IL-33 downregulates FcεRI expression on mast cell progenitors (MCPs). (A) Quantification of FcεRI expression on pre–mast cells after 5 days in culture with the indicated cytokines. Points in each group represent individual buffy coats. The levels were normalized to the IL-3 condition of each donor. The study used 10 ng/mL IL-3, 20 ng/mL IL-5, 10 ng/mL GM-CSF, 1 U/mL EPO, 50 ng/mL thrombopoietin (TPO), 50 ng/mL Flt3L, and 22.5 to 100 ng/mL IL-33 unless otherwise specified. (B) Flow cytometry analysis of FACS-isolated MCPs cultured with 10 ng/mL IL-3 alone or in combination with 100 ng/mL IL-33 for 5 days. Cultures of FcεRI⁻ progenitors are shown in supplemental Figure 8. (C) Quantification of panel B (n = 3). Two-tailed one-sample t test in panels A and C, hypothetical value 1.0. ***P < .001, ****P < .0001. MFI, median fluorescent intensity.