The identification of fibrosis-driving myofibroblast precursors reveals new therapeutic avenues in myelofibrosis

Myofibroblasts are fibrosis-driving cells and are well characterized in solid organ fibrosis, but their role and cellular origin in bone marrow fibrosis remains obscure. Recent work has demonstrated that Gli1(+) and LepR(+) mesenchymal stromal cells (MSCs) are progenitors of fibrosis-causing myofibroblasts in the bone marrow. Genetic ablation of Gli1(+ )MSCs or pharmacologic targeting of hedgehog (Hh)-Gli signaling ameliorated fibrosis in mouse models of myelofibrosis (MF). Moreover, pharmacologic or genetic intervention in platelet-derived growth factor receptor alpha (Pdgfr alpha) signaling in Lepr(+) stromal cells suppressed their expansion and ameliorated MF. Improved understanding of cellular and molecular mechanisms in the hematopoietic stem cell niche that govern the transition of MSCs to myofibroblasts and myofibroblast expansion in MF has led to new paradigms in the pathogenesis and treatment of MF. Here, we highlight the central role of malignant hematopoietic clone-derived megakaryocytes in reprogramming the hematopoietic stem cell niche in MF with potential detrimental consequences for hematopoietic reconstitution after allogenic stem cell transplantation, so far the only therapeutic approach in MF considered to be curative. We and others have reported that targeting Hh-Gli signaling is a therapeutic strategy in solid organ fibrosis. Data indicate that targeting Gli proteins directly inhibits Gli1(+) cell proliferation and myofibroblast differentiation, which results in reduced fibrosis severity and improved organ function. Although canonical Hh inhibition (eg, smoothened [Smo] inhibition) failed to improve pulmonary fibrosis, kidney fibrosis, or MF, the direct inhibition of Gli proteins ameliorated fibrosis. Therefore, targeting Gli proteins directly might be an interesting and novel therapeutic approach in MF.