Calcium/Calmodulin Dependent Protein Kinase Kinase 2 Regulates the Expansion of Tumor-Induced Myeloid-Derived Suppressor Cells

Abstract

Myeloid-derived suppressor cells (MDSCs) are a hetero geneous group of cells, which can suppress the immune response, promote tumor progression and impair the efficacy of immunotherapies. Consequently, the pharmacological targeting of MDSC is emerging as a new immunotherapeutic strategy to stimulate the natural anti-tumor immune response and potentiate the efficacy of immunotherapies. Herein, we leveraged genetically modified models and a small molecule inhibitor to validate Calcium-Calmodulin Kinase Kinase 2 (CaMKK2) as a druggable target to control MDSC accumulation in tumor-bearing mice. The results indicated that deletion of CaMKK2 in the host attenuated the growth of engrafted tumor cells, and this phenomenon was associated with increased antitumor T cell response and decreased accumulation of MDSC. The adoptive transfer of MDSC was sufficient to restore the ability of the tumor to grow in Camkk2-/- mice, confirming the key role of MDSC in the mechanism of tumor rejection. In vitro studies indicated that blocking of CaMKK2 is sufficient to impair the yield of MDSC. Surprisingly, MDSC generated from Camkk2-/- bone marrow cells also showed a higher ability to terminally differentiate toward more immunogenic cell types (e.g inflammatory macrophages and dendritic cells) compared to wild type (WT). Higher intracellular levels of reactive oxygen species (ROS) accumulated in Camkk2-/- MDSC, increasing their susceptibility to apoptosis and promoting their terminal differentiation toward more mature myeloid cells. Mechanistic studies indicated that AMP-activated protein kinase (AMPK), which is a known CaMKK2 proximal target controlling the oxidative stress response, fine-tunes ROS accumulation in MDSC. Accordingly, failure to activate the CaMKK2-AMPK axis can account for the elevated ROS levels in Camkk2-/- MDSC. These results highlight CaMKK2 as an important regulator of the MDSC lifecycle, identifying this kinase as a new druggable target to restrain MDSC expansion and enhance the efficacy of anti-tumor immunotherapy.

Increased CaMKK2 expression is an adaptive response that maintains the fitness of tumor-infiltrating natural killer cells

“…1A-B, S1A-B) 15,24 . The results from this experiment confirmed that the eGFP reporter is highly active in myeloid cells, especially macrophages 14,16 . In contrast, eGFP expression was not observed in B cells or any T-cell subset (Fig.…”

“…CaMKK2 remains highly expressed in monocytes and monocyte-derived macrophages, and deletion of this kinase attenuates macrophage-mediated inflammatory responses 14 . Recently, we showed that pharmacologic inhibition of CaMKK2 prevents the accumulation of M2-like tumor-associated macrophages (TAMs) 15 while decreasing the number of myeloid-derived suppressor cells (MDSC) in the tumor microenvironment 16 . Thus, CaMKK2 contributes to tumor biology through its ability to regulate cancer cell-intrinsic biology and through its actions in specific myeloid populations [8][9][10][15][16][17] .…”

“…Recently, we showed that pharmacologic inhibition of CaMKK2 prevents the accumulation of M2-like tumor-associated macrophages (TAMs) 15 while decreasing the number of myeloid-derived suppressor cells (MDSC) in the tumor microenvironment 16 . Thus, CaMKK2 contributes to tumor biology through its ability to regulate cancer cell-intrinsic biology and through its actions in specific myeloid populations [8][9][10][15][16][17] . However, the role of CaMKK2 in other tumor-infiltrating immune populations has yet to be studied, limiting our ability to develop a complete model of how CaMKK2 influences tumor pathobiology.…”

Systemic Ablation of Camkk2 Impairs Metastatic Colonization and Improves Insulin Sensitivity in TRAMP Mice: Evidence for Cancer Cell-Extrinsic CAMKK2 Functions in Prostate Cancer

“…Previously, a different cancer cell-extrinsic role was reported in breast cancer involving the regulation of the anti-tumor immune response through CAMKK2’s functions in macrophages [ 46 ], and more recently in a preclinical model of lymphoma, myeloid-derived suppressor cells (MDSCs) [ 47 ]. While our initial histological analysis did not note any overt changes in tumor immune cell infiltration, more in-depth profiling is required to determine if an immunosuppressive switch is occurring within the prostate cancer tumor microenvironment.…”

Systemic ablation of Camkk2 impairs metastatic colonization and improves insulin sensitivity in TRAMP mice: Evidence for cancer cell-extrinsic CAMKK2 functions in prostate cancer

“…Previously, a different cancer cell-extrinsic role was reported in breast cancer involving the regulation of the anti-tumor immune response through CAMKK2’s functions in macrophages [44] and more recently in a preclinical model of lymphoma, myeloid-derived suppressor cells (MDSCs) [45]. While our initial histological analysis did not note any overt changes in tumor immune cell infiltration, more in-depth profiling is required to determine if an immunosuppressive switch is occurring within the prostate cancer tumor microenvironment.…”