B cell progenitors are a source of IL-33 in injured BM. (A) Flow cytometry of collagenase-treated BM samples from IL-33–GFP mice. In the histogram, IL-33–GFP levels in Lin⁻CD45⁻CD31⁺Sca1⁺ ECs of IL-33–GFP (red line) and control (Ctrl) WT (gray-tinted line) mice are shown. Results shown are representative of three independent experiments. (B) Flow cytometry of flushed BM cells from WT mice on day 0 (Ctrl) and day 2 after 200 mg/kg 5-FU injection. In upper panels, gated fraction represents CD45^Lo BM cell. Lower histogram shows the expression level of CD19 in CD45^Lo cells of homeostatic WT mice. Results shown are representative of three independent experiments. (C) CD19⁺ B cells from BM of WT mice were seeded in the presence of 5-FU. IL-33 levels of supernatants and control culture medium were measured by ELISA; n = 3, representative of two independent experiments. (D) Immunohistochemical analyses of BM sections from 5-FU–treated mice (200 mg/kg, day 2). Scale bars, 50 µm. Results shown are representative of two independent experiments. (E) WT and IL-33^GFP/GFP mice were treated with 150 mg/kg 5-FU. BM cellularity was analyzed on day 8. The numbers of CMPs, GMPs, and MEPs from femurs and tibias are shown; n = 7 in WT, n = 11 in IL-33^GFP/GFP. Data are pooled from two independent experiments. In the bar charts, the results are shown as mean ± SEM, and each dot represents an individual mouse. Statistical significance was determined by unpaired Student’s t test (C and E).