Guangwu Xu

Mesenchymal Stem Cell-Mediated Immunosuppression Occurs via Concerted Action of Chemokines and Nitric Oxide

Mesenchymal stem cells (MSCs) can become potently immunosuppressive through unknown mechanisms. We found that the immunosuppressive function of MSCs is elicited by IFNgamma and the concomitant presence of any of three other proinflammatory cytokines, TNFalpha, IL-1alpha, or IL-1beta. These cytokine combinations provoke the expression of high levels of several chemokines and inducible nitric oxide synthase (iNOS) by MSCs. Chemokines drive T cell migration into proximity with MSCs, where T cell responsiveness is suppressed by nitric oxide (NO). This cytokine-induced immunosuppression was absent in MSCs derived from iNOS(-/-) or IFNgammaR1(-/-) mice. Blockade of chemokine receptors also abolished the immunosuppression. Administration of wild-type MSCs, but not IFNgammaR1(-/-) or iNOS(-/-) MSCs, prevented graft-versus-host disease in mice, an effect reversed by anti-IFNgamma or iNOS inhibitors. Wild-type MSCs also inhibited delayed-type hypersensitivity, while iNOS(-/-) MSCs aggravated it. Therefore, proinflammatory cytokines are required to induce immunosuppression by MSCs through the concerted action of chemokines and NO.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and T-cell responses: what we do and don't know

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is an important hematopoietic growth factor and immune modulator. GM-CSF also has profound effects on the functional activities of various circulating leukocytes. It is produced by a variety of cell types including T cells, macrophages, endothelial cells and fibroblasts upon receiving immune stimuli. Although GM-CSF is produced locally, it can act in a paracrine fashion to recruit circulating neutrophils, monocytes and lymphocytes to enhance their functions in host defense. Recent intensive investigations are centered on the application of GM-CSF as an immune adjuvant for its ability to increase dendritic cell (DC) maturation and function as well as macrophage activity. It is used clinically to treat neutropenia in cancer patients undergoing chemotherapy, in AIDS patients during therapy, and in patients after bone marrow transplantation. Interestingly, the hematopoietic system of GM-CSF-deficient mice appears to be normal; the most significant changes are in some specific T cell responses. Although molecular cloning of GM-CSF was carried out using cDNA library of T cells and it is well known that the T cells produce GM-CSF after activation, there is a lack of systematic investigation of this cytokine in production by T cells and its effect on T cell function. In this article, we will focus mainly on the immunobiology of GM-CSF in T cells.

Apoptosis signaling pathways and lymphocyte homeostasis.

It has been almost three decades since the term “apoptosis” was first coined to describe a unique form of cell death that involves orderly, gene-dependent cell disintegration. It is now well accepted that apoptosis is an essential life process for metazoan animals and is critical for the formation and function of tissues and organs. In the adult mammalian body, apoptosis is especially important for proper functioning of the immune system. In recent years, along with the rapid advancement of molecular and cellular biology, great progress has been made in understanding the mechanisms leading to apoptosis. It is generally accepted that there are two major pathways of apoptotic cell death induction: extrinsic signaling through death receptors that leads to the formation of the death-inducing signaling complex (DISC), and intrinsic signaling mainly through mitochondria which leads to the formation of the apoptosome. Formation of the DISC or apoptosome, respectively, activates initiator and common effector caspases that execute the apoptosis process. In the immune system, both pathways operate; however, it is not known whether they are sufficient to maintain lymphocyte homeostasis. Recently, new apoptotic mechanisms including caspase-independent pathways and granzyme-initiated pathways have been shown to exist in lymphocytes. This review will summarize our understanding of the mechanisms that control the homeostasis of various lymphocyte populations.

Immunosuppressive properties of cloned bone marrow mesenchymal stem cells

Mesenchymal stem cells (MSCs), derived from adult tissues, are multipotent progenitor cells, which hold great promise for regenerative medicine. Recent studies have shown that MSCs are immunosuppressive in vivo and in vitro in both animals and humans. However, the mechanisms that govern these immune modulatory functions of MSCs remain largely elusive. Some studies with bulk populations of MSCs indicated that soluble factors such as PGE2 and TGFβ are important, while others support a role for cell-cell contact. In this study, we intended to clarify these issues by examining immunosuppressive effects of cloned MSCs. We derived MSC clones from mouse bone marrow and showed that the majority of these clones were able to differentiate into adipocytes and osteoblast-like cells. Importantly, cells from these clones exhibited strong inhibitory effects on TCR activation-induced T cell proliferation in vitro, and injection of a small number of these cells promoted the survival of allogeneic skin grafts in mice. Conditioned medium from MSC cultures showed some inhibitory effect on anti-CD3 induced lymphocyte proliferation independent of PGE2 and TGFβ. In comparison, direct co-culture of MSCs with stimulated lymphocytes resulted in much stronger immunosuppressive effect. Interestingly, the suppression was bi-directional, as MSC proliferation was also reduced in the presence of lymphocytes. Taking together, our findings with cloned MSCs demonstrate that these cells exert their immunosuppressive effects through both soluble factor(s) and cell-cell contact, and that lymphocytes and MSCs are mutually inhibitory on their respective proliferation.

CD11b facilitates the development of peripheral tolerance by suppressing Th17 differentiation

Antigen-induced immune suppression, like T cell activation, requires antigen-presenting cells (APCs); however, the role of APCs in mediating these opposing effects is not well understood, especially in vivo. We report that genetic inactivation of CD11b, which is a CD18 subfamily of integrin receptors that is highly expressed on APCs, abolishes orally induced peripheral immune tolerance (oral tolerance) without compromising APC maturation or antigen-specific immune activation. The defective oral tolerance in CD11b−/− mice can be restored by adoptive transfer of wild-type APCs. CD11b deficiency leads to enhanced interleukin (IL) 6 production by APCs, which subsequently promotes preferential differentiation of naive T cells to T helper 17 (Th17) cells, which are a T cell lineage characterized by their production of IL-17. Consequently, antigen feeding and immunization of CD11b−/− mice results in significant production of IL-17 within the draining lymph nodes that interferes with the establishment of oral tolerance. Together, we conclude that CD11b facilitates oral tolerance by suppressing Th17 immune differentiation.

Interleukin-17 enhances immunosuppression by mesenchymal stem cells

IL-17 is one of the most potent and most actively investigated proinflammatory cytokines. In this study, we examined the effect of IL-17 on mesenchymal stem cells (MSCs) under the influence of inflammatory cytokines. Ironically, IL-17 dramatically enhanced the immunosuppressive effect of MSCs induced by IFNγ and TNFα, revealing a novel role of IL-17 in immunosuppression. Interestingly, we found that this action of IL-17 was dependent on the promoted expression of a key immune suppressive molecule, inducible nitric oxide synthase (iNOS), in MSCs. In a concanavalin A (ConA)-induced hepatitis mouse model, we found that IL-17 also enhanced the in vivo immunosuppressive effect of MSCs in an iNOS-dependent manner. Moreover, this promoting effect of IL-17 was found to be exerted through enhancing mRNA stability by modulating the protein level of ARE/poly(U)-binding/degradation factor 1 (AUF1), a well-known factor that promotes mRNA decay. In auf1−/− MSCs, IFNγ and TNFα could induce maximal immunosuppressive effect, both in vitro and in vivo, without the need for IL-17. Thus, our studies demonstrated that in the presence of MSCs, IL-17 promotes immunosuppression.

Th17 Cells Undergo Fas-Mediated Activation-Induced Cell Death Independent of IFN-γ

IL-17-secreting CD4+ T cells (Th17 cells) play a critical role in immune responses to certain infections and in the development of many autoimmune disorders. The mechanisms controlling homeostasis in this cell population are largely unknown. In this study, we show that murine Th17 cells undergo rapid apoptosis in vitro upon restimulation through the TCR. This activation-induced cell death (AICD), a common mechanism for elimination of activated T cells, required the Fas and FasL interaction: Fas was stably expressed, while FasL was up-regulated upon TCR reactivation of Th17 cells; Ab ligation of Fas induced Th17 cell death; and AICD was completely absent in Th17 cells differentiated from gld/gld CD4+ T cells. Thus, the Fas/FasL pathway is essential in regulating the AICD of Th17 cells. Interestingly, IFN-γ, a cytokine previously found to be important for the AICD of T cells, did not affect Th17 cell apoptosis. Furthermore, Th17 cells derived from mice deficient in IFN-γ receptor 1 (IFN-γR1−/−) underwent AICD similar to wild-type cells. Thus, AICD of Th17 cells occurs via the Fas pathway, but is independent of IFN-γ.

Combinatorial mRNA binding by AUF1 and Argonaute 2 controls decay of selected target mRNAs

The RNA-binding protein AUF1 binds AU-rich elements in 3′-untranslated regions to regulate mRNA degradation and/or translation. Many of these mRNAs are predicted microRNA targets as well. An emerging theme in post-transcriptional control of gene expression is that RNA-binding proteins and microRNAs co-regulate mRNAs. Recent experiments and bioinformatic analyses suggest this type of co-regulation may be widespread across the transcriptome. Here, we identified mRNA targets of AUF1 from a complex pool of cellular mRNAs and examined a subset of these mRNAs to explore the links between RNA binding and mRNA degradation for both AUF1 and Argonaute 2 (AGO2), which is an essential effector of microRNA-induced gene silencing. Depending on the specific mRNA examined, AUF1 and AGO2 binding is proportional/cooperative, reciprocal/competitive or independent. For most mRNAs in which AUF1 affects their decay rates, mRNA degradation requires AGO2. Thus, AUF1 and AGO2 present mRNA-specific allosteric binding relationships for co-regulation of mRNA degradation.

C/EBPβ Mediates Synergistic Upregulation of Gene Expression by Interferon‐γ and Tumor Necrosis Factor‐α in Bone Marrow‐Derived Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) are potent immunoregulators and have shown clinical utility in suppressing immunity. MSC function is modulated by cytokines, since inflammatory cytokines, such as interferon‐γ (IFNγ) concomitant with tumor necrosis factor‐α (TNFα), induce their immunoregulatory capability. Here, we show that IFNγ and TNFα act synergistically to induce high levels of expression of interleukin‐6 (IL‐6) and several other immune‐related molecules in MSCs in vitro. We further found that, while either IFNγ or TNFα alone induced minor expression of C/EBPβ in MSCs, this transcription factor was dramatically upregulated when these cytokines were added together. A causal relationship between C/EBPβ upregulation and IL‐6 expression was demonstrated by small interfering RNA knockdown of C/EBPβ. C/EBPβ knockdown also inhibited the synergistic expression of CXCL1, inducible nitric oxide synthase, and CCL5 in response to concomitant IFNγ and TNFα. We conclude that C/EBPβ is a key transcription factor in synergistic gene upregulation by IFNγ and TNFα. Importantly, C/EBPβ similarly mediated synergistic gene induction in response to IFNγ accompanied by IL‐1β or lipopolysaccharide, suggesting that synergy between IFNγ and other stimuli share C/EBPβ as common mechanism. Furthermore, while STAT1 is critical in IFNγ signaling, we found that STAT1 knockdown in MSCs did not affect C/EBPβ expression or the synergistic induction of IL‐6 and CXCL1 by IFNγ and TNFα. Thus, C/EBPβ is not regulated by STAT1. These results demonstrate the importance of cytokine interactions in MSC immunobiology, a better understanding of which will allow improved clinical application of these cells. STEM CELLS 2009;27:942–948

A novel subset of helper T cells promotes immune responses by secreting GM-CSF.

Helper T cells are crucial for maintaining proper immune responses. Yet, they have an undefined relationship with one of the most potent immune stimulatory cytokines, granulocyte macrophage-colony-stimulating factor (GM-CSF). By depleting major cytokines during the differentiation of CD4+ T cells in vitro, we derived cells that were found to produce large amounts of GM-CSF, but little of the cytokines produced by other helper T subsets. By their secretion of GM-CSF, this novel subset of helper T cells (which we have termed ThGM cells) promoted the production of cytokines by other T-cell subtypes, including type 1 helper T cell (Th1), type 2 helper T cell (Th2), type 1 cytotoxic T cell (Tc1), type 2 cytotoxic T cell (Tc2), and naive T cells, as evidenced by the fact that antibody neutralization of GM-CSF abolished this effect. ThGM cells were found to be highly prone to activation-induced cell death (AICD). Inhibitors of TRAIL or granzymes could not block AICD in ThGM cells, whereas inhibition of FasL/Fas interaction partially rescued ThGM cells from AICD. Thus, ThGM cells are a novel subpopulation of T helper cells that produce abundant GM-CSF, exhibit exquisite susceptibility to apoptosis, and therefore play a pivotal role in the regulation of the early stages of immune responses.