Mesenchymal stem cells polarize macrophages to an anti-inflammatory phenotype to ameliorate diabetic nephropathy

Abstract: Background Diabetic nephropathy is closely related to immune-regulation, in which macrophages play a crucial role. In diabetic nephropathy, the classically activated macrophages (M1) increased while the alternatively activated macrophages (M2) decreased in kidney. Mesenchymal stem cells (MSCs) administration can alleviate diabetic nephropathy, however, the mechanisms still remain unclear. MSCs have been shown to stimulate macrophages from a M1 phenotype to a M2 phenotype. Thus, we aimed to investigate whether the polarization of M1/M2 induced by MSCs was involved in diabetic nephropathy (DN). Methods In our study, we injected human umbilical cord mesenchymal stem cells (UC-MSCs) into type 2 diabetic nephropathy rats induced by high fat diet combined with a low dose of streptozotocin. To clarify the effect of MSCs on macrophages polarization, peritoneal macrophages were extracted and directed into M1 macrophages by lipopolysaccharides (LPS) in vitro. Then we co-cultured UC-MSCs with M1 macrophages, and evaluated the effect on differentiation. We also co-cultured rat glomerular mesangial cells (HBZY-1) in high-glucose DMEM with LPS-stimulated macrophages (M1 macrophages) or UC-MSCs-induced M2 macrophages in a trans-well system to clarify the complex mechanisms by which UC-MSCs-induced M2 macrophages improve the progression of DN. Results The UC-MSCs infusion reduced the infiltration of M1 macrophages, and increased the infiltration of M2 macrophages in the glomerulus, thereby attenuating histopathological renal damage and improving renal inflammation and fibrosis in diabetic nephropathy rats. After coculturing UC-MSCs with M1 macrophages, we found that the M1 macrophage maker inducible nitric oxide synthase (NOS2) and the mRNA and protein levels of the related pro-inflammatory cytokines TNF-α, TGF-β and IL-1β decreased. However, the expression of the M2 macrophage markers CD163 and CD206, as well as the anti-inflammatory cytokine IL-10 increased observably. Furthermore, UC-MSCs increased the expression of IL-4Rα on macrophages by secreting IL-6; blocking IL-6 secretion inhibited the UC-MSCs effect on M2 macrophage polarization. Then we explored the mechanism by which M2 macrophages ameliorate diabetic nephropathy in vitro and found that UC-MSCs-induced M2 macrophages attenuated the secretion of the chemokine monocyte chemoattractant protein-1(MCP-1) in hyperglycemia-induced mesangial cells, which led to reduce macrophage recruitment and infiltration. Moreover, UC-MSCs-induced M2 macrophages inhibited TGF-β in glomerular mesangial cells, thereby reducing the synthesis of collagen I and collagen IV. Conclusions Our study proposes and discusses a mechanism by which MSCs promote the polarization of macrophages from M1 into M2 in the kidney, thereby ameliorating diabetic nephropathy.