Abstract
Myeloid-derived suppressor cells (MDSCs), which accumulate in tumor bearers, are known to suppress anti-tumor immunity and thus promote tumor progression. MDSCs are considered a major cause of resistance against immune checkpoint inhibitors in patients with cancer. Therefore, MDSCs are potential targets in cancer immunotherapy. In this study, we modified an in vitro method of MDSC differentiation. Upon stimulating bone marrow (BM) cells with granulocyte-macrophage colony-stimulating factor in vitro, we obtained both lymphocyte antigen 6G positive (Ly-6G+) and negative (Ly-6G-) MDSCs (collectively, hereafter referred to as conventional MDSCs), which were non-immunosuppressive and immunosuppressive, respectively. We then found that MDSCs differentiated from Ly-6G- BM (hereafter called 6G- BM-MDSC) suppressed T-cell proliferation more strongly than conventional MDSCs, whereas the cells differentiated from Ly-6G+ BM (hereafter called 6G+ BM-MDSC) were non-immunosuppressive. In line with this, conventional MDSCs or 6G- BM-MDSC, but not 6G+ BM-MDSC, promoted tumor progression in tumor-bearing mice. Moreover, we identified that activated glutathione metabolism was responsible for the enhanced immunosuppressive ability of 6G- BM-MDSC. Finally, we showed that Ly-6G+ cells in 6G- BM-MDSC, which exhibited weak immunosuppression, expressed higher levels of Cybb mRNA, an immunosuppressive gene of MDSCs, than 6G+ BM-MDSC. Together, these data suggest that the depletion of Ly-6G+ cells from the BM cells leads to differentiation of immunosuppressive Ly-6G+ MDSCs. In summary, we propose a better method for MDSC differentiation in vitro. Moreover, our findings contribute to the understanding of MDSC subpopulations and provide a basis for further research on MDSCs.