The regulatory role of dendritic cells in the induction and maintenance of T-cell tolerance

Targeted exosome coating gene-chem nanocomplex as "nanoscavenger" for clearing α-synuclein and immune activation of Parkinson's disease

The most critical problem in the treatment of neurodegenerative diseases is brain neuronal protection, which can be overcome by clearing pathological substances and regulating the immune environment. In the above treatment strategies, the traditional poor drug delivery problem is inevitable. Here, we show an engineering core-shell hybrid system named rabies virus glycoprotein (RVG) peptide-modified exosome (EXO) curcumin/phenylboronic acid-poly(2-(dimethylamino)ethyl acrylate) nanoparticle/small interfering RNA targeting SNCA (REXO-C/ANP/S). It is a nanoscavenger for clearing α-synuclein aggregates and reducing their cytotoxicity in Parkinson's disease neurons. The motor behavior of Parkinson's disease mice is substantially improved after REXO-C/ANP/S treatment. In particular, we demonstrate that REXO-C/ANP/S is also a nanoscavenger for clearing immune activation due to its natural immature dendritic cell EXO coating. Our findings show that REXO-C/ANP/S may serve as a platform for neurodegenerative diseases treatment.

Dendritic cells modulate c-kit expression on the edge between activation and death

Dendritic cells (DCs) are key players in immunity and tolerance. Some DCs express c-kit, the receptor for stem cell factor (SCF), nevertheless c-kit functional role and the regulation of its expression in DCs are incompletely defined. We recently demonstrated that autocrine SCF sustains a pro-survival circuit, and that SCF increases phospho-AKT in c-kit+ mouse bone marrow-derived DCs (BMdDCs). Herein we observed that CpG and PolyI:C, two stimuli mimicking bacterial and viral nucleic acids respectively, strongly inhibited c-kit expression by BMdDCs and spleen DCs in vitro and in vivo. Experiments in IFNARI^-/- mice showed that IFN-I pathway was required for c-kit down-regulation in cDC1s, but only partially supported it in cDC2s. Furthermore, CpG and PolyI:C strongly inhibited c-kit mRNA expression. In agreement with the reduced c-kit levels, SCF pro-survival activity was impaired. Thus in the presence of exogenously provided SCF, either PolyI:C or CpG induced spleen DC death in 2 days, while at earlier times IL-6 and IL-12 production were slightly increased. In contrast, SCF improved survival of unstimulated spleen DCs expressing high c-kit levels. Our studies suggest that c-kit down-modulation is a previously neglected component of DC response to CpG and PolyI:C, regulating DC survival and ultimately tuning immune response.

Translating Mechanism of Regulatory Action of Tolerogenic Dendritic Cells to Monitoring Endpoints in Clinical Trials

Tolerogenic dendritic cells (tolDCs) have reached patients with autoimmune and inflammatory disease, at least in clinical trials. The safety of tolDCs as intervention therapy has been established, but the capacity to modulate autoimmune response in vivo remains to be demonstrated. Studies have revealed a diversity of regulatory mechanisms that tolDCs may employ in vivo. These mechanisms differ between various types of modulated tolDC. The most often foreseen action of tolDCs is through regulatory polarization of naïve T cells or activation of existing regulatory T cells, which should ultimately diminish autoimmune inflammation. Yet, selection of a target autoantigen remains critical to expedite tissue specific tolerance induction, while measuring immune modulation incited by tolDCs in vivo provides a great challenge. We will discuss the regulatory action of different types of tolDCs and the possible methods to monitor immunological efficacy endpoints for the next generation clinical trials.

The Importance of Dendritic Cells in Maintaining Immune Tolerance

Immune tolerance is necessary to prevent the immune system from reacting against self, and thus to avoid the development of autoimmune diseases. In this review, we discuss key findings that position dendritic cells (DCs) as critical modulators of both thymic and peripheral immune tolerance. Although DCs are important for inducing both immunity and tolerance, increased autoimmunity associated with decreased DCs suggests their nonredundant role in tolerance induction. DC-mediated T cell immune tolerance is an active process that is influenced by genetic variants, environmental signals, as well as the nature of the specific DC subset presenting Ag to T cells. Answering the many open questions with regard to the role of DCs in immune tolerance could lead to the development of novel therapies for the prevention of autoimmune diseases.

GM-CSF Inhibits c-Kit and SCF Expression by Bone Marrow-Derived Dendritic Cells

Stem cell factor (SCF), the ligand of c-kit, is a key cytokine for hematopoiesis. Hematopoietic precursors express c-kit, whereas differentiated cells of hematopoietic lineage are negative for this receptor, with the exception of NK cells, mast cells, and a few others. While it has long been recognized that dendritic cells (DCs) can express c-kit, several questions remain concerning the SCF/c-kit axis in DCs. This is particularly relevant for DCs found in those organs wherein SCF is highly expressed, including the bone marrow (BM). We characterized c-kit expression by conventional DCs (cDCs) from BM and demonstrated a higher proportion of c-kit⁺ cells among type 1 cDC subsets (cDC1s) than type 2 cDC subsets (cDC2s) in both humans and mice, whereas similar levels of c-kit expression were observed in cDC1s and cDC2s from mouse spleen. To further study c-kit regulation, DCs were generated with granulocyte-macrophage colony-stimulating factor (GM-CSF) from mouse BM, a widely used protocol. CD11c⁺ cells were purified from pooled non-adherent and slightly adherent cells collected after 7 days of culture, thus obtaining highly purified BM-derived DCs (BMdDCs). BMdDCs contained a small fraction of c-kit⁺ cells, and by replating them for 2 days with GM-CSF, we obtained a homogeneous population of c-kit⁺ CD40^hi MHCII^hi cells. Not only did BMdDCs express c-kit but they also produced SCF, and both were striking upregulated if GM-CSF was omitted after replating. Furthermore, a small but significant reduction in BMdDC survival was observed upon SCF silencing. Incubation of BMdDCs with SCF did not modulate antigen presentation ability of these cells, nor it did regulate their membrane expression of the chemokine receptor CXCR4. We conclude that the SCF/c-kit-mediated prosurvival circuit may have been overlooked because of the prominent use of GM-CSF in DC cultures in vitro, including those human DC cultures destined for the clinics. We speculate that DCs more prominently rely on SCF in vivo in some microenvironments, with potential implications for graft-versus-host disease and antitumor immunity.

Immunotherapeutic potential of extracellular vesicles

Extracellular vesicle or EV is a term that encompasses all classes of secreted lipid membrane vesicles. Despite being scientific novelties, EVs are gaining importance as a mediator of important physiological and pathological intercellular activities possibly through the transfer of their cargo of protein and RNA between cells. In particular, exosomes, the currently best characterized EVs have been notable for their in vitro and in vivo immunomodulatory activities. Exosomes are nanometer-sized endosome-derived vesicles secreted by many cell types and their immunomodulatory potential is independent of their cell source. Besides immune cells such as dendritic cells, macrophages, and T cells, cancer and stem cells also secrete immunologically active exosomes that could influence both physiological and pathological processes. The immunological activities of exosomes affect both innate and adaptive immunity and include antigen presentation, T cell activation, T cell polarization to regulatory T cells, immune suppression, and anti-inflammation. As such, exosomes carry much immunotherapeutic potential as a therapeutic agent and a therapeutic target.

Indoleamine 2,3-dioxygenase: expressing cells in inflammatory bowel disease-a cross-sectional study

Aim. To characterise and enumerate IDO⁺ cells, Tregs, and T cell subsets in patients with ulcerative colitis (UC) and Crohn's disease (CD) with regard to their clinical activity. Methods. Ten active UC (aUC), 10 inactive UC (iUC), 6 aCD, and 8 iCD patients and 10 healthy individuals were included in the study. Circulating Foxp3-, IDO-, IL-17A-, IL-4-, IFN-γ-, and IL-10-expressing CD4⁺ T cells were quantitated by flow cytometry. Interleukin-17-expressing cells, CD25⁺/Foxp3⁺ Tregs, and CD123⁺/IDO⁺ plasmacytoid dendritic cells were evaluated in intestinal biopsies from 10 aUC, 6 aCD, and 10 noninflamed tissues. Results. All CD4⁺ T subsets were increased in aIBD patients compared with healthy donors. Meanwhile, frequency of CD8α⁺/CD16⁺/IDO⁺, CD8α⁺/CD56⁺/IDO⁺, CD8α⁺/CD80⁺/IDO⁺, CD8α⁺/CD123⁺/IDO⁺ large granular nonlymphoid cells, and CCR6⁺/CD123⁺/IDO⁺ plasmacytoid dendritic cells was higher in aIBD patients versus healthy donors or iIBD patients. Tissue IL-17A⁺ cells were present in higher amounts in aIBD versus noninflamed controls. IDO- and Foxp3-expressing cells were increased in aUC versus aCD patients and noninflamed tissues. Conclusions. The findings represent an original work in Mexican Mestizo patients with IBD. It shows that Tregs and IDO-expressing cells are increased with regard to disease activity. These cells could significantly shape inflammatory bowel disease pathophysiology, severity, and tolerance loss.

[Vitamin D and autoimmunity]

Vitamin D acts at several levels in the immune systems to maintain immune tolerance. Vitamin D deficiency is a plausible environmental risk factor for autoimmune disease. Basic, genetic and epidemiological studies indicate a potential role of vitamin D in the prevention and the treatment of autoimmune diseases. Most of the epidemiological studies are cross-sectional, so that they are insufficient to establish a direct link between vitamin D deficiency and both disease risk and disease activity. Randomized, controlled trials are necessary.

Immature dendritic cell-derived exosomes: a promise subcellular vaccine for autoimmunity

Exosomes, 60-90-nm-sized vesicles, are produced by a large number of cell types, including tumor cells, neurons, astrocytes, hemocytes, intestinal epithelial cells, and so on. Dendritic cell (DC), the most potent professional antigen-presenting cell in the immune system, produces exosomes in the course of maturation. Mature DCs produce exosomes with the ability to elicit potent immunoactivation, resulting in tumor eradication and bacterial or virus elimination. Given the notion that exosomes are stable and easy to be modified artificially, autologous mature DC-derived exosomes have been vaccinated into patients with malignant diseases. In clinical trials utilizing exosomes as therapeutic approaches, researchers observed considerable curative effect with little side effect. However, immature or suppressive DC-derived exosomes harbor anti-inflammatory properties distinct from mature DC-derived exosomes. In murine models of autoimmune disease and transplantation, immature DC-derived exosomes reduced T cell-dependent immunoactivation, relieved clinical manifestation of autoimmune disease, and prolonged survival time of transplantation. Although the exact mechanism of how immature DC-derived exosomes function in vivo is still unclear, and there are no clinical trials regarding application of exosome vaccine into patients with autoimmune disease, we will analyze the promise of immature DC-derived exosomes as a subcellular vaccine in autoimmunity in this review.

Mechanisms of CpG-induced CD40 expression on murine bone marrow-derived dendritic cells

Aberrant CD40 expression by dendritic cells (DCs), induced by microbial stimuli, such as CpG, contributes to the pathogenesis of many human/murine diseases, particularly autoimmune and inflammatory diseases. Given the importance of CD40 in these diseases, and the contribution of DCs to the diseases process, it is very important to investigate the mechanisms of CD40 expression induced by CpG on DCs. In this study, we made the observation that CpG-B is a potent inducer on CD40 expression on murine bone marrow-derived DCs. Based on this finding, we undertook an analysis of the molecular basis of CpG-induced CD40 expression on DCs. By using selective inhibitors, it was demonstrated that MAPKs (JNK and p38 MAPK but not ERK) and NF-κB were involved in CpG-induced CD40 expression on DCs. In addition, RNA interference analysis revealed that IRF8 was a key transcription factor in the basal expression of CD40 upon CpG stimulation. Moreover, up-regulating miRNA-146a in DCs effectively decreased CD40 expression by targeting TRAF6 and IRAK1. Thus, our results have elucidated the molecular mechanisms underlying CpG-induced CD40 expression and DC maturation.

Macrophages and fibrosis: How resident and infiltrating mononuclear phagocytes orchestrate all phases of tissue injury and repair

Certain macrophage phenotypes contribute to tissue fibrosis, but why? Tissues host resident mononuclear phagocytes for their support to maintain homeostasis. Upon injury the changing tissue microenvironment alters their phenotype and primes infiltrating monocytes toward pro-inflammatory macrophages. Several mechanisms contribute to their deactivation and macrophage priming toward anti-inflammatory and pro-regenerative macrophages that produce multiple cytokines that display immunosuppressive as well as pro-regeneratory effects, such as IL-10 and TGF-beta1. Insufficient parenchymal repair creates a tissue microenvironment that becomes dominated by multiple growth factors that promote the pro-fibrotic macrophage phenotype that itself produces large amounts of such growth factors that further support fibrogenesis. However, the contribution of resident mononuclear phagocytes to physiological extracellular matrix turnover implies also their fibrolytic effects in the late stage of tissue scaring. Fibrolytic macrophages break down fibrous tissue, but their phenotypic characteristics remain to be described in more detail. Together, macrophages contribute to tissue fibrosis because the changing tissue environments prime them to assist and orchestrate all phases of tissue injury and repair. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Reduced numbers of dendritic cells with a tolerogenic phenotype in the prediabetic pancreas of NOD mice

The NOD mouse is a widely used animal model of autoimmune diabetes. Prior to the onset of lymphocytic insulitis, DCs accumulate at the islet edges. Our recent work indicated that these DCs may derive from aberrantly proliferating local precursor cells. As CD8α(+) DCs play a role in tolerance induction in steady-state conditions, we hypothesized that the autoimmune phenotype might associate with deficiencies in CD8α(+) DCs in the prediabetic NOD mouse pancreas. We studied CD8α(+) DCs in the pancreas and pLNs of NOD and control mice, focusing on molecules associated with tolerance induction (CD103, Langerin, CLEC9A, CCR5). mRNA expression levels of tolerance-modulating cytokines were studied in pancreatic CD8α(+) DCs of NOD and control mice. In the NOD pancreas, the frequency of CD8α(+)CD103(+)Langerin(+) cells was reduced significantly compared with control mice. NOD pancreatic CD8α(+)CD103(+)Langerin(+) DCs expressed reduced levels of CCR5, CLEC9A, and IL-10 as compared with control DCs. These alterations in the CD8α(+)CD103(+)Langerin(+) DC population were not present in pLNs. We demonstrate local abnormalities in the CD8α(+) DC population in the prediabetic NOD pancreas. These data suggest that abnormal differentiation of pancreatic DCs contributes to loss of tolerance, hallmarking the development of autoimmune diabetes.

The p50 subunit of NF-κB orchestrates dendritic cell lifespan and activation of adaptive immunity

Dendritic cells play a central role in keeping the balance between immunity and immune tolerance. A key factor in this equilibrium is the lifespan of DC, as its reduction restrains antigen availability leading to termination of immune responses. Here we show that lipopolysaccharide-driven DC maturation is paralleled by increased nuclear levels of p50 NF-κB, an event associated with DC apoptosis. Lack of p50 in murine DC promoted increased lifespan, enhanced level of maturation associated with increased expression of the proinflammatory cytokines IL-1, IL-18 and IFN-β, enhanced capacity of activating and expanding CD4⁺ and CD8⁺ T cells in vivo and decreased ability to induce differentiation of FoxP3⁺ regulatory T cells. In agreement, vaccination of melanoma-bearing mice with antigen-pulsed LPS-treated p50^−/− BM-DC boosted antitumor immunity and inhibition of tumor growth. We propose that nuclear accumulation of the p50 NF-κB subunit in DC, as occurring during lipopolysaccharide-driven maturation, is a homeostatic mechanism tuning the balance between uncontrolled activation of adaptive immunity and immune tolerance.

microRNA-181a represses ox-LDL-stimulated inflammatory response in dendritic cell by targeting c-Fos

Oxidized LDL (ox-LDL) activates dendritic cells (DCs), thereby initiating inflammation responses in atherosclerosis, yet the modulatory mechanisms remain unclear. MicroRNAs (miRNAs) are important regulators for DC functions. This study evaluated the regulation by miRNAs of the ox-LDL-induced DC immune response. In CD11c⁺ DCs from ApoE-deficient mice with hyperlipidemia, microRNA miR-181a was significantly up-regulated. In cultured bone marrow-derived DCs (BMDCs), ox-LDL promoted DC maturation and up-regulated miR-181a expression. Abundance of miR-181a attenuated ox-LDL-induced CD83 and CD40 expression, inhibited the secretion of interleukin (IL)-6 and TNF-α, and up-regulated IL-10, an important anti-inflammatory cytokine that was inhibited by ox-LDL. Inhibition of the endogenous miR-181a reversed the effects on CD83 and CD40 as well as the effects on IL-6 and TNF-α. The putative target genes of miR-181a were evaluated by gene ontology assessment, and the c-Fos-mediated inflammation pathway was identified. miR-181a targeted the 3′ untranslated region of c-Fos mRNA by luciferase experiments. Thus, abundance of miR-181a reduced c-Fos protein, whereas inhibition of miR-181a increased c-Fos protein in BMDCs. We therefore suggest that miR-181a attenuates ox-LDL-stimulated immune inflammation responses by targeting c-Fos in DCs.

Activation-induced FoxP3 expression regulates cytokine production in conventional T cells stimulated with autologous dendritic cells

A defining feature of dendritic cells (DCs) is their ability to induce the proliferation of autologous T cells in the absence of foreign antigen-a process termed the "autologous mixed leukocyte reaction" (AMLR). We report that equine monocyte-derived DCs, but not macrophages, are potent inducers of the AMLR. The response is contact dependent and major histocompatibility complex class II dependent and primarily involves CD3(+) CD4(+) CD8(-) T cells. Upon stimulation with DCs or the mitogen concanavalin A, a subset of the proliferating T cells expresses the regulatory T-cell (Treg) transcription factor FoxP3. Although many of these FoxP3(+) T cells are capable of producing the effector cytokines interleukin-4 (IL-4) and gamma interferon (IFN-γ), they are more likely to produce IL-10 and less likely to produce IFN-γ than equivalent FoxP3(-) cells. Therefore, FoxP3 expression is an inherent component of equine T cell activation and is associated with a more immunosuppressive cytokine profile. These results confirm that FoxP3 expression in the horse, in contrast to the mouse, is regulated similarly to FOXP3 expression in humans and provide evidence that FoxP3 expression by conventional T cells may help regulate the developing immune response.

Functional specialization of islet dendritic cell subsets

Dendritic cells (DC) play important roles in both tolerance and immunity to β cells in type 1 diabetes. How and why DC can have diverse and opposing functions in islets remains elusive. To answer these questions, islet DC subsets and their specialized functions were characterized. Under both homeostatic and inflammatory conditions, there were two main tissue-resident DC subsets in islets, defined as CD11b(lo/-)CD103(+)CX3CR1(-) (CD103(+) DC), the majority of which were derived from fms-like tyrosine kinase 3-dependent pre-DC, and CD11b(+)CD103(-)CX3CR1(+) (CD11b(+) DC), the majority of which were derived from monocytes. CD103(+) DC were the major migratory DC and cross-presented islet-derived Ag in the pancreatic draining lymph node, although this DC subset displayed limited phagocytic activity. CD11b(+) DC were numerically the predominant subset (60-80%) but poorly migrated to the draining lymph node. Although CD11b(+) DC had greater phagocytic activity, they poorly presented Ag to T cells. CD11b(+) DC increased in numbers and percentage during T cell-mediated insulitis, suggesting that this subset might be involved in the pathogenesis of diabetes. These data elucidate the phenotype and function of homeostatic and inflammatory islet DC, suggesting differential roles in islet immunity.

Tolerogenic dendritic cells as a target for the therapy of immune thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease in which platelets are destroyed by special antiplatelet autoantibodies produced by B cells. Dendritic cells (DCs) are professional antigen-presenting cells involved in humoral immunity and cellular immunity and among them DCs that induce autoimmune tolerance are called tolerogenic DCs (tDCs). As a promising immunotherapeutic strategy for ITP, tDCs have received increasing attention. In this review, we describe the significant role of DCs in regulating autoimmune balances, introduce the manipulation strategies to generate tDCs, summarize recent progress on the experimental application of tDCs for ITP therapy, and finally discuss the perspectives of tolerogenic vaccination for ITP treatment in the clinic.

Maturation profiles of peripheral blood dendritic cells in patients with endogenous uveitis

To determine whether or not maturation of dendritic cells (DCs) is associated with pathogenesis of endogenous uveitis, we analyzed expression of maturation markers, including CD80, CD86, and human leukocyte antigen (HLA)-DR, in peripheral blood (PB) DCs for comparison between healthy controls (HCs) and uveitis patients. A total of 21 patients and 16 HCs were included. Flow cytometric analysis was performed using PB DCs during the active phase of intraocular inflammation. CD86 and HLA-DR expression was higher in PB DCs from uveitis patients versus HCs, whereas that of CD80 was not significantly different. Levels of CD86 and HLA-DR expression tended to parallel those of inflammatory activity, and decreased after anti-inflammatory therapy. However, expression of CD86 and HLA-DR, even in remission, was not completely down-regulated to the low levels found in HCs. Our results indicate the maturation of DCs may play a role in the pathogenesis of endogenous uveitis. The relatively high expression of HLA-DR and co-stimulatory molecules even in the quiescence of inflammation suggests maturation of DCs may be associated with chronicity and recurrence of uveitis.

Dendritic cells and the maintenance of self-tolerance

Dendritic cells (DC) play important roles in the initiation of immune responses and in the maintenance of self-tolerance. We have been studying the role of DC in the pathogenesis of type 1 diabetes and exploring the ability of specific DC subsets to prevent diabetes in non-obese diabetic (NOD) mice. DC presenting low doses of antigen are capable of inducing and expanding T-regulatory (Treg) cells that have potent suppressive function. We review here our recent findings in this area and highlight the ability of semi-mature therapeutic DC to induce Treg expansion in the absence of exogenous antigen. We discuss how the presentation of endogenous self-antigen by DC may represent a natural mechanism for peripheral self-tolerance that can be harnessed to prevent autoimmunity.

Indoleamine 2,3-dioxygenase-expressing peripheral cells in rheumatoid arthritis and systemic lupus erythematosus: a cross-sectional study

BACKGROUND

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-degrading enzyme which suppresses T lymphocyte activity and induces Foxp3+ CD4+ regulatory T cells (Tregs) polarisation. The aim of this study was to evaluate the expression of IDO in freshly isolated peripheral cells as well as to enumerate Tregs and Th17 subpopulation in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) patients.

MATERIALS AND METHODS

The percentage of IDO-expressing cells as well as Tregs and Th17 was evaluated in 14 active RA- (aRA), 13 inactive RA- (iRA), 7 aSLE-, 12 iSLE-treated patients and 11 healthy donors (controls). Intracellular IDO was analysed by flow cytometry in CD14+, CD8α+, CD16+ and CD123+ cell subpopulations. Tregs and Th17 were assessed by intracellular of Foxp3 and IL17A detection in CD4+ CD14- cells. A total of 50,000 events were recorded for each sample.

RESULTS

The amounts of CD14+/CD16-/IDO+, CD14-/CD16+/IDO+ and CD14+/CD16+/IDO+-expressing peripheral cells were slightly lower in inactive vs. active disease in RA and SLE patients. Notwithstanding, only inactive patients had statistically significant lower percentages when compared to controls. aRA and iRA showed a statistically significant decrease in CD8α+/CD123+/IDO+ vs. controls. Meanwhile, only iSLE patients had lower CD8α+/CD123+/IDO+ cells vs. aSLE patients and controls. Th17 subset was present in higher amounts in aRA and iRA patients vs. controls. Tregs showed an increase in aRA patients vs. controls.

CONCLUSIONS

A decreased percentage of IDO-expressing peripheral cells were determined in iRA and iSLE compared to controls. It could play a critical role in tolerance loss in these diseases.