Background/Purpose: The synovium is primarily formed by fibroblast-like synoviocytes (FLS). Its multicellularity requires precise coordination to generate a tissue that confers specialized functions critical to joint homeostasis. Cell-to-cell communication facilitates the concerted behavior of FLS within the synovium. Using a 3D model of the synovium, we analyzed FLS capacity for exchange of cytoplasmic content.

Methods: Human FLS were prepared from synovial tissues obtained as discarded specimens following joint arthroplasty. Cells were cultured in spherical matrigel micromasses with an average size of 2 mm Ø. Data was acquired by confocal live cell imaging and transmission electron microscopy. Analysis of the resulting 4D movies was done with Imaris® software.

Results: To examine intercellular cytoplasmatic transfer, we labeled 50% of FLS with red cell tracker dye and loaded the other 50 % with green non-degradable microspheres. In a time series (8 days), we found that microspheres do indeed appear in red labeled cells. First evidence was found on Day 1 and over the course of the following days microspheres accumulated in red labeled cells with a transfer rate of 10 % of newly affected cells/day. Additionally, red vesicles also appeared in green labeled cells. They first occurred on day 1, however, their transfer rate reached a steady state at about 20%, presumably due to degradation following transfer in the receiving cell. A similar experiment in 2D demonstrated microsphere movement within interconnecting nanotubes. Transfer rates for microspheres into red cells were identical. By contrast, rates for red vesicles into green cells were much higher than in 3D cultures. Thus, cells in 2D culture may have easier access to released vesicles as compared to 3D tissues. Transmission Electron Microscopy revealed transfer via exocytosed vesicles as well as open intercellular connections.

Conclusion: These studies suggest transfer of cytoplasmic cargo between FLS. We identified two ways for cytoplasmic transfer between cells; 1) through vesicles and 2) through interconnecting open nanotubes. Further studies will demonstrate the significance of directed cargo exchange for cellular cooperation and the function of the normal as well as the diseased synovium.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/along-those-lines-synoviocyte-cell-to-cell-communication-via-nanotubes/