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Rheumatology 2016;55:391–402
doi:10.1093/rheumatology/keu469
Advance Access publication 23 December 2014
RHEUMATOLOGY
RA: from risk factors and
pathogenesis to prevention
Gene, environment, microbiome and mucosal
immune tolerance in rheumatoid arthritis
Anca I. Catrina1, Kevin D. Deane2 and Jose U. Scher3
RA is a complex multifactorial chronic disease that transitions through several stages. Multiple studies
now support that there is a prolonged phase in early RA development during which there is serum elevation of RA-related autoantibodies including RF and ACPAs in the absence of clinically evident synovitis.
This suggests that RA pathogenesis might originate in an extra-articular location, which we hypothesize is
a mucosal site. In discussing this hypothesis, we will present herein the current understanding of mucosal
immunology, including a discussion about the generation of autoimmune responses at these surfaces. We
will also examine how other factors such as genes, microbes and other environmental toxins (including
tobacco smoke) could influence the triggering of autoimmunity at mucosal sites and eventually systemic
organ disease. We will also propose a research agenda to improve our understanding of the role of
mucosal inflammation in the development of RA.
Key words: rheumatoid arthritis, environmental risk factors, genetic susceptibility, microbiome, break in
tolerance.
Rheumatology key messages
Environmental and microbial challenges at mucosal sites might initiate autoimmunity in genetically susceptible
hosts.
. Mucosal autoimmunity precedes disease onset in RA and may contribute to disease pathogenesis.
. Natural history studies are needed to understand the role of mucosal sites in RA development.
.
Introduction
RA is one of the most common forms of inflammatory
arthritis. Notable advances in the understanding of its
pathogenesis have been achieved in recent years. One
of the major concepts developed by multiple lines of investigation posits that there is a period in the development
of seropositive RA during which there is elevation of autoantibodies, including RF and ACPAs, several years prior to
1
Rheumatology Unit, Department of Medicine, Karolinska University
Hospital and Institutet, Stockholm, Sweden, 2Division of
Rheumatology, University of Colorado, School of Medicine, Aurora,
CO and 3Division of Rheumatology, Department of Medicine, New
York University School of Medicine and Hospital for Joint Diseases,
New York, NY, USA
Submitted 14 April 2014; revised version accepted 22 October 2014
Correspondence to: Anca I. Catrina, Rheumatology Unit, Department
of Medicine, Karolinska University Hospital, Karolinska Institutet,
Stockholm, Sweden. E-mail: [email protected]
Anca I. Catrina, Kevin D. Deane and Jose U. Scher contributed equally
to this work.
a diagnosis of RA. This autoantibody production occurs in
the absence of synovitis (as determined either by physical
examination, imaging or, in some cases, synovial biopsy)
[1–13].
While the specific aetiology of RA remains elusive,
this preclinical period of RA development implies that
the disease is initiated at a site outside of the joints.
While the exact location is unknown, established and
emerging data discussed below support the central
hypothesis for our current studies, namely that events
leading to RA autoimmunity might originate at a mucosal
site.
In this review we will discuss this hypothesis by first succinctly describing the structure, development and function
of mucosal surfaces. In particular we will examine the role
of microbes in shaping mucosal and systemic immune responses. We will also outline how microbes, as well as
other factors such as smoking, can trigger immune responses at mucosal sites and eventually lead to RA.
Finally, we propose a research agenda for greater
! The Author 2014. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: [email protected]
R EV I E W
Abstract
Anca I. Catrina et al.
understanding of the role of mucosal immune responses in
the initiation of autoimmunity and RA.
Mucosal structure and function
There are a variety of mucosal sites in humans, including
the eye, respiratory tract, gastrointestinal tract and genitourinary tract, as well as mammary glands and serosal
sites such as the pleural and peritoneal cavities [14–16]. In
addition, there are multiple subsites with unique immunological characteristics. As examples, the oral cavity has
salivary glands as well as subgingival spaces; the respiratory tract can be divided into the nasopharynx (and even
the middle ear), the airways (including the trachea and
bronchi) and the aveolar spaces [17].
In general, the mucosa consists of epithelial cells that
form a surface barrier, with hair (in some areas) and a
coating of mucus contributing to that barrier (Fig. 1). In
some sites, such as the larger airways in the lung, these
epithelial cells have cilia that contribute to the removal of
foreign material. On the luminal surface there is a variety
of components of the immune system, including immunoglobulins, complement and cells that include neutrophils,
macrophages, dendritic cells and T and B cells [15,
18–23]. Intermixed with the mucosal epithelial cells are
cells that produce mucus (e.g. goblet cells), cells with
endocrine function (e.g. enteroendocrine cells in the intestine), cells that participate in a non-specific fashion in host
defence (e.g. Paneth cells in the gut) and cells that participate in antigen recognition and presentation, including
dendritic cells and M cells. In addition, there are intraepithelial lymphocytes that serve to maintain mucosal
homeostasis and are similar to T cells [24]. Underlying
the epithelial cell layer in the lamina propria are blood
vessels and lymphatics as well as cells that include neutrophils, mast cells, macrophages, dendritic cells, T cells
(including effector T cells producing IL-17, Th17 cells and
Tregs), B cells, as well as NK cells [25].
The mucosal surfaces sit at the interface between the
environment and the host and have multiple means that
involve both innate and adaptive factors to manage not
only potential threats from the environment, such as
toxins and pathogens, but also factors that may be beneficial to the organism, such as commensal bacteria.
Among the innate factors, lysozyme, lactoferrin, complement, immunoglobulin and cellular elements such as neutrophils and macrophages play important roles. In
addition, areas of submucosal organized lymphatic
tissue called mucosa-associated lymphoid tissue (MALT)
[20, 26] are present in most mucosal surfaces. In the gut,
MALT and in particular Peyer’s patches form in utero and
with influence from endogenous factors [14]; in contrast,
MALT tissue in the nasopharynx begins development only
after the tissue is exposed to exogenous flora [27]. In welldeveloped MALT, cells such as M cells, dendritic cells and
macrophages can sample antigens and lead to immune
responses. In the lung, an ectopic lymphatic tissue called
bronchus-associated lymphatic tissue can form, with local
production of antibodies and class switching that can aid
in clearance of local insults, apparently only in the
392
presence of inflammation or as a consequence of microbial pathogens [28–30]. Immune responses generated in
MALT and ectopic lymphoid structures can then traffic
first to regional lymphatics, then systemically and finally
back through the circulation to mucosal sites (such as the
gut lamina propria) where they can perform effector functions [17, 19]. In particular, several molecules including
a4b7 integrin are known to facilitate effector cell homing
to the gut mucosa [31]. However, little is known about the
specific factors that may induce effector cell homing in
other tissues, although these factors likely exist [31].
Immunoglobulins are central players in mucosal immunity. All of the immunoglobulin isotypes (IgA, IgD, IgE, IgG
and IgM) may be present at mucosal surfaces [19]; however, the hallmark of mucosal immune responses is the
presence of IgA, which is typically in its secretory form
(sIgA). IgG is also present at mucosal sites, and can
arrive by active transport typically through the neonatal
Fc receptor, diffusion from the circulation or local production [17]. IgM is also present at mucosal surfaces, typically
in its secretory form. IgD may also play an important role
in mucosal responses, including a role in basophil activation and cytokine secretion (IL-4, IL-13) and in particular is
present in secretions from the upper airway and nares and
in human breast milk [17].
Overall, mucosal immunological structure and function
allow for protection against invasion of harmful factors
through both mechanical barriers and immune responses.
In addition, the mucosa contributes to the generation of
beneficial immune responses of protective immunity to
many natural infections, allowing the use of oral vaccines,
enteric viruses and pathogens and of a nasal vaccine
against influenza [32]. However, immune responses that
initiate at mucosal surfaces can also lead to harm and in
the next sections we discuss in detail how the mucosa
balances defence with homeostasis and cooperation
with common environmental factors, including the microbiome, and how these relationships may go awry and lead
to autoimmunity.
Microbiome physiology in mucosal sites
The microbiome, as defined by Joshua Lederberg, is composed of the totality of the ecological communities of symbiotic, commensal and pathogenic microorganisms (and
their genomes) that literally share our body space [33]. It
has been estimated that about 100 trillion microorganisms
live in and on our body spaces and surfaces, outnumbering human cells by a factor of 10 and total protein-coding
genes by a factor of 100. Importantly, each mucosal site
harbours its own set of distinct microbial communities that
exist in the unique mucosal environments.
This characterization of the human microbiome in health
and disease states has been catapulted by advances in
bacterial DNA-sequencing technologies [34]. In fact, fewer
than 20% of bacterial species can be cultured using classical microbiological approaches. Largely due to efforts
such as those of the National Institutes of Health Human
Microbiome Project [35] and the European Metagenomics
of the Human Intestinal Tract consortium, an almost
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Mucosal immune tolerance in RA
FIG. 1 Overview of the immune response in the tonsil and regional lymph node as an example of the mechanisms of
immunity at mucosal sites
Tonsils, adenoid tissue and the intestine contain M cells that mediate antigen uptake into the tissue rich with lymphoid
follicles (A) in which the primary expansion of naive B cells occurs (dendritic cells can also uptake antigen at mucosal
sites through processes that extend into the lumen). Antigen presentation is followed by the subsequent generation of
memory B cells that populate other lymphoid tissues, especially regional lymph nodes. Dark and light zones containing
centroblasts and centrocytes and the mantle zone, in which dendritic cells, B cells and T cells collaborate for B cell
activation, are shown in the upper right of (B). A similar expansion of memory (and naive) B cells can occur with secondary exposure in the lymph nodes that are draining the airways as well as other mucosal sites. FDC: follicular dendritic
cells; HEV: high endothelial venule. Figure reprinted from Kato A et al., B-lymphocyte lineage cells and the respiratory
system. J Allergy and Clinical Immunol 2013;131:933-57 [17], with permission from Elsevier.
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Anca I. Catrina et al.
complete catalogue of oral, airways, intestinal and skin
microbial communities is now available. This characterization of bacterial communities and its biological relationship to mucosal immunology responses have led to new
advances in our understanding of their role in health and
disease [36]. It has also opened new fields of research
suggesting that the microbiome could potentially serve
as an environmental factor leading to autoimmunity and
related clinical manifestations, as demonstrated by several studies in IBD, psoriasis and inflammatory arthritis
[37–39].
For the most part, however, our microbiome fulfils complementary physiological functions vital for our survival,
including assisting with metabolic activity and nutrition.
In addition, as discussed in more detail below, the microbiome is fundamental for the development of the mucosal
immune system and defence against luminal pathogens.
Development of the mucosal immune
system: relationship with the microbiome
Although humans undergo embryogenesis under sterile
conditions, immediately postpartum the newborn’s body
is populated by a range of microbes originating in the
surrounding environment. Thereafter, and for life, this
vast and dynamic community of microorganisms coexists
with us in a complex but mutually beneficial relationship.
Initially, however, a period of floral instability is the norm,
particularly in the gastrointestinal tract [40]. Slowly, and
during the first year of life, both the taxonomic richness
and diversity of bacterial species increase. When solid
foods are introduced, the gut microbiota expands, becomes more stable and begins to mimic the characteristics of the adult communities [41]. The neonatal period is
critical in the establishment of the microbiome and its relationship with the host. Microbial colonization of the intestinal lumen and other sites has a profound effect on the
development and function of the immune system. Animals
kept under germ-free conditions have impaired development of the mucosal immune system, including lymphoid
tissue genesis and organization of Peyer’s patches and
lymphoid follicles, secretion of antimicrobial and bactericidal peptides by epithelial cells and mucosal accumulation of immune cells. Immunoglobulins (sIgA) delivered by
breastfeeding prevent the translocation of aerobic bacteria from the neonatal gut into draining lymph nodes
and results in a protective pattern of intestinal epithelial
cell gene expression in adult mice [42]. Throughout adult
life, the microbiota continues to affect the host immune
system utilizing multiple signal mechanisms, including microbial components and their metabolites. In turn, the
immune system is capable of recognizing these factors
by activating innate immune receptors. The armamentarium used to prevent tissue damage and antigen translocation includes cellular repairing factors, antimicrobial
proteins and secretory sIgA [43–45].
Mucosal microbiota influences innate immune recognition and leads to the development of a diverse and specific mucosal lymphocyte repertoire [46]. Recent
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paradigm-shifting studies using gnotobiotic experiments
have demonstrated how individual components of the
microbiota can induce specific populations of immune
cells and alter the balance between pro-inflammatory
and Tregs at mucosal sites and in the periphery. This
has challenged the assumption that microbes and/or
their components are only able to activate the innate
immune system. The gut commensal segmented filamentous bacterium (SFB), for example, is sufficient to activate
Th17 cells in the lamina propria [47] and eventually trigger
autoimmunity and inflammatory arthritis [48] (see section
Environmental and genetic factors contributing to the generation of autoimmunity at mucosal sites below). One
plausible explanation derives from a recent study showing
that the TCR repertoire of intestinal Th17 cells in SFBcolonized mice is highly specific and that most Th17
cells, but not other T cells, recognize antigens encoded
by SFB. This explains potential mechanisms of Th17 cell
induction by microbiota and how gut-induced Th17 cells
can contribute to distal organ-specific autoimmunity [49].
For their part, Bacteroides species and their molecule
polysaccharide A are unique in that they appear to be
specific inducers of Tregs in the mucosa [50] and provide
protection against the development of IBD [51] and multiple sclerosis in an animal model [52].
In humans, microbiota composition is influenced by
diet, antibiotic use, infections and possibly the host
genome. When this fine equilibrium is altered in an unfavourable way, a state of dysbiosis ensues, typically characterized by an overgrowth of potentially pathogenic
bacteria (termed pathobionts) and/or a decrease in the
number of beneficial bacteria [53, 54]. A growing body
of evidence has shown a correlation between dysbiosis,
autoimmunity and systemic inflammation. This is true for
both animal models and human studies and has been reported in various conditions, including not only IBD [55,
56], but also systemic autoimmune diseases such as type
1 diabetes [57], encephalomyelitis [58] and RA [48, 59, 60].
The following section provides detailed evidence for the
implication of the microbiome in local and systemic autoimmune processes.
Environmental and genetic factors
contributing to the generation of
autoimmunity at mucosal sites
Several indirect lines of evidence support mucosal surfaces as sites of generation of RA-related autoimmunity.
In particular, Barra et al. [61] demonstrated that the proportion of IgA ACPAs was higher than IgG in subjects at
risk for future RA; intriguingly, in patients with established
RA, the proportion of IgG ACPAs was higher than IgA. This
finding supports the notion that early RA-related autoimmunity may be triggered by mucosal processes generating IgA-related autoimmunity, later transitioning into an
IgG response that eventually leads to clinical manifestations of disease (i.e. synovitis). There have also been
associations (albeit controversial) between serological evidence for certain infections, including Proteus and
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Mucosal immune tolerance in RA
TABLE 1 Mechanisms by which the mucosa may be involved in the development of autoimmunity
The mucosal surface serves as site of initial contact and as portal of entry for foreign antigens.
The mucosal surfaces contain regional lymph structures (MALT or regional lymph nodes) and in certain conditions ectopic
lymphoid tissue.
Cross-reactivity might occur between foreign antigens (which may be microbial in origin) and self (e.g. rheumatic fever).
Mucosal alteration of self-proteins (e.g. citrullination by pathogen-mediated inflammation or carbamylation through microbial-related respiratory burst.
Mucosa serves to educate the immune system, leading to a host that is more susceptible to autoimmunity through alteration
of regulatory cells.
Mucosal sites might be targeted by systemic autoimmunity resulting in local immune-mediated injury.
MALT: mucosa-associated lymphoid tissue.
Mycoplasma species, and risk for RA [62–64]. In addition,
Mikuls et al. [65] demonstrated an association between
RA-related autoantibodies in healthy, first-degree relatives
of probands with RA and serum antibody titres against
Porphyromonas gingivalis—an organism implicated in
human periodontal disease (PD)—suggesting that
immune responses to P. gingivalis may play a role in
early RA-related autoimmunity.
There are several mechanisms by which the mucosa
might contribute to the development of RA-associated
autoimmunity (Table 1). Cross-reactivity may occur between mucosal antigens (which may be microbial in
origin) and self-proteins, a process thought to occur in
rheumatic fever, where pharyngeal infection with certain
species of Streptococcus leads to the generation of
antibodies that cross-react with self-antigens in the
heart and other tissues [66]. In addition, mucosal
generation of neo-antigens (e.g. citrullination) by pathogen-mediated inflammation and peptidylarginine deiminase activation, or carbamylation, could occur at
mucosal sites through microbial-related respiratory burst
[67–69].
In RA, the finding of local enrichment of ACPA in both
bronchoalveolar fluid of early untreated RA patients and in
induced sputum of arthritis-free individuals at risk of
developing RA [70] (in some cases, even in the absence
of the same antibodies in the blood) supports the notion
that autoimmunity might indeed originate at mucosal
sites. In line with this, tissue inflammation and ectopic
lymphoid structures were described in bronchial biopsies
of patients with early untreated ACPA-positive RA in the
absence of any other associated lung disease [71]. This
was also shown in lung biopsies of ACPA-positive individuals with chronic lung disease but no signs of joint inflammation [72]. Interestingly, similar structures containing
citrullinated protein-binding B cells have also been
reported in patients with established RA and associated lung disease [73]. Signs of lung inflammation
have further been described by high-resolution CT
imaging of both ACPA-positive healthy individuals at
high risk for RA development, even in the absence of
smoking [8], and patients with early untreated ACPApositive RA [74]. An alternative explanation for ectopic
lymphoid tissue formation is a secondary immune
injury of the lungs induced by ACPA in the presence
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of citrullinated proteins in the lungs. However, these two
different scenarios are complementary and not mutually
exclusive.
In additional support of a mucosal-based trigger for RA,
exposure to tobacco smoke is the strongest environmental risk factor associated with RA, with some estimates
that smoking explains 30% of the risk for ACPA-positive
RA [75]. However, despite this association, the role of
smoking in disease pathogenesis is still uncertain.
Original studies showed that smoking increases the expression of citrullinated proteins in the lungs of healthy
smokers [76]. This was later confirmed for early untreated
RA patients as well [74]. Interestingly, increased expression of citrullinated proteins was present not only in smokers, but also in ACPA-positive non-smokers, suggesting
that factors other than smoking might also contribute to
the generation of citrullinated epitopes in the lungs.
The presence of susceptibility genes (and in particular
HLA-DR SE) in smokers further increases the risk of developing seropositive RA, as demonstrated by several
large epidemiological investigations [77–83]. This might
be explained through specific interaction of the susceptibility genes with citrullinated but not native (i.e. arginine)
epitopes [84–86]. In addition, peripheral blood B cells of
both RA patients and healthy individuals carrying the SE
alleles show increased ACPA production when exposed
to smoking, suggesting that both environment and genetics influence the pool of autoreactive B cells in healthy
subjects [87]. Somewhat controversial data have recently
emerged from studies in animal models exposed to
chronic cigarette smoking [88]. In these experiments, exposure of HLA-DR4 transgenic mice to cigarette smoking
unexpectedly suppressed CIA, while enhancing innate immunity and mounting a robust response to citrullinated
vimentin. In contrast, similar exposure in HLA-DQ8 transgenic mice (occurring in linkage with DR4 in humans) not
only augmented the antigen-specific adaptive T cell responses to native and citrullinated proteins, but also worsened the course of CIA.
One interpretation of these findings is that DR4 contributes to autoimmunity by enhancing innate immune responses potentially following bacterial challenge, while
DQ8 might contribute to antigen-specific autoreactive
processes. However, the arthritis-suppressive effect of
smoking in HLA-DR transgenic mice and the relevance
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of these findings in human disease remain to be
elucidated.
Microbial factors contributing to the
generation of autoimmunity at mucosal
sites and their interaction with
environmental and genetic factors
As mentioned above, inconclusive studies have implicated infections with specific organisms in the pathogenesis of RA [62, 63, 89, 90]. The emerging complexity of the
human microbiome, coupled with new methods for culture-independent microbial DNA sequencing, today
allows studies on how the microbiome interacts with genetic and environmental factors and contributes to disease
[60, 91].
Microbiome composition is partially determined by the
host genome, as first suggested by early studies in twins
[92, 93], although some conflicting observations also exist
[94, 95]. Subsequent studies in congenic mouse strains
have revealed that MHC genes might have a prominent
effect in determining the composition of the gastrointestinal microbiota [96]. While these observations do not necessarily imply the same genetic associations as for RA,
studies in HLA-DR transgenic mice have shown differences in the relative abundances of gut microbiota
between arthritis-susceptible HLA-DRB1 0401 and arthritis-resistant HLA-DRB1 0402 transgenic mice [97].
Moreover, germ-free conditions lead to abrogation
of spontaneous arthritis in IL-1 receptor antagonist
knockout mice [59] and attenuation of spontaneous
arthritis in K/BxN TCR transgenic mice. Interestingly, the
wild-type animals do not develop arthritis, even in the
presence of pro-arthritogenic gut flora [48]. It has also
been shown that new-onset untreated seropositive RA
patients have an overexpansion of gut Prevotella copri
[60], a recently described species with connection to
IBD and atherosclerosis [98]. Curiously, RA patients with
a greater abundance of P. copri are mostly those with
negative SE alleles.
Beyond the gut, the oral and respiratory tract microbiome, and in particular changes in the oral microbiome
related to PD, have long been implicated in the pathogenesis of RA. It is well known that PD shares multiple risk
factors with RA, including smoking and the genetic association with HLA-DR, but the exact causality or association between these two disease states is not yet
completely understood [99]. Interestingly, a-enolase of
both bacterial (P. gingivalis derived) and human origin
was immunogenic in HLA-DR transgenic mice, with generation of antibodies recognizing both native and citrullinated forms of human enolase. However, an arthritogenic
effect could not be replicated. A possible explanation for
this discrepancy may be attributed to local environmental
conditions, in particular the pathogen status in different
specific pathogen-free facilities [100, 101].
Importantly, despite data documenting a relevant role
for the lower respiratory tract as a mucosal initiating site of
RA-associated autoimmunity [70, 74, 102], no study to
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investigate the lung microbiome in RA is currently available. Preliminary data suggest that the lung microbiome is
different in asymptomatic subjects with elevated risk of
future RA when compared with healthy controls [103]. It
should be noted that environmental factors—and smoking
in particular—have a large effect on the mucosal microbiome composition [104–111]. These are issues that will
need to be explored in future studies.
Expansion of localized autoimmunity to
joints and distal sites
As mentioned above, antibodies are released in the peripheral blood and circulated in the body for years before
any clinical sign of joint inflammation [5–7, 112–115]. This
strongly suggests that antibodies are generated at extraarticular sites, but does not completely exclude the possibility that they might still be produced in the joints in the
absence of any macroscopic and/or microscopic signs of
inflammation. These healthy individuals lack not only clinical complaints, but also any clear-cut sign of joint inflammation [9, 116], raising the possibility that antibodies are
passive bystanders of the disease. However, ACPAs are
able to exacerbate existing minimal joint disease by passive transfer in mice [117] and possess several effector
pathogenic functions. ACPAs activate the complement
system [118] and promote macrophage activation when
incorporated in immune complexes via either Fcg receptors or TLR-4-dependent mechanisms [119]. More recently, antibodies against mutated citrullinated vimentin
were shown to promote bone resorption in vitro and to
induce osteoclastogenesis by adoptive transfer into
mice [120]. ACPAs can also stimulate neutrophils to release neutrophil-derived extracellular traps and promote
inflammation [121] (Fig. 2).
Despite these advances in understanding how ACPAs
might contribute to perpetuation of joint inflammation, it
remains unclear what event or series of events is required
for the delayed initiation of this joint inflammation, although there are several possibilities. First, it is plausible
that a yet unidentified second hit (such as minor trauma or
transient infection/microbiota community alteration) could
lead to expression of citrullinated proteins in an otherwise
citrullinated protein-poor healthy joint [67] and that these
antigens would then be targeted by the pre-existing circulating autoantibodies, ultimately leading to clinical signs
of joint inflammation. This would imply that circulating
autoantibodies that may have been generated in response
to mucosal antigens would target similar antigens in the
joint. Supporting this, it has recently been shown that the
same citrullinated peptides could be identified by mass
spectrometry in the lungs and joints of RA patients
[122]. Second, it is possible that sites other than the synovial membrane are the first joint component to be affected, with secondary synovial involvement being an
epiphenomenon. In line with this, recent studies utilizing
microcomputer tomography showed that signs of bone
destruction are present before clinical onset of synovial
inflammation [121]. Third, progressive epitope spreading
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Mucosal immune tolerance in RA
FIG. 2 A schematic representation of how mucosal disequilibrium might lead to generation of autoimmunity and later to
joint disease development
Complex mechanisms dependent on environmental exposure and the host microbiome are responsible for maintaining
homeostasis at mucosal surfaces (such as the respiratory and gastrointestinal tract). In genetically susceptible hosts,
failure of these mechanisms may lead to mucosal disequilibrium and molecular changes such as post-translational
modifications (citrullination), with subsequent antigen presentation by professional antigen presenting cells, activation of
immune effector T cells (such as Th17) and relative deficiency of Tregs. These changes lead to activation of B cells and
generation of antibodies (such as ACPAs) by plasma cells. These antibodies undergo somatic hypermutation and epitope
spreading, leading to joint disease initiation and perpetuation through several mechanisms (complement activation, cell
surface receptor activation, ligation of cell surface components and neutrophil activation).
[123] and the emergence of subclinical inflammation
(as seen for certain cytokines and chemokines) [124,
125] might be needed to alter the number and/or specificity profile required by ACPAs to gain these effector functions. In this context, it is worth mentioning that all
available evidence showing direct pro-arthritogenic properties of ACPAs have been obtained using antibodies purified from peripheral blood and/or SF of patients with
already established disease. This leaves open the possibility that antibodies might emerge from mucosal interactions first as non-pathogenic immunoglobulins during
the preclinical phase only to gain arthritogenic properties
through epitope spreading or pathogenic changes in avidity, affinity or Fc function at a later time point. These
changes could potentially allow ACPAs to target the
joints and cause inflammation. These issues remain to
be addressed in high-quality natural history studies of
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RA that can include broad evaluations of innate and adaptive immunity at mucosal, systemic and joint sites.
Summary and future directions
As discussed above, several aspects of the mucosal
system biology, including its relationship to a variety of
commensal as well as pathogenic organisms, make it an
attractive frontier in rheumatological research. However,
several challenges on how to explore this frontier are still
elusive (Table 2). Perhaps the most important factor in
advancing the understanding of the role of mucosal biology in the development of rheumatic disease will be the
careful utilization of well-characterized human cohorts followed longitudinally in various phases of development of
rheumatic disease. These should range from a healthy
state to preclinical disease (i.e. where there is evidence
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Anca I. Catrina et al.
TABLE 2 Research agenda for the study of mucosal biology in relation to rheumatic diseases
High-quality longitudinal natural history studies of rheumatic disease that can be used to evaluate the temporal relationship
between mucosal inflammation, exposure to environmental risk factors (including the microbiome) and genetics on the
development of autoimmunity.
Robust, safe and feasible methods to assess mucosal biology in human subjects. Methods to reliably obtain high-quality
biospecimens that can be tested for a variety of factors, including metabolic, immune and microbiome, need to be
established for each mucosal region.
Robust analytical methods to evaluate the relationship between the microbiome and mucosal inflammation and autoimmunity. These methods should include means to assess microbial relationships between sites (e.g. oral and lung, gut and
genitourinary) and allow for robust assessment of changes over time.
Identification of specific mechanisms by which autoimmunity is generated at a mucosal site (molecular mimicry, alteration of
human proteins or creation of an inflammatory environment in which autoimmunity develops).
Identification of humoral and cellular immune mechanisms by which autoimmunity may start at a mucosal site then affect
distant tissue (lymphoid trafficking, circulating immune complexes or a second hit in the target organ).
Identification of mucosal-based therapy for the treatment and/or prevention of rheumatic disease (potential role of antibiotics,
probiotics, mucosal target of specific immune pathways, mucosal generation of tolerance).
Evaluation of how known risk factors for rheumatic disease (sex, smoking, shared epitope) impact mucosal immunity and fit
with a model of disease development at a mucosal site.
Identification of mechanisms by which a mucosal surface (such as the lungs) may be a site of initiation of autoimmunity as
well as a target of immune-mediated inflammation and damage.
Development of highly relevant animal models to test specific hypotheses about the role of the mucosa in rheumatic disease.
of autoimmunity but in the absence of clear target organ
injury) and established disease (both before and after the
initiation of immunomodulatory therapies that can alter
mucosal inflammation and the microbiome). Given the
great promise that the study of mucosal immunity holds
for understanding the development of rheumatic and
other autoimmune diseases, understanding this biological
frontier should be in the vanguard of research in the field
of human rheumatology, with the ultimate goal of
elucidating pathways that can lead to preventive tools
and strategies.
Acknowledgements
A.I.C. was supported by the Swedish Foundation for
Strategic Research, Innovative Medicine Initiative BTCu
re (115142-2), FP7th framework program Euro-TEAM
(305549-2), the Initial Training Networks 7th framework
program Osteoimmune (289150) and the Swedish
Research Council. K.D. was supported by grants from
the National Institutes of Health (AI103023), the
Rheumatology Research Foundation and the Walter S.
and Lucienne Driskill Foundation. J.U.S. was supported
by grants from National Institutes of Health/NIAMS (5
K23 AR064318-02) and the Arthritis Foundation.
Funding: No specific funding was received from any funding bodies in the public, commercial or not-for-profit sectors to carry out the work described in this manuscript.
Disclosure statement: The authors have declared no
conflicts of interest.
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