BACKGROUND & AIMS: In liver fibrosis, myofibroblasts derive from hepatic stellate cells (HSCs) and as yet undefined mesenchymal cells. We aimed to identify portal mesenchymal progenitors of myofibroblasts.
APPROACH & RESULTS: Portal mesenchymal cells were isolated from mouse bilio-vascular tree and analyzed by scRNAseq. Thereby, we uncovered the landscape of portal mesenchymal cells in homeostatic mouse liver. Trajectory analysis enabled inferring a small cell population further defined by surface markers used to isolate it. This population consisted of portal fibroblasts with mesenchymal stem cell features (PMSCs), i.e., high clonogenicity and tri-lineage differentiation potential, that generated proliferative myofibroblasts, contrasting with non-proliferative HSC-derived myofibroblasts (-MFs). Using bulk RNAseq, we built oligogene signatures of the two cell populations, that remained discriminant across myofibroblastic differentiation. SLIT2, a prototypical gene of PMSC/PMSC-MF signature, mediated pro-fibrotic and angiogenic effects of these cells, whose conditioned medium promoted HSC survival and endothelial cell tubulogenesis. Using PMSC/PMSC-MF 7-gene signature and SLIT2 FISH, we showed that PMSCs display a perivascular portal distribution in homeostatic liver and largely expand with fibrosis progression, contributing to the myofibroblast populations that form fibrotic septa, preferentially along neo-vessels, in murine and human liver disorders, irrespective of aetiology. We also unraveled a 6-gene expression signature of HSCs/HSC-MFs that did not vary in these disorders, consistent with their low proliferation rate.
Perivascular portal fibroblasts with stem cell features generate expansive myofibroblasts, expressing SLIT2, which promotes angiogenesis and the survival of hepatic stellate cell-derived myofibroblasts, thereby underlying the formation of fibrotic septa (FS).
CONCLUSIONS: PMSCs form a small reservoir of expansive myofibroblasts, which in interaction with neo-vessels and HSC-MFs that mainly arise via differentiation from a preexisting pool, underlie the formation of fibrotic septa in all types of liver diseases.