Yingyu Zhang

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.

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.

Identification of G-protein-coupled receptor 120 as a tumor-promoting receptor that induces angiogenesis and migration in human colorectal carcinoma

G-protein-coupled receptor 120 (GPR120) functions as a receptor for unsaturated long-chain free fatty acids and has an important role in regulating lipid and glucose metabolism. However, a role for GPR120 in the development of tumors has not yet been clarified. Here, we show that GPR120 signaling promotes angiogenic switching and motility of human colorectal carcinoma (CRC) cells. We show that the expression of GPR120 is significantly induced in CRC tissues and cell lines, which is associated with tumor progression. Activation of GPR120 signaling in human CRC promotes angiogenesis in vitro and in vivo, largely by inducing the expression and secretion of proangiogenic mediators such as vascular endothelial growth factor (VEGF), interleukin-8 and cyclooxygenase-2-derived prostaglandin E2. The PI3K/Akt–NF-κB pathway is activated by GPR120 signaling and is required for GPR120 signaling-induced angiogenic switching in CRC cells. And, GPR120 activation enhances the motility of CRC cells and induces epithelial–mesenchymal transition. Furthermore, in vivo study shows that activation of GPR120 promotes angiogenesis and tumor growth. Finally, we find that GPR120 expression is positively correlated with VEGF expression and inversely correlated with the epithelial marker E-cadherin in CRC tissues. Collectively, our results demonstrate that GPR120 functions as a tumor-promoting receptor in CRC and, therefore, shows promise as a new potential target for cancer therapeutics.

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.

Immune activation induces immortalization of HTLV-1 LTR-Tax transgenic CD4+ T cells

Infection with the human T-cell leukemia virus-1 (HTLV-1) results in a variety of diseases including adult T-cell leukemia/lymphoma (ATL). Although the pathogenesis of these disorders is poorly understood, it involves complex interactions with the host immune system. Activation of infected T cells may play an important role in disease pathogenesis through induction of the oncogenic HTLV-1 Tax transactivator protein. To test this hypothesis, we employed transgenic mice in which Tax is regulated by the HTLV-1 LTR. T-cell receptor stimulation of LTR-Tax CD4(+) T cells induced Tax expression, hyper-proliferation, and immortalization in culture. The transition to cellular immortalization was accompanied by markedly increased expression of the antiapoptotic gene, mcl-1, previously implicated as important in T-cell survival. Immortalized cells exhibited a CD4(+)CD25(+)CD3(-) phenotype commonly observed in ATL. Engraftment of activated LTR-Tax CD4(+) T cells into NOD/Shi-scid/IL-2Rγ null mice resulted in a leukemia-like phenotype with expansion and tissue infiltration of Tax(+), CD4(+) lymphocytes. We suggest that immune activation of infected CD4(+) T cells plays an important role in the induction of Tax expression, T-cell proliferation, and pathogenesis of ATL in HTLV-1-infected individuals.

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

Bone marrow stromal cells induce apoptosis of lymphoma cells in the presence of IFNγ and TNF by producing nitric oxide

Bone marrow stromal cells (BMSCs) have been shown to promote the growth and survival of a wide variety of tumors. However, in the present study, we found that BMSCs induced apoptosis of lymphoma cells in the presence of INFgamma and TNF. IFNgamma and TNF dramatically induced the expression of inducible nitric oxide synthase (iNOS) by BMSCs in culture, and BMSCs generated from iNOS knockout mice did not induce apoptosis of lymphoma cells in the presence of IFNgamma and TNF. In addition, we found that IFNgamma and TNF also increased IL-6 expression by BMSCs, and anti-IL-6 further increased the killing of tumor cells by BMSCs. Taken together, our findings indicate that BMSCs induce apoptosis of lymphoma cells in the presence of IFNgamma and TNF, and that the proapoptotic effect of BMSCs is mediated by nitric oxide. Our findings suggest a possibility to harness this proapoptotic feature of BMSCs for the development of novel therapeutic strategy to eliminate tumor cells, especially tumor cells in bone marrow.

Activation of HTLV-1 expression in chronically-infected CD4+ T cells: mechanisms and implications for pathogenesis

Infection with the human T-cell leukemia virus-1 (HTLV-1) is associated with a range of outcomes ranging from asymptomatic infection, to the development of HAM/TSP and adult T cell leukemia/lymphoma. Pathogenesis of these disorders involves complex interactions with the host immune system. Our previous studies showed that phorbol ester (PMA) and T cell receptor (TCR)-mediated activation of chronically infected CD4 T cells increased expression of HTLV-1 gene products [1]. We hypothesized that immune activation of infected T cells may play an important role in disease pathogenesis through induction of Tax expression resulting in increased survival and proliferation. To test this, we employed transgenic mice in which Tax is regulated by the HTLV-1 LTR. These mice develop neurofibromas, but do not express Tax in T cells and do not develop lymphomas [2]. TCR stimulation of transgenic LTR-Tax CD4+ T cells induced Tax expression, early hyper-proliferation, and long-term growth in culture. Survival of these cells was accompanied by transiently increased expression of mcl-1. Long-term surviving cells exhibited a CD4+CD25+CD3- phenotype commonly observed in ATL. Engraftment of immune-activated LTR-Tax CD4+ T-cells into NOD/Shi-scid/IL-2Rγ null mice resulted in a leukemia-like phenotype. Immune activated Tax CD4+ T cells express characteristics of several different CD4+ T cell subtypes, suggesting that HTLV-1 Tax induces changes in the normal pattern of CD4+ subtype specification. We also investigated mechanisms of HTLV-1 activation following PMA stimulation. Surprisingly, PMA treatment was associated with a rapid, marked stabilization of HTLV-1 tax/rex mRNA. Increased RNA stability represents a novel mechanism for increasing HTLV-1 gene expression in chronically infected cells.