New Suitable Eµ-Tcl1 Mouse Model for Research on BH3-Mimetics Therapy Response In Vivo

Introduction: Generation of appropriate mouse models to study clinically active compounds is inevitable. Currently, the commonly used mouse model for CLL is the TCL1 transgenic model, which shows an expression of the human TCL1 oncogene in B cells. While this model resembles many features of human CLL, it is resistant to venetoclax, a BCL2 inhibitor which represents an important treatment option for CLL and other hematological malignancies. The lack of therapeutic response to venetoclax is an important limitation of the TCL1 mouse model. As a result, the effectiveness, the mechanisms, and the development of venetoclax resistance and other BH3 mimetics cannot be investigated in the TCL1 transgenic setting.

Methods and results: To overcome the stated limitations, a novel mouse model was generated expressing human BCL2 in B cells under the control of the TCL1 promoter. We established a new mouse model (TBC) by crossbreeding Eµ-Tcl1^tg/wt mice with mice containing a B cell specific conditional Bcl-2^Rosa26/wt; Cd19Cre^Cre/wt overexpression (control group Eµ-Tcl1^tg/wt; Cd19Cre^Cre/wt: TC). Initial data show a strong leukocytosis (p<0,0001) and splenomegaly in TBC mice leading to a significantly shortened overall survival compared to TC mice (log-rank: p=0,0028). Immunophenotyping revealed a population of class-switched IgM^- B cells within TBC mice and only a small population of Cd19⁺/Cd5^dim cells. Transcriptomics followed by gene enrichment analysis showed a development of GCB-type DLBCL in TBC mice compared to TC mice.

Apoptosis assays revealed high sensitivity towards BH3 mimetics, especially BCL2 inhibitor venetoclax. In vitro experiments demonstrated that B cells from TBC mice are 10 times more sensitive towards venetoclax treatment. Based on this fact, we treated 5 TBC mice for 4 weeks with venetoclax. Even in this short treatment time frame, there was a strong loss of leukocytes in peripheral blood of about 60%, leading to normal ranges of leukocytes in TBC mice after even 14 days of treatment with venetoclax. Interestingly, we observed a massive change of Cd4⁺/Cd8⁺ ratio towards Cd4⁺ cells due to venetoclax treatment, indicating that our novel model resembles human settings.

Conclusions: We were able to show a strong therapy response towards treatment with venetoclax in vivo in TBC mice leading to a loss of leukocytes, a gain of CLL typical Cd19⁺/Cd5^dim cells and a strong effect on tumormicroenvironment with a significantly changed Cd4⁺/Cd8⁺ ratio. This makes the new mouse model very suitable for testing and observing venetoclax therapy response, its side effects and outcome in vivo, to provide our patients with the best possible therapy.