Phenotype in Pulmonary Hypertension Proinflammatory/Profibrotic Macrophage Adventitial Fibroblasts Induce a Distinct

Macrophage accumulation is not only a characteristic hallmark but is also a critical component of pulmonary artery remodeling associated with pulmonary hypertension (PH). However, the cellular and molecular mechanisms that drive vascular macrophage activation and their functional phenotype remain poorly defined. Using multiple levels of in vivo (bovine and rat models of hypoxia-induced PH, together with human tissue samples) and in vitro (primary mouse, rat, and bovine macrophages, human monocytes, and primary human and bovine fibroblasts) approaches, we observed that adventitial fibroblasts derived from hypertensive pulmonary arteries (bovine and human) regulate macrophage activation. These fibroblasts activate macrophages through paracrine IL-6 and STAT3, HIF1, and C/EBPb signaling to drive expression of genes previously implicated in chronic inflammation, tissue remodeling, and PH. This distinct fibroblast-activated macrophage phenotype was independent of IL-4/IL-13–STAT6 and TLR– MyD88 signaling. We found that genetic STAT3 haplodeficiency in macrophages attenuated macrophage activation, complete STAT3 deficiency increased macrophage activation through compensatory upregulation of STAT1 signaling, and deficiency in C/EBPb or HIF1 attenuated fibroblast-driven macrophage activation. These findings challenge the current paradigm of IL-4/IL-13–STAT6–mediated alternative macrophage activation as the sole driver of vascular remodeling in PH, and uncover a cross-talk between adventitial fibroblasts and macrophages in which paracrine IL-6–activated STAT3, HIF1a, and C/EBPb signaling are critical for macrophage activation and polarization. Thus, targeting IL-6 signaling in macrophages by completely inhibiting C/EBPb or HIF1a or by partially inhibiting STAT3 may hold therapeutic value for treatment of PH and other inflammatory conditions characterized by increased IL-6 and absent IL-4/IL-13 signaling. S tudies in animal models of pulmonary hypertension (PH) and humans with pulmonary arterial hypertension (PAH) have provided convincing evidence that early and persistent inflammation is an essential component of pulmonary vascular disease (1–7). The extent of the vascular inflammatory infiltrate in PH was shown to correlate directly with parameters of vascular remodeling and hemodynamics (3, 4, 6). Importantly, as described extensively by our group (5, 8–12) and other investigators (3), PH-associated vascular inflammation is largely perivascular/ adventitial in nature and is characterized by a robust influx of leukocytes, primarily macrophages, into the adventitial compartment. An essential role for these cells in the PH process was demonstrated in experiments in which in vivo depletion of macrophages attenuated pulmonary vascular remodeling (8). We documented that, in both experimental hypoxia-induced PH and human PAH, the pulmonary artery (PA) adventitia harbors activated The online version of this article contains supplemental material. fibroblasts (termed " PH-Fibs …