Reciprocal regulation of fibroblast-macrophage equilibrium governs skin integrity.

Fibroblast-macrophage crosstalk is well-established in vitro, and fibroblast-derived colony-stimulating factor 1 (CSF1) supports macrophages in select tissues. However, whether macrophages regulate fibroblasts in vivo remains unknown. Leveraging genetic mouse models, single-cell multi-omics, flow cytometry and imaging, we show that fibroblast depletion or loss of fibroblast-derived growth factors impacts skin macrophage populations in the dermis and hypodermis. Conditional deletion of Csf1 in Dpt+ fibroblasts progressively decreases CD64+ and CD11c+ macrophages, impairing skin wound healing. Reduced macrophage abundance disrupts fibroblast cell cycle regulation, metabolism and immune signaling, and increases fibroblast abundance, affirming a reciprocal relationship. In human systemic sclerosis (scleroderma), elevated fibroblast-derived CSF1 and increased macrophage abundance correlate with disease severity, implicating the CSF1-CSF1R axis in pathology. These findings provide in vivo evidence of macrophage regulation of fibroblasts, revealing a bidirectional interplay that advances understanding of tissue homeostasis and immune regulation in skin.