Defects in CD54 and CD86 up-regulation by plasmacytoid dendritic cells during pregnancy

Physiological modulation of the immune system is required for foetal tolerance during pregnancy. However, this immune regulation might lead to impaired self-defence against pathogens. Indeed, pregnant women are more susceptible to newly encountered viruses comparing to non-pregnant women, as exemplified by the prevalence of severe complications in pregnant women infected with the pandemic influenza virus in 2009. Plasmacytoid dendritic cells (pDCs) are specialized dendritic cells that recognise viral antigens and initiate both innate and adaptive immune responses. We therefore sought to determine whether the number and/or the functions of peripheral blood pDCs are regulated during pregnancy. pDC maturation and interferon (IFN)-α production were analysed in response to Toll-like receptor (TLR) stimulation of peripheral blood mononuclear cells from pregnant and non-pregnant women. Our results reveal that pDC frequency is slightly decreased, while the IFN-α production in response to TLR stimulation increases during pregnancy. Interestingly, the up-regulation of the co-stimulatory receptors CD54 (ICAM1) and CD86 is significantly decreased in pDCs from pregnant women as compared to controls, suggesting a possible impact on T-cell responses. In conclusion, we propose that the modulation of CD54 and CD86 expression on peripheral blood pDCs during pregnancy might decrease the initiation of adaptive antiviral immune responses.     

Triptolide inhibits IL-12 production and T cell-stimulatory capacity of dendritic cells

Extracts of Tripterygium wilfordii Hook F.(TWHF ) are effective in traditional Chinesemedicine for treatment of autoimmune diseasessuch as rheumatoid arthritis, systemic lupus ery thematosus, nephritis and asthma [1, 2]. Trip tolide, a diterpenoid triepoxide, is derived fromTWHF and is responsible for most of the immuno suppressive and anti inflammatory effects ofTWHF. Triptolide has been shown to be effectivein the treatment of autoimmune diseases, …     

Modulation of regulatory T cell-Th17 balance by plasmacytoid dendritic cells

Tregs represent an interesting therapeutic tool to modulate immune responses that could be deleterious in autoimmune diseases and in transplantation. However, phenotype and functions of Tregs do not seem to be stable, and recent data suggest that FoxP3-expressing Tregs can be driven to produce IL-17. In this study, we have analyzed the role of pDCs versus cDCs on Treg responses and underlined that pDCs have an intrinsic, unique capacity to induce IL-17 secretion from T cells. We showed in rats that FoxP3(+) Tregs were able to secrete IL-17 only when stimulated by allogeneic, mature pDCs but not cDCs. In addition, in rats and mice, mature pDCs but not cDCs inhibited in vitro Treg-suppressive functions and in the presence of Tregs, supported Th17 differentiation from naive T cells through secretion of high amounts of IL-6. These data suggest an important role for pDCs in modulating or switching Treg function and allowing Th17 differentiation.     

Immunopathology associated with Epstein-Barr virus (EBV) infection: Evidence for interactions with T-lymphocyte EBV receptor CD21

Epstein-Barr virus (EBV), also termed Human Herpes Virus 4, is the causative agent of infectious mononucleosis and may be a cofactor in some human cancers. The virus has also been suggested to play a role in human autoimmune diseases including rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis. X-linked lymphoproliferative syndrome caused by the deficiency of the Sarc homology 2 domain protein 1A, also termed signaling lymphocyte activation marker-associated protein, can result in immune dysfunction and death after EBV infection. The EBV-related immunopathology in X-linked lymphoproliferative syndrome and prototypical autoimmune syndromes is summarized in this review. A novel model of viral interaction with complement receptor CD21, which is also the receptor for EBV, is proposed to account for both the immunological abnormalities of X-linked lymphoproliferative syndrome and autoimmune diseases associated with EBV infection. The pathogenesis of both X-linked lymphoproliferative syndrome and EBV-associated immune diseases is proposed to result from increased direct infection of T cells by EBV through the T-cell complement receptor CD21 expressed on T cells. A prediction of this model is that therapy designed to decrease CD21-mediated EBV infection of T lymphocytes could also be beneficial in the treatment of some autoimmune diseases.     

Natural mannosylation of HIV-1 gp120 imposes no immunoregulatory effects in primary human plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) play a vital role in activation of anti-HIV-1 immunity, and suppression of pDCs might mitigate immune responses against HIV-1. HIV-1 gp120 high-mannose has been attributed immunosuppressive roles in human myeloid DCs, but no receptors for high-mannose have so far been reported on human pDCs. Here we show that upon activation with HIV-1 or by a synthetic compound triggering the same receptor in human pDCs as single-stranded RNA, human pDCs upregulate the mannose receptor (MR, CD206). To examine the functional outcome of this upregulation, inactivated intact or viable HIV-1 particles with various degrees of mannosylation were cultured with pDCs. Activation of pDCs was determined by assaying secretion of IFN-alpha, viability, and upregulation of several pDC-activation markers: CD40, CD86, HLA-DR, CCR7, and PD-L1. The level of activation negatively correlated with degree of mannosylation, however, subsequent reduction in the original mannosylation level had no effect on the pDC phenotype. Furthermore, two of the infectious HIV-1 strains induced profound necrosis in pDCs, also in a mannose-independent manner. We therefore conclude that natural mannosylation of HIV-1 is not involved in HIV-1-mediated immune suppression of pDCs.     

Defective expression and function of the ILT2/CD85j regulatory receptor in dendritic cells from patients with systemic lupus erythematosus

Dendritic cells (DCs) play an important role in inducing immune tolerance. Inhibitory receptors, such as ILT2/CD85j, are involved in the tolerogenic effect of DCs and previous studies have indicated the important role of these receptors on the pathogenesis of autoimmune diseases. The aim of this study was to evaluate the expression and function of ILT2 in DCs from SLE patients. Peripheral blood from fifty patients with SLE and 38 healthy volunteers was obtained. ILT2 expression was assessed by flow cytometry. DCs were generated in the absence or presence of an anti-ILT2 antibody. The effect of ILT2 in the immunogenic ability of SLE DCs was also evaluated. We observed that SLE patients had significant higher levels of circulating plasmacytoid DCs (pDCs), and that these levels correlated with disease activity. In contrast, the expression of ILT2 was diminished in both pDCs and myeloid DCs (mDCs) from these patients. However, under our experimental conditions, the in vitro differentiation of DCs was not apparently affected by ILT2 engagement. In contrast, the immunogenic capability of monocyte-derived DCs was not down-regulated by ILT2 cross-linking in a significant proportion of SLE patients. Our results suggest that ILT2 participates in the defective immune-regulation observed in patients with SLE.     

Unraveling the functions of plasmacytoid dendritic cells during viral infections,autoimmunity, and tolerance

Summary: Plasmacytoid dendritic cells (pDCs) are bone marrow-derived cells that secrete large amounts of type I interferon (IFN) in response to viruses. Type I IFNs are pleiotropic cytokines with antiviral activity that also enhance innate and adaptive immune responses. Viruses trigger activation of pDCs and type I IFN responses mainly through the Toll-like receptor pathway. However, a variety of activating and inhibitory pDC receptors fine tune the amplitude of type I IFN responses. Chronic activation and secretion of type I IFN in the absence of infection can promote autoimmune diseases. Furthermore, while activated pDCs promote immunity and autoimmunity, resting or alternatively activated pDCs may be tolerogenic. The various roles of pDCs have been extensively studied in vitro and in vivo with depleting antibodies. However, depleting antibodies cross-react with other cell types that are critical for eliciting protective immunity, potentially yielding ambiguous phenotypes. Here we discuss new approaches to assess pDC functions in vivo and provide preliminary data on their potential roles during viral infections. Such approaches would also prove useful in the more specific evaluation of how pDCs mediate tolerance and autoimmunity. Finally, we discuss the emergent role of pDCs and one of their receptors, tetherin, in human immunodeficiency virus pathogenesis.     

Activation of plasmacytoid dendritic cells and B cells with two structurally different Toll-like receptor 7 agonists

Synthetic Toll-like receptor (TLR) 7 agonists have been suggested as immune modulators in a range of conditions. In contrast, self-derived TLR7 activators, such as RNA-containing immune complexes (RNA-IC), can contribute to autoimmune diseases due to endogenous immune activation. The exact difference in immune cell response between synthetic and endogenous TLR7 triggers is only partly known. An understanding of these differences could aid in the development of new therapeutic agents and provide insights into autoimmune disease mechanisms. We therefore compared the stimulatory capacity of two TLR7 agonists, RNA-IC and a synthetic small molecule DSR-6434, on blood leucocytes, plasmacytoid dendritic cells (pDCs) and B cells from healthy individuals. IFN-α, IL-6, IL-8 and TNF levels were measured by immunoassays, and gene expression in pDCs was analysed by an expression array. DSR-6434 triggered 20-fold lower levels of IFN-α by pDCs, but higher production of IL-6, IL-8 and TNF, compared to RNA-IC. Furthermore, IFN-α and TNF production were increased with exogenous IFN-α2b priming, whereas IL-8 synthesis by B cells was reduced for both stimuli. Cocultivation of pDCs and B cells increased the RNA-IC-stimulated IFN-α and TNF levels, while only IL-6 production was enhanced in the DSR-6434-stimulated cocultures. When comparing pDCs stimulated with RNA-IC and DSR-6434, twelve genes were differentially expressed (log₂ fold change >2, adjusted P-value <.05). In conclusion, RNA-IC, which mimics an endogenous TLR7 stimulator, and the synthetic TLR7 agonist DSR-6434 trigger distinct inflammatory profiles in immune cells. This demonstrates the importance of using relevant stimuli when targeting the TLR7 pathway for therapeutic purposes.     

Disruption of the Notch pathway aggravates airway inflammation by inhibiting regulatory T cell differentiation via regulation of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) regulate immunity and promote tolerance in asthma. Notch signalling is a highly conserved pathway that regulates the immune response; however, its role in pDC-mediated asthmatic airway inflammation is unclear. This study clarified the effects of Notch signalling on pDC-mediated airway inflammation using murine models of ovalbumin-sensitized allergic asthma. RBP-J-deficient pDCs (RBP-J^−/− pDCs) were co-cultured with naïve CD4⁺ T cells and supernatants and T cell subtypes were analysed. RBP-J^−/− pDCs were intranasally transferred to the airways of ovalbumin-sensitized recipient mice. Lung samples of all mice were subjected to tests for histopathology, cytokine profile of bronchoalveolar lavage fluid, airway hyperactivity and expression of T helper type 1 (Th1)/Th2 cells, regulatory T cells and type 2 innate lymphoid cells (ILC2s). The results showed that pDCs with and without RBP-J deficiency significantly differed in expression levels of cluster of differentiation 83 (CD83), but not CD80, CD86 and major histocompatibility complex class II. Co-culturing pDCs with naïve T cells revealed a poorer immunosuppressive effect of RBP-J^−/− pDCs. This may be attributed to the lower expression levels of inducible co-stimulator ligand and lower production of interleukin 10 in RBP-J^−/− pDCs than in control pDCs, which impeded T cell activation and Treg suppression. RBP-J^−/− pDCs were associated with high ILC2 expression and severe Th2 immune responses and airway inflammation. Therefore, Notch signalling is critical for pDC-dependent immunoregulation, and RBP-J deficiency reduces pDC-based immunosuppression via T cell activation and Th cell differentiation. Thus, this pathway may be a therapeutic target for pDC-based anti-asthma immunotherapy.     

CD4+CD25+ regulatory T cells in systemic sclerosis and other rheumatic diseases

Systemic sclerosis (SSc) is a generalized connective tissue disorder, characterized by a wide spectrum of microvascular and immunological abnormalities, leading to a progressive thickening and fibrosis of the skin and other organs, such as the lungs, GI tract, heart and kidneys. SSc is thought to be an autoimmune disease owing to the presence of high affinity antibodies and possible clinical overlap with other autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. Autoimmune diseases arise because of a breakdown in immunological self tolerance. Self tolerance is maintained via multiple regulatory mechanisms within the immune system, including the thymic deletion of self-reactive T cells and mechanisms of peripheral tolerance. In recent years, the presence of CD4(+)CD25(+)FOXP3(+) Tregs has been identified as a major mechanism of peripheral tolerance, and accumulating evidence indicates that alterations in Treg frequencies and/or function may contribute to autoimmune diseases. Here, we will review recent data on the percentage, function and phenotype of CD4(+)CD25(+) Tregs in rheumatic disease, and discuss how recent developments may guide research in this area in SSc.     

Deficiency of Mouse CD4~+CD25~+Foxp3~+ Regulatory T Cells in Xenogeneic Pig Thymus-Grafted Nude Mice Suffering from Autoimmune Diseases

Xenogeneic thymus transplantation can efficiently induce specific immune tolerance to donor antigens in athymic recipients.However,many nude mice snffer from autoimmune diseases(AID) for over 10 weeks after xenogeneic thymus transplantation.CD4 CD25 Foxp3 regulatory T (Treg)cells were recently determined to play a pivotal role in keeping immune tolerance in humans and mice.Thus,we investigated this subpopulation of Treg cells in the periphery of pig thymus-grafted nude mice suffering from AID.Our results showed that the expression of Foxp3, CTLA-4 and GITR on mouse CD4 CD25 T cells and the ratio of CD4 CD25 Foxp3 Treg cells to CD4 T cells were significantly decreased in the periphery of pig thymus-grafted nude mice snfiering from AID,compared with healthy pig or mouse thymus-grafted nude mice.Furthermore,mouse CD4 CD25 T cells in pig thymus-grafted nude mice Sufiering from AID showed more severe deficiency in immunosuppressive function compared with the counterpart in xenogeneic pig or syngeneic thymus-grafted nude mice without AID.Thus,the decreased frequency, altered phenotype and functional deficiency of mouse CD4 CD25 Treg cells in pig thymus-grafted nude mice may contribute to the development of AID in this model.     

Ligand and cytokine dependence of the immunosuppressive pathway of tryptophan catabolism in plasmacytoid dendritic cells

Murine plasmacytoid dendritic cells (pDCs) have been credited with a unique ability to express indoleamine 2,3-dioxygenase (IDO) function and mediate immunosuppression in specific settings; yet, the conditions of spontaneous versus induced activity have remained unclear. We have used maneuvers known to up-regulate IDO in different cell types and have examined the relative efficacy and mechanisms of the induced activity in splenic pDCs, namely, after specific receptor engagement by CTLA-4-Ig, CD200-Ig or CD28-Ig, the latter in combination with silenced expression of the suppressor of cytokine signaling 3 (SOCS3) gene. We found that pDCs (CD11c+ mPDCA-1+ 120G8+) do not express IDO and are not tolerogenic under basal conditions. B7-1 engagement by CTLA-4-Ig, CD200R1 engagement by CD200-Ig and B7-1/B7-2 engagement by CD28-Ig in SOCS3-deficient pDCs were each capable of initiating IDO-dependent tolerance via different mechanisms. IFN-gamma was the major cytokine responsible for CTLA-4-Ig effects, and type I IFNs for those of CD200-Ig. Immunosuppression by CD28-Ig in the absence of SOCS3 required IFN-gamma induction and IFN-like actions of IL-6. Therefore, although pDCs do not mediate IDO-dependent tolerance constitutively, multiple ligands and cytokines will contribute to the expression of a tolerogenic phenotype by pDCs in the mouse.     

CD49d/CD29‐integrin controls the accumulation of plasmacytoid dendritic cells into the CNS during neuroinflammation

Plasmacytoid dendritic cells (pDCs) are found in the CNS during neuroinflammation and have been reported to exert regulatory functions in multiple sclerosis (MS) and its animal model experimental autoimmune encephalomyelitis (EAE). However, the mechanisms of entry of pDCs into the CNS as well as their phenotype and innate functional properties, once recruited into the CNS, have not been thoroughly examined. Herein, we show that pDCs rapidly accumulate into the brain and spinal cord during the acute phase of EAE, and maintain the expression of numerous phenotypic markers typical of peripheral pDCs. Functionally, CNS‐pDCs constitutively expressed IRF7 and were able to rapidly produce type I IFNs and IL‐12p40 upon ex vivo TLR‐9 stimulation. Using adoptive transfer experiments, we provide evidence that CNS‐pDC are recruited from the blood and accumulate into the CNS during the acute phase of EAE. Accumulation of pDCs into the CNS was strongly inhibited in the absence of CD29, but not CD18, suggesting a major role for ß1 but not ß2 integrins. Indeed, blocking the CD49d α4‐integrins during acute EAE drastically diminished CNS‐pDC numbers. Together, our results demonstrate that circulating pDCs are actively recruited into the CNS during acute EAE through a mechanism largely dependent on CD49d/CD29‐integrins.     

Differentiation of human dendritic cell subsets for immune tolerance induction

Objectives

Since no further progress was achieved, in order to improve the long-term organ transplantation outcome, the immune tolerance appears as an interesting therapeutic goal. Dendritic cells (DCs) are specialized cells participating in the homeostasis of the immune response. Moreover, subsets of DCs, identified in humans, appear to have their respective competences in immune response modulation. Our objective is to purify from PBMC or to differentiate DC subsets from monocytes using several strategies and evaluate their IL10 secretion.

CD28 Function: A Balance of Costimulatory and Regulatory Signals

Both the recognition of MHC/antigen complex by the T-cell receptor and engagement of costimulatory molecules are necessary for efficient T-cell activation. CD28 has been widely recognized as the major costimulation pathway for naive T-cell activation, and the CD28/B7 pathway plays a central role in immune responses against pathogens, autoimmune diseases, and graft rejection. In this review, we will summarize evidence that CD28 is also prominent in the regulation of immune responses and the maintenance of peripheral tolerance. Indeed, CD28 engagement increases the expression of the down-modulatory molecule CTLA-4, induces the differentiation of Th2 cells that have a protective function in autoimmunity, and has an obligatory role in the homeostasis of regulatory T cells. Therefore, CD28/B7 interactions induce a balance of costimulatory and regulatory signals that have opposite outcomes on immune responses. This new perspective on CD28 function suggests that caution should be taken in the development of immunotherapies targeting costimulatory pathways.     

CTLA-4: a key regulatory point in the control of autoimmune disease

Summary: Chronic autoimmune disease in humans is the result of a failure to control autoreactive immune cells in the periphery. This control is largely achieved by inhibition of newly activated and memory cells. A number of negative immune regulatory pathways have been characterized. The cell surface coreceptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) has emerged as a critical attenuator of T-cell activation and an essential component of the regulatory systems that serve to maintain peripheral tolerance. CTLA-4 expression is induced on the surface of T cells after they have received a costimulatory signal from antigen-presenting cells (APCs) via engagement of CD28 on the T-cell surface. CTLA-4 attenuates this costimulation by competing for CD28 ligands and through direct effects on APCs via the same ligands utilized by CD28. A large number of genetic association studies suggest that the CTLA-4 gene is a locus of susceptibility to autoimmune disease. However, specific functional defects in the CTLA-4 gene in patients have not been identified to date. Elucidating the role of CTLA-4 in immune tolerance has also led to a number of therapeutic applications, particularly in the treatment of malignancy and autoimmune disease.     

Molecular mechanisms of T-cell tolerance

CD4(+) T cells are the master regulators of adaptive immune responses, and many autoimmune diseases arise due to a breakdown of self-tolerance in CD4(+) T cells. Activation of CD4(+) T cells is regulated by not only the binding of peptide-major histocompatibility complexes to T-cell receptor but also costimulatory signals from antigen-presenting cells. Recently, there has been progress in understanding the extracellular and intracellular mechanisms that are required for implementation and maintenance of T-cell tolerance. Understanding of the molecular mechanisms underlying T-cell tolerance will lead to development of pharmacological approaches either to promote the tolerance state in terms of autoimmunity or to break tolerance in cancer.