Characterization of Synovial Fibroblast Subtypes


Disease modifying anti-rheumatic drugs have revolutionized RA therapy, expanded life expectancy, and dramatically improved the quality of patients' lives. However, it is estimated that a significant number of patients display inadequate response, whereas others experience severe side effects. As evidence grows that RA patients with a "fibroid pathotype" respond less effectively to treatment, the need to develop therapeutic targets for influencing the activated stroma increases. Previous studies have highlighted the potential role of four distinct synovial fibroblast subtypes in the propagation of RA inflammation, warranting further investigation. This project aim is to further explore the functional differences of the synovial fibroblast subtypes and assess whether POSTN+ SF and CXCL14+SF are functional antagonists with opposite roles in the inflammatory process in RA. Furthermore, we aim to elucidate the spatial distribution of the fibroblast subtypes as well as their interaction with immune cells in the microenvironment of the synovial tissue from RA patients. Our approach would provide insight into the pathogenic role of the stroma and the different fibroblast subtypes and could be the trigger for uncovering novel therapeutic targets.

Facts and Figures

Project Lead
M Toitou
University Hospital of Heraklion
FOREUM research grant: € 50.000

Meet the Team

Project Lead

M Toitou
University Hospital of Heraklion
Prof. dr. med. C Ospelt
University Hospital Zurich
Prof. Dr. P Sidiropoulos
University of Crete

Final results

In the study's first aim, we explored the topographical organization of synovial fibroblast (SF) subtypes in rheumatoid arthritis (RA) and osteoarthritis (OA) synovium using immunohistochemistry (IHC). The POSTN+ SF subtype was located in the sublining layer with perivascular distribution. MFAP5+ SFs expanded throughout the sublining layer, while CXCL12+ SFs were associated with inflammatory infiltrates and were more prevalent in RA compared to OA synovium. In the second aim, functional analysis after FACS isolation revealed distinct cytokine secretion patterns, with CD74+ SFs showing increased IL-6 production and immune-effector functions, activating macrophages. This effect was lost after 6 days in culture and CD74+ SFs lost their CD74 expression. Recreating the CD74+ phenotype in vitro allowed independent investigation and showcased its preservation in a synovial organoid model, influencing healthy macrophage activation. The study provides insights into SF subtype characteristics and their potential cross-talk with immune cells.

Lay Summary

According to the World Health Organisation, Rheumatoid arthritis (RA) is a systemic autoimmune disease primarily impacting joints and multiple body systems. Osteoarthritis (OA) is a degenerative disease that also affects the joints leading to their destruction. More than 500 million people worldwide are affected by these two conditions.
In both conditions, the synovial membrane, a tissue in the joints made up of cells called Synovial Fibroblasts (SF), gets disturbed. The study delved into distinct SF subtypes—PRG4, POSTN, MFAP5, and CD74—in individuals with RA and OA. By subtypes, we refer to different categories of SFs, each with its role. The PRG4, was notably present in areas invading the cartilage. The POSTN was located around blood vessels, while MFAP5 was distributed across the joint sublining. The CD74 was more prevalent in RA than OA and was associated with inflammatory regions. Several experiments have shown that CD74 cells secreted inflammatory proteins and exhibited an impact on the immune system, particularly in activating specific immune cells. However, these distinctions among subtypes diminished after prolonged cultivation.
To gain deeper insights into the CD74 subpopulation and overcome the restrictions, we replicated the cellular conditions in a laboratory setting. It has been shown that CD74 cells, following interaction with certain immune cells, acquired their distinctive characteristics. When introduced this subtype into a model simulating the joint environment, these cells maintained their characteristics and influenced the behaviour of other immune cells. The research provides insights into the functions of different SF subtypes and their interplay with immune cells, with future prospects aimed at uncovering the mechanisms through which CD74 interacts with immune cells, holding promise for potential therapeutic targets.

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