Interleukin-6 downregulation with mesenchymal stem cell differentiation results in loss of immunoprivilege

Allogeneic mesenchymal stem cell (MSC) transplantation improves cardiac function, but cellular differentiation results in loss of immunoprivilege and rejection. To explore the mechanism involved in this immune rejection, we investigated the influence of interleukin-6 (IL-6), a factor secreted by MSCs, on immune privilege after myogenic, endothelial and smooth muscle cell differentiation induced by 5-azacytidine, VEGF, and transforming growth factor-β (TGF-β), respectively. Both RT-PCR and ELISA showed that myogenic differentiation of MSCs was associated with significant downregulation of IL-6 expression (P < 0.01), which was also observed following endothelial (P < 0.01) and smooth muscle cell differentiation (P < 0.05), indicating that IL-6 downregulation was dependent on differentiation but not cell phenotype. Flow cytometry demonstrated that IL-6 downregulation as a result of myogenic differentiation was associated with increased leucocyte-mediated cell death in an allogeneic leucocyte co-culture study (P < 0.01). The allogeneic reactivity associated with IL-6 downregulation was also observed following MSC differentiation to endothelial and smooth muscle cells (P < 0.01), demonstrating that leucocyte-mediated cytotoxicity was also dependent on differentiation but not cell phenotype. Restoration of IL-6 partially rescued the differentiated cells from leucocyte-mediated cell death. These findings suggest that rejection of allogeneic MSCs after implantation may be because of a reduction in cellular IL-6 levels, and restoration of IL-6 may be a new target to retain MSC immunoprivilege.