RA is characterised by infiltration of inflammatory cells and fibroblast proliferation in the synovial joints, leading to chronic inflammation and a progressive destruction of bone and cartilage [13]. The cells in the RA joint produce elevated levels of cytokines, including TNF, IL-1 and IL-6, that support Th17 cell differentiation and suppress the differentiation of regulatory T lymphocytes, further perpetuating the inflammatory environment [14]. The past decade has seen the treatment of RA transformed by the use of biological therapies such as anti-TNF. Whilst a clear improvement over conventional treatments such as methotrexate, many patients fail to respond adequately to anti-TNF and can become unresponsive to treatment over time. More recently tociluzimab, an IL-6 receptor antibody, has shown great promise in this patient group [15]. However, the manufacturing cost of biological therapies remains a factor that severely restricts their use. On cessation of these therapies the disease reactivates in time, demonstrating that cytokine blockade does not modify the upstream disease mechanisms but instead works by dampening the resulting inflammation.

The pathogenic mechanisms are still poorly understood. There is increasing evidence from studies in both human and animal models of RA that TLRs and NLRs may contribute to this process [16,17]. Although it has long been proposed that infection could play a role in the initiation of RA, no specific pathogen has ever been linked to disease. However, the identification of elevated levels of endogenous PRR ligands, such as HMGB1, fibronectin and, more recently, the heat shock protein GP96, in the RA joint has highlighted the possibility that activation of PRRs may play an important role in the maintenance of inflammation in RA [18].

A variety of animal models have been used to identify the potential roles of TLRs in RA with mixed results. IL-1 receptor antagonist-deficient (IL-1Ra−/−) mice develop spontaneous arthritis, but when crossed with TLR4−/− mice, disease severity is reduced [19]. These results are consistent with work from the same group demonstrating that a naturally occurring TLR4 antagonist from Bartonella Quintana had therapeutic effects in both the IL-1Ra−/− model and the collagen-induced arthritis (CIA) model [20]. Conversely, TLR2−/− mice crossed with IL-1Ra−/− mice produced more severe arthritis, whilst crossing with a TLR9−/− mouse had no effect on disease [19]. In the IL-1Ra−/− mice TLR2 appears to be protective whilst TLR4 is linked with disease development. However, in a streptococcal cell wall-induced model of arthritis, TLR2-deficient mice show a reduced disease severity, highlighting the variability of results between arthritis models [21].

Conflicting roles have also been reported for TLR3. Activation of TLR3 suppressed arthritis in the mouse CIA and K/BxN serum transfer models [22]. However, TLR3 activation increased disease severity in the rat pristane-induced arthritis and rat CIA models, where upregulation of TLR3 was associated with disease and downregulation via small interfering RNA improved disease symptoms [23]. In a recent study, treatment of pristane-induced arthritis rats with a miR-26a mimic decreased TLR3 expression and disease symptoms, confirming a role for TLR3 in this model [24]. TLR7 has also been suggested to drive disease maintenance in the rat CIA model where intra-articular knockdown of TLR7 decreased disease activity [25]. In agreement with this study, we found that although TLR7−/− CIA mice developed arthritis symptoms, the disease did not progress, leading to a reduction in clinical score and paw swelling compared with wild-type mice [26]. Together these studies suggest the potential for a contribution from TLRs to the maintenance of inflammation in arthritis models. However, there are many conflicting animal model studies in the literature, highlighting the need for caution when interpreting these results in relation to human disease.

Many human studies have been conducted on synovial tissue removed from patients during joint replacement surgery. Human RA synovial tissue is composed of a mixed population of cells, including RA synovial fibroblasts (RASFs), macrophages, T lymphocytes, B lymphocytes and dendritic cells, and has been demonstrated to be auto-stimulatory in culture. Expression of most of the TLRs has been demonstrated in the RA synovial membrane. In our own studies, we have demonstrated that activation of RA synovial membrane cultures with ligands for TLR1/2, 2/6, 3, and 4 and particularly TLR8 increased TNF production [27,28]. The major cell type in the synovium is the RASFs expressing mainly TLR2, 3, 4 and 9, which on stimulation of TLR3 or 4 can promote Th1 and Th17 cell expansion [29]. Interestingly, IL-17 has been reported to increase the expression of TLR2, 3 and 4 in RASFs, creating the potential for a positive feedback [30].

In comparison to OA tissue where there is a lower level of inflammation, TLR2, 3, 4, 5, 7 and 9 are known to be expressed at elevated levels in RA tissue [31-34]. TLR5 expression is also elevated on RA peripheral blood monocytes and correlates with disease activity [34]. In addition to elevated expression of TLRs in the RA synovium, we have previously demonstrated a functional role for TLRs in generating inflammation in the RA synovium from data generated using dominant negative versions of the TLR adaptor proteins MyD88 and Mal. These dominant negative adaptor proteins prevent downstream signalling and lead to a significant reduction in spontaneous cytokine and matrix metalloprotease production from RA synovial membrane cultures [28]. Furthermore, we were also able to inhibit spontaneous cytokine production from RA cultures by addition of chloroquine or mianserin, both small molecule inhibitors of the endosomal TLR3, 7, 8 and 9 [35]. Mianserin is thought to inhibit TLRs via an off-target mechanism and as such does not represent a suitable drug for clinical trial; however, other inhibitors of the endosomal TLRs are being developed for SLE, such as DV1179 (Dynavax, Berkeley, CA, USA) and CPG52364 (Pfizer, New York, NY, USA), which may also be of therapeutic benefit in RA (Table 2). Anti-malarials such as hydroxychloroquine have long been used to treat RA and it is believed that their effects are mediated through inhibition of the endosomal TLRs. However, in clinical practice they are often given in combination with other therapies as they cannot be used at high doses due to ocular toxicity. Indeed, other anti-rheumatic agents traditionally used in the treatment of RA have since been shown to possibly work via modulation of TLR function. For example, auranofin, an organogold compound, has also been shown to inhibit TLR4 dimerization and TLR3-dependent TRIF signalling [36]. Another approach to inhibiting TLR function in RA is by neutralising antibodies (biologicals); Opsona Therapeutics (Dublin, Ireland) have developed a TLR2 neutralising antibody which they have shown can reduce spontaneous cytokine production from RA synovial explants [37]. However, this antibody is currently in clinical trials for other conditions and not yet for RA (Table 2). There are now several immune modulators at various stages of development that target TLRs with a potential role in RA (Table 2); though as yet none have been approved for use in the clinic.

In addition to TLRs, evidence is emerging that the NLRs may also have a role and could also represent potential therapeutic targets for RA [38]. Both NOD1 and NOD2 are expressed in synovial tissue and expression of NOD1 is increased in RA synovium compared with OA [17]. Silencing of NOD1 in RASFs decreased TLR2- and IL-1β-induced IL-6 production, suggesting that NOD1 can synergize with TLRs in potentiating inflammation in RA [39]. More recently, the NLRP3 inflammasome has been implicated in a murine model of RA, where deletion of A20 triggers spontaneous erosive polyarthritis through an increase in NLRP3 activation [40]. Several inhibitors of the NLRP3 inflammasome are currently in preclinical development and will be discussed below in more detail in relation to gout.