A composition comprising TGFβ, an inflammatory agent and a tryptophan indoleamine-2,3 dioxygenase (IDO) metabolite, in particular, TGFβ, IFNγ and kynurenine, is provided, as well as regulatory mesenchymal stem cell lines or myeloid-derived suppressor cell lines obtained by contacting mesenchymal stem cell lines or myeloid-derived suppressor cell lines, respectively, with the composition. Methods for inhibiting proliferation of T cells, reducing Th17 and Tc17 differentiation of activated T cells and inflammation or for treating inter alia graft-versus-host disease (GVHD), comprising administering the cell lines, are further provided.
