The Voltage-Gated Hv1 H⁺ Channel Is Expressed in Tumor-Infiltrating Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are key determinants of the immunosuppressive microenvironment in tumors. As ion channels play key roles in the physiology/pathophysiology of immune cells, we aimed at studying the ion channel repertoire in tumor-derived polymorphonuclear (PMN-MDSC) and monocytic (Mo-MDSC) MDSCs. Subcutaneous tumors in mice were induced by the Lewis lung carcinoma cell line (LLC). The presence of PMN-MDSC (CD11b⁺/Ly6G⁺) and Mo-MDSCs (CD11b⁺/Ly6C⁺) in the tumor tissue was confirmed using immunofluorescence microscopy and cells were identified as CD11b⁺/Ly6G⁺ PMN-MDSCs and CD11b⁺/Ly6C⁺/F4/80^-/MHCII^- Mo-MDSCs using flow cytometry and sorting. The majority of the myeloid cells infiltrating the LLC tumors were PMN-MDSC (~60%) as compared to ~10% being Mo-MDSCs. We showed that PMN- and Mo-MDSCs express the Hv1 H⁺ channel both at the mRNA and at the protein level and that the biophysical and pharmacological properties of the whole-cell currents recapitulate the hallmarks of Hv1 currents: ~40 mV shift in the activation threshold of the current per unit change in the extracellular pH, high H⁺ selectivity, and sensitivity to the Hv1 inhibitor ClGBI. As MDSCs exert immunosuppression mainly by producing reactive oxygen species which is coupled to Hv1-mediated H⁺ currents, Hv1 might be an attractive target for inhibition of MDSCs in tumors.