Identification of Progenitors Committed to Basophil and/or Mast Cell Lineages.

Eosinophils, 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 downstream of granulocyte/monocyte progenitors (GMPs). Since both basophils and mast cells express the αβγ2 form of FcεRI on their surface, we hypothesized that early progenitors restricted to these lineages may have already upregulated these molecules. Thus, FcεRIα-expressing cells were searched within the Lin⁻CD34⁺ bone marrow and spleen cells. Lin⁻CD34⁺ bone marrow cells contained a small fraction of cells expressing a high level of FcεRIα that were all c-Kit⁻. Purified Lin⁻CD34⁺FcεRIα^hic-Kit⁻ cells were cultured, and they gave rise exclusively to pure basophil colonies, which were named as basophil progenitors (BaPs). In contrast, the spleen had a small fraction of Lin⁻CD34⁺ cells expressing a low level of FcεRIα and a high level of c-Kit. Strikingly, single Lin⁻CD34⁺FcεRIα^loc-Kit^hi cells formed colonies containing both basophils and mast cells as well as pure mast cell or basophil colonies. This indicates that at least a fraction of Lin⁻CD34⁺FcεRIα^loc-Kit^hi cells are bipotent for basophils and mast cells. We thus named the Lin⁻CD34⁺FcεRIα^loc-Kit^hi cells as BMCPs. Identification of BMCPs formally proves for the first time that basophils and mast cells share a common progenitor stage. After 3-day culture, BMCPs gave rise to Lin⁻CD34⁺FcεRIα^hic-Kit⁻BaPs and Lin⁻CD34⁺FcεRIα^hic-Kit⁺ cells which exclusively formed pure mast cell colonies. Lin⁻CD34⁺FcεRIα^hic-Kit⁺ cells were named as mast cell progenitors (MCPs). All of these progenitors are located downstream of GMPs since GMPs gave rise to BMCPs, BaPs and MCPs in vitro after 3-day culture with SCF, IL-3, and IL-9. The intestine is known to collect mast cell colony-forming activity. Since MCPs were not isolatable as a distinct population in the bone marrow or the spleen, we searched for MCPs in the intestine by using similar markers. We newly identified Lin⁻CD34⁺FcεRIα^loc-Kit^lo cells in the intestine, and these cells exclusively formed pure mast cell colonies. In mice sensitized with OVA to induce allergic reaction, BMCPs, BaPs and intestinal MCPs expanded by1.5- to 5-fold in number after OVA administration. This strongly suggests that these populations constitute critical stages in physiological pathways for each lineage development. Taken together, it is likely that the initial commitment into basophil/mast cell lineages occurs in the spleen, and that spleen BMCPs may migrate into the bone marrow to become BaPs or into the intestine to become MCPs. These progenitor populations 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.