Myelodysplastic syndromes (MDS) is generally referred as a heterogenous group of clonal hematopoietic diseases characterized by ineffective hematopoiesis resulting in peripheral blood cytopenia potentially shifting to acute myeloid leukemia (AML) [
80]. MDS patients display different degrees of cytopenia and dysplasia, therefore constituting the basis for Word health organization MDS classification criteria [
81]. To date, no clinically effective treatment is available for preventing progression to AML. Half of patients show cytogenetic alteration, while nearly 90% of them harbor at least one somatic mutation affecting specific genes involved in spliceosome, transcription factors and epigenetics. Despite the clonal dominance, these mutations don’t provide a determined advantage for malignant cell growth, as suggested by their coexistence alongside normal HSPCs [
82]. Therefore, MDS cells get an extrinsic support from BMME important for malignant cell cloning. Notably, support from BM milieu is essential to maintain MDS cells
ex vivo. Concerning MSCs, MDS stromal cells are reprogramed to support uniquely MDS clones at the expense of normal HSPCs [
35]. MDS-MSCs are characterized by a slower proliferation rate, which is independent by cell cycle distribution and apoptotic events [
83]. Therefore, the reason characterizing this phenotype might involve cytogenetic aberrations, which have been difficult to characterize due to the lack of a specific isolation protocol allowing the comparison between different MSCs subpopulations. This goal has been achieved upon Aanei and colleagues published a robust immunoselection-based isolation protocol through two specific mesenchymal-associated markers, STRO-1 and CD73 [
84]. Therefore, MDS-MSC cytological characterization highlighted genomic gains involving genes taking part in cell-cell adhesion processes and tumor development. In addition, MSCs isolated from patients harboring 5q- cytogenetic shared common traits including the overexpression of some genomic regions as 7p22.3, 19p13.3, and 19p13.11 [
83]. Despite a cytogenetic signature characterizing MDS-MSCs is still missing, it is widely reported how these cells display all the typical marker related to cell senescence [
85]. In this context, Fei et al. reported that isolated MDS-MSCs display a profound change in cytoskeletal architecture, in turn showing an increased size, longer podia and a disordered distribution of F-actin [
86]. Moreover, MDS-MSCs also display an increased DNA damage level [
87]. Coherently with this outcome, an hyperactivation of p53 signaling cascade has been detected in MDS-MSCs, therefore providing a further mechanism leading to MSC senescence [
86]. Despite the efforts showing the essential role covered by BMME in MDS, the question asking who the first cell population is to impair the basal crosstalk, therefore triggering MDS pathogenesis, is still standing. A partial answer has been given by the studies describing that, in patients showing a complete hematological remission, the treatment was able to restore a MSC functionality comparable to healthy donors [
88]. However, this study has the intrinsic assumption that the treatment has the HSC compartment as only target, excluding a direct effect on MSCs themselves. In this context, it has been recently demonstrated that the antileukemic activity of azacytidine in part depend on its direct effect on the hematopoietic supportive capacity of MDS-MSCs favoring expansion of healthy over malignant hematopoiesis [
89]. These data highlighted the crucial role of an epigenetic treatment of dysfunctional MSCs. Other studies corroborate the hypothesis involving the crosstalk between stromal and hematopoietic compartments as driver of MDS pathogenesis, in turn rearranging the surrounding microenvironment to support the expansion of the malignant clone. Indeed, murine models depleted for Dicer or Sbds gene expression exclusively in stromal compartment have shown to develop an MDS-like phenotype characterized by ineffective hematopoiesis, marked dysplasia and leukemic progression, despite having no mutation in their HSPCs [
90,
91]. Medyouf and colleagues described a scenario in which MSCs are instructed by malignant HSCs to acquire MDS-MSC-like properties, eventually promoting the progression of the malignant clone over the healthy one [
35]. This data introduced the “hematopoietic niche unit”, sustaining MDS progression also by the establishment of an altered secretome profile, where abundant levels of TNF-α, IFN-γ, IL-1α, IL-6, IL-17 and TGF-β have been detected [
92]. These factors account for the establishment of an inflammatory BMME, in turn triggering genetic and epigenetics modifications on BM resident cell populations. Corroborating this, MDS-MSCs show several differentially methylated genes associated to alterations of their phenotype [
36]. For instance, HHIP (Hh-interacting protein) gene is hypermethylated in MDS-MSCs [
93]. Its downregulation accompanied by activation of the Hedgehog pathway in stromal cells sustain survival of MDS cells. Recently, our group have shown the relevance of an epigenetic-inflammatory interplay in MDS-MSCs supported by macroH2A1/TLR4 axis, prompting a replicative senescent phenotype, hypermethylation and metabolic rewiring which contribute to ineffective hematopoiesis [
94]. In agreement, cellular stress and upregulation of inflammatory molecules with inhibitory effects on normal hematopoiesis has been described in MDS-MSCs [
95]. In particular, activation of NFkB signaling in MSCs from patients with lower risk MDS (LR-MDS) attenuates normal hematopoiesis in accordance with cytopenia observed in these patients [
96]. Moreover, overexpression of the alarmins S100A8/9 in stromal cell compartment has been shown to activate NFkB and a genotoxic stress in HSPCs associated to leukemic evolution in a subset of LR-MDS patients [
91]. Supporting the crucial role played by the BM niche in MDS evolution, it has been proposed that the overexpression of CXCL12, in synergy with its receptor CXCR4, keep the myelodysplastic anchored inside the BM niche, in turn providing them with protection and support [
97]. In this scenario an in vitro study highlighted the overproduction of IL-6, an interleukin possibly linked to the mechanism promoting MSCs senescence and chronic inflammation [
98]. The crosstalk between MDS cells and MSCs is also orchestrated by a plethora of factors, as part of the two populations’ secretome. Releasing alarmins such as S100A9 and S100A8, tumor cells activate the inflammasome in stromal cells which results in higher secretion of pro-inflammatory cytokine [
99]. Also, EVs secreted by MDS cells have been demonstrated to reduce the hematopoietic supportive capacity of MSCs inhibiting osteolineage differentiation of MSCs [
100]. This perturbation of bone metabolism enables MDS clones to outcompete normal HSPCs. In turn, MDS-MSCs have been described to release EVs carrying miRNA, such as miR10a and miR15a, which increase the viability and clonogenicity of MDS cells [
101]. Therefore, the multifaced aspects accounting for MSCs significancy need to be further dissected to provide more efficient strategies counteracting MDS progression.