Development of effective and durable antitumor immunity requires the activation, expansion and maintenance of function of tumor antigen-specific effector T cells. The mechanism of action of recently FDA approved monoclonal antibody (mAb) therapies which target immune checkpoints such as PD-1 and CTLA-4 and uncouple inhibitory pathways from the activation of antigen-specific T cells underlines the critical requirement for antigen-specific priming to elicit potent and long-lasting anti-tumor immunity. However, immune checkpoint blockade results in significant clinical benefit largely in malignancies with high mutational burden, where the tumor itself can initiate T cell priming, but is less effective when used as a single agent among cancers with lower mutational burden, associated typically with a low level of lymphocyte infiltration into the tumor microenvironment (TME). We are evaluating clinical immunotherapy regimens combining immune checkpoint blockade with recombinant live-attenuated double-deleted strains of the intracellular bacterium Listeria monocytogenes (LADD), based on a hypothesis that effective immunotherapy will result from agents that in combination re-polarize the TME to facilitate immune effector cell function, prime functional tumor-specific T cells in the appropriate context, and block inhibitory signaling pathways. Here we show that tumor antigen-expressing LADD therapy enhanced CD8+ T cell effector function, resulting in significant tumor eradication in several preclinical mouse models. LADD treatment in addition induced favorable changes in the TME, as shown by enhanced CD8+ T effector function, recruitment of antigen presenting cells and reduction of regulatory T cells. Treatment regimens combining LADD-based immunotherapy with PD-1 immune checkpoint blockade significantly enhanced antitumor efficacy in CT26, 4T1 and MC38 tumor models. Together these data support the rationale for integrating LADD-based immunotherapy into clinical regimens with immune checkpoint blockade on the basis that TME modification and priming of tumor Ag-specific T cells significantly enhances the activity of mAb therapies blocking T cell inhibitory pathways.

Citation Format: Weiwen Deng, Weiqun Liu, Thomas Hudson, Chris S. Rae, Ed Lemmens, Anthony Desbien, Bill Hanson, Pete Lauer, Thomas W. Dubensky, Jr., Meredith Leong. Synergistic antitumor efficacy in mice with immunotherapy regimens combining recombinant live-attenuated Listeria with immune checkpoint inhibitors. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2016 Oct 20-23; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2017;5(3 Suppl):Abstract nr B50.