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Plasmacytoid dendritic cells in autoimmunity
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ScienceDirect Plasmacytoid dendritic cells in autoimmunity Santosh K Panda, Roland Kolbeck and Miguel A Sanjuan Plasmacytoid dendritic cells (pDC) is a unique cell population that produces large amounts of type I interferon upon recognition of nucleic acids placing them at the crossroad of both innate and adaptive immunity. Their ability to produce interferon makes them central to anti-viral responses. They are also responsive to circulating autoantibodies bound to nuclear antigens and in that scenario the release of interferons initiate self-directed immune responses. There are now a growing number of autoimmune disorders where unabated activation of pDC is suspected to be pathogenic. Here, we discuss the different mechanisms responsible for breaking tolerance to self-nucleic acids by pDC, including the novel role of IgE autoantibodies in systemic lupus erythematosus. We also summarized the recent progress on therapies undergoing clinical testing that target either pDC or type I interferons. Address Dept of Respiratory, Inflammation & Autoimmunity, MedImmune LLC, Gaithersburg, MD, USA Corresponding author: Sanjuan, Miguel A ([email protected])
Current Opinion in Immunology 2016, 44:20–25 This review comes from a themed issue on Innate immunity Edited by Marco Colonna
http://dx.doi.org/10.1016/j.coi.2016.10.006 0952-7915/(c) 2016 Elsevier Ltd. All rights reserved.
Introduction Plasmacytoid dendritic cells (pDC) correspond to a subset of dendritic cells that originate at the bone marrow and exhibit plasma cell morphology. Currently, there are three cell surface markers that are unique to human pDC, the blood-derived dendritic cell antigen-2 (BDCA-2), BDCA4 and immunoglobulin-like transcript 7 (ILT7) [1,2]. They also express HLA-DR, CD4 and CD123 but not CD11c, a common dendritic cell marker. Through their production of type I interferons (IFNs) and other proinflammatory cytokines, pDC initiate an immune response that involves the activation of other myeloid cells, as well as B cells, T cells and natural killer (NK) cells. Viral nucleic acids are sensed in the endosome by Toll-like receptor (TLR)7 and TLR9. Activation of these receptors leads to pDC maturation and the initiation of an inflammatory response that is characterized by the secretion of large quantities of IFN-a [3] which ultimately results in the upregulation of IFN-induced genes [4], the activation Current Opinion in Immunology 2017, 44:20–25
of B cells and generation of antibody secreting plasma cells [5]. Along with the generation of a robust interferon response, mature human pDC secrete the proinflammatory cytokines TNF and IL-6, chemokines, and express costimulatory molecules at the cell surface that allows them to present antigens to T cells. Altogether, activation of TLR7 and TLR9 result in the initiation of a complex anti-viral program that places pDC at the center of both innate and adaptive immune responses [6]. Increased expression of genes regulated by type I IFNs, termed the interferon gene signature has been found in the blood and/or involved tissues of patients with autoimmune disorders [7,8]. Interferons are key drivers of autoimmunity, as they support the functions of monocytes and T cells, the activation and proliferation of B cells, and the differentiation of plasma cells into autoantibody-producing cells [9]. Administration of interferons result in lupuslike syndrome [10] and their pathogenic role in autoimmunity has now been clinically validated in two phase 2 clinical trials [11,12]. The secretion of type I IFN can be induced by autoantibodies that bind to nucleic acids or proteins that in turn bind to nucleic acids like nucleosomes. Upon binding to self-antigens released by dead cells, these autoantibodies form immune complexes (ICs) that activate pDC and initiate interferon responses. Most studies have focused on describing the pathogenic role of autoantibodies of the IgG subclass. Recent studies have now also associated autoantibodies of the IgE subclass to systemic lupus erythomatosus (SLE) pathogenesis. IgE facilitates degranulation of mast cells and basophils and promote Th2 immunity, mechanisms that are central to allergies. Recently, others and we have shown that IgE autoantibodies that bind to nucleic acids are present in SLE and induce robust type I IFN responses by pDC. Here, we will review the latest understanding of the links between pDC and autoimmunity, including their recently described ability to respond to IgE autoantibodies. We will also summarize the latest clinical trials focusing on targeting pDC or type I IFN in autoimmunity.
PDC in infection The importance of pDC on anti-viral response has been recently demonstrated in mice models. Depletion of pDC in adult animals reduced early type I interferon production and resulted in increased viral burden in early stage of both MCMV and VSV infection [13]. Conditional targeting of pDC-specific transcription factor Tcf4 severely reduce interferon response in pDC and its migration to peripheral lymphoid organs and tissues [14]. Constitutive loss of pDC in this model produced a impaired response to chronic LCMV infection characterized by reduced www.sciencedirect.com
Plasmacytoid dendritic cells in autoimmunity Panda, Kolbeck and Sanjuan 21
number and function of CD4 and CD8 T cells [14]. Type I IFNs released by pDC are central to controlling viral infections [15]. IFNs restrict viral infections by inhibiting viral replication and inducing apoptosis of the infected cells [16,17]. Type I IFN induces cross priming of CD8 T cells, enhances the clonal expansion of antigen specific CD8 T cells [18], and also acts synergistically with IL-27 to differentiate naı¨ve T cells into Th1 cells during viral infections [19]. While the potent antiviral role of type I IFNs is clear, their role in host defense against bacterial infections is enigmatic and may exert opposite functions depending on the bacterium, and the stage of infection. Low levels of type I IFNs may be required at an early stage of infection to initiate cell-mediated immune responses. High concentrations however may be detrimental to control bacterial infection due to reduced Th17 responses [20] and macrophage activation [15].
PDC in autoimmunity Evidence linking pDC to autoimmunity
Infiltration of pDC into involved tissues and evidence of interferon responses have been found in a number of autoimmune disorders including SLE, Sjogren’s syndrome, systemic sclerosis, psoriasis, and alopecia areata [3,7,8,21,22]. The expression of type I IFN genes in blood correlates with disease activity and plasma levels of antinuclear autoantibodies in SLE [23–25]. pDC are the main source of IFN-a and once activated by nucleic acidcontaining ICs they migrate from the blood into inflamed tissues including the skin [26] and kidney [27]. The central role of pDC in autoimmunity has been demonstrated in lupus prone mouse models. Mono allelic loss of the transcription factor Tcf4 causes specific impairment of pDC functions (such as ability to produce IFN-a in response to TLR9 agonist). In a murine SLE-prone background, the genetic impairment of pDC function induced by Tcf4 haplodeficiency nearly abolished key disease manifestations such as anti-DNA antibody production and glomerulonephritis [28]. In a different model, transient ablation of pDC reduced splenomegaly and lymphadenopathy, impaired expansion and activation of T and B cells, reduced antibodies against nuclear autoantigens (ANA) and improved kidney pathology. Amelioration of pathology coincided with decreased transcription of interferon-regulated genes in tissues [29]. The role of pDC in lupus seems to be largely driven by type I IFNs as lupus prone animals lacking the ability to sense nucleic acid containing autoantibodies due to defective endosomal TLR signaling showed an absence of autoantibody formation, reduced lymphadenopathy and splenomegaly, and extended survival [30].
Mechanism of self-nucleic acid recognition in pDC Sensing nucleic acids through endosomal TLRs is probably the most important mechanism by which pDC recognize and respond to ongoing viral infections. These www.sciencedirect.com
ligands however, are a feature shared by both host and pathogens. It is then paramount to have safeguards in place that avoid undesirable recognition of self-nucleic acids. Compartmentalization of TLR7 and TLR9 inside the cells is one of those measures. Endolysosomal location drastically limits their ability to respond to host extracellular nucleic acids that are shed by dead cells. The importance of concealing these receptors intracellularly is highlighted by the lethal autoinflammatory disease caused by the expression of a modified version of TLR9 at the cell surface in hematopoietic stem cells in mouse [31]. There is now ample evidence showing that autoantibodies that bind directly or indirectly to host nucleic acids form ICs that bypass those safeguards and cause autoimmunity. Nucleic acid containing ICs initiate phagocytosis by binding to FcgRIIa (CD32A) at the plasma membrane of pDC (Figure 1) [32,33]. The engulfed nucleic acids are then delivered into a phagosomal compartment where TLR7 and TLR9 sense them. Activation of either of these intracellular TLRs triggers the recruitment of the adaptor protein Myd88 and the subsequent activation of the transcription factor NF-kB, which results in the production of pro-inflammatory cytokines (such as TNF and IL-6) and chemokines (such as CXCL8, or CXCL10) [6]. As the phagosome matures into an acidic compartment the transcription factor interferon regulatory factor 7 (IRF7) is then activated and initiates the secretion of large amounts of type I IFNs. Activation of IRF7 is dependent on the recruitment of the autophagy protein microtubule-associated protein 1A/1B-light chain 3 (LC3) to the phagosome, a process that is known as LC3associated phagocytosis (LAP) to distinguish it from canonical autophagy [34]. While LAP mediates IFN responses from large compartments such as phagosomes containing pathogenic ICs, adaptor protein 3 (AP3) is responsible for initiating responses from smaller endosomal compartments [35]. Genome-wide association studies have identified polymorphisms in the autophagy gene Atg5 as a marker of predisposition for SLE [36,37], suggesting an important role for autophagy proteins and potentially LAP in SLE. Nonetheless, autophagy functions are not likely to be restricted to pDC and the IFN pathway. As shown by recent studies autophagy in B cells and macrophage may be also important for autoimmunity. In a Tlr7 transgenic mouse model, ablation of autophagy in B cells largely reduced inflammatory markers in the serum (such as type I IFNs, type III IFNs, IL-12 and IL6), and the formation of ANA. As a result, glomerulonephritis was ameliorated and survival extended [38]. Specific ablation of LAP components in macrophages however resulted in an inflammatory lupus-like response. This was largely due to a defect in phagosomal maturation and defective clearance of dead cells [39], the latest being a recognized driver of SLE pathology [40]. As new data and models arise focusing on investigating the role of autophagy in autoimmunity, it is becoming evident that those roles are bound to be context and cell dependent. Current Opinion in Immunology 2017, 44:20–25
22 Innate immunity
The breakdown of tolerance to self-nucleic acid does not appear to be exclusively driven by self-reactive IgG autoantibodies. HMGB1, a nuclear DNA-binding protein that is released by dying cells has the ability to trigger type I IFN production upon binding RAGE (receptor for advanced glycation end-products) at the cell surface [41]. Upon exposure to SLE derived anti-ribonucleoprotein antibodies, neutrophils die through netosis, a process that is characterized by the release of extracellular fibers, which are primarily composed of DNA. These neutrophil extracellular traps (NETs) are also decorated by LL37, a cationic anti-microbial protein that protects DNA from extracellular nuclease [42]. DNA bound to LL37 induces IFN responses by pDC in a TLR9-dependent manner [42,43] and are thought to contribute to the type I IFN flares that affect SLE patients [44]. As these molecules do
not appear in isolation, it is likely that during pathophysiological processes, both HMBG1 and LL37 act in conjunction with the anti-DNA autoantibodies that are found in patients (Figure 1). Along with IgG autoantibodies, SLE patients also present with IgE autoantibodies against nucleic acids. We have recently shown that these autoantibodies are also capable of inducing robust IFN responses, TNF, and IL-6 from pDC to a similar degree as their IgG counterparts [45] (Figure 1). Anti-DNA IgE antibodies are found in a majority of patients and their levels correlate with disease activity [46]. The levels of anti-DNA IgE antibodies are significantly lower than anti-DNA IgG autoantibodies in patients. In vitro experiments however have shown that at those levels, and even lower, IgE can synergize with IgG
Figure 1
HMBG1 RAGE IgE DNA IgG
IgG
IgG
LL37
RNA IgG
Antigen presentation
MHCII CD80 CD86
CCR7 Lymphoid tissue trafficking
Chemokines (IP10, IL-8)
Cytokines (TNF, IL-6)
IFN-α Current Opinion in Immunology
Mechanism of self-nucleic acid recognition by pDC. Expression of FceRI and FcgRII allows pDC to internalize ICs formed by autoantibodies bound to self-nucleic acids. These nucleic acids are then delivered to either TLR7 or TLR9 at the phagosome. Triggering of endosomal TLR7/9 result in the recruitment of the adaptor protein Myd88. In the early phagosome, Myd88 activates the transcription factor NF-kB. That leads to the formation of pro-inflammatory cytokine, the release of chemokines and the upregulation of costimulatory receptors. Maturation of the phagosome into an acidic compartment is mediated by the recruitment of LC3, which allows its fusion with lysosomes. Upon acidification of the compartment, Myd88 becomes capable of activating the transcription factor IRF-7, which upon nuclear translocation initiates interferon responses. DNA also promotes interferon production by binding to HMBG1 released by dead cells and LL37 found in neutrophil NETs. HMBG1 binds RAGE at the surface of pDC. In the context of autoimmunity, it is likely that HMBG1 and LL37 work in concert with circulating anti-DNA autoantibodies to deliver self-nucleic acids inside pDC. Current Opinion in Immunology 2017, 44:20–25
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Plasmacytoid dendritic cells in autoimmunity Panda, Kolbeck and Sanjuan 23
Table 1 Clinical Trials focusing on pDC and Type I IFN pathway inhibition Therapeutic candidates
Antigen
Format
Status
Disease
Anifrolumab
IFNAR1
Blocking antibody
Phase 3
SLE
Anifrolumab
IFNAR1
Blocking antibody
Phase 2
Sifalimumab*
IFN-a
Blocking antibody
Phase 2
Lupus Nephritis SLE
Rontalizumab
IFN-a
Blocking antibody
Phase 2
SLE
IFNa-Kinoid DV1179
IFN-a TLR7/9
Vaccine Oligonucleotide inhibitor
Phase 2b Phase 2a
SLE SLE
BIIB059
BDCA2
Functional antagonist
Phase 2
SLE, CLE
CPG-52365 PF-06650833 MEDI7734
TLR7/8/9 IRAK4 ILT7
Small molecule inhibitor Small molecule inhibitor ADCC
Phase 1 Phase 1 Phase I
SLE SLE DM, PM, Sjogren’s, SLE, SSc
Findings
References
Phase 2 met primary endpoint. Phase 3 ongoing Ongoing
NCT02446912, and [12] NCT02547922
Phase 2 met primary endpoint (not progressing in favor of anifrolumab) Primary endpoint not met in Phase 2 (discontinued) Ongoing Primary pharmacodynamic endpoints not met Ongoing
NCT01283139, and [11]
Phase 1 completed Ongoing Ongoing
[53] NCT02665364 – NCT02847598, and [54] NCT02485769 NCT02485769 NCT00547014
IFNAR1, Type I IFN receptor subunit-1; SSc, systemic sclerosis; CLE, cutaneous lupus erythematosus; DM, dermatomyositis; PM, polymyositis. Despite meeting the primary endpoint, sifalimumab is not progressing in favor of anifrolumab.
*
[45]. Cooperation of the Fc receptors at the surface of pDC that bind these IC, FcgRIIa and FceRI, leads to larger delivery of DNA inside the phagosome and ultimately stronger activation of TLR9. Consistently anti-IgE blocking antibodies reduces the ability of patient’s serum to induce IFN-a secretion in PBMCs, supporting the notion that (as it was already described for IgG) IgE autoantibodies have an active role in SLE pathogenesis [45]. Finally, it is worth noting that allergies are not more common in SLE [46] and that allergens are not likely to induce IFN responses by pDC as they largely lack the ability of forming nucleic acid-containing ICs capable of triggering TLR7 and TLR9. In fact, cross-linking of FceRI at the cell surface of pDC has been reported to inhibit IFN responses [46–48], and it has been speculated that this inhibition is partly responsible for the broadly impaired antiviral immunity in asthma. The opposing actions of IgE on promoting or blocking IFN responses follow a similar pattern to what was described previously for IgG [45]. These data suggest that the activation of Fc-receptors accompanied by intracellular delivery of nucleic acids induce IFN responses. In the absence of nucleic acids the same triggers block the ability of TLR7 and TLR9 to respond to their ligands. In addition to FcgRIIa and FceRI, crosslinking of BDCA-2, and ILT7 at the cell surface has also been shown to inhibit IFN responses by pDC [49].
Perspective of targeting pDC functions in SLE Many of the current therapeutic options for SLE patients are considered to be inadequate because of toxicities, accrual of organ damage, or insufficient control of the underlying disease pathology. Steroids are effective at suppressing flares, but are associated with short- and www.sciencedirect.com
long-term toxicity. Irreversible and cumulative organ damage arises from both the underlying disease and concomitant chronic steroid use [50]. Similarly, conventional immunosuppressant drugs have untargeted broad effects on the immune system, not universally effective, and are associated with toxicities. Belimumab, a B-cell activating factor (BAFF) blocking antibody demonstrated modest improvement in two placebo-controlled Phase III trials in non-renal SLE becoming the first treatment approved for use in SLE in over 60 years [51]. Although not approved for the treatment of SLE, rituximab (anti-CD20 B-cell depleting antibody) is used extensively off label [52]. Improved understanding of SLE pathogenesis and immunopathology has led to the identification of new treatment targets. Some of these new targets blocks pDC or the IFN pathway downstream of pDC (Table 1). The most advanced molecule in that group, anifrolumab, blocks the type I IFN receptor and is currently progressing in phase 3 clinical trials in SLE and in phase 2 trial in lupus nephritis. In a phase 2b clinical trial, anifrolumab met its primary endpoint and demonstrated consistent benefit across multiple global and organ-specific measures after one year of treatment in SLE patients. A higher frequency of influenza (most unconfirmed) and a dosage-dependent increase in herpes zoster were observed for patients receiving anifrolumab [12]. A clinical trial using an IFN-a blocker [11] has also shown promise in SLE. Together these trials have demonstrated that in patients the type I IFN pathway, and in particular IFN-a, are central to SLE pathogenesis. A number of new molecules are also progressing in the pipeline focusing on depleting or inhibiting pDC. These molecules bind to surface receptors (such as BDCA2 or ILT7), block endosomal TLRs, or Current Opinion in Immunology 2017, 44:20–25
24 Innate immunity
TLR’s downstream signaling (Table 1). Along with blocking the release of IFN-a, depletion or inhibition of pDC has the potential of blocking additional pDC functions such as the production of TNF, IL-6, type III IFNs, chemokines and antigen presentation that is triggered by pathogenic nucleic acid containing ICs. As this group of molecules advance in their development, we will gain further understanding of the relative importance of these additional pDC functions in SLE pathogenesis.
Conclusions
6.
Swiecki M, Colonna M: The multifaceted biology of plasmacytoid dendritic cells. Nat Rev Immunol 2015, 15:471-485.
7.
Baechler EC, Batliwalla FM, Reed AM, Peterson EJ, Gaffney PM, Moser KL, Gregersen PK, Behrens TW: Gene expression profiling in human autoimmunity. Immunol Rev 2006, 210:120-137.
8.
Higgs BW, Liu Z, White B, Zhu W, White WI, Morehouse C, Brohawn P, Kiener PA, Richman L, Fiorentino D et al.: Patients with systemic lupus erythematosus, myositis, rheumatoid arthritis and scleroderma share activation of a common type I interferon pathway. Ann Rheum Dis 2011, 70:2029-2036.
9.
Crow MK: Interferon-alpha: a therapeutic target in systemic lupus erythematosus. Rheum Dis Clin North Am 2010, 36:173-186 x.
Current treatment approaches in SLE are based on nonspecific immunosuppression. Different lines of research are placing pDC and the production of IFN-a at the center of the immunological abnormalities observed in SLE making them novel targets for therapeutic intervention. Their ability to be induced by not only IgG autoantibodies but also by low concentration of IgE autoantibodies makes them key responders to the pathogenic nucleic acid-containing ICs that are present in patients. Drug development has been challenging in SLE due to the high degree of heterogeneity in clinical manifestations. Today, new promising molecules are being tested in clinical trials that were designed as a result of our improved understanding of disease pathogenesis and molecular mechanisms. Among them, molecules blocking the type I IFN pathway have shown promising results in recent clinical studies and have the potential of offering new therapeutic options to patients with autoimmune disease.
10. Di Domizio J, Cao W: Fueling autoimmunity: type I interferon in autoimmune diseases. Expert Rev Clin Immunol 2013, 9:201-210.
Funding sources
15. McNab F, Mayer-Barber K, Sher A, Wack A, O’Garra A: Type I interferons in infectious disease. Nat Rev Immunol 2015, 15:87-103. Here the authors review the importance of type-1 interferons in infectious diseases and recent controversies.
All authors are full-time employees of MedImmune, which is developing drugs in autoimmunity.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
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33. Lovgren T, Eloranta ML, Bave U, Alm GV, Ronnblom L: Induction of interferon-alpha production in plasmacytoid dendritic cells by immune complexes containing nucleic acid released by necrotic or late apoptotic cells and lupus IgG. Arthritis Rheum 2004, 50:1861-1872. 34. Henault J, Martinez J, Riggs JM, Tian J, Mehta P, Clarke L, Sasai M, Latz E, Brinkmann MM, Iwasaki A et al.: Noncanonical autophagy is required for type I interferon secretion in response to DNAimmune complexes. Immunity 2012, 37:986-997. 35. Sasai M, Linehan MM, Iwasaki A: Bifurcation of Toll-like receptor 9 signaling by adaptor protein 3. Science 2010, 329:1530-1534. 36. Harley JB, Alarcon-Riquelme ME, Criswell LA, Jacob CO, Kimberly RP, Moser KL, Tsao BP, Vyse TJ, Langefeld CD, Nath SK et al.: Genome-wide association scan in women with systemic lupus erythematosus identifies susceptibility variants in ITGAM, PXK, KIAA1542 and other loci. Nat Genet 2008, 40:204-210. 37. Han JW, Zheng HF, Cui Y, Sun LD, Ye DQ, Hu Z, Xu JH, Cai ZM, Huang W, Zhao GP et al.: Genome-wide association study in a Chinese Han population identifies nine new susceptibility loci for systemic lupus erythematosus. Nat Genet 2009, 41:1234-1237. 38. Weindel CG, Richey LJ, Bolland S, Mehta AJ, Kearney JF, Huber BT: B cell autophagy mediates TLR7-dependent autoimmunity and inflammation. Autophagy 2015, 11:1010-1024. www.sciencedirect.com
49. Swiecki M, Colonna M: Unraveling the functions of plasmacytoid dendritic cells during viral infections, autoimmunity, and tolerance. Immunol Rev 2010, 234:142-162. 50. Yildirim-Toruner C, Diamond B: Current and novel therapeutics in the treatment of systemic lupus erythematosus. J Allergy Clin Immunol 2011, 127:303-312 quiz 313-304. 51. Lutalo PM, D’Cruz DP: Update on belimumab for the management of systemic lupus erythematosus. Expert Opin Biol Ther 2014, 14:1701-1708. 52. Rovin BH, Furie R, Latinis K, Looney RJ, Fervenza FC, SanchezGuerrero J, Maciuca R, Zhang D, Garg JP, Brunetta P et al.: Efficacy and safety of rituximab in patients with active proliferative lupus nephritis: the Lupus Nephritis Assessment with Rituximab study. Arthritis Rheum 2012, 64:1215-1226. 53. Kalunian KC, Merrill JT, Maciuca R, McBride JM, Townsend MJ, Wei X, Davis JC Jr, Kennedy WP: A Phase II study of the efficacy and safety of rontalizumab (rhuMAb interferon-alpha) in patients with systemic lupus erythematosus (ROSE). Ann Rheum Dis 2016, 75:196-202. 54. Pellerin A, Otero K, Czerkowicz JM, Kerns HM, Shapiro RI, Ranger AM, Otipoby KL, Taylor FR, Cameron TO, Viney JL et al.: Anti-BDCA2 monoclonal antibody inhibits plasmacytoid dendritic cell activation through Fc-dependent and Fcindependent mechanisms. EMBO Mol Med 2015, 7:464-476. Current Opinion in Immunology 2017, 44:20–25
ScienceDirect Plasmacytoid dendritic cells in autoimmunity Santosh K Panda, Roland Kolbeck and Miguel A Sanjuan Plasmacytoid dendritic cells (pDC) is a unique cell population that produces large amounts of type I interferon upon recognition of nucleic acids placing them at the crossroad of both innate and adaptive immunity. Their ability to produce interferon makes them central to anti-viral responses. They are also responsive to circulating autoantibodies bound to nuclear antigens and in that scenario the release of interferons initiate self-directed immune responses. There are now a growing number of autoimmune disorders where unabated activation of pDC is suspected to be pathogenic. Here, we discuss the different mechanisms responsible for breaking tolerance to self-nucleic acids by pDC, including the novel role of IgE autoantibodies in systemic lupus erythematosus. We also summarized the recent progress on therapies undergoing clinical testing that target either pDC or type I interferons. Address Dept of Respiratory, Inflammation & Autoimmunity, MedImmune LLC, Gaithersburg, MD, USA Corresponding author: Sanjuan, Miguel A ([email protected])
Current Opinion in Immunology 2016, 44:20–25 This review comes from a themed issue on Innate immunity Edited by Marco Colonna
http://dx.doi.org/10.1016/j.coi.2016.10.006 0952-7915/(c) 2016 Elsevier Ltd. All rights reserved.
Introduction Plasmacytoid dendritic cells (pDC) correspond to a subset of dendritic cells that originate at the bone marrow and exhibit plasma cell morphology. Currently, there are three cell surface markers that are unique to human pDC, the blood-derived dendritic cell antigen-2 (BDCA-2), BDCA4 and immunoglobulin-like transcript 7 (ILT7) [1,2]. They also express HLA-DR, CD4 and CD123 but not CD11c, a common dendritic cell marker. Through their production of type I interferons (IFNs) and other proinflammatory cytokines, pDC initiate an immune response that involves the activation of other myeloid cells, as well as B cells, T cells and natural killer (NK) cells. Viral nucleic acids are sensed in the endosome by Toll-like receptor (TLR)7 and TLR9. Activation of these receptors leads to pDC maturation and the initiation of an inflammatory response that is characterized by the secretion of large quantities of IFN-a [3] which ultimately results in the upregulation of IFN-induced genes [4], the activation Current Opinion in Immunology 2017, 44:20–25
of B cells and generation of antibody secreting plasma cells [5]. Along with the generation of a robust interferon response, mature human pDC secrete the proinflammatory cytokines TNF and IL-6, chemokines, and express costimulatory molecules at the cell surface that allows them to present antigens to T cells. Altogether, activation of TLR7 and TLR9 result in the initiation of a complex anti-viral program that places pDC at the center of both innate and adaptive immune responses [6]. Increased expression of genes regulated by type I IFNs, termed the interferon gene signature has been found in the blood and/or involved tissues of patients with autoimmune disorders [7,8]. Interferons are key drivers of autoimmunity, as they support the functions of monocytes and T cells, the activation and proliferation of B cells, and the differentiation of plasma cells into autoantibody-producing cells [9]. Administration of interferons result in lupuslike syndrome [10] and their pathogenic role in autoimmunity has now been clinically validated in two phase 2 clinical trials [11,12]. The secretion of type I IFN can be induced by autoantibodies that bind to nucleic acids or proteins that in turn bind to nucleic acids like nucleosomes. Upon binding to self-antigens released by dead cells, these autoantibodies form immune complexes (ICs) that activate pDC and initiate interferon responses. Most studies have focused on describing the pathogenic role of autoantibodies of the IgG subclass. Recent studies have now also associated autoantibodies of the IgE subclass to systemic lupus erythomatosus (SLE) pathogenesis. IgE facilitates degranulation of mast cells and basophils and promote Th2 immunity, mechanisms that are central to allergies. Recently, others and we have shown that IgE autoantibodies that bind to nucleic acids are present in SLE and induce robust type I IFN responses by pDC. Here, we will review the latest understanding of the links between pDC and autoimmunity, including their recently described ability to respond to IgE autoantibodies. We will also summarize the latest clinical trials focusing on targeting pDC or type I IFN in autoimmunity.
PDC in infection The importance of pDC on anti-viral response has been recently demonstrated in mice models. Depletion of pDC in adult animals reduced early type I interferon production and resulted in increased viral burden in early stage of both MCMV and VSV infection [13]. Conditional targeting of pDC-specific transcription factor Tcf4 severely reduce interferon response in pDC and its migration to peripheral lymphoid organs and tissues [14]. Constitutive loss of pDC in this model produced a impaired response to chronic LCMV infection characterized by reduced www.sciencedirect.com
Plasmacytoid dendritic cells in autoimmunity Panda, Kolbeck and Sanjuan 21
number and function of CD4 and CD8 T cells [14]. Type I IFNs released by pDC are central to controlling viral infections [15]. IFNs restrict viral infections by inhibiting viral replication and inducing apoptosis of the infected cells [16,17]. Type I IFN induces cross priming of CD8 T cells, enhances the clonal expansion of antigen specific CD8 T cells [18], and also acts synergistically with IL-27 to differentiate naı¨ve T cells into Th1 cells during viral infections [19]. While the potent antiviral role of type I IFNs is clear, their role in host defense against bacterial infections is enigmatic and may exert opposite functions depending on the bacterium, and the stage of infection. Low levels of type I IFNs may be required at an early stage of infection to initiate cell-mediated immune responses. High concentrations however may be detrimental to control bacterial infection due to reduced Th17 responses [20] and macrophage activation [15].
PDC in autoimmunity Evidence linking pDC to autoimmunity
Infiltration of pDC into involved tissues and evidence of interferon responses have been found in a number of autoimmune disorders including SLE, Sjogren’s syndrome, systemic sclerosis, psoriasis, and alopecia areata [3,7,8,21,22]. The expression of type I IFN genes in blood correlates with disease activity and plasma levels of antinuclear autoantibodies in SLE [23–25]. pDC are the main source of IFN-a and once activated by nucleic acidcontaining ICs they migrate from the blood into inflamed tissues including the skin [26] and kidney [27]. The central role of pDC in autoimmunity has been demonstrated in lupus prone mouse models. Mono allelic loss of the transcription factor Tcf4 causes specific impairment of pDC functions (such as ability to produce IFN-a in response to TLR9 agonist). In a murine SLE-prone background, the genetic impairment of pDC function induced by Tcf4 haplodeficiency nearly abolished key disease manifestations such as anti-DNA antibody production and glomerulonephritis [28]. In a different model, transient ablation of pDC reduced splenomegaly and lymphadenopathy, impaired expansion and activation of T and B cells, reduced antibodies against nuclear autoantigens (ANA) and improved kidney pathology. Amelioration of pathology coincided with decreased transcription of interferon-regulated genes in tissues [29]. The role of pDC in lupus seems to be largely driven by type I IFNs as lupus prone animals lacking the ability to sense nucleic acid containing autoantibodies due to defective endosomal TLR signaling showed an absence of autoantibody formation, reduced lymphadenopathy and splenomegaly, and extended survival [30].
Mechanism of self-nucleic acid recognition in pDC Sensing nucleic acids through endosomal TLRs is probably the most important mechanism by which pDC recognize and respond to ongoing viral infections. These www.sciencedirect.com
ligands however, are a feature shared by both host and pathogens. It is then paramount to have safeguards in place that avoid undesirable recognition of self-nucleic acids. Compartmentalization of TLR7 and TLR9 inside the cells is one of those measures. Endolysosomal location drastically limits their ability to respond to host extracellular nucleic acids that are shed by dead cells. The importance of concealing these receptors intracellularly is highlighted by the lethal autoinflammatory disease caused by the expression of a modified version of TLR9 at the cell surface in hematopoietic stem cells in mouse [31]. There is now ample evidence showing that autoantibodies that bind directly or indirectly to host nucleic acids form ICs that bypass those safeguards and cause autoimmunity. Nucleic acid containing ICs initiate phagocytosis by binding to FcgRIIa (CD32A) at the plasma membrane of pDC (Figure 1) [32,33]. The engulfed nucleic acids are then delivered into a phagosomal compartment where TLR7 and TLR9 sense them. Activation of either of these intracellular TLRs triggers the recruitment of the adaptor protein Myd88 and the subsequent activation of the transcription factor NF-kB, which results in the production of pro-inflammatory cytokines (such as TNF and IL-6) and chemokines (such as CXCL8, or CXCL10) [6]. As the phagosome matures into an acidic compartment the transcription factor interferon regulatory factor 7 (IRF7) is then activated and initiates the secretion of large amounts of type I IFNs. Activation of IRF7 is dependent on the recruitment of the autophagy protein microtubule-associated protein 1A/1B-light chain 3 (LC3) to the phagosome, a process that is known as LC3associated phagocytosis (LAP) to distinguish it from canonical autophagy [34]. While LAP mediates IFN responses from large compartments such as phagosomes containing pathogenic ICs, adaptor protein 3 (AP3) is responsible for initiating responses from smaller endosomal compartments [35]. Genome-wide association studies have identified polymorphisms in the autophagy gene Atg5 as a marker of predisposition for SLE [36,37], suggesting an important role for autophagy proteins and potentially LAP in SLE. Nonetheless, autophagy functions are not likely to be restricted to pDC and the IFN pathway. As shown by recent studies autophagy in B cells and macrophage may be also important for autoimmunity. In a Tlr7 transgenic mouse model, ablation of autophagy in B cells largely reduced inflammatory markers in the serum (such as type I IFNs, type III IFNs, IL-12 and IL6), and the formation of ANA. As a result, glomerulonephritis was ameliorated and survival extended [38]. Specific ablation of LAP components in macrophages however resulted in an inflammatory lupus-like response. This was largely due to a defect in phagosomal maturation and defective clearance of dead cells [39], the latest being a recognized driver of SLE pathology [40]. As new data and models arise focusing on investigating the role of autophagy in autoimmunity, it is becoming evident that those roles are bound to be context and cell dependent. Current Opinion in Immunology 2017, 44:20–25
22 Innate immunity
The breakdown of tolerance to self-nucleic acid does not appear to be exclusively driven by self-reactive IgG autoantibodies. HMGB1, a nuclear DNA-binding protein that is released by dying cells has the ability to trigger type I IFN production upon binding RAGE (receptor for advanced glycation end-products) at the cell surface [41]. Upon exposure to SLE derived anti-ribonucleoprotein antibodies, neutrophils die through netosis, a process that is characterized by the release of extracellular fibers, which are primarily composed of DNA. These neutrophil extracellular traps (NETs) are also decorated by LL37, a cationic anti-microbial protein that protects DNA from extracellular nuclease [42]. DNA bound to LL37 induces IFN responses by pDC in a TLR9-dependent manner [42,43] and are thought to contribute to the type I IFN flares that affect SLE patients [44]. As these molecules do
not appear in isolation, it is likely that during pathophysiological processes, both HMBG1 and LL37 act in conjunction with the anti-DNA autoantibodies that are found in patients (Figure 1). Along with IgG autoantibodies, SLE patients also present with IgE autoantibodies against nucleic acids. We have recently shown that these autoantibodies are also capable of inducing robust IFN responses, TNF, and IL-6 from pDC to a similar degree as their IgG counterparts [45] (Figure 1). Anti-DNA IgE antibodies are found in a majority of patients and their levels correlate with disease activity [46]. The levels of anti-DNA IgE antibodies are significantly lower than anti-DNA IgG autoantibodies in patients. In vitro experiments however have shown that at those levels, and even lower, IgE can synergize with IgG
Figure 1
HMBG1 RAGE IgE DNA IgG
IgG
IgG
LL37
RNA IgG
Antigen presentation
MHCII CD80 CD86
CCR7 Lymphoid tissue trafficking
Chemokines (IP10, IL-8)
Cytokines (TNF, IL-6)
IFN-α Current Opinion in Immunology
Mechanism of self-nucleic acid recognition by pDC. Expression of FceRI and FcgRII allows pDC to internalize ICs formed by autoantibodies bound to self-nucleic acids. These nucleic acids are then delivered to either TLR7 or TLR9 at the phagosome. Triggering of endosomal TLR7/9 result in the recruitment of the adaptor protein Myd88. In the early phagosome, Myd88 activates the transcription factor NF-kB. That leads to the formation of pro-inflammatory cytokine, the release of chemokines and the upregulation of costimulatory receptors. Maturation of the phagosome into an acidic compartment is mediated by the recruitment of LC3, which allows its fusion with lysosomes. Upon acidification of the compartment, Myd88 becomes capable of activating the transcription factor IRF-7, which upon nuclear translocation initiates interferon responses. DNA also promotes interferon production by binding to HMBG1 released by dead cells and LL37 found in neutrophil NETs. HMBG1 binds RAGE at the surface of pDC. In the context of autoimmunity, it is likely that HMBG1 and LL37 work in concert with circulating anti-DNA autoantibodies to deliver self-nucleic acids inside pDC. Current Opinion in Immunology 2017, 44:20–25
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Plasmacytoid dendritic cells in autoimmunity Panda, Kolbeck and Sanjuan 23
Table 1 Clinical Trials focusing on pDC and Type I IFN pathway inhibition Therapeutic candidates
Antigen
Format
Status
Disease
Anifrolumab
IFNAR1
Blocking antibody
Phase 3
SLE
Anifrolumab
IFNAR1
Blocking antibody
Phase 2
Sifalimumab*
IFN-a
Blocking antibody
Phase 2
Lupus Nephritis SLE
Rontalizumab
IFN-a
Blocking antibody
Phase 2
SLE
IFNa-Kinoid DV1179
IFN-a TLR7/9
Vaccine Oligonucleotide inhibitor
Phase 2b Phase 2a
SLE SLE
BIIB059
BDCA2
Functional antagonist
Phase 2
SLE, CLE
CPG-52365 PF-06650833 MEDI7734
TLR7/8/9 IRAK4 ILT7
Small molecule inhibitor Small molecule inhibitor ADCC
Phase 1 Phase 1 Phase I
SLE SLE DM, PM, Sjogren’s, SLE, SSc
Findings
References
Phase 2 met primary endpoint. Phase 3 ongoing Ongoing
NCT02446912, and [12] NCT02547922
Phase 2 met primary endpoint (not progressing in favor of anifrolumab) Primary endpoint not met in Phase 2 (discontinued) Ongoing Primary pharmacodynamic endpoints not met Ongoing
NCT01283139, and [11]
Phase 1 completed Ongoing Ongoing
[53] NCT02665364 – NCT02847598, and [54] NCT02485769 NCT02485769 NCT00547014
IFNAR1, Type I IFN receptor subunit-1; SSc, systemic sclerosis; CLE, cutaneous lupus erythematosus; DM, dermatomyositis; PM, polymyositis. Despite meeting the primary endpoint, sifalimumab is not progressing in favor of anifrolumab.
*
[45]. Cooperation of the Fc receptors at the surface of pDC that bind these IC, FcgRIIa and FceRI, leads to larger delivery of DNA inside the phagosome and ultimately stronger activation of TLR9. Consistently anti-IgE blocking antibodies reduces the ability of patient’s serum to induce IFN-a secretion in PBMCs, supporting the notion that (as it was already described for IgG) IgE autoantibodies have an active role in SLE pathogenesis [45]. Finally, it is worth noting that allergies are not more common in SLE [46] and that allergens are not likely to induce IFN responses by pDC as they largely lack the ability of forming nucleic acid-containing ICs capable of triggering TLR7 and TLR9. In fact, cross-linking of FceRI at the cell surface of pDC has been reported to inhibit IFN responses [46–48], and it has been speculated that this inhibition is partly responsible for the broadly impaired antiviral immunity in asthma. The opposing actions of IgE on promoting or blocking IFN responses follow a similar pattern to what was described previously for IgG [45]. These data suggest that the activation of Fc-receptors accompanied by intracellular delivery of nucleic acids induce IFN responses. In the absence of nucleic acids the same triggers block the ability of TLR7 and TLR9 to respond to their ligands. In addition to FcgRIIa and FceRI, crosslinking of BDCA-2, and ILT7 at the cell surface has also been shown to inhibit IFN responses by pDC [49].
Perspective of targeting pDC functions in SLE Many of the current therapeutic options for SLE patients are considered to be inadequate because of toxicities, accrual of organ damage, or insufficient control of the underlying disease pathology. Steroids are effective at suppressing flares, but are associated with short- and www.sciencedirect.com
long-term toxicity. Irreversible and cumulative organ damage arises from both the underlying disease and concomitant chronic steroid use [50]. Similarly, conventional immunosuppressant drugs have untargeted broad effects on the immune system, not universally effective, and are associated with toxicities. Belimumab, a B-cell activating factor (BAFF) blocking antibody demonstrated modest improvement in two placebo-controlled Phase III trials in non-renal SLE becoming the first treatment approved for use in SLE in over 60 years [51]. Although not approved for the treatment of SLE, rituximab (anti-CD20 B-cell depleting antibody) is used extensively off label [52]. Improved understanding of SLE pathogenesis and immunopathology has led to the identification of new treatment targets. Some of these new targets blocks pDC or the IFN pathway downstream of pDC (Table 1). The most advanced molecule in that group, anifrolumab, blocks the type I IFN receptor and is currently progressing in phase 3 clinical trials in SLE and in phase 2 trial in lupus nephritis. In a phase 2b clinical trial, anifrolumab met its primary endpoint and demonstrated consistent benefit across multiple global and organ-specific measures after one year of treatment in SLE patients. A higher frequency of influenza (most unconfirmed) and a dosage-dependent increase in herpes zoster were observed for patients receiving anifrolumab [12]. A clinical trial using an IFN-a blocker [11] has also shown promise in SLE. Together these trials have demonstrated that in patients the type I IFN pathway, and in particular IFN-a, are central to SLE pathogenesis. A number of new molecules are also progressing in the pipeline focusing on depleting or inhibiting pDC. These molecules bind to surface receptors (such as BDCA2 or ILT7), block endosomal TLRs, or Current Opinion in Immunology 2017, 44:20–25
24 Innate immunity
TLR’s downstream signaling (Table 1). Along with blocking the release of IFN-a, depletion or inhibition of pDC has the potential of blocking additional pDC functions such as the production of TNF, IL-6, type III IFNs, chemokines and antigen presentation that is triggered by pathogenic nucleic acid containing ICs. As this group of molecules advance in their development, we will gain further understanding of the relative importance of these additional pDC functions in SLE pathogenesis.
Conclusions
6.
Swiecki M, Colonna M: The multifaceted biology of plasmacytoid dendritic cells. Nat Rev Immunol 2015, 15:471-485.
7.
Baechler EC, Batliwalla FM, Reed AM, Peterson EJ, Gaffney PM, Moser KL, Gregersen PK, Behrens TW: Gene expression profiling in human autoimmunity. Immunol Rev 2006, 210:120-137.
8.
Higgs BW, Liu Z, White B, Zhu W, White WI, Morehouse C, Brohawn P, Kiener PA, Richman L, Fiorentino D et al.: Patients with systemic lupus erythematosus, myositis, rheumatoid arthritis and scleroderma share activation of a common type I interferon pathway. Ann Rheum Dis 2011, 70:2029-2036.
9.
Crow MK: Interferon-alpha: a therapeutic target in systemic lupus erythematosus. Rheum Dis Clin North Am 2010, 36:173-186 x.
Current treatment approaches in SLE are based on nonspecific immunosuppression. Different lines of research are placing pDC and the production of IFN-a at the center of the immunological abnormalities observed in SLE making them novel targets for therapeutic intervention. Their ability to be induced by not only IgG autoantibodies but also by low concentration of IgE autoantibodies makes them key responders to the pathogenic nucleic acid-containing ICs that are present in patients. Drug development has been challenging in SLE due to the high degree of heterogeneity in clinical manifestations. Today, new promising molecules are being tested in clinical trials that were designed as a result of our improved understanding of disease pathogenesis and molecular mechanisms. Among them, molecules blocking the type I IFN pathway have shown promising results in recent clinical studies and have the potential of offering new therapeutic options to patients with autoimmune disease.
10. Di Domizio J, Cao W: Fueling autoimmunity: type I interferon in autoimmune diseases. Expert Rev Clin Immunol 2013, 9:201-210.
Funding sources
15. McNab F, Mayer-Barber K, Sher A, Wack A, O’Garra A: Type I interferons in infectious disease. Nat Rev Immunol 2015, 15:87-103. Here the authors review the importance of type-1 interferons in infectious diseases and recent controversies.
All authors are full-time employees of MedImmune, which is developing drugs in autoimmunity.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest
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