Human mast cells are derived from CD34+ hematopoietic cells present in cord blood, bone marrow, and peripheral blood. However, little is known about the properties of the CD34+ cells. We demonstrated here that mast cell progenitors that have distinct phenotypes from other hematopoietic cell types are present in cord blood by culturing single, sorted CD34+ cells in 96-well plates or unsorted cells in methylcellulose. The CD34+ mast cell-committed progenitors often expressed CD38 and often lacked HLA-DR, whereas CD34+ erythroid progenitors often expressed both CD38 and HLA-DR and CD34+ granulocyte-macrophage progenitors often had CD33 and sometimes expressed CD38. We then cultured single cord blood-derived CD34+CD38+ cells under conditions optimal for mast cells and three types of myeloid cells, ie, basophils, eosinophils, and macrophages. Of 1,200 CD34+CD38+ cells, we were able to detect 13 pure mast cell colonies and 52 pure colonies consisting of either one of these three myeloid cell types. We found 17 colonies consisting of two of the three myeloid cell types, whereas only one colony consisted of mast cells and another cell type. These results indicate that human mast cells develop from progenitors that have unique phenotypes and that committed mast cell progenitors develop from multipotent hematopoietic cells through a pathway distinct from myeloid lineages including basophils, which have many similarities to mast cells.
Human cultured mast cells (HCMCs) grown from cord blood mononuclear cells in the presence of stem cell factor (SCF) and interleukin-6 (IL-6) expressed tryptase but no or low chymase in their cytoplasm. The addition of IL-4 to these cells strikingly increased chymase expression. Consequently, the activity of chymase was significantly higher in IL-4–treated mast cells than that in IL-4–nontreated mast cells, whereas the activity of tryptase and histamine content were comparable in both cells. Electron microscopic immunocytochemistry also showed that secretary granules containing chymase increased in IL-4–treated mast cells. Interestingly, the IL-4–induced increase of chymase expression in HCMCs was accompanied by morphological maturation of the cells. Cytoplasmic projections were few in IL-4–nontreated HCMCs, and a small number of secretary granules were observed, most of which were empty or partially filled with discrete scrolls with rough particles showing immaturity. In contrast, IL-4–treated HCMCs had extremely abundant cytoplasmic projections and had many secretary granules filled with electron-dense crystal materials. Taken together, immature HCMCs grown only with SCF and IL-6 expressed tryptase with no or a low amount of chymase, and addition of IL-4 promoted cell maturation together with the expression of both tryptase and a high amount of chymase. Our findings will raise a possibility of a linear pathway of human mast cell development from tryptase single positive mast cells into tryptase and chymase double positive mast cells as the cells mature and will suggest that this maturation process is promoted by IL-4.
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