Modified method for differentiation of myeloid-derived suppressor cells enhances immunosuppressive ability via glutathione metabolism.

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 method of MDSC differentiation. Upon stimulating bone marrow (BM) cells with granulocyte-macrophage colony-stimulating factor , 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 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 . Moreover, our findings contribute to the understanding of MDSC subpopulations and provide a basis for further research on MDSCs.