Evolution of the Tumor Microenvironment throughout Progression and Transformation of EZH2 Mutant Follicular Lymphoma

The genetic hallmarks of follicular lymphomas include BCL2 translocations and somatic mutations of epigenetic modifier genes such as EZH2. Histologically, FLs typically feature a rich microenvironment, most notably featuring extensive follicular dendritic cell (FDC) networks with dendrites making extensive contact with lymphoma cells. In recent work we showed that the main effect of EZH2 gain-of-function mutations in GC B-cells is to enable them to become less dependent of T-cell help and strengthen their immune synapse formation with FDCs, which induces aberrant proliferation and survival of GC centrocytes and hence formation of a unique lymphoma-permissive immune niche. However, it is still unknown how interactions between EZH2 mutant GC B-cells and other immune cells change throughout the progression of the disease. To evaluate the evolution of the tumor microenvironment (TME) throughout EZH2 mutant lymphoma progression, we developed a genetically engineered mouse model designed for conditional expression of gain-of-function Ezh2 mutant and BCL2 in GC B-cells, Rosa26 LSL.BCL2.IRES.GFP;Ezh2 Y641F;Cγ1Cre (hereafter BCL2/Ezh2 Y641F), in which Cγ1-driven Cre excision of a STOP cassette in the Rosa26 locus leads to overexpression of BCL2 and GFP, and expression of the endogenous Ezh2 mutant Y641F. This mouse model develops low-grade follicular-like lymphoma, characterized by expanded follicles composed largely of centrocyte neoplastic GC B-cells and extensive FDC meshwork, along with presence of CD4 +, TFH and regulatory T cells (FOXP3 +), as depicted by multiparametric in situ imaging and multiparametric flow cytometry. At cytological level, cells are predominantly small with condensed chromatin, irregular nuclei, scant cytoplasm, and inconspicuous nucleoli, without sheets of large cells. Over time, these low grade FLs progress to advanced grade, characterized by disruption of follicle structures, expansion of centroblast-like large tumor cells and reduced CD4 + T cell infiltration and FDC meshwork. Furthermore, we have developed a murine transformed FL cell line by sequential passages of an original BCL2/Ezh2 Y641F low-grade FL into immunodeficient Rag1KO mice. We have transduced this BCL2/Ezh2 Y641F cell line with luciferase, and it successfully engrafted and homed to lymphoid organs when injected i.v. into immunocompetent C57BL6 mice. The cell line consists of high proliferative GC B-cells with multiple and irregular nuclei, open chromatin and prominent