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 CD11bhiLy6ChiLy6Glo cells into immunosuppressive CD11bmidLy6CmidLy6Glo cells. Herein, we analyzed MSC-induced CD11bmidLy6Cmid cells using single-cell RNA sequencing and compared them with CD11bhiLy6Chi cells. Our investigation revealed 7 distinct immune cell types including myeloid-derived suppressor cells (MDSCs) in the CD11bmidLy6Cmid cells, while CD11bhiLy6Chi 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-1RhiCD11bmidLy6Cmid and CSF-1RloCD11bmidLy6Cmid cells demonstrated that the former had a homogeneous monocyte morphology and produced elevated levels of IL-10. Functionally, these CSF-1RhiCD11bmidLy6Cmid cells, compared with the CSF-1RloCD11bmidLy6Cmid 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. We purified CD11bmidLy6Cmid cells from BM cells cocultured with MSCs as well as CD11bhiLy6Chi cells from BM cells without MSC coculture by fluorescence-activated cell sorting (FACS), and subjected these two FACS-sorted live cell populations to scRNA-seq using the 10x Genomics Chromium platform.