Abstract 2283: Novel mouse model for mature B cell lymphoma reveals the requirement of Fas downregulation in lymphomagenesis

Abstract

Abstract Non-Hodgkin's lymphoma includes mostly mature B cell lymphomas such as Burkitt lymphoma (BL), diffuse large cell lymphoma (DLBCL), and follicular lymphoma which are all derived from germinal center B (GCB) cells. Clarifying the molecular mechanisms during lymphomagenesis requires appropriate mouse models with short latency that can be easy to monitor the whole process from the cell of origin to full-blown lymphoma. Here, we developed a novel mouse model for mature B cell lymphoma originated from GCB cells by using ex vivo culture system with retroviral transduction. Naive B cells harvested from Ink4a/Arf-/- mice were cultured and stimulated with IL-4 and anti-CD40 antibody to differentiate into GCB cells. GCB cells were retrovirally transduced with c-Myc, highly expressed in most human lymphomas, and then transplanted into irradiated mice. Around 60% of mice transplanted via intraperitoneal injection exhibited splenomegaly and enlarged lymph nodes in the axilla, inguinal and intestinal regions, and finally died. Lymphoma cells and tissues from those mice showed mature B cell immunophenope (B220+IgM+IgD+) and histopathology similar to human Burkitt lymphoma. Interestingly, lymphoma cells from all analyzed mice did not show Fas expression at protein and mRNA levels in contrast to the original GCB cells that highly express Fas. Furthermore, spontaneously developed lymphoma cells in all λ-Myc mice (a transgenic mouse carrying c-Myc under the control of the Igλ light chain enhancer) also showed no Fas expression. Fas is a cell death receptor which has an important role in negative selection of GCB cells with high affinity antibody. To clarify the role of Fas downregulation in lymphomagenesis, Ink4a/Arf-/- GCB cells were transduced with c-Myc and shRNA for Fas and then transplanted. All mice transplanted with shFas-treated GCB cells developed lymphoma and died faster than mice transplanted with shControl-treated GCB cells. These results suggest that Fas downregulation is required for lymphomagenesis. In primary culture of the lymphoma cells, Fas expression was restored after CD40 ligand treatments, suggesting that reduced CD40 signaling may be involved in Fas downregulation. We further found that 8 out of 10 human lymphoma cell lines including 2 BL and 6 DLBCL exhibited low Fas expression whereas 2 cell lines exhibited high Fas expression. Among those Fas-downregulated cells, Fas expression in some cell lines was restored not only after Fas ligand treatment, but also after a DNA methyltransferase inhibitor 5-Aza-dC treatment or an HDAC inhibitor TSA treatment, indicating that Fas expression is also regulated by epigenetic mechanisms in human lymphoma cells. Collectively, these findings suggest that Fas downregulation is a crucial event for mature B cell lymphomagenesis to escape from immune surveillance and that restoration of Fas expression could be a new strategy for lymphoma treatment. Citation Format: Eiji Sugihara, Norisato Hashimoto, Sayaka Ueno, Hideyuki Saya. Novel mouse model for mature B cell lymphoma reveals the requirement of Fas downregulation in lymphomagenesis. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2283. doi:10.1158/1538-7445.AM2015-2283