Extended flow cytometry characterization of normal bone marrow progenitor cells by simultaneous detection of aldehyde dehydrogenase and early hematopoietic antigens: implication for erythroid differentiation studies
© Mirabelli et al; licensee BioMed Central Ltd. 2008
Received: 22 November 2007
Accepted: 29 May 2008
Published: 29 May 2008
Aldehyde dehydrogenase (ALDH) is a cytosolic enzyme highly expressed in hematopoietic precursors from cord blood and granulocyte-colony stimulating factor mobilized peripheral blood, as well as in bone marrow from patients with acute myeloblastic leukemia. As regards human normal bone marrow, detailed characterization of ALDH+ cells has been addressed by one single study (Gentry et al, 2007). The goal of our work was to provide new information about the dissection of normal bone marrow progenitor cells based upon the simultaneous detection by flow cytometry of ALDH and early hematopoietic antigens, with particular attention to the expression of ALDH on erythroid precursors. To this aim, we used three kinds of approach: i) multidimensional analytical flow cytometry, detecting ALDH and early hematopoietic antigens in normal bone marrow; ii) fluorescence activated cell sorting of distinct subpopulations of progenitor cells, followed by in vitro induction of erythroid differentiation; iii) detection of ALDH+ cellular subsets in bone marrow from pure red cell aplasia patients.
In normal bone marrow, we identified three populations of cells, namely ALDH+CD34+, ALDH-CD34+ and ALDH+CD34- (median percentages were 0.52, 0.53 and 0.57, respectively). As compared to ALDH-CD34+ cells, ALDH+CD34+ cells expressed the phenotypic profile of primitive hematopoietic progenitor cells, with brighter expression of CD117 and CD133, accompanied by lower display of CD38 and CD45RA. Of interest, ALDH+CD34- population disclosed a straightforward erythroid commitment, on the basis of three orders of evidences. First of all, ALDH+CD34- cells showed a CD71bright, CD105+, CD45- phenotype. Secondly, induction of differentiation experiments evidenced a clear-cut expression of glycophorin A (CD235a). Finally, ALDH+CD34- precursors were not detectable in patients with pure red cell aplasia (PRCA).
Our study, comparing surface antigen expression of ALDH+/CD34+, ALDH-/CD34+ and ALDH+/CD34- progenitor cell subsets in human bone marrow, clearly indicated that ALDH+CD34- cells are mainly committed towards erythropoiesis. To the best of our knowledge this finding is new and could be useful for basic studies about normal erythropoietic differentiation as well as for enabling the employment of ALDH as a red cell marker in polychromatic flow cytometry characterization of bone marrow from patients with aplastic anemia and myelodysplasia.
Aldehyde dehydrogenase (ALDH) is a family of enzymes involved in metabolism of aldehydes to their corresponding carboxylic acids . It plays an important role in metabolism of vitamin A as well as in mechanisms of resistance to alchylating agents, e.g. cyclophosphamide . For these reasons, ALDH is considered a protecting or detoxifying enzyme, able to preserve stem cells from cytotoxic effects [2–4]. One of the accepted technologies to identify human hematopoietic stem cells (HSC) is based upon flow cytometry (FCM) detection of ALDH enzymatic activity . In particular, Storms et al  designed a substrate for ALDH, termed BODIPY aminoacetaldehyde (BAAA), which consists of an aminoacetaldehyde moiety bonded to the BODIPY fluorochrome. Once BAAA diffuses freely into cells and it is converted by ALDH into BODIPY aminoacetate (BAA), it remains trapped intracellularly, so emitting green fluorescence .
Recently, the functional role of ALDH has been elucidated in a study, in which the specific inhibitor of ALDH, diethylaminobenzaldehyde (DEAB), was able to alter the molecular and cellular mechanisms that control self-renewal capacity of human HSC .
The evidence of ALDH involvement in the physiology of HSC was further highlighted by a series of studies devoted to purification/analysis of highly immature progenitor cells, particularly in human cord blood (CB) as well as in murine bone marrow (BM) [6–8]. The importance of ALDH in human hematopoiesis was also testified by a recent study in which the Authors tried to purify HSCs by combining FCM cell sorting and Hoechst-33342 efflux ability (the so called "Side Population") . At variance with previous findings obtained in mouse, human BM hematopoietic cells able to exclude Hoechst-33342 did not correspond to highly immature HSCs. On the other hand, the Authors proposed that ALDH activity had to be considered as the reference method for the detection of immature HSCs in human BM, at the same time emphasizing the need of studies about expression pattern of ALDH in comparison with other hematopoietic cell markers in this tissue .
Thanks to the relevant advances deriving from studies performed by Gentry and coworkers [10, 11], it was definitely understood that ALDHbright cell population in banked cord blood is highly enriched in hematopoietic colony forming cells (CFC-H), compared to ALDHdim cell populations . Moreover, cell sorting of ALDHbright cells in human BM enables a marked enrichment of CFC-H as well as of endothelial and fibroblast colonies .
Starting from these literature data, we carried out a comprehensive FCM characterization of human normal BM by detecting ALDH in combination with other hematopoietic cell markers. To this aim, we used a multidimensional polychromatic approach, according to international guidelines .
The goal of our work was to better define the expression of surface antigens classically associated with early steps of hematopoiesis such as CD38, CD45RA, CD71, CD105, CD117 and CD133 in three BM cellular compartments identified on the basis of ALDH and CD34 expression. We were also interested to derive information about the possible erythroid lineage commitment of these identified subpopulations of cells.
Normal BM samples were obtained, following informed consent, from 23 patients with non-Hodgkin lymphoma (age 20–81, median 45) who underwent BM aspiration in the context of routine clinical practice. All samples showed complete absence of neoplastic cells when analyzed by smear morphology, BM biopsy and FCM. Peripheral blood counts were in normal ranges. Thus, all BM aspirates included in the study were operationally considered as normal. Written informed consent was obtained from all patients before the study.
Three patients with pure red cell aplasia (PRCA, age 36–40) were also included in the study. Also in this case, BM samples were obtained following informed consent.
In the first phase of the study, we analyzed BM cells by four-color FCM, detecting ALDH activity along with the expression of a series of hematopoietic antigens. Directly conjugated fluorescent antibodies were used reacting with the following antigens: CD34, CD38, CD45, CD45RA, CD71, CD105, CD117, CD133 and CD235a. In particular, PE conjugated monoclonal antibodies (MoAb) against CD133, CD117, CD45RA and CD105 were obtained from Miltenyi Biotec (Bergisch Gladbach, Germany), Caltag (Burlingame, CA, USA), Beckman-Coulter (Miami, FL, USA) and R&D Systems (Minneapolis, MN, USA), respectively. PE-conjugated anti-CD38, anti-CD34 and anti-CD235a, along with APC-conjugated anti-CD71, PE-Cy7 conjugated anti CD34 and PerCP-conjugated anti-CD45 were purchased from Becton Dickinson (San Jose, CA, USA).
For all antibody staining experiments, 50 μl of whole BM sample were incubated at 4°C for 30 min in the presence of appropriate amount of MoAbs in PBS. Successively, the mixtures were diluted 1:20 in ammonium chloride (NH4Cl) lysing solution, then incubated at room temperature for 5 minutes and finally washed with staining media prior to flow cytometry analysis. In experiments where BM samples were stained with BAAA and additional antibodies, the antibody reagents were incubated with the cells at a concentration of 106 cells/ml for 15 minutes prior to BAAA staining. Cells with low side scatter and high levels of BAAA staining were defined as "Side Scatter Low ALDH Bright" (SSCloALDHbr) cells. ALDH activity was assessed by the BODIPY-based ALDEFLUOR assay (Becton Dickinson), according to the manufacturer method. As negative control, we used DEAB, a potent inhibitor of ALDH activity. Antibody and BAAA stained cells were analyzed on a FACSCalibur flow cytometer (Becton Dickinson) equipped with a 488-nm argon laser and a 635-nm red diode laser. BODIPY fluorescence was excited at 488 nm and fluorescence emission was detected using a standard fluorescein isothiocyanate (FITC) 530/30 bandpass filter. Levels of CD antigen expression were displayed as mean fluorescence intensity (MFI). The software used for cytometric analysis was Paint-a-gate (Becton Dickinson).
Cell sorting experiments were performed by the cell sorter FACSAria (Becton Dickinson) equipped with blue, red, and violet lasers. Dead cells were excluded by analyzing forward scatter (FSC) vs side scatter (SSC) dot plots. Doublets were excluded by FSC-H vs FSC-A dot plots. Three populations of cells were sorted: ALDH-CD34+, ALDH+CD34+ and ALDH+CD34-. In all experiments, purity was higher than 95% of desired cells.
Immunophenotyping of patients with PRCA was performed by the flow cytometer FACSCanto (Becton Dickinson), by using a 5-colour strategy (1st channel: ALDH; 2nd channel: CD235a-PE; 3rd channel: CD45-PerCP; 4th channel: CD71-APC; 5th channel: CD34-PE-Cy7).
Induction of differentiation studies were performed according to Malik et al (13). In brief, ALDH+CD34+ and ALDH+CD34- cells were purified from three different normal bone marrow samples and were cultured at a density of 105 cells/mL in Iscove's Modified Dulbecco's Medium (IMDM; GIBCO, Grand Island, NY), 1% deionized bovine serum albumin (BSA; Sigma, St Louis, MO), 10-4 mol/L 2-mercaptoethanol, 10-6 mol/L hydrocortisone, 100 U/mL penicillin-streptomycin, and 2 mmol/L L-glutamine with the following recombinant cytokines: 10 U/mL of recombinant human (rH) Epo (Amgen, Thousand Oaks, CA), 0.001 ng/mL rH granulocyte-macrophage colonystimulating factor (GM-CSF), and 0.01 U/mL rH interleukin-3 (IL-3; Immunex Corp, Seattle, WA) and incubated in 5% CO2 at 37°C. We strictly followed the indications of the reported method , culturing ALDH+CD34+ cells and ALDH+CD34- cells in the aforementioned culture conditions for 4 days in order to demonstrate our hypothesis that ALDH+CD34- cells were committed erythroid precursors.
Descriptive statistics included medians, lowest and highest values, and 95% CI for the medians. Statistical significance was assessed by paired Wilcoxon test.
Starting from this point, the work was carried out with the aim of identifying the immunophenotype of the three aforementioned cell populations.
Expression of surface antigens on CD34+ALDH+ and CD34+ALDH- progenitor cells.
In spite of the existence of a series of studies describing ALDH expression in human CB [4, 6, 8, 10], peripheral blood stem cells  and even acute myeloid leukemia (AML) BM cells  data aiming to characterize normal BM ALDH+ cells are still limited to a few recent reports [9, 11].
The use of NH4Cl lysed, unseparated bone marrow for immunophenotypic studies is strongly recommended by clinico-hematological guidelines [12, 15]. In this work we decided to follow these recommendations, so that BM samples were not processed by density gradient separation to the aim of avoiding unwanted selection of cell subsets, and, at the same time, of not interfering with assay interpretation .
Our interest in ALDH detection relies on the consideration that FCM based ALDH activity assessment is exploited in order to evidence a conserved stem cell function, rather than to merely identify a stem cell antigen .
The first result of this study, based upon the simultaneous identification of ALDH activity and a detailed immunophenotype profile, is that the contemporary expression of CD34 and high levels of ALDH is able to identify primitive BM hematopoietic progenitors, clearly expressing high amounts of CD117 and CD133 as well as low amounts of CD38 and CD45RA. These data are in keeping with findings reported by Hess, Pearce and Gentry groups on CB cells [4, 9, 10]. They found that ALDHbright cells were able to give multilineage hematopoietic repopulation and, when compared to their ALDHlo counterparts, they expressed higher amounts of CD133, CD31, CD117 and lower amounts of CD38 [4, 9, 10].
Our study shares some arguments with the work by Gentry et al on human bone marrow . Similarly to our work, these Authors characterized ALDHbright cells with a series of MoAbs directed to surface antigens, e.g. CD34, CD117, CD133, CD105, CD166, CD127, CD38, HLA-DR and CD45. Nevertheless, there is one difference between our study and Gentry's work. While Gentry and coworkers  used a 3-color plus 7-AAD (7-amino-actinomycin D) scheme, thus comparing the expression of ALDH with 2 surface antigens, we combined ALDH analysis with 3 surface antigens. Such a multidimensional strategy allowed us to obtain the core finding of our work, i.e. the detection of a consistent population of cells characterized by the simultaneous presence of CD105, CD71 and ALDH, in the absence of CD34. In this regard, if ALDH+/CD34- cell subset may also contain a very rare, primitive stem cell remains to be demonstrated and it is conceivable that a specific marker for the selection of human CD34-negative stem cells needs to be identified before definite conclusions are made, as suggested by other research groups [9, 17, 18]. However, in our study the majority of cells we identified by ALDH+/CD34-/CD45- immunophenotype appeared to be committed, lineage antigen positive, non-stem cells. In particular, we demonstrated that these cells display precise erythroid features, i.e. CD105 and CD71bright expression in all 23 normal BM samples analyzed. The expression of these two antigens along with CD34 negativity is consistent with erythroid precursors [19, 20]. This finding is intriguing because little direct information is available about the phenotypic features of erythroid precursors, as these cells are both sparse and difficult to isolate in sufficient numbers to be studied .
In order to further characterize ALDH+CD34- cells we performed cell sorting isolation of ALDH+CD34+ and ALDH+CD34- cell subsets, followed by short term cell culture experiments in which we were able to induce the expression of GPA only on ALDH+CD34- cells. These experiments, which strictly followed indications by Malik et al  who recapitulated the entire erythropoietic process in vitro, were the proof of eythroid commitment of ALDH+CD34- cells in normal bone marrow.
We also decided to further confirm erythroid commitment of ALDH+CD34- cells in a disease model. Interestingly, we demonstrated that three PRCA patients were completely devoid of this cellular subset, thus allowing us to hypothesize the use of this cytometric approach as a diagnostic tool in this kind of disease.
Taken together, our data indicate ALDH as a suitable tool to identify early steps of erythropoiesis as well as to isolate erythroid precursors by cell sorting. Moreover, given the lack of sufficient numbers of reliable erythroid markers in diagnostic panels [15, 21] ALDH could also have possible application in the field of PRCA and myelodysplasia FCM diagnosis.
Finally, a possible application of ALDH detection by FCM to the field of acute leukemia may derive from the study of Cheung et al , in which the authors described ALDH expression in AML. They noted that in AML patients in complete remission, a relevant population of cells characterized by high ALDH activity remained . In this regard, our data concerning multidimensional expression profile of ALDH combined with other hematopoietic antigens in normal BM precursors could represent the basis to distinguish by FCM leukemic from normal ALDH+ cells.
In this study we demonstrated that ALDH+CD34- cells are mainly committed towards erythropoiesis. This finding is new and could be relevant for human physiology studies about erythropoietic differentiation as well as for proposing the utilization of ALDH as a red cell marker in multidimensional flow cytometry characterization of bone marrow from patients with aplastic anemia (mainly PRCA) and myelodysplasia.
The authors Peppino Mirabelli, Rosa Di Noto, Giovanna Abate, Marisa Gorrese, Maddalena Raia, Caterina Pascariello, Giulia Scalia, Marica Gemei and Elisabetta Mariotti would like to acknowledge Professor Francesco Salvatore for sustaining their professional growth.
This work was supported by AIL Salerno – ONLUS – Sezione "Marco Tulimieri" and AIL Benevento – ONLUS – Sezione "Stefania Mottola".
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