Differentiation Of Bone Marrow Progenitors Research Articles

HuMSC-EV induce monocyte/macrophage mobilization to orchestrate neovascularization in wound healing process following radiation injury

This study aims to investigate the mechanisms of human mesenchymal stem cell-derived extracellular vesicles (HuMSC-EV)-induced proangiogenic paracrine effects after radiation injury. HuMSC-EV were locally administered in mice hindlimb following 80-Gy X-ray irradiation and animals were monitored at different time points. HuMSC-EV improved neovascularization of the irradiated tissue, by stimulating angiogenesis, normalizing cutaneous blood perfusion, and increasing capillary density and production of proangiogenic factors. HuMSC-EV also stimulated vasculogenesis by promoting the recruitment and differentiation of bone marrow progenitors. Moreover, HuMSC-EV improved arteriogenesis by increasing the mobilization of monocytes from the spleen and the bone marrow and their recruitment into the muscle, with a pro-inflammatory potential. Importantly, monocyte depletion by clodronate treatment abolished the proangiogenic effect of HuMSC-EV. The critical role of Ly6C(hi) monocyte subset in HuMSC-EV-induced neovascularization process was further confirmed using Ccr2−/− mice. This study demonstrates that HuMSC-derived EV enhances the neovascularization process in the irradiated tissue by increasing the production of proangiogenic factors, promoting the recruitment of vascular progenitor cells, and the mobilization of innate cells to the injured site. These results support the concept that HuMSC-EV might represent a suitable alternative to stem cells for therapeutic neovascularization in tissue repair.

Macrophage-specific deletion of neuropilin-2 inhibits inflammatory signalling and attenuates atherosclerotic plaque development in apolipoprotein e-deficient mice

Abstract Background Atherosclerosis is the underlying pathology in most cases of ischaemic heart disease, which itself remains the leading cause of mortality worldwide. Atherosclerosis is a multifaceted vascular disease characterised by arterial wall inflammation resulting from the subendothelial retention, and modification, of lipoproteins by macrophages. Macrophages exhibit a high degree of plasticity, and undergo polarisation to a pro-inflammatory phenotype in response to pro-inflammatory cytokines and other environmental stimuli, making them an attractive therapeutic target. Neuropilin-2 (NRP2) is a cell-surface co-receptor with essential roles in angiogenesis and axonal guidance, and is also expressed by macrophages. However, little is known about the role of NRP2 in macrophage function or atherosclerosis. Purpose To investigate whether NRP2 is important for macrophage involvement in atherosclerosis, we characterised the effects of conditional NRP2 deletion on macrophage function and atherosclerotic plaque development. Methods We generated mice with macrophage-specific (LysM-Cre) deletion of Nrp2, and lineage tracing via EYFP transgene expression, on the pro-atherogenic Apolipoprotein E-deficient background (Nrp2-KOMac, Apoe−/−, EYFP). These mice were fed a high-fat diet (HFD) for 16 weeks before their aortas were stained with Oil-red-O to assess plaque coverage. Plaque content was then characterised by sectioning and staining the aortic roots. The role of NRP2 in modulating pro-inflammatory macrophage polarisation and signalling, was assessed using functional assays and transcriptome analysis in Nrp2-KO bone marrow-derived macrophages (BMDMs). Results HFD-induced plaque development was significantly reduced in Nrp2-KOMac, Apoe−/−, EYFP mice, with 17.4% total aortic plaque coverage in Nrp2-KOMac, Apoe−/−, EYFP mice, compared to 24.9% in WTApoe−/−, EYFP mice (p=0.0021, n=10). Plaques from Nrp2-KOMac, Apoe−/−, EYFP mice also displayed features consistent with increased plaque stability, including reduced necrotic core area and plaque lipid content, and increased cap thickness. NRP2 was significantly upregulated upon M-CSF-mediated differentiation of bone marrow progenitors into BMDMs, and further upregulated by pro-inflammatory polarisation using LPS and IFN-gamma. Furthermore, upregulation of Il1β, Tnfα and Il6, induced by LPS and IFN-gamma, was significantly reduced in Nrp2-KO BMDMs, and MCP-1 induced migration was also decreased in these cells. Transcriptome analysis revealed that NF-kappaB signalling pathway genes, and genes regulating monocyte chemotaxis, were downregulated in Nrp2-KO BMDMs. Conclusion Macrophage-derived NRP2 is pro-atherogenic, likely resulting from its role in positively regulating pro-inflammatory signalling and macrophage migration. Targeting NRP2 expressed on the surface of macrophages could therefore offer a novel therapeutic approach for reducing the disease burden associated with atherosclerosis. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): British Heart Foundation (MB PhD Studentship)

GM-CSF: A Promising Target in Inflammation and Autoimmunity.

The cytokine, granulocyte macrophage-colony stimulating factor (GM-CSF), was firstly identified as being able to induce in vitro the proliferation and differentiation of bone marrow progenitors into granulocytes and macrophages. Much preclinical data have indicated that GM-CSF has a wide range of functions across different tissues in its action on myeloid cells, and GM-CSF deletion/depletion approaches indicate its potential as an important therapeutic target in several inflammatory and autoimmune disorders, for example, rheumatoid arthritis. In this review, we discuss briefly the biology of GM-CSF, raise some current issues and questions pertaining to this biology, summarize the results from preclinical models of a range of inflammatory and autoimmune disorders and list the latest clinical trials evaluating GM-CSF blockade in such disorders.

S1P₄ Regulates Passive Systemic Anaphylaxis in Mice but Is Dispensable for Canonical IgE-Mediated Responses in Mast Cells.

Mast cells are key players in the development of inflammatory allergic reactions. Cross-linking of the high-affinity receptor for IgE (FcεRI) on mast cells leads to the generation and secretion of the sphingolipid mediator, sphingosine-1-phosphate (S1P) which is able, in turn, to transactivate its receptors on mast cells. Previous reports have identified the expression of two of the five receptors for S1P on mast cells, S1P1 and S1P2, with functions in FcεRI-mediated chemotaxis and degranulation, respectively. Here, we show that cultured mouse mast cells also express abundant message for S1P4. Genetic deletion of S1pr4 did not affect the differentiation of bone marrow progenitors into mast cells or the proliferation of mast cells in culture. A comprehensive characterization of IgE-mediated responses in S1P4-deficient bone marrow-derived and peritoneal mouse mast cells indicated that this receptor is dispensable for mast cell degranulation, cytokine/chemokine production and FcεRI-mediated chemotaxis in vitro. However, interleukin-33 (IL-33)-mediated enhancement of IgE-induced degranulation was reduced in S1P4-deficient peritoneal mast cells, revealing a potential negative regulatory role for S1P4 in an IL-33-rich environment. Surprisingly, genetic deletion of S1pr4 resulted in exacerbation of passive systemic anaphylaxis to IgE/anti-IgE in mice, a phenotype likely related to mast cell-extrinsic influences, such as the high circulating levels of IgE in these mice which increases FcεRI expression and consequently the extent of the response to FcεRI engagement. Thus, we provide evidence that S1P4 modulates anaphylaxis in an unexpected manner that does not involve regulation of mast cell responsiveness to IgE stimulation.

Unacylated ghrelin modulates circulating angiogenic cell number in insulin-resistant states

BackgroundType 2 diabetes (T2D) is associated with reduced numbers and impaired function of circulating angiogenic cells (CAC) which contributes to the progression of atherosclerosis and microvascular disease. Previous studies suggest that short-term infusion of unacylated ghrelin (UAG) normalizes CAC number in patients with T2D. To determine dose-dependent effects of short-term infusion of UAG in T2D patients using a cross-over model, and of long-term infusion of UAG in obese mice, on differentiation of monocyte progenitors into CAC.MethodsEight overweight T2D patients were infused overnight with 3 and 10 µg/kg/h of UAG in a double-blind, placebo-controlled cross-over study. To assess the effects of long-term UAG treatment, obese mice were infused with UAG for 4 weeks. Monocyte progenitors were assessed for their ability to differentiate into CAC in vitro.ResultsIn T2D patients, UAG treatment caused a reduction in differentiation of CAC, dependent on UAG dose and differentiation method. However, mice treated with UAG showed a significant increase in differentiation of bone marrow progenitors into CAC.ConclusionUAG causes a minor suppressive effect on CAC development after short-term treatment in humans, but experiments in mice suggest that long-term treatment has beneficial effects on CAC formation.The Netherlands Trial Register: TC=2487

The mitochondrial fusion-related proteins Mfn2 and OPA1 are transcriptionally induced during differentiation of bone marrow progenitors to immature dendritic cells.

The shape and activity of mitochondria are tightly regulated by fusion and fission processes that are essential for maintaining normal cellular function. However, little is known about the involvement of mitochondrial dynamics in the development of the immune system. In this study, we demonstrate that mitochondrial dynamics play a role in the differentiation and migration of immature dendritic cells (imDCs). We show that mitochondrial elongation is induced during GM-CSF-stimulated differentiation of bone marrow progenitors to imDCs accompanied by upregulation of mitochondrial fusion proteins. These processes precede the changes in mitochondrial morphology and connectivity that occur during differentiation. Mfn2 and OPA1, but not Mfn1, are transcriptionally upregulated during differentiation; however, knockdown of Mfn2 and OPA1 does not induce any change in expression of CD11c, CDC80, or CD86. Notably, knockdown of Mfn2 or OPA1 by siRNA in imDCs significantly reduces CCR7 expression and CCL19-mediated migration. These results suggest that the mitochondrial fusion-related proteins Mfn2 and OPA1 are upregulated during bone marrow progenitor differentiation and promote the migration of imDCs by regulating the expression of CCR7.

Role of the P2Y13 Receptor in the Differentiation of Bone Marrow Stromal Cells into Osteoblasts and Adipocytes

Accumulating evidence indicates that extracellular nucleotides, signaling through purinergic receptors, play a significant role in bone remodeling. Mesenchymal stem cells (MSCs) express functional P2Y receptors whose expression level is regulated during osteoblast or adipocyte differentiation. P2Y13 -deficient mice were previously shown to exhibit a decreased bone turnover associated with a reduction in the number of both osteoblasts and osteoclasts on the bone surfaces. We therefore examined whether P2Y13 R activation was involved in the osteogenic differentiation of MSC. Our study demonstrated that ADP stimulation of P2Y13 R(+/+) (but not P2Y13 R(-/-) ) adherent bone marrow stromal cells (BMSCs) increased significantly the formation of alkaline phosphatase-colony-forming units (CFU-ALP) as well as the expression of osteoblastic markers (osterix, alkaline phosphatase, and collagen I) involved in the maturation of preosteoblasts into osteoblasts. The number of CFU-ALP obtained from P2Y13 R(-/-) BMSC and the level of osteoblastic gene expression after osteogenic stimulation were strongly reduced compared to those obtained in wild-type cell cultures. In contrast, when P2Y13 R(-/-) BMSCs were incubated in an adipogenic medium, the number of adipocytes generated and the level of adipogenic gene expression (PPARγ2 and Adipsin) were higher than those obtained in P2Y13 R(+/+) MSC. Interestingly, we observed a significant increase of the number of bone marrow adipocytes in tibia of P2Y13 R(-/-) mice. In conclusion, our findings indicate that the P2Y13 R plays an important role in the balance of osteoblast and adipocyte terminal differentiation of bone marrow progenitors. Therefore, the P2Y13 receptor can be considered as a new pharmacological target for the treatment of bone diseases like osteoporosis. STEM Cells 2013;31:2747-2758.

The PI3K Inhibitor GDC-0941 Attenuates Disease in a KrasG12D Mouse Model of CMML and JMML.

AbstractAbstract 2862▪▪This icon denotes a clinically relevant abstractChronic and juvenile myelomonocytic leukemias (CMML and JMML) are overlap myelodysplastic/myeloproliferative neoplasia (MDS/MPN) syndromes that respond poorly to conventional treatment regimens. Both diseases are characterized by aberrant N-Ras, K-Ras, Cbl, and SHP-2 proteins, which are not attractive drug target candidates. Focus has shifted to downstream effector pathways, which include Raf/MEK/ERK, phosphoinositide-3-OH kinase (PI3K)/Akt, and Ral-GDS/Ral-A cascades. However, it is unclear which pathways, if any, should be targeted. In part to address this question, we previously developed a mouse model of CMML and JMML by expressing a conditional “knock-in” KrasG12D oncogene in bone marrow. Our earlier studies showed that inhibition of MEK yields a significant reduction in disease burden in this model, including reduced leukocytosis, improved anemia and enhanced survival. Here, we interrogate the role of the PI3K/Akt pathway in KrasG12D driven MPN and further explore which specific pathways are responsible for leukemogenesis due to hyperactive Ras. We administered GDC-0941, a selective PI3K inhibitor, to Mx1-Cre, KrasG12D mutant mice Mice with well established MPN and wild-type (WT) littermates were randomly chosen to receive daily oral administration of GDC-0941 or a control vehicle. Treated mice exhibited dramatic corrections of leukocytosis and anemia as well as decrease in splenomegaly. Flow cytometry of bone marrow and peripheral populations also imply that GDC-0941 treatment corrects the aberrant proliferation, amends differentiation of bone marrow progenitors, and revives ineffective erythropoiesis found in KrasG12D mice. Treatment also resulted in markedly improved survival of KrasG12D mice; virtually all KrasG12Dmice in the treatment arm outlived their control counterparts. Our data suggest PI3K inhibition may play a role in suppressing hematologic dysfunction in JMML and CMML patients. Potential crosstalk between PI3K and MEK signaling further suggest that combinatorial activities of PI3K and MEK inhibition should be investigated. [Display omitted] Disclosures:Friedman:Genentech, Inc.: Employment. Sampath:Genentech, Inc.: Employment.

VEGF Induces Neuroglial Differentiation in Bone Marrow-Derived Stem Cells and Promotes Microglia Conversion Following Mobilization with GM-CSF

Evaluation of potential tropic effects of vascular endothelial growth factor (VEGF) on the incorporation and differentiation of bone-marrow-derived stem cells (BMSCs) in a murine model of anterior ischemic optic neuropathy (AION). In the first approach, small-sized subset of BMCs were isolated from GFP donors mice by counterflow centrifugal elutriation and depleted of hematopoietic lineages (Fr25lin(-)). These cells were injected into a peripheral vein (1 × 10(6) in 0.2 ml) or inoculated intravitreally (2 × 10(5)) to syngeneic mice, with or without intravitreal injection of 5 μg/2μL VEGF, simultaneously with AION induction. In a second approach, hematopoietic cells were substituted by myelablative transplant of syngeseic GFP + bone marrow cells. After 3 months, progenitors were mobilized with granulocyte-macrophage colony-stimulating factor (GM-CSF) followed by VEGF inoculation into the vitreous body and AION induction . Engraftment and phenotype were examined by immunohistochemistry and FISH at 4 and 24 weeks post-transplantation, and VEGF receptors were determined by real time PCR. VEGF had no quantitative effect on incorporation of elutriated cells in the injured retina, yet it induced early expression of neuroal markers in cells incorporated in the RGC layer and promoted durable gliosis, most prominent perivascular astrocytes. These effects were mediated by VEGF-R1/Flt-1, which is constitutively expresses in the elutriated fraction of stem cells. Mobilization with GM-CSF limited the differentiation of bone marrow progenitors to microglia, which was also fostered by VEGF. VEGF signaling mediated by Flt-1 induces early neural and sustained astrocytic differentiation of stem cells elutriated from adult bone-marrow, with significant contribution to stabilization retinal architecture following ischemic injury.

Macrophage Activation and Differentiation Signals Regulate Schlafen-4 Gene Expression: Evidence for Schlafen-4 as a Modulator of Myelopoiesis

BackgroundThe ten mouse and six human members of the Schlafen (Slfn) gene family all contain an AAA domain. Little is known of their function, but previous studies suggest roles in immune cell development. In this report, we assessed Slfn regulation and function in macrophages, which are key cellular regulators of innate immunity.Methodology/Principal FindingsMultiple members of the Slfn family were up-regulated in mouse bone marrow-derived macrophages (BMM) by the Toll-like Receptor (TLR)4 agonist lipopolysaccharide (LPS), the TLR3 agonist Poly(I∶C), and in disease-affected joints in the collagen-induced model of rheumatoid arthritis. Of these, the most inducible was Slfn4. TLR agonists that signal exclusively through the MyD88 adaptor protein had more modest effects on Slfn4 mRNA levels, thus implicating MyD88-independent signalling and autocrine interferon (IFN)-β in inducible expression. This was supported by the substantial reduction in basal and LPS-induced Slfn4 mRNA expression in IFNAR-1−/− BMM. LPS causes growth arrest in macrophages, and other Slfn family genes have been implicated in growth control. Slfn4 mRNA levels were repressed during macrophage colony-stimulating factor (CSF-1)-mediated differentiation of bone marrow progenitors into BMM. To determine the role of Slfn4 in vivo, we over-expressed the gene specifically in macrophages in mice using a csf1r promoter-driven binary expression system. Transgenic over-expression of Slfn4 in myeloid cells did not alter macrophage colony formation or proliferation in vitro. Monocyte numbers, as well as inflammatory macrophages recruited to the peritoneal cavity, were reduced in transgenic mice that specifically over-expressed Slfn4, while macrophage numbers and hematopoietic activity were increased in the livers and spleens.Conclusions Slfn4 mRNA levels were up-regulated during macrophage activation but down-regulated during differentiation. Constitutive Slfn4 expression in the myeloid lineage in vivo perturbs myelopoiesis. We hypothesise that the down-regulation of Slfn4 gene expression during macrophage differentiation is a necessary step in development of this lineage.

Effects of carboxymethylcellulose and hydroxypropylmethylcellulose on the differentiation and activity of osteoclasts and osteoblasts

Carboxymethylcellulose (CMC) and hydroxypropylmethylcellulose (HPMC) serve as carriers for growth factors and bone substitutes. Although both carriers are placed into the defect sites, their impacts on bone regeneration are unclear. Herein, we examined whether CMC and HPMC affect the differentiation of bone marrow progenitors into osteoclasts and osteoblasts. We therefore induced osteoclastogenesis and osteoblastogenesis in murine bone marrow progenitors in the presence of CMC and HPMC, respectively. Measures of osteoclastogesis were based on the number and activity of tartrate-resistant acid-phosphatase-positive (TRAP(+)) multinucleated cells and expression of marker genes. Osteoblastogenesis was determined by the number and activity of alkaline-phosphatase-positive (AP(+)) colonies and relevant marker genes. Viability was assessed by colorimetric measurement of formazan formation. We report that CMC at 1% caused a significant reduction in the number and activity of TRAP(+) multinucleated cells. Changes in viability were not responsible for the observed effects. HPMC showed no remarkable impact on osteoclastogenesis; however, the concentration was limited to 0.5% because of the high viscosity. The ability of bone marrow progenitors to form AP(+) colonies was not affected by either of the two carriers. Together, these results suggest that CMC and possibly also HPMC can decrease osteoclastogenesis while osteoblastogenesis remains unchanged in vitro. These observations raise the possibility that these carriers might affect the cellular process of bone regeneration.

Toll‐like Receptors (TLRs) are expressed by myeloid leukaemia cell lines, but fail to trigger differentiation in response to the respective TLR ligands

Toll‐like Receptors (TLRs) are expressed by myeloid leukaemia cell lines, but fail to trigger differentiation in response to the respective TLR ligands

Lentiviral tracking of vascular differentiation in bone marrow progenitor cells

Lentiviral vectors encoding for identifiable marker genes controlled by lineage-specific promoters can be used to track differentiation of bone marrow progenitors into endothelial cells and/or smooth muscle cells. Human VE-Cadherin and Smoothelin-B promoters were cloned into a self-inactivating lentiviral vector (HR-VECad and HR-SMTHB) and used to drive expression of green fluorescent protein (eGFP). These constructs demonstrated specific promoter activity in mature endothelial and smooth muscle cells respectively in vitro. Lin − bone marrow progenitor cells (Lin − BMCs) in culture were used to test vector ability to track vascular differentiation. HR-VECad transduced Lin − BMCs were plated on collagen-coated slides and grown in endothelial media, while HR-SMTHB transduced Lin − BMCs were cultured on fibronectin-coated slides and grown in smooth muscle media. For in vivo differentiation assessment, lentiviral transduced Lin − BMCs resuspended in Matrigel were injected subcutaneously into C57BL/6J mice. Explants were evaluated for eGFP expression. Lin − BMCs grown in endothelial differentiation media produced groups of polygonal endothelial-like cells by days 16–21. When transduced with HR-VECad vector, these expressed eGFP in distinct cells within the colony by days 18–21, and coexpressed VE-Cadherin and eNOS. Lin − BMCs grown in smooth muscle differentiation media produced spindle-shaped cells between days 10–14 in culture. When transduced with the HR-SMTHB vector, these cells showed eGFP expression at ∼12 days, which increased over time and coexpressed αSMA, calponin and myosin heavy chain. Within Matrigel plugs containing HR-VECad transduced cells, eGFP + constituted 0.4±0.2% of total cells. In contrast, within Matrigel plugs containing HR-SMTHB transduced cells, eGFP + cells constituted 0.2±0.1% of total cells. These data demonstrate the feasibility of selectively marking BMC populations for cell fate determination.

Activation of peroxisome proliferator‐activated receptor gamma suppresses mast cell maturation involved in allergic diseases

Mast cells play a central role in allergic and inflammatory diseases. Several reports indicated role of peroxisome proliferator-activated receptor gamma (PPARgamma) on mast cell function. However, there is no report about the role of PPARgamma on differentiation of mast cells from the progenitors. In this study, we investigated the role of PPARgamma in regulating bone marrow-derived mast cell maturation and the therapeutic implications for mast cell-related diseases such as atopic or contact dermatitis. We used in vitro cell culture system for mast cell differentiation from bone marrow-progenitors using specific ligands and lentiviral-mediated short hairpin RNA of PPARgamma, and in vivo murine dermatitis models. Activation of PPARgamma inhibited the maturation of bone marrow progenitors into connective tissue-type mast cells (CTMCs) through up-regulation of GATA-4 and GATA-6 resulting in a decrease in expression of histidine decarboxylase and mast cell histamine content. In comparison, the differentiation of bone marrow progenitors into CTMCs was significantly accelerated by the knockdown of PPARgamma expression by lentiviral-mediated short hairpin RNA. Peroxisome proliferator-activated receptor gamma ligand administration to mice inhibited the maturation of mast cells resulting in attenuation of atopic and contact dermatitis via diminishment of the number of mature mast cells. Our results indicate that PPARgamma is one of master regulators on mast cell maturation and potentially useful for the therapy in various disorders involving mast cell activation.

Immunosuppressant Sinomenine Promotes Differentiation of Bone Marrow Progenitors to IL-10-Producing Ialow Regulatory Dendritic Cells Leading to the Prolongation of Allograft Survival.

Sinomenine (SN), an immunosuppressive compound derived from the Chinese medicinal plant Sinomenium acutum, effectively inhibits both proinflammatory factor production and lymphocyte proliferation. However, the effects of SN on dendritic cells (DC), the professional antigen-presenting cells, have not been elucidated. In this study, we examined the effects of SN on DC generation, maturation and function. We found that SN affected DC in a dose dependent manner; significantly inhibiting surface expression of MHC class II, CD86, and CD40, production of IL-12, TNF-a, IL-1b, and T cell-allostimulatory activity. SN targets antigen-presenting capacity of DC not only via the MHC class II pathway, but also impairs DC migration by inhibiting LPS-induced up-regulation of CCR7 and CXCR4 expression. These inhibitory effects on DC may be partially due to SN-mediated suppression of NF-kB and p38 MAPK pathways. Further study showed that SN-treated DC (DC-SN) induce donor-specific T cell hyporesponsiveness and trigger generation of IL-10-producing T regulatory-like cells in vitro. In vivo transplant studies revealed that, compared to pretreatment with immature DC, pretreatment of recipients with DC-SN could significantly prolong allograft survival, although long-term allograft acceptance was not achieved. Importantly, pretreatment of recipients with DC-SN in combination with SN administration profoundly prolonged allograft survival, resulting in long-term survival of 30% of allografts. Increased generation of CD4+CD25+ Treg cells and enhanced microchimerism in recipients brought on by combination treatment may contribute to this extension of allograft lifespan. Our results demonstrate that SN can promote bone marrow progenitors differentiation to IL-10-producing (Ialow) regulatory DC which contribute to induction of tolerance by increase generation of Treg cells.

Mesenchymal Stem Cells Inhibit the Differentiation of Dendritic Cells Through an Interleukin-6-Dependent Mechanism

Mesenchymal stem cells (MSC) are of particular interest for their potential clinical use in tissue engineering as well as for their capacity to reduce the incidence and severity of graft-versus-host disease in allogeneic transplantation. We have previously shown that MSC-mediated immune suppression acts via the secretion of soluble factor(s) induced upon stimulation. The aim of this study was to identify the molecule(s) involved and the underlying mechanism(s). We show that murine MSC secrete high levels of interleukin (IL)-6 and vascular endothelial growth factor, which are directly correlated to the inhibition of T-cell proliferation. The T-cell activation is partially restored upon addition of a neutralizing anti-IL-6 antibody or the prostaglandin E2 inhibitor indomethacin. Interestingly, no indoleamine 2,3-dioxygenase activity was detected in our conditions. Instead, we show that MSC reduce the expression of major histocompatibility complex class II, CD40, and CD86 costimulatory molecules on mature dendritic cells (DC), which was responsible for a decrease in T-cell proliferation. Moreover, we show that the differentiation of bone marrow progenitors into DC cultured with conditioned supernatants from MSC was partly inhibited through the secretion of IL-6. Altogether, these data suggest that IL-6 is involved in the immunoregulatory mechanism mediated by MSC through a partial inhibition of DC differentiation but is probably not the main mechanism. Disclosure of potential conflicts of interest is found at the end of this article.