Innate Immunity and
Rheumatoid Arthritis
Angelica Gierut, MD, Harris Perlman, PhD, Richard M. Pope, MD*
Although environmental insults such as smoking have been implicated in the initiation
of rheumatoid arthritis (RA) in patients who express the shared epitope, the understanding of the role of innate immunity in the pathogenesis of this disease is also
expanding. The clinical picture of pain, stiffness, swelling, and joint destruction seen
in RA is a result of chronic inflammation of the synovium, characterized by interactions
of fibroblast-like synoviocytes with cells of the innate immune system, including
macrophages, dendritic cells (DCs), mast cells, and natural killer (NK) cells, as well
as cells of the adaptive immune system, B and T lymphocytes.1 Also present are
immune complexes; proteins of the complement system; autocrine- and paracrineacting cytokines; as well as chemokines that have inflammatory, homeostatic, and
even antiinflammatory properties.2 As knowledge of the complexities of RA grows,
gaps in the understanding of its pathogenesis are filled and new potential therapeutic
targets are uncovered.
The best-known function of the innate immune system is the initial recognition of
microbial pathogens. On encounter with nonself, primarily by macrophages and
DCs via membrane-bound or intracellular pattern-recognition receptors (PRRs), cells
of the innate system become activated, leading to the production of inflammatory
cytokines and chemokines. Effector cells and molecules of the innate system are
recruited locally, and if unable to overcome the pathogen alone, macrophages and
DCs travel to local lymphoid tissues where processed antigens are presented by major
histocompatibility complex (MHC) molecules to naive T cells, thus initiating an adaptive response complete with lasting immunologic memory. On clearance of the
organism, with the help of opposing antiinflammatory mediators, the inflammatory
response is terminated.3 In RA, however, ‘‘self’’ is either the primary target or an innocent bystander that then becomes the focus of attack. In RA, there is abundant
evidence that the innate immune system is persistently activated, as evidenced by
the continual expression of macrophage-derived cytokines, such as tumor necrosis
This work is supported by grants from the National Institutes of Health: AR050250, AI067590,
AR054796 to HP and AR049217 to RMP.
Division of Rheumatology, Department of Medicine, Feinberg School of Medicine, Northwestern University, 240 East Huron Street, McGaw M300, Chicago, IL 60611, USA
* Corresponding author.
E-mail address: rmp158@northwestern.edu
KEYWORDS
Macrophage Fibroblast Receptor
Rheum Dis Clin N Am 36 (2010) 271–296
doi:10.1016/j.rdc.2010.03.004 rheumatic.theclinics.com
0889-857X/10/$ – see front matter. Published by Elsevier Inc.
factor a (TNF-a), interleukin (IL) 1, and IL-6. As the understanding of the innate immune
system in RA continues to expand, enticing targets for new therapeutic interventions
continue to be identified. This article focuses on cells of myelomonocytic origin, their
receptors, and factors that interact with them.
MONOCYTES AND MACROPHAGES
Background and Role in RA
Macrophages, together with osteoclasts and myeloid DCs, are derived from myelomonocytic origins and are key cellular components of the innate immune system.
Macrophages differentiate from circulating monocytes and have primary roles in
tissues as phagocytes of invading pathogens and as scavengers of apoptotic debris.
In addition, macrophage activation results in the expression of chemokines and cytokines, such as TNF-a and IL-1b, that helps to attract other cells and proteins to the
sites of inflammation.3 The central role of macrophages in RA pathogenesis is supported by the fact that conventional therapies, including methotrexate and cytokine
inhibitors, act to decrease the production of cytokines that are produced primarily
by macrophages.4 Indeed, a correlation has been found between synovial macrophage infiltration and subsequent radiographic joint destruction.5 A remarkable fact
is that a reduction in the number of sublining macrophages in RA synovial tissue
has been shown to strongly correlate with the degree of clinical improvement, regardless of the type of therapy chosen.6 In addition to local effects of macrophages in the
synovial tissue, systemic consequences of macrophage-mediated inflammation in RA
may be manifested by damage to other areas such as the subendothelial space where
macrophages become foam cells contributing to atherosclerotic plaques.7
Mechanisms for Increased Macrophage Number in RA Tissue
Possible mechanisms for the increased number of macrophages in diseased tissue
include increased chemotaxis8,9 and reduced emigration.10 Some studies also
suggest local proliferation of macrophages in areas of inflammation.11–14 Decreased
apoptosis may also contribute to the accumulation of macrophages in the RA joint.
Several studies have shown that induction of synoviocyte apoptosis in animal models
of inflammatory arthritis ameliorates joint inflammation and joint destruction.15 In both
experimental arthritis and synovial tissue from patients with RA, reduced expression of
the proapoptotic Bcl-2 family member, Bim, was seen in macrophages and corresponded to the increased expression of IL-1b by macrophages. Furthermore, administration of a Bim mimetic dramatically reduced the incidence of arthritis and
successfully ameliorated established arthritis in mice.16 This result suggests that
therapies that restore the homeostasis between survival and cell death of RA
macrophages may be successful in ameliorating arthritis in patients.
Heterogeneity of Monocyte and Macrophage Populations
Within monocyte and macrophage populations, there is a great deal of heterogeneity.
For example, 2 human monocyte populations have been defined based on their
surface marker expression: the CD141CD16 and the CD14lowCD161 subsets.17
CD16 is a receptor for immunoglobulin (Ig) G, FcgIIIA, which binds to IgG-containing
immune complexes (see later discussion). The number of CD14lowCD161 monocytes
is elevated in RA peripheral blood, and CD14lowCD161 macrophages are enriched in
RA synovial tissue.18 CD14lowCD161 monocytes produce more TNF-a in response to
the microbial toll-like receptor (TLR) 4 ligand lipopolysaccharide (LPS) compared with
the CD141CD16 subset.19,20 These observations suggest that the proinflammatory
272 Gierut et al
CD14lowCD161 monocytes migrate to the RA joint and become highly responsive
macrophages. However, CD141CD16 monocytes also express the chemokine
receptor (CCR) 2, which binds monocyte chemotactic protein 1, and thereby
promotes monocyte migration to the site of inflammation. Because the RA joint is
rich in this chemokine, it is possible that CD141CD16 and CCR21 monocytes are
recruited to the joint where CD16 expression is then induced. Nonetheless, because
monocytes migrate from the peripheral blood into RA synovial tissue, identification
of circulating monocyte subpopulations may be an extremely useful clinical tool for
tracking disease activity and for identifying additional therapeutic targets.20–22
In addition, diversity in activation states of macrophages has been found.23 In
general, macrophages exhibiting a more inflammatory phenotype have been named
M1, or classically activated macrophages, whereas those that trend toward a more
antiinflammatory and repair role are known as M2, or alternatively activated macrophages.23 Most macrophages in the RA joint express proinflammatory cytokines
and are thus most consistent with classically activated macrophages. Therapies
that promote the balance in favor of an M2 phenotype may be useful in RA.8
Therapies Targeting Macrophages
Conventional therapies such as prednisone, methotrexate, leflunomide, sulfasalazine,
and TNF-a inhibitors have been shown to decrease the number of CD681 macrophages in the synovial sublining.24 Another study of synovial tissue response to rituximab found a significant reduction in the number of sublining macrophages at 16
weeks, providing evidence for synovial tissue sublining macrophage reduction after
B-cell depletion therapy in RA as well.25 Furthermore, a reduction in the number of
synovial sublining macrophages correlated clinically with the improvement of the
values of disease activity score (DAS) 28, suggesting an association between sublining
CD681 macrophages and therapeutic efficacy.24 The positive correlation between the
change in RA clinical activity and CD68 expression in the synovial sublining has been
independently confirmed.26
Specifically, targeting activated macrophages at sites of inflammation would be
a way of circumventing the potential untoward effects of systemic macrophage depletion. The bisphosphonate clodronate, encapsulated within liposomes, has been used
to specifically deplete macrophages. After injecting rats intraperitoneally with streptococcal cell-wall (SCW) fragments to induce arthritis, intravenous (IV) liposomal clodronate suppressed the development of chronic arthritis for up to 26 days after
treatment. Treatment was also associated with the depletion of synovial and hepatic,
but not splenic, macrophages, as well as a reduction in articular IL-1b, IL-6, TNF-a,
and matrix metallopeptidase (MMP) 9 levels.27 Similarly, in the K/B N serum transfer
model of arthritis, where spontaneously produced anti–glucose phosphate isomerase
(GPI) antibodies from a K/B N mouse are injected into a naive host, treatment with
liposomal clodronate before serum transfer caused depletion of macrophages in the
bone marrow and liver, and the treated mice were completely resistant to arthritis.
Resistance to arthritis was reversed when the macrophage-depleted mice were
reconstituted with macrophages from naive animals and immediately injected with
K/B N serum.28 In rabbits with established antigen-induced arthritis (AIA), repeated
intraarticular administrations of low, noncytotoxic doses of liposomal clodronate led to
an early reduction of joint swelling, delay in radiographic progression, and decrease in
the number of synovial lining macrophages. However, no difference was seen in pannus formation or radiographic erosions at 8 weeks.29 In patients with RA undergoing
knee replacement surgery, a single intraarticular dose of clodronate liposomes significantly reduced the number of CD681 cells and the expression of adhesion molecules
Innate Immunity and Rheumatoid Arthritis 273
in the synovial lining. In contrast, no immunohistologic difference was observed in the
control group.30 These observations suggest that depletion of synovial tissue macrophages may be an important therapeutic goal in RA.
Systemic depletion of all macrophages could have serious consequences in
patients, and this may be avoided by specifically targeting receptors present on activated macrophages. Folate receptor b (FRb) has been described on both activated
macrophages from RA synovial fluid and animal models of arthritis but not on resting
or quiescent macrophages or normal cells of the body except for the proximal tubule
cells of the kidneys.31–33 The FRb has been used to deliver folate-conjugated imaging
agents to inflamed joints in patients with RA34 and is a target for novel therapeutic
agents. Several new-generation folate antagonists are selectively taken up by the
FRb and show growth inhibition capabilities against FRb-expressing cells, thus circumventing the systemic effects.35 In addition, antibodies or fragments of antibodies
against FRb linked with immunotoxins have been developed and have been shown to
reduce the number of macrophages and levels of IL-6 and to increase the number of
apoptotic cells in RA synovial tissue engrafted into severe combined immunodeficient
mice.36,37 Another approach involved the conjugation of folate to superoxide dismutase and catalase, 2 enzymes that scavenge the damaging reactive oxygen species
secreted by activated macrophages. Folate conjugation dramatically enhanced the
ability of catalase and superoxide dismutase to scavenge reactive oxygen species
produced by activated macrophages in cell culture experiments and the uptake of
the enzyme catalase into activated macrophages.38 Folate has also been conjugated
to small molecules, or haptens such as fluorescein, and given to rodents previously
immunized to the hapten after the onset of experimental arthritis. The folate-hapten
conjugate selectively ‘‘decorated’’ and promoted immune-mediated elimination of
activated macrophages and decreased paw swelling, spleen size, systemic inflammation, arthritis score, and bone erosion.39 Thus, the presence of select folate receptors
on activated macrophages offers exciting potential to target activated macrophages in
the RA joint.
DENDRITIC CELLS
Background
DCs, along with macrophages and B cells, have the ability to present antigen to T
cells, and therefore play a central role in the development of innate and adaptive
immune responses. In the periphery, immature DCs are stimulated to undergo differentiation by an array of pathogens, mainly via the activation of TLRs by exogenous or
endogenous stimuli, and also in response to cytokines or immune complexes
produced during the inflammatory response. TLR signaling results in a significant
change in chemokine receptors expressed by DCs, allowing for maturation of DCs
and migration to the lymphoid tissue, where mature DCs display antigen on MHC
molecules to naive T cells. DCs also express the critical costimulatory molecules,
CD80 and CD86, which interact with CD28 on T cells, completing the necessary signal
for antigen-specific effector T-cell maturation to occur. In addition to stimulation of
naive T cells, DCs can process and display antigen in local tissues and contribute
to the inflammatory response by the production of cytokines, such as TNF-a, IL-1b,
and IL-6. Furthermore, DCs can direct the formation of distinct T helper (TH) cells by
producing key cytokines, such as IL-12 and IL-18 for TH1 cells and IL-23 for TH17
cells.3 Finally, DCs are important in the development of both central and peripheral
tolerance, and their depletion in animal models is associated with the onset of fatal
autoimmune-type disease.40 In the thymus, DCs present endogenous self-antigens
274 Gierut et al
to T cells and delete those that are strongly reactive, whereas in the periphery, interaction between autoreactive T cells and immature DCs bearing self-antigen may result
in anergy, apoptosis, or differentiation into regulatory T cells.3 Deviations in this
pathway, either failed clearance of dead cells or exposure of DCs bearing self-antigens to maturation signals, can abrogate their tolerogenic ability and are implicated
in the development of autoimmunity.41
DCs may be categorized into subtypes based on the expression of various cellsurface markers.42,43 Functionally, however, DCs may be separated into 2 main
classes: classical or conventional DCs (cDCs), which are resident in lymphoid tissues
or migratory in nonlymphoid tissues, and plasmacytoid DCs (pDCs). Both types may
be activated by particular TLRs that induce the molecules necessary to promote
antigen presentation, T-cell stimulation, and cytokine production. cDCs express
CD11c:CD18, also known as complement receptor (CR) 4, and all known TLRs,
except for TLR9. cDCs are the main participants in antigen presentation and activation
of naive T cells as well as the mediators of peripheral tolerance. pDCs, on the other
hand, are particularly important in modifying the immune response toward viruses.
They do not express high levels of CD11c and have been identified by the expression
of specific markers, such as blood dendritic cell antigen 2. In addition, pDCs express
TLRs 1, 7, and 9 and other TLRs to a lesser degree.3 In response to stimuli such as
viruses, pDCs are able to generate abundant amounts of type I interferons (IFNs)
(IFN-a and IFN-b) and other cytokines such as TNF-a and IL-12. These cytokines
increase the production of inflammatory mediators by macrophages, and in the
case of IL-12, they can direct a potent TH1 response.44
DCs and RA
The vast majority of studies that have examined the role of DCs in RA have relied on
immunohistochemical techniques or have isolated DCs from peripheral blood and
characterized their phenotype and function. In RA synovial tissue, the number of
pDCs that are localized to perivascular lymphocytic infiltrates45 correlates with anti–
cyclic citrullinated peptide (anti-CCP) antibodies.46 These pDCs produced B-cell activating factor, IL-18, and IFN-a/-b, whereas the cDCs secreted IL-12 and IL-23.46 The
total numbers of cDCs or pDCs or the number of mature DCs in RA synovial tissue
were not significantly different from patients with osteoarthritis (OA) or psoriatic
arthritis,46,47 although there was a statistical increase in the ratio of pDC/cDC in RA
synovial tissue.46 These data suggest that the number of DCs in synovial tissue may
not reflect their true contribution to RA pathogenesis.
Few cDCs or pDCs are detected in peripheral blood of patients with RA, and the
numbers are lower compared with healthy controls. The expression of the inhibitory
FcgRIIB on DCs derived from peripheral blood monocytes of patients with RA correlated with disease activity. In addition, DCs expressing higher levels of FcgRIIB
inhibited T-cell proliferation and promoted the T-regulatory phenotype after TLR and
Fc receptor (FcR) stimulation in coculture studies.48 Treatment with methotrexate or
infliximab dramatically affects the number, maturation, and function of the DC. DCs
derived from monocytes of patients treated with infliximab displayed an antiinflammatory phenotype49 and increased numbers of cDCs in the circulation.50 In addition, the
numbers of pDCs were increased in the peripheral blood in patients in clinical remission induced by either methotrexate or infliximab. Further, isolation of these pDCs and
coculture with naive T cells led to an induction of the T-regulatory phenotype, which
was capable of inhibiting autologous T-cell proliferation.51 In contrast, there was
a reduction in both circulating cDC and pDC numbers in anti–IL-6 receptor–treated
patients with RA, which was not observed in anti-TNF or cytotoxic T-lymphocyte
Innate Immunity and Rheumatoid Arthritis 275
antigen (CTLA) 4 immunoglobulin–treated patients with RA.52 These data suggest that
clinically relevant information may be gleaned from examining circulating DCs in
patients with RA and that there are differences related to the mode of therapy.
DCs and Murine Models of RA
The use of murine models of inflammatory arthritis has supported the human studies
on the roles of DCs in RA. Follicular DCs are required for the development of the K/B
N mouse model of arthritis.53 In contrast, selective depletion of pDCs enhanced the
severity and pathology of collagen-induced arthritis (CIA).54 These data suggest that
pDCs may prevent the break of tolerance and that the follicular DC or cDC may be
the central culprit that leads to the activation of autoreactive lymphocytes. Adoptive
transfer of DCs from CTLA4 immunoglobulin–treated mice was sufficient to inhibit
arthritis in CIA-recipient mice.55 In addition, adoptive transfer of TLR-stimulated
DCs after immunization reduced CIA.56 Thus, similar to patients with RA, modification
of the DC function by biologic therapies may lead to a skewing of T-cell development
toward a T-regulatory phenotype mediated by DCs.
PATTERN-RECOGNITION RECEPTORS
Background
There are several mechanisms by which macrophages and other innate immune cells
become activated. One way is via PRRs that are designed to recognize simple and
regular nonself patterns of molecular structure, conserved during evolution, called
pathogen-associated molecular patterns (PAMPs). Furthermore, when cells are under
duress, such as in chronic inflammation, they may express danger-associated molecular patterns (DAMPs), such as uric acid, adenosine triphosphate, heat shock proteins
(HSPs), or glycoprotein 96 (gp96), that may also be recognized by PRRs.57–60 PRRs
may be membrane bound or soluble plasma proteins. Examples include mannosebinding lectin (MBL) that is important in the lectin pathway of complement activation,
the transmembrane PRRs composed of 10 known human TLRs that may be activated
on cell surfaces or within endosomal compartments, and the cytosolic PRRs that
include the nucleotide-binding oligomerization domain (NOD)–like receptors (NLRs)
and the retinoic acid–inducible gene I (RIG-I)–like receptors (RLRs).
Toll-like Receptors
The cell-surface TLRs include TLR1, 2, 4, 5, and 6, with the recognition motifs outside
the cell, whereas TLR3, 7, 8, and 9 are on the endosomal membrane, with the PRR
recognition motifs within the endosomal compartment. The TLR system recognizes
PAMPs, including LPS (TLR4), peptidoglycans (PGNs) (TLR2 together with TLR1 or
6), unmethylated CpG DNA motifs (TLR9) from bacteria, and single-stranded RNA
(TLR7) and double-stranded RNA (TLR3) from viruses. The cytoplasmic domain of
the TLR is called the Toll–IL-1 receptor (TIR) motif because it is also present in the IL-
1 receptor. TLR signals are mediated through the TIR, which interacts with adapter
molecules. All TLRs except TLR3 signal through the adapter molecule myeloid differentiation factor 88 (MyD88), whereas TLR3 signals only through the adapter molecule TIR
domain-containing adapter-inducing IFN-b (TRIF), and TLR4 signals through both
MyD88 and TRIF. Signaling through the MyD88 leads to the activation of nuclear factor
kB (NF-kB) and the mitogen-activated protein (MAP) kinases, c-Jun N-terminal kinase
and p38. Activation of NF-kB and the MAP kinases leads to the transcription of genes
involved in inflammation, proliferation, and protection against apoptosis,3,61,62 whereas
activation through TRIF results in the expression of type I IFNs, IFN-a and -b.63
276 Gierut et al
Clinically, further elucidation of TLR signaling cascades is important because they offer
attractive targets for intervention.
TLR Expression in RA
TLRs are expressed in the RA joint. In RA synovial tissue, CD161 synovial lining
macrophages also expressed TLR2,64 and the expression of both TLR2 and TLR4
was significantly higher in RA tissue than in samples from patients with OA.65 RA synovial tissues have a pronounced expression of TLR2 messenger RNA (mRNA) in the
synovial lining and at sites of attachment and invasion into cartilage or bone tissue.66
In addition, TLR3 and TLR7 were also found to be highly expressed in RA synovium,67
and samples of tissue from patients with either early or long-standing RA showed
similar levels of TLR3 and TLR4, both of which were significantly higher than those
in patients with OA.68
In RA synovial fibroblasts, levels of baseline mRNA for TLR2 and TLR4 did not differ
compared with OA tissue fibroblasts. However, compared with OA fibroblasts, RA
synovial fibroblasts demonstrated a significantly increased expression of TLR2 after
treatment with IL-1b, TNF-a, LPS, or synthetic bacterial lipopeptide, leading to a strong
increase of NF-kB translocation into the nucleus.66 In contrast, another study found
that levels of both TLR2 and TLR4 mRNA were significantly higher in RA synovial fibroblasts compared with those from patients with OA and normal skin fibroblasts.69 In
contrast to whole synovial tissue, TLR7 mRNA expression by synovial fibroblasts
was not seen, suggesting that previous TLR7 staining may have been reflecting
expression by macrophages or DCs.68
Peripheral blood monocytes also express TLRs. Both TLR2 and TLR4 were increased
on CD161 and CD16 peripheral blood monocytes from patients with RA versus
healthy controls, and the TLR2 expression on the CD161 subset was higher than that
on the CD16 subset.64 Furthermore, IFN-g increased the expression of TLR2 and
TLR4 on RA peripheral blood monocytes.65 In RA synovial fluid, CD141 macrophages
demonstrated increased expression of TLR2 and TLR4 compared with peripheral blood
monocytes or control macrophages differentiated in vitro from normal monocytes.70
Activation of Cells from the RA Joint by Microbial TLR Ligands
Isolated RA synovial fibroblasts treated with microbial TLR2 and TLR4 ligands demonstrated a marked increase in the osteoclast activator RANKL, at both the mRNA and
protein levels.69 Inflammatory cytokines IL-6 and IL-8, as well as MMP-1 and -3, were
induced by stimulation of RA synovial fibroblasts with bacterial PGN.71 Furthermore,
RA synovial fibroblasts demonstrated increased expression of vascular endothelial
growth factor after stimulation with bacterial PGN and increased IL-15 after stimulation by TLR2 and TLR4 ligands.72 Stimulation of RA synovial fibroblasts with the
synthetic TLR3 ligand, poly I:C, led to the production of IL-6 and TNF-a, which was
significantly enhanced when the cells were preincubated with IFN-a compared with
cells stimulated with poly I:C alone.73 These observations suggest that in RA, synovial
fibroblasts may be activated through the TLR pathway.
RA synovial fluid macrophages demonstrate an increased response to microbial
TLR2 and TLR4 ligands compared with control macrophages differentiated in vitro
from normal monocytes, macrophages from the joints of patients with other forms
of inflammatory arthritis, or RA peripheral blood monocytes.70 In addition, treatment
of RA synovial fluid macrophages with a microbial TLR2 ligand significantly increased
the levels of IFN-g and IL-23 mRNA compared with in vitro–differentiated control
macrophages.74 It is possible that alterations of intracellular signaling pathways,
such as those regulated by IFN-g or IL-10, might be responsible.75
Innate Immunity and Rheumatoid Arthritis 277
Endogenous TLR Ligands in RA
Because microbial ligands are not likely the cause of TLR signaling in the RA joint,
studies have examined RA synovial fluids and tissues for the presence of potential
endogenous TLR ligands. RA synovial fluid activated human embryonic kidney 293
cells only when these cells expressed TLR4, suggesting the presence of endogenous
TLR4 ligands in RA synovial fluid.76 In addition, the authors have demonstrated that RA
synovial fluid is capable of activating normal macrophages and that this activation was
mediated through TLR2 and TLR4 (Pope and colleagues, unpublished data, 2010).
Together, these observations suggest that endogenous TLR ligands or DAMPs may
be released in response to inflammation in early RA and result in continuous persistence of inflammatory mediators through activation of cells of the innate immune
system.
Several potentially relevant endogenous TLR ligands or DAMPs have been identified
in the RA joint. Ligands such as HSP22, tenascin-C, high-mobility group box chromosomal protein 1, serum amyloid A protein, and fragments of hyaluronic acid are highly
expressed in the RA joint and are capable of activating monocytic cells through TLR2,
or TLR4, or both. Another DAMP, gp96, was also highly expressed in RA synovial fluid
and synovial tissue. The addition of gp96 in vitro to RA synovial fluid macrophages
induced significantly higher levels of TNF-a, IL-8, and TLR2 compared with control
macrophages.60 Although gp96 bound to both TLR2 and TLR4, macrophage activation was mediated primarily through TLR2. The quantity of TLR2 expression on synovial fluid macrophages strongly correlated with the level of gp96 in the same synovial
fluids. Further studies, using a murine model of arthritis mediated by immune
complexes, demonstrated that in the normal joint, an extracellular matrix (ECM) glycoprotein, tenascin-C, was not expressed. However, tenascin-C was induced during the
early phase of the arthritis, and it contributed to the progression of the arthritis, mediated through TLR4 activation of macrophages and synovial fibroblasts.77 These
observations support a potentially important role of endogenous TLR ligands in the
persistent activation of macrophages and synovial fibroblasts that is observed in
the joints of patients with RA.
TLR Signaling as a Target in RA
Understanding the potential role of TLRs in inflammation has led to its therapeutic
exploitation. In mice with CIA, in which intradermal injections of type II collagen leads
to the priming and the effector phases of inflammatory arthritis, treatment with a TLR4
antagonist both before the onset of disease and during established arthritis led to
a significant reduction of arthritis.78 Furthermore, there was decreased histologic
destruction of the cartilage matrix and infiltration of inflammatory cells into the joint
space. In addition, IL-1 receptor antagonist–deficient mice developed a spontaneous
chronic arthritis, which was ameliorated when treated with a TLR4 antagonist.78 The
results of these animal models support the potential benefits of suppressing TLR
signaling as a therapeutic approach in RA.
In ex vivo synovial tissue cultures from patients with RA, the addition of a TLR4
antagonist suppressed the spontaneous secretion of IL-1b and TNF-a, thus supporting the role of TLR4 in the production of inflammatory cytokines.76 Currently available
antirheumatic therapies that are known to have a suppressive effect on the TLR
signaling pathway include hydroxychloroquine (TLR7, 8, and 9) and auranofin
(TLR4). Antagonists of TLR4 are being studied for possible use in sepsis and endotoxemia. Lipid A, the innermost of the 3 regions of LPS, was created in a stable, synthetic
form called E5564 (eritoran).79 It is currently undergoing clinical trials and may become
a viable agent in RA. Another TLR4 receptor antagonist, chaperonin 10 (HSP10), has
278 Gierut et al
been studied in a randomized, double-blind, multicenter study of 23 patients with
moderate to severe active RA who received twice weekly IV therapy at different
concentrations for 12 weeks. All 3 treatment groups tolerated the therapy well and
had significant improvement in the primary endpoint of clinical improvement as
measured by the DAS28 score. The effect seemed to be dose dependent, with 4 of
7 patients in the highest group achieving an American College of Rheumatology
(ACR) 50 response and 2 of 7 achieving an ACR 70 response. The highest treatment
group also had significant improvement in all secondary endpoint measures, including
swollen and tender joint count, patient’s assessment of pain on the visual analog
scale, disability index on the health assessment questionnaire, and morning stiffness.80 In summary, the TLR signaling pathway, which may be activated by endogenous TLR ligands or DAMPs, is a novel target for therapeutic intervention in patients
with RA.
Nucleotide-binding Oligomerization Domain–like Receptors
Similar to TLRs, the NLRs are intracellular, cytosolic receptors that sense PAMPs or
DAMPS and mediate an inflammatory response. Thus far, there are 22 proteins in
the NLR family, including the NOD and NTPases implicated in apoptosis and multihistocompatability complex transcription leucine rich repeat protein (NALP) subfamilies,
with the 14 NALPs characterizing the largest subfamily. Common features of the NLRs
include a central nucleotide-binding domain, a C-terminal leucine-rich repeat domain,
and N-terminal caspase-recruitment and pyrin domains.81 The NOD proteins recognize fragments of bacterial cell-wall proteoglycans and activate the transcription factor
NF-kB. Although NOD proteins are expressed in phagocytes along with TLRs, they are
especially important activators of the innate response in epithelial cells, where expression of TLRs is weak or absent.3 Members of the NALP family, including NALP1, 3, and
12, are capable of forming functional caspase-1 activation complexes, or inflammasomes, that are important in the processing and release of IL-1b and IL-18 in response
to PAMPs or DAMPs.
It has been shown that the induction of SCW-driven arthritis in NOD2 gene–deficient
mice results in reduced joint swelling and decreased levels of TNF-a and IL-1b,
whereas the opposite effect was seen in NOD1–deficient mice, suggesting an antiinflammatory role for NOD1. Moreover, the microbial ligand for NOD2, muramyl dipeptide (MDP), has been detected in RA synovium, whereas none was found in synovium
from patients with OA.82 NOD2 was expressed by both macrophages and synovial
fibroblasts, as detected by immunohistochemistry, but not by lymphocytes or blood
vessels. Transcription of NOD2 mRNA by RA synovial fibroblasts was induced by
TNF-a, poly I:C, and LPS. Adding MDP to other inflammatory stimuli augmented RA
synovial fibroblast production of inflammatory cytokines and matrix-degrading
enzymes compared with the stimuli alone. These observations suggest that TLR activation may induce NOD2 expression and that TLRs and NOD2 may act synergistically
in promoting inflammation and matrix destruction in the RA joint.83 Further studies will
be required to determine whether NODs may potentially become effective therapeutic
targets in RA.
The Inflammasome and RA
In macrophages, the essential link between pro-IL-1b and pro-IL-18 and their bioactive counterparts is the protease caspase 1, which cleaves these molecules and
generates the active cytokines that are then released from the cell.84 Activation of caspase 1 depends on the formation of inflammasomes, which are multiprotein
complexes consisting of an NLR protein such as an NALP, the adapter molecule
Innate Immunity and Rheumatoid Arthritis 279
apoptosis-associated speck-like protein containing a caspase recruitment domain
(ASC), and caspase 1, and which are assembled in response to cellular recognition
of DAMPs or PAMPs.85 Inflammasomes are implicated in diseases such as systemic
onset juvenile idiopathic arthritis and familial Mediterranean fever.86,87 In addition,
mutations in NALP3 are responsible for cryopyrin-associated periodic syndromes,88
and NALP3-containing inflammasomes are activated by monosodium urate (MSU)
and calcium pyrophosphate dihydrate (CPPD) crystals in gout and pseudogout,
respectively.89 In addition to MSU and CPPD crystals, other ligands of the NALP3
inflammasome include Alzheimer disease–associated amyloid deposits (amyloid-b)
and the ECM components, biglycan and hyaluronan.90
NALP3 is expressed in the RA joint. Using real-time PCR, it has been shown that
NALP3 mRNA levels were increased in RA synovium compared with OA and that
monocyte-derived macrophages from healthy donors differentiated in vitro increased
NALP3 expression when stimulated by TNF-a.91 Another study did not find any differences between RA and OA synovial expression of NALP1, NALP3, NALP12, or ASC
using densitometric analysis of Western blots. However, using enzyme-linked immunosorbent assay, caspase-1 levels were significantly enhanced in RA synovial tissues,
even though there was no difference in concentrations of IL-1b.92 Thus, further studies
are needed to clarify the role of the inflammasome in RA pathogenesis.
Caspase 1 is not the only IL-1b converting enzyme (ICE) involved in IL-1b processing, which is likely the reason why inhibition of caspase 1 only partially inhibits experimental models of RA, whereas deficiency of IL-1b completely ameliorates the
arthritis.85 Alternative ICEs, such as elastase or proteinase 3 in neutrophils or chymase
in mast cells, may be involved in the early stages of inflammatory arthritis by contributing to the processing of IL-1, whereas caspase 1 may play more of a role in the
chronic stage of the arthritis.93,94
Retinoic Acid–inducible Gene I–like Receptors
Three genes encode RLRs in human and mouse genomes.95 One of these genes, RIGI, encodes an RNA helicase protein whose expression is induced by IFN-g and which
is found in cells such as endothelial cells, bronchial epithelial cells, smooth muscle
cells, and macrophages.96–98 It is a sensor of viral RNAs and activates cells of the
innate immune system.99 It is also associated with various chronic inflammatory
diseases, including lupus nephritis, and is considered important in mediating reactions
induced by IFN-g.100 Recently, high levels of RIG-I expression have been found in RA
synovial tissues compared with OA controls. Treatment of RA synovial fibroblasts with
IFN-g significantly induced the expression of RIG-I, and knockdown of RIG-I in RA
synovial fibroblasts with small interfering RNA (siRNA) resulted in the inhibition of
the expression of the chemokine CXCL10.101 In addition, RIG-I was expressed in
RA synovial fibroblasts stimulated with TNF-a and knockdown of RIG-I with siRNA
suppressed TNF-a–induced RANTES (chemokine (C-C motif) ligand 5 [CCL5]) expression, suggesting a possible role for the TNF-a/RIG-I/CCL5 pathway in RA pathogenesis.102 Further studies are needed to show whether RIG-I is a plausible target for
therapy in RA.
COMPLEMENT PATHWAYS
Background
The complement system is a key mediator of inflammation in the effector phase of RA.
It is composed of a family of plasma proteins that act alone or in concert with antibodies to opsonize pathogens and dying cells of the host, to enhance phagocytosis,
280 Gierut et al
and to recruit effector cells to areas of inflammation. In addition, the coating of antigens with complement facilitates uptake by antigen-presenting cells, and thus
enhances the presentation of antigen to the adaptive immune system. Several
complement proteins, including C3 and C5, are present in inactive states called zymogens. A cleaved zymogen yields 2 fragments. The larger fragment is a serine protease
that remains covalently bound to the immune complex or pathogen surface (eg, C3b),
whereas the smaller peptide fragment acts locally as a mediator of inflammation (eg,
C3a). On activation, the proteases cleave other complement proteins into their active
forms, thus amplifying the response.
There are 3 pathways of complement activation. The classical pathway is initiated
by the binding of C1 to antigen:antibody complexes, either circulating or tissue bound
or on pathogen surfaces, or directly to the surface components of some bacteria.
Once activated, C1 cleaves C2 and C4, which together forms C4b2a, the C3 convertase of the classical pathway. The lectin pathway is initiated by the binding of carbohydrate-binding proteins, such as MBL, to arrays of carbohydrates on the surface of
pathogens. This pathway also leads to the creation of the C3 convertase formed by
C4b2a. The alternative pathway primarily amplifies activation that is initiated by the
other 2 pathways. C3b generated by the cleavage of C3 by the classical or lectin pathways is able to bind factor B that is cleaved by factor D, ultimately forming the alternative pathway C3 convertase, C3bBb. The most important activity of the C3
convertase is to cleave large numbers of C3 molecules into C3b fragments that opsonize pathogens, dying cells, and immune complexes and amplify the complement
cascade. Surfaces opsonized by C3b and its derivatives are recognized by effector
cells bearing CRs and can stimulate phagocytosis augment inflammatory signals in
innate and adaptive cells.
Formation of the C3 convertases is the merging point of the 3-complement pathways. The next step involves the formation of the C5 convertase, which is capable
of generating the most potent inflammatory peptide, C5a. Finally, the terminal complement proteins, C5b-C9, form the membrane attack complex, which constructs pores
in the cell membranes of some pathogens, thus causing death. The enzymatic
complement cascade is balanced by membrane and plasma proteins, such as
decay-accelerating factor (DAF) or factor I, that inhibit the formation of the C3 convertase or promote its dissociation, thus preventing complement activation on normal
host cells. Deficiencies in these proteins may lead to excessive complement activation
and inflammation, as well as complement depletion and susceptibility to recurrent
bacterial infections.3
Evidence of Complement Activation in Murine Models of RA
Findings from experimental models suggest roles for the alternative and classical
pathways in the development of arthritis. In the CIA model, mice deficient in C3
demonstrate a significantly lower arthritis score than controls, whereas the arthritis
was intermediate in mice deficient in the factor B, suggesting that C3 activation occurs
via classical and alternative C3 pathways in CIA.103 In contrast, another study found
no resistance to CIA in C4-deficient mice, suggesting a more important role for the
alternative pathway in this model.104 Intriguingly, however, both CIA and a passive
serum transfer model showed that injection of C4-binding protein was effective at
delaying the onset of arthritis and reducing the severity of already established disease
by inhibiting activity of both the classical and the alternative pathways.105 Mice with
C5 deficiency were highly resistant to CIA despite evidence of normal cellular and
humoral immune responses to type II collagen and intra-articular deposition of IgG
antibodies.106 In the K/B N serum transfer model, anti-GPI antibodies were effective
Innate Immunity and Rheumatoid Arthritis 281
at causing arthritis in mice deficient in C4, whereas no arthritis was seen in C3- or C5-
deficient mice, suggesting that the pathogenesis of K/B N serum–induced arthritis
relies on activation of the alternative pathway.107,108 Together, these observations
support a major role for the alternative pathway in CIA and anti-GPI–mediated arthritis.
Evidence of Complement Activation in RA
Complement-activating immune complexes are abundant in the joints of patients with
RA and seem to be the crucial mediators of the effector phase of inflammation in the
pathogenesis of RA.109–111 In RA synovial fluid, a decrease in C3 and C4 along with
increased cleavage products of C3 suggests increased complement consumption
within the joint.112,113
The concentration of C5a in plasma and synovial fluid of patients with RA is significantly higher than that in patients with OA,114 and levels of C5a in RA synovial fluid are
sufficient to induce both neutrophil chemotaxis and microvascular plasma protein
leakage, 2 important features of inflammation in RA.115 Neutrophils that migrate into
inflamed joints demonstrate upregulated expression of CRs that enhance phagocytosis of material opsonized by C3b.116 The terminal complement proteins, C5b-C9
have also been implicated in RA. Plasma levels of C5b-C9 are significantly higher in
RA than in controls.117 Compared with crystal-induced arthritis and OA, RA had significantly higher synovial fluid levels of the C5b-9 complex and were positively correlated
with synovial fluid C3a and Bb levels.118
Further supporting the importance of the classical pathway, microparticles from RA
synovial fibroblasts were found to contain abundant quantities of bound C1q, C4, and
C3, as well as IgM and IgG antibodies.119 Components of the ECM exposed during
cartilage damage can also activate the classical pathway. ECM-stabilizing proteins,
fibromodulin and osteoadherin, are able to bind to and activate C1q by its globular
head domain, resulting in a significant deposition of C3b and C4b.120 In addition,
new activation products have recently been described in the classical complement
pathway. One such molecule is a covalent complex between C1q and activated C4.
The plasma levels of C1q-C4 complex were found to be significantly higher in patients
with active RA compared with patients with RA in clinical remission.121 Finally, it has
been shown that human anti-CCP antibodies activate both the classical and the
alternative pathways of complement in vitro.122
In addition to plasma and RA synovial fluid, complement-mediated inflammation is
evident in RA synovial tissue. On RA synovial fibroblasts and macrophages, the cellsurface expression of the C5a receptor (C5aR) was elevated and positively correlated
with joint swelling, erythrocyte sedimentation rate, and C-reactive protein levels.123
Furthermore, aside from the classic role of the C5b-C9 complex in cell lysis, it has
been shown that synovial fibroblasts are activated when exposed to sublytic concentrations of C5b-C9 in vitro. Thus, enhanced activation of synovial fibroblasts provides
an additional mechanism by which complement promotes inflammation in RA.124
Together, these observations support an important role for the classical and the
alternative pathways of complement activation in the pathogenesis of RA.
Therapeutic Strategies Targeting Complement in RA
Therapies that target complement activation may be effective in ameliorating disease
given that it is one of the key mediators of inflammation in the effector phase of RA. In
mice, it was previously shown that inhibition of C5 with a monoclonal antibody was
successful at preventing the onset of CIA and was also effective at ameliorating established disease.125 This led to the development of eculizumab, which prevents the
release of C5a and the formation of C5b-C9 complexes by inhibiting cleavage of C5
282 Gierut et al
into C5a and C5b.126 Eculizumab has been most beneficial for patients suffering with
paroxysmal nocturnal hemoglobinuria. In phase 2 trials, published only in abstract
form, significant improvement in RA clinical score was seen after 3 months of treatment with eculizumab, and the best responders were those patients who had high
baseline levels of C5b-9 complexes in the serum.127,128 Another approach used
soluble CR1 (sCR1) derivatives that act as cofactors for complement inhibitory
proteins, DAF and factor I.129 In mice with established CIA, gene therapy with sCR
inhibited the progression of CIA, reduced anticollagen antibody levels, and inhibited
T-cell response to type II collagen in vitro.130 In addition, rats with AIA were concomitantly treated with a single intra-articular dose of a membrane-targeting complement
regulator derived from human CR1 (APT070) or vehicle buffer only. Animals treated
with APT070 demonstrated significantly less clinical and histologic disease compared
with controls after 14 days.131 APT070 is a truncated version of sCR1 with the addition
of a membrane-targeting moiety that improves protein retention at the site of inflammation, and it is currently undergoing phase 1/2 clinical trials in RA. Other potential
therapies targeting complement activation have shown positive results in animal
studies and include serine protease inhibitors, C3a and C5a receptor antagonists,
and synthetic regulatory proteins involved in complement inhibition.132
FC RECEPTORS
Background
FcRs are surface molecules on myeloid cells and B cells that are capable of interacting
with the Fc portion of immunoglobulin molecules. FcRs help to bridge the adaptive
and innate immune responses and function as mediators of effector cell activation
and inhibition, antibody-dependent cellular cytotoxicity (ADCC), and release of inflammatory mediators. For human IgG, there are 3 known activating receptors: FcgRI
(CD64); FcgRIIA (CD32a); and FcgRIIIA (CD16); and 1 inhibitory receptor, FcgRIIB
(CD32b). The activation receptors are distinguished by the presence of a cytoplasmic
immunoreceptor tyrosine-based activation motif (ITAM), whereas the inhibitory
receptor possesses an immunoreceptor tyrosine-based inhibitory motif (ITIM). Initiation of the activation pathway leads to phosphorylation of ITAM sequences by Src
family kinases and recruitment and activation of spleen tyrosine kinase (Syk), ultimately resulting in activation of downstream signaling pathways such as MAP kinases
that are important for cellular proliferation. On the other hand, engagement of the
inhibitory pathway and phosphorylation of ITIM sequences leads to the prevention
of calcium influx and thus blocks calcium-dependent processes such as degranulation, phagocytosis, ADCC, cytokine release, and proinflammatory activation. The critical step in effector cell activation by FcRs occurs via the cross-linking of the receptors
by immunoglobulin. Thus, the cross-linking of 2 ITAM-bearing FcRs leads to an activation signal, whereas cross-linking of ITIM-bearing FcRs results in the arrest of
effector responses. In general, both the activating and inhibitory receptors are coexpressed on myeloid cells and are engaged at the same time by circulating immune
complexes or cells that have been opsonized by immunoglobulin. The threshold of
effector cell activation is determined by the levels of expression of each receptor class
on effector cells, as well as the ratio of an antibody’s binding affinity for an activating
receptor to an inhibitory receptor (A/I ratio). Therefore, either increased expression of
FcgIIB or higher avidity of immunoglobulin subclass for FcgIIB results in a higher
threshold for effector cell activation. Activating and inhibitory receptors may be upand downregulated by inflammatory or inhibitory cytokines. In addition, polymorphisms of FcgRs have been described, which affect the binding affinities for specific
Innate Immunity and Rheumatoid Arthritis 283
IgG subclasses and result in greater or lesser activation of effector cells when stimulated. Disequilibrium between activating and inhibitory FcR pathways can result in
pathologic responses, and further understanding of perturbations in the FcR system
that contribute to RA will aid in the development of new therapeutic strategies that
target this system.133
FcRs in Murine Models of RA
Studying mice that are deficient in various FcRs has led to better understanding of how
these receptors contribute to experimental arthritis. A role for the FcgRIIB receptor in
preventing arthritis has been demonstrated using several models of arthritis. Mice with
a haplotype that confers resistance to CIA were rendered susceptible to arthritis when
they were made FcgRIIB deficient.134 Similarly, FcgRIIB-deficient mice developed
accelerated arthritis when they were administered anti-GPI antibody from K/B N
mice.135 In contrast, deficiencies in activating receptors have been shown to reduce
the severity of arthritis. Mice deficient in the common activating FcRg chain were protected from CIA.136,137 Another study showed that, although FcgRIII was critical in
early development of CIA, both FcgRIII and FcgRI were dispensable for the progression to destructive joint disease, whereas the FcRg chain was not, implying a likely role
for FcgRIV, which is not expressed in humans but in mice.138 Finally, studies using the
K/B N serum transfer model of arthritis showed absence of clinical arthritis in Fcg
chain–deficient mice, but erosive lesions in the bone still developed, suggesting separate mechanisms for inflammation and bony erosions.135 Together, these observations support an important role for the FcgR signaling pathway in the pathogenesis
of experimental models of RA.
FcRs in Patients with RA
FcRs are found on cells from patients with RA and may be associated with activation
or inhibition of innate effector cells. Levels of activating receptors in plasma shed from
macrophages and NK cells, as well as membrane-bound activating receptors on
peripheral blood monocytes, were increased in patients with RA compared with
healthy controls.139,140 In addition, increases in the expression of activating FcRs on
RA peripheral blood monocytes correlated with higher sedimentation rates, and
disease-modifying antirheumatic drug (DMARD)–naive patients had significantly
higher levels of activating FcgRIIA compared with patients with RA taking DMARDs,
supporting an association between expression of activating FcRs and disease
activity.140 The stimulation of activating receptors on macrophages with immune
complexes results in the expression of proinflammatory molecules. In vitro stimulation
of healthy peripheral blood monocyte–derived macrophages with immune complexes
formed by anti–citrullinated protein antibodies in sera from patients with RA resulted in
significantly increased TNF-a secretion via engagement of FcgRIIA. Similar findings
were seen when such cells were stimulated in vitro with immune complexes derived
from RA synovial fluid.141,142 In RA synovial tissue, significantly increased levels of
activating FcRs were found to correlate with the number of synovial macrophages
as well as the expression of TNF-a and matrix metalloproteinases.143 No difference
was seen between levels of inhibitory FcgRIIB expression on peripheral blood monocytes from patients and controls with RA,144 whereas in the synovial tissue, the
expression of all FcgRs, including FcgRIIB, was found to be significantly elevated in
patients with RA compared with biopsies from healthy volunteers.145 Thus, upregulation of activating receptors, rather than a paucity of inhibitory receptors, seems to
contribute to the increased activation of monocytes and macrophages in RA. In addition, C-reactive protein, which is increased in RA, is capable of binding FcRs
284 Gierut et al
Fig. 1. Various current and novel innate immunity-directed therapeutics and their targets. IL-1R, interleukin 1 receptor; IVIG, intravenous immunoglobulin; Mf, macrophage; sCR1, soluble complement receptor 1.
Innate Immunity and Rheumatoid Arthritis 285
promoting the expression of proinflammatory cytokines, possibly contributing to the
persistent expression of macrophage cytokines in RA.146,147
It has been demonstrated that FcgRIIB was selectively upregulated on monocytederived DCs from patients with RA with low disease activity. In vitro incubation of
FcgRIIB-bearing DCs with a TLR4 agonist and immune complexes inhibited T-cell
proliferation and promoted the development of regulatory T cells. Furthermore, the
addition of FcgRIIB-specific blocking antibody abrogated regulatory T-cell development, suggesting that FcgRIIB expression on DCs in patients with RA with low activity
may be important in the maintenance of tolerance.48
FcRs as Targets for Therapy in RA
Altering the balance of FcgRs in favor of inhibitory pathways is an attractive therapeutic strategy in RA. Pooled IgG from multiple donors (IV immunoglobulin) is used
to treat various autoimmune diseases, and suggested mechanisms of action include
blockade of activating FcRs and upregulation of FcgRIIB on effector macrophages.148
In mice, treatment with soluble FcgRIIB significantly reduced the severity of CIA
compared with controls.149 Clinical improvement in patients with RA on DMARD
therapy may be associated with changes in FcgR expression or binding affinities. In
one study, the levels of activating FcgRs on peripheral blood monocytes were significantly decreased after 16 weeks in patients with RA receiving methotrexate.150 In
addition, recent data suggest that polymorphisms of activating FcgRs in patients
with RA may influence outcomes of treatment with TNF-a blocking agents.151,152
Novel approaches are being developed that target FcRs and downstream signaling
pathways associated with FcR activation. Inflammatory macrophages from RA synovial fluid treated in vitro with toxin-conjugated antibodies against FcgRI were efficiently
eliminated via apoptotic cell death, resulting in a reduction in TNF-a level.153,154
Finally, a novel small molecule Syk inhibitor, R788, and its active metabolite R406
have been shown to suppress clinical arthritis, bone erosions, pannus formation,
and synovitis in experimental arthritis.155 Further, in a 12-week, randomized,
placebo-controlled trial with 158 active patients with RA, twice-daily oral doses of
R788 showed significant improvement in ACR 20, 50, and 70 scores compared with
controls. Clinical efficacy was noted as early as 1 week after initiation of therapy
and correlated with serum reductions in IL-6 and MMP-3 levels.156 Therefore, targeting the FcR signaling pathway may be an effective therapeutic strategy in RA.
SUMMARY
Innate immunity, with macrophages playing a central role, is critically important in the
pathogenesis of RA (Fig. 1). Experimental models document the importance in innate
immune cells in the initiation of many experimental models of arthritis, promoting the
development of adaptive immunity, which results in autoantibody production. In
patients with RA, the presence of rheumatoid factors and anti-CCP antibodies supports
the importance of innate immunity in the initiation of RA. In addition, in the joints of
patients with RA, there is abundant evidence for the presence of immune complexes
and the activation of complement, which directly contributes to the pathogenesis of
disease. Further, immune complexes bind to FcgRs, which are capable of activating
macrophages and DCs. The importance of this process is supported by the preliminary
observations demonstrating that suppression of the FcgR activation pathway may be
effective in treating patients with RA. Finally, synovial macrophages have clearly
been demonstrated to be critically important in the pathogenesis of RA, and effective
therapies result in a reduction in the number of synovial macrophages, regardless of
286 Gierut et al
the biologic pathway targeted. The mechanisms contributing to the persistent activation of macrophages may be a result of the expression of endogenous TLR ligands,
suchasgp96andtenascin-C, which areupregulatedin RAandarecapableofactivating
macrophages through TLR signaling, thus creating a self-perpetuating, progressive
chronic inflammatory process. Thus, targeting innate immunity has already proved
beneficial in RA, and targeting additional pathways such as complement, the FcGR,
and TLR signaling pathways holds promise for further therapeutic advances.
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