In addition, lactic acid produced by tumor cells has been recently demonstrated to induce Arg1 expression in macrophages via an hypoxia-inducible factor 1α (HIF1α)-dependent pathway [43] and that IL-6 produced by fibroblasts in a model of pulmonary hypertension induce Arg1 in macrophages via STAT3- and C/EBPβ-dependent pathways [44]. Here, STAT3 is linked to pulmonary arterial hypertension.