Inhibition of Overactive TGF-β-Induced Fibrotic Scar Formation Repairs Mouse Spinal Cord Injury

Spinal cord injury (SCI) often causes disability in humans and other mammals. Here we report that fibrotic scar formation at injury sites prevents recovery after SCI and that the inhibition of fibrotic scar formation significantly improved recovery in adult mice. Neonatal mice were able to fully recover from SCI because they do not experience fibrotic scar formation. Active transforming growth factor-β 1 (TGF-β1) was significantly elevated at SCI sites to recruit mesenchymal stromal/stem cells (MSCs) and induce fibroblast differentiation. Eliminating macrophage lineage cells in LysM-cre::iDTRflox/flox mice significantly decreased TGF-β activity, suggesting macrophages as primary source of TGF-β1. Moreover, TGF-β activity was significantly decreased after selective deletion of TGF-β1 in macrophages in LysM-cre::Tgfb1flox/flox mice. Knocking out of TGF-β type 2 receptor in pericytes of Glast-creERT2::Tgfbr2flox/flox mice also decreased fibrotic scar formation. Interestingly, TGF-β–neutralizing antibody 1D11 treatment induced a greater inhibition of scar formation than any of the knockout mice. Single-cell RNA sequencing revealed that TGF-β downstream signaling was significantly enriched in MSCs, fibroblasts, pericytes, and endothelial cells after SCI, and the lineage trajectory indicates that pericytes differentiate to fibroblasts via MSCs. Most importantly, neonatal mice did not have active TGF-β at the injury site and they recovered completely after SCI. Thus, fibrotic scar may be the primary obstacle that prevents recovery after SCI.