCL2+ B cells were similar to B cells stimulated in the presence of FK506 as well, suggesting that repressed terminal differentiation in Eμ-BCL2+ B cells was partly caused by the suppressed activity of NF-AT. In summary, BCL2-deregulated B cells preferentially differentiate into follicular B cells, and as a result of decreased terminal differentiation in addition to their anti-apoptotic property, they may be obliged to survive and recirculate as memory B cells, and accumulate genetic abnormalities while they repeatedly pass through the germinal center. As the germinal center is the particular site where they can counterbalance the cell cycle-retarding effect of BCL2, it may be a specific place for generating lymphoma triggered by BCL2/IGH translocation. Our results emphasize the importance of the microenvironment of pre-malignant cells during transformation process, and suggest that a simple transgenic mouse model may not be always appropriate for the study of oncogenesis.