Myelofibrosis (MF) associated with myeloproliferative neoplasms (MPN) has been considered to be a reactive phenomenon caused by mesenchymal stromal cells (MSCs) stimulated by cytokines such as TGFb-1 overproduced by neoplastic megakaryocytes (MKs) and platelets. TGFb-1 stimulates non-neoplastic mesenchymal cells to produce collagen and fibronectin and to induces bone marrow (BM) fibrosis. However, the involvement of neoplastic fibrocyte in MF has recently been reported (Verstovsek et al. JEM 2016), and among blood cells, monocytes in particular are considered to be the main source of neoplastic fibrocytes. In this study, we assesed the role of neoplastic fibrocytes using a mouse model of MPN induced by Jak2V617F (Shide et al. Leukemia 2008).

First, the distribution of neoplastic fibrocyte in the BM of Jak2V617F transgenic (TG) mice was examined. We transplanted wild-type (WT) or Jak2V617F TG cells (B6-CD45.2), together with WT BM cells (B6-CD45.1) into irradiated WT recipient mice (B6-CD45.1). Only recipient mice transplanted with a mixture of Jak2V617F cells and WT cells developed BM fibrosis. In immunofluorescent staining of fibrotic BM, cells expressing the fibrocyte marker CD45/Collagen-1(Col-1) were observed much more than cells expressing the fibroblast marker CD90(usually positive for MSCs)/Col-1. As for CD45/Col-1 positive cells, cells expressing CD45.2/Col-1 were much more than cells expressing CD45.1/Col-1, clearly indicating that these cells were derived from Jak2V617F mutant blood cells. On the other hand, in the BM of recipient mice transplanted with control WT cells, few cells expressing CD45/Col-1 or CD90/Col-1 were present. To examine the differentiation ability of Jak2V617F blood cells to fibrocytes directly, peripheral blood (PB) mononuclear cells (MNC) of Jak2V617F mice or WT mice were cultured in vitro. After 5 days of culture, PB MNCs from Jak2V617F mice differentiated into mature fibrocytes exhibiting a long spindle shape with Col-1 expression. On the other hand, there were very few fibrocytes differentiated from PB MNC from WT mice.

Next, we depleted monocytes, the main source of fibrocytes, and observed its effects on BM fibrosis in vivo. Jak2V617F TG mice were mated with CD11b-diphtheria toxin receptor (DTR) TG mice (Duffield et al. JCI 2005) to obtain Jak2V617F/CD11b-DTR double TG mice. Mice transplanted with BM cells from Jak2V617F/CD11b-DTR double TG mice (hereinafter called Jak2V617F/CD11b-DTR mice) exhibit leukocytosis, thrombocytosis, anemia, splenomegaly, and BM fibrosis with increased megakaryocytes. Jak2V617F/CD11b-DTR mice was administered diphtheria toxin (DT) intraperitoneally to deplete monocytes. One day after DT administration, the number of PB monocytes (CD11b+/F4/80+) drastically decreased in Jak2V617F/CD11b-DTR mice, and the reduction of monocyte was maintained by every-other-day DT administration. After 8 weeks DT treatment, mice were sacrificed and analyzed. As a control group, Jak2V617F/CD11b-DTR mice treated with PBS were examined. DT treatment drastically decreased the number of neoplastic fibrocytes expressing CD45.2/Col-1 in BM and spleen of Jak2V617F/CD11b-DTR mice compared with control mice treated with PBS. Consistently, reticulin fibers were eliminated almost completely and collagen fibers almost fully disappeared in BM, which led to a reversal of the decrease in BM cellularity, although the number of MKs was not affected. Similar findings were observed in the spleen, although not completely. Plasma TGF-b1 level were about 2-fold higher in Jak2V617F/CD11b-DTR mice than in WT mice. Neoplastic monocyte depletion significantly decreased TGF-b1 level. Since MK numbers did not change, this indicates that fibrocytes are one of the main sources of TGF-b1. In other features of MF in Jak2V617F/CD11b-DTR mice, splenomegaly was ameliorated by DT treatment. Microscopic analysis revealed an improvement in the damaged spleen architecture and the disappearance of splenic fibrosis.

In summary, most collagen-producing cells in BM were neoplastic fibrocytes in Jak2V617F-induced MPN, indicating that neoplastic fibrocytes played an essential role and mesenchymal fibroblasts had a minor contribution in fibrosis in MPN. Depletion of neoplastic monocytes also improved splenomegaly as well as BM fibrosis in mice, and this cell fraction could be a promising therapeutic target.

Disclosures

No relevant conflicts of interest to declare.

Author notes

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