Abstract SY33-01: Senescence-related stroma: Implications in lymphoma development and therapy
Authors: Clemens A. Schmitt
Journal: Cancer Research (2011)

Abstract Tumor/stroma interactions of malignant lymphomas are distinct from the environmental interferences solid cancers or leukemias exhibit. In carcinomas, for instance, the disruption of anatomical borders such as the basal membrane not only represents the critical step of invasiveness but also marks unrestricted exposure of cancer cells to potential novel interaction partners, e.g., stromal fibroblasts, endothelial cells, various types of host immune cells, and acellular stromal structures. The complex network of cell-cell interactions and secretable factors during carcinogenesis is subject of intense scientific investigation, and distinct components of this network may have different, even opposing functions during early tumor development and later during tumor progression. In contrast, acute leukemias originate from cells with high similarities to normal, niche-residing hematopoietic stem cells, and expand to a full-blown disease without the requirement to invade foreign tissue compartments. Nevertheless, signals from the leukemic stem cell niche and crosstalk with the host immune system comprise important factors in leukemogenesis as well. Malignant lymphomas arise as “tumors of the immune system” in functionally distinct zones of secondary lymphatic organs, for example in the germinal center of lymph follicles. Consequently, they share tumor/stroma features with both solid organ tumors and “liquid,” leukemia-like hematological malignancies. The architecture of the normal lymph node already reflects a rich network of cellular and chemokine-/cytokine-based interactions between many types of immune cells. While cell-autonomous defects in the key steps of lymphoid ontogenesis – V(D)J rearrangement, class switch recombination, somatic hypermutation and elimination of improper cells by default apoptosis – are undoubtedly central in lymphoma development, the interaction of lymphoma-prone progenitors with surrounding host immune cells and secreted factors is increasingly appreciated as a critical co-factor of lymphomagenesis. Indeed, recent publications address the prognostic implications nonneoplastic components of the host immune system might have in various lymphoma types. For example, a macrophage-enriched gene expression signature as well as the frequency of tumor-infiltrating macrophages reportedly predict inferior outcome in follicular lymphoma, while a T-cell-enriched signature correlated with superior outcome [1,2]. Very recently, a similar observation has been described for Hodgkin's lymphoma, a B-cell malignancy in which the actually transformed cells (Hodgkin-Reed-Sternberg and Hodgkin cells) typically represent a small minority within an infiltrate of inflammatory cells: patients whose initial biopsies exhibited particular low frequencies of lymphoma-infiltrating macrophages had a higher likelihood of survival [3]. In contrast, gene expression profiling of diffuse large B-cell lymphomas (DLBCL), the most frequent entity of aggressive non-Hodgkin's lymphomas, found a macrophage-enriched stromal signature to be associated with a superior outcome of DLBCL patients treated with standard immunochemotherapy [4]. By utilizing transgenic mouse lymphoma models, we are particularly interested in dissecting the cell-autonomous vs. non-cell autonomous tumor-suppressive contributions of cellular failsafe mechanisms such as apoptosis and senescence, a terminal cell-cycle arrest program. Apoptosis is known to act as a crucial countermeasure against Myc-driven lymphoma development [5], as most prominently seen in the constitutively Myc-active Burkitt's lymphoma, where a significant steady-state fraction of apoptotic lymphoma cells is constantly cleared by macrophages, corresponding to the “starry sky” histology as the hallmark histological finding of this entity. More recently, we discovered cellular senescence to operate as the key barrier against Ras-driven lymphoma formation in vivo [6]. Based on the finding that the senescent G1 arrest is mediated by a retinoblastoma protein-bound histone H3 lysine 9 (H3K9) methyltransfersase activity [7], we identified the H3K9 methyltransferase Suv39h1 as an essential senescence mediator in this process. Next, we asked whether Suv39h1-controlled senescence might also suppress Myc-driven tumorigenesis [8]. Indeed, lack of Suv39h1 alleles dramatically accelerated lymphoma onset in the Eµ-myc transgenic mouse lymphoma model by virtually completely ablating a surprisingly high fraction of senescent cells detectable in Suv39h1-proficient Myc-tumors. However, we found Myc – unlike oncogenic Ras – to evoke a rather weak direct senescence response. Since data from gene expression analyses pointed toward activated TGF-β signaling in Eµ-myc lymphomas in vivo, we investigated the role of macrophages as the prime source of pro-senescent TGF-β in Myc-initiated, but non-cell autonomous senescence. Indeed, apoptotic lymphoma cells attracted macrophages to the lymphoma site [9], and, following phagocytic ingestion of the apoptotic corpses, activated macrophages dramatically increased their TGF-β secretion, which, in turn, induced lymphoma cell senescence in their vicinity. We were able to confirm causal dependencies within this chain of events by genetic interference with all its individual components, i.e., lymphoma cell apoptosis, availability of macrophages, and TGF-β action. Moreover, proof-of-concept examination of human DLBCL samples underscored a close correlation between the overall proliferation rate and the frequency of apoptotic lymphoma cells, the extent of tumor-infiltrating macrophages, evidence of activated TGF-β signaling in lymphoma cells, and, ultimately, the presence of lymphoma cells with features of senescent cells. Thus, oncogenic Myc may evoke a non-cell autonomous, macrophage-dependent and TGF-β-mediated tumor suppressive senescent response. Importantly, these results are in good agreement with the aforementioned stromal gene expression findings in DLBCL, where not only a macrophage but explicitly a TGF-β pathway signaling gene expression signature was linked to superior outcome [4]. Macrophages and other host immune cells impact on lymphomagenesis and lymphoma biology but also critically interfere with treatment responses, because nonneoplastic immune cells serve as components or secondary targets of chemotherapy-induced apoptosis and cellular senescence. Moreover, as known from oncogene-induced senescence [10-12], therapy-induced senescent tumor cells secrete a plethora of senescence-associated cytokines that directly affect lymphoma biology but also modulate composition and function of the non-neoplastic cells in the lymphoma environment. Data will be presented at the meeting that demonstrate how host immune cells enhance or terminate drug-induced senescence. Functional implications of the resulting profound tumor remodeling for the long-term outcome to cancer therapy will be discussed. References: 1. Dave SS, Wright G, Tan B et al. (2004). Prediction of survival in follicular lymphoma based on molecular features of tumor-infiltrating immune cells. 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Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr SY33-01. doi:10.1158/1538-7445.AM2011-SY33-01