MiR-6991-3p is identified as a novel suppressor in the expansion and activation of myeloid-derived suppressor cells in hepatoma-bearing mice.

Objective: Myeloid-derived suppressor cells (MDSCs) are a heterogeneous group of cells derived from bone marrow, which has a significant ability in inhibition of immune cell response. In this study, the role of miR-6991-3p in regulating function of MDSCs was investigated.

Methods: MDSCs were isolated from different tissues of the control and hepatoma-bearing mice, and then expression of miR-6991-3p was detected with qPCR. Then, the miR-6991-3p mimic and inhibitor were respectively transfected into MDSCs, and behaviors of MDSCs were evaluated, including expansion, apoptosis, and production of inflammatory factors. Furthermore, we explored the underlying mechanism from which miR-6991-3p regulated MDSC functions.

Results: Expression miR-6991-3p was markedly decreased in the MDSCs derived from spleen and further decreased in the MDSCs derived from the tumor tissue. MiR-6991-3p mimic transfection suppressed expansion and promoted apoptosis of MDSCs, accompanied by a significant decrease in the production of IL-6 and GM-CSF that are identified as stimulators in MDSC expansion. In contrast, miR-6991-3p inhibitor transfection displayed the opposite effect. miR-6991-3p bound with and negatively regulated expression of LGALS9, a newly identified immune checkpoint gene and activator of STAT3, suppressing production of multiple factors that were customarily used to characterize the activation of MDSCs. MiR-6991-3p-accommodated MDSCs displayed less suppression on T cells, while miR-6991-3p inhibitor enhanced the suppression of MDSCs on T cells.

Conclusion: MiR-6991-3p is identified as a novel suppressor in the expansion and activation of myeloid-derived suppressor cells, which may be regarded as a promising target for modulating the function of MDSCs.