In vitro (Fig. 4e), a significant four-fold increased IL6 concentration compared to controls (1.1 ± 0.56 vs. 4.1 ± 1.95, p = 0.01) was measured in the cell-free supernatants of PAH MVECs after 24-h BMP9 stimulation. Here, IL6 is linked to pulmonary arterial hypertension.