Spleens of myelofibrosis patients contain malignant hematopoietic stem cells

Abstract

Cancer stem cell behavior is thought to be largely determined by intrinsic properties and by regulatory signals provided by the microenvironment. Myelofibrosis (MF) is characterized by hematopoiesis occurring not only in the marrow but also in extramedullary sites such as the spleen. In order to study the effects of these different microenvironments on primitive malignant hematopoietic cells, we phenotypically and functionally characterized splenic and peripheral blood (PB) MF CD34 + cells from patients with MF. MF spleens contained greater numbers of malignant primitive HPCs than PB. Transplantation of PB MF CD34 + cells into immunodeficient (NOD/SCID/IL2Rγ null ) mice resulted in a limited degree of donor cell chimerism and a differentiation program skewed toward myeloid lineages. By contrast, transplanted splenic MF CD34 + cells achieved a higher level of chimerism and generated both myeloid and lymphoid cells that contained molecular or cytogenetic abnormalities indicating their malignant nature. Only splenic MF CD34 + cells were able to sustain hematopoiesis for prolonged periods (9 months) and were able to engraft secondary recipients. These data document the existence of MF stem cells (MF-SCs) that reside in the spleens of MF patients and demonstrate that these MF-SCs retain a differentiation program identical to that of normal hematopoietic stem cells.

Characterization of the TGF-β1 signaling abnormalities in the Gata1low mouse model of myelofibrosis

“…30 This concept was recently validated by the observation that the spleen of PMF patients is uniquely enriched for PMF stem cells. 50 The great numbers of PMF stem cells observed in the spleen of the patients and the different TGF-b1 alteration signatures of BM and spleen described here suggest a model for the development of MF, according to which TGF-b1 released by the MK progeny of MF stem cells induce different TGF-b1 expression signatures in BM and spleen, which suppress normal hematopoiesis in BM and activate MF hematopoiesis in spleen (supplemental Figure 4). This model predicts that, in spite of the encouraging results obtained with JAK2 inhibitors in PMF, the disease will be better controlled by therapies combining JAK2 inhibitors with drugs, still to be identified, and that may reactivate normal hematopoiesis in BM.…”

The microenvironment in human myeloid malignancies: emerging concepts and therapeutic implications

“…Notably, propagation of human myeloid leukemia in vivo remains challenging, even for a subset of AML, a fully penetrant and deadly disease in patients. Besides the notoriously poor engraftment of MDS 37,140-142 and MPNs, 143,144 ;50% of AMLs fail to engraft immune-compromised hosts. 145 This fraction of "engrafter" cases could be further increased in AML by using new mouse lines that express a number of human cytokines that are poorly or not at all cross-reactive between mice and humans.…”

The characteristics of vessel lining cells in normal spleens and their role in the pathobiology of myelofibrosis

“…44 In addition, our laboratory has also reported the presence of greater numbers of malignant stem cells in the spleen compared with the peripheral blood of patients with MF. 16 Splenic endothelial cells express many hematopoietic factors, which may be insufficient to maintain the normal hematopoietic progenitor potential. MF hematopoietic progenitors, in contrast, are more sensitive to hematopoietic growth factors and microenvironmental cues because of the characteristic driver mutations and the subsequent activation of the JAK/STAT pathway.…”

“…The vascular lining cells within MF spleens are thought to serve as a supportive microenvironment for MF hematopoietic cells. [13][14][15][16] Barosi and coworkers have reported that the capillary, but not sinusoidal, vascular density was increased within the MF spleen, (CD206), and STAB2 (Stabilin-2) (H); RHOU (J). The expression of all genes was normalized to GAPDH, and the relative expression of a LC gene was calculated against that of SVEC, which was set as 1.…”