Ultrastructure of synovial changes in rheumatoid disease and in seronegative inflammatory arthropathies

The synovium as a privileged site in rheumatoid arthritis: cadherin-11 as a dominant player in synovial pathology

Rheumatoid arthritis is a systemic disease in which an autoimmune response translates primarily into joint inflammation with attendant joint destruction. While evidence implicates both the adaptive and innate immune system in rheumatoid synovitis, several lines of evidence now support the concept that the synovial tissue itself actively participates in the destructive inflammatory processes of arthritis. Specifically, the resident mesenchymal cells, the fibroblast-like synoviocytes (FLSs), frame a synovial microenvironment that responds to, augments and perpetuates the inflammatory process. Moreover, the FLSs have been recognised as the dominant cells mediating joint destruction. The identification of cadherin-11 expression on FLS provided an opportunity to unravel molecular mechanisms by which these resident mesenchymal cells govern processes that result in destructive synovitis in the context of systemic autoimmune disease. Herein, we discuss the unfolding biology of the synovial cadherin with its implications for the synovial pathology in arthritis, especially rheumatoid arthritis.

Cadherin-11 in synovial lining formation and pathology in arthritis

The normal synovium forms a membrane at the edges of joints and provides lubrication and nutrients for the cartilage. In rheumatoid arthritis, the synovium is the site of inflammation, and it participates in an organized tissue response that damages cartilage and bone. We identified cadherin-11 as essential for the development of the synovium. Cadherin-11-deficient mice have a hypoplastic synovial lining, display a disorganized synovial reaction to inflammation, and are resistant to inflammatory arthritis. Cadherin-11 therapeutics prevent and reduce arthritis in mouse models. Thus, synovial cadherin-11 determines the behavior of synovial cells in their proinflammatory and destructive tissue response in inflammatory arthritis.

Mast cells in inflammatory arthritis

Mast cells are present in limited numbers in normal human synovium, but in rheumatoid arthritis and other inflammatory joint diseases this population can expand to constitute 5% or more of all synovial cells. Recent investigations in a murine model have demonstrated that mast cells can have a critical role in the generation of inflammation within the joint. This finding highlights the results of more than 20 years of research indicating that mast cells are frequent participants in non-allergic immune responses as well as in allergy. Equipped with a diversity of surface receptors and effector capabilities, mast cells are sentinels of the immune system, detecting and delivering a first response to invading bacteria and other insults. Accumulating within inflamed tissues, mast cells produce cytokines and other mediators that may contribute vitally to ongoing inflammation. Here we review some of the non-allergic functions of mast cells and focus on the potential role of these cells in murine and human inflammatory arthritis.

Ultrastructural characteristics of the synovial membrane in osteoarthritic temporomandibular joints

PURPOSE

This study analyzed the ultrastructural characteristics of the synovial membrane in various stages of osteoarthritis (OA) of the temporomandibular joint (TMJ), and developed a classification of this involvement based on these morphologic characteristics.

PATIENTS AND METHODS

Synovial membrane biopsies were performed during unilateral arthroscopy in 40 patients. Thirty-one TMJs constituted the OA group; nine TMJs that were not involved by OA constituted the control group. During light microscopic (LM) examination, various variables were recorded and related to the duration of clinical signs and symptoms. Ten synovial membranes from osteoarthritic joints showing histologically visible pathologic changes in various stages and one control synovial membrane were selected for electron microscopic examination.

RESULTS

The initial, early, and intermediate stages of synovial membrane involvement in TMJ OA were characterized by intima hyperplasia. In the initial and early stages, active and hypertrophic intimal cells are found. In the intermediate stage, an increased number of both intracytoplasmic and extracellular filaments was predominant. Fibrosis of the subintimal tissue was initiated by an increased number of active fibroblasts. The late stage of synovial membrane involvement in TMJ OA was characterized by a relatively normal synovial intima of normal thickness, whereas extensive fibrosis was seen in the subintimal tissues.

CONCLUSIONS

Synovial membrane involvement in TMJ OA is characterized by an early proliferative phase with probable growth factor-mediated increases in the cellular activity of the synovial intima cells (resulting in hyperplasia and hypertrophy), of fibroblasts (resulting in increased production of collagen fibrils and fibrosis), and of endothelial cells (resulting in blood vessel growth and hypervascularity). The late phase is characterized by extensive fibrosis of the subintimal tissue, whether caused by sustained production of growth factors or by chronic venous insufficiency, with normal or little cellular activity.

Reactive oxygen species and iron--a dangerous partnership in inflammation

Cells of nearly all forms of life require well-defined amounts of iron for survival, replication and expression of differentiated processes. It has a central role in erythropoiesis but is also involved in many other intracellular processes in the tissues of the body. It is the fourth most abundant element in the Earth's crust and the most abundant transition metal in living organisms for which its characteristic chemistry endows it with a series of properties enabling it to fulfil certain biological reactions especially those involving redox mechanisms. It is involved in the transport of oxygen, in electron transfer, in the synthesis of DNA, in oxidations by oxygen (O2) and hydrogen peroxide (H2O2) and in many other processes maintaining normal structure and function of virtually all mammalian cells. Because an iron atom can exist in two valency states, ferrous and ferric, iron became the primordial partner of oxygen in evolution. However, as de Sousa et al. (1989) state, such long standing partnerships have to use protective devices to ensure that the toxicity of neither partner is expressed in the presence of the other. Here, we discuss this dangerous partnership and its relevance to inflammation. The main themes of this review are the known roles of iron in the generation of reactive oxygen intermediates and new developments, including iron and transcription and the reaction of iron with nitric oxide. We also consider the widening recognition of the importance of oxygen metabolites in hypoxia-reperfusion injury and disease of the skin and joint.

Fine structure of the human synovial lining cell in osteoarthritis: its prominent cytoskeleton

The cytoskeleton of the human osteoarthritic synovial lining cell (SLC) consists of an extensive number of vimentin intermediate filaments (IFs) in addition to microfilaments and microtubules. The IFs are especially prevalent in the SLC processes, but are commonly seen in a paranuclear arrangement. Processes, ending in numerous microvilli and blebs, project into the joint space. Scanning electron microscopy (SEM) further reveals the processes that may parallel the synovium surface for a short distance. IFs extend to the termination of such Numerous pinocytotic vesicles and extensive rough endoplasmic reticulum (rER) are characteristic of the type B cells. Lysosomes and long microvilli identify the type A cell. Punctate adherens, gap junctions, and cilia are the cell membrane specializations of the osteoarthritis (OA) synovium. A comparison with synovium from rheumatoid arthritis (RA) patients is made in order to assess the effect o this inflammatory disease on the SLC cytoskeleton, cell type relationship, and cell arrangement. The prominent cytoskeleton appears to play an important role in the architecture of the synovium. Our findings are further presented in the form of a drawing which in some aspects could describe the morphology of the normal synovium.

Correlation of anti-cytoskeleton antibody activities in synovial fluid with interleukin-6 in patients with osteoarthritis and inflammatory joint disease

Synovial fluids and sera from patients with rheumatoid arthritis, psoriatic arthritis, yersinia arthritis, Behçet's syndrome, Crohn's disease, and osteoarthritis were tested for antinuclear antibodies and antibodies to five cytoskeletal components in sensitive enzyme-linked immunosorbent assay (ELISA) systems and for IL-6 concentrations in a proliferation assay (IL-6 dependent hybridoma cell line B13.29, subclone B9). Statistically significant correlations between antibody activities and IL-6 levels were found for vimentin antibodies (r = 0.56; p less than 0.05) and actin antibodies (r = 0.44; p less than 0.05). In patients with chronic and active disease like rheumatoid arthritis and psoriatic arthritis, optical densities measured by vimentin- and actin-ELISA were significantly different from those measured in patients with osteoarthritis. To date only a few reports exist concerning the incidence of antibodies in synovial fluids. We have shown to our knowledge for the first time that IL-6 seems to induce synovial fluid antibody activities restricted to cytoskeletal components of synoviocytes (i.e., vimentin and actin). Synovial fluid antibody activities against vimentin and actin appear to be markers of activity in patients with inflammatory joint disease.

Ultrastructural changes in the rectal mucosa of patients with rheumatoid arthritis

The rectal mucosa from patients with severe rheumatoid arthritis has been examined by electron microscopy. Initially the patients were biopsied for investigation into the diagnosis of amyloidosis or vasculitis, which proved to be negative. A range of ultrastructural abnormalities was found which included proliferation of the rough endoplasmic reticulum, mitochondrial inclusions, association of the mitochondria with cytoplasmic filaments, and marked increase in the number of iron containing siderosomes. Most of these abnormalities have already been reported in rheumatoid synovial tissue, and attention is drawn to the similarity in findings between these two sites.