Pulmonary arterial hypertension (PAH) is characterized by progressive increase of pulmonary vascular resistance and obliterative vascular remodeling that causes right heart failure and premature death. Given the underlying molecular mechanisms of obliterative vascular remodeling remain enigmatic, current therapies have not targeted the fundamental disease modifying mechanisms and hence only resulted in a modest improvement in the morbidity and mortality. We have recently shown that Egln1Tie2Cre mice exhibit unprecedented spontaneous severe PAH and obliterative pulmonary vascular remodeling including neointima formation, and vascular occlusion as seen in patients with idiopathic PAH (IPAH) and PHD2 expression is diminished in ECs of occlusive pulmonary vessels of IPAH patients. This renewal proposal is based on our fundamental new observations that 1) genetic depletion of macrophages (Mφ) with the MaFIA mice inhibited obliterative vascular remodeling and attenuated PAH in Egln1Tie2Cre/MaFIA mice; 2) Single cell RNA sequencing analysis identified an emerging subpopulation of PAH-specific progenitor Mφ in Egln1Tie2Cre lungs; 3) CD206+ Mφ accumulated in the vascular lesions of Egln1Tie2Cre lungs expressed abundantly angiogenic factors inducing PASMC proliferation; 4) genetic disruption of the GPCR-dependent p110γ isoform of PI3K in Egln1Tie2Cre mice inhibited the production of these factors and obliterative vascular remodeling. Thus, we hypothesize that marked accumulation of CD206+ Mφ derived from progenitor Mφ in pulmonary vascular lesions induces obliterative pulmonary vascular remodeling and severe PAH through p110γPI3K-dependent induction of pro-PAH factors via EC-Mφ interaction. Studies in Aim 1 will determine the fundamental role of CD206+ Mφ subpopulation in promoting obliterative pulmonary vascular remodeling and severe PAH. We will employ single cell RNA sequencing, lineage tracing and genetic depletion approaches to determine the causal role of CD206+ Mφ and pMφ in mediating obliterative pulmonary vascular remodeling. In Aim 2, we will delineate the molecular basis of CD206+ Mφ in mediating obliterative pulmonary vascular remolding and explore the translational potential of targeting these molecular mechanisms for treatment of PAH. The molecular mechanisms identified in mouse models will be validated in SuHx PAH rats. The clinical relevance will also be determined with PASMCs and lung tissues of IPAH patients. We expect that the proposed studies have significant translational potential by delineating the molecular and cellular mechanisms of obliterative vascular remodeling, identifying druggable targets, and exploring novel pharmacological agents that can pharmacologically inhibit/reverse vascular remodeling for the prevention and treatment of PAH in patients.