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 BM cell differentiation from pro-inflammatory CD11bhi Ly6Chi Ly6Glo cells into immunosuppressive CD11bmid Ly6Cmid Ly6Glo cells. Herein, we analyzed MSC-induced CD11bmid Ly6Cmid cells using single-cell RNA sequencing and compared them with CD11bhi Ly6Chi cells. Our investigation revealed 7 distinct immune cell types including myeloid-derived suppressor cells (MDSCs) in the CD11bmid Ly6Cmid cells, while CD11bhi Ly6Chi 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-1Rhi CD11bmid Ly6Cmid and CSF-1Rlo CD11bmid Ly6Cmid cells demonstrated that the former had a homogeneous monocyte morphology and produced elevated levels of IL-10. Functionally, these CSF-1Rhi CD11bmid Ly6Cmid cells, compared with the CSF-1Rlo CD11bmid Ly6Cmid cells, inhibited CD4+ T cell proliferation and promoted CD4+ CD25+ Foxp3+ Treg expansion in culture and in a mouse model of experimental autoimmune uveoretinitis. 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.