BACKGROUND:Pulmonary hypertension (PH) is a lethal disease characterized in part by progressive pulmonary arteriole (PA) remodeling. Excessive PA fibrosis and macrophage infiltration are often present in PH, but the potential associations are obscure. We investigated the link between interstitial macrophage (iM Phi) infiltration and PA fibrosis in PH and idiopathic pulmonary arterial hypertension.METHODS:Lung tissue samples from patients with idiopathic pulmonary arterial hypertension and experimental PH animals were obtained to analyze the extent of fibrosis and iM Phi infiltration in the different layers of PAs and their correlation with disease severity. Single-cell RNA sequencing, lineage tracing, histological analyses, iM Phi and PA smooth muscle cell coculture, and transgenic animal experiments were used to investigate the cell heterogeneity and origins and molecular mechanisms by which iM Phi s promote PA fibrosis.RESULTS:We found that increased collagen deposition and fibrosis in the PA media were most strongly related to the severity of PH, and medial iM Phi infiltration may be involved in these pathological processes. Single-cell transcriptomics revealed that major histocompatibility complex class II (high) lymphatic vessel endothelial hyaluronan receptor 1 (low) C-C motif chemokine receptor 2 (high) (MHCIIhiLYVE1loCCR2hi) iM Phi s were the major type of iM Phi that expanded upon Sugen-5416 and hypoxia plus normoxia stimulation and were responsible for PA medial fibrosis. Lineage tracing experiments suggested that these medial iM Phi s were largely from recruited monocytes. Mechanistically, MHCIIhiLYVE1loCCR2hi iM Phi s promoted the transition of PA smooth muscle cells to a fibroblast-like phenotype through the WNT11 (wingless member 11)/planar cell polarity (PCP) pathway. Wnt11 deletion in iM Phi s from PH rats normalized the fibrotic PA smooth muscle cell phenotype and decreased PA medial fibrosis, thereby improving vascular compliance and protecting against PH. Moreover, myeloid-specific Ccr2 deficiency in PH-PAs inhibited the medial infiltration of MHCIIhiLYVE1loCCR2hi iM Phi s, which also relieved PH.CONCLUSIONS:This study demonstrates that the recruitment of MHCIIhiLYVE1loCCR2hi iM Phi s leads to medial fibrosis in PH-PAs associated with PH severity and that inhibition of their pathogenicity or recruitment reverses PA medial fibrosis and PH.