Stat3 is constitutively activated in diverse cancers and acts as a critical mediator of tumor immune evasion. Previously, we described in murine breast cancer (BC) models that blockade of Stat3 activation induces cellular senescence. Although senescent cells are growth arrested, they remain metabolically active and develop a senescence-associated secretory phenotype (SASP) that can have pro- as well as antitumorigenic effects. Our objectives were to characterize the composition and activity of the SASP induced by Stat3 blockade (SASP-Stat3) and to develop an immunotherapy (IT) based on this SASP. Here we report that Stat3 knockdown induced senescence and growth arrest only in tumor cells that exhibit constitutive activation of this oncogene (oncogene addiction), such as 4T1 BC and B16-OVA (herein “B16”) melanoma cells. Moreover, we observed that the SASP-Stat3 from 4T1 and B16 cells had several layers of antitumor effects. One of the effects relied on inhibition of tumor cell proliferation, migration and angiogenic activity. The other enhanced T-cell proliferation and activation in vitro. Furthermore, these effects were not observed with the conditioned medium (CM) from Stat3-blocked MCA cells, which are not Stat3 addicted. In order to translate these findings to a potential clinical application, we designed an immunization protocol based on the administration of irradiated wild-type cancer cells together with a depot of the SASP-Stat3. Therapeutic IT with SASP-Stat3 in mice bearing 4T1 or B16 tumors decreased tumor growth compared with control CM (CM-Control). In 4T1 tumors, we also observed a decrease in pulmonary metastasis vs. CM-Control. In addition, combination of the SASP-Stat3-based IT with an anti PD-1 antibody enhanced the antitumor activity against B16 tumor growth. We observed that this synergistic antitumor effect was the result of the involvement of different subsets of immune cells: SASP-Stat3-based IT activates CD4+ T cells and NK cells, while anti PD-1 therapy targets CD8+ T cells. Next, we studied the composition of the SASP-Stat3 through cytokine array and proteomic studies and disclosed several T cells and NK cells-attracting chemokines, IFNγ-induced cytokines and IFN-associated proteins (IP-10, RANTES, IL-15, MCP-1, ISG15 and IFI35). Taken together, these results demonstrate that Stat3 blockade in tumor cells that are addicted to this oncogene results in the induction of cellular senescence with the production of a SASP that has antitumoral and immune-stimulating activities. Cytokines and proteins from the SASP can be used to formulate an effective adjuvant to enhance the antitumor effect of anti-PD-1 antibodies.

Citation Format: Mara De Martino, Mercedes Tkach, María F. Mercogliano, Mauro E. Cenciarini, María F. Chervo, Cecilia J. Proietti, Patricia V. Elizalde, Eliane Piaggio, Roxana Schillaci. Blockade of Stat3 oncogene addiction induces cellular senescence and reveals a cell-nonautonomous activity suitable for cancer immunotherapy [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B25.