Fibrocytes in Early and Longstanding Rheumatoid Arthritis

The intimal lining of a synovial joint consists of unique cells called fibroblast-like synoviocytes (FLS cells) with interspersed macrophage-like synoviocytes. In the healthy joint macrophage-like synoviocytes are implicated in the innate immune defence and support adaptive immunity, while FLS cells function to regulate the release of nutrients and molecules, including hyaluronan, into the joint cavity.

Rheumatoid arthritis (RA) is a chronic systemic disease with autoimmune traits, which primarily affects synovial joints. The intimal lining of the synovium expands and exhibits characteristics of unregulated growth and loss of contact inhibition. The expanding synovium degrades cartilage and erodes into the bone at the cartilage-bone interface, creating the hallmark of RA, the joint erosion.

The RA FLS cell, has a markedly different phenotype than the healthy cell, and plays a key role in the destructive process by the release of pro-inflammatory cytokines and matrix destructive enzymes. FLS cells expand during the joint destructive process, however, there is minimal mitotic activity. The origin of the expanding FLS population is therefore uncertain.

Possible explanations to this phenomenon could be decreased senescence, migration of mesenchymal stem cells from the circulation, epithelial to mesenchymal cell-transition or expansion of a stem cell pool in the synovium. FLS precursors could also migrate to the synovium through pores in cortical bone, as has been demonstrated in mice with collagen-induced arthritis. In a murine model, it has further been shown that RA FLS cells can spread via the systemic circulation from one joint and attack previously unaffected joints.

A possible FLS cell precursor has been identified in peripheral blood and called fibrocytes. Fibrocytes are spindle-shaped cells that express surface receptors of both stromal and hematopoietic cells. The fibrocytes are a rare (~0.5% of leukocytes) population of bone marrow derived mesenchymal progenitor cells. A combination of markers that distinguish fibrocytes from monocytes, macrophages and fibroblasts has been described. In vitro circulating human peripheral blood mononuclear cells and monocytes (CD14+) can differentiate into fibrocytes. Fibrocytes can differentiate in vitro along multiple mesenchymal lineages into adipocytes, chondrocytes, myofibroblasts and osteoblasts. It is unknown which differentiation pathway occurs in vivo or if multiple differentiation profiles can occur .

Increased numbers of circulating fibrocytes have been found in murine RA models. In a small sample of RA patients (six patients, ten controls), peripheral blood fibrocytes had elevated phosphorylation activation compared to controls, and cells were demonstrated in the synovium. Here fibrocyte activation was correlated with arthritis disease activity (DAS28). In a murine model, activated circulating fibrocytes where found to migrate to arthritis joints in the preclinical phase of the disease, and to accumulate in the FLS cell layer of the synovium. Adoptive transfer of circulating fibrocytes in this murine model, led to worsening of joint disease indicating that circulating fibrocytes are involved in RA joint pathology, possibly as a FLS-cell precursor. Further, fibrocytes have also been implicated in extra articular manifestations of RA, e.g. pericarditis (murine model), atherosclerosis, and interstitial lung disease.

Studies on synovial biopsies have been critical in the understanding of RA pathogenesis. A new method of collecting synovial tissue, ultrasound-guided synovial biopsy is safe and well tolerated by patients and reliable for collecting high quality synovial tissue from large and small joints.