Murine Mast Cells Express Mpl, the Thrombopoietin Receptor, and Thrombopoietin Is a Potent Regulator of Mast Cell Differentiation.
Authors: Martelli Fabrizio , Amabile Giovanni , Ghinassi Barbara , Lorenzini Rodolfo , Vannucchi Alessandro M. , Rana Rosa Alba , Nishikawa Mitsuo , Migliaccio Giovanni , Migliaccio Anna Rita 
Published: April 2020

Abstract
Mast cells are hematopoietic cells localized in extramedullary sites where they engage themselves in the process of allergic response and in the immune reaction against parasites. Mast cells derive from multilineage c-KitlowCD34lowSca-1pos progenitor cells present in the marrow. These cells give rise to Linnegc-KitposSca-1neg T1/ST2pos mast cell restricted progenitor cells (MCP) whose futher maturation in the marrow remains limited under steady state conditions. MCP migrate through the blood in extramedullary sites were they mature into tissue-retricted c-KitposFceRIpos mast cells characterized by a specific mast cell protease (MMCP) profiling (dermal, mucosal and serosal mast cells in skin, gut and peritoneal cavity, respectively). The molecular mechanism that, in normal mice, restricts the mastocytopoietic potential of progenitor cells to the extramedullary sites, as well as the factors that guide the tissue-restricted differentiation of these cells, are unknown. Thrombopoietin (TPO)-Mpl interactions play an important role in the regulation of hematopoietic stem/progenitor cell proliferation and differentiation in the marrow. Here we report that mast cells, and their precursors, express Mpl (both as mRNA and cell surface protein) (see Table). Furthermore, targeted deletion of this gene (Mplnull mutation) decrease the number of MCP (by 1-log) and increases that of mast cells in dermis (by 3-fold), peritoneal cavity (by 3-fold), bone marrow (2-log) and spleen (2-log). Furthermore, because of their higher (by 2-log) MMCP-7 expression, serosal Mplnull mast cells resemble more wild-type dermal rather than serosal mast cells. On the other hand, either treatment of mice with TPO or addition of TPO to bone marrow-derived mast cell cultures induces mast cell apoptosis (by Tunel and Annexin staining) and severely hampers mast cell differentiation (by expression profiling). These data are consistent with a regulatory mechanism for murine mastocytopoiesis according to which TPO favours the transition from multilineage progenitors to CMP but blocks differentiation of MCP to mature mast cells. We propose TPO as the growth factor that restrict mast cell differentiation to extramedullaty sites and that control the switch between serosal vs dermal mast cell differentiation.
Mpl expression mRNA 2-ΔCt Protein (AFU) Cy7-A Protein (AFU) Cy7-AMM2 AFU= arbitrary fluorescence intensity. p&lt; 0.01 with respect to Cy7-A (irrilevant antibody) Wild type Marrow B cells (B220pos) b.d. 120±4 205±4 Wild type Marrow Megakaryocytes (CD61pos/CD41pos) 5.0±0.1 × 10-2 178±3 978±74* Wild type Marrow MCP (cKitpos/T1ST2pos) 1.3±0.01 × 10-2 139±16 1658±73* Wild-type Marrow Mast Cells (cKitpos/Fcε RIpos) 1.9±0.1 × 10-2 110±1 868±71* Serosal Mast Cells (cKitpos/FcεRIpos) 7.2±2.1 × 10-4 393±1 1374±25* Mplnull Marrow Megakaryocytes (CD61pos/CD41pos) b.d. 365±28 469±50 Mplnull Marrow Mast Cells (cKitpos/FcεRIpos) b.d 107±1 109±3