FcεRI⁺ hematopoietic progenitors exhibit mast cell–forming potential in health and disease. (A) Experimental setup to analyze mast cell differentiation. The study used 10 ng/mL IL-3, 50 ng/mL IL-6, and 100 ng/mL stem cell factor (SCF). (B-C) Flow cytometry analysis of cultured progenitors. Filled histograms show isotype control staining. The results in panel B are quantified in Ci-ii (n = 6). (D) May-Grünwald Giemsa staining. (E) Tryptase staining (day 0, one experiment; days 7-28 from an individual experiment, culture experiment performed 6 times). Microscope: Olympus AX70 with XC10 camera. (Fi) Functional analysis of mast cells derived from hematopoietic progenitors. The CD63 gate that defines mast cell activation was set to ensure that the IgE-only group showed <1% positive events. (Fii) Quantification of activated mast cells in panel Fi (n = 3). Lines link samples from the same donor. (G) Progenitors from patients with systemic mastocytosis were sorted and cultured as indicated in panel A. Cultured FcεRI⁻ progenitors and cultured population A (Pop A) cells from each patient were analyzed by using flow cytometry on the same day (days 28-30, depending on donor). Analysis of aberrant markers was not performed. (H) Quantification of the results in panel G (n = 3). Lines link samples from the same donor. Two-tailed unpaired t tests; *P < .05, **P < .01, ****P < .0001. MCs, mast cells; ns, not significant.