Mesenchymal stromal cells from JAK2 (V617F) myeloproliferative neoplasms support healthy and malignant hematopoiesis in a humanized scaffold model in vivo.

Myeloproliferative Neoplasms (MPN) are malignancies of hematopoietic stem and progenitor cells (HSPCs) that lead to the overproduction of mature blood cells. These disorders include Essential Thrombocythemia (ET), Polycythemia Vera (PV), and Primary Myelofibrosis (PMF), primarily driven by somatic mutations such as JAK2 (V617F) . Research indicates that mesenchymal stromal cells (MSCs) support fibrosis in PMF, though their role in ET and PV remains less clear. Furthermore, in vivo studies of ET/PV HSPCs remain a challenge due to low engraftment levels in xenograft models. We employed a 3D scaffold model to create an MPN humanized xenograft mouse model, enabling in vivo functional studies of primary MPN progenitor cells and the supportive role of human MSCs. Using this model, we first demonstrated robust hematopoietic support of healthy (HD) HSPCs by PV and ET MSCs. We then investigated the role of MSCs in sustaining JAK2 (V617F) mutant cells by using a CRISPR-Cas9 editing model, along with primary PV and ET HSPCs. Our results showed consistent engraftment of CRISPR-edited JAK2 (V617F) mutant HSPCs and PV and ET patient-derived HSPCs in scaffolds seeded with HD, PV, and ET stroma, providing the first in vivo evidence that PV and ET MSCs can sustain both healthy and MPN-associated hematopoiesis. Furthermore, HD MSCs were also capable of sustaining PV and ET HSPCs in vivo. Overall, we present the first humanized MPN xenograft model that offers valuable insights into how human BM MSCs interact with JAK2 (V617F) mutant clones.