Role of inflammatory factors and adipose tissue in pathogenesis of rheumatoid arthritis and osteoarthritis. Part II: Inflammatory background of osteoarthritis.

Osteoarthritis is the most common rheumatoid disease. It may develop as a primary disease of the motor organ or as a secondary one in the course of other inflammatory joint diseases. Similarly to the majority of rheumatoid conditions, the pathogenesis of osteoarthritis has not been fully explained. The fact that its development is determined by adipocytokines, which are inflammatory mediators produced in the adipose tissue, has been known for several years. Additionally, inflammatory processes taking place in the adipose tissue that lead to degenerative changes are the main subject of studies conducted by various immunological laboratories. Degenerative changes in patients with osteoarthritis are frequently accompanied by secondary inflammation with cellular infiltrations in the synovial membrane. In numerous cases, the intensification of inflammatory lesions resembles changes seen in arthritis, particularly in rheumatoid arthritis, which inhibits the differential diagnosis by means of imaging examinations. This may have significant clinical implications, e.g. with respect to sonography, which is the basic imaging examination in diagnosing rheumatoid arthritis, monitoring the efficacy of implemented treatment or confirming remission. This article discusses the pathogenesis of three elements of osteoarthritis, i.e. synovitis (due to the difficulties in differentiation of synovitis in the course of osteoarthritis and in rheumatoid arthritis) as well as osteophytes and subchondral sclerosis (due to the significance of the inflammatory factor in their development).

Introduction

Similarly to rheumatoid arthritis (RA) discussed in Part I of this article(, histopathological and immunological examinations of the adipose tissue collected from patients with osteoarthritis (OA) reveal the presence of cellular infiltrations containing mainly macrophages, lymphocytes, mastocytes and natural killers (NK cells)(. Furthermore, it has been shown that the adipose tissue produces growth factors and adipocytokines, i.e. various soluble factors, including: classic adipocytokines (such as leptin, adiponectin and resistin produced mainly by adipocytes), proinflammatory cytokines and components of the complement system. All of them may affect the metabolism of the cartilage and synovium. In the example of Hoffa's body (infrapatellar fat pad, IPFP), the presence of inflammatory cells was observed in 36% of patients with OA. When compared with RA patients, such infiltrations are less numerous and do not form an organized ectopic lymphatic tissue. Nonetheless, they may modulate cellular function in auto-, para- and endocrine manners* and all of them may affect the metabolism of the cartilage and synovium as well as sustain the inflammatory reaction(.

Role of inflammatory factors in etiopathogenesis of OA

Osteoarthritis is a common disease of old age characterized by damage to the hyaline cartilage and presence of proliferative bone changes. The pathogenesis of OA is not fully understood but the role of mechanical, genetic, hormonal, metabolic and, finally, inflammatory and immunological factors is emphasized. Recently, the role of the adipose tissue, as the source of inflammatory immune cells, has been taken into account(.

This article discusses the pathogenesis of three elements of OA picture, i.e. synovitis (due to the difficulties in differentiation of synovitis in the course of OA and RA) as well as osteophytes and subchondral sclerosis (due to the significance of the inflammatory factor in their development).

The lesions in OA affect all structures of a joint i.e. articular cartilage, subchondral bone layers, synovium and periarticular structures(. Irrespective of the etiological factor, in the first stadium, the damage involves the hyaline cartilage (fig. 1) and leads to the changes of phenotype/ functions of chondrocytes and composition of the extracellular matrix.

Fig. 1

Hyaline cartilage of OA patient. Numerous degenerative lesions in the forms of ruptures and fissures (long arrows) and presence of fibrin collections on the surface (short arrows). 400× magnification; hematoxylin and eosin stain (H&E)

The cartilage is composed of chondrocytes (2–10% of the cartilage) and matrix which is primarily composed of water (65–80%) as well as collagen (10–30%) and proteoglycans (5–10%), the remaining elements are non-collagen proteins and low number of lipids. Proteoglycans are composed of protein core with attached glycosaminoglycan chains (polysaccharides). The main proteoglycan of the cartilage is aggrecan which contains chains of chondroitin sulfate and keratan sulfate. The dominant collagen type is type II (and to a lesser degree, types IX and XI). Collagen forms a scaffold for the cartilage matrix. It is also responsible for the cohesiveness and mechanical resistance of the cartilage and gives it shape.

Degenerative lesions in the cartilage, which begin the cascade of destructive changes, are primarily induced by inflammatory factors produced by the synovium, adipose tissue and activation of mechanoreceptors in chondrocytes and osteoblasts.

Pathogenesis of synovitis in patients with OA

The majority of OA patients manifest features of synovitis on histopathological examination(. In RA patients, presumable factors responsible for the proliferation of the lining layer and inflammatory infiltration of the sublining are environmental factors such as smoking and Porphyromonas gingivalis bacteria(. In the case of OA, however, the trigger points appear to be the molecules/products which are released when cartilage is damaged and which probably operate through the receptors of innate immune system, so-called, pattern recognition receptors (PRRs), e.g. Tolllike receptors, and secondarily, also through other proinflammatory factors, such as cytokines, which irritate the synovial membrane causing its inflammation and inducing immune response(. Apart from the pathogen-associated patterns, PRRs also recognize damage-associated molecular patterns (DAMPs). DAMPs that are typical of OA are released fragments of the articular cartilage (biglycans, fibronectin, fragments of hyaluronic acid chains or tenascin C), some plasma proteins (such as α1-microglobulin, α2-microglobulin or fibrinogen), intracellular alarmins (among others high-mobility group box 1, HMGB1) and crystals released from subchondral bone layers. DAMPs attach to PRRs on the surface of macrophages, fibroblasts of the synovial membrane and chondrocytes. This leads to their activation and production of inflammatory mediators(.

Activated macrophages produce various growth factors, cytokines and enzymes which contribute to the proliferation of the synovium, induce joint effusion by dilating vessels, affect the formation of osteophytes and probably have an effect on the metabolism of the subchondral tissue (see below). Neutrophils enhance cartilage degradation and lead to necrosis of the adipose tissue by producing IL-6 and IL-8 cytokines and metalloproteinase MMP8(. Eosinophils and basophils release histamine as well as enhance the production of enzymes and proinflammatory mediators in synovial fibroblasts and cartilage, which degrade the matrix(. In OA patients, T-helper cells (lymphocytes Th1) show expression of IL-2, IL-3, IFNs and GM-CSF, which is typical of this cell line, and may degrade the cartilage directly or indirectly by macrophage activation(.

Another significant factor in the development of synovitis is the adipose tissue(. Both immune cells that infiltrate it (fig. 2) and adipocytokines secreted by the adipose tissue may stimulate macrophages to induce the proliferation of the synovial membrane.

Fig. 2

Hoffa's fat pad in OA: A. angiogenesis in the adipose tissue (arrows); B. slight inflammatory infiltration by lymphocytes and plasma cells (thin arrow) and features of angiogenesis (thick arrow); C. focal inflammatory infiltration by lymphocytes (arrow). 200× magnification; H&E stain

The histopathological picture of the synovium in patients with OA is usually heterogeneous: within one joint, one may observe fibrosis next to active inflammatory lesions resembling RA synovitis, i.e. hyperplasia (proliferation) of the synovial lining cells, inflammatory infiltrations consisting mainly of T-cells and monocytes in the sublining as well as neovascularization( (fig. 3). Contrary to synovitis in the course RA, in OA, there is no ectopic lymphatic tissue.

Fig. 3

Proliferation of villi (thin arrows), slight proliferation of synoviocytes, average inflammatory infiltrations of lymphocytes and plasma cells (thick arrows), features of angiogenesis. 200× magnification; H&E stain

Therefore, in imaging examinations, the differentiation of RA and OA synovitis is not feasible. This refers to all types of arthritis in which, based on current immunological and histopathological knowledge, the presentation of synovitis is very similar. One of the distinguishing features is the presence of ectopic lymphatic tissue in RA. This has serious clinical implications not only in terms of differential diagnosis of arthritis types, including undifferentiated types, but also in terms of treatment monitoring or stating the remission – a certain element of synovitis will not necessarily indicate persisting inflammation connected with the underlying disease but may be a manifestation of degenerative changes.

A particular example of the coexistence of proliferative changes/osteophytes and synovitis is degeneration of the carpometacarpal joint of the thumb (rhizarthrosis) (fig. 4). Due to its frequent involvement by OA, Larsen (the author of one of the most popular scales of RA advancement) excluded it from his classification in 1995. No one, however, will guarantee that in a patient with a clinical suspicion of RA, the features of synovitis in this joint attest to degenerative process, even when osteophytes are observed. Hence, the carpometacarpal joint of the thumb remains one of the elements of wrist and hand assessment in RA diagnosis in most of radiological classifications, despite their modifications (e.g. in Sharp/van der Heijde score or SENS)(. The problem in differentiating between degenerative changes and rheumatoid process may also concern other joints (fig. 5).

Fig. 4

A, B. Advanced osteoarthritis in the carpometacarpal joint of the thumb in grey scale examination (A) and in power Doppler ultrasound (B): large osseous proliferations on the edges of articular surfaces. Thickened synovial membrane in the joint cavity with single vessels of inflammatory process – features of chronic synovitis (rhizarthrosis)

Fig. 5

US examination: advanced proliferative changes in the PIP joint with thickened and intensely vascularized synovium as well as erosions

Role of inflammatory factors in the formation of osteophytes

The inflammatory factor, next to genetic factors, has its role in osteophyte formation, which particularly concerns rheumatoid patients (fig. 6). Osteophytes reveal the presence of growth factor TGF-β, IGF-1 and leptin. The TGF-β family plays a vital role in inducing osteoblast activity by promoting stem cell differentiation in the bone marrow within the osteoblastic cell line (osteoblastogenesis). It is also a mitogen of human precursors of osteoblasts. Intraarticular administration of TGF-β to rats induced the formation of large osteophytes(. What is more, macrophages of the synovium lining layer constitute an important secondary factor in osteophyte formation. They are significant producers of proinflammatory cytokines (TNF-α and IL-1β), chemokines (chemotactic cytokines) and growth factors regulating chondrogenesis, which, next to the extracellular matrix protein, affect mesenchymal cell differentiation within chondrocyte cell line into a new cartilage and bone(. Some of these compounds are capable of inducing synovitis in animal studies(.

Fig. 6

Lateral X-ray of the knee: subchondral sclerosis

Immunological research confirmed that osteophytes are formed from the mesenchymal cells that are localized in the periosteum or its vicinity(.

Some of the activities of macrophages, such as producing TNF-α cytokines, are inhibited by peripheral sympathetic neurotransmitters – norepinephrine, neuropeptide Y (NPY) and adenosine(. In synovitis and in the adipose tissue of patients with RA and OA, the density of sympathetic nerve fibers is dramatically reduced but the density of proinflammatory peptidergic C sensory fibers that secrete substance P is increased(. Similarly to TNF-α and IL-1β, substance P induces nitric oxide which is an inflammatory mediator in rheumatoid synoviocytes and in experimental models of the inflammation(. It activates inflammatory cells and synoviocytes, stimulates IL-1 production and enhances its effects on tissues(. The concentration of substance P in articular tissues of patients is correlated with IL-6 activity(. What is more, together with several other neurotransmitters (e.g. noradrenaline), it stimulates IL-6 and IL-8 in vitro synthesis in synoviocytes. Moreover, in numerous tissues, it enhances the production of: IL-1β, TNF-α, nuclear factor kappa B and superoxide anion 02-(. Furthermore, it exerts strong activating effects on fibroblasts and on the production of extracellular matrix(. It also causes dilation of vessels which leads to the extravasation of the immune system cells and, subsequently, to edema of the Hoffa's fat pad – IPFP. IPFP edema frequently results in jamming of the fat pad which, in turn, may lead to its ischemia and necrosis(. Ischemia induces neurotropin release – a natural growth factor that activates substance P release. This is how a vicious circle is created which is probably responsible for persisting inflammation of the Hoffa's body(. IPFP in patients with OA contains more nerves with low diameters, where substance P is present, than with medium or large ones. This means that IPFP contains a part of sensory endings of the knee and may be the source of severe pain in patients with OA(. In persons experiencing pain in the anterior aspect of the knee and in RA patients, the number of fibers secreting substance P is ever 8 times greater than the number of sympathetic nerve fibers secreting anti-inflammatory norepinephrine and endogenous opioids which inhibit the perception of pain by acting on substance P in an antagonistic manner(.

Role of inflammatory factors in the formation of subchondral sclerosis

Inflammatory processes also cause changes in the subchondral tissue (fig. 6). Until recently, it was thought that the thickening and sclerosis of subchondral layers, which are observed in the course of OA, are a secondary phenomenon(. For several years, however, it has been known that the lesions in the subchondral layers occur simultaneously and sometimes, even precede the development of cartilage lesions (analogically to the changes in the bone marrow in the course of RA or in spondyloarthropathy/sacroiliac arthritis)(. Cytokines and growth factors produced in the subchondral tissue lead to cartilage degradation by stimulating chondrocytes to produce increased amounts of matrix metalloproteinase and inhibiting aggrecan synthesis(. Furthermore, they may induce hypertrophic differentiation of chondrocytes or stimulate chondrocytes in a paracrine manner(. This entails progressive sclerosis and correlated bone ankylosis, which may be conductive to the development of new articular cartilage damage resulting from improper distribution of mechanical tension or microfractures(. What is more, progressive sclerosis is not accompanied by adequate increase in mineralization, which may indicate the disorder in the process of bone remodeling in the course of OA(. A range of data confirm the existence of malfunctions of osteoblasts originating from the subchondral layers of involved bones. The presence of, among others, improper response to the stimulation with parathormone, vitamin D3, prostaglandin E2 and certain growth factors has been noticed. As a result, there are disorders in the production of collagen and noncollagen proteins, including osteocalcin(.

Conclusion

The ability of a diagnostic method to identify false positive cases is a determinant of its accuracy. Unfortunately, histological and immunological studies reveal that in a certain disease stadium, imaging examinations, in this case sonography and magnetic resonance imaging (MRI), preclude the differentiation between rheumatic synovitis and synovitis developing in the course of OA. The early stage of RA, prior to the implementation of treatment, is an exception. In this case, all elements of inflammation, particularly hyperemia visualized by imaging examinations, are more intensified than in other rheumatic entities, including OA. However, arthrifollowing the commencement of treatment, a certain level of thickening and increased vascularity of the synovium may result either from chronic RA or from intensified degenerative lesions with the presence of synovitis which is caused by the “irritation” of the synovial membrane by fragments of damaged cartilage. This is of significant clinical importance with respect to sonography which in clinical practice, constitutes the fundamental imaging examination applied for the purposes of monitoring the efficacy of RA treatment or stating remission in rheumatoid patients. A certain element of synovitis may be of degenerative nature and probably it will not be feasible to identify it in certain stages of these diseases. Nevertheless, the application of dynamic MRI (DCEMRI) shows promising possibilities. So far, out of imaging examinations, this method has proven to be the most accurate in visualizing the processes taking place in joints and in the bone marrow as well as in monitoring them during treatment in qualitative and quantitative manners( (fig. 7). The initial results of Cimmino et al.( indicate the possibility to differentiate between RA and psoriatic arthritis based on the differences in the localization of inflammatory lesions of the extensor sheaths, volume of inflamed synovium and exacerbation of synovitis. These results are initial and require confirmation among a larger population. Nevertheless, they are promising in terms of differential diagnosis of early, and particularly, unclassified (undifferentiated) forms of arthritis.

Fig. 7

Maximal enhancement of the synovium following contrast i.v. injection in MR study of the knee in patients with RA (A) and OA (B)