TGF-Beta Signaling in Mesenchymal Stromal Cells Contributes to Myelofibrosis but Not Hematopoietic Phenotypes in Myeloproliferative Neoplasms

The development of myelofibrosis in patients with myeloproliferative neoplasms (MPNs) is associated with a dismal prognosis. The mechanisms responsible for the progression to myelofibrosis are unclear, limiting the development of therapies to treat or prevent it. The cell of origin responsible for the increased collagen deposition is controversial, with recent studies implicating Gli1⁺ or leptin receptor⁺ mesenchymal stromal cells, monocytes, or even endothelial cells. Moreover, the signals generated by malignant hematopoietic cells in MPN that induce increased collagen expression are uncertain. There is some evidence that elevated expression of cytokines/chemokines in the bone marrow microenvironment of patients with MPN may contribute. In particular, recent studies have implicated transforming growth factor-β (TGF-β), platelet-derived growth factor and CXCL4 in the development of myelofibrosis. Here, we test the specific hypothesis that TGF-β signaling in mesenchymal stromal cells is required for the development of myelofibrosis. Moreover, we hypothesize that TGF-β signaling, by altering the expression of key niche factors by mesenchymal stromal cells, contributes to the myeloid expansion in MPN.

To test this hypothesis, we abrogated TGF-β signaling in mesenchymal stem/progenitor cells (MSPCs) by deleting Tgfbr2 using a doxycycline-repressible Sp7 (osterix)-Cre transgene (Osx-Cre), which targets all mesenchymal stromal cells in the bone marrow, including CXCL12-abundant reticular (CAR) cells, osteoblasts, adipocytes, or arteriolar pericytes. We previously showed that TGF-β signaling plays a key role in the lineage specification of MSPCs during development (2017 ASH abstract #2438). In contrast, we show that post-natal deletion of Tgfbr2, by removing doxycycline at birth, is not associated with significant changes in mesenchymal stromal cells in the bone marrow. Moreover, expression of key niche factors, including Cxcl12 and stem cell factor, and basal hematopoiesis were normal in these mice. Thus, we used the post-natal Osx-Cre; Tgfbr2-deleted mice as recipients to assess the role of TGF-β signaling in mesenchymal stromal cells on the hematopoietic and myelofibrosis phenotype in Jak2^V617For MPL^W515Lmodels of MPN. Specifically, we transplanted hematopoietic cells from Mx1-Cre; Jak2^V617Fmice (4 weeks after pIpC treatment) or hematopoietic cells transduced with MPL^W515Lretrovirus into irradiated wildtype or post-natal Osx-Cre; Tgfbr2-deleted mice. Both MPN models have elevated Tgfb1 expression in the bone marrow. As reported previously, transplantation of MPL^W515Ltransduced hematopoietic cells into wildtype recipients produced a rapidly fatal MPN characterized by neutrophilia, erythrocytosis, thrombocytosis, splenomegaly, and reticulin fibrosis in the bone marrow. A similar hematopoietic phenotype was observed in Osx-Cre; Tgfbr2^fl/flrecipients. However, a trend to decreased reticulin fibrosis was observed in Osx-Cre; Tgfbr2^fl/flcompared to wildtype recipients (reticulin histology score: 0.5 versus 1.1, respectively, n=5, p=0.23). Likewise, the degree of neutrophilia, erythrocytosis, thrombocytosis, and splenomegaly in wildtype and Osx-Cre; Tgfbr2^fl/flrecipients of Jak2^V617Fcells was similar. As reported previously, we did not observe overt myelofibrosis in this model (as measured by reticulin staining). However, we were able to detect increased collagen III deposition using immunofluorescence staining in 4 of 5 wildtype recipients compared to 1 of 4 Osx-Cre Tgfbr2^fl/flrecipients of Jak2^V617Fcells (p=0.21).

In conclusion, our data suggest that TGF-β signaling in mesenchymal stromal cells contributes, but is not absolutely required, for the development of myelofibrosis. Alterations in mesenchymal stromal cells induced by increased TGF-β signaling do not appear to be a major driver of the myeloid expansion in MPN. The contribution of increased TGF-β signaling in hematopoietic cells or other bone marrow stromal cell populations to the MPN phenotype is under investigation.