Hepatoma cells inhibit the differentiation and maturation of dendritic cells and increase the production of regulatory T cells

Tumor cells may escape from the immune responses because of defective differentiation of dendritic cells (DC). Recent studies have found an increased number of regulatory T cells (Treg) in both peripheral blood and tissues from patients with hepatocellular carcinoma. In the present study, we used tumor culture supernatants (TSN) from hepatoma-derived cell lines to investigate whether TSN interfere with the differentiation of human monocyte-derived DC and/or their ability to increase Treg. The results showed that exposure to TSN significantly inhibited the differentiation of monocytes into DC with retained CD14 molecule and reduced expression of CD1a. These TSN-exposed immature DC also produced significant amount of immunosuppressive cytokine IL-10 and displayed an increased expression of co-stimulatory molecules. Upon stimulation with LPS, however, the TSN-exposed DC failed to undergo full maturation, with a blockage of the upregulation of co-stimulatory molecules on their surface and a switch to an IL-10(high)IL-12(low)TNF-alpha(low) phenotype. Moreover, exposure of DC to TSN selectively inhibited their capacity to stimulate the proliferation of allogeneic CD8(+) T cells, but promoted the generation of CD4(+)CD25(hi)Foxp3(+) Treg cells. These findings, together with previous clinical studies showing that CD4(+)CD25(hi) Treg cells are concentrated within hepatocellular carcinoma tissue, suggest that the local tumor microenvironment may favor the induction of Treg cells through inhibiting the differentiation and maturation of DC.     

Emodin inhibits the differentiation and maturation of dendritic cells and increases the production of regulatory T cells

The aim of this study was to characterize the effects of emodin on dendritic cells (DCs) and CD4?CD25? regulatory T cells (Tregs). Myeloid DCs were prepared from peripheral blood mononuclear cells of healthy human donors and treated with emodin at different concentrations. The phenotype and T cell stimulatory capacity of these DCs were analyzed. The expression ratios of CD80 and CD83 in DCs in the presence of emodin (100 μg/ml) were significantly decreased compared with that in DCs without emodin treatment (P<0.05). IL-12p70 production of DCs decreased significantly with emodin treatment (P<0.05). Furthermore, an approximately 2-fold decrease was observed in the ability of DCs pre-treated with emodin to induce T-lymphocyte proliferation. In addition, we found that emodin treatment increased the number of Tregs, which expressed lower levels of human leukocyte antigen (HLA-DR), glucocorticoid-induced tumor necrosis factor receptor (GITR), and cytotoxic T-lymphocyte associated antigen-4 (CTLA-4) as compared to cells without emodin treatment. Our results suggest that emodin inhibits the differentiation and maturation of DCs and induces Tregs, which may be helpful for the modulation of the immune rejection after liver transplantation.     

调节性DC与调节性T细胞的相互作用

近年来研究发现调节性树突状细胞(Dendritic cells,DC)能够下调免疫应答和介导外周免疫耐受,调节性DC诱导的免疫耐受与其未成熟或半成熟状态密切相关。大量研究表明调节性DC和调节性T细胞(Regulatory T cells,Treg)之间存在着复杂的双向调控:调节性DC可扩增和诱导产生Treg,还可影响Treg向局部组织和外周淋巴器官归巢;Treg则妨碍DC与非调节性T细胞的结合并抑制DC的活化、成熟和刺激T细胞增殖的能力。总之,调节性DC与Treg相互协同以精细调控机体的免疫应答。     

Origin and differentiation of dendritic cells

Despite extensive, recent research on the development of dendritic cells (DCs), their origin is a controversial issue in immunology, with important implications regarding their use in cancer immunotherapy. Although, under defined experimental conditions, DCs can be generated from myeloid or lymphoid precursors, the differentiation pathways that generate DCs in vivo remain unknown largely. Indeed, experimental results suggest that the in vivo differentiation of a particular DC subpopulation could be unrelated to its possible experimental generation. Nevertheless, the analysis of DC differentiation by in vivo and in vitro experimental systems could provide important insights into the control of the physiological development of DCs and constitutes the basis of a model of common DC differentiation that we propose.     

Characterization of dendritic cells that induce tolerance and T regulatory 1 cell differentiation in vivo

Active suppression is mediated by a subpopulation of CD4(+) T cells that prevents autoimmunity. However, the mechanisms involved in their differentiation in vivo are currently under intensive research. Here we show that in vitro culture of bone marrow cells in the presence of IL-10 induces the differentiation of a distinct subset of dendritic cells with a specific expression of CD45RB. These CD11c(low)CD45RB(high) DCs are present in the spleen and lymph nodes of normal mice and are significantly enriched in the spleen of IL-10 Tg mice. These natural or in vitro-derived DCs display plasmacytoid morphology and an immature-like phenotype, and secrete high levels of IL-10 after activation. OVA peptide-pulsed CD11c(low)CD45RB(high) DCs specifically induce tolerance through the differentiation of Tr1 cells in vitro and in vivo. Our findings identify a natural DC subset that induces the differentiation of Tr1 cells and suggest their therapeutic use.     

Splenic stroma drives mature dendritic cells to differentiate into regulatory dendritic cells

The fates of dendritic cells (DCs) after antigen presentation have been studied extensively, but the influence of lymphoid microenvironments on DCs is mostly unknown. Here, using splenic stromal cells to mimic the immune microenvironment, we show that contact with stromal cells promoted mature DCs to proliferate in a fibronectin-dependent way and that both stromal cell contact and stromal cell-derived transforming growth factor-beta induced their differentiation into a new regulatory DC subset. We have identified an in vivo counterpart in the spleen with similar phenotype and functions. These differentiated DCs secreted nitric oxide, which mediated the suppression of T cell proliferation in response to antigen presentation by mature DCs. Thus, our findings identify an important mechanism by which the microenvironment regulates immune responses.     

Trypanosoma cruzi induces regulatory dendritic cells in vitro

A main feature of acute infection with Trypanosoma cruzi is the presence of immunological disorders. A previous study demonstrated that acute infection with the virulent RA strain downregulates the expression of major histocompatibility complex class II (MHC-II) on antigen-presenting cells and impairs the T-cell stimulatory capacity of splenic dendritic cells (DC). In the present work, we assessed the ability of trypomastigotes (Tp) to modulate the differentiation stage and functionality of bone marrow-derived DC in vitro. We observed that the Tp stage of T. cruzi failed to activate DC, which preserved their low expression of MHC-II and costimulatory molecules, as well as their endocytic activity. We also show that Tp induced transforming growth factor beta (TGF-beta) secretion by DC and enhanced the gap between interleukin-10 (IL-10) and IL-12p70 production, showing a higher IL-10/IL-12p70 ratio upon lipopolysaccharide (LPS) treatment. In addition, we observed that Tp prevented DC full activation induced by LPS, thereby downregulating their MHC-II surface expression and inhibiting their capacity to stimulate lymphocyte proliferation. In vitro IL-10 neutralization during the differentiation process of DC with Tp+LPS showed a reversion of their inhibitory effect during mixed lymphocyte reaction. In contrast, only simultaneous neutralization of IL-10 and TGF-beta, after DC differentiation, was involved in the partial restitution of lymphocyte proliferation. Since both TGF-beta and IL-10 are immunosuppressive cytokines essential in the modulation of the immune response and important in the induction of tolerance, our results suggest for the first time that Tp are responsible for the generation of regulatory DC in vitro.     

Allogeneic apoptotic thymocyte-stimulated dendritic cells expand functional regulatory T cells

Dendritic cells (DCs) play an important role in the clearance of apoptotic cells. The removal of apoptotic cells leads to peripheral tolerance, although their role is still not clear. We show that the uptake of apoptotic thymocytes by DCs converts these cells into tolerogenic DCs resistant to maturation by lipopolysaccharide, modulating the production of interleukin-12 and up-regulating the expression of transforming growth factor-β(1) latency associated peptide. We also observed that DCs pulsed with apoptotic cells in the allogeneic context were more efficient in the expansion of regulatory T cells (Tregs), and that this expansion requires contact between DCs and the T cell. The Tregs sorted from in vitro culture suppressed the proliferation of splenocytes in vitro in a specific and non-specific manner. In the in vivo model, the transfer of CD4(+) CD25(-) cells to Nude mice induced autoimmunity, with cell infiltrate found in the stomach, colon, liver and kidneys. The co-transfer of CD4(+) CD25(-) and CD4(+) CD25(+) prevented the presence of cell infiltrates in several organs and increased the total cell count in lymph nodes. Our data indicate that apoptotic cells have an important role in peripheral tolerance via induction of tolerogenic DCs and CD4(+) CD25(+) Foxp3(+) cells that present regulatory functions.     

Role of dendritic cells in the generation of regulatory T cells

The induction of antigen-specific T-cell tolerance in the thymus and its maintenance in the periphery is crucial for the prevention of autoimmunity. It was recently proposed that cells of the dendritic family not only control immunity but also maintain tolerance to self-antigens, two complementary functions that would ensure the integrity of the organism in an environment full of pathogens. The tolerogenic function of dendritic cells has been shown to be dependent on certain maturation stages and subsets of different ontogenies, and can be influenced by immunomodulatory agents. Here we discuss the current knowledge of these tolerogenic dendritic cells and how might the understanding of the function and characterization of tolerance-inducing dendritic cells be relevant to therapeutic applications.     

Control of intestinal homeostasis by regulatory T cells and dendritic cells

Many different pathways contribute to the maintenance of tolerance to harmless antigens in the intestine. When these important pathways are compromised, chronic intestinal inflammation can develop. In particular, naturally occurring CD4+CD25+ regulatory T cells have been shown to play an important role in the prevention and cure of colitis in animal models of intestinal inflammation. These regulatory T cell responses may be influenced by the local environment in the intestine. For example, functionally specialised populations of dendritic cells exist in the intestine which may favour regulatory type responses. Understanding how these pathways intersect may lead to the development of more specific therapies for the treatment of inflammatory bowel disease.     

Immunomodulatory function of regulatory dendritic cells induced by mesenchymal stem cells

Mesenchymal stem cells (MSCs) provide an excellent model for development of stem cell therapeutics, and their potential treatment in the immunopathogenic diseases have gained further interest after demonstration of immunomodulatory effects on complicated interactions between T cells and even dendritic cells (DCs). However, the mechanisms underlying these immunoregulatory effects of MSCs are poorly understood. In this study, we show that bone marrow derived MSCs can differentiate mature DCs (mDCs) into a distinct regulatory DC population. Compared with mDCs, they have lower expression of CD1a, CD80, CD86 and CD40, but higher expression of CD11b. MSCs induced DCs (MSC-DCs) can hardly stimulate T-cell proliferation even when MSC-DCs are stimulated by LPS. In addition, high endocytosic capacity, low immunogenicity, and strong immunoregulatory function of MSC-DCs are also observed. Moreover, MSC-DCs can efficiently generate CD4+CD25+Foxp3+ Treg cells from CD4+CD25-Foxp3-T cells. The inhibitory function of MSC-DCs is mediated not only through TGF-β1, but also by inducing the production of Treg cells or T-cell anergy. These results demonstrate that the immunomodulatory effects of regulatory DCs induced by MSCs provide efficacious treatment for immunopathogenic diseases.     

TLR-mediated induction of negative regulatory ligands on dendritic cells

Dendritic cells (DCs) shape T-cell response patterns and determine early, intermediate, and late outcomes of immune recognition events. They either facilitate immunostimulation or induce tolerance, possibly determined by initial DC activation signals, such as binding Toll-like receptor (TLR) ligands. Here, we report that DC stimulation through the TLR3 ligand dsRNA [poly(I:C)] limits CD4 T-cell proliferation, curtailing adaptive immune responses. CD4+ T cells instructed by either lipopolysaccharide (LPS) or poly(I:C)-conditioned DCs promptly upregulated the activation marker CD69. Whereas LPS-pretreated DCs subsequently sustained T-cell clonal expansion, proliferation of CD4+ T cells exposed to poly(I:C)-pretreated DCs was markedly suppressed. This proliferative defect required DC-T cell contact, was independent of IFN-alpha, and was overcome by exogenous IL-2, indicating T-cell anergy. Coinciding with the downregulation, CD4+ T cells expressed the inhibitory receptor PD-1. Antibodies blocking the PD-1 ligand PD-L1 restored proliferation. dsRNA-stimulated DCs preferentially induced PD-L1, whereas poly(I:C) and LPS both upregulated the costimulatory molecule CD86 to a comparable extent. Poly(dA-dT), a ligand targeting the cytoplasmic RNA helicase pattern-recognition pathway, failed to selectively induce PD-L1 upregulation, assigning this effect to the TLR3 pathway. Poly(I:C)-conditioned DCs promoted accumulation of phosphorylated SHP-2, the intracellular phosphatase mediating PD-1 inhibitory effects. The ability of dsRNA to bias DC differentiation toward providing inhibitory signals to interacting CD4+ T cells may be instrumental in viral immune evasion. Conversely, TLR3 ligands may have therapeutic value in silencing pathogenic immune responses.     

Induction of anergic and regulatory T cells by plasmacytoid dendritic cells and other dendritic cell subsets

The induction of antigen-specific tolerance is critical for maintaining immune homeostasis and preventing autoimmunity. Because the central tolerance that eliminates potentially harmful autoreactive T cells is incomplete, peripheral mechanisms for suppressing self-reactive T cells play an important role. Dendritic cells (DCs) are professional antigen-presenting cells, which have an extraordinary capacity to stimulate naïve T cells and initiate primary immune responses. Recent accumulating evidence indicates that several subsets of human DCs also play a critical role in the induction of peripheral tolerance by anergizing effector CD4⁺ and CD8⁺ T cells or by inducing the differentiation of naïve T cells into T-regulatory cells, which produce interleukin (IL)-10. Human DC subsets with the property of suppressing an antigen-specific T-cell response include plasmacytoid DCs, which are either in an immature state or in a mature state induced by CD40 ligand stimulation, and monocyte-derived DCs, which are either in an immature state or have had their state modulated by treatment with IL-10 or CD8⁺CD28⁻ T cells. These “tolerogenic” DCs may be relevant to therapeutic applications for autoimmune and allergic diseases as well as organ transplant rejection.     

Tuning microenvironments: induction of regulatory T cells by dendritic cells

The body requires the generation of regulatory T (Treg) cells to preserve its integrity. Each microenvironment is controlled by a specific set of regulatory elements that have to be finefrly and constantly tuned to maintain local homeostasis. These environments could be site specific, such as the gut environment, or induced by chronic exposure to microbes or tumors. Various populations of dendritic cells (DCs) are central to the orchestration of this control. In this review, we will discuss some new findings associating DCs from defined compartments with the induction of antigen-specific Treg cells.     

Role of regulatory dendritic cells in allergy and asthma

PURPOSE OF REVIEW: Dendritic cells are the most efficient inducers of all immune responses, and are capable of inducing either productive immunity or maintaining the state of tolerance to self-antigens and allergens. The present review summarizes the emerging literature on dendritic cells, with the emphasis on regulatory function of dendritic cells in allergy and asthma. In particular we summarize recent data regarding the relationship between dendritic cell subsets and Th1, Th2 and regulatory T (TReg) cells. RECENT FINDINGS: The diverse functions of dendritic cells have been attributed to distinct lineages of dendritic cells, which arise from common immature precursor cells that differentiate in response to specific maturation-inducing or local microenvironment conditions. These subsets induce different lineages of T cells such as Th1, Th2 and TReg cells, including Th1Reg and Th2Reg cells, which regulate allergic diseases and asthma. SUMMARY: Subsets of dendritic cells regulate the induction of a variety of T-cell subtypes, which suppress the development of allergy and asthma, thus providing antiinflammatory responses and protective immunity.     

IDO-expressing regulatory dendritic cells in cancer and chronic infection

Immune evasion and T cell tolerance induction have been associated both with malignant disease and chronic infection. In recent years, increasing evidence has been accumulated that antigen-presenting cells such as dendritic cells (DC) play a major role in immune regulation. They are not only involved in the induction of immunity but also can inhibit immune responses. Interesting parallels for major molecular mechanisms involved in turning DC from stimulatory to regulatory cells have been uncovered between malignant disease and chronic infection. Apparently, not only inhibitory cytokines such as IL-10 seem to play a role, but also metabolic mechanisms dysregulating tryptophan metabolism, thereby, leading to inhibition of T cells and pathogens. We focus here on recent findings establishing the tryptophan catabolizing enzyme indoleamine-pyrrole 2,3 dioxygenase (IDO) as a central feature of DC with regulatory function both in cancer and chronic infection. Induction of enzymatically active IDO can be triggered by various soluble and membrane-bound factors, and in general, require interferon (IFN) signaling. In addition, based on the most recently established link between tumor necrosis factor alpha (TNFalpha), prostaglandin E2 and IDO, a new model of regulation of IDO in context of cancer and infection is proposed. In light of the increasing use of anti-TNFalpha drugs, these findings are also of great interest to the clinician scientist.     

Notch signaling in differentiation and function of dendritic cells

Hematopoietic stem cells give rise to multiple lineages of cells. This process is governed by a tightly controlled signaling network regulated by cytokines and a direct cell-cell contact. Notch signaling represents one of the major pathways activated during direct interaction between hematopoietic progenitor cells and bone marrow stroma. A critical role of Notch signaling in differentiation of T- and B-lymphocytes has now been established. Until recently, the role of Notch signaling in the development of myeloid cells and particular dendritic cells remained unclear. In this review, we discuss recent exciting findings that shed light on the critical role of Notch in differentiation and the function of dendritic cells and its impact on immune responses.