Xenotransplantation of Primary CD34+CD38- Hematopoietic Stem Cells Results in an In Vivo Model of Idiopathic Myelofibrosis.

Idiopathic myelofibrosis (IMF) is characterized by clonal proliferation of abnormal myelomonocytic cells and megakaryocytes. These abnormal cells secrete various cytokines resulting in reactive fibrosis and increased collagen content in the bone marrow (BM), and lead to extramedullary hematopoiesis and the appearance of CD34+ cells in the peripheral blood (PB). Although IMF is thought to originate at the level of hematopoietic stem cell (HSC), this has not been demonstrated directly in primary human IMF. To demonstrate the involvement of HSCs in the pathogenesis of IMF and to establish an in vivo model of IMF, we used the newborn NOD/SCID/IL2rg-null xenotransplantation model. We purified PB CD34+ cells from six IMF patients, transplanted 1–10 x10e4 cells intravenously into newborn NOD/SCID/IL2rg-null recipients and analyzed PB and BM human CD45+ hematopoietic cell chimerism, degree of suppression of murine hematopoiesis, presence of hallmark BM fibrosis and plasma TGF-b1 levels in the recipients at 6 months post-transplantation. Primary IMF PB CD34+ cells from five out of six patients engrafted in twelve out of twelve recipients. BM of all engrafted recipients demonstrated fibrotic changes associated with increased proliferation of murine fibroblasts, the presence of human megakaryocytes and elevated plasma TGF-b1 levels, recapitulating the clinical features of IMF. Three distinct patterns of human hematopoietic reconstitution were observed among the engrafted recipients:

1. Predominantly malignant myelomonocytic engraftment in the PB and BM (n=4),

2. Reconstitution of both normal human hematopoiesis (with mature B and T cells, myeloid cells and platelets) and malignant myelomonocytic cells (n=6) and

3. Development of acute leukemia (n=2).

Fibrotic change was seen even in the BM of recipients that showed normal human hematopoietic reconstitution, showing that in IMF, there is co-existence of both normal and malignant hematopoietic stem/progenitor cells in the PB CD34+ fraction. Furthermore, when 5–10 x 10e3 sorted PB CD34+CD38– cells from three patients were transplanted into six newborn NOD/SCID/IL2rg-null recipients, reconstitution with human myelomonocytic cells associated with BM fibrosis was demonstrated in all recipients, with compatible level of PB and BM chimerism with those transplanted with PB CD34+ cells. These findings demonstrate that the IMF-initiating cells are contained within the HSC fraction. The newborn NOD/SCID/IL2rg-null xenotransplantation model provides an in vivo model of primary human IMF that may lead to better understanding of the mechanisms of IMF pathogenesis including the identification of IMF stem cells and may be useful for development of novel therapeutic agents for IMF.