ACPA mediates the interplay between innate and adaptive immunity in rheumatoid arthritis

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Review

ACPA mediates the interplay between innate and adaptive immunity in rheumatoid arthritis Xiwen Donga,b,c,1, Zhaohui Zhenga,b,1, Yue Zhaia,b,c,1, Yan Zhenga,b, Jin Dinga,b, Jianli Jiangb,c, ⁎ Ping Zhua,b, a

Department of Clinical Immunology, Branch of Immune Cell Biology, State Key Discipline of Cell Biology, PLA Specialized Research Institute of Rheumatology & Immunology, Xijing Hospital, Fourth Military Medical University, No. 127 West Changle Road, Xi'an 710032, Shaanxi Province, China b National Translational Science Center for Molecular Medicine, Xi'an 710032, China c Department of Cell Biology, State Key Discipline of Cell Biology, Fourth Military Medical University, Xi'an 710032, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Rheumatoid arthritis Anti-citrullinated protein antibodies Adaptive immunity Innate immunity

The production of anti-citrullinated peptide antibodies (ACPAs) requires the participation of both innate immunity and adaptive immunity. On the one hand, activated innate immunity is able to produce citrullinated auto-antigens that fuel autoimmunity and provide an inflammatory environment that facilitates the breach of self-tolerance, proliferation of self-reactive T/B cells and the production of ACPAs. On the other hand, after their production by plasma B cells, ACPAs are also able to interact with innate immunity to exacerbate the manifestation and chronicity of rheumatoid arthritis (RA). This article discusses the roles of citrullinated peptides and ACPA played in innate immunity and autoimmunity. In addition, we emphasise the relationships between environmental factors and innate immunity, as well as the pathogenic function of ACPAs per se. In doing so, we hope to provide fundamental knowledge of RA pathogenesis and reveal potential therapeutic targets in RA treatment.

1. Introduction Anti-citrullinated peptide antibodies (ACPAs) are a family of autoantibodies that specifically target proteins containing peptidylcitrulline, a deiminated form of peptidylarginine [1,2]. In 1998, Schellekens et al. first reported the existence of specific antibodies targeting synthetic citrullinated peptides in sera of rheumatoid arthritis (RA) patients [3]. With the initial peptide variants used for ELISA, ACPAs were identified in 76% of RA sera with a specificity of 96% [3]. Ever since the identification of ACPAs, these newly found autoantibodies have been studied, leading to successive discoveries of their irreplaceable diagnostic values due to the robust associations of ACPAs with the pathogenesis, radiographic progression and extra-articular complications of RA [4]. Accordingly, ACPAs tests were included in the 2010 RA classification criteria [5,6]. Beyond extraordinary clinical utility, the origin of ACPAs and their potential role in coordinating the immune and non-immune system advanced our appreciation for RA pathogenesis. The manufacture of a pathogenic autoantibody requires the

participation of environmental factors, the alteration of self-tissue, and the activation of autoimmunity. The interplay between them was first verified in 1934 by Schwentker and Rivers who immunised rabbits with autolytic rabbit brain emulsions, inducing the production of “anti-brain antibodies” and neurological manifestations such as paralysis, whereas rabbits treated with fresh brain emulsions did not exhibit autoimmunity [7]. For the first time, autoantibody production was shown to require the activation of autoimmunity targeting altered tissues generated by pathogens or chemical agents [8]. After the development of this hypothesis for almost a century, it still applies to ACPA autoimmunity but has evolved in the following aspects. First, citrullinated proteins/peptides generated by exogenous or endogenous factors are the main targets of ACPAs and the fuel for ACPA autoimmunity. Second, citrullinated proteins/peptides can be presented by antigen-presenting cells (APCs) to auto-reactive lymphocytes under certain predisposing major histocompatibility complex (MHC) genetic backgrounds, thereby igniting ACPA autoimmunity [9]. Finally, ACPAs participate in the localisation of the clinical manifestation of RA and the resulting deterioration.

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Corresponding author at: Department of Clinical Immunology, Branch of Immune Cell Biology, State Key Discipline of Cell Biology, PLA Specialized Research Institute of Rheumatology & Immunology, Xijing Hospital, Fourth Military Medical University, No. 127 West Changle Road, Xi'an 710032, Shaanxi Province, China. E-mail address: [email protected] (P. Zhu). 1 Xiwen Dong, Zhaohui Zheng and Yue Zhai contributed equally to this work. https://doi.org/10.1016/j.autrev.2018.02.014 Received 14 February 2018; Accepted 20 February 2018 1568-9972/ © 2018 Elsevier B.V. All rights reserved.

Please cite this article as: Dong, X., Autoimmunity Reviews (2018), https://doi.org/10.1016/j.autrev.2018.02.014

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Fig. 1. Metabolism of protein citrullination and free citrulline metabolism.

innate immune responses that can induce extracellular Ca2+ influx are able to trigger PAD activation [17]. To live up to the role as the first line of defence against pathogens, innate immune cells (especially macrophages) express families of detecting proteins, collectively termed pattern recognition receptors (PRRs) [18]. PRRs, which include Toll-like receptors (TLRs), nucleotide-binding and oligomerisation domain (NOD)-like receptors (NLRs), retinoic-acid-induced gene I (RIG-1)-like helicases and C-type lectin receptors, serve as sentinels to detect arrays of pathogen-associated molecular patterns (PAMPs; e.g., lipopolysaccharide (LPS), pathogenic RNA and DNA) and damage-associated molecular patterns (DAMPs) derived from stressed or dying cells released from cell death-related pathways [18–20]. The functions of these PRRs (especially TLRs) have been extensively studied in RA through in vitro and in vivo experiments [21]. For example, both TLR2 and TLR4 have been reported to be elevated in RA synovial tissue (ST) lining macrophages and fibroblasts and sublining macrophages together with elevated levels of their ligands (e.g., peptidoglycan (PG), LPS and heat shock proteins (HSPs)) [22–24]. The presence of PAMPs (such as LPS and glucan [25]) is recognised by PRRs, which induce a subsequent Ca2+ influx and activate PADs. In addition, some kinds of DAMPs, especially ATP, which is actively secreted by stressed cells or passively released by dead cells, bind to P2XR channels and allow sodium and calcium influx and potassium efflux [26]. Meanwhile, the PAMP and DAMP are also able to induce apoptosis. During apoptosis, the unlimited influx of Ca2+ triggers PADs, causing citrullination within cells, which is essential for the preparation of intermediate filament disassembly in the apoptosis process [27]. For instance, ectopic expression of PAD4 leads to chromatin decondensation and promotes DNA cleavage, whereas Pad4−/− mice exhibited resistance to radiation-induced apoptosis in the thymus, confirming the crucial role of citrullination in apoptosis [28]. Over-activated apoptosis or inefficient clearance of apoptotic elements may be responsible for the leakage of citrullinated proteins [29,30]. Hence, innate immune responses facilitate PAD activation and promote the induction of

Therefore, this review will discuss the role of ACPA in the interactions of innate immunity and autoimmunity in detail, with a focus on environmental factors and innate immunity in RA pathogenesis and the pathogenic function of ACPA per se. 2. The induction of citrullination: citrullinated peptides and innate immunity Citrulline metabolism in humans includes free citrulline metabolism and citrullinated protein production (Fig. 1). Free citrulline metabolism involves nitic oxide (NO) synthase (NOS), ornithine carbamoyltransferase (OCT), and argininosuccinate synthetase (ASS) and results in urea production [10]. Protein citrullination occurs in some autoimmune diseases such as RA. Citrullinated proteins (e.g., citrullinated vimentin (CV), fibrinogen and histone), which are the main targets of ACPA, are produced under the influence of environmental factors with the help of peptidyl arginine deiminase (PAD) [3,11]. Citrullination is a post-translational modification of proteins in which arginines are converted into citrulline with the help of Ca2+-dependent PADs, causing an increase in molecular mass of 0.984 Da and a decrease of one positive charge [12]. Five different isoforms of PADs (including PAD 1, 2, 3, 4, 6) are distributed in the human body over a wide range of tissues and are characterised by their fine specificity. Due to the distinct enzymatic preference of different PAD isoforms, not every arginine on a peptide is equally susceptible to catalysis by a specific PAD isoform [13]. For instance, PAD4 in neutrophils is able to catalyse histone H3 citrullination, while citrullinated β/γ-actin can only be achieved by PAD2 [14]. The molecular basis for such preference lies in the varied recognition of potential citrullination sites due to the amino acids flanking the central arginine [15,16]. 2.1. Activation of PADs Normally, PADs cannot be activated under physiological intracellular calcium levels. However, recent findings revealed that some 2

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secreting pro-inflammatory cytokines and chemokines (such as tumour necrosis factor alpha (TNF-α), IL-1β, etc.) [46–48]. Indeed, in vivo experiments have proven that, in principle, infections can spark autoimmune responses [48]. However, this process must be distinguished from the elicitation of overt autoimmune disease because of the huge leap between the presence of auto-reactive responses and the appearance of clinical manifestations (discussed in the next section).

citrullination. Another acknowledged crucial source of citrullinated proteins is neutrophil extracellular trap (NET)osis, which is uniquely observed in neutrophils and plays a vital role in confrontation with infection and the clearance of cell corpses (especially those of necrotic cells) [31,32]. NETosis can be activated under circumstances in which the size and/or number of the invading pathogens is excessive for phagocytosis, leading to the release of chromatin NETs, which contain decondensed chromatin fibres, antimicrobial matter, and cytoplasmic proteins [33]. NETs can be released as a part of a suicide pathway, which requires reactive oxygen species (ROS) produced by nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, or a non-suicide pathway that is NADPH oxidase-independent. These two pathways are distinct in the speed and direction of NET release, as well as the maintenance of chemotaxis and phagocytosis in neutrophils [34,35]. The expelled NETs can attach to the microorganisms and serve as a scaffold that enables the antimicrobial enzymes to efficiently damage the invaders. The relatively large abundance of activated neutrophils in RA synovial fluid (SF) led scientists to focus on the excessive activation of NETosis. As an effector in cigarette smoke, nicotine is able to drive NET formation and accelerate collagen-induced arthritis [36]. Accelerated NETosis has been observed in RA patients compared with that in healthy controls and osteoarthritis (OA) patients and is closely correlated with higher ACPA levels [37]. In addition, higher concentrations of citrullinated proteins (mainly histones) with the ability to bind ACPAs have been reported in RA patients, who are also characterised by increased spontaneous NETosis compared with that in healthy controls [38]. These findings indicate that NETs may contribute to the production of ACPAs acting as a crucial source of citrullinated auto-antigens. Later, the hypothesis was confirmed by the fact that a variety of proteins are deiminated by PAD4, which is abundantly expressed in granulocytes and could be strongly activated by intrinsic signals and enzymes of NETosis [39] or extracellular Ca2+ influx [40]. In addition, the existence of NET-associated PADs verifies the role of NETs serving as a scaffold to accelerate protein citrullination (especially histones) and externalization [40]. As a result, proteins citrullinated by PAD4 in NETosis form a pool of auto-antigens that fuels autoimmunity [14, 40–42]. Although a large spectrum of citrullinated proteins produced by stimulated neutrophils has been identified in SF, there are significantly fewer in RA sera than in in the SF [43]. This finding emphasises that citrullination induced by NETosis is locally restricted to the inflammatory milieu. Indeed, NETs enhance the expression of proinflammatory genes and the production of pro-inflammatory cytokines (such as interleukin (IL)-6, IL-8), chemokines and adhesion molecules from RA synovial fibroblasts [37]. Through the regulation of local innate immune cells, such as macrophages and plasmacytoid dendritic cells, the pro-inflammatory milieu is preserved, leading to continuous joint deterioration. Under NET stimulation, the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome in macrophages is activated, inducing the production of IL-1β and mobilisation of immune cells to the milieu, in which T/B lymphocytes and plasma cells form germinal centre (GC)-like structures whereby self-tolerance breaks down [44,45]. Therefore, NETs vigorously participate in autoimmunity by providing an autoimmune-prone environment and abundant source of citrullinated auto-antigens.

3. The production of ACPA: an adaptive process under innate influence Citrullination catalysed by PADs incurs conformational alterations to the primary proteins, changing their inherent functions and interactions with other peptides [9]. Notably, the transformation of peptidylarginine generates new epitopes with distinct and augmented antigenicity. Therefore, the production of citrullinated peptides was initially regarded as an RA-specific phenomenon. However, identification of citrullinated proteins in the ST of patients with distinct inflammatory arthritis (namely, inflammatory osteoarthritis, reactive arthritis, and other inflammatory joint diseases originated from trauma or unknown reasons) negated this hypothesis [49]. The follow-up localisation of citrullinated proteins in muscular tissues from polymyositis patients, colonic tissues from intestinal bowel disease (IBD) patients, tonsillar tissues from chronic tonsillitis patients, brain tissues from multiple sclerosis patients, and islets from diabetes-prone non-obese diabetic mice [50–52] supported a much broader distribution of citrullinated proteins in an expanded spectrum of disease. Even though the production of ACPA remains an RA-specific event, these observations proved that citrullination is more an inflammation-dependent process than an RA-specific one, whereas the presence of ACPA is more likely to be RA-dependent. Herein, we discuss how the presentation of citrullinated auto-antigens to adaptive immune cells and the triggering of tolerance breakdown distinguish RA from other diseases. 3.1. Presentation of citrullinated auto-antigens and production of ACPA Prior to antigen presentation, peptides must be produced by the cell's own translational system or released after protein decomposition via the endo-lysosomal vesicular system. In fact, recent research demonstrated that autophagy is a key cellular event involved in this process, as autophagy was shown to participate in the generation of citrullinated peptides (such as CV, citrullinated α-enolase, and citrullinated filaggrin in synoviocytes [53]) and the presentation of peptidylcitrulline but not peptidylarginine in APCs [53–55]. Antigen presentation by MHC proteins is essential for adaptive immunity. An interesting question is why such a small modification (from peptidylarginine to peptidylcitrulline) dramatically enhances the antigenicity of a peptide. A possible explanation is that citrullination can enhance the binding preferences for APCs [56]. As a matter of fact, Scally et al. confirmed that the citrullinated form of vimentin is able to be accommodated within the P4 pocket of human leukocyte antigen (HLA)-DRB1*04:01/04, a shared susceptibility epitope (SE) of the RArelated HLA-DRB1 locus, while the arginine form interacts with the RAresistant HLAD-RB1*04:02 allomorph. In addition, CD4+ T cells against CV have been located in the peripheral blood of HLA-DRB1*04:01-positive RA patients and healthy controls [57]. After being activated, CD4+ Th cells further facilitate the activation of B cells, which in turn, differentiate into plasma cells, resulting in the production of ACPA.

2.2. Formation of an auto-reactive environment Apart from serving as an inducer of citrullination, innate immunity may also provide an autoimmune-prone environment after its activation. Therefore, these PAMPs and DAMPs serve as adjuvants for the immune response. Activated innate immune cells further promote autoimmunity in the following ways: first, by enhancing the antigen presentation of APCs and increasing expression of co-stimulatory molecules (e.g., B7, cluster of differentiation 40 ligand (CD40L)) on the cell surface and, second, by

3.2. Breach of self-tolerance Several mucosal sites (such as lung, oral cavity, and gut) have been speculated to be possible initiating sites for the breach of self-tolerance, partially due to the fact that environmental factors such as cigarette smoke, nano particles [58], silica [59], dust [60], and microorganisms, which are thought to participate in ACPA production and RA 3

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critical role the initiation and deterioration of RA and is believed to perpetuate ACPA autoimmunity [77]. Currently, P. gingivalis is the only known microorganism that possesses PADs (known as PPADs) that are not Ca2+-dependent and are easily activated by elevated pH [78]. After polypeptides are cleaved into short peptides by arginine gingipains, PPAD preferentially and rapidly citrullinates C-terminal arginines, imbuing P. gingivalis with the ability to trigger an immunological response in RA patients with HLA-SE status [79]. Apart from providing citrullinated auto-antigens, P. gingivalis may also be able to alter lymphocyte cell reactivity, acting as a super-antigen [73]. A crucial way in which microorganisms drive auto-reactive T and B lymphocyte activation and tolerance breakdown is molecular mimicry. Molecular mimicry broadens the pathogen-specific reactivity to allow cross-reactivity with self-antigens in conditions in which self-antigens and foreign antigens share structural similarity [80]. Such ubiquitous molecular structural similarity between microbial and self-antigens is supported by the evidence that no human protein is exempt from bacterial motifs, where all human proteins contain a penta- or hexapeptide bacteria motif [81]. In addition, the degeneracy of T-cell receptors (TCRs) implies that T cells activated by microbial antigens possess the potential to cross-react with self-antigens, which can result in tolerance breakdown. For instance, anti-citrullinated peptide autoimmunity can be evoked by bacterial enolase citrullinated by PPADs or PADs [78,82]. Similarly, citrullinated peptides of the Epstein-Barr nuclear antigen 1 (EBNA-1) protein from Epstein-Barr virus (EBV) also present as a protein mimicking human citrullinated fibrinogen, suggesting that citrullinated bacterial or viral peptides participate in the initiation of ACPA via molecular mimicry [83]. After their activation by the citrullinated enolase (cit-enolase) of P. gingivalis through molecular mimicry, self-reactive B cells produce ACPAs that initially target the cit-enolase of P. gingivalis and cross-react with the human cit-enolase; however, with the progression of the ongoing immune reaction, the epitopes recognised by these ACPAs spread to other citrullinated human proteins, known as epitope spreading [84]. Such a scenario is representative of prototypical autoimmune disease initiation via sequential participation of molecular mimicry and epitope spreading. Recent findings demonstrated that ACPAs in patients with only bronchiectasis (BR, an infectious lung disease and a potent model for RA autoimmunity induction) showed higher degrees of cross-reactivity with unmodified epitopes (peptidylarginine) than those of BR patients with RA, suggesting that the specificity of ACPAs may not be achieved during the early stages of tolerance breakdown in BR but rather accumulate through epitope spreading within RA development [65]. These studies suggest that microorganisms may serve as initiating factors via molecular mimicry and expand influence through ACPA epitope spreading. Other microorganisms such as parvovirus [85], alphaviruses [86], Proteus mirabilis [87] could take the place of P. gingivalis in the induction of ACPA autoimmunity. Notably, Tsuda recently reported that 38 human, 56 viral, 1383 fungal, 547 bacterial, and 1072 plant proteins shared essential epitopes that could react with the monocolonal ACPA obtained by screening the peripheral B lymphocytes of RA patients [88]. This evidence prompts a new hypothesis that not only microbial proteins but also numerous botanic proteins assist in the breakdown of self-tolerance and the generation of cross-reactive ACPA via molecular mimicry in the initiation of RA [88]. Although the gut contains the most abundant amount of bacterial species [89] and has been investigated in RA pathogenesis since the 1950s, the most direct evidence for an interaction between microbiota and RA appeared recently. Zhang et al. collected faecal, dental and salivary samples from individuals with RA and healthy controls in multiple clinical centres for a meta genome-wide association study (MGWAS) to determine the differences among their microbiota. The analysis found overlapping and detectable dysbiosis with increased abundance of Lactobacillus salivarius and decreased levels of Haemophilus spp. in the gut and oral cavity within RA patients who could be partially improved by disease-modifying anti-rheumatic drug (DMARD)

pathogenesis, manifest within these mucosal sites [61,62]. 3.2.1. Lung and smoking-induced chronic inflammation Of all of the mucosal sites, the lung is the first and most studied in RA pathogenesis. The first evidence of a gene–environment interaction between smoking and the HLA-DR SE in seropositive RA patients revealed that lungs may be crucial in the interaction between certain MHC-II alleles and smoking, resulting in a high risk of ACPA-positive RA [63]. Such a hypothesis was further supported by evidence that irritants (especially smoking) are able to facilitate the appearance of citrullinated antigens in the lung. Klareskog et al. discovered that the lung participated in the production of citrullinated antigens by showing that smoking increased the presence of citrullinated proteins in bronchoalveolar lavage (BAL) cells [64]. Furthermore, signs of immune activation and local inflammation were identified in the bronchial tissue of untreated early RA. Gudrun et al. found that lymphocyte infiltration was more frequently found in ACPA-positive patients' bronchial biopsies, in which GCs, B cells and plasma cells were exclusively found [45]. Meanwhile, BAL samples from patients with ACPA-positive RA had significantly higher numbers of lymphocytes and expressed higher levels of activation markers than samples from controls [45]. Based on the preliminary results, environment factors (such as smoking) were first thought to be potent enough to directly trigger autoimmunity that could lead to tolerance breakdown. However, accumulating evidence suggested more complicated roles of environmental factors in ACPA autoimmunity. Taking cigarette smoking as an example, Lugli et al. found that the increased levels of citrullination and PAD activity between smokers and non-smokers were non-significant; on the contrary, citrullination in samples from chronic obstructive pulmonary disease (COPD) patients was significantly amplified compared with those in patients without airflow limitation. Others found that smoking-induced respiratory infection (such as chronic bronchiectasis) could directly generate ACPA production before the onset of RA [65–67]. These findings suggested that smoking participated in RA pathogenesis through initiating chronic inflammatory events in the lungs. Smoking-induced chronic inflammation, in turn, promotes the activation of PADs from phagocytes, enhances the proliferation of autoreactive T/B cells and induces the production of ACPA under predisposing genetic backgrounds [68]. 3.2.2. Digestive system and microorganism factors In the digestive system, the involvement of microorganisms plays a more critical role in tolerance breakdown and RA pathogenesis. A coexistence and coevolution of microbiota (a commensal and symbiotic ecological community of microorganisms) and the immune system have maintained the homeostasis of the local environment, which was also crucial in the peripheral education of the immune system. On the one hand, the immune system intervenes in the inhabitation, proliferation and function of microorganisms through barrier defence and phagocytosis, secreting antimicrobial substances of the humoural immune response and inducing a photokilling effect in the cellular immune response [69,70]. The microbiota, on the other hand, reshapes immune responses, modulates the development of immune cells and alters the immune phenotype [71]. For example, microbiota-transferred metabolites (taurine, histamine, and spermine) co-modulate NLRP6 inflammasome signalling and IL-18 secretion, which affects the quality and quantity of downstream antimicrobial peptide (AMP) profiles orchestrating the host-microbiome interface and microenvironment homeostasis [72]. When homeostasis is jeopardised from either side, dysbiosis occurs, which has been extensively reported to be associated with autoimmune diseases, such as RA, SLE, IBD, etc. [73–75]. Periodontitis, a typical dysbiosis in oral mucosa, co-exists and is mutually aggravated by RA and is able to induce the production of ACPAs [76]. Both periodontitis and RA patients share an overlapping risk factor. The most notorious oral bacteria contributing to periodontal disease is Porphyromonas gingivalis (P. gingivalis), which also plays a 4

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also be activated by ACPA-IC or ACPA alone. Citrullinated H2B-IC activates neutrophils measured by Sytox green quantification [41], while anti-CV antibodies induce NETs in neutrophils [37]. Anti-CV antibodies recognise CV externalized in NETosis and accelerate the formation of such antigens [37]. On the other hand, ACPA-ICs activate mast cells and increase cytokine production (mainly IL-8) by co-stimulating FcγRIIa and TLR, which then enhance inflammation in RA ST [101].

treatment [90]. The change in gut microbiota may give rise to an altered redox environment as well as varied transportation and metabolism of nutrients altered in RA patients [90]. However, how these microbiota alterations contribute to the sabotage of self-tolerance and production of ACPA has yet to be clearly elucidated. In addition, whether faecal microbiota transplantation (FMT), which has been proven to be useful in many interstitial diseases such as IBD and recurrent Clostridium difficile infection (RCDI) [91], is helpful in RA treatment still needs further verification in future clinical practice [50,66,67].

4.2. Collaboration with the complement system Accumulated evidence has demonstrated the critical participation of complement in the pathophysiology of RA [102]. In the 1990s, reduced levels of native complement components together with elevated levels of complement metabolites, such as sC5b-9, Bb etc. [103], were identified in the plasma, SF and ST of RA patients [104]. Furthermore, genetic evidence also demonstrated that a variation of C1q was correlated with C1q levels and predisposed carriers to RA [105], indicating a pathogenic role of complement in RA pathogenesis. Indeed, both ACPA and complement contribute to the pathogenesis of RA, and the collaboration between them has attracted attention. The first question scientists wanted to answer was whether ACPAs could activate complement. Since classical activation of the complement system relies on the formation of IC by immunoglobulin M (IgM) or immunoglobulin G (IgG), ACPAs were naturally hypothesised to activate complement through the classical pathway. However, in vivo evidence indicated otherwise, as studies reported that inflammation and joint destruction in mice underwent passive transfer of arthritogenic antibodies (not specifically ACPAs), which was fully dependent on the alternative pathway of complement, not the classical pathway as expected [106,107]. Whether this finding applies to the activation of complement specifically by ACPAs in human bodies requires further investigation. A year after the publication of the mouse model results, Trouw et al. isolated immunoglobulin from a pool of anti-cyclic citrullinated peptide (CCP)-positive sera to verify whether ACPAs could activate complement and through which pathways the activation was realised [108]. Because C4 could not be activated in alternative pathways, comparisons between C3 and C4 deposition were analysed to discriminate alternative activation from classical and lectin pathways. Additionally, a special buffer that allows for alternative pathway activation but blocks the classical pathway and lectin were utilised to measure complement activation at several concentrations of normal human serum. With these approaches, anti-CCP antibodies were shown to activate the complement system in a dose-dependent manner through either the classical or alternative pathway but not the lectin pathway. Furthermore, such activation proceeded until the formation of the membrane attack complex (MAC), suggesting that all activation steps in the complement system were achieved [108]. In support of these findings, Zhao et al. found that C1q could bind to ACPA-ICs that also contain citrullinated fibrinogen in 50% of anti-CCP-positive patients, and the colocalisation of these ICs with C3 in RA synovium was indicative of their contribution to the activation of complement in synovitis [109]. However, ACPAs from different patients recruit different pathways for complement activation, and not all isotypes of antibodies activate complement to the same extent [108]. Normally, IgM, IgG3 and IgG1 are the most potent activating isotypes, while IgG4 and immunoglobulin E (IgE) lack such an ability. Although IgG and IgM are traditionally thought to activate complement only through the classical pathway, studies have demonstrated that distinct glycosylation statuses of IgG and IgM are able to activate the alternative pathway or lectin pathway [110]. Apart from glycosylation status, the avidity of ACPAs also influences its activation ability. Low-avidity ACPAs have been suggested to display more complement activation than high-avidity ACPAs [111], potentially related to the ability of low-avidity ACPAs to easily detach from their antigens, leading to in situ ACPA tethering in which one immunoglobulin arm swings around to bind other antigens,

4. The effect of ACPA: interplay of innate and adaptive immunity Due to its relatively high sensitivity and specificity in predicting RA, ACPA was initially recognised as a superior alternative to rheumatoid factor (RF) as a marker for RA [92]. After ten years follow-up of 238 RA patients in a cohort study, researchers revealed that the presence of ACPAs was the strongest independent predictor of radiographic progression (odds ratio (OR) = 4.0) [93]. Radiographic progression was more likely to be found in patients with low to moderate levels of ACPAs (OR = 2.6) and those with high levels of ACPAs (OR = 9.9) than in ACPA-negative patients [93]. Another study demonstrated that the presence of an increased modified Sharp score (radiographic evaluation of hands and feet bone erosion) in a five-year follow-up was significant among ACPA-positive patients (OR = 2.5) and antiperinuclear factor (APF)-positive patients (OR = 2.4) but not RF-positive patients [94]. Patients with ACPAs or APF had relatively higher values of radiographic damage, bone erosion and joint narrowing scores than those without [94]. These clinical associations indicated that ACPAs may not only be a specific autoantibody crucial for RA diagnosis but also a pathogenic factor leading to RA deterioration, which began a new era of the study of ACPA pathogenic function. 4.1. Stimulation of innate immune cells In 2008, Clavel et al. first identified ACPAs as the prime candidate directly activating mono/macrophages [95]. Immune complexes (ICs) containing citrullinated fibrinogen (cFb) and corresponding ACPAs are able to stimulate human monocyte-derived macrophages to secrete high levels of TNF-α in a dose-dependent manner. Such stimulation mediated by Fcγ receptor (FcγR) IIa was systematically higher in macrophages than in homologous monocytes [96,97]. In addition to FcγR, citrullinated histone (mainly 2A and 2B)-containing ICs co-stimulate TLR4 and FcγR, leading to enhanced production of TNF-α in monocytederived macrophages [41]. Additionally, cFb alone is able to stimulate macrophage TNF production via the TLR4/MyD88 pathway and such stimulation of cFb-IC is dependent on the engagement of both TLR4/ MyD88 and FcγR [21,98]. Whether such macrophage stimulation is related to the fine specificity of ACPAs and citrullinated peptides is still unclear. These results provide evidence supporting the hypothesis that ICs containing citrullinated peptides may serve as DAMPs to stimulate innate immune cells; however, the roles of other PRRs and the differences between citrullinated proteins and other DAMPs still need to be elucidated. ACPAs are able to induce interferon regulatory factor (IRF) 4 and 5, thereby polarizing macrophages to the pro-inflammatory M1 subset, leading to a higher M1/M2 ratio in RA patients than in OA patients [99]. In addition, the existence of macrophage colony-stimulating factors (MCSF) in the local inflammatory milieu polarizes macrophages to a ACPA-sensitive subset with an increased expression of CD16 and CD163. After ACPA-IC stimulation, this subset of macrophages secreted the highest levels of pro-inflammatory cytokines, with a high TNF-α: IL10 ratio and the lowest IL-1Ra: IL-1β ratio compared to other macrophages [100]. Such cytokines further promote Th1/17 cells in the peripheral blood. Other innate immune cells, such as neutrophils and mast cells can 5

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Fig. 2. Panorama from ACPA production to manifestation.

suggesting that citrullination is an indispensable process in osteoclast differentiation and metabolism [115]. Therefore, the preferential binding of osteoclasts and circulating ACPAs originates from their specific dependency on citrullination [116]. Among the vast majority of citrullinated target peptides, CV could be a promising target, since human antibodies against mutated CV were able not only to bind to the osteoclast surface but also induce osteoclastogenesis and enhance bone resorptive activities [114]. Adoptive transfer of these antibodies to mice induces osteopenia (lower bone mineral density), resulting from the increased production of TNF-α from the rising number and enhanced activity of osteoclast precursor cells [114]. However, the following cellular events and corresponding intracellular alterations after ACPA binding are still unclear. Importantly, increased secretion of the autocrine growth factor, IL8, was clearly observed in human osteoclasts after ACPA incubation, as well as during normal differentiation of human osteoclasts [117,118]. Intriguingly, other pro-inflammatory factors such as IL-1, IL-6, and TNF-α were not induced upon ACPA stimulation [119]. Thus, IL-8 may possess a more important role than other cytokines in ACPA-mediated osteoclast differentiation and activation, which deserves special attention. Although the existing hypothesis is able to partially explain the presence of osteolysis and joint pain, the perpetuation of synovial inflammation still requires further investigation. Notably, the sole existence or passive transfer of ACPAs may not be sufficient to induce RA synovitis [119,120]. However, adoptive transfer of ACPAs to mice that undergo mild joint inflammation may extensively intensify existing synovitis, deteriorating into severe joint inflammation that resembles human RA [41]. These observations indicate that the combined existence of ACPA and an additional inflammatory environment is required to induce chronic synovitis, the interactions between synovial cells and ACPA during which require further study. These findings indicate that after activation, cytotoxic cells or compartments may lead to tissue destruction, exposing and citrullinating self-peptides through death-related pathways (as discussed before). Some proteins released from dead cells, such as HSP60 and 70 and high-mobility group box (HMGB) 1, stimulate innate immunity DAMPs through binding to PRRs [121,122]. In those inflammatory milieu, a higher frequency of differentiated plasmablasts and plasma

while the another arm is attached to the antigen surface [112]. Furthermore, the presence of low-avidity ACPAs in patients is correlated with a higher frequency of joint destruction compared with that of high-avidity ACPAs [111]. These findings indicate that low-avidity ACPAs mediate more severe biological effects, possibly because lowavidity ACPAs are able to penetrate into deeper tissues than highavidity ACPAs, which are more likely to be trapped in ICs [49]. Since low-avidity ACPAs are able to penetrate into deeper tissues and bind, detach and rebind easily at different locations, they produce a more profound pathological effect than high-avidity ACPAs, and this phenomenon deserves further investigation. Notably, the activity of the MAC/perforin pathway generated by the activation of complement may induce citrullinated auto-antigens that completely overlap with citrullinated auto-antigens found in RA SF cells [55]. 4.3. Stimulation of osteoclasts An intriguing question in RA pathogenesis is how the activated autoimmunity homes to the particular compartment. Recently, evidence demonstrated that such a joint-homing mechanism may be mediated by the preferential binding between ACPA and osteoclasts, which is responsible for osteolysis and bone erosion. Therefore, attention should be paid to osteoclasts, for they narrow the gap between ACPA autoimmunity and the clinical manifestation. Osteoclasts are multinucleated cells derived from the fusion of mononuclear haematopoietic myeloid lineage cells in the bone marrow [113]. In 2012, Harre et al. first reported that ACPA can preferentially bind to osteoclasts [114]. During the experiment, purified IgG ACPAs targeting different citrullinated peptides were isolated from RA patient serum or SF and were shown to be able to cause in vitro differentiation and activation of osteoclasts. Such a phenomenon could not be induced by controlled polyclonal IgGs, indicating that the stimulation ability was related to the specific recognition of citrullinated peptides. Following this idea, studies then focused on the identification of target molecules on osteoclasts, namely, citrullinated peptides. Notably, detailed studies showed that with or without the presence of ACPA, both PADs and citrullinated peptides were detectable among all stages of osteoclast differentiation under a physiological development condition, 6

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cells with prolonged longevity has been discovered [123]. Therefore, PAMPs, DAMPs and even ACPAs are able to activate APCs and stimulate the activation and proliferation of autoimmune T and B lymphocytes via bystander activation in an inflammatory environment [48]. Then, the activated APCs uptake and present citrullinated self-antigens from cell debris to auto-reactive T and B cells, accelerating epitope spreading and triggering extensive tissue damage, thereby developing a vicious circle.

[16]

[17]

[18]

[19]

5. Conclusion

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In this review, we summarised the findings that describe the role of ACPA in the interplay between the innate immune system and autoimmunity. First, environmental factors activate the innate immune system, leading to the production of citrullinated peptides and local inflammation, which contributes to the breach of self-tolerance. Second, externalized citrullinated proteins or its mimic molecule are presented by APCs to auto-reactive T/B lymphocytes, triggering the differentiation and proliferation of naïve T cells to Th or cytolytic T lymphocyte (CTL) cells and the maturation of plasma cells. Finally, ACPA produced by plasma cells evolves through molecular mimicry and epitope spreading and further activates innate immune cells, collaborates with the complement system and stimulates osteoclasts, thereby perpetuating inflammation (Fig. 2). We described a panorama from ACPA production to manifestation. The elucidation of these sequential events in RA may guide the use of proper medical intervention within the appropriate window of specific developmental phases.

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AcknowledgementsFunding

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[21] [22]

[23]

[25] [26] [27]

[28]

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This work was supported by National Basic Research Program 973 Grants (2015CB553704).Role of the funding source The funding supports in the writing of this article.Declarations of interest None.

[31] [32]

[33]

References

[34]

[1] Uysal H, Nandakumar KS, Kessel C, Haag S, Carlsen S, Burkhardt H, et al. Antibodies to citrullinated proteins: molecular interactions and arthritogenicity. Immunol Rev 2010;233:9–33. [2] Willemze A, Trouw LA, Toes RE, Huizinga TW. The influence of ACPA status and characteristics on the course of RA. Nat Rev Rheumatol 2012;8:144–52. [3] Schellekens GA, de Jong BA, van den Hoogen FH, van de Putte LB, van Venrooij WJ. Citrulline is an essential constituent of antigenic determinants recognized by rheumatoid arthritis-specific autoantibodies. J Clin Invest 1998;101:273–81. [4] Schmidt L. Rheumatoid arthritis. Sykepl Fag 2009;373:659–72. [5] Kastbom A, Strandberg G, Lindroos A, Skogh T. Anti-CCP antibody test predicts the disease course during 3 years in early rheumatoid arthritis (the Swedish TIRA project). Ann Rheum Dis 2004;63:1085–9. [6] Aletaha D, Neogi T, Silman AJ, Funovits J, Felson DT, Bingham 3rd CO, et al. 2010 rheumatoid arthritis classification criteria: an American College of Rheumatology/ European League Against Rheumatism collaborative initiative. Arthritis Rheum 2010;62:2569–81. [7] Schwentker FF, Rivers TM. The antibody response of rabbits to injections of emulsions and extracts of homologous brain. J Exp Med 1934;60:559–74. [8] Zoutendyk A, Gear J. Auto-antibodies in the pathogenesis of disease; a preliminary study of auto-sensitization of red cells in various diseases. S Afr Med J 1951;25:665–8. [9] Valesini G, Gerardi MC, Iannuccelli C, Pacucci VA, Pendolino M, Shoenfeld Y. Citrullination and autoimmunity. Autoimmun Rev 2015;14:490–7. [10] Curis E, Nicolis I, Moinard C, Osowska S, Zerrouk N, Benazeth S, et al. Almost all about citrulline in mammals. Amino Acids 2005;29:177–205. [11] Fox DA. Citrullination: a specific target for the autoimmune response in rheumatoid arthritis. J Immunol 2015;195:5–7. [12] Gyorgy B, Toth E, Tarcsa E, Falus A, Buzas EI. Citrullination: a posttranslational modification in health and disease. Int J Biochem Cell Biol 2006;38:1662–77. [13] Pratesi F, Panza F, Paolini I, Petrelli F, Puxeddu I, Casigliani-Rabl S, et al. Fingerprinting of anti-citrullinated protein antibodies (ACPA): specificity, isotypes and subclasses. Lupus 2015;24:433–41. [14] Darrah E, Rosen A, Giles JT, Andrade F. Peptidylarginine deiminase 2, 3 and 4 have distinct specificities against cellular substrates: novel insights into autoantigen selection in rheumatoid arthritis. Ann Rheum Dis 2012;71:92–8. [15] Assohou-Luty C, Raijmakers R, Benckhuijsen WE, Stammen-Vogelzangs J, de Ru A,

[35] [36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

[44]

[45]

[46]

7

van Veelen PA, et al. The human peptidylarginine deiminases type 2 and type 4 have distinct substrate specificities. Biochim Biophys Acta 1844;2014:829–36. Stensland ME, Pollmann S, Molberg O, Sollid LM, Fleckenstein B. Primary sequence, together with other factors, influence peptide deimination by peptidylarginine deiminase-4. Biol Chem 2009;390:99–107. Asaga H, Yamada M, Senshu T. Selective deimination of vimentin in calcium ionophore-induced apoptosis of mouse peritoneal macrophages. Biochem Biophys Res Commun 1998;243:641–6. Baccala R, Gonzalezquintial R, Lawson BR, Stern ME, Kono DH, Beutler B, et al. Sensors of the innate immune system: their mode of action. Nat Rev Rheumatol 2009;5:448. Matzinger P. Tolerance, danger, and the extended family. Annu Rev Immunol 1994;12:991–1045. Miyake K. Innate immune sensing of pathogens and danger signals by cell surface toll-like receptors. Semin Immunol 2007;19:3–10. Elshabrawy HA, Essani AE, Szekanecz Z, Fox DA, Shahrara S. TLRs, future potential therapeutic targets for RA. Autoimmun Rev 2017;16:103–13. Huang Q, Ma Y, Adebayo A, Pope RM. Increased macrophage activation mediated through toll-like receptors in rheumatoid arthritis. Arthritis Rheum 2007;56:2192–201. Radstake TR, Roelofs MF, Jenniskens YM, Oppers-Walgreen B, van Riel PL, Barrera P, et al. Expression of toll-like receptors 2 and 4 in rheumatoid synovial tissue and regulation by proinflammatory cytokines interleukin-12 and interleukin-18 via interferon-gamma. Arthritis Rheum 2004;50:3856–65. Huang Q, Sobkoviak RC, Ar Shi B, Tak P, Nicchitta C, Pope R. Heat shock protein 96 is elevated in rheumatoid arthritis and activates macrophages primarily via TLR2 signaling. J Immunol 2009;182:4965–73. Fric J, Zelante T, Wong AY, Mertes A, Yu HB, Ricciardi-Castagnoli P. NFAT control of innate immunity. Blood 2012;120:1380–9. Venereau E, Ceriotti C, Bianchi ME. DAMPs from cell death to new life. Front Immunol 2015;6:422. Inagaki M, Takahara H, Nishi Y, Sugawara K, Sato C. Ca2+-dependent deimination-induced disassembly of intermediate filaments involves specific modification of the amino-terminal head domain. J Biol Chem 1989;264:18119–27. Tanikawa C, Espinosa M, Suzuki A, Masuda K, Yamamoto K, Tsuchiya E, et al. Corrigendum: regulation of histone modification and chromatin structure by the p53-PADI4 pathway. Nat Commun 2012;3:676. Radic M, Marion T, Monestier M. Nucleosomes are exposed at the cell surface in apoptosis. J Immunol 2004;172:6692–700. Cocca BA, Cline AM, Radic MZ. Blebs and apoptotic bodies are B cell autoantigens. J Immunol 2002;169:159–66. Jorgensen I, Rayamajhi M, Miao EA. Programmed cell death as a defence against infection. Nat Rev Immunol 2017;17:151. Biermann MHC, Podolska MJ, Knopf J, Reinwald C, Weidner D, Maueröder C, et al. Oxidative burst-dependent NETosis is implicated in the resolution of necrosis-associated sterile inflammation. Front Immunol 2016;7:557. Brinkmann V, Reichard U, Goosmann C, Fauler B, Uhlemann Y, Weiss DS, et al. Neutrophil extracellular traps kill bacteria. Science 2004;303:1532–5. Papayannopoulos V, Metzler KD, Hakkim A, Zychlinsky A. Neutrophil elastase and myeloperoxidase regulate the formation of neutrophil extracellular traps. J Cell Biol 2010;191:677–91. Brinkmann V, Zychlinsky A. Neutrophil extracellular traps: is immunity the second function of chromatin? J Cell Biol 2012;198:773–83. Lee J, Luria A, Rhodes C, Raghu H, Lingampalli N, Sharpe O, et al. Nicotine drives neutrophil extracellular traps formation and accelerates collagen-induced arthritis. Rheumatology (Oxford) 2017;56:644–53. Khandpur R, Carmona-Rivera C, Vivekanandan-Giri A, Gizinski A, Yalavarthi S, Knight JS, et al. NETs are a source of citrullinated autoantigens and stimulate inflammatory responses in rheumatoid arthritis. Sci Transl Med 2013;5:178ra40. Dwivedi N, Neeli I, Schall N, Wan H, Desiderio DM, Csernok E, et al. Deimination of linker histones links neutrophil extracellular trap release with autoantibodies in systemic autoimmunity. FASEB J 2014;28:2840–51. Neeli I, Khan SN, Radic M. Histone deimination as a response to inflammatory stimuli in neutrophils. J Immunol 2008;180:1895–902. Spengler J, Lugonja B, Ytterberg AJ, Zubarev RA, Creese AJ, Pearson MJ, et al. Release of active peptidyl arginine deiminases by neutrophils can explain production of extracellular citrullinated autoantigens in rheumatoid arthritis synovial fluid. Arthritis Rheum 2015;67:3135–45. Sohn DH, Rhodes C, Onuma K, Zhao X, Sharpe O, Gazitt T, et al. Local joint inflammation and histone citrullination in a murine model of the transition from preclinical autoimmunity to inflammatory arthritis. Arthritis Rheum 2015;67:2877–87. Wegner N, Lundberg K, Kinloch A, Fisher B, Malmstrom V, Feldmann M, et al. Autoimmunity to specific citrullinated proteins gives the first clues to the etiology of rheumatoid arthritis. Immunol Rev 2010;233:34–54. Pratesi F, Dioni I, Tommasi C, Alcaro MC, Paolini I, Barbetti F, et al. Antibodies from patients with rheumatoid arthritis target citrullinated histone 4 contained in neutrophils extracellular traps. Ann Rheum Dis 2014;73:1414–22. Kahlenberg JM, Carmona-Rivera C, Smith CK, Kaplan MJ. Neutrophil extracellular trap-associated protein activation of the NLRP3 inflammasome is enhanced in lupus macrophages. J Immunol 2013;190:1217–26. Reynisdottir G, Olsen H, Joshua V, Engstrom M, Forsslund H, Karimi R, et al. Signs of immune activation and local inflammation are present in the bronchial tissue of patients with untreated early rheumatoid arthritis. Ann Rheum Dis 2016;75:1722–7. Pandey S, Kawai T, Akira S. Microbial sensing by toll-like receptors and

Autoimmunity Reviews xxx (xxxx) xxx–xxx

X. Dong et al.

Rheumatol 2014;26:101–7. [74] Edwards CJ, Costenbader KH. Epigenetics and the microbiome: developing areas in the understanding of the aetiology of lupus. Lupus 2014;23:505–6. [75] Huttenhower C, Kostic AD, Xavier RJ. Inflammatory bowel disease as a model for translating the microbiome. Immunity 2014;40:843–54. [76] Kaur S, White S, Bartold M. Periodontal disease as a risk factor for rheumatoid arthritis: a systematic review. JBI Libr Syst Rev 2012;10:1–12. [77] Nesse W, Westra J, van der Wal JE, Balsma J, Abbas F, Brouwer E, et al. The periodontium contains citrullinated proteins, PAD-2 enzymes and HC Gp-39. Arthritis Rheum 2009;60:S434–5. [78] Wegner N, Wait R, Sroka A, Eick S, Nguyen KA, Lundberg K, et al. Peptidylarginine deiminase from Porphyromonas gingivalis citrullinates human fibrinogen and alphaenolase: implications for autoimmunity in rheumatoid arthritis. Arthritis Rheum 2010;62:2662–72. [79] Marotte H, Farge P, Gaudin P, Alexandre C, Mougin B, Miossec P. The association between periodontal disease and joint destruction in rheumatoid arthritis extends the link between the HLA-DR shared epitope and severity of bone destruction. Ann Rheum Dis 2006;65:905–9. [80] Arleevskaya MI, Kravtsova OA, Lemerle J, Renaudineau Y, Tsibulkin AP. How rheumatoid arthritis can result from provocation of the immune system by microorganisms and viruses. Front Microbiol 2016;7:1296. [81] Trost B, Lucchese G, Stufano A, Bickis M, Kusalik A, Kanduc D. No human protein is exempt from bacterial motifs, not even one. Self Nonself 2010;1:328–34. [82] Kinloch A, Tatzer V, Wait R, Peston D, Lundberg K, Donatien P, et al. Identification of citrullinated alpha-enolase as a candidate autoantigen in rheumatoid arthritis. Arthritis Res Ther 2005;7:R1421–9. [83] Cornillet M, Verrouil E, Cantagrel A, Serre G, Nogueira L. In ACPA-positive RA patients, antibodies to EBNA35-58Cit, a citrullinated peptide from the Epstein-Barr nuclear antigen-1, strongly cross-react with the peptide beta60-74Cit which bears the immunodominant epitope of citrullinated fibrin. Immunol Res 2015;61:117–25. [84] Lundberg K, Wegner N, Yucel-Lindberg T, Venables PJ. Periodontitis in RA-the citrullinated enolase connection. Nat Rev Rheumatol 2010;6:727–30. [85] Saal JG, Steidle M, Einsele H, Muller CA, Fritz P, Zacher J. Persistence of B19 parvovirus in synovial membranes of patients with rheumatoid arthritis. Rheumatol Int 1992;12:147–51. [86] Hogeboom C. Peptide motif analysis predicts alphaviruses as triggers for rheumatoid arthritis. Mol Immunol 2015;68:465–75. [87] Christopoulos G, Christopoulou V, Routsias JG, Babionitakis A, Antoniadis C, Vaiopoulos G. Greek rheumatoid arthritis patients have elevated levels of antibodies against antigens from Proteus mirabilis. Clin Rheumatol 2017;36:527–35. [88] Tsuda R, Ozawa T, Kobayashi E, Hamana H, Taki H, Tobe K, et al. Monoclonal antibody against citrullinated peptides obtained from rheumatoid arthritis patients reacts with numerous citrullinated microbial and food proteins. Arthritis Rheum 2015;67:2020–31. [89] Tomasello G, Mazzola M, Jurjus A, Cappello F, Carini F, Damiani P, et al. The fingerprint of the human gastrointestinal tract microbiota: a hypothesis of molecular mapping. J Biol Regul Homeost Agents 2017;31:245–9. [90] Zhang X, Zhang D, Jia H. The oral and gut microbiomes are perturbed in rheumatoid arthritis and partly normalized after treatment. Nat Med 2015;21:895–905. [91] Barbut F, Collignon A, Butel MJ, Bourlioux P. Fecal microbiota transplantation: review. Ann Pharm Fr 2015;73:13–21. [92] Szodoray P, Szabó Z, Kapitány A, Gyetvai A, Lakos G, Szántó S, et al. Anti-citrullinated protein/peptide autoantibodies in association with genetic and environmental factors as indicators of disease outcome in rheumatoid arthritis. Autoimmun Rev 2010;9:140–3. [93] Syversen SW, Gaarder PI, Goll GL, Odegard S, Haavardsholm EA, Mowinckel P, et al. High anti-cyclic citrullinated peptide levels and an algorithm of four variables predict radiographic progression in patients with rheumatoid arthritis: results from a 10-year longitudinal study. Ann Rheum Dis 2008;67:212–7. [94] Meyer O, Labarre C, Dougados M, Goupille P, Cantagrel A, Dubois A, et al. Anticitrullinated protein/peptide antibody assays in early rheumatoid arthritis for predicting five year radiographic damage. Ann Rheum Dis 2003;62:120–6. [95] Abrahams VM, Cambridge G, Lydyard PM, Edwards JC. Induction of tumor necrosis factor alpha production by adhered human monocytes: a key role for Fcgamma receptor type IIIa in rheumatoid arthritis. Arthritis Rheum 2000;43:608–16. [96] Clavel C, Nogueira L, Laurent L, Iobagiu C, Vincent C, Sebbag M, et al. Induction of macrophage secretion of tumor necrosis factor alpha through Fcgamma receptor IIa engagement by rheumatoid arthritis-specific autoantibodies to citrullinated proteins complexed with fibrinogen. Arthritis Rheum 2008;58:678–88. [97] Yu X, Lazarus AH. Targeting FcgammaRs to treat antibody-dependent autoimmunity. Autoimmun Rev 2016;15:510–2. [98] Sokolove J, Zhao X, Chandra PE, Robinson WH. Immune complexes containing citrullinated fibrinogen costimulate macrophages via toll-like receptor 4 and Fcgamma receptor. Arthritis Rheum 2011;63:53–62. [99] Zhu W, Li X, Fang S, Zhang X, Wang Y, Zhang T, et al. Anti-citrullinated protein antibodies induce macrophage subset disequilibrium in RA patients. Inflammation 2015;38:2067–75. [100] Clavel C, Ceccato L, Anquetil F, Serre G, Sebbag M. Among human macrophages polarised to different phenotypes, the M-CSF-oriented cells present the highest pro-inflammatory response to the rheumatoid arthritis-specific immune complexes containing ACPA. Ann Rheum Dis 2016;75:2184–91. [101] Suurmond J, Rivellese F, Dorjee AL, Bakker AM, Rombouts YJ, Rispens T, et al. Toll-like receptor triggering augments activation of human mast cells by anti-citrullinated protein antibodies. Ann Rheum Dis 2015;74:1915–23.

intracellular nucleic acid sensors. Cold Spring Harb Perspect Biol 2014;7:a016246. [47] Ozinsky A, Underhill DM, Fontenot JD, Hajjar AM, Smith KD, Wilson CB, et al. The repertoire for pattern recognition of pathogens by the innate immune system is defined by cooperation between toll-like receptors. Proc Natl Acad Sci U S A 2000;97:13766–71. [48] Munz C, Lunemann JD, Getts MT, Miller SD. Antiviral immune responses: triggers of or triggered by autoimmunity? Nat Rev Immunol 2009;9:246–58. [49] Vossenaar ER, Smeets TJ, Kraan MC, Raats JM, van Venrooij WJ, Tak PP. The presence of citrullinated proteins is not specific for rheumatoid synovial tissue. Arthritis Rheum 2004;50:3485–94. [50] Makrygiannakis D, af Klint E, Lundberg IE, Lofberg R, Ulfgren AK, Klareskog L, et al. Citrullination is an inflammation-dependent process. Ann Rheum Dis 2006;65:1219–22. [51] Bradford CM, Ramos I, Cross AK, Haddock G, McQuaid S, Nicholas AP, et al. Localisation of citrullinated proteins in normal appearing white matter and lesions in the central nervous system in multiple sclerosis. J Neuroimmunol 2014;273:85–95. [52] Rondas D, Crevecoeur I, D'Hertog W, Ferreira GB, Staes A, Garg AD, et al. Citrullinated glucose-regulated protein 78 is an autoantigen in type 1 diabetes. Diabetes 2015;64:573–86. [53] Sorice M, Iannuccelli C, Manganelli V, Capozzi A, Alessandri C, Lococo E, et al. Autophagy generates citrullinated peptides in human synoviocytes: a possible trigger for anti-citrullinated peptide antibodies. Rheumatology (Oxford) 2016;55:1374–85. [54] Ireland JM, Unanue ER. Processing of proteins in autophagy vesicles of antigenpresenting cells generates citrullinated peptides recognized by the immune system. Autophagy 2012;8:429–30. [55] Romero V, Fert-Bober J, Nigrovic PA, Darrah E, Haque UJ, Lee DM, et al. Immunemediated pore-forming pathways induce cellular hypercitrullination and generate citrullinated autoantigens in rheumatoid arthritis. Sci Transl Med 2013;5:209ra150. [56] Carrasco-Marin E, Paz-Miguel JE, Lopez-Mato P, Alvarez-Dominguez C, LeyvaCobian F. Oxidation of defined antigens allows protein unfolding and increases both proteolytic processing and exposes peptide epitopes which are recognized by specific T cells. Immunology 1998;95:314–21. [57] Scally SW, Petersen J, Law SC, Dudek NL, Nel HJ, Loh KL, et al. A molecular basis for the association of the HLA-DRB1 locus, citrullination, and rheumatoid arthritis. J Exp Med 2013;210:2569–82. [58] Mohamed BM, Verma NK, Davies AM, McGowan A, Crosbie-Staunton K, PrinaMello A, et al. Citrullination of proteins: a common post-translational modification pathway induced by different nanoparticles in vitro and in vivo. Nanomedicine (London) 2012;7:1181–95. [59] Stolt P, Yahya A, Bengtsson C, Kallberg H, Ronnelid J, Lundberg I, et al. Silica exposure among male current smokers is associated with a high risk of developing ACPA-positive rheumatoid arthritis. Ann Rheum Dis 2010;69:1072–6. [60] Too CL, Muhamad NA, Ilar A, Padyukov L, Alfredsson L, Klareskog L, et al. Occupational exposure to textile dust increases the risk of rheumatoid arthritis: results from a Malaysian population-based case-control study. Ann Rheum Dis 2016;75:997–1002. [61] Klareskog L, Catrina AI, Paget S. Rheumatoid arthritis. Lancet 2009;373:659–72. [62] Ytterberg AJ, Joshua V, Reynisdottir G, Tarasova NK, Rutishauser D, Ossipova E, et al. Shared immunological targets in the lungs and joints of patients with rheumatoid arthritis: identification and validation. Ann Rheum Dis 2015;74:1772–7. [63] Padyukov L, Silva C, Stolt P, Alfredsson L, Klareskog L. A gene-environment interaction between smoking and shared epitope genes in HLA-DR provides a high risk of seropositive rheumatoid arthritis. Arthritis Rheum 2004;50:3085–92. [64] Klareskog L, Stolt P, Lundberg K, Kallberg H, Bengtsson C, Grunewald J, et al. A new model for an etiology of rheumatoid arthritis: smoking may trigger HLA-DR (shared epitope)-restricted immune reactions to autoantigens modified by citrullination. Arthritis Rheum 2006;54:38–46. [65] Quirke AM, Perry E, Cartwright A, Kelly C, De Soyza A, Eggleton P, et al. Bronchiectasis is a model for chronic bacterial infection inducing autoimmunity in rheumatoid arthritis. Arthritis Rheum 2015;67:2335–42. [66] Lugli EB, Correia RE, Fischer R, Lundberg K, Bracke KR, Montgomery AB, et al. Expression of citrulline and homocitrulline residues in the lungs of non-smokers and smokers: implications for autoimmunity in rheumatoid arthritis. Arthritis Res Ther 2015;17:9. [67] Klareskog L, Catrina AI. Autoimmunity: lungs and citrullination. Nat Rev Rheumatol 2015;11:261–2. [68] Anderson R, Meyer PW, Ally MM, Tikly M. Smoking and air pollution as proinflammatory triggers for the development of rheumatoid arthritis. Nicotine Tob Res 2016;18:1556–65. [69] Hildebrand F, Nguyen TL, Brinkman B, Yunta RG, Cauwe B, Vandenabeele P, et al. Inflammation-associated enterotypes, host genotype, cage and inter-individual effects drive gut microbiota variation in common laboratory mice. Genome Biol 2013;14:R4. [70] Thaiss CA, Levy M, Suez J, Elinav E. The interplay between the innate immune system and the microbiota. Curr Opin Immunol 2014;26:41–8. [71] Shamriz O, Mizrahi H, Werbner M, Shoenfeld Y, Avni O, Koren O. Microbiota at the crossroads of autoimmunity. Autoimmun Rev 2016;15:859–69. [72] Levy M, Thaiss CA, Zeevi D, Dohnalova L, Zilberman-Schapira G, Mahdi JA, et al. Microbiota-modulated metabolites shape the intestinal microenvironment by regulating NLRP6 inflammasome signaling. Cell 2015;163:1428–43. [73] Brusca SB, Abramson SB, Scher JU. Microbiome and mucosal inflammation as extra-articular triggers for rheumatoid arthritis and autoimmunity. Curr Opin

8

Autoimmunity Reviews xxx (xxxx) xxx–xxx

X. Dong et al.

[113] Boyce BF. Advances in the regulation of osteoclasts and osteoclast functions. J Dent Res 2013;92:860–7. [114] Harre U, Georgess D, Bang H, Bozec A, Axmann R, Ossipova E, et al. Induction of osteoclastogenesis and bone loss by human autoantibodies against citrullinated vimentin. J Clin Invest 2012;122:1791–802. [115] Rogoz K, Kato J, Sandor K, Su J, Jimenez-Andrade JM, Finn A, et al. Identification of a novel chemokine-dependent molecular mechanism underlying rheumatoid arthritis-associated autoantibody-mediated bone loss. Ann Rheum Dis 2016;75:721–9. [116] Catrina AI, Svensson CI, Malmstrom V, Schett G, Klareskog L. Mechanisms leading from systemic autoimmunity to joint-specific disease in rheumatoid arthritis. Nat Rev Rheumatol 2017;13:79–86. [117] Rothe L, Collin-Osdoby P, Chen Y, Sunyer T, Chaudhary L, Tsay A, et al. Human osteoclasts and osteoclast-like cells synthesize and release high basal and inflammatory stimulated levels of the potent chemokine interleukin-8. Endocrinology 1998;139:4353–63. [118] Kopesky P, Tiedemann K, Alkekhia D, Zechner C, Millard B, Schoeberl B, et al. Autocrine signaling is a key regulatory element during osteoclastogenesis. Biol Open 2014;3:767–76. [119] Wigerblad G, Bas DB, Fernades-Cerqueira C, Krishnamurthy A, Nandakumar KS. Autoantibodies to citrullinated proteins induce joint pain independent of inflammation via a chemokine-dependent mechanism. Ann Rheum Dis 2016;75:730–8. [120] Kuhn KA, Kulik L, Tomooka B, Braschler KJ, Arend WP, Robinson WH, et al. Antibodies against citrullinated proteins enhance tissue injury in experimental autoimmune arthritis. J Clin Invest 2006;116:961–73. [121] Kol A, Lichtman AH, Finberg RW, Libby P, Kurt-Jones EA. Cutting edge: heat shock protein (HSP) 60 activates the innate immune response: CD14 is an essential receptor for HSP60 activation of mononuclear cells. J Immunol 2000;164:13–7. [122] Hreggvidsdottir HS, Ostberg T, Wahamaa H, Schierbeck H, Aveberger AC, Klevenvall L, et al. The alarmin HMGB1 acts in synergy with endogenous and exogenous danger signals to promote inflammation. J Leukoc Biol 2009;86:655–62. [123] Kerkman PF, Kempers AC, van der Voort EI, van Oosterhout M, Huizinga TW, Toes RE, et al. Synovial fluid mononuclear cells provide an environment for long-term survival of antibody-secreting cells and promote the spontaneous production of anti-citrullinated protein antibodies. Ann Rheum Dis 2016;75:2201–7.

[102] Ballanti E, Perricone C, di Muzio G, Kroegler B, Chimenti MS, Graceffa D, et al. Role of the complement system in rheumatoid arthritis and psoriatic arthritis: relationship with anti-TNF inhibitors. Autoimmun Rev 2011;10:617–23. [103] Konttinen YT, Ceponis A, Meri S, Vuorikoski A, Kortekangas P, Sorsa T, et al. Complement in acute and chronic arthritides: assessment of C3c, C9, and protectin (CD59) in synovial membrane. Ann Rheum Dis 1996;55:888–94. [104] Guc D, Gulati P, Lemercier C, Lappin D, Birnie GD, Whaley K. Expression of the components and regulatory proteins of the alternative complement pathway and the membrane attack complex in normal and diseased synovium. Rheumatol Int 1993;13:139–46. [105] Trouw LA, Daha N, Kurreeman FA, Bohringer S, Goulielmos GN, Westra HJ, et al. Genetic variants in the region of the C1q genes are associated with rheumatoid arthritis. Clin Exp Immunol 2013;173:76–83. [106] Banda NK, Thurman JM, Kraus D, Wood A, Carroll MC, Arend WP, et al. Alternative complement pathway activation is essential for inflammation and joint destruction in the passive transfer model of collagen-induced arthritis. J Immunol 2006;177:1904–12. [107] Katschke Jr. KJ, Helmy KY, Steffek M, Xi H, Yin J, Lee WP, et al. A novel inhibitor of the alternative pathway of complement reverses inflammation and bone destruction in experimental arthritis. J Exp Med 2007;204:1319–25. [108] Trouw LA, Haisma EM, Levarht EW, van der Woude D, Ioan-Facsinay A, Daha MR, et al. Anti-cyclic citrullinated peptide antibodies from rheumatoid arthritis patients activate complement via both the classical and alternative pathways. Arthritis Rheum 2009;60:1923–31. [109] Zhao X, Okeke NL, Sharpe O, Batliwalla FM, Lee AT, Ho PP, et al. Circulating immune complexes contain citrullinated fibrinogen in rheumatoid arthritis. Arthritis Res Ther 2008;10:R94. [110] Arnold JN, Wormald MR, Suter DM, Radcliffe CM, Harvey DJ, Dwek RA, et al. Human serum IgM glycosylation: identification of glycoforms that can bind to mannan-binding lectin. J Biol Chem 2005;280:29080–7. [111] Suwannalai P, Britsemmer K, Knevel R, Scherer HU, Levarht EW, van der Helmvan Mil AH, et al. Low-avidity anticitrullinated protein antibodies (ACPA) are associated with a higher rate of joint destruction in rheumatoid arthritis. Ann Rheum Dis 2014;73:270–6. [112] Kaufman EN, Jain RK. Effect of bivalent interaction upon apparent antibody affinity: experimental confirmation of theory using fluorescence photobleaching and implications for antibody binding assays. Cancer Res 1992;52:4157–67.

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