Abstract The CD39-CD73-adenosine pathway is an emerging regulator of the immune antitumor response. CD39 is expressed within tumors and the tumor microenvironment by several cell population including immune and cancer cells. In tumor tissues, the pathway leads to the accumulation of immunosuppressive adenosine together with decreased levels of immunoactivating peritumoral ATP. We reported previously that CD39 blockade increased T cell and NK cell-mediated cytotoxic activity in vitro and disclose, during this meeting, the development of the first human-CD39-blocking humanized antibody (S. Augier et al., Preclinical development of a humanized blocking antibody targeting the CD39 immune checkpoint for cancer immunotherapy). Here we demonstrated that this pathway is involved in tumor-induced resistance to various cancer therapies in syngeneic mouse melanoma, colon cancer and fibrosarcoma models. We used therapy-resistant mouse models or inefficacious treatment regimens in the CD39 knockout mice to assess the capacity of CD39 to affect the response to chemotherapies, tumor associated antigen (TAA)-targeting antibodies and immunomodulators such as anti-PD1 antibodies. We achieved increased response rates, increased response duration and some complete and long lasting tumor regressions in the CD39 deficient context. These preclinical proof-of-concept studies highlight the role of the CD39 immune checkpoint pathway in limiting the efficacy of various anticancer therapies in syngeneic mouse models and thereby support the potential clinical value of the humanized CD39-neutralizing antibody under development. Citation Format: Marion Lapierre, Cécile Dejou, Carine Paturel, Henri-Alexandre Michaud, Laurent Gros, Armand Bensussan, Jean-François Eliaou, Jeremy Bastid, Nathalie Bonnefoy. Disruption of the CD39 immune checkpoint pathway increases the efficacy of various anticancer therapies in syngeneic mouse models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3218.