August H. Westra

High Stem Cell Frequency in Acute Myeloid Leukemia at Diagnosis Predicts High Minimal Residual Disease and Poor Survival

Purpose: In CD34-positive acute myeloid leukemia (AML), the leukemia-initiating event originates from the CD34+CD38− stem cell compartment. Survival of these cells after chemotherapy may lead to minimal residual disease (MRD) and subsequently to relapse. Therefore, the prognostic impact of stem cell frequency in CD34-positive AML was investigated. Experimental Design: First, the leukemogenic potential of unpurified CD34+CD38− cells, present among other cells, was investigated in vivo using nonobese diabetic/severe combined immunodeficient mice transplantation experiments. Second, we analyzed whether the CD34+CD38− compartment at diagnosis correlates with MRD frequency after chemotherapy and clinical outcome in 92 AML patients. Results:In vivo data showed that engraftment of AML blasts in nonobese diabetic/severe combined immunodeficient mice directly correlated with stem cell frequency of the graft. In patients, a high percentage of CD34+CD38− stem cells at diagnosis significantly correlated with a high MRD frequency, especially after the third course of chemotherapy. Also, it directly correlated with poor survival. In contrast, total CD34+ percentage showed no such correlations. Conclusions: Both in vivo data, as well as the correlation studies, show that AML stem cell frequency at diagnosis offers a new prognostic factor. From our data, it is tempting to hypothesize that a large CD34+CD38− population at diagnosis reflects a higher percentage of chemotherapy-resistant cells that will lead to the outgrowth of MRD, thereby affecting clinical outcome. Ultimately, future therapies should be directed toward malignant stem cells.

Standardization of flow cytometry in myelodysplastic syndromes: report from the first European LeukemiaNet working conference on flow cytometry in myelodysplastic syndromes

This article decribes the results of the first European LeukemiaNet working conference on flow cytometry immunophenotyping in myelodysplastic syndrome. This report is a very comprehensive analysis of the topic, and provides detailed information on what is currently known in the field. See related perspective article on page 1041. The myelodysplastic syndromes are a group of clonal hematopoietic stem cell diseases characterized by cytopenia(s), dysplasia in one or more cell lineages and increased risk of evolution to acute myeloid leukemia (AML). Recent advances in immunophenotyping of hematopoietic progenitor and maturing cells in dysplastic bone marrow point to a useful role for multiparameter flow cytometry (FCM) in the diagnosis and prognostication of myelodysplastic syndromes. In March 2008, representatives from 18 European institutes participated in a European LeukemiaNet (ELN) workshop held in Amsterdam as a first step towards standardization of FCM in myelodysplastic syndromes. Consensus was reached regarding standard methods for cell sampling, handling and processing. The group also defined minimal combinations of antibodies to analyze aberrant immunophenotypes and thus dysplasia. Examples are altered numbers of CD34+ precursors, aberrant expression of markers on myeloblasts, maturing myeloid cells, monocytes or erythroid precursors and the expression of lineage infidelity markers. When applied in practice, aberrant FCM patterns correlate well with morphology, the subclassification of myelodysplastic syndromes, and prognostic scoring systems. However, the group also concluded that despite strong evidence for an impact of FCM in myelodysplastic syndromes, further (prospective) validation of markers and immunophenotypic patterns are required against control patient groups as well as further standardization in multi-center studies. Standardization of FCM in myelodysplastic syndromes may thus contribute to improved diagnosis and prognostication of myelodysplastic syndromes in the future.

MRD parameters using immunophenotypic detection methods are highly reliable in predicting survival in acute myeloid leukaemia

Outgrowth of minimal residual disease (MRD) in acute myeloid leukaemia (AML) is responsible for the occurrence of relapses. MRD can be quantified by immunophenotyping on a flow cytometer using the expression of leukaemia-associated phenotypes. MRD was monitored in follow-up samples taken from bone marrow (BM) of 72 patients after three different cycles of chemotherapy and from autologous peripheral blood stem cell (PBSC) products. The MRD% in BM after the first cycle (n=51), second cycle (n=52) and third cycle (n=30), as well as in PBSC products (n=39) strongly correlated with relapse-free survival. At a cutoff level of 1% after the first cycle and median cutoff levels of 0.14% after the second, 0.11% after the third cycle and 0.13% for PBSC products, the relative risk of relapse was a factor 6.1, 3.4, 7.2 and 5.7, respectively, higher for patients in the high MRD group. Also, absolute MRD cell number/ml was highly predictive of the clinical outcome. After the treatment has ended, an increase of MRD% predicted forthcoming relapses, with MRD assessment intervals of ⩽3 months. In conclusion, MRD parameter assessment at different stages of disease is highly reliable in predicting survival and forthcoming relapses in AML.

Identification of a small subpopulation of candidate leukemia-initiating cells in the side population of patients with acute myeloid leukemia.

In acute myeloid leukemia (AML), apart from the CD34+CD38− compartment, the side population (SP) compartment contains leukemic stem cells (LSCs). We have previously shown that CD34+CD38− LSCs can be identified using stem cell‐associated cell surface markers, including C‐type lectin‐like molecule‐1 (CLL‐1), and lineage markers, such as CD7, CD19, and CD56. A similar study was performed for AML SP to further characterize the SP cells with the aim of narrowing down the putatively very low stem cell fraction. Fluorescence‐activated cell sorting (FACS) analysis of 48 bone marrow and peripheral blood samples at diagnosis showed SP cells in 41 of 48 cases that were partly or completely positive for the markers, including CD123. SP cells in normal bone marrow (NBM) were completely negative for markers, except CD123. Further analysis revealed that the SP fraction contains different subpopulations: (a) three small lymphoid subpopulations (with T‐, B‐, or natural killer‐cell markers); (b) a differentiated myeloid population with high forward scatter (FSChigh) and high sideward scatter (SSChigh), high CD38 expression, and usually with aberrant marker expression; (c) a more primitive FSClow/SSClow, CD38low, marker‐negative myeloid fraction; and (d) a more primitive FSClow/SSClow, CD38low, marker‐positive myeloid fraction. NBM contained the first three populations, although the aberrant markers were absent in the second population. Suspension culture assay showed that FSClow/SSClow SP cells were highly enriched for primitive cells. Fluorescence in situ hybridization (FISH) analyses showed that cytogenetically abnormal colonies originated from sorted marker positive cells, whereas the cytogenetically normal colonies originated from sorted marker‐negative cells. In conclusion, AML SP cells could be discriminated from normal SP cells at diagnosis on the basis of expression of CLL‐1 and lineage markers. This reveals the presence of a low‐frequency (median, 0.0016%) SP subfraction as a likely candidate to be enriched for leukemia stem cells.

Functional characterization of minimal residual disease for P-glycoprotein and multidrug resistance protein activity in acute myeloid leukemia

Relapse is common in acute myeloid leukemia (AML) due to persistence of residual leukemia cells: minimal residual disease (MRD). In 102 out of 127 patients (80%), cells at diagnosis displayed one or more leukemia-associated phenotypes (LAP), ie combinations of cell surface markers which are absent in normal cells and can thus be used to detect MRD at follow-up. Functional characterization of MRD cells for P-glycoprotein (Pgp) and multidrug resistance protein (MRP) activity is essential to investigate the role of these drug transport proteins in multidrug resistance in AML. A fluorescent probe assay using Syto16/PSC833 and calcein-AM/probenecid as substrate/ modulator of the Pgp and MRP pump, respectively, and subsequent labeling of cells with monoclonal antibodies for LAP detection allowed simultaneous detection of LAP and Pgp or MRP activity. Validation of this assay is shown for 30 newly diagnosed AML and 11 MRD situations. In addition, no significant differences were found when comparing fresh and cryopreserved de novo AML for LAP expression (n = 43), Pgp (n = 30) and MRP (n = 24) function and for MRD samples for simultaneous LAP expression and Pgp/MRP activity (n = 10). This approach enables longitudinal and multicenter studies on the detection, quantification and functional characterisation of MRD cells.

A flow cytometric method to detect apoptosis-related protein expression in minimal residual disease in acute myeloid leukemia *

Minimal residual disease (MRD) cells are thought to be responsible for the persistence and relapse of acute myeloid leukemia (AML). Flow cytometric MRD detection by the establishment of a leukemia-associated phenotype (LAP) at diagnosis can be used in 80% of AML patients, allowing detection and functional characterization of MRD in follow-up bone marrow. One of the mechanisms contributing to inefficient chemotherapy is apoptosis resistance. Measuring apoptosis parameters in MRD cells will help to unravel the importance of apoptosis resistance in AML. We therefore developed a four-color flow cytometry method that enables establishment of apoptosis-related protein expression such as Bcl-2, Bcl-xL, Mcl-1 and Bax at diagnosis and in MRD. Firstly, validation of this assay using Western blot analysis in five leukemia cell lines showed a significant correlation (R=0.70: P<0.0001). Secondly, the influence of the permeabilization procedure on LAP expression was investigated in 38 AML samples at diagnosis and in 42 MRD samples. Quantification of the frequency of LAP+ cells with and without permeabilization showed no significant differences (diagnosis: P= 0.57, follow-up: P= 0.43). The flow cytometric protocol thus enables analysis of apoptosis-related proteins at different stages of the disease, which will lead to a better understanding of the role of apoptosis resistance in the emergence of MRD in AML.

Implementation of flow cytometry in the diagnostic work-up of myelodysplastic syndromes in a multicenter approach: report from the Dutch Working Party on Flow Cytometry in MDS

Flow cytometry (FC) is recognized as an important tool in the diagnosis of myelodysplastic syndromes (MDS) especially when standard criteria fail. A working group within the Dutch Society of Cytometry aimed to implement FC in the diagnostic work-up of MDS. Hereto, guidelines for data acquisition, analysis and interpretation were formulated. Based on discussions on analyses of list mode data files and fresh MDS bone marrow samples and recent literature, the guidelines were modified. Over the years (2005-2011), the concordance between the participating centers increased indicating that the proposed guidelines contributed to a more objective, standardized FC analysis, thereby ratifying the implementation of FC in MDS.

Chemotherapeutic treatment of bone marrow stromal cells strongly affects their protective effect on acute myeloid leukemia cell survival

Bone marrow stromal cells (BMSCs) have been found to support leukemic cell survival; however, the mechanisms responsible are far from elucidated yet. Therefore, the effect of BMSCs on both proliferation and apoptosis characteristics of acute myeloid leukaemia (AML) cells was investigated as well as the effect of BMSCs exposure to chemotherapy on the stromal supportive capacity. Leukemic HL-60 and primary AML cells were either untreated or treated with cytarabine and subsequently cultured for 3 – 4 days, in the presence or absence of untreated or cytarabine-treated BMSCs. The effect on proliferation and apoptosis was investigated with flow cytometry using CFSE labeling and Syto16 and 7AAD staining. BMSCs were found to maintain cytarabine-exposed primary AML cells by protection against spontaneous apoptosis. Accordingly, an increase in phosphorylated-AKT and Bcl-2 expression was found. Concomitant exposure of BMSCs to cytarabine resulted in a dose-dependent decrease of protective capacity of BMSCs. Thus, inhibition of spontaneous apoptosis of leukemic cells mediated by phosphorylation of AKT/Bcl-2 pathway results in protection of leukemic cells by BMSCs, which decreases after BMSCs exposure to chemotherapy. Targeting both the tumor cells and intervening in their interaction with the bone marrow microenvironment may thus affect clinical outcome in AML.

Minimal residual disease cells in AML patients have an apoptosis-sensitive protein profile

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Minimal residual disease in acute myeloid leukemia is predicted by P-glycoprotein activity but not by multidrug resistance protein activity at diagnosis

Minimal residual disease in acute myeloid leukemia is predicted by P-glycoprotein activity but not by multidrug resistance protein activity at diagnosis