A humanized ossicle model of myelofibrosis reveals THPO-driven fibrosis, osteosclerosis and SPP1-dependent microenvironmental remodeling

Myelofibrosis (MF) is the most severe myeloproliferative neoplasm. Current therapies - except for allogeneic stem cell transplantation - are largely supportive which highlights the need for improved disease models and novel therapeutic targets. Here, we established a humanized MF model by transplanting thrombopoietin (THPO)-overexpressing human bone marrow CD34{square} cells into humanized bone marrow ossicles generated in immunodeficient NSG mice. THPO overexpression induced progressive reticulin fibrosis in vivo, accompanied by myeloid skewing, increased megakaryocyte clustering, and redistribution of human hematopoietic cells to murine spleen and femur, consistent with extramedullary hematopoiesis. THPO-driven ossicles also exhibited features of osteosclerosis, including increased trabecular bone and osteoid formation, indicating active pathological remodeling of the niche. Mechanistically, fibrosis was associated with increased SPP1/OPN expression, which was also observed in bone marrow biopsies from MF patients. Importantly, in vivo neutralization of SPP1 attenuated myeloid skewing, reduced megakaryocyte expansion, and decreased fibrosis severity, highlighting SPP1-driven niche remodeling as a potential therapeutic target in MF. This humanized MF model thus provides a translationally relevant platform to dissect microenvironment-driven MF pathogenesis and evaluate targeted therapies.