Defining Mesenchymal Stem/stromal Cell-induced Myeloid-derived Suppressor Cells Using Single-cell Transcriptomics

Mesenchymal stem/stromal cells (MSCs) modulate the immune response through interactions with innate immune cells. We previously demonstrated that MSCs alleviate ocular autoimmune inflammation by directing bone marrow cell differentiation from pro-inflammatory CD11bLy6CLy6G cells into immunosuppressive CD11bLy6CLy6G cells. Herein, we analyzed MSC-induced CD11bLy6C cells using single-cell RNA sequencing and compared them with CD11bLy6C cells. Our investigation revealed seven distinct immune cell types including myeloid-derived suppressor cells (MDSCs) in the CD11bLy6C cells, while CD11bLy6C cells included mostly monocytes/macrophages with a small cluster of neutrophils. These MSC-induced MDSCs highly expressed Retnlg, Cxcl3, Cxcl2, Mmp8, Cd14, and Csf1r as well as Arg1. Comparative analyses of CSF-1RCD11bLy6C and CSF-1RCD11bLy6C cells demonstrated that the former had a homogeneous monocyte morphology and produced elevated levels of interleukin-10. Functionally, these CSF-1RCD11bLy6C cells, compared with the CSF-1RCD11bLy6C cells, inhibited CD4 T cell proliferation and promoted CD4CD25Foxp3 Treg expansion in culture and in a mouse model of experimental autoimmune uveoretinitis. Resistin-like molecule (RELM)-γ encoded by Retnlg, one of the highly upregulated genes in MSC-induced MDSCs, had no direct effects on T cell proliferation, Treg expansion, or splenocyte activation. Together, our study revealed a distinct transcriptional profile of MSC-induced MDSCs and identified CSF-1R as a key cell-surface marker for detection and therapeutic enrichment of MDSCs.