ABSTRACT Pulmonary hypertension (PH), arising from primary pulmonary vascular disease or secondary to many other conditions, is associated with high morbidity and mortality. Despite recent therapeutic advancements, PH hospitalization and mortality continue to rise. Development of effective therapy is needed but is limited by a fundamental knowledge gap in our understanding of cells and mechanisms that regulate pulmonary vascular remodeling underlying PH development. Macrophage (MØ) accumulation is observed in animal models and human PH suggesting a role for monocytes/MØs in regulating PH pathogenesis, My K08 award centered on defining the contribution of monocytes/MØs to PH pathogenesis. I demonstrated that non-classical monocytes (NcMos) sense hypoxia, through hypoxia inducible factor-1α (HIF1α), and differentiate into pro-vascular remodeling interstitial MØs (IMØs) that promote PH development. Effective targeting of these disease promoting cells to alleviate disease will require an understanding of mechanisms by which they regulate vascular remodeling. I identified paired immunoglobulin-like type 2 receptor beta (PILRβ) as a candidate NcMo-derived IMØ-specific disease mediator. We hypothesize that, in response to hypoxia and activation of HIF1α: 1) PILRβ expression in infiltrating IMØs is increased and 2) PILRβ engages its receptor, CD99, on vascular structural cells (i.e. endothelial cells, vascular smooth muscle cells, and fibroblasts) to increase the production of cytokines (e.g. TNF, IL-1β, and IL-6) that promote pulmonary vascular remodeling and PH development. Our preliminary studies show that gene expression of PILRβ is decreased in Hif1α-deficient IMØs. This decrease is associated with lower expression of pro-vascular remodeling cytokines (i.e. TNF, IL-1, and IL6), as well as reduced severity of vascular remodeling and PH. These findings suggest that PILRβ is a NcMo/IMØ-specific mediator that modulates PH pathogenesis. Utilizing transgenic animal and human PAH lung tissue collection developed during my K08 award, we propose the following specific aims: Aim 1. To determine the expression patterns of PILRβ receptor complex and its regulation by HIF1α in a murine model and human PH. NcMo-specific Hif1α knockout mice will be examined in a model of hypoxia-induced PH to define PILRβ expression pattern over the course of PH development. PILRβ expression in human PAH lung will also be examined. Aim 2. To determine the cells that express CD99, a ligand for PILRβ, and ligand expression in response to chronic hypoxia and in human PH. CD99 expression will be examined over the course of PH development in hypoxia-induced PH and human PAH lung explants. Aim 3. To determine cytokine response during PH development that is modulated by infiltrating IMØs to promote vascular remodeling. Cytokine responses over the course of PH development will be examined in NcMo-specific Hif1α knockout mice and in human PAH lung explants. The proposed studies will provid...