Flow cytometric analysis of Thy-1 expression in CD34-positive acute leukemia.

We analyzed the expression of the Thy-1 antigen (CD90) in CD34+ acute leukemia using two-color flow cytometry. Leukemic cells were obtained from the bone marrow (BM) and/or the peripheral blood (PB) of 57 patients: 37 with acute myelogenous leukemia (AML) including nine with secondary AML following myelodysplastic syndrome (MDS/AML), and 20 with acute lymphoblastic leukemia (ALL) including three with chronic myelogenous leukemia in blast crisis (CML-BC) of the lymphoid type. Among these patients, one (3.6%) with de novo AML, two (22.2%) with MDS/AML, three (17.6%) with de novo ALL, and two (66.7%) with CML-BC coexpressed CD34 and Thy-1 (CD34+ Thy-1+) on more than 20% of the mononuclear cells within 'lymph' plus 'blast' window. Thy-1 was rarely expressed in de novo acute leukemia especially in AML. Interestingly, in 1 patient with CML-BC, the leukemic cells in BM were divided into two subpopulations (CD34+ Thy-1low and CD34+ Thy-1high), whereas most of the CD34+ leukemic cells in PB were Thy-1high. However, the mechanism for the mobilization of CD34+ Thy-1high leukemic cells into the PB is unknown.