Sniping the Scout: Targeting the key molecules in dendritic cell functions for treatment of autoimmune diseases

Abstract

Dendritic cells (DCs) are a power tool for manipulating immune system. They play important roles in the induction of immunity as well as inducing intrathymic and peripheral tolerance. After generated from stem cells in the bone marrow, DCs traffic to the peripheral tissues, where they capture and process antigens, express lymphocyte co-stimulators, migrate to the secondary lymph organs and present the processed antigen to naive T cells to either activate or tolerize them.

These processes are modulated subtly and influenced by various factors. Aberrant regulation of the processes may cause autoimmunity. Investigation into the biology of DCs and the molecules and mechanisms that regulate them helps us understanding the pathogenesis of autoimmune diseases and reveals numerous steps for pharmacological manipulation. In this review, we made a sketch line of the critical events of DC biology that are potential pharmacologic targets for the treatment of autoimmune diseases.

Introduction

The immune system provides protection against infection and maintains immune homeostasis. Dendritic cells (DCs) play a central role in the induction of immunity as well as inducing intrathymic and peripheral tolerance. Clinical autoimmunity arises as a result of aberrant regulation of the immune responses [1].

The development of autoimmune diseases, such as rheumatoid arthritis (RA), systemic lupus erythematosus(SLE), requires three different but related processes: disruption of immune tolerance, development of chronic inflammation in one or several organs, and tissue destruction and their harmful effects [2].

Classical treatment for autoimmune diseases is usually a combination of anti-inflammatory, immunosuppressive drugs and biological agents that are designed to block the activity of pro-inflammatory cytokines and lymphocytes as the primary cellular targets.

However, the strategy is not so effective in controlling symptoms and disease progression. Accumulating evidence indicates that the central role of DCs in immunity and tolerance is an indispensable component in the pathogenesis of autoimmune diseases [3].

Therefore, targeting DC functions maybe a potential and valuable therapy for the control of autoimmune diseases.

DCs represent a heterogeneous population of uniquely well-equipped antigen-presenting cells (APCs) with distinct developmental origins, phenotype markers and immunological functions. After generated from stem cells in the bone marrow, DCs are generally immature and traffic to the peripheral tissues, where they undergo several molecular events (namely antigen uptake, phenotypic and functional maturation, migration to the secondary lymphoid organs and present acquired antigens to naive T lymphocytes).

Then, they instruct naive T cells to differentiate toward T helper (Th) -1(Th1), Th2, Th17 or regulatory T cell (Treg) cells as well as to promote maturation of antibody-producing B cells [4, 5]. These processes enable DCs to behave as critical sentinels to eliminate invading pathogens, to induce and regulate of most adaptive immune responses and to maintain tolerance and immune homeostasis [6-9].

Disregulation of these processes may cause imbalance of immune responses and lead to autoimmunity. Animal studies indicate that DCs play important roles in the initiation and perpetuation of autoimmune diseases [10]. In this review, we present a description of key molecules and mechanisms that regulate DC differentiation, endocytosis, maturation and migration, especially focusing on their significances or potentials as pharmacological drugs or agents in anti-inflammation or immunosuppression for autoimmune diseases.

Molecular control of DC biology in autoimmunity

DC differentiation in autoimmunity

After several decades of research, we have known that DCs stem from a hematopoietic lineage distinct from other leukocytes, establishing the DC system as an unique hematopoietic branch [10, 11].

Accumulated evidence suggests different DC lineages play different roles in the maintenance of immune homeostasis via induction of immune tolerance and regulation. Under the mediation of some factors, DC precursors can be instructed to differentiate into immunogenic DCs and tolerogenic DCs.

Immunogenic DCs have the ability of up-regulation the expression of co-stimulatory molecules (CD40, CD80 and CD86), MHC class II molecules and proinflammatory cytokines such as interleukin-12 (IL-12), tumor necrosis factor (TNF-α) and IL-6, and presenting antigens to specific T cells to elicit adaptive immune responses. Conversely, although tolerogenic DCs retain the ability of presenting antigens to specific T cells, they downregulate the expression of co-stimulatory molecules and proinflammatory cytokines, upregulate the expression of inhibitory molecules and anti-inflammatory cytokines and are resistant to maturation-inducing signals.

The process is crucial for maintenance of immune homeostasis and control of autoimmune disorders. Hence, molecular control of DC differentiation maybe the pharmacological potential targets for the control of autoimmune diseases. Previous reviews have reported that pharmacological effects on DC differentiation and expansion can be achieved by immunomodulatory drugs, such as corticosteroids [12, 13], vitamin D3 [14-16], or corticosteroids plus vitamin D3 [13, 17-22], rapamycin [23-27], aspirin [28-30], N-acetyl-L-cysteine [31], mycophenolate mofetil [32, 33], butyric acid [34-36], rosiglitazone [37, 38], rabeximod [39] , n-3 fatty acid [40] and serial chinese herbal medicine [41].

These agents that have been widely used in the treatment of autoimmune diseases are previously reported to block DC differentiation quantitatively by antagonizing the effects of several key molecules, such as hematopoietic cytokines. Generally different hematopoietic molecules control the differentiation and expansion of bone-marrow(BM) cells toward DC specific lineage and maintain DC homeostasis in the periphery [10]. Here, we will present key hematopoietic cytokines that control DC differentiation and their significance in the treatment of autoimmune diseases.

FMS-like tyrosine kinase-3 Ligand (Flt3L) is a key mediators for DC generation [42]. Loss of Flt3 expression in hematopoietic progenitors correlates with the loss of DC differentiation potential [43, 44]. Treatment with Flt3L tyrosine kinase inhibitors or genetic deletion of Flt3L gene in mice leads to 10-fold decrease of pDCs and DCs in lymphoid organs [45], whereas enforcement of Flt3L expression in progenitors that lack the ability to develop into DC restores some DC developmental potential [46].

Prior studies showed that Flt3L was associated with autoimmune pathogenesis. In rheumatoid arthritis, Flt3L is increased in synovial fluid and correlated with active lesions [47]. In collagen-induced arthritis, Flt3L deficiency mice showed a marked increase in the scores and incidence of arthritis in both acute and chronic phases of CIA model mice as compared with wild type mice [48].

A Flt3-Flt3L pathway inhibitor sunitinib can reduce the intensity of synovitis and the incidence of bone destruction in bovine serum albumin (mBSA)-induced arthritis, which was achieved partly by the inhibition on DC differentiation [49].

Whereas increasing the number of DCs by Flt3L injection leads to an increase in the number of regulatory T cells, which contributes to the protection of mice from developing type 1diabetes(T1D) [50]. Due to the dual effects of Flt3L in both promoting and suppressing autoimmunity, the effects of Flt3L on DC precursors at their different developmental stages during the progress of autoimmune diseases need further investigation.

Granulocyte-macrophage colony stimulating factor(GM-CSF), a hematopoietic growth factor that regulates the differentiation of the myeloid lineage genitor cells, plays a critical role in the differentiation of mouse and human hematopoietic progenitors and monocytes into DCs. Accumulated evidence has shown that GM-CSF affects DCs on multiple levels during the pathogenesis of autoimmune diseases [51-54].

Under the influence of elevated GM-CSF, circulating inflammatory monocytes develops into inflammatory DCs that can present self-antigens to T cells and elicit autoimmune responses [55, 56]. Moreover, after antigen uptake and stimulation by the GM-CSF, DCs acquire enhanced life span and presentation ability to active T cells in the lymph nodes and cause autoimmunity [55].

Prior experiments in vitro showed that the GM-CSF stimulated monocyte-derived DC (moDC) maintained their inflammatory property and increased the secretion of pro-inflammatory cytokines TNF-α and IL-6 [56, 57]. And GM-CSF could stimulate immature myeloid cells to differentiate into DCs in the central nervous system (CNS) and exacerbate experimental autoimmune encephalomyelitis (EAE) [58, 59].

Whereas apart from its key roles in the initiation and progression of DC-mediated autoimmunity, GM-CSF also suppresses the immune responses via its effect on DCs. Previous studies have reported that low-dose GM-CSF prevents the development of various autoimmune diseases in mice, through induction of tolerogenic CD8- myeloid DCs from bone marrow precursors and expansion of Foxp3+Tregs [60, 61].

Due to crucial role of GM-CSF in the differentiation and expansion of DCs, currently three anti-GM-CSF antibodies (CAM-3001, MOR103 and KB003) are tested for the pre-clinical treatment of diseases such as multiple sclerosis(MS), RA and asthma. The results showed rapid and significant beneficial effects on autoimmune diseases with no or less unexpected safety concerns [62-64].

Macrophage colony-stimulating factor (M-CSF), also named CSF-1, is a hematopoietic cytokine that mediates the proliferation and differentiation of macrophages [65]. And, M-CSF binds specifically to its receptor(CSF-1R) and partially regulates the differentiation or survival of nonlymphoid DCs [66, 67].

Epidermal langerhans cell (LCs) are totally absent in Csf-1R deficiency mice [68]. In the presence of M-CSF, circulating inflammatory monocytes develop into tolerogenic DCs that maintain the ability of antigen presentation, whereas downregulate expression of co-stimulatory molecules and proinflammatory cytokines [69]. In addition, injection of M-CSF increases both lymphoid organ pDCs and DCs in mice [70, 71].

Taken together, given their crucial roles in DC and autoimmunity, hematopoietic cytokines(e.g Flt3L, GM-CSF, M-CSF) can serve as potential targets to affect DC genesis as well as functions to prevent the development of autoimmune disease.

DC endocytosis in autoimmunity

Haematopoietic stem cells differentiate into immature DCs that are recruited to peripheral tissues, where they continuously take up and process antigens by different endosomal pathways [11]. Previous studies have reported that several immunosuppressive mediators potently manipulate the function of DC endocytosis via distinct mechanisms.

Some agents, like corticosteroids [12], vitamin D3 [14, 15] ,aspirin [34] and fumarates [72] inhibit phenotypic and functional DC maturation, and consequently enhance endocytic capacity. Whereas some other immunosuppressors suppress DC endocytosis by down-regulating the expression of DC endocytosis receptors in a DC-maturation-independent manner.

The main agents are rapamycin, sanglifehrin, cyclosporine A (CsA) [34, 73, 74] and quercetin [75]. In the steady state, immature DCs selectively express plenty of endocytosis receptors, such as Fcγ receptors(FcγRs), mannose receptor(MRs), and heat shock protein receptors, which could be used to target antigens for processing and presentation in vivo [8].

Fcγ receptors(FcγRs), a family of endocytotic receptors for the Fc portion of immunoglobulin, confer the protective effects of the immune system by recognition of IgG bound pathogens and play a critical role in autoimmune diseases [76]. Both human and mouse immature DCs express several FcγRs, and the activating FcγRs(FcγR and FcγRⅢ) enhanc DC activation via immunoreceptor tyrosine-based activation motif (ITAM) ,while the inhibitory FcR (FcγR ) regulates DC function to promote Treg induction but inhibit effector T cell responses via immunoreceptor tyrosine-based inhibitory motif (ITIM) [77, 78].

Prior studies suggest that activating FcγR(FcγR and FcγRⅢ) deficiency protects from autoimmune diseases, such as arthritis and lupus [79]. TG19320, a tetrameric peptide interfere with IgG/FcγR interaction, attenuates kidney damage and prolongs the survival rate of lupus-prone MRL/lpr mice [80]. Moreover, mouse anti-human FcγRⅢ mAb 3G8 has been used to successfully treat immune thrombocytopenia (ITP) [81, 82].

Conversely, inhibitory FcR(FcγR Ⅱ ) suppresses the expression of proinflammatory cytokines of DCs stimulated by toll-like receptor (TLR) 4 via lipid kinase phosphoinositied 3-OH kinase(PI3K)/AKT signaling pathway [83]. And FcγR Ⅱ -overexpressing DCs dramatically prevents glomerulonephritis and enhances the survival rate of lupus-prone MRL/lpr mice [84].

Additionally, FcγRⅡspecific small chemical entities suppress arthritis development in a collagen-induced arthritis(CIA) model, an suppressive effect even stronger than methotrexate(MTX), a classical drug for RA in human [85]. Currently, FcγRs are appealing targets in the treatment of inflammatory autoimmune diseases. Targeting approaches include blocking activating FcγR and activating inhibitory Fc γ RⅡ.

DEC205 is an endocytotic receptor and highly expressed on several subsets of DCs [86]. Accumulating evidence suggests that DEC205 plays an important role for the induction of immunity and tolerance [87].

Prior studies have suggested that selective delivery of antigens to DEC205 on steady-state DCs confers a powerful method to induce antigen-specific T cell tolerance [88]. And recombinant anti-DEC205 Abs fused to disease-relevant autoantigens have been developed and used successfully for tolerogenic protection from autoimmunity in mouse models of autoimmune diabetes, encephalomyelitis and arthritis [89-91].

Dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN) is a typeⅡC-type lectin receptor that is abundantly expressed on DCs, especially immature DCs. Apart from being an adhesion molecule, DC-SIGN has emerged as a key player in the regulation of immune responses by recognition with several endogenous and exogenous antigens [92, 93]. As DC-specific phenotypic marker, DC-SIGN commonly recognizes intestinal commensal, which promotes DC induced generation of Tregs and prevents the development of T cell-dependent colitis [94]. Hence, DC-SIGN maybe a potential and valuable target on DC endocytosis in autoimmunity.

DC maturation in autoimmunity

DCs are specialized sentinels responsible for coordinating innate and adaptive immunity. DC function is dependent upon their sensitivity to environmental inflammatory signs that promotes cellular maturation, phenotype alteration, activation and migration. We now appreciate that various molecules, signals, and mechanisms control the maturation of steady-state DC. Corruption of these steady-state operatives has diverse immunological consequences and pinpoints DCs as potent therapy targets of autoimmune diseases [95].

DC migration in autoimmunity

The ability of DCs to initiate and orchestrate adaptive immune responses is a consequence of their localization within tissues and their specialized capacity for migration. Accumulating evidence indicates that trafficking molecule matrix metalloproteinases (MMPs) [182, 183], chemokines and their receptors [184] and adhesion molecules [185] play critical roles in orchestrating DC migration. Once a mobilization signal has triggered detachment from parenchymal tissues, maturing DC secrete MMPs to disintegrate extracellular matrix (ECM) barriers.

Then under the regulation of chemokines and adhesion molecules, DCs traffic to secondary lymphoid organs to present the antigen to T cells and elicit the adaptive immune responses, and consequently result in autoimmune diseases.

Hence, pharmacological inhibition on DC migration will contribute to the prevention of autoimmune diseases. Prior studies have reported that pharmacological effects on DC migration can be achieved by immunomodulatory agents, including immunosuppressant such as corticosteroids [34, 186], vitamin D3 [34], CsA [34, 73, 74], mycophenolate [187] and leflunomide [188, 189]; Chinese herbal medicine such as triptolide [190], apigenin [191] and quercetin [75]; cytokines such as IL-10 [192], IL-21 [193], M-CSF [194].

These agents modulate DC migration primarily via control of trafficking molecules. Here, we review the key molecular traffic signals that govern the migration of DCs and provide novel insights into the significance of DC migration in the treatment of autoimmune diseases.

Others molecules

Factors present in inflammatory sites, such as lipid mediators, particularly prostaglandin E2 (PGE2) [238], and the high mobility group box-1 (HMGB1) protein [239], have been shown to induce migration of maturing DCs. The role of PGE2 in DC migration is strongly supported by the fact that mice deficient in the PGE2 receptor EP4 or wild-type mice treated with an EP4 antagonist exhibit reduced DC migration to lymph nodes [240].

The PGE2-induced DC migration is mediated through up-regulates MMP-9 expression, including both secreted and membrane- bound MMP-9 [183]. Besides, Faure-Andre and colleagues recently noted that DC migration was regulated by Ii, a key intracellular chaperone of MHC class II molecules that directs MHC class II endosomal localization and enables peptide loading. Ii-mediated association and activation of myosin light chain, which mediates cell migration, may enable DCs to effectively probe the tissue microenvironment [241].

DCs express cannabinoid receptor 2 (CBR2) and CBR2 agonists decrease autoimmune inflammation and DC production of pro-inflamamtory cytokines and capacity to stimulate CD4+ T cells. CBR2 agonists were found to inhibit DC migration from peripheral tissues by blocking MMP9 expression [242] .

Conclusion

As the most powerful APC, DCs serve as the key bridge between innate and adaptive immunity. DCs also function as a critical switcher between immune activation and immune tolerance. To be a truly effective immune system manipulator, DCs must endure many steps of evolution.

And each step was regulated subtly by a variety of molecules. These steps, from differentiation, endocytosis to maturation and migration, are closely associated with the function of DCs and the outcome of immune response.

Impaired endocytosis of dead cells by DCs is implicated in the pathogenesis of SLE; Different DC maturation state determines the immune system to be activated or tolerant; Migration is the prerequisite step for DCs to intact with T cells/B cells in LNs and orchestrate them subsequently.

The complexity of DC evolution process and the flexibility of their property determine the biological basis of their variety functions and the ability to react with different environments efficiently. And also provide us with plenty therapeutic targets to manipulate them. Further studies in the mechanisms of current drugs or novel molecules in DC functions will be of great significance for developing unpreceding protocols for the treatment of autoimmune diseases. Fostamatinib