Rheumatoid arthritis is a systemic autoimmune disease that is characterized by chronic inflammation of joints, leading to synovial hyperplasia, infiltration of leukocytes and progressive destruction of cartilage and bone . Various humoral factors are involved in the development of RA. One of these is autoantibodies, which activate complement systems by formation of immune complexes in combination with self-antigens and induce recruitment of inflammatory cells into synovium [1, 2]. Another important factor is inflammatory mediators such as cytokines, chemokines and proteases that induce inflammation and tissue destruction [1, 3-6]. In particular, proinflammatory cytokines including TNF-α, IL-1β, IL-6 and IL-17 amplify inflammation by inducing production of various additional inflammatory mediators and induction of recruitment, differentiation and proliferation of inflammatory cells. These inflammatory mediators are detectable in high concentrations in synovial fluid or serum of RA patients [7-14]. Although some biologics targeting proinflammatory cytokines have improved the effectiveness of RA therapy , some patients are unresponsive to such therapy. Thus, the pathogenesis of RA is complex and has not yet been fully elucidated.
Collagen-induced arthritis is a common animal model with similarities to RA in both cause and pathology. Synovial hyperplasia, infiltration of inflammatory cells and destruction of cartilage and bone occur in the joint tissue of both CIA mice and humans with RA . In addition, an autoantibody, namely anti-CII antibody, triggers CIA . Injection of a cocktail with monoclonal anti-CII antibody induces arthritis in mice [18, 19]. Furthermore, proinflammatory cytokines play important roles in the pathogenetic process of CIA. Antibodies against TNF-α, IL-1β and IL-6 are effective against CIA [20-22]. In addition, it is well known that CIA is accelerated by LPS, a major component of the outer membrane of gram-negative bacteria [23-26]. LPS-CIA is more useful than CIA alone for screening for anti-rheumatic drugs because it enables not only shortening the duration of experiments but also more accurate evaluation of incidence and severity . On the other hand, the characteristics of some pathogenetic mechanisms are similar in LPS-CIA and RA. It is well known that some patients with RA have acute onset and exacerbations of their disease. Administration of LPS reactivates paw inflammation in CIA mice, suggesting LPS plays a role in the exacerbation of autoimmune diseases, including RA [27, 28]. Likewise, LPS-CIA induces acute onset of inflammation after LPS injection in CII-immunized mice, in contrast with unmodified CIA, which develops slowly . Therefore, LPS-CIA is a valuable disease model for investigating the acute phase of RA. However, few studies have analyzed the pathogenetic mechanisms of LPS-CIA. In particular, there are no detailed analyses focusing on the period before the onset of arthritis.
In this study, we have demonstrated an association between induction of paw inflammation and both gene expression of inflammatory mediators and production of anti-CII antibody prior to the onset of arthritis. In addition, we propose that TNF-α contributes to the development of LPS-CIA by both initiation of inflammation and production of anti-CII antibody. Our findings suggest that rapid and continuous production of inflammatory mediators and autoantibodies is involved in the mechanisms of RA exacerbation.
- Top of page
- 1 MATERIALS AND METHODS
- 2 RESULTS
- 3 DISCUSSION
- 4 DISCLOSURE
Autoimmune diseases sometimes have acute onsets and periods of exacerbation, the causes for which have not yet been clarified. Because a number of studies have shown that LPS may play a role in the exacerbation of autoimmune diseases including RA, LPS-CIA is a valuable disease model for investigation of the acute phase of RA [27, 28]. In this study, we elucidated details of the pathogenesis of LPS-CIA from the perspective of the sequence and timing of production of inflammatory mediators and anti-CII antibody and related this to histopathological findings.
Inflammatory mediators such as cytokines, chemokines and proteases are involved in the development of RA through induction of inflammation and destruction of tissues. Upregulation of inflammatory mediators has been observed in both RA and CIA [7-14, 29]. Previous studies have reported increased expression of mRNA of IL-1β and MMPs in arthritic paws of animals with LPS-CIA [30, 31]. In the present study, we clearly demonstrated that LPS-CIA mice have marked increases in the degree of gene expression of inflammatory mediators, including IL-1β, MMP-3 and MMP-9 in their arthritic paws. The amount of mRNA in them increased in parallel with the development of arthritis and correlated positively with clinical severity. Importantly, TNF-α, IL-1β and MIP-2 were rapidly and continuously expressed after direct activation of NF-κB and before the onset of arthritis. In particular, the degree of expression of TNF-α was greater before the onset of arthritis than after its onset. However, we did not observe these responses for MMP-3 and MMP-9, which are overexpressed as a result of aspects of joint inflammation such as proliferation of synovial cells and infiltration of inflammatory cells . These findings suggest that upregulation of TNF-α, IL-1β and MIP-2 is involved in the mechanisms of induction of LPS-CIA. Furthermore, IL-17A was upregulated in parallel with the development of arthritis in the paws of LPS-CIA mice. It is well known that Th17 cells produce IL-17A, which is involved in human RA and arthritis in animal models . This finding suggests that Th17 cells play important roles in the pathogenesis of LPS-CIA.
We confirmed that arthritic paws of LPS-CIA mice are characterized by similar histopathological features as human RA; these include bone and cartilage destruction, synovial change and infiltration of inflammatory cells. Moreover, expression of inflammatory mediators is likely to correlate with histopathological findings. There is consensus that synovial hyperplasia caused by a marked increase in macrophage-like and fibroblast-like synoviocytes is a hallmark of RA . We observed proliferation of both macrophage-like and fibroblast-like synovial cells in parallel with the development of LPS-CIA, which demonstrates that LPS-CIA has this pathological hallmark in common with RA. In addition, we observed the increase in synovial cells from the early stages of arthritis. The increase in MMP-3 we observed 24 hrs after LPS injection may reflect the proliferation of synovial cells, which generally takes 1 day. Notably, infiltration of neutrophils accompanied by edema occurred first; this coincided with the onset of arthritis, suggesting that neutrophil infiltration is the main cause of the inflammation in LPS-CIA. Recent studies have highlighted the possible contribution of neutrophils in the early phases of RA physiopathology. Although it is well known that mononuclear cells such as T cells, B cells and macrophages infiltrate the synovium in RA, neutrophils migrate to the synovial fluid before mononuclear cells do [33-35]. Previous studies have reported accumulation or participation of neutrophils in commonly-used arthritis models, including CIA [29, 36, 37]. In addition, the histopathology of LPS-CIA is similar to that of anti-CII antibody and LPS-induced arthritis, in both of which neutrophils, but not T and B cells, play crucial roles in the development of arthritis . Thus, neutrophil infiltration may cause the acute inflammation in LPS-CIA; this suggests that LPS-CIA has pathogenetic mechanisms in common with the early phase of RA. Upregulation of MIP-2 may play a part in recruitment of neutrophils to inflammatory sites. Likewise, TNF-α and IL-1β may result in edema formation by initiating acute inflammation with enhanced vascular permeability. In addition, the increase in osteoclasts associated with bone and cartilage destruction is consistent with increased expression of RANKL. Similarly, overexpression of MMP-3 and MMP-9 may cause cartilage destruction.
The rapid and continuous increase in serum anti-CII antibody concentrations may, in part, contribute to the development of LPS-CIA. CII is the major constituent protein of the cartilage of joints and autoimmunity to CII occurs in RA patients . The rapidity of increase in production of serum anti-CII antibody induced by LPS may be mediated via a direct effect on B cells. LPS injection results in production of polyclonal antibodies via stimulation of B cells [38, 39]. Anti-CII antibody has the potential to induce arthritis, as evidenced by induction of articular inflammation by serum transfer or collagen antibody cocktail injection [18, 19]. Complex formation between anti-CII antibody and CII in cartilage activates the complement systems and initiates recruitment of neutrophils and macrophages, which are activated by FcγR ligation and secrete inflammatory mediators . In the present study, we detected a second increase in gene expression of some inflammatory mediators 2–8 hrs after LPS injection, this interval of time being just after the increase in serum anti-CII IgG. Additionally, a previous study has shown that LPS injection of mice without CII immunization does not induce arthritis [23, 26]. Thus, production of anti-CII antibody and the consequent induction of continuous expression of inflammatory mediators may be necessary for acute onset of LPS-CIA. Furthermore, T cells might be involved in LPS-CIA via modulation of pathogenic B cells, which produce anti-CII antibody.
The expression pattern of its mRNA did not decrease after LPS injection and before the onset of arthritis, implying an important role for TNF-α in the development of LPS-CIA. Previous studies have shown that antibodies and soluble TNF-α receptor fusion protein significantly ameliorate CIA [20, 40, 41]. Additionally, anti-TNF-α antibody was effective in LPS-CIA mice which had been given a booster injection of CII 7 days before LPS injection , this procedure differing slightly from ours. In the present study, we confirmed the effectiveness of anti-TNF-α antibody. Our findings indicate that TNF-α is an important molecule in the pathogenesis of LPS-CIA. A single intra-articular injection of TNF-α accelerates the onset of arthritis in rats immunized with CII . In addition, comparable to LPS, administration of recombinant IL-1β can induce arthritis in CII-immunized mice . However, TNF-α did not induce severe arthritis when it was injected i.v. in place of LPS after CII immunization. Interestingly, this experiment showed TNF-α has the potential to induce production of anti-CII antibody. This finding suggests that TNF-α contributes to the development of LPS-CIA by production of anti-CII antibody. On the other hand, because TNF-α alone is not sufficient to induce arthritis, our findings raise the possibility that involvement of other proinflammatory cytokines, such as IL-1β and IL-6, could be necessary for the development of LPS-CIA.
The development and severity of RA are linked to genetic and environmental factors. Microbial infections, which may be a potent source of LPS, may be one of the triggers for autoimmune diseases, including RA [43-45]. It is well known that systemic infections cause acute exacerbations of RA. One of the multiple possible mechanisms by which infections may trigger autoimmunity is the adjuvant effects of pathogens mediated by TLRs and other pattern-recognition receptors on antigen-presenting cells; this would lead to production of inflammatory mediators which in turn would lead to tissue damage . A number of studies suggest a possible role for LPS or its cellular receptor, TLR4, in RA. The serum and synovial fluids of RA patients contain higher concentrations of antibodies against E. coli or LPS-binding protein than do those of healthy subjects [46, 47]. In animal models, the spontaneous arthritis of IL-1Ra−/− mice is markedly suppressed by crossing with TLR4-deficient mice . Moreover, blocking of TLR4 by receptor antagonists suppresses experimental arthritis in mice [48, 49]. We found that, after LPS injection, LPS-CIA mice had increased inflammatory mediators in their paws, similar to that associated with infection. Taken together with these findings, LPS-CIA may have pathogenetic mechanisms in common with infection-associated RA.
In summary, we have demonstrated that LPS induces arthritis in CII-immunized mice and that the onset of that arthritis is preceded by rapid and continuous production of inflammatory mediators and anti-CII antibody. This suggests that production of these humoral factors induces the development of LPS-CIA. In particular, TNF-α may contribute to the development of LPS-CIA via both initiation of inflammation and production of anti-CII antibody. It is possible that the rapid and continuous production of inflammatory mediators and autoantibodies is the mechanism by which infections exacerbate RA.