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One central factor in hepatopulmonary syndrome (HPS) pathogenesis is intravascular accumulation of activated macrophages in small pulmonary arteries. However, molecular mechanism underlying the macrophage accumulation in HPS is unknown. In this study, we aimed to explore whether elevated COX-2 induces the Bone morphogenic protein-2 (BMP-2)/Crossveinless-2 (CV-2) imbalance and then activation of BMP signaling pathway promotes the macrophage accumulation in Common Bile Duct Ligation (CBDL) rat lung.The COX-2/PGE2 signaling activation, the BMP-2/CV-2 imbalance and the activation of Smad1 were evaluated in CBDL rat lung and in cultured pulmonary microvascular endothelial cells (PMVECs) under the HPS serum stimulation. The effects of Parecoxib (COX-2 inhibitor), BMP-2 and CV-2 recombinant proteins on 4-week CBDL rat lung were determined, respectively.The COX-2/PGE2 signaling pathway was activated in CBDL rat lung in vivo and PMVECs in vitro, which was due to the activation of NF-κB P65. The inhibition of COX-2 by Parecoxib reduced macrophage accumulation, decreased lung angiogenesis and improved HPS. Meanwhile, the CBDL rat lung secreted more BMP-2 but less CV-2, and the imbalance between BMP-2 and CV-2 exacerbated the BMP signaling activation thus promoting the macrophage accumulation and lung angiogenesis. The BMP-2/CV-2 imbalance is dependent on the COX-2/PGE2 signaling pathway, and thus the effects of this imbalance can be reversed by adminstration of Parecoxib.Our findings indicate that inhibition of COX-2 by parecoxib can improve the HPS through the repression of BMP signaling and macrophage accumulation.COX-2/PGE2 signaling is strongly activated in lung after CBDL.COX-2/PGE2 activation after CBDL promotes the development of HPS via macrophage accumulation.The BMP-2/CV-2 imbalance induced BMP signaling promotes macrophage accumulation after CBDL.COX-2/PGE2 signaling promotes the imbalance of BMP-2/CV-2 in PMVECs and lung.Parecoxib improves HPS by the inhibition of BMP-2/CV-2 imbalance and macrophage accumulation, indicating its clinical value.