Sox11-induced reprogramming of B2 cells to a B1a-like phenotype promotes Mantle Cell Lymphoma in mice

Abstract Background: Mantle cell lymphoma (MCL) is a highly aggressive B cell lymphoma that accounts for 6% of all non-Hodgkin lymphomas and is characterized by abnormal proliferation of mature antigen- naive B lymphocytes. Despite recent therapeutic advances, MCL remains incurable due to frequent relapses and resistance to therapy. SOX11 is a key transcription factor in the pathogenesis of MCL and is highly expressed in conventional MCL, but it is not expressed in the normal B cells or in the indolent leukemic nonnodal MCL subtype. Although SOX11 was identified as crucial oncogene that supports maintenance of MCL, its role in the initiation of MCL remains largely unknown. Aims: We want to investigate if SOX11 overexpression is a driver MCL-like lymphoma in mice. Furthermore, we want to identify the cell-of-origin of this murine MCL model and uncover the molecular mechanisms underlying SOX11-driven B2-to-B1a reprogramming and formation of MCL-like disease. Methods: We developed a novel mouse model for condition overexpression of SOX11 and an eGFP reporter. We used different Cre-lines to restrict SOX11 expression in space (pre-B cells or common lymphoid progenitors (CLPs)) and time (tamoxifen-dependent Cre). Transplantation experiments are used to identify the cell-of-origin and asses B-cell reprogramming. We used a multi-omics approach, including scRNAseq and ATACseq, to unravel SOX11-based transcriptional activity. Results: We found that elevated levels of SOX11 in B-cells are sufficient to drive MCL-like lymphomas in mice. Moreover, SOX11 expression synergized with loss of p53 and overexpression of Ccnd2 to form aggressive MCL-like CD19+CD5+CD23-IgM+IgD+ lymphomas, with a median survival of 314 (p53Mb1; n=11), 165 (SOX11/p53Mb1; n=19), and 134 (SOX11/Ccnd2/p53Mb1; n=15) days. These SOX11-driven lymphomas show transcriptional, immunophenotypic and functional similarities with both murine B1a cells and MCL patients. scRNAseq analysis shows that SOX11 overexpression from CLP stage blocks T-cell development and skews B-cell development towards the B1a and marginal zone B-cell (MZB) lineage. Strikingly, while SOX11-induced B1a cells were dependent on MALT1 protease activity, the SOX11-induced MZBs were not. In contrast to our cyclin D2-driven MCL-like model, SOX11is inducing a pre-lymphoma stage, in which there is a progressive accumulation of B1a cells in peripheral blood, peritoneal cavity, spleen and bone-marrow. The pre-lymphoma B1a cells have a BCR repertoire which was biased towards binding of self-antigens, such as phosphatidylcholine, and have increased BCR signaling activity. To investigate how elevated SOX11 levels contribute to the pre-lymphoma phase, we explored three potential mechanisms: (1) expansion of existing B1a cells, (2) a developmental bias towards B1a cells, (3) or reprogramming of B2-cells towards a B1a cell fate. To test the first two hypothesis, we transplanted embryonic-derived B cells from fetal liver or bone marrow-derived B2 progenitors with and without SOX11 overexpression into secondary recipients. We found that only SOX11-expressing cells were capable of initiating MCL-like lymphomas. To evaluate the reprogramming potential, we crossed SOX11/p53 mice with a tamoxifen-inducible CreERT2 line and transplanted different sorted B1 and B2 subsets in immunocompromised mice followed by tamoxifen administration via diet. These experiments demonstrate that SOX11 overexpression is sufficient to reprogram mature B2 cells into a B1a-like phenotype, which subsequently expands and serves as the primary source of B1a/MCL-like lymphomagenesis Summary/Conclusion SOX11 functions as a key regulator of the B1a lineage in mice, and its aberrant expression drives the reprogramming of B2 to B1a cells, leading to the development of MCL-like lymphomas.