Intracavitary fibrin clots may initiate pannus formation and the immunopathology of RA. Two critical steps, probably host dependent, may determine the development of RA: an altered regulation of extravascular haemostasis or an aberrant reactivity of synovial fibroblasts to the adhered fibrin clots. Current treatments for RA target events downstream of fibrin deposition, perhaps agents acting at an earlier stage should be tried.
- rheumatoid arthritis
- MMPs, matrix metalloproteinases
- PA, plasminogen activator
- RA, rheumatoid arthritis
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Currently, it is considered that mononuclear cells recruited to the sublining are the principal inducers of synovial cell activation in rheumatoid arthritis (RA).1 None the less, a small amount of evidence of T cell mediators at the start of the disease has led some experts to suggest that joint destruction and recruitment of immunocompetent cells may rely on innate processes primarily involving synovial fibroblasts.2 These cells are arranged at the intimal lining as an epithelium, separating a cavity from a substratum. The cavity acts as the final receptacle for cell debris and exudates resulting from the cascade of intrasynovial inflammatory events. Because of the accumulation of cytokines, free radicals, or metalloproteinases at this site, processes at the cavity have a role in disease amplification. In addition to its contribution to disease progression, we propose that both inflammatory and immunological processes of RA are initiated at the joint space and result from the activation of synovial fibroblasts by their contact with intracavitary fibrin. Figure 1A shows the pathogenic sequence that we propose. In parallel with the scheme, fig 1B shows the microarchitecture of areas associated with fibrin deposition in a representative rheumatoid synovial tissue.
COAGULATION INSIDE THE JOINT SPACE
The rheumatoid process may be started by any external factor which can induce joint swelling.1 As major candidates, several viruses, mycoplasms, mycobacteria, and enterobacteria have been evoked. Genetic factors associated with the appearance of RA and its severity would contribute at this point by selecting susceptible hosts to develop a severe enough response to potential aggressors.3 From the beginning of the inflammatory reaction, plasma components, including fibrinogen and zymogens, drain to the joint cavity (fig 1A, step 1). Several inflammatory mediators have the ability to cleave zymogens to their active form, triggering haemostasis and fibrin clotting inside the joint space (fig 1A, step 2). Indeed, there is evidence of activation of both extrinsic and intrinsic pathways of haemostasis in the synovial fluid (reviewed by Busso and Hamilton4). But haemostasis activation seems to be a “normal” response to joint injury, because it occurs in other inflammatory conditions.5 What may differentiate RA from other joint diseases is an imbalance between coagulation and fibrinolysis within the rheumatoid joint.6–8
“An imbalance between coagulation and fibrinolysis may be present in RA”
Most likely, non-rheumatoid hosts can maintain a proportional rate of fibrin formation and dissolution that facilitates clearance of the inflammatory exudates. Rheumatoid patients, on the other hand, have difficulties in the removal of fibrin clots.9–11 This imbalance is not dependent on the existence of an inefficacious plasminogen system in rheumatoid hosts, and in fact fibrinolysis seems reasonably activated in their joints.12 Several factors can account for the difficult dissolution of fibrin clots. For instance, it has been shown that a significant amount of the single chain urokinase (scuPA) molecule present in the synovial fluid from patients with RA is cleaved by thrombin into a two chain form (tcuPA/T), which is inactive as a plasminogen activator. Presumably, tcuPA/T maintains its affinity for the urokinase cell receptor (uPAR), and could act through this pathway as a mediator of tissue damage.13 Additionally, inhibitors of the plasminogen system are more abundant in rheumatoid than in non-rheumatoid synovial fluids.14,15 Whatever mechanisms are involved, the final result of this imbalance is that, in patients with RA, fibrin clots tend to increase in size, and are subsequently deposited on the synovial intima (fig 1A, step 3).16–18
ROLE OF FIBRIN DEPOSITS IN THE ACQUISITION OF AN INVASIVE PHENOTYPE
Adhesion of fibrin clots to the synovial surface may induce migration of intimal cells into and around the deposits (fig 1A, step 4), as we have recently shown in antigen induced arthritis.19 In this experimental model, invasion of the deposits by synovial cells ended with a complete encircling and incorporation of fibrin into the tissue. Migration of fibroblasts through fibrin matrices is a naturally occurring phenomenon in wound healing processes, and also likely to take place in RA. The ability of fibronectin to cross link fibrin matrices, found both in wounds and in the inflamed joint, may be relevant for the induction of fibroblast invasiveness.20–22 Fibronectin provides binding sites for cell attachment, and dynamic interactions between integrins and their matrix ligands allow fibroblast locomotion through the network.23 Interestingly, in RA, fibrin-fibronectin complexes have been associated with pannus formation and with an erosive tendency.24,25
“Adhered fibrin clots may act as scaffolds for pannus formation”
Indeed, fibrin deposits could constitute the substrate for pannus development, eliciting several activation pathways in synovial fibroblasts (fig 1A, step 5). Pannus is defined as a transformed tissue, rich in synovial fibroblasts and interstitial matrix, fibrotic, hyperplasic, and invasive. It is not uniformly distributed throughout the rheumatoid synovium, but appears as focal hypertrophic areas frequently close to cartilage erosions. According to our model, pannus would form around adhered fibrin clots, which would act as scaffolds for synovial focal growth.26–28 As long as there is fibrin clotting inside the joint space, this mechanism of synovial growth may self perpetuate through the adhesion of new fibrin and its invasion by migrating cells. Moreover, several clot components, such as thrombin, urokinase, and matrix glycoproteins, can induce cell proliferation, and trigger the expression of cytokines and inflammatory mediators through the binding of surface receptors of synovial fibroblasts.29–33 Some features related to pannus, such as angiogenesis and recruitment of macrophages, may appear as the response to cell interactions with clotting factors and fibrinogen.34,35 With regard to the aggressive behaviour of the transformed tissue, the interaction between fibrin and infiltrating fibroblasts results in the coexistent activation of different proteolytic pathways which are potent effectors of joint destruction, especially the plasminogen system and the family of matrix metalloproteinases (MMPs). There is certain overlapping in the functions of these groups of proteins; thus, while thrombin and uPA can erode cartilage and the extracellular matrix,36 most MMPs display fibrinolytic activity.37 Both the uPA-uPAR complex and the membrane anchored MMPs have been implicated in the pericellular matrix degradation necessary for the fibrin-invasive activity of fibroblasts. Additionally, plasminogen activators may cleave pro-MMPs to their active forms, thus increasing the destructive potential of fibrin areas infiltrated by fibroblasts.38
INTRASYNOVIAL DIGESTION OF FIBRIN MATRICES MAY ACCOUNT FOR IMMUNOGENESIS IN RA
The synovial fibroblasts primed by their contact with fibrin constitute a front of cell migration moving towards the luminal surface of the deposits. As a consequence of cell advance, fibrinous material becomes incorporated to the synovium (fig 1A, step 6). Figure 1B illustrates the typical synovial microarchitecture under a fibrin deposit, in which a fringe of hypercellular scarring tissue appears between the deposit and normal synovium. This granulation tissue presumably results from progressive remodelling of the incorporated fibrin by interstitial fibroblasts and macrophages recruited to these areas.
“Insolubility of fibrin accounts for a prolonged exposure of its residues to immunocompetent cells inside the joint”
Acquisition of autoantigenicity by components of the fibrin networks may be a direct consequence of tissue remodelling (fig 1A, step 7). Autologous proteins need to undergo modifications in their molecules to become immunogenic. Because fibrin is an insoluble protein, its degradation inside the synovium is a laborious process, and clearance of its residues is difficult in comparison with clearance of soluble substrates. These facts favour the appearance of structural alterations in fibrin chains and their prolonged exposure to immunocompetent cells inside the joint. In the inflamed environment part of the fibrin degradation is carried out by the action of MMPs, instead of through the constitutive route dependent on plasmin. This may account for the generation of new epitopes in the molecule of fibrin.37 Another mechanism of protein transformation, which may facilitate proteolytic cleavage of the substrate, is citrullination. This process consists of the substitution of arginine residues by citrulline, by the enzyme arginine peptidyl deiminase. Not only are citrullinated forms of the fibrin chains found inside the inflamed synovium39 but also the production of antibodies against citrullinated substrates is being recently recognised as specific to RA.40 On the whole, this suggests that digestion of fibrin networks generates transformed peptides that can be phagocytosed and subsequently externalised at the surface of antigen presenting cells for the recognition of T lymphocytes. The latter would further orchestrate the immunological response typical of established RA.
CRITICAL STEPS IN THE FIBRIN INDUCED PATHOGENIC SCHEME OF RA
In summary, according to our proposition, RA is an inflammatory state which generates an excess of intracavitary fibrin, and the typical processes of the disease are due to the interaction of fibrin clots with intimal cells. It is likely that the formation of intracavitary fibrin clots or even their interaction with the intimal layer happens in a non-specific way after the incidence of any triggering agent. The development of RA could then occur in those hosts with a genetically determined alteration of extravascular haemostasis, leading to an excessive local fibrin formation. On the other hand, individual factors probably play a part in determining the cellular response to the adhered fibrin clots. An aberrant response resulting in the rheumatoid transformation could be due to multiple mechanisms, such as an overexpression of adhesion molecules or an abnormal generation of proinflammatory isoforms of fibronectin. Because migration of synovial fibroblasts into the fibrin networks is central to the pathogenic process here described, mechanisms controlling fibroblast locomotion could also be important in patients with RA. None the less, patients with RA do not show relevant alterations in their systemic coagulation activity or in their wound healing processes outside the joint. Hence, any of these alterations must be restricted to the inflamed environment, or possibly induced by the initiating agent.
According to the pathogenic sequence we propose here, current treatments for RA target events downstream of fibrin deposition. It would be interesting to test the clinical efficacy of agents that might act at earlier stages in the disease process, such as anticoagulants, blockers of cell-matrix adhesion processes, and agents interfering with the intracellular fibrillar system of fibroblasts.