SPP1 macrophages promote sFRP2 expressing fibroblasts in epidural fibrosis

Post-spinal surgical lower back pain is a primary complication resultant from excessive epidural fibrosis exerting compression on the lumbar nerves. The underlying mechanism of epidural fibrosis is currently not well understood. Here, we identified a profibrotic macrophage population, termed SPP1+ macrophages, which characterized by the expression of Spp1, MMP12 and Arg1. We determined that SPP1+ macrophages expanded in mice and patients after laminectomy. GESA analysis showed enrichments of toll-like receptor signaling pathway in SPP1+ macrophages. Then we verified SAA3 to be among the top upregulated genes in operation mice, which is a functional ligand of toll-like receptor. Forward and reverse experiments in vitro confirmed that SAA3 induced SPP1+ macrophages via TLR4/MyD88/NF-KB signaling pathway. In the mouse spinal operation model, SPP1 knockout, macrophage specific SPP1 knockdown or anti-SPP1 antibody alleviated epidural fibrosis. Furthermore, we discovered a profibrotic fibroblast population, termed sFRP2+ fibroblasts, which appeared in mice and patients post laminectomy. SFRP2 enhancer lentivirus and sFRP2 CRISPR sgRNA determined the profibrotic effects in vitro. CellphoneDB analysis revealed the interaction between SPP1+ macrophages and sFRP2+ fibroblasts. Co-cultured system verified that SPP1+ macrophages induced sFRP2+ fibroblasts by producing IL-1b, which could be partially blocked by IL-1 receptor inhibitor. Lastly, IL-1 receptor inhibition mitigated epidural fibrosis in the mouse model of laminectomy. Overall, our results provide a potential therapeutic strategy for epidural fibrosis post laminectomy by targeting SPP1+ macrophages and sFRP2+ fibroblasts.