Search

Aberrant B-cell receptor/NF-κB signaling is a hallmark feature of B-cell non-Hodgkin lymphomas, especially in diffuse large B-cell lymphoma (DLBCL). Recurrent mutations in this cascade, for example, in CD79B, CARD11, or NFKBIZ, and also in the Toll-like receptor pathway transducer MyD88, all deregulate NF-κB, but their differential impact on lymphoma development and biology remains to be determined. Here, we functionally investigate primary mouse lymphomas that formed in recipient mice of Eµ-myc transgenic hematopoietic stem cells stably transduced with naturally occurring NF-κB mutants. Although most mutants supported Myc-driven lymphoma formation through repressed apoptosis, CARD11- or MyD88-mutant lymphoma cells selectively presented with a macrophage-activating secretion profile, which, in turn, strongly enforced transforming growth factor β (TGF-β)-mediated senescence in the lymphoma cell compartment. However, MyD88- or CARD11-mutant Eµ-myc lymphomas exhibited high-level expression of the immune-checkpoint mediator programmed cell death ligand 1 (PD-L1), thus preventing their efficient clearance by adaptive host immunity. Conversely, these mutant-specific dependencies were therapeutically exploitable by anti–programmed cell death 1 checkpoint blockade, leading to direct T-cell–mediated lysis of predominantly but not exclusively senescent lymphoma cells. Importantly, mouse-based mutant MyD88- and CARD11-derived signatures marked DLBCL subgroups exhibiting mirroring phenotypes with respect to the triad of senescence induction, macrophage attraction, and evasion of cytotoxic T-cell immunity. Complementing genomic subclassification approaches, our functional, cross-species investigation unveils pathogenic principles and therapeutic vulnerabilities applicable to and testable in human DLBCL subsets that may inform future personalized treatment strategies.

A cell's genomic integrity is at risk when DNA-damaging stress, evoked by mitogenic oncogenes or genotoxic treatment modalities such as radiation or chemotherapy, apply. If the DNA repair machinery fails to fix the damaged site during a temporary cell-cycle arrest, or if massive genotoxic stress overwhelmed the repair capacity, cellular failsafe programs such as apoptosis or senescence will be triggered to limit aberrant propagation of these damaged and potentially harmful cells. After decades of scientific focusing on apoptosis, cellular senescence is increasingly recognized as an equally important but biologically and fundamentally different type of ultimate cell-cycle exit program, because of its lastingly persistent nature and cell-intrinsic and extrinsic roles within the tissue and tumor microenvironment. We established primary apoptosis-compromised, Bcl2-expressing Eμ-myc transgenic mouse lymphomas as a versatile and clinically relevant model system to study therapy-induced senescence (TIS). Given the lack of a single specific senescence-defining marker, we previously exploited co-staining of senescence-associated β-galactosidase (SA-β-gal) activity with immunohistochemical detection of trimethylated histone H3 lysine 9 (H3K9me3), an established S-phase gene expression-controlling, repressive chromatin mark, and the proliferation marker Ki67. This biomarker panel is instrumental to characterize cells as senescent via their high SA-β-gal activity, strong nuclear H3K9me3 expression and Ki67-negative profile. In this chapter, we demonstrate the detection of viable senescent cells by novel methods based on a fluorescent version of the SA-β-gal (fSA-β-gal) assay, combined with immuno-fluoroscence staining of H3K9me3 or Ki67, or analysis of the DNA replication status by incorporating 5-ethynyl-2'-deoxyuridine (EdU) detection into the protocol. Notably, while most senescence markers, irrespective of their specificity and sensitivity, may only be assessed in endpoint assays, we would like to emphasize here the strength of viable fSA-β-gal to track single-cell fate in senescent populations over time.

Cellular senescence is a stress-responsive cell-cycle arrest program that terminates the further expansion of (pre-)malignant cells. Key signalling components of the senescence machinery, such as p16(INK4a), p21(CIP1) and p53, as well as trimethylation of lysine 9 at histone H3 (H3K9me3), also operate as critical regulators of stem-cell functions (which are collectively termed 'stemness'). In cancer cells, a gain of stemness may have profound implications for tumour aggressiveness and clinical outcome. Here we investigated whether chemotherapy-induced senescence could change stem-cell-related properties of malignant cells. Gene expression and functional analyses comparing senescent and non-senescent B-cell lymphomas from Eμ-Myc transgenic mice revealed substantial upregulation of an adult tissue stem-cell signature, activated Wnt signalling, and distinct stem-cell markers in senescence. Using genetically switchable models of senescence targeting H3K9me3 or p53 to mimic spontaneous escape from the arrested condition, we found that cells released from senescence re-entered the cell cycle with strongly enhanced and Wnt-dependent clonogenic growth potential compared to virtually identical populations that had been equally exposed to chemotherapy but had never been senescent. In vivo, these previously senescent cells presented with a much higher tumour initiation potential. Notably, the temporary enforcement of senescence in p53-regulatable models of acute lymphoblastic leukaemia and acute myeloid leukaemia was found to reprogram non-stem bulk leukaemia cells into self-renewing, leukaemia-initiating stem cells. Our data, which are further supported by consistent results in human cancer cell lines and primary samples of human haematological malignancies, reveal that senescence-associated stemness is an unexpected, cell-autonomous feature that exerts its detrimental, highly aggressive growth potential upon escape from cell-cycle blockade, and is enriched in relapse tumours. These findings have profound implications for cancer therapy, and provide new mechanistic insights into the plasticity of cancer cells.

Therapy-induced senescence (TIS), a lasting chemotherapy-evoked proliferative arrest of tumor cells, has gained increasing attention by cancer researchers because of its' profound biological implications, and by clinical oncologists due to its potential contribution to the long-term outcome of cancer patients post-treatment. Although both apoptosis and senescence represent therapy-inducible, ultimate cell-cycle exit programs, mediated via DNA damage response signaling, apoptotic cell death as the faster and often quantitatively more prominent tumor response has been in the scientific focus for decades. The more recently recognized TIS as another "safeguard" response of cancer cells that were never primed for or failed to execute apoptosis, not only reflects a more complex "arrest-plus-other features" cell-autonomous condition but produces non-cell-autonomous phenotypes at the tumor site, collectively impinging on tumor control and clinical outcome. Hence, TIS research is gaining pivotal interest from both a tumor biological and a therapeutic perspective, and the development of non-DNA damaging, senescence-evoking therapeutics is about to become a major research objective. In this chapter, we describe a well-characterized, genetically controlled TIS model system based on primary BCL2-expressing Eμ-myc transgenic lymphoma cells harboring defined genetic lesions and provide protocols for co-staining of either senescence-associated β-galactosidase (SA-β-gal) activity or trimethylated lysine 9 of histone H3 (H3K9me3) together with Ki67 to detect the senescent status of therapy-exposed cancer cells.