Dysfunctional bone marrow endothelial progenitor cells are involved in patients with myelodysplastic syndromes

Myelodysplastic syndromes (MDS) are a group of heterogeneous myeloid clonal disorders characterized by ineffective haematopoiesis and immune deregulation. Emerging evidence has shown the effect of bone marrow (BM) endothelial progenitor cells (EPCs) in regulating haematopoiesis and immune balance. However, the number and functions of BM EPCs in patients with different stages of MDS remain largely unknown. Patients with MDS (N = 30), de novo acute myeloid leukaemia (AML) (N = 15), and healthy donors (HDs) (N = 15) were enrolled. MDS patients were divided into lower-risk MDS (N = 15) and higher-risk MDS (N = 15) groups according to the dichotomization of the Revised International Prognostic Scoring System. Flow cytometry was performed to analyse the number of BM EPCs. Tube formation and migration assays were performed to evaluate the functions of BM EPCs. In order to assess the gene expression profiles of BM EPCs, RNA sequencing (RNA-seq) were performed. BM EPC supporting abilities of haematopoietic stem cells (HSCs), leukaemia cells and T cells were assessed by in vitro coculture experiments. Increased but dysfunctional BM EPCs were found in MDS patients compared with HDs, especially in patients with higher-risk MDS. RNA-seq indicated the progressive change and differences of haematopoiesis- and immune-related pathways and genes in MDS BM EPCs. In vitro coculture experiments verified that BM EPCs from HDs, lower-risk MDS, and higher-risk MDS to AML exhibited a progressively decreased ability to support HSCs, manifested as elevated apoptosis rates and intracellular reactive oxygen species (ROS) levels and decreased colony-forming unit plating efficiencies of HSCs. Moreover, BM EPCs from higher-risk MDS patients demonstrated an increased ability to support leukaemia cells, characterized by increased proliferation, leukaemia colony-forming unit plating efficiencies, decreased apoptosis rates and apoptosis-related genes. Furthermore, BM EPCs induced T cell differentiation towards more immune-tolerant cells in higher-risk MDS patients in vitro. In addition, the levels of intracellular ROS and the apoptosis ratios were increased in BM EPCs from MDS patients, especially in higher-risk MDS patients, which may be therapeutic candidates for MDS patients. Our results suggest that dysfunctional BM EPCs are involved in MDS patients, which indicates that improving haematopoiesis supporting ability and immuneregulation ability of BM EPCs may represent a promising therapeutic approach for MDS patients.

Background Myelodysplastic syndromes (MDS) refer to a group of heterogeneous myeloid clonal disorders characterized by abnormal development of myeloid cells, which manifest as ineffective haematopoiesis and a tendency to transform into acute myeloid leukaemia (AML) [1][2][3]. The pathogenesis of MDS is complex and diverse and includes inherent genetic abnormalities in myeloid progenitor cells, changes in the bone marrow (BM) microenvironment, and chronic immune stimulation [1,4,5]. Current strategies to treat MDS rely on allogeneic haematopoietic stem cell transplantation (allo-HSCT), hypomethylating agents, immunosuppressive therapy and chemotherapy [6][7][8][9], which have not achieved satisfactory clinical efficacy [10,11]. Taking allo-HSCT as an example, only 40 to 50% of patients survive for 5 years [12,13]. Therefore, further study of MDS pathogenesis is urgently needed to provide new treatment strategies for MDS patients. The growth and spread of malignant clones represent the dominant pathophysiological process of MDS [14,15]. With disease progression, malignant cells gradually replace normal haematopoietic stem cells (HSCs) and eventually dominate the BM [16,17]. Progression of MDS to AML is thought to result from a shift from apoptosis to proliferation of these malignant clones [18]. As another important pathogenic mechanism of MDS, immune deregulation has been shown in a great number of MDS patients [19,20]. For example, T helper (Th) 17 cells were increased in lower-risk MDS, and regulatory T cells (Tregs) were increased in higher-risk MDS [21,22], which suggests that progression is facilitated by immune suppression. Therefore, understanding the mechanisms of ineffective haematopoiesis and immune deregulation in MDS patients is of considerable importance.
Emerging evidence has shown important roles of the BM microenvironment in regulating haematopoiesis and immune balance [23][24][25]. Previous murine studies [24,26,27] and our previous studies [28][29][30] have reported that endothelial progenitor cells (EPCs) are an important component of the normal BM microenvironment to support HSCs. However, Sophia et al. reported that increased BM EPCs may contribute to inferior haematopoietic function in a MDS murine model [31]. In vitro experiments have shown a poor ability of endothelial colony forming cells from the peripheral blood of MDS patients to support HSCs [32]. These evidences suggest that EPCs demonstrate inferior supporting ability to normal HSCs in MDS whereas the supporting abilities in patients with different stages of MDS remain to be comprehensively depicted. In terms of malignant cells, intrinsic apoptotic signals like clonal chromosomal changes or gene mutations contribute to progression to AML [16]. However, extrinsic apoptotic signals which are related to immune and microenvironment are also considered the pivotal reasons of the transformation of clonal progenitors from MDS to AML [1]. Mice transplantation experiment proved that leukemia-derived endothelial cells are capable of giving rise to AML in normal mice [33]. However, the supporting abilities of BM EPCs to malignant cells in MDS remain unclear. Furthermore, although normal human EPCs exhibit immunosuppressive properties [34][35][36], the immunomodulatory effect of BM EPCs has not been reported in MDS patients. Therefore, the current study was performed to explore the functions of BM EPCs in MDS patients, including lower-risk MDS and higher-risk MDS patients. The number and functions of BM EPCs from MDS patients, de novo AML patients and healthy donors (HDs) were analysed. Bulk RNA sequencing (RNA-seq) was used to further explore the expression profile of BM EPCs, and in vitro coculture strategies were used to evaluate the abilities of BM EPCs to regulate haematopoiesis and immunity. Our aim was to provide a potential therapeutic strategy for MDS patients.
BM adherent cells at day 7 of cultivation were further functionally characterized as BM EPCs for their capacity to uptake Acetylated low-density lipoprotein (Ac-LDL) and to bind Ulex europaeus agglutinin I (UEA I) [28-30, 40, 41]. The adherent cells were incubated with DiI-AcLDL (Life Technologies, Gaithersburg, USA) at 37 °C. After 4 h, the cells were fixed with 4% prechilled paraformaldehyde and incubated with 10 μg/ml fluorescein isothiocyanate-labeled-labelled UEA I (FITC-UEA I; Sigma, St. Louis, USA) for 1 h. To evaluate the numbers of double-positive-stained EPCs, three power fields were randomly counted using a fluorescence microscope (Olympus, Tokyo, Japan).

Tube formation and migration assays
A total of 4 × 10 4 EPCs at day 7 of cultivation were transferred to matrigel-coated (Corning, New York, USA) plates and incubated for 48 h at 37 °C in 5% CO 2 . Tube formation [28,29,39,40] was measured by determining the relative tube length per field of view using an inverted light microscope. All cells were counted in three random fields.
Cell migration [28-30, 39, 40, 43] was assayed using a transwell chamber (Corning, New York, USA). The EPCs after 7 days of culture were seeded in the upper chambers at a density of 4 × 10 4 cells per well, while 500 μl medium was added to the lower chamber. The cells were cultured for 24 h, and migrated cells were fixed with paraformaldehyde for 30 min. Then, cells on the bottom surface of the membrane were stained with crystal violet for 20 min and counted manually in three random fields/sample. Cell images were obtained on a phase-contrast microscope (Olympus, Tokyo, Japan).

HSCs, T cells or HL-60 cells were cocultured with BM EPCs
In order to evaluate the effect of supporting ability of BM EPCs to normal HSCs, T cells or HL-60 cells, the cultivated BM EPCs from HDs or MDS or AML patients were cocultured with normal CD34 + cells, or normal CD3 + cells, or HL-60 cells [28-30, 40, 44]. HSCs or T cells were isolated from BMMNCs of HDs using CD34 or CD3 MicroBead kits (Miltenyi Biotec, Bergisch Gladbach, Germany). Cultivated EPCs (described before) were plated onto gelatinized 24-well culture plates at 1 × 10 5 cells/well and cultured overnight to achieve confluence. Then, HSCs (1 × 10 5 per well) or T cells (1 × 10 5 per well) or HL-60 cells (Manassas, Virginia, USA; 5 × 10 4 per well) were added in direct contact with confluent and adherent EPCs. EPC-HSC cocultures were maintained in StemSpan ™ SFEM (Stem Cell Technologies, Vancouver, Canada) for 5 days. EPC-T cells or HL-60 cells cocultures were maintained in RPMI 1640 medium supplemented with 10% FBS for 3 days or 5 days. Appropriate controls of T cell culture alone were also included.
To detect ROS levels on day 7 of cultivated BM EPCs, adherent cells were incubated with 1 μg/ml DCFH-DA at 37 °C for 20 min. Images were obtained in three random fields/sample using a fluorescence microscope (Olympus, Tokyo, Japan). The fluorescence intensity of ROS was analysed via the mean grey value using ImageJ 1.52v (National Institutes of Health, Bethesda, USA).

Analysis of T cell subsets
After 3 days of coculture, CD3 + T cells were stimulated with a cell stimulation cocktail (eBioscience, San Diego, USA) to induce CD3 + T cell activation and cytokine secretion. Lymphocyte subpopulations were quantified via flow cytometry as previously described [46][47][48][49]. Th1, Th2, Th17 cells, and Tregs were identified as The details of antibodies were in Additional file 1: Table S2.

RNA-seq and data analysis
RNA-seq analyses were performed to analyse the 7-day cultivated BM EPCs from HDs, lower-risk MDS, higher-risk MDS or AML patients. The accession number of whole transcriptome RNA-seq data is GSE197907. Differential gene expression (DEGs), principal component analysis (PCA), hierarchical clustering analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment plot were executed by the DESeq2, clusterProfiler, pheatmap, and ggplot2 packages in R (1.16.1). The top 20 up and down regulated genes in higher-risk MDS BM EPCs than lowerrisk MDS BM EPCs were list in Additional file 1: Table S3 and in AML BM EPCs than higher-risk MDS BM EPCs were list in Additional file 1: Table S4. Gene set enrichment analysis (GSEA) analysis was performed with respect to MSigDB (version 5.1) genesets C5 GO biological processes [50].

Statistical analysis
Analyses were performed using GraphPad Prism 6.0. Statistical analyses were performed using Mann-Whitney U test. The relative mRNA analyses and paired analyses were using Wilcoxon matched-pairs signed rank test. The results are expressed as the means ± SEM, and P-values < 0.05 were considered statistically significant.

Decreased angiogenic potential but increased migration ability of BM EPCs in MDS patients
To evaluate the angiogenic potential and migration of BM EPCs from different types of MDS patients, tube formation and migration were analysed on day 7 of culture [28][29][30]40]. Higher-risk MDS BM EPCs showed a significantly increased tube formation ability compared with lower-risk MDS BM EPCs ( Fig. 1D and E, 4375 ± 321 vs.1874.7 ± 118.9, P < 0.0001) and a markedly decreased tube formation ability compared with AML patients ( Fig. 1D and E, 4375 ± 321 vs. 6403 ± 370.4, P = 0.001).
The AML BM EPCs on day 7 of culture showed increased migrated cells (Fig. 1F and G, 172.4 ± 29.8 vs. 82.6 ± 7.9, P = 0.01) compared with lower-risk MDS BM EPCs. These results demonstrated that the decreased angiogenic potential but increased migration capability in BM EPCs from MDS patients.

RNA-seq indicates the progressive change and differences of haematopoiesis-and immune-related pathways and genes in MDS BM EPCs
To uncover the underlying mechanism of the variant dysfunctions in different types MDS BM EPCs, the lowerrisk MDS BM EPCs (N = 3), higher-risk MDS BM EPCs (N = 3), AML patients BM EPCs (N = 3) and primary BM EPCs from HDs (HD BM EPCs) (N = 3) were analysed via RNA-seq ( Fig. 2A). The results of PCA (Fig. 2B) and hierarchical clustering analysis (Fig. 2C) showed that the entire population was clearly separated into two distinct subpopulations, HD and disease BM EPCs. More importantly, the heatmap (Fig. 2C) of different groups showed the progressive change in the total RNA expression profile of HD BM EPCs, lower-risk MDS BM EPCs, and higher-risk MDS BM EPCs to AML BM EPCs. There were 886 different genes between lower-risk MDS and higher-risk MDS BM EPCs (Fig. 2D) and 825 different genes between higher-risk MDS and AML BM EPCs (Fig. 2E). KEGG pathway enrichment analysis of Organismal Systems class ( Fig. 2F and G) indicated that haematopoiesis-and immune-related pathways were enriched in MDS BM EPCs. For example, haematopoietic cell lineage was enriched in both lower-risk MDS BM EPCs vs. higher-risk MDS BM EPCs and higher-risk MDS BM EPCs vs. AML BM EPCs. Th17 and Th2 cell differentiation and Th17 cell differentiation were enriched in higher-risk MDS BM EPCs vs. AML BM EPCs.
In addition, the previously reported haematopoiesisand immunomodulation-related genes [51][52][53][54][55][56] in BM EPCs were detected by RNA-seq (Fig. 2H) and verified by qRT-PCR. As shown in Fig. 2H-J, mRNA levels of haematopoiesis-related genes CXCL12, KITLG, NFKB1 and immune-related genes HAVCR2, LGALS9, CIITA were increased in higher-risk MDS and AML BM EPCs compared with lower-risk MDS BM EPCs. These results suggested markedly enriched haematopoiesis-and immunomodulatory-related pathways and upregulation of related genes in higher-risk MDS BM EPCs. However, normal or malignant haematopoiesis-related genes could not be distinguished via RNA-seq analysis. Therefore, we further carried out in vitro experiments to analyse the haematopoiesis and immunomodulatory function of BM EPCs from MDS patients.

Decreased ability of BM EPCs to support HSCs in higher-risk MDS patients
To investigate the effects of BM EPCs on HSCs in vitro, we sorted CD34 + cells from BMMNCs of HDs with magnetic beads and cocultured them with BM EPCs. After 5 days, we analysed the apoptosis rates, intracellular ROS levels and CFU plating efficiencies of HSCs (Fig. 3A). Compared with the lower-risk MDS group, the higher-risk MDS group exhibited a marked increase in the apoptosis rates of HSCs after coculture ( Fig. 3B and C, 21.09 ± 1.21% vs. 16.79 ± 0.97%, P = 0.01). The ROS level (Fig. 3D, 6039 ± 654.4 vs. 4308 ± 394.7, P = 0.03) of CD34 + cells after coculture with AML BM EPCs was significantly higher than that of lower-risk MDS BM EPCs. HSCs cocultured with higher-risk MDS BM EPCs had lower CFU plating efficiencies than those cocultured with lower-risk MDS BM EPCs (Fig. 3E), as determined by CFU-E, BFU-E (12.9 ± 1.67 vs. 26.9 ± 3.62, P = 0.003), CFU-GM, and CFU-GEMM. The AML group had lower CFU-E, BFU-E, CFU-GM and CFU-GEMM plating efficiencies than the higher-risk MDS group. These data suggest that of MDS BM EPCs were less able to support HSCs in more severe type of MDS patients.

BM EPCs induce T cell differentiation towards more immune-tolerant cells in higher-risk MDS patients
To further investigate the immunoregulatory effects of BM EPCs in vitro, we sorted CD3 + cells from BMMNCs of HDs and cocultured them without or with HD BM EPCs, lower-risk MDS BM EPCs, higher-risk MDS BM EPCs and AML BM EPCs. After 3 days, we analysed differences in T cell subsets after coculture or not (control of T cell alone cultures) (Fig. 4A). The subtypes of Th1 cells, Th2 cells, Th17 cells and Tregs were characterized by positive expression of CD4 + IFN-γ + , CD4 + IL-4 + , CD4 + IL-17 + and CD4 + CD25 + Foxp3 + , respectively (Fig. 4B). There was a markedly lower percentage of Th1 cells in the AML group than in the higher-risk MDS group (Fig. 4C, 4.86% ± 0.54% vs. 8.07% ± 0.51%, P = 0.004) and in the higher-risk MDS group than in the lower-risk MDS group (Fig. 4C, 8.07% ± 0.51% vs. 9.95% ± 0.6%, P = 0.04). A significantly lower percentage of Th17 cells was observed in the higher-risk MDS group than in the lower-risk MDS group (Fig. 4D, 6.54% ± 0.43% vs. 9.35% ± 0.92%, P = 0.004) and in the AML group than in the higher-risk MDS group (Fig. 4D, 4.48% ± 0.58% vs. 6.54% ± 0.43%, P = 0.04). However, higher percentages of Th2 cells (Fig. 4E, 8.77% ± 1.0% vs. 5.67% ± 0.31%, P = 0.02) in the higher-risk MDS group than in the lowerrisk MDS group. In addition, higher percentages of Tregs (Fig. 4F, 4.74% ± 0.59% vs. 3.14% ± 0.31%, P = 0.01) were observed in the AML group than in the higher-risk MDS group. As a result, the ratio of Th1/Th2 cells was markedly lower in the higher-risk MDS group than in the lower-risk MDS group (Fig. 4G, 1.0 ± 0.1 vs. 1.81 ± 0.17, P = 0.004), whereas that was significantly lower in the AML group than in the higher-risk MDS group (Fig. 4G, 0.51 ± 0.06 vs. 1.0 ± 0.1, P = 0.02). These data showed that lower-risk MDS BM EPCs regulated T cell differentiation into Th17 cells, whereas higher-risk MDS and AML BM EPCs regulated T cell differentiation into Th2 cells and Tregs. Our data suggested that BM EPCs might be inclined to induce T cell differentiation towards more immune-tolerant cells in higher-risk MDS patients.

Increased ability of BM EPCs to support leukaemia cells in higher-risk MDS patients
To investigate the effect of BM EPCs on leukaemia cells in vitro, we assessed the proliferation, apoptosis and CFU-L plating efficiency of HL-60 cells after coculture with BM EPCs (Fig. 5A). In addition, the relative mRNA expression levels of apoptosis-and cell cycle-related genes were detected in HL-60 cells after coculture. The EdU -positive rate of HL-60 cells was significantly higher in the higher-risk MDS group than in the lower-risk MDS group ( Fig. 5B and C, 45.79% ± 1.12% vs. 36.52% ± 2.20%, P = 0.003). More importantly, CFU-L plating efficiencies were markedly increased in AML group compared with higher-risk MDS group ( Fig. 5D and E, 374.1 ± 37.2 vs. 257.6 ± 33.2, P = 0.04). However, the apoptotic rate of HL-60 cells was notably decreased in the AML group compared with the lower-risk MDS group (Fig. 5F, 2.6% ± 0.52% vs. 4.38% ± 0.46%, P = 0.01). The CASP2, CASP3, BAX, TP53 and CDKN1A mRNA levels in HL-60 cells were downregulated, whereas the CCNE1 and MCL1 mRNA levels were upregulated in the higher-risk MDS and AML groups than the lower-risk MDS group (Fig. 5G). These data suggest that BM EPCs may become more supportive of leukaemia cells in higher-risk MDS patients.

The levels of ROS and the apoptosis ratio were increased in BM EPCs from higher-risk MDS patients
To further explore the internal changes in dysfunctional EPCs and identify underlying therapeutic targets, we investigated damage related pathways from the RNAseq data. GSEA highlighted abnormalities of mitochondrial signaling pathway (Fig. 6A, B), so we verified them by detecting ROS levels of precultivated and cultivated BM EPCs. The ROS level of precultivated AML BM EPCs was significantly higher than that in higher-risk MDS BM EPCs (Fig. 6C, 5369 ± 426.0 vs. 3039 ± 335.3, P = 0.0002).
The ROS level in the cultivated AML BM EPCs was higher than that in the cultivated higher-risk MDS BM EPCs, whereas the ROS level of the cultivated higherrisk MDS BM EPCs was higher than that in the lowerrisk MDS BM EPCs (Fig. 6D, E). Moreover, the pathway of apoptotic nuclear changes was identified in GSEA (Fig. 6F). The apoptosis rate (Fig. 6G, 43.79% ± 2.77% vs. 20.63% ± 3.01%, P < 0.0001) was significantly higher in AML BM EPCs than in lower-risk MDS EPCs. The relative mRNA expression levels of apoptosis-related genes in BM EPCs were further analysed by qRT-PCR. The CASP2 mRNA level was significantly upregulated in AML BM EPCs (Fig. 6H, 6.43 ± 0.89-fold vs. 2.28 ± 0.56fold, P = 0.03) compared to higher-risk MDS BM EPCs. The CASP3 (Fig. 6H, 2.09 ± 0.26-fold vs. 0.88 ± 0.3fold, P = 0.03) and BAX (Fig. 6H, 2.0 ± 0.35-fold vs. 1.08 ± 0.14-fold, P = 0.03) mRNA levels were significantly upregulated in higher-risk MDS BM EPCs compared to lower-risk MDS BM EPCs. Together, elevated levels of ROS and apoptosis of BM EPCs were found in MDS patients, especially in higher-risk MDS patients, which may be the underlying repair targets for MDS patients (Fig. 7).

Discussion
The current study firstly demonstrated increased but dysfunctional BM EPCs in patients with MDS. The abilities of BM EPCs from higher-risk MDS patients to support HSCs decreased, whereas those from higher-risk MDS patients to support leukaemia cells increased. Furthermore, BM EPCs might be inclined to induce T cell differentiation towards more immune-tolerant cells in more severe type of MDS patients. In addition, excessive production of ROS and apoptotic pathway activation may be the underlying mechanisms of dysfunctional BM EPCs. Our data indicate that repair of dysfunctional BM EPCs may be a potential therapeutic approach for MDS patients.
Increased BM angiogenesis has been reported in MDS patients [32,[57][58][59]. Immunohistochemical studies have demonstrated an increased microvessel density (MVD) in the biopsies of MDS patients [58][59][60]. The results concerning the correlation between BM MVD and classification are controversial. Some studies reported a higher MVD preferentially among higher-risk MDS patients [58,61], while other studies failed to find such a correlation [60]. Other studies regarding circulating endothelial cells and peripheral EPCs are consistently increased in MDS patients compared with normal controls [32,57].
In summary, the number of BM EPCs in MDS patients and their correlation with the different risk degrees are not clear. More importantly, the functions of BM EPCs in patients with different risk groups of MDS are largely unknown. Our study firstly demonstrated increased BM EPCs with dysfunctions in MDS patients, especially more severe dysfunctions of BM EPCs in patients with higher-risk MDS. We speculated that the increased and dysfunctional EPCs in MDS patients were responsible for more nutrient and metabolite turnover, and may secrete more cytokines to better support malignant hematopoiesis. Based on our previous work and the current study, the impairment of EPCs was associated with elevated intracellular ROS levels and an elevated apoptosis ratio in MDS patients [28,29,39,62]. Therefore, it is worth investigating the efficacy of therapeutic strategies, such as N-acetyl-l-cysteine (a reactive oxygen species scavenger) [28,29,62,63] or inhibitor of EPC-targeted apoptosis pathways, to improve the prognosis of MDS patients by enhancing BM EPCs in the future. A great deal of researches have certified the critical role of EPCs in regulating haematopoiesis [24,26,27], whereas Hatfield et al. found that EPCs supported leukaemia cells by directly enhancing the proliferation and inhibiting apoptosis of AML blasts [45]. However, it is unclear whether the abilities of BM EPCs from MDS patients to support HSCs or leukaemia cells are the same or different. Surprisingly, we found antipodal results for normal HSCs and leukaemia cells. The CFU plating efficiency of HSCs declined with disease progression, which suggested BM EPCs from patients with MDS or AML could not support normal differentiation of HSCs. On the contrary, the CFU-L plating efficiency increased, which implied the possible role of BM EPCs as an oncogenic driver or facilitator of MDS. On the other hand, the progression of MDS is also facilitated by immune deregulation [1]. However, the correlation between BM EPCs and immune deregulation in MDS is largely unknown. Consistent with previous clinical reports of T cell subtypes in MDS patients [21,22], our data demonstrated that BM EPCs from lower-risk MDS patients regulate T cell differentiation into inflammatory Th17 cells but BM EPCs from higher-risk MDS patients regulate T cells into more immune tolerant cells.
We are aware that the underlying mechanism on how BM EPCs regulate T cells and the precise T cell subset need to be further explored in the future. However, our data indicated that the dysregulated immunomodulatory function of EPCs may also contribute to ineffective haematopoiesis and evasion from antitumoural immunity in MDS. Therefore, in conjunction with existing therapies, improvement of BM EPCs may be a potential therapeutic strategy for patients with MDS.

Conclusions
In summary, the current study demonstrated that dysfunctional BM EPCs were involved in MDS patients. Changes in the BM microenvironment may be a primary driver of human haematological malignancies, as has been suggested by animal models [64,65]. Although further validation is required, our findings indicate that improving BM EPCs may represent a potential therapeutic approach for MDS patients.