Role of neoplastic monocyte-derived fibrocytes in primary myelofibrosis.

Primary myelofibrosis (PMF) is a fatal neoplastic disease characterized by clonal myeloproliferation and progressive bone marrow (BM) fibrosis thought to be induced by mesenchymal stromal cells stimulated by overproduced growth factors. However, tissue fibrosis in other diseases is associated with monocyte-derived fibrocytes. Therefore, we sought to determine whether fibrocytes play a role in the induction of BM fibrosis in PMF. In this study, we show that BM from patients with PMF harbors an abundance of clonal, neoplastic collagen- and fibronectin-producing fibrocytes. Immunodeficient mice transplanted with myelofibrosis patients' BM cells developed a lethal myelofibrosis-like phenotype. Treatment of the xenograft mice with the fibrocyte inhibitor serum amyloid P (SAP; pentraxin-2) significantly prolonged survival and slowed the development of BM fibrosis. Collectively, our data suggest that neoplastic fibrocytes contribute to the induction of BM fibrosis in PMF, and inhibiting fibrocyte differentiation with SAP may interfere with this process.

Bone marrow stroma-secreted cytokines protect JAK2(V617F)-mutated cells from the effects of a JAK2 inhibitor.

Signals emanating from the bone marrow microenvironment, such as stromal cells, are thought to support the survival and proliferation of the malignant cells in patients with myeloproliferative neoplasms (MPN). To examine this hypothesis, we established a coculture platform [cells cocultured directly (cell-on-cell) or indirectly (separated by micropore membrane)] designed to interrogate the interplay between Janus activated kinase 2-V617F (JAK2(V617F))-positive cells and the stromal cells. Treatment with atiprimod, a potent JAK2 inhibitor, caused marked growth inhibition and apoptosis of human (SET-2) and mouse (FDCP-EpoR) JAK2(V617F)-positive cells as well as primary blood or bone marrow mononuclear cells from patients with polycythemia vera; however, these effects were attenuated when any of these cell types were cocultured (cell-on-cell) with human marrow stromal cell lines (e.g., HS5, NK.tert, TM-R1). Coculture with stromal cells hampered the ability of atiprimod to inhibit phosphorylation of JAK2 and the downstream STAT3 and STAT5 pathways. This protective effect was maintained in noncontact coculture assays (JAK2(V617F)-positive cells separated by 0.4-µm-thick micropore membranes from stromal cells), indicating a paracrine effect. Cytokine profiling of supernatants from noncontact coculture assays detected distinctly high levels of interleukin 6 (IL-6), fibroblast growth factor (FGF), and chemokine C-X-C-motif ligand 10 (CXCL-10)/IFN-γ-inducible 10-kD protein (IP-10). Anti-IL-6, -FGF, or -CXCL-10/IP-10 neutralizing antibodies ablated the protective effect of stromal cells and restored atiprimod-induced apoptosis of JAK2(V617F)-positive cells. Therefore, our results indicate that humoral factors secreted by stromal cells protect MPN clones from JAK2 inhibitor therapy, thus underscoring the importance of targeting the marrow niche in MPN for therapeutic purposes.