Together, these studies showed that allogeneic ABCB5+ PD-1+ MSCs can exert, in a stem cell graft PD-1/recipient PD-L1-dependent fashion, significant in vivo inhibitory effects on alloreactive T-cell activation, along with induction of tolerogenic CD4+CD25+Foxp3+ Treg responses [10], providing a rationale for their potential therapeutic development in settings of unwanted allo-activation such as, for example, allograft rejection or graft-versus-host disease (GvHD). Here, PDCD1 is linked to graft versus host disease.