GM-CSF and STAT5 signaling axis as regulator of Trem2+ macrophage fate specification in the heart

PROJECT SUMMARY Cardiovascular disease is the leading cause of death worldwide. While current heart failure therapies primarily target neurohormonal pathways driving adverse remodeling, they do not address inflammation, a key contributor to cardiac dysfunction and fibrosis. Recent studies have highlighted the importance of monocyte- derived macrophages marked by C-C chemokine receptor 2 (CCR2) in cardiac inflammation and remodeling. These CCR2+ macrophages arise from highly plastic monocytes and form a heterogeneous population with both inflammatory and reparative functions. However, little is understood about the mechanisms that govern monocyte fate specification in the heart. This project investigates the GM-CSF/STAT5 signaling axis as a critical regulator of CCR2+ monocyte fate specification during cardiac injury. GM-CSF (Csf2) signaling, which drives STAT5-mediated transcription of inflammatory genes, is upregulated following cardiac injury. My central hypothesis is that GM-CSF signaling to heart-infiltrating monocytes regulates monocyte differentiation, controlling the specification between inflammatory and reparative macrophage states, a process critical for cardiac repair. Supporting this hypothesis, I found that Stat5 deletion in CCR2+ monocytes reduced cardiac inflammation and improved cardiac function in two distinct murine heart failure models. Importantly, I used single-cell RNA sequencing to identify cardiac macrophage states, and I demonstrated that Stat5 deletion reduces inflammatory macrophage subsets and enriches a macrophage population expressing high levels of Trem2 (triggering receptor expressed on myeloid cells 2). Trem2+ macrophages are implicated in resolving inflammation and promoting tissue repair in other organs, but their specification and functional roles in the heart remain unexplored. Aim 1 of this proposal elucidates mechanisms of Trem2+ macrophage specification and fate maps their lineage in the heart. Aim 2 investigates the functional roles of Trem2+ macrophages in murine models of heart failure. By addressing critical gaps in our understanding of CCR2+ monocyte fate specification and the identity and function of Trem2+ macrophages, this work will inform novel therapeutic strategies targeting specific macrophage states to mitigate inflammation in cardiac disease—maximizing treatment efficacy while minimizing off-target effects.