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CD123 Immunohistochemical Expression Is a Specific But Insensitive Marker of Early T-precursor Leukemia

Early T-cell precursor leukemia (ETPL) is a prognostically unfavorable subgroup of T-lymphoblastic leukemia/ lymphoma (T-LBL) associated with a high risk of induction chemotherapy failure or relapse.1 ETPL accounts for up to 15% of T-LBL and can be recognized by a distinctive immunophenotype: CD1a , CD8 , CD5 with myeloid or stem cell markers. The genetic landscape identified in ETPL is suggestive of a stem cell–like leukemia, and it is thought to be derived from a hematopoietic stem cell progenitor less mature than that of non-ETP T-LBL, possibly accounting for its poor response to conventional lymphoid-directed chemotherapy.4 CD123, or interleukin-3 receptor a chain, is expressed in a subset of acute myeloid leukemias, B-lymphoblastic leukemias, and other hematologic malignancies. Although considered a leukemic stem cell marker, CD123 expression has not been well characterized in the ETPL subtype of T-LBL. Lhermitte et al5 found that approximately 16% (35/ 217) of T-LBL expressed CD123 by