IntroductionHuman bone marrow stromal cells (MSCs), more recently referred to as mesenchymal stem cells, are capable of differentiating along multiple mesenchymal lineages in addition to supporting hematopoiesis. 1,2 Due to their potential for differentiation into osteocytes, chondrocytes, myocytes, and adipocytes, MSCs have emerged as a promising tool for clinical applications such as tissue engineering and cell and gene therapy. 3,4 MSCs are of inherently low immunogenicity and, more importantly, are capable of inhibiting allogeneic T-cell responses. 5-8 These intriguing observations have prompted clinical studies to investigate cotransplantation of MSCs in allogeneic hematopoietic stem cell transplantation (HSCT) in order to promote hematopoietic engraftment by preventing host-versusgraft reactivity and to suppress graft-versus-host reactions. 9,10 As of yet, the molecular mechanisms responsible for the immunosuppressive effects of MSCs have not been unequivocally identified. The reports describing a potential role of transforming growth factor-␤1 and hepatocyte growth factor as mediators of T-cell inhibition remain controversial, but most studies agree that soluble factors are involved. [6][7][8]11 In professional antigen-presenting cells (APCs), expression of indoleamine 2,3-dioxygenase (IDO) induced by interferon-␥ (IFN-␥) and other proinflammatory cytokines catalyzes conversion from tryptophan to kynurenine and has recently been identified as a major immunosuppressive effector pathway that inhibits T-cell responses to autoantigens and fetal alloantigens in vivo. [12][13][14][15][16] Based on these findings, we investigated whether MSCs exhibit IFN-␥-inducible IDO activity and whether this mechanism contributes to T-cell inhibition mediated by MSCs. Study design Culture of human bone marrow-derived MSCsBone marrow aspirates were harvested from volunteer donors who had provided informed consent; the study was approved by the institutional review board of the University Clinic, Düsseldorf, Germany. Primary human MSCs were generated as previously described 17 except that culture medium was supplemented with 3 ng/mL basic fibroblast growth factor (R&D Systems, Minneapolis, MN). Mixed lymphocyte reactions (MLRs)Standard 5-day MLR cultures were set up with 5 ϫ 10 4 mitomycin C-treated human peripheral blood mononuclear cells (PBMCs) as stimulators and 2 ϫ 10 5 human T cells purified using sheep red blood cell rosetting as responder cells. 5,11 In MSC/MLR coculture experiments, MLRs were performed on a layer of either 5 ϫ 10 3 or 2 ϫ 10 4 MSCs seeded one day before. IFN-␥ concentration was determined in MSC/MLR coculture supernatants using a commercially available enzyme-linked immunosorbent assay (ELISA; R&D Systems) according to manufacturer's instructions. Detection of IDO expression and activityMSCs were stimulated with IFN-␥ (R&D Systems) and assayed for IDO expression and function. Standard Western blot analysis for IDO protein expression was performed. 18 IDO enzyme activity following IFN-␥ stimulation of ...

Despite recent advances in understanding microbial diversity in skin homeostasis, the relevance of microbial dysbiosis in inflammatory disease is poorly understood. Here we perform a comparative analysis of skin microbial communities coupled to global patterns of cutaneous gene expression in patients with atopic dermatitis or psoriasis. The skin microbiota is analysed by 16S amplicon or whole genome sequencing and the skin transcriptome by microarrays, followed by integration of the data layers. We find that atopic dermatitis and psoriasis can be classified by distinct microbes, which differ from healthy volunteers microbiome composition. Atopic dermatitis is dominated by a single microbe (Staphylococcus aureus), and associated with a disease relevant host transcriptomic signature enriched for skin barrier function, tryptophan metabolism and immune activation. In contrast, psoriasis is characterized by co-occurring communities of microbes with weak associations with disease related gene expression. Our work provides a basis for biomarker discovery and targeted therapies in skin dysbiosis.

Human multipotent mesenchymal stromal cells (MSCs) exhibit multilineage differentiation potential, support hematopoiesis, and inhibit proliferation and effector function of various immune cells. On the basis of these properties, MSC are currently under clinical investigation in a range of therapeutic applications including tissue repair and immune-mediated disorders such as graft-versus-host-disease refractory to pharmacological immunosuppression. Although initial clinical results appear promising, there are significant concerns that application of MSC might inadvertently suppress antimicrobial immunity with an increased risk of infection. We demonstrate here that on stimulation with inflammatory cytokines human MSC exhibit broad-spectrum antimicrobial effector function directed against a range of clinically relevant bacteria, protozoal parasites and viruses. Moreover, we identify the tryptophan catabolizing enzyme indoleamine 2,3-dioxygenase (IDO) as the underlying molecular mechanism. We furthermore delineate significant differences between human and murine MSC in that murine MSC fail to express IDO and inhibit bacterial growth. Conversely, only murine but not human MSC express inducible nitric oxide synthase on cytokine stimulation thus challenging the validity of murine in vivo models for the preclinical evaluation of human MSC. Collectively, our data identify human MSC as a cellular immunosuppressant that concurrently exhibits potent antimicrobial effector function thus encouraging their further evaluation in clinical trials.

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