Development of a New Mouse Model for Mature B Cell Lymphoma and Resistance to Fas Restoration-Triggered Apoptosis

Non-Hodgkin's lymphomas (NHLs) include mature B cell lymphomas such as Burkitt lymphoma (BL) and diffuse large B cell lymphoma (DLBCL), which are derived from germinal center (GC) B cells. The pro-apoptotic receptor Fas (CD95) is normally expressed in GC B cells and has been considered to be implicated in the pathogenesis of lymphomas. However, little is known about how Fas is regulated during lymphomagenesis.

In this study, we developed a new ex vivo-based simple mouse model for mature B cell lymphoma by the transplantation of Ink4a/Arf (Cdkn2a)-deficient GC B cells that were retrovirally transduced with c-Myc. We found that Fas expression was downregulated at protein and mRNA levels in all formed lymphomas. To determine the role of Fas downregulation in lymphomagenesis and established lymphoma cells, we performed shRNA-mediated knockdown of Fas in c-Myc-GC B cells and retroviral transduction of Fas in lymphoma cells. As a result of transplantations, Fas downregulation was critical for both lymphomagenesis and maintenance of lymphoma cell survival, suggesting that GC-derived lymphomas require sustained Fas downregulation, probably to escape immune surveillance. We further found that CD40 signal activation in mouse lymphoma cells restored Fas expression thorough multiple signaling pathways including NFkB, PI3K and MAPKs (SAPK, MEK and p38). Restored Fas expression significantly induced apoptosis after FasL treatment, suggesting that Fas restoration is a potential therapeutic strategy for lymphomas.

Similarly, human BL and DLBCL cell lines mostly demonstrated Fas downregulation, which was restored by CD40L stimulation. While half of the lymphoma cell lines exhibited sensitivity to FasL treatment upon Fas restoration, the other cell lines were resistant to it. We identified that Livin, a member of IAP family, is highly expressed in these resistant cell lines and is a poor prognostic factor for BL and DLBCL patients. Knockdown of Livin by shRNA and an inhibitor targeting Livin sensitized the resistant cells to Fas restoration-triggered cell death. Thus, the resistant lymphoma cells may acquire Livin during lymphoma development.

Correctively, these findings suggest that Fas can be restored in lymphoma cells and thereby induce apoptosis with FasL treatment, and that Livin is a promising therapeutic target for NHLs resistant to Fas restoration-triggered apoptosis.