Characterization of the DNA-synthesizing cells in rheumatoid synovial tissue

The synovial fluid fibroblast-like synoviocyte: A long-neglected piece in the puzzle of rheumatoid arthritis pathogenesis

Rheumatoid arthritis (RA) is a systemic autoimmune disease during which fibroblast-like synoviocytes (FLS) contribute to both joint inflammation and destruction. FLS represent the core component of the synovial membrane. Following inflammation of this membrane, an effusion of cell-rich synovial fluid (SF) fills the joint cavity. Unlikely, SF has been shown to contain fibroblasts with some shared phenotypic traits with the synovial membrane FLS. These cells are called SF-FLS and their origin is still unclear. They are either brought into the synovium via migration through blood vessels, or they could originate within the synovium and exist in projections of the synovial membrane. SF-FLS function and phenotype are poorly documented compared to recently well-characterized synovial membrane FLS subsets. Furthermore, no study has yet reported a SF-FLS single-cell profiling analysis. This review will discuss the origin and cellular characteristics of SF-FLS in patients with RA. In addition, recent advances on the involvement of SF-FLS in the pathogenesis of RA will be summarized. Current knowledge on possible relationships between SF-FLS and other types of fibroblasts, including synovial membrane FLS, circulating fibrocytes, and pre- inflammatory mesenchymal (PRIME) cells will also be addressed. Finally, recent therapeutic strategies employed to specifically target SF-FLS in RA will be discussed.

Circulating fibrocytes contribute to the pathogenesis of collagen antibody-induced arthritis

OBJECTIVE

Rheumatoid arthritis (RA) is a systemic autoimmune disease resulting in joint inflammation. Fibroblast-like synoviocytes in affected joints are responsible for pannus formation and cytokine/chemokine production, resulting in leukocyte recruitment and bone/cartilage destruction. Previously, we identified a multipotent stem cell population of activated fibrocytes in the blood of patients with RA that may have a role in disease pathogenesis, perhaps as fibroblast-like synoviocyte precursors. The aim of this study was to further characterize the contribution of circulating fibrocytes to the pathogenesis of RA.

METHODS

Circulating fibrocytes were isolated from mice with collagen-induced arthritis and transferred intravenously into recipient mice with collagen antibody-induced arthritis (CAIA). The activation status of circulating fibrocytes was determined using multidimensional phosphoflow cytometric analysis of the signaling effectors STAT-5, STAT-1, AKT, and JNK. Circulating fibrocyte trafficking and matrix metalloproteinase (MMP) activity were assessed in real time using fluorescence molecular tomography, specifically labeling circulating fibrocytes with CellVue Maroon and measuring MMP activity using MMPSense 680.

RESULTS

The numbers of circulating fibrocytes were increased early during the onset of CAIA, concomitant with their activation, as measured by phosphorylation of STAT-5. Adoptive transfer of circulating fibrocytes augmented disease scores and increased class II major histocompatibility complex expression and peripheral blood phosphoactivation profiles in recipient mice with CAIA. Notably, adoptively transferred fluorescence-labeled circulating fibrocytes rapidly migrated into the affected joints of recipient mice with CAIA, and this was associated with augmented neutrophil recruitment into affected joints and MMP activation.

CONCLUSION

Circulating fibrocytes migrate to joints and influence the onset of disease processes in arthritis.

Fibrocyte activation in rheumatoid arthritis

OBJECTIVES

RA is a common, relapsing autoimmune disease primarily affecting the joints. Fibroblast-like synovial (FLS) cells are thought to be responsible for pannus formation and secretion of factors that recruit leucocytes to affected joints, thereby promoting bone and cartilage destruction. Fibrocytes are multipotent circulating stem cells that may have a role in RA pathogenesis, perhaps as the precursors of the FLS cells, or by regulating FLS cell function.

METHODS

We utilized multidimensional phospho-specific flow cytometry to characterize the activation status of peripheral blood (PB) fibrocytes derived from human RA patients at different stages of disease and from mice with CIA.

RESULTS

Human PB fibrocytes from RA patients exhibited phosporylation activation of the p44/42 and p38 MAP kinases (MAPKs), and STAT3 (signal transducer and activator of transcription) and STAT-5 early in disease, within the first year of diagnosis. Similarly, in murine CIA, an increase in the total number of PB phosphoSTAT5-positive fibrocytes was observed at early time points in disease. Notably, in the affected paws of mice with CIA, we identified an increased number of fibrocytes, in contrast to the paws of control mice.

CONCLUSIONS

These data suggest that activated fibrocytes may influence the disease process in RA and may serve as surrogate markers for disease in the PB of affected patients.

The fibroblast-like synovial cell in rheumatoid arthritis: a key player in inflammation and joint destruction

Although multiple cell types are present in the rheumatoid joint, the fibroblast-like synovial cell (FLS) is among the most prominent. It is now appreciated that the FLS is not only space-filling, but is directly responsible for cartilage destruction, and also drives both inflammation and autoimmunity. In this article, we consider the normal role of the FLS in healthy joints, and review evidence that implicates the FLS as a central player in the propagation of rheumatoid arthritis.

Local proliferation of fibroblast-like synoviocytes contributes to synovial hyperplasia. Results of proliferating cell nuclear antigen/cyclin, c-myc, and nucleolar organizer region staining

OBJECTIVE

To test the hypothesis that local proliferation contributes significantly to the hyperplasia of rheumatoid synovium.

METHODS

Immunohistologic and chemical staining was used to identify 3 markers of cell proliferation: proliferating cell nuclear antigen, c-myc proto-oncogene, and nucleolar organizer regions. Synovium from 21 patients with rheumatoid arthritis, 34 with degenerative joint disease, and 7 with joint trauma was examined.

RESULTS

All 3 markers indicated substantial, active proliferation of synovial lining cells in synovium with hyperplasia. Proliferating cells showed type I procollagen immunoreactivity but were negative for CD68, a monocyte/macrophage marker. Proliferation was greater in rheumatoid arthritis than in the other conditions evaluated.

CONCLUSION

In situ proliferation of fibroblast-like synoviocytes in the synovium lining contributes considerably to the increase in cell numbers in rheumatoid synovium.

The preferential accumulation of helper-inducer T lymphocytes in inflammatory lesions: evidence for regulation by selective endothelial and homotypic adhesion

The mechanisms which lead to the accumulation of T lymphocytes into inflammatory lesions are not clearly understood. We have previously shown that synovial CD4 T lymphocytes are mostly CDw29+UCHL1+ (helper-inducer cells) and very few carry the CD45R antigen which identifies the suppressor-inducer subset. Synovial CD8+ cells are also CDw29+UCHL1+CD45R-. In the present study, lymphocytes from pleural and peritoneal inflammatory infiltrates were shown to have a similar phenotypic pattern. Furthermore, it was demonstrated that the CDw29+UCHL1+ subset had a greater ability than CD45R+ cells to adhere to endothelial cells and to form homotypic clusters. Differential surface expression of LFA-1 on the two subsets was also shown, but this could not account for the demonstrated adhesion differences. Differences in adhesion between CDw29+/UCHL1+ and CD45R+ cells may explain the preferential accumulation of CDw29+/UCHL1+ cells in inflammatory infiltrates and underlie some of the functional differences between cells taken from sites of chronic inflammation and those from peripheral blood.

Phenotypic characterization of 3H-thymidine incorporating cells in rheumatoid arthritis synovial membrane

Tritiated-thymidine incorporating cells in synovial tissue samples from ten patients with definite or classic rheumatoid arthritis (RA) were studied by combining two techniques. Tritiated-thymidine-labelled cells were seen in autoradiography and simultaneously the subtype of them was determined with immunoperoxidase staining using monoclonal antibodies. Tritiated-thymidine-labelled cells comprised 0.8 +/- 0.4% of all the inflammatory cells in RA synovial membrane. Of all 3H-thymidine-labelled cells 34 +/- 17% were positive for OKT8 and 19 +/- 8% for OKT4 monoclonal antibodies. OKM1-positive cells comprised 7 +/- 3% of all 3H-thymidine labelled cells, whereas only a few (3 +/- 4%) of them were positive for pan-B monoclonal antibody. This study emphasizes the importance of activated OKT8 lymphocytes in RA synovial membrane.

The application of quantitative cytochemistry to the study of diseases of the connective tissues

The connective tissues are a complex organisation of tissues, cells and intercellular materials spread throughout the body and are subject to a large number of diseases. Such complexity makes the study of the metabolism of the connective tissues in health and more particularly in disease states difficult if one uses conventional biochemical methodology. Fortunately the techniques of quantitative cytochemistry, as developed in recent years, have made it possible to study the metabolism of even such complex and refractory connective tissues as bone. Using properly validated assays of enzyme activity in unfixed sections from various tissues a number of the diseases of the connective tissues have been studied. For example the synovia from patients with rheumatoid arthritis and related conditions have been studied using these techniques and marked alterations in the metabolism of the synovial lining cell population of this tissue have been demonstrated. These alterations in metabolism are believed to be related to the destruction of cartilage and bone found in such diseases. Investigations of the metabolism of the chondrocytes of articular cartilage in a strain of mice which spontaneously develops osteoarthritis has revealed a lack of certain key enzymes of carbohydrate metabolism in precisely those areas where degradation of the matrix of articular cartilage begins suggesting a causal relationship between these events. These same techniques have been used to study the cellular kinetics and metabolism of the dermis and epidermis in the disfiguring disease, psoriasis. The metabolism of healing bone fractures, the diagnosis and treatment of the mucopolysaccharidoses and the metabolic effects of currently used anti-inflammatory and anti-rheumatic drugs have also been examined. Perhaps the most exciting aspect of these studies has been the development and use of the technique of the cytochemical bioassay (CBA) to study hormonally mediated diseases of the connective tissues. Such studies have recently shed new light on the molecular lesion in pseudohypoparathyroidism. Though still in their relative infancy the studies described in this review show the potential inherent in the use of quantitative cytochemistry for the study of diseases of the connective tissues.

Evidence for cell division in synoviocytes in acutely inflamed rabbit joints

DNA synthesis has been measured both by Feulgen cytophotometry, quantified by the DNA synthesis index, and by tritiated thymidine autoradiography, quantified by the labelling index. In the early acute inflammation resulting from the intra-articular challenge of ovalbumin in sensitised rabbits both indices rose considerably, so that at least 1 in 10 synoviocytes was heavily labelled 3 days after challenge. The results are compatible with the concept that even such apparently differentiated synoviocytes are capable of cell division.

The origin of phagocytic cells in the joint and bone

Studies dealing with the concept of the mononuclear phagocyte system are described, and the origin of synovial type A cells and osteoclasts is discussed in some detail. All of the available evidence indicates that both derive from precursor cells in the bone marrow. It seems justifiable to conclude that monocytes transform into type A synovial cells and fuse to form osteoclasts.

DNA synthesis in human rheumatoid and nonrheumatoid synovial lining

The DNA content per nucleus has been measured in the lining cells of 12 specimens of rheumatoid and 12 of nonrheumatoid synovial tissue. Optimal conditions for this reaction on these cells have been defined, and an index of DNA synthesis has been applied to give a quantitative measure of the degree of synthesis. This has given evidence of some DNA synthesis in both types of synoviocytes, compatible with the amount found in a slowly self-replacing tissue. There was no difference in the amount of synthesis in the rheumatoid and nonrheumatoid synoviocytes.

Degradation of thymidine to thymine by rheumatoid arthritis synovial tissue eluate

A study was made of the effect of rheumatoid synovial tissue eluate on phytohaemagglutinin (PHA) stimulation of peripheral blood lymphocytes. It was found that rheumatoid synovial tissue eluate caused marked inhibition of 3H-thymidine incorporation by lymphocytes stimulated with PHA. Synovial tissue from traumatic joints had no effect. The inhibitor was a heat-sensitive and nondialysable substance. The inhibitory effect of PHA stimulation was diminished by thymine. In thin-layer chromatography it showed the capacity to convert thymidine to thymine. Because thymine is not incorporated by DNA-synthesizing cells, this report explains why negative results are obtained when lymphocytes are stimulated by rheumatoid synovial tissue using 3H-thymidine incorporation as an indicator of lymphocyte activation.