In mice, elimination of Tregs decreased the production of several vascular protection-related proteins in ECs, such as cyclooxygenase 2 (COX-2), prostaglandin I2 synthase (PTGIS; prostacyclin synthase), programmed death ligand 1 (PD-L1) and heme oxygenase 1 (HO-1) and worsened pulmonary hypertension. Here, HMOX1 is linked to pulmonary arterial hypertension.