ARG1 and pulmonary arterial hypertension: Further studies have shown that in the hypoxic PAH calf model, IL-6 released from pulmonary artery explants activates the STAT3–C/EBPβ axis rather than the classical IL-4/IL-13–STAT6 pathway in macrophages, thereby inducing hallmark molecules such as Arg1 and giving rise to a disease-associated, non-classical alternatively activated state (75).