Abstract A51: Impact of CaMKK2 inhibition in tumor-associated myeloid cells on CD8+ cytotoxic T-cell recruitment into mammary tumors

The development of cancer immunotherapy has reached an important inflection point, whereby such therapies have been administered with considerable clinical success in many cancers. However, despite some success in triple-negative breast cancer, the overall efficacy of existing immunotherapies in breast cancer has remained disappointing. This is largely due to low levels of tumor antigen-specific CD8+ tumor-infiltrating lymphocytes (TILs) in breast tumors, as a result of which mammary tumors are unable to establish a potent antitumor immune response. The efficacy of immune checkpoint inhibitors positively correlates with the intratumoral content of cytotoxic T cells and on the activity of processes that increase trafficking of these cells into the tumor microenvironment (TME). This study has been undertaken to identify and exploit mechanisms that facilitate recruitment of TILs to tumors to develop strategies rendering weakly immunogenic mammary tumors sensitive to contemporary immunotherapies. Our studies have shown that the serine/threonine kinase Calcium/Calmodulin Kinase Kinase 2 (CaMKK2) is expressed in all breast tumor subtypes, and its overexpression is directly correlated to poor clinical outcome. In previous studies we reported that primary tumor growth was dramatically reduced when tumors were propagated syngenically in Camkk2−/− mice or mice harboring a myeloid-specific CaMKK2 knockout as compared to wild-type mice. Taking this observation forward, we conducted immune cell profiling, which revealed that relative to WT mice, the number of cytotoxic CD8+T cells was significantly increased in tumors derived from Camkk2−/− mice. These results pointed to a cell-extrinsic function of CaMKK2 in breast cancer whereby depletion of CaMKK2 in stromal cells facilitated changes in the immune cell repertoire, specifically the increase of intratumoral effector T cells, thereby resulting in a more immunogenic tumor microenvironment. Furthermore, we found evidence of increased CXCL10 expression in bone marrow-derived macrophages (BMDMs) generated from mice harboring a myeloid-specific CaMKK2 knockout as compared to wild-type mice. Given the role of the CXCL10-CXCR3 signaling arc in intratumoral lymphocyte recruitment, our studies directly point to the role of myeloid-specific CaMKK2 in attenuating lymphocyte trafficking into mammary tumors by inhibiting the expression of promigratory chemokines. Indeed, transwell assays showed significantly higher number of T cells migrating towards supernates collected from Camkk2−/− BMDM than those from WT mice. These results were supported by additional data from a second study using antigen-specific CD8+T cells (OTI-T). We anticipate that our study will open up a new avenue of research into facilitating the entry of activated effector T cells into the tumor microenvironment, thereby enhancing the efficacy of established immune checkpoint inhibitors in cancer immunotherapy. Citation Format: Debarati Mukherjee, Robert Baldi, Ching-Yi Chang, Luigi Racioppi, Donald P. McDonnell. Impact of CaMKK2 inhibition in tumor-associated myeloid cells on CD8+ cytotoxic T-cell recruitment into mammary tumors [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A51.