IKKβ inhibition by ML120B depletes all stages of B cells in the marrow and thymocytes but maintains granulocytes in vivo. (A) Histologic and flow cytometric analysis of bone marrow and thymus after twice daily oral dosing of ML120B. Representative bone marrow and thymus sections and flow cytometry after 4 days of dosing (100 mg/kg) with ML120B or vehicle alone. (B) Total cells, B220⁺ (B cells), GR-1⁺ (granulocytes), and B-cell subsets (C) per femur after oral dosing twice per day at indicated doses. All stages of B-cell development analyzed were depleted at 100 mg/kg and 300 mg/kg. (D) Bones were flushed and red blood cells (RBCs) were lysed and plated in semisolid media with IL-7 for B-cell colony growth or IL-6, IL-3, and SCF for myeloid colony growth. Colony-forming units (CFUs) were scored on day 7 and total number per individual femur was calculated (n = 4 per group). B-cell progenitors (CFU-Bs) are depleted at 100 mg/kg and 300 mg/kg. Myeloid progenitors (CFU-G/Ms) are unaffected at 100 mg/kg. (E) Thymocyte CD4, CD8, CD4/CD8 double positive (DP), and CD4/CD8 double negative (DN) numbers after 4 days of oral dosing at 100 mg/kg. (F) Numbers of T cells (CD4 and CD8⁺), B cells (B220⁺), and granulocytes (GR-1⁺) in peripheral blood after 7 days of ML120B at the indicated doses. Representative blood smears are shown from blood obtained in vehicle and a 300 mg/kg ML120B. Error bars indicate SD. Original magnifications: (A, bone marrow) 200 × (objective 20 ×/0.50 NA); (A, thymus) 40 × (objective 4 ×/0.13 NA); and (F) 400 × (objective 40 ×/0.75 NA).