A number of preclinical reports support the use of MSCs for improvement of HSCT engraftment and/or GVHD, with mechanisms including suppression of effector T cells and improvement of survival in murine in vivo GVHD models through MSC‐expressed IDO and PD‐L1 in response to inflammatory cytokines such as IFN‐γ, TNF‐α, and IL‐1β, all inflammatory cytokines which are elevated in GVHD patients.53, 54. Here, IDO1 is linked to graft versus host disease.