Serious adverse events noticed in some of mesenchymal stem cell-treated patients could be explained by the fact that mesenchymal stem cells either suppress or promote inflammation in dependence of inflammatory environtment to which they are exposed to. When mesenchymal stem cells are transplanted in the tissue with high levels of pro-inflammatory cytokines (IFN-γ, TNF-α, IL-12, IL-6, IL-17, and IL-23), mesenchymal stem cells adopt an immuno-suppressive phenotype and modify maturation of DCs, promote conversion of macrophages in anti-inflammatory M2 phenotype and suppress proliferation and activation of T lymphocytes, BK and NKT cells. In the presence of low levels of inflammatory cytokines, mesnchymal stem cells adopt a pro-inflammatory cytokines that promote neutrophil and T cell activation and enhance immune response and inflammation.