FIP1L1/PDGFRa Synergizes with SCF/c-Kit Signaling To Induce Systemic Mastocytosis in a Chronic Eosinophilic Leukemia Murine Model

Systemic mastocytosis (SM) associated with chronic eosinophilic leukemia (CEL)/hypereosinophilic syndrome (HES) is a result of expression of the Fip1-like1 (FIP1L1)/platelet-derived growth factor receptor alpha (PDGFRa) (F/P) fusion gene. We have previously described a murine CEL/HES model (CEL-like mice) induced by F/P fusion gene transduction and T-cell overexpression of IL-5 (Yamada Y et al., Blood 2006). We have now validated a preclinical murine model of F/P-induced SM/CEL and analyzed the pathogenesis of SM in this model. F/P⁺ mast cells (MC, defined as EGFP⁺/c-kit⁺/FceRI⁺) were significantly increased in the small intestine, bone marrow (BM) and spleen of CEL-like mice compared to wild-type mice (Table). CEL-like mice also developed cutaneous MC infiltration. In addition, mMCP-1 serum levels, which correlate well with MC expansion and activation in vivo, were significantly higher in CEL-like mice than in wild-type mice (64,000 ± 23,800 and 38 ± 41.4 pg/ml, respectively). F/P induces increased expansion of BM-derived MC in vitro (∼2,000-fold) and F/P⁺ BM-derived MC survive longer than wild-type MC in cytokine-deprived medium (28.0 ± 2.3% vs. 8.7 ± 3.1% 7AAD⁻/Annexin V⁻ cells after 48 hours). This correlated with increased Akt phosphorylation in the F/P⁺ MC. Since c-kit mutations are the most frequent cause of SM, we analyzed the possible synergistic role of SCF and F/P signaling. F/P and SCF/c-kit signaling indeed synergize in the development of BM-derived MC (16-fold greater expansion than in the absence of SCF) and F/P⁺ BM-derived MC showed a 3.7-fold greater migratory response to SCF than wild-type BM-derived MC. In order to determine the role of SCF/c-kit signaling in F/P⁺ MC development, activation and tissue infiltration in vivo,these responses were evaluated in mice that were treated with a blocking anti-c-kit blocking antibody, ACK-2, or an isotype-matched control antibody. ACK-2 treatment suppressed intestinal MC infiltration and elevated plasma levels of mMCP-1 induced by F/P expression by 95 ± 6.0% and 98 ± 0.76%, respectively, whereas MC and plasma mMCP-1 were completely undetectable in wild-type mice treated with ACK2. This suggests that SCF/c-kit interactions may synergize with F/P to induce SM. In summary, mice with CEL-like disease also develop SM. F/P-induced SM is a result of increased in vivo MC proliferation, survival, activation and tissue infiltration. SCF/c-kit signaling synergizes with F/P in vivo and in vitro to promote mast cell development, activation and survival.