Abstract Body: Fibrosis and inflammation are crucial for heart healing following ischemic injury, but their prolonged activation contributes to cardiovascular disease progression. While myofibroblasts are primarily thought to originate from cardiac fibroblasts in heart failure (HF), we previously identified the role of pro-fibrotic platelet-derived growth factor (PDGF) receptor (PDGFR)-expressing Sca1⁺ cardiac mesenchymal cells in chronic HF. PDGF, a key mediator of tissue fibrosis, signals through PDGFRα and PDGFRβ. Whereas cardiac fibroblasts express PDGFRα which is important for their survival, the role of PDGFRβ in cardiac fibroblasts is not known. Preliminary scRNA-Seq analysis of mouse hearts identified cardiac fibroblast mesenchymal cells (cFMSCs) as multipotent fibroblasts with mesenchymal stem cell-like traits with elevated PDGFRβ expression that expand during HF. Flow cytometry-sorted cFMSCs (CD45^-CD31^-PDGFRα⁺Sca1⁺) differentiate into mesodermal lineage cells and exhibit gene signatures characteristic of activated fibroblasts. We hypothesize that enhanced cFMSC-PDGFRβ expression during chronic HF drives maladaptive left ventricular (LV) remodeling, fibrosis, and inflammation. To test this, we generated transgenic (Tg) mice with an inducible PDGFRβ-activating mutation in cardiac fibroblasts (Tcf21^+/MerCreMer-PDGFRβ^+/-D849) that activate PDGFRβ signaling in cardiac fibroblasts upon tamoxifen administration in the absence of myocardial injury. In preliminary studies, Tg mice were fed tamoxifen-containing diet for 8 w. Echocardiographic analysis showed no significant changes in cardiac function, but global longitudinal strain (GLS), an early predictor of cardiac dysfunction, was elevated in Tg mice. GLS changes were associated with increased myocardial interstitial fibrosis, cardiomyocyte hypertrophy, and macrophage abundance. These findings suggest that cFMSC-PDGFRβ activation is sufficient to induce LV dysfunction by promoting fibrosis and inflammation, potentially through cFMSC differentiation into inflammatory myofibroblasts. Modulating PDGFRβ expression in cardiac fibroblasts may offer a therapeutic avenue for chronic HF.