Donor MSCs derived from wild-type (WT) mice or heme oxygenase-1 (HO-1) null mice (Hmox1(KO)) conferred partial protection from PAH when transplanted into WT or Hmox1(KO) recipients, whereas treatment with MSCs isolated from transgenic mice harboring a human HO-1 transgene under the control of surfactant protein C promoter (SH01 line) reversed established disease in WT recipients.