IDENTIFICATION OF LEUKEMIC STEM CELLS IN ACUTE MYELOID LEUKEMIA BY FLOW CYTOMETRY AND ITS CORRELATION WITH RESPONSE TO INDUCTION THERAPY

Laboratory data suggest that acute myeloid leukemia AML originates from a rare population of cells, termed leukemic stem cells (LSCs) or leukemia-initiating cells, which are capable of self-renewal, proliferation and differentiation into malignant blasts. There’s a universal agreement that LSCs lie within the CD34+ compartment of hemopoietic cells and most of leukemic stem cells express the interleukin-3 alpha chain receptor,CD123 and lack CD38.Aim of this study: To assess the percentage of LSCs identified by expression of CD34 and CD123 and lack of expression of CD38 in AML patients and to clarify their relation with the response to induction therapy; and to define the role of flow cytometric myeloperoxidase (MPO) in the diagnosis of AML and its relationship with the response to induction therapy.Patients and Methods:
A cohort of 41 patients older than 15 years with newly diagnosed de novo AML were enrolled in this study. They were obtained from the National center of hematology (NCH) and Baghdad teaching hospital, between February and July 2013. Hematological parameters were obtained using an automated hematology analyzer (CELLDYN Ruby).The expression of CD34, CD38 and CD123 was assessed by multi-color flow cytometry. LSC positive (LSC+) samples must express CD34 and CD123 and lack of expression of CD38 in >1% of cells. To determine the positivity of the studied markers, a cut-off value of 20% was used for surface markers CD34 and CD123; whereas 10% was used for the cytoplasmic marker myeloperoxidase. French American British (FAB) classification system was used in this study. Patients who survived were followed up for morphological response four weeks after the start of induction therapy. Regarding response to induction therapy; three grouped were found: those who reached the complete morphological remission (CR), those who failed to reach CR and those who died before assessment of morphological remission. The last two groups were merged for statistical purposes. Results:
Patients were stratified into 6 age groups with 10 years interval in each age group, the peak age of incidence was in the younger age group 15-24 years old (26.83%) and lowest age of incidence was >65 years (4.87%). The male to female ratio was 1.41:1. According to FAB classification, M3 was the most common subtype (26.83%) followed by M2. After the course of induction therapy, 41.46% of patients had complete morphological remission while 58.54% of the studied patients failed to reach complete remission. The complete remission (CR) rate was 53.33% in patients who were LSC- and 34.61% in the LSC+ group. The rate of complete remission in CD123- patients was 64.28%. Regarding CD34, 39.39% of CD34+ patients achieved CR, while 60.61% failed to achieve CR. Regarding myeloperoxidase (MPO) 70.37% of patients who expressed MPO in <20% of cells failed to achieve CR, while 61.54% of patients who had >20% expression of MPO achieved CR (p value=0.05).Conclusions: 1. LSCs were expressed in 63.41% of AML cases and were distributed among FAB subtypes without preference to any FAB subtype. 2.MPO was expressed in 90% of cases including all FAB subtypes except in 50% of M5 cases.
3. The expression of LSC phenotype was associated with poor response to induction therapy in AML patients.4. The expression of MPO was associated with better response to induction therapy.