Identification of Bipotent Basophil/Mast Cell Progenitors in Adult Murine Hematopoiesis.
Authors: Arinobu Yojiro , Iwasaki Hiromi , Gurish Michael F. , Mizuno Shinichi , Shigematsu Hirokazu , Ozawa Hidetoshi , Tenen Daniel G. , Austen Frank K. , Akashi Koichi 
Published: April 2020

Abstract
Basophils and mast cells are multifunctional hematopoietic effectors that co-operate to mount a variety of allergic and innate immune responses. Their origin and developmental relationships, however, have not yet been resolved, and remain as one of the major issues in the biology of hematopoiesis. Here we report that progenitors bipotent for basophils and mast cells(basophil/mast cell progenitors: BMCPs) are prospectively isolatable within murine spleen. We have shown that the β7-integrin(β7) is an essential molecule for tissue-specific homing of putative precursors for intestinal mast cells (J Exp Med194:1243, 2001). To identify a candidate population that seeds intestinal progenitors for mast cells, we searched for β7+ cells in the bone marrow and the spleen. Lin−c-Kit+ spleen cells contained a fraction of cells expressing β7 at high levels. They also expressed FcγRII/III, but the majority of these cells did not express FcεRIα. These Lin−c-Kit+FcγRII/III+β7hiFcεRIα−/locells exclusively differentiated into mature mast cells and basophils. Strikingly, single Lin−c-Kit+FcγRII/III+β7hiFcεRIα−/locells formed colonies containing both basophils and mast cells as well as pure mast cell or basophil colonies. We thus named these cells as BMCPs. In 2-day cultures, purified BMCPs upregulated FcεRIα, giving rise to Lin−CD34+FcεRIαhic-Kit+ and Li− CD34+FcεRIαhic-Kit− blastic cell populations, and they differentiated exclusively into mast cells and basophils, respectively. Based on this phenotype, we searched for precursors committed to either lineage in vivo. The Lin−CD34+ bone marrow cells contained FcεRIαhic-Kit−cells, which differentiated exclusively into basophils. We named this population as basophil progenitors (BaPs). Since mast cell progenitors(MCPs) were not isolatable as a distinct population in the bone marrow or the spleen, we searched for MCPs in the intestine. We newly identified CD45+Li− CD34+β7hiFcεRIαlocells in the intestine, and these cells exclusively formed pure mast cell colonies, which were named as intestinal MCPs. Since the expression of C/EBPα was dramatically increased in BaPs but was downregulated in MCPs, we hypothesized that it plays a key role in the basophil versus the mast cell lineage commitment. To test this hypothesis, we disrupted or overexpressed C/EBPα at the BMCP stage. BMCPs disrupted with C/EBPα gave rise exclusively to mast cells, while, BMCPs overexpressing C/EBPα differentiated mainly into basophils, suggesting that C/EBPα plays a primary role in deciding the basophil vs. mast cell fate at the BMCP stage. Thus, differentiation of the BMCPs into committed progeny may lead to selective migration, BaPs to the bone marrow or MCPs to peripheral tissues, and this fate decision is controlled principally by C/EBPα. These newly identified progenitors should be useful to analyze the mechanism of commitment into each of these lineages, and could also be therapeutic targets for a variety of allergic and autoimmune disorders.