Therapy-Inducible Senescence Activates Both Innate and Adaptive Immune Mechanisms That Control Lymphoma Growth In Vivo

Premature senescence is a cellular failsafe mechanism which is induced upon various cellular insults, such as oncogene activation or exposure to DNA damaging chemotherapy. It suppresses tumor formation and acts as a barrier to tumor progression in vivo. In contrast to apoptotic cells, senescent cells are viably arrested in the G1 phase of the cell cycle. They continue to take up nutrients and interact with tumor and host cells. To what extent senescent cells alter the tumor environment and tumor-host interactions remains largely unsolved. Here, we analyze lymphoma cells with defined genetic lesions, e.g. deletion of the histone H3 lysine 9 methyltransferase Suv39h1 (controlling senescence) and p53 (mediating both apoptosis and senescence), for their influence on immunological tumor-host interactions as a consequence of therapy-induced senescence (TIS) in the Eμ-myc mouse lymphoma model. Our data demonstrate for the first time a senescence-primed T-cell response against lymphoma cells in vitro and in vivo. Methods: Lymphoma cells (LCs) from different genetic were retrovirally transduced with the bcl2 gene to block apoptosis. Subsequently, they were treated with the DNA damaging anticancer agent adriamycin in vitro or the alkylating agent cyclophosphamide upon lymphoma formation in normal immunocompetent mice in vivo. Therapy-inducible senescence (TIS) was detected based on senescence-associated b-galactosidase activity (SA-b-gal), Ki67 staining and BrdU incorporation. The cytokine profile of senescent LCs was analysed by gene expression and protein arrays. Infiltration and activation of immune cells in TIS lymphomas was analysed by immunohistochemistry and flow cytometry with leukocyte-specific antibodies. Immune responses elicited upon TIS induction in vivo were further analysed in gld (generalized lymphoproliferative disease) mice, which lack functional FasL and by systemic depletion of macrophages after clodronate administration. Pharmaceutical inhibitors of FasL and perforin and IFNg knockout mice were used to analyze T-cell mediated cytotoxity in vitro. Results: TIS lymphoma cells, but not Suv39h1- or p53-deficient LCs, upregulate the secretion of pro-inflammatory cytokines, such as IL6 and IL12, with pro-inflammatory on tumor and bystander cells. In vivo, TIS correlates with the attraction of immune cells, particularly macrophages and T cells, to the tumor site. Senescent LCs became sensitive to both macrophage engulfment and death receptor (Fas)-mediated apoptosis. Activation of both CD4 and CD8 T cells leads to production of IFNg and clearing of senescent cells. Clearance can be attenuated by systemic depletion of macrophages and interference with T cell-mediated programmed cell death. T-cells specifically primed by TIS cells in vivo potently killed both senescent and proliferating LCs after isolation and co-incubation in vitro. In vivo clearance of TIS LCs was attenuated by systemic depletion of macrophages or by interference with T-cell-mediated programmed cell death. Lymphoma-bearing gld mice presented with a reduced overall survival when compared to wild-type host mice. Discussion: This study demonstrates that therapy-induced senescence drives a profound remodeling of the tumor site after therapy and unveils functional interactions of senescent LCs with different immune cell subsets in vitro and in vivo. Senescent cells secrete a cytokine program, which stimulates immune cell attraction and an adaptive and presumably lastingly protective immune response. Thus, TIS is a highly dynamic and interdependent process whose paracrine effects and immune cell interactions account for regression of the senescent mass and present an attractive target network for novel therapeutic strategies.

No relevant conflicts of interest to declare.