Contribution of dendritic cells to the autoimmune pathology of systemic lupus erythematosus

Summary

Systemic lupus erythematosus (SLE) is a heterogeneous disease in which excessive inflammation, autoantibodies and complement activation lead to multisystem tissue damage. The contribution of the individual genetic composition has been extensively studied, and several susceptibility genes related to immune pathways that participate in SLE pathogenesis have been identified. It has been proposed that SLE takes place when susceptibility factors interact with environmental stimuli leading to a deregulated immune response. Experimental evidence suggests that such events are related to the failure of T‐cell and B‐cell suppression mediated by defects in cell signalling, immune tolerance and apoptotic mechanism promoting autoimmunity. In addition, it has been reported that dendritic cells (DCs) from SLE patients, which are crucial in the modulation of peripheral tolerance to self‐antigens, show an increased ratio of activating/inhibitory receptors on their surfaces. This phenotype and an augmented expression of co‐stimulatory molecules is thought to be critical for disease pathogenesis. Accordingly, tolerogenic DCs can be a potential strategy for developing antigen‐specific therapies to reduce detrimental inflammation without causing systemic immunosuppression. In this review article we discuss the most relevant data relative to the contribution of DCs to the triggering of SLE.

Abbreviations

ANA

anti‐nuclear antibodies

APCs

antigen‐presenting cells

cDCs

conventional dendritic cells

DCs

dendritic cells

EAE

experimental autoimmune encephalomyelitis

IC

immune complex

IFN

interferon

IL

interleukin

IRF

interferon regulatory factor

PD‐1

programmed death 1

pDCs

plasmacytoid dendritic cells

SLAM

signalling lymphocyte activation molecule

SLE

systemic lupus erythematosus

Th

T helper

TLRs

Toll‐like receptors

tolDC

tolerogenic dendritic cells

Treg

regulatory T

Introduction

Immunological tolerance is crucial for the development and maintenance of functional and non‐harmful T and B cells.1 Although the aetiology of autoimmune diseases is unknown and may involve several factors, it can be initiated when specific gene products interact with environmental stimuli, resulting in a deregulated immune response.2, 3 Alteration in T‐cell and B‐cell signalling, immune tolerance and clearance of apoptotic cells may result in immune failure, leading to autoimmunity in susceptible individuals.2 Although physiological expression of inhibitory as well as activating receptors is fundamental for antigen‐presenting cells (APCs) to trigger a protective immune response or to promote immune tolerance, the precise molecular mechanisms responsible for peripheral T‐cell tolerance remain to be elucidated.4

Systemic lupus erythematosus (SLE) is a chronic and heterogeneous disease that majorly affects joints, kidneys, nervous systems, skin and mucosa, in which innate and adaptive immune cells are involved.5, 6, 7 It has been shown that dendritic cells (DCs) from patients with SLE exhibit an altered expression of CD40, CD86 and Fcγ receptors (FcγRs) compared with healthy controls.6 Similarly, it has been found that the anti‐inflammatory enzyme haemeoxygenase‐1 is less expressed in monocytes from SLE compared with healthy controls, suggesting that phenotypic alterations in APCs may impact T‐cell homeostasis, resulting in immune‐mediated diseases.6, 8 Recent advances in the modulation of DCs in vitro has increased the potential of cell‐based approaches for autoimmune disease treatment, in which immunotherapy is restricted to autoantigens involved in tissue damage.9, 10 Several autoimmune diseases show an imbalance in the homeostasis of DC subpopulations. Indeed, there is increasing evidence supporting the notion that DCs may play a key role in SLE pathogenesis.11 Herein, we discuss recent data concerning how phenotypic alterations in DCs may drive SLE.

DC phenotype in SLE patients

The diversity of DC function is the result of their complex differentiation and maturation mechanisms, which lead to the generation of specialized DCs that conform to wide subsets of different subpopulations.12 In addition, several findings related to a more immunogenic phenotype have been reported in DCs from SLE patients.6, 13 It was first reported the 1980s that patients with SLE had increased levels of serum interferon‐α (IFN‐α), which correlates with disease activity and anti‐dsDNA antibody titres.14, 15 A consistent finding is that the majority of patients with SLE show an increased expression of IFN‐α‐stimulated genes, which is known as IFN‐α signature.16

It has been reported that plasmacytoid DCs (pDCs) from patients with SLE show an enhanced expression of interferon regulatory factor 3 (IRF3) and IRF5 compared with healthy controls, which is associated with higher circulating levels of IFN‐α 13 (Fig. 1). Signalling lymphocyte activation molecule (SLAM) forms a family of receptors expressed by several immune cells and some polymorphisms in their locus have been associated with SLE diseases.17, 18 Interestingly, the expression of CD319 and CD48 (SLAM family receptors) is regulated by RNA containing immune complexes (ICs) and is diminished in pDCs from patients with SLE compared with healthy controls19 (Fig. 1). In addition, DCs from patients with active SLE show an altered homeostasis of the inducible programmed death ligand‐1 (PD‐L1), which plays a crucial role in immune tolerance and T‐cell suppression when binding to its ligand programmed death‐1 (PD‐1).20 Dendritic cells from patients with active SLE failed to induced PD‐L1 expression in vitro.20 Strikingly, DCs from SLE patients show a remarkable decrease of PD‐L1 expression during disease flares, whereas the expression of CD80/CD86 is increased. In contrast, DCs from SLE patients during remission show greater expression of PD‐L120 (Fig. 1).

Figure 1.

Human dendritic cell (DC) phenotype during systemic lupus erythematosus (SLE). DC modulation is mediated by different signals that may lead to DC activation and initiation of the immune response, or may lead to immunosuppression. DCs from SLE patients showed an increased expression of interferon‐α (IFN‐α), IFN regulatory factor 3 (IRF3), IRF5 and IFN‐induced genes, which correspond to the IFN signature. Similarly, SLE DCs show an increased expression of co‐stimulatory molecules, such as CD40, CD86 and activating FcγRs, showing a mature phenotype. In contrast, the expression of immunoregulatory molecules such as SLAM (CD319, CD48), the co‐inhibitor molecule PD‐L1 and the inhibitory FcγR are decreased in SLE. This mature‐type phenotype of DCs with higher expression of activating (and lower expression of inhibitory) signals may promote naive T‐cell priming and B‐cell activation, favouring a deregulated T helper type differentiation and antibody production.

Interestingly Mer, a surface immunoregulatory receptor involved in apoptotic cell recognition and removal, is increased in DCs from SLE patients.21 This phenotype correlates with serum levels of IFN‐α.22 Furthermore, prednisone‐treated SLE patients show higher expression of Mer in DCs than do patients that did not receive steroids, suggesting a role for Mer in inducing a tolerogenic response by DCs in the presence of apoptotic antigens22 (Fig. 1).

It has also been reported that patients with SLE show reduced numbers of conventional DCs (cDCs) in their bloodstream compared with healthy controls whereas pDCs were increased.23, 24 Both, cDCs and pDCs can sense pathogen‐associated molecular patterns by pathogen recognition receptors, such as Toll‐like receptor 7 (TLR7) and TLR9, contributing to protective immunity against viral and bacterial infections.25, 26, 27 TLR7 and TLR9 ligands (such as Imiquimod and CpG) induce the production of large amounts of type I IFN by pDCs via the IRF signalling pathway.28, 29 The fact that TLR7 ligand stimulation of pDCs induces high expression of interleukin‐1β (IL‐1β) and IL‐23, leading to a T helper 17 (Th17) differentiation, highlights the potential role of pDCs at modulating immunity and tolerance.27 In contrast, some studies suggest that DCs from patients with SLE produce fewer proinflammatory cytokines in response to CpG compared with DCs from healthy controls.30

Interestingly, it has been reported that serum from SLE patients induces monocytes from healthy controls to differentiate into DCs in a type I IFN‐dependent manner. Furthermore, authors demonstrated that IFN‐α and serum from patients with SLE induce the expression of CCR7 in monocytes from healthy subjects, suggesting that SLE serum may prime monocytes to migrate to lymphatic nodes such as DCs.31

We have shown that DCs from SLE patients show a higher expression of co‐stimulatory molecules such as CD40 and CD86, as well as an altered ratio of activating/inhibitory FcγRs compared with healthy controls, which may impact T‐cell priming leading to SLE6 (Fig. 1). However, some studies demonstrate that CD40 expression on SLE patient DCs can be decreased.30 All these data from human studies underscore the role of DCs in SLE pathogenesis and support the strategy of designing new therapies based on DC modulation and depletion.

Murine models of SLE to identify the role of DCs and IFN‐α in disease onset

Several murine strains have been reported to study the immunopathogenesis of SLE. These murine models of SLE include the F₁ hybrid between the New Zealand Black (NZB) and New Zealand White (NZW) strains (NZB/W F₁), the MRL.Fas^lpr, FcγRIIb knockout and BXSB/Yaa (TLR7 gene duplication) and the strains in which autoimmunity develops from polygenic factors, apoptosis failure, inhibitory receptor deficiency and gene duplication.32, 33, 34, 35

For instance, murine models have been key to determine the role of Blimp‐1 in SLE pathogenesis. In both murine models and humans, genome‐wide studies have determined that a polymorphism of Blimp‐1 can be associated with SLE susceptibility.36, 37 Plasmacytoid DCs stimulated with IFN‐α induced the expression of miRNAs that regulate Blimp‐1, suggesting that this mediator may be involved in SLE pathogenesis.38 Female mice lacking Blimp‐1 on DCs show an expansion of follicular helper T cells with an enhanced germinal center response and the development of anti‐nuclear antibodies (ANAs) and an SLE‐like syndrome, which is dependent on the production of IL‐6.36

Also, the relevant role of DCs in driving SLE pathogenesis is highlighted by the fact that DCs loaded with apoptotic cells could initiate an autoreactive immune response with the development of SLE‐like symptoms, such as ANAs and glomerulonephritis.39, 40, 41 Interestingly, IFN‐α can reduce the suppressive effect of apoptotic cells on DCs, promoting SLE pathogenesis through DC activation.42 In addition, the administration of adenovirus expressing IFN‐α to SLE NZB/NZW mice accelerated disease onset, increased serum levels of anti‐dsDNA antibodies being associated with increased B‐cell activating factor, IL‐6 and tumour necrosis factor‐α serum levels.43

In contrast, deleting DCs in SLE‐prone MRL.Fas^lpr mice ameliorates disease progression, and decreases inflammation and glomerulonephritis, highlighting the essential role of DCs in SLE pathogenesis and autoantibody development.11 Similarly, a transient ablation of pDCs in the BXSB/Yaa SLE mice ameliorates disease, and reduces lymphoproliferation and ANA development, which correlates with lower IFN‐α/β‐induced gene expression, suggesting a necessary role of DCs in SLE onset and progression.44 In addition, in NZB/W F₁ mice the subpopulation of DCs that produce IFN‐α shows an altered phenotype, consisting of a higher production of IL‐12 and expression of TLR9 mRNA.45

Dendritic cells from B6.NZMSle1/Sle2/Sle3 (a lupus murine model derived from NZB/W F₁) SLE‐prone mice show an overexpression of IFN‐responsive genes (IFN‐β and CXCL10) and members of the IFN signalling pathway (signal transducer and activator of transcription 1 and 2, and IRF7) that hyper‐respond to IFN‐α and TLR7‐TLR9 ligands.46 Similarly, DCs also show an IFN signature in vivo before SLE onset, suggesting that DCs may have a crucial role in SLE pathogenesis.46 In contrast, chronic administration of anti‐IFN‐α/β receptor antibodies to male BXSB/Yaa mice reduces autoimmunity, suggesting that IFN signalling is crucial for SLE pathology.47

Role of pathogen recognition receptors and immune complexes in SLE pathogenesis

Activation of TLRs could be triggered by either endogenous or foreign molecules, such as danger‐associated molecular patterns and pathogen‐associated molecular patterns, respectively. Some of most studied danger‐associated molecular patterns are mRNA, ssRNA, high mobility group box protein 1 (HMGB1), heat‐shock protein 60 (HSP 60), fibronectin, fibrinogen, hyaluronic acid fragments and chromatin.48, 49, 50 Plasmacytoid DCs activated by CpG suppress the function of regulatory T (Treg) cells, induce inflammatory cytokines (IL‐6, transforming growth factor‐β and IFN‐α) and promote Th17 polarization.51, 52, 53, 54, 55, 56 Danger‐associated molecular patterns binding to TLR initiate an innate immune response that contributes to tissue damage, as observed when circulating ICs containing self nucleic acids stimulate DCs and promote cell activation and tissue injury in SLE.57 Strikingly, HMGB1 bound to circulating nucleosome‐containing ICs triggers TLR2 on APCs and induces pathogenic anti‐dsDNA in SLE49 (Fig. 2). It has been shown that ICs containing nucleosomes derived from the plasma of patients with SLE carry HMGB1 tightly attached to the chromatin of apoptotic cells.58, 59 These HMGB1‐loaded ICs induce the secretion of IL‐1β, IL‐6, IL‐10 and tumour necrosis factor‐α and the expression of co‐stimulatory molecules in DCs via TLR258 (Fig. 2).

Figure 2.

Hypothetical circuits and interactions between dendritic cells (DCs), neutrophils and B cells in systemic lupus erythematosus (SLE). Hypothetical circuits, interactions and mechanism between DCs, neutrophils, B cells and the stimulatory effects of nucleic acid containing immune complexes (ICs). Apoptotic antigens are recognized by autoantibodies present in SLE‐susceptible hosts. ICs containing self‐DNA or ‐RNA are internalized by FcγRs targeting Toll‐like receptor 9/7 (TLR9/7) expressing endosomes, a process enhanced by the presence of LL‐37. Extracellular ICs containing self‐DNA or ‐RNA need to be transported into TLR7/9‐containing endosomes to fully activate DCs. TLR9/7 ligation promotes DC activation with production of pro‐inflammatory cytokines, such as interferon‐α (IFN‐α), interleukin‐6 (IL‐6), IL‐12 and IL‐23, which may interact with B cells inducing cellular activation, plasma cell differentiation and antibody production. Also, ICs containing HMGB1 activate DCs by a TLR2‐dependent mechanism. The recognition of autoantibodies against neutrophil peptides (LL‐37) exposed on the cell surface may trigger neutrophil extracellular trap (NET)‐osis with the subsequent releasing of nuclear antigens and new ICs formation when interacting with anti‐nuclear antibodies produced by stimulated plasma cells.

It has also been reported that nucleosomes induce neutrophil activation, IL‐8 secretion and increased phagocytic activity independent of IC formation.60 Neutrophils from patients with SLE are sensitive to nucleosome‐induced activation, suggesting that nucleosomes may link innate immunity with loss of peripheral tolerance during SLE pathogenesis.60

Similarly, the activation of autoreactive B cells is mediated by DNA‐containing ICs upon ligation of both B‐cell receptor and TLR9, enhancing the immune response and promoting the development of SLE.50 To be recognized by TLR9, DNA‐ICs need to be taken up by cells through the endosome pathway and translocated to TLR9‐expressing vesicles, a process that might be mediated by FcγRIIa. Subsequently, DNA‐containing ICs are able to activate TLR9 in pDCs, leading to the production of IFN‐α 61 (Fig. 2). Although TLR9 promotes pDC activation and anti‐DNA autoantibody development in lupus mouse models, major features of lupus disease such as hypergammaglobulinaemia and glomerulonephritis are also observed in TLR9‐deficient mice.62, 63 Remarkably, the critical role of TLR9 in the development of anti‐DNA antibodies is supported by the fact that targeting TLR9 in different SLE‐prone mice decreases autoreactivity.64, 65 Similarly, TLR7 expression is necessary for the development of anti‐Smith (Sm) autoantibodies in the MRL. Fas^lpr mouse model, which is consistent with the presence of anti‐Sm antibodies in SLE patients63, 66 (Fig. 2). These mechanisms were corroborated in the MyD88‐deficient mice, in which anti‐DNA and anti‐RNA antibodies were absent, suggesting that TLRs could be involved in this process.67, 68

Recently, a new mechanism for IC pathogenesis has been described. The antimicrobial peptide LL‐37, a cathelicidin polypeptide, binds self‐DNA and directs this molecule to TLR9‐containing endosomes in DCs.69 TLR9 ligation leads to DC activation, IFN‐α production and possibly IC‐dependent pathogenesis during SLE and systemic autoimmune disease.69, 70, 71, 72

Hydroxychloroquine (HCQ), which has been demonstrated to decrease SLE flares and mortality, is able to prevent pDC activation and IFN‐α production by limiting acidification and maturation of endosomes after stimulation with ligands for TLR7 and TLR9.73, 74 HCQ may inhibit TLR ligation of internalized self‐DNA/RNA ICs.61 New compounds are being studied to antagonize TLR7 and TLR9 signalling by IC containing DNA/RNA, which accumulate in TLR‐expressing endosomes and decrease the affinity of nucleic acid to TLR7 and TLR9.75, 76 Interestingly, when female MRL.Fas^lpr mice were treated with the drug E6446, a TLR9 antagonist, development of ANAs was suppressed in a dose‐dependent manner.75

Immune cells, including DCs, differentially express several FcγRs, such as FcγRI, FcγRIIa, FcγRIIb, FcγRIIc, FcγRIIIa and FcγRIIIb in humans, and FcγRI, FcγRIIb, FcγRIII and FcγRIV in mice.77, 78, 79 In both SLE patients and murine models of this disease, ICs binding to FcγRs on DCs induce migration to lymph nodes, up‐regulating CCR7 expression, which is essential in this process to expand the immune response to self‐antigens captured by DCs.80 Furthermore, absence of the inhibitory receptor FcγRIIb enhanced this process, suggesting that fine tuning between activating/inhibitory signalling from different FcγRs and co‐stimulatory molecules is crucial for keeping the homeostasis of the immune system.80, 81

As previously mentioned, DNA/RNA containing ICs lead to DC/B‐cell activation and might initiate SLE.61, 82, 83 The pDCs are activated through TLR signalling by ICs from SLE patients containing IgG and DNA/RNA after FcγRIIa‐mediated internalization. In contrast, IgGs from healthy controls do not induce TLR‐mediated activation of DCs.61, 83, 84

Similarly, ICs binding to FcγRI and FcγRIII (activating receptors) on DCs leads to maturation. In contrast, engagement of FcγRIIb, an inhibitory receptor, keeps DCs in an immature state.85, 86, 87 The inhibitory FcγRIIb is expressed by innate and adaptive immune cells including monocytes, DCs and B cells.88 FcγRIIb deficiency in DCs improves T‐cell priming and leads to protective tumour immunity.87 Interestingly, FγRIIb knockout mice develop spontaneous SLE‐like disease with production of anti‐DNA antibodies, IC deposition in kidneys and glomerulonephritis, as well as increased susceptibility to other autoimmune diseases, such as collagen‐induced arthritis and experimental autoimmune encephalomyelitis (EAE).89 In contrast, FcγRIII knockout mice are more resistant to EAE induction than WT mice.81

As discussed above, cDCs from SLE patients show a higher ratio of activating/inhibitory FcγRs expression, suggesting that these cells are more susceptible to being activated by ICs.6 Furthermore, the increased ratio of activating/inhibitory FcγRs in DCs correlates with the activity of SLE6 (Fig. 1).

The pentraxin C‐reactive protein is a serum molecule that binds phosphorilcolin on apoptotic cells.90 Interestingly, C‐reactive protein can prevent DC activation and IFN‐α production after stimulation with ICs containing anti‐U1 RNP‐snRNP and anti‐DNA‐DNA.91 In addition, C‐reactive protein may modify intracellular trafficking of endocytosed self‐antigens to prevent TLR engagement in DCs.91

In addition, it has been reported that sera from SLE patients show ICs containing neutrophil extracellular trap components, such as antimicrobial peptides and self‐DNA that could promote pDC activation.92 Interestingly, the production of neutrophil extracellular traps by SLE neutrophils is higher than in healthy donors, suggesting that this mechanism enhances DC immunogenicity.93 The underlying mechanism could be mediated in the production of anti‐LL37 autoantibodies that bind surface‐exposed specific ligands and trigger the release of neutrophil extracellular traps93 (Fig. 2).

The selective engagement of inhibitory FcγRs with ICs containing immunodominant self‐antigens to modulate DC function may restore antigen‐specific tolerance in immune‐mediated and autoimmune diseases.94

Control of DC immunogenicity by co‐stimulatory molecules

In autoimmune susceptible individuals, central and/or peripheral tolerance could be experimentally overcome by an altered APC function that primes self‐reactive T cells.1 The expression of a wide spectrum of co‐stimulatory molecules by DCs constitutes a major aspect in T‐cell activation.95, 96 CD28 (T cells)/CD80/CD86 (DCs) interaction is crucial for clonal expansion and survival of antigen‐specific T cells by promoting optimal mRNA stabilization and IL‐2 production.97, 98 As previously mentioned, our group and others have shown that cDCs obtained from SLE patients show greater expression of co‐stimulatory molecules (such as CD86 and CD40) than healthy controls.99, 100 These data suggest a more immunogenic state for these APCs and an increased capacity to prime T cells.6

Similarly, activating signals from the interaction between CD40L and CD40 are important for T‐cell/B‐cell cooperation and T‐cell/DC crosstalk with subsequent cellular activation.101 The relevance of this interaction is highlighted by the fact that pharmacological blockade with anti‐CD40L monoclonal antibodies ameliorates autoimmune diseases.102, 103 OX40 ligand (OX40L) is an inducible co‐stimulatory molecule expressed on APCs and non‐immune cells that binds to OX40 on T cells and promotes cellular survival as well as Th2 polarization.104, 105 Similarly as observed with CD40/CD40L interaction, a deficiency (or blockade) of OX40/OX40L delays the onset of autoimmune symptoms by inhibiting the expression of the effector cytokines IFN‐γ and IL‐4.106 Dendritic cells also express ICOS‐L (B7‐H2), which binds to ICOS on T cells, partially modulating T‐cell fate.107, 108 The blockade of ICOS/ICOS‐L interaction limits the production of IL‐10 release without affecting IL‐2 secretion, suggesting an active role in immune regulation and peripheral tolerance.109 Interestingly a Treg cell subpopulation expresses ICOS, suggesting that this molecule is also involved in Treg–DC crosstalk and immune suppression mediated by IL‐10.110, 111 ICOS‐L deficiency can impair Th2 bias, mainly limiting IL‐4 and IL‐10 production by T cells.112

Flt3L, a growth factor involved in development and differentiation of DCs, has also been associated with autoimmune pathogenesis, such as rheumatoid arthritis. In this disease, Flt3L is increased in synovial fluid and correlated with active lesions.113, 114, 115, 116

DC modulation by co‐inhibitory molecules

The most studied interaction between co‐inhibitors on T cells and DCs is the PD‐1/PD‐L1 (and PD‐L2) axis.117, 118, 119 PD‐L1 expressed on DCs drives T‐cell inhibition.120, 121 In addition, Treg cell development is also enhanced by PD‐L1 expression on DCs, which may prevent autoimmunity as observed with PD‐1⁺ Treg cells during EAE.117, 122 Similarly, PD‐1/PD‐L1 interaction can suppress Th1 differentiation during EAE.123 Interestingly, PD‐1/PD‐L1 deficiency in mice promotes IFN‐γ overproduction by T cells and the activation of CD8⁺ T‐cell responses, making these animals more susceptible to autoimmunity and SLE‐like disease.124, 125 TIM‐3, an exhausted cell marker, has been reported to display an inhibitory function on DCs that are resistant to maturation.126, 127 TIM‐3 deficiency in DCs leads to overproduction of pro‐inflammatory cytokines after stimulation with TLR ligands.126

Because no single DC/T‐cell interaction is fully immunogenic or tolerogenic, further research is needed to understand the activation/inhibitory signalling network controlling DC immunogenicity. Strategies based on interfering DC/T‐cell interactions are of clinical interest mainly in autoimmune diseases, where the immune response is deregulated.

DC interaction with B cells and Treg cells during SLE pathogenesis

Dendritic cells could also interact with the main factor responsible for tissue damage during SLE pathogenesis: B cells.5 The DCs and macrophages efficiently transfer conformational antigens such as particulates and ICs and present them to naive B cells in lymphoid organs.128 In addition, it has been shown that DCs induced surface IgA expression on B cells, which is enhanced by IL‐10 and transforming growth factor‐β, suggesting that DCs directly modulate B‐cell activation and differentiation.129 Similarly, activated DCs in SLE‐prone mice enhanced B‐cell proliferation, IL‐6 and IFN‐γ production and ANA production.130, 131 Moreover, SLE DCs induce chemokine receptor expression on B cells that target them to initiate germinal centre responses with subsequent IgG production.130 The pDCs can induce plasma cell differentiation and promote antibody secretion through type 1 IFN and CD70.132 Also, activated DCs produce BLyS and APRIL, which are key mediators in B‐cell homeostasis and may induce immunoglobulin class‐switch DNA recombination.133

Recently, it has been reported that a subset of splenic regulatory CD11b^hi Ia^low DCs induce B‐cell differentiation into an IL‐10 producing regulatory CD19^hi FcγIIb^hi B cells, which could prevent T‐cell response via IL‐10.134 These data highlight the potential of tolerogenic DC (tolDC) ‐based therapy to promote immunesuppression in immunoglobulin‐mediated diseases such as SLE.

It has been reported that activated DCs impair Treg function.135 TLR ligand stimulation of DCs prevents Treg cell suppression, leading to T‐cell priming by an IL‐6‐dependent mechanism.135, 136 During SLE, elevated cytokines such as IFN‐α, IL‐6 and IL‐18 might affect the capacity of Treg cells to modulate DC immunogenicity, promoting T‐cell proliferation.137, 138, 139 Accordingly, pharmacological blockade of IL‐6 by monoclonal antibodies in experimental SLE mouse models improves clinical symptoms, highlighting the role of this pro‐inflammatory cytokine in DC interaction with B cells and Treg cells during systemic autoimmunity.140

Targeting DC function as a therapeutic approach for autoimmune diseases

Pharmacological targeting of DCs to restore tolerance by either Treg cell promotion or phenotype skewing of autoantigen‐specific T‐cell responses has been studied.141 For T‐cell tolerance induction, DCs might exhibit low expression of different surface molecules involved in T‐cell priming, such as MHC‐II, CD40, CD80, CD86, and a reduced production of pro‐inflammatory cytokines while promoting the secretion of the anti‐inflammatory cytokine IL‐10.142, 143 The generation of tolDCs could be assessed by DC modulation with chemicals, biological agents and gene therapy, mainly targeting the maturation capacity.143, 144

Dexamethasone is one of the most studied inhibitors of DC maturation. Dexamethasone‐treated DCs show a semi‐mature phenotype with low expression of co‐stimulatory molecules, such as MHC‐II and CD86; are resistant to maturation (keeping IL‐10 production unaffected); induce Treg cell differentiation; suppress T‐cell priming; and modulate nuclear factor‐κB signalling.10, 143, 145 Similarly, 1α,25‐dihydroxyvitamin D3 and acetylsalicylic acid (aspirin) induce tolDCs with low co‐stimulatory molecule expression and resistant to maturation.10, 146 However, due to the capacity of dexamethasone and 1α,25‐dihydroxyvitamin D3 to induce cell death that could cause immune suppression, further research is required to confirm a direct role of these drugs in DC tolerance induction.147, 148, 149

Rapamycin‐ or rosiglitazone‐treated DCs show a tolerogenic phenotype that induces Treg cell differentiation and prevents pro‐inflammatory cytokine production.150 Dendritic cells treated with BAY‐117085 and andrographolide, two nuclear factor‐κB blockers, show a tolerogenic phenotype inducing Treg cell expansion and ameliorating autoimmune diseases.151, 152 Our group has reported that the chronic administration of andrographolide and rosiglitazone to FcγRIIb knockout mice prevents SLE onset. Splenic DCs from treated mice show lower CD40/CD86 expression, ANA levels and IC deposition in kidneys than untreated mice.153 Cobalt protoporphyrin promotes a tolerogenic phenotype on DCs by inducing the expression of hemeoxygenase‐1.154

Some biological compounds such as anti‐inflammatory cytokines are potent tolerogenic inducers. DCs treated with IL‐10 show a potent tolerogenic phenotype preventing T‐cell proliferation, decreasing pro‐inflammatory cytokine production, while increasing the expression of immunoglobulin‐like transcript‐2, which is an inhibitory receptor.144, 155, 156, 157 Similarly, transforming growth factor‐β induces tolDCs and prevents the expression of CD80/CD86, IL‐12 production and improved survival of grafted β‐cell islets in a diabetes model.158

Interference RNA and gene therapy technology provide a wide and new spectrum of therapeutic strategies for autoimmune diseases.159 Gene silencing of different pro‐inflammatory molecules, such as CD40, CD80, CD86 and IL‐12 in DCs promotes a tolerogenic phenotype that may improve autoimmune diseases, mainly by suppressing the activation of T and B cells and expanding Treg cell subsets.159, 160, 161 These data underscore the potential use of DC manipulation with lentivirus transduction expressing interference RNA for co‐stimulatory molecules, to induce a tolerogenic phenotype.162, 163

Although there are several studies reporting the generation of tolDCs from patients with multiple sclerosis, which induce hyporesponsiveness in myelin‐specific autologous T cells, to date there are no clinical trials with tolDC therapy in multiple sclerosis.164, 165, 166 Nevertheless, this strategy is a promising cell therapy for the treatment of immune‐mediated diseases.

Targeting DCs in clinical application

Advances in immune intervention with tolDCs, as well as the identification of the immunodominant self‐antigen in SLE, are crucial for designing an efficient and specific therapy based on autologous tolDCs.167 A Phase I randomized placebo‐controlled trial using tolDCs as a therapy for type 1 diabetes, an organ‐specific autoimmune disease, has already been published (ClinicalTrials.gov identifier NCT00445913).168 This approach was based on the administration of tolDCs induced ex vivo by the administration of anti‐sense oligonucleotides for the co‐stimulatory molecules CD40, CD80, and CD86 in patients with type 1 diabetes.168 These researchers reported that the tolDC transfer was well‐tolerated, with no adverse events after 1 year of follow up.168

A Phase I clinical trial in patients with rheumatoid arthritis evaluating the feasibility and safety of autologous tolDC therapy and describing its clinical and immune effects is being conducted. Autologous tolDCs were generated by stimulating DCs with BAY11‐7082, a nuclear factor‐κB inhibitor, and loading DCs with different citrullinated peptides. Only mild adverse effects were reported and, strikingly, disease activity improved in the treated group.169, 170 Another randomized, placebo‐controlled Phase I clinical trial based on tolDC therapy in patients with rheumatoid arthritis is currently recruiting participants [Autologous Tolerogenic Dendritic Cells for RheumatoidArthritis(AutoDECRA); ClinicalTrials.gov Identifier: NCT01352858]. In this study TolDCs from blood monocytes will be generated with dexamethasone and 1α,25‐dihydroxyvitamin D3 and will be administered to affected joints, using arthroscopy.

It has been reported that IL‐10 treatment of DCs from SLE patients may promote a tolerogenic phenotype, supporting the feasibility of using tolDCs in therapeutic clinical trials for patients with SLE.99

Concluding remarks

Development of more efficient therapies for SLE, avoiding systemic immunosuppression, is one of the major goals of rheumatologists. The inhibitory effects of tolDCs in T‐cell priming and B‐cell differentiation underlie the role of DCs in maintaining peripheral tolerance, promoting its potential use in clinical studies. The therapeutic efficacy of a tolDC‐based approach during immune pathogenesis in experimental autoimmune models such as EAE, type 1 diabetes and rheumatoid arthritis highlights the value of testing this strategy in human SLE. The achievement of tolDC intervention in SLE as a therapeutic approach includes specificity avoiding adverse effects from systemic immunosuppression. Identification of the immunodominant self‐antigens involved in SLE pathogenesis, as well as the understanding of DC function are crucial for designing more efficient therapies targeting DCs, which may have a major clinical impact in chronic autoimmune diseases such as SLE.

Disclosures

The authors declare no financial or commercial conflict of interest.