Secretome of Hypoxic Endothelial Cells Stimulates Bone Marrow-Derived Mesenchymal Stem Cells to Enhance Alternative Activation of Macrophages

We intended to explore the cellular interaction between mesenchymal stem cells (MSCs) and injured endothelial cells leading to macrophage alternative polarization in healing kidney ischemic reperfusion injury. In vivo, the amounts of recruited macrophages were significantly mitigated by MSCs in the injured tissues, especially in the group using hematopoietic cell E- and L-selectin ligand (HCELL)-positive MSCs. Compared to controls, MSCs also enhanced expression of CD206 and CD163, which was further enhanced by HCELL expression. In vitro, analysis of cytokines involving macrophage polarization showed IL-13 rather than IL-4 from MSCs agreed with expression of macrophage CD206 in the presence of hypoxic endothelial cells. Among them, HCELL-positive MSCs in contact with hypoxic endothelial cells produced the greatest response, which were reduced without HCELL or using a transwell to prevent cell contact. With blockade of the respective cytokine, downregulated MSCs secretion of IL-13 and CD206 expression were observed using inhibitors of IFN-γ and TNF-α, but not using those of TGF-β and NO. With IFN-γ and TNF-α, MSCs IL-13 secretion and CD206 expression were upregulated. In conclusion, hypoxia induces endothelial cells producing multiple cytokines. Among them, IFN-γ and TNF-α that stimulate MSCs to secrete IL-13 but not IL-4, leading to alternative polarization.