Stem Cell Factor Signaling Collaborates in the Development of FIP1L1/PDGFRα Induced Chronic Eosinophilic Leukemia in a Murine Model

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We have previously demonstrated the synergistic role of stem cell factor (SCF) in systemic mastocytosis associated with FIP1L1/PDGFRA (F/P)- chronic eosinophilic leukemia (CEL) (Yamada Y et al., Blood 2008). The aim of this study is to determine if SCF is also required for eosinophilia development in CEL. First, F/P- or vector control-transduced bone marrow hematopoietic stem cell/progenitors (BM-HSC/P) were cultured in SCF in the presence of hydrocortisone (10−5M, HC-SCF) for 2 or 3 weeks. Expression of F/P in HSC/P induced a 27,000-fold higher cell expansion in the presence of HC-SCF compared with HC-SCF -cultured control vector-transduced HSC/P. The cultured cells were then assayed in an IL-5-containing methylcellulose for analysis of CFU-Eosinophil formation, while the culture was continued with SCF in the absence or presence of HC. Only F/P+ HSC/P formed CFU-Eosinophil after ex-vivo culture in the presence of SCF (19 ± 2.2 CFU-eosinophils/105 cells) and HC-SCF-cultured BM-HSC/P quickly differentiated into mast cells after deprivation of HC (98% c-kit+/EGFP+ cells at two weeks of culture after HC withdrawal), suggesting that SCF in collaboration with F/P fusion, may expand progenitor cells of both eosinophil and mast cell lineages. In addition, we analyzed the role of SCF/ckit signaling in disease development of F/P+ CEL in vivo. CEL mice were developed using retroviral introduction of F/P fusion into HSC/P of IL-5 transgenic mice as previously reported (Yamada Y et al., Blood 2006). CEL mice were treated with a blocking anti-c-kit antibody, ACK-2, or an isotype-matched control antibody. ACK-2 treatment suppressed the circulating leukocyte count, eosinophil count and spleen weight significantly in CEL mice, whereas control (vector control-IL-5 transgenic BM-HSC/P transplanted wild type mice) did not show significant diference between ACK-2 treatment and isotype control one (Table). These results suggest that SCF signaling is required for eosinophilic leukemia development induced by FIP1L1/PDGFRa.Table Circulating total leukocyte and eosinophilc counts and spleen weight of mice treated with antibodiesControlCELisotype-controlACK-2isotype-controlACK-2WBC(mm3)8805±43775480±1433176278±1210†9602±12177±68.3**Eosinophil(mm3)2609±671804±23529940±18735†3480±4292±68.3**Spleen weight (mg)137±11.389.9±211.22±11.2**222.8±68.3**Data are shown as mean ± SD, **p < 0.001 compared with isotype control, † p<0.001 compared with control WBC, white blood cells, CEL, chronic eosinophilic leukemia.