Bone marrow cellular composition at baseline and 12 weeks after treatment with dexpramipexole. (A) Eosinophils, mast cells, CD34⁺ cells, and eosinophil precursors in the 4 responders and 6 delayed or nonresponders at baseline (predrug) and again at week 12 (on dexpramipexole). (B-I) Bone marrow aspirate and biopsy specimens in a responder (subject 15) at baseline and again at 12 weeks on dexpramipexole. Pretreatment bone marrow aspirate (B; Wright-Giemsa staining) and biopsy (D; hematoxylin and eosin staining) show increased eosinophils. Week 12 (on dexpramipexole) counterparts (C,E) demonstrate absence of eosinophils. Rare eosinophilic promyelocytes were noted after treatment (E, inset). The number of tryptase-positive mast cells (F-G) and CD34⁺ precursors (H-I) in the biopsy specimens did not to change with treatment. Original magnification ×500 for panels B-E; magnification ×200 for panels F-I. Cell enumeration and flow cytometric assay details are included in supplemental Methods. BM, bone marrow; W, week.