Interleukin-13- and Tansforming Growth Factor-β- Producing Mast Cells Are Associated with Fibrosis in Bone Marrow
Authors: Nakayama Shoko , Yokote Taiji , Hiraoka Nobuya , Akioka Toshikazu , Miyoshi Takuji , Iwaki Kazuki , Fumimoto Ayami , Nishiwaki Uta , Masuda Yuki , Hatooka Jun , Fujimoto Mayumi , Tsuji Motomu 
Published: October 2019

Abstract
Background: Fibrosis in bone marrow is a lethal condition, which currently has no effective treatments. Transforming growth factor (TGF)-β, produced by megakaryocytes and blasts in myelodisplastic syndrome (MDS) with bone marrow fibrosis, is reportedly linked to this condition; however, the underlying mechanism of fibrosis in bone marrow remains uncharacterized. Recently, interleukin (IL)-13- and TGF-β- producing mast cells have been reported to participate in the development of fibrosis in certain fibrotic diseases, such as the IgG4-related diseases.
Aim: This study aimed to investigate the pathogenesis of fibrosis in bone marrow through histological examination of mast cell infiltration and the expression of cytokines associated with fibrosis.
Methods: We analyzed 22 bone marrow samples from patients with confirmed fibrosis; of these, eight patients had been diagnosed with primary myelofibrosis (PMF), five with post-polycythemia vera (PV)/essential thrombocythemia (ET) MF, and nine with MDS with bone marrow fibrosis. These were compared with 15 control bone marrow samples, using immunohistochemical staining of mast cell tryptase, IL-13, and TGF-β, as well as using dual immunofluorescence assays for the following pairs: mast cell tryptase/ IL-13 and mast cell tryptase/ TGF-β. The extent of fibrosis in bone marrow samples was analyzed by Masson's trichrome staining and silver impregnation staining.
Results: Among eight PMF cases, five were identified as grade 2 and three as grade 3; among five post-PV/ET cases, four were grade 2 and one was grade 3; and among nine MDS patients with bone marrow fibrosis, six were grade 2 and three were grade 3. Grading was performed according to the European consensus on grading of bone marrow fibrosis. The number of mast cells in bone marrow samples was significantly higher in cases with fibrosis than in controls, as analyzed by t-test (p < 0.0001) (Figure 1A). Each group of bone marrow samples with fibrosis (primary MF, post-PV/ET MF, and MDS with bone marrow fibrosis) also had significantly more infiltrating mast cells than controls, using multiple comparison with the Tukey's honestly significant difference test (p = 0.0470, < 0.0001, 0.0005, respectively) (Figure 1B). Bone marrow samples with higher fibrotic grades exhibited greater amounts of infiltrating mast cells; for instance, bone marrows with fibrotic grades of 2 or 3 had more infiltrating mast cells than controls (p = 0.0010, < 0.0001, respectively). In addition, bone marrows with a fibrotic grade of 3 had more infiltrating mast cells than those with fibrotic grade 2 (p = 0.0010) (Figure 2). Mast cells were positive for IL-13 (Figure 3) and TGF-β (Figure 4) in dual immunofluorescence assays of mast cell tryptase and IL-13 or TGF-β; however, megakaryocytes and blasts of MDS with bone marrow fibrosis were negative for IL-13 and TGF-β.
Conclusion: Based on these findings, we hypothesize that IL-13- and TGF-β- producing mast cells are key elements in the development of fibrosis in bone marrow. Therefore, mast cells are potential therapeutic targets for the treatment of fibrosis in bone marrow.

Disclosures
Nakayama: the promotion and mutual aid corporation for private schools of Japan: Research Funding.