Bone Marrow Research of HSCs cultured with MSCs was higher in aza-D or TSA than in any culture conditions without MSCs, while most of HSCs cultured with cytokine treatment but without MSCs would lose their pluripotency and then diﬀerentiate, though they were induced to proliferate eﬀectively [60]. It suggested that the co-culture of CD34+ cells with MSCs might not simply deliver the proliferation signals but also stemness and survival signals, and overlap the action of epigenetic regulators [57, 60]. 4. Application of Bone Marrow Mesenchymal Stem Cells in Hematopoietic Stem Cell Transplantation HSCs were primarily used in the treatment of patients with hematological malignancies. During the course of treatment, patients’ cancerous cells are ﬁrst destroyed by chemo/radiotherapy and then replaced with BM or PB/G- CSF transplant from a human leukocyte antigen- (HLA-) matched donor [61, 62]. In most cases, autologous HSCs are collected prior to the treatment and reinfused into the patients, but the patient’s cancerous cells may be inadver- tently collected and reinfused back into the patients along with HSCs [63]. Allogeneic marrow transplants have also been used in the treatment of hereditary blood disorders including aplastic anemia, β-thalassemia, Wiskott-Aldrich syndrome, and SCID, as well as inborn errors of metabolism disorders such as Hunter’s syndrome and Hurler’s syndrome [64–68]. One of the major challenges with HSC transplants is failure to engraft, which is mediated by donor T cells as a result of graft-versus-host disease (GVHD). Graft-versus- tumor eﬀect of allogeneic HSC transplants may be a result of an immune reaction between donor cytotoxic T cells and patient’s malignant cells [69]. MSCs are known to interact with HSCs and immune cells, and represent potential cellular therapy to enhance allogeneic hematopoietic engraftment and prevent GVHD [70–72]. Coculture of MSCs and HSCs could cause a signiﬁcantly increase in CD34+ cells [73]. Aside from BM-derived MSCs, MSCs from adipose tissue can also be applied in hematopoietic engraftment, which would be an innovative supplement for cellular therapies [74, 75]. Cotransplantation studies in animal models as well as in humans showed that primary or culture-expanded MSCs promote the engraftment of HSCs. Cotransplantation of MSCs and cord blood or mobilized peripheral blood CD34+ cells resulted in a signiﬁcantly higher level of engraftment than transplantation of CD34+ cells only [35, 37, 76–81]. This enhancement was greater after cotransplantation of GM-CSF and SCF-transfected MSCs, indicating that these growth factors relate to engraftment, though the mechanism of the enhancing eﬀect is still unknown [35]. It is likely that the ability to promote engraftment is maintained along lineage diﬀerentiation [76]. Several lines of evidence suggest that MSCs produce several essential hematopoietic growth factors, adhesion molecules [28, 52–54], and extracellular matrix (ECM) proteins (such as VCAM1, E-selectin, collagen I, and ﬁbronectin) that are known to play an important role in HSC homing [35, 37]. Selective adhesion of progenitors 3 and cytokines to ECM components or stromal cells then result in the colocalization of progenitors at a speciﬁc stage of diﬀerentiation with a speciﬁc array of cytokines in so- termed niches [77]. This provides a level of growth and diﬀerentiation regulation [37]. Although it would mean exposure to allogeneic donor antigens, allogeneic MSCs can provide equal enhancement of engraftment as autologous cells. Cotransplanted MSCs shift the diﬀerentiation pattern from a lymphoid to a myeloid predominance and enhance megakaryocytic engraftment [78]. The cotransplantation of HSCs and MSCs enhanced engraftment as the dose of MSCs increased whereas an excessive dose of MSCs might decrease engraftment eﬃciency [79]. Besides, human allogeneic MSC layers in a serum-free culture system enabled the ex vivo expansion/maintenance of human HSCs [80], which indicates that MSCs may be used as a universal and reproducible stromal feeder layer to eﬃciently expand and maintain human BM HSCs ex vivo [81]. MSCs produce a microenvironment supporting hema- topoiesis and may contribute to immune tolerance because of low immunogenicity and the suppressive eﬀect of allore- activity [75, 82]. MSCs had a potent immunosuppressive eﬀect in vivo after allogeneic stem-cell transplantations [26]. The CXCL12-α secreted by MSCs could reduce the production of a variety of inﬂammatory cytokines and chemokines, including IL-13, IL-3 Rβ, IL-4, IL-5, IL-9, IL-10, L-selectin, MIP-3α/β, TCA3/CCL1, TNF-a, IL-1β, lymphotactin/CXCL1, L-selectin, leptin receptor, eotaxin-2, CTACK/CCL27, CRG-2/CXCL10, and CD30L [33]. In allo- geneic transplantation, the simultaneous infusion of MSCs may promote hematopoietic engraftment across the major histocompatibility complex (MHC) barrier and decrease the incidence of GVHD, even though the exact mechanisms have not been clariﬁed [83–85]. MSCs are lack of MHC class II and most of classical costimulatory molecules [86, 87]. Moreover, MSCs directly inhibit the expansion and activa- tion of alloreactive Tlymphocytes and this T cell-suppressive eﬀect may have important therapeutic implications in preventing or treating acute and chronic GVHD [70]. MSCs can signiﬁcantly reduced the expression of activation markers CD25 (interleukn-2 receptor), CD38, and CD69 on phytohaemagglutinin- (PHA-) stimulated lymphocytes, making allogeneic HSCs and MSCs escape from recognition by alloreactive T-cells, because the expression of CD25 (IL-2 receptor), CD38 and CD69 was unchanged. Besides, MSC suppressed the proliferation of PHA-stimulated CD3+, CD4+, and CD8+ lymphocytes [87–89]. However, MSCs inhibit na¨ıve and memory T-cell responses to their cognate antigens by the engagement of the inhibitory molecule PD-1 while the expression of MHC molecules and the presence in culture of antigen-presenting cells (APCs) or CD4+/CD25+ regulatory T cells were not required for MSCs to inhibit preferentially [87–91]. MSCs can regulate B-cell functions including migration, proliferation, and immunoglobulin(Ig) synthesis. For example, MSCs inhibit the proliferation of B- cells by arresting them at G0/G1 phase of the cell cycle, and the production of IgM, IgA, and IgG of B-cells [88, 92]. Dendritic cells (DCs) play an important role in supporting antigen-speciﬁc CD4+ T-cell proliferation and modulating