Single-cell transcriptomics reveals the identity and regulators of human mast cell progenitors

Mast cell accumulation is a hallmark of a number of diseases including allergic asthma and systemic mastocytosis. IgE-mediated crosslinking of the FcεRI receptors causes mast cell activation and contributes to disease pathogenesis. The mast cell lineage is one of the least studied among the hematopoietic cell lineages and there are still controversies about the identity of the mast cell progenitor, i.e., whether FcεRI expression appears during the hematopoietic progenitor stage or in maturing mast cells. Here, we used single-cell transcriptomics to reveal a temporal association between the appearance of FcεRI and the mast cell gene signature in CD34+ hematopoietic progenitors. In agreement with these data, the FcεRI+ hematopoietic progenitors formed morphologically, phenotypically and functionally mature mast cells in long-term culture assays. Single-cell transcriptomics analysis further revealed the expression patterns of prospective cytokine receptors regulating mast cell progenitor development. Culture assays showed that IL-3 and IL-5 promoted disparate effects on progenitor cell proliferation and survival, respectively, whereas IL-33 caused robust FcεRI downregulation. Taken together, we have demonstrated that FcεRI appears during the hematopoietic progenitor stage of mast cell differentiation and that external stimuli may regulate the FcεRI expression. Thus, the results resolve the controversy regarding the appearance of FcεRI during mast cell development.One-sentence summarySingle-cell analysis of human hematopoiesis uncovers the stage at which FcεRI appears during mast cell differentiation and reveals disparate effects of IL-3, IL-5 and IL-33 on mast cell progenitor proliferation, survival, and suppression of FcεRI expression.

KIT signaling is dispensable for human mast cell progenitor development

Key Points SCF and KIT signaling are dispensable for the survival, proliferation, and maturation of human mast cell progenitors.

Effect of Neopterin on the Hematopoietic System

Neopterin is produced by monocytes and is a useful biomarker for inflammation. We recently found that neopterin is biologically active against hematopoiesis during inflammatory processes. Namely, neopterin stimulates bone marrow myelopoiesis but suppresses B-lymphopoiesis and erythropoiesis. Neopterin also enlarges the splenic mast cell progenitor pool. These hematopoietic responses are not due to a direct effect of neopterin on hematopoietic stem cells; rather, they are an indirect effect mediated by the stromal cell production of cytokines. These hematopoietic responses are compatible with the hematopoietic response that occurs during inflammatory processes such as infection. These findings suggest that neopterin reinforces the hematopoietic response during inflammation and contributes to the potential upregulation of the inflammatory process as a host defense. Thus, neopterin acts as a potent hematopoietic regulatory factor by activating stromal cell functions, although further study is needed to elucidate the mechanism of action.

In Vitro Effect of Neopterin on Splenic Mast Cell Development in Senescence Accelerated Mice (SAM)

Neopterin is produced by monocytes and is a useful biomarker of inflammatory responses. We found that neopterin enhances granulopoiesis, but suppresses B-lymphopoiesis triggered by the positive- and negative regulations of cytokines produced by stromal cells in mice. Furthermore, neopterin suppressed the colony formation of mast cell progenitor (CFU-mast) from bone marrow cells in in vitro culture system. In this study, neopterin was also found to regulate the proliferation and differentiation of splenic CFU-mast in vitro as observed in that from bone marrow, which was confirmed in the mouse model of senescent stromal-cell impairment (SCI). In non- SCI mice (=less senescent stage of SCI mice), neopterin also decreased the number of colonies of interleukin-3 (IL-3)-dependent mast-cell progenitor cells (CFU-mast) from unfractionated spleen cells, but not that from the lineage-negative (fractionated) spleen cell population without stromal cells in a semisolid in vitro culture system. In contrast, in a case of SCI mice, the treatment with neopterin did not decrease the number of colonies of IL-3- dependent mast-cell progenitor cells (CFU-mast) from unfractionated spleen cells. These results suggest that, firstly, neopterin decrease the number of colonies of IL-3-dependent CFU-mast by stimulating splenic stromal cells, and secondly, such neopterin function becomes declined during senescence because of an impaired stromalcell function.