Mesenchymal Stem/Stromal Cells Induce Myeloid-Derived Suppressor Cells in the Bone Marrow via Activation of Jnk Signaling Pathway

Our previous study demonstrated that mesenchymal stem/stromal cells (MSCs) induced the differentiation of myeloid-derived suppressor cells (MDSCs) in the bone marrow (BM) under inflammatory conditions. In the present study, we aimed to explore the signaling pathway in BM cells responsible for MDSC generation. RNA-seq revealed that the mitogen-activated protein kinase (MAPK) pathways, among biochemical pathways, exhibited the highest number of differentially expressed genes in BM cells with MSC coculture relative to those without. Western blot confirmed strong phosphorylation of c-Jun N-terminal kinase (JNK) in BM cells cocultured with MSCs under GM-CSF stimulation, whereas p38 kinase activation remained unaltered by MSCs. Inhibition of JNK by SP600125 significantly abolished the expression of Arg1 and Nos2, the hallmark genes of MDSCs, as well as Hif1α, a molecule mediating monocyte functional re-programming into suppressive phenotype, in MSC-cocultured BM cells, while increasing TNF-α in response to lipopolysaccharides stimulation. Furthermore, JNK inhibition markedly abrogated the effect of MSCs on the production of TGF-β1, TGF-β2 and IL-10 in BM cells. These results suggest that MSCs induce MDSC differentiation in BM during inflammation, at least in part, through the activation of the JNK MAPK signaling pathway.