Immunohistochemical analysis of bone marrow biopsies.

Immunohistochemistry was performed as described in detail in “Methods.” (A) Approximately 40% to 50% of leukemic blasts (black arrows) showed coexpression of CD34 and CD117. Erythropoietic cells (white arrow) are negative for CD34 and CD117. (B) Day 118 after imatinib mesylate therapy: leukemic blasts with coexpression of CD34 and CD117 are not present. Some regenerating immature myeloid precursor cells (black arrows) show CD117 positivity. Endothelial cells of marrow arterioles show reactivity for CD34 (red arrow), serving as an internal positive control. Erythropoietic cells (white arrow) are negative for CD34 and CD117. (C) Positive staining by means of an antiphospho–c-Kit (Tyr719) antibody indicated activation of c-Kit. Black arrows show phospho–c-Kit⁺ blasts. Erythropoietic cells (white arrow) serve as internal negative control. (D) Prior to imatinib mesylate, approximately 10% of leukemic blasts showed expression of Fas (black arrow). Upon complete remission, fewer than 5% of blasts were left for analysis. In this situation, only an occasional Fas⁺ blast was found (data not shown). Original magnifications × 100 (A-C) and × 150 (D).