Background/Purpose: Fibroblast-like synoviocytes (FLS) form a complex tissue network via long-distance intercellular connections with wide intercellular matrix spaces. The adaptive synovial tissue response to inflammation likely depends upon the concerted activity of FLS. Using an in-vitro synovial organ culture system, we explore mechanisms of FLS directional cell-cell cooperation.

Methods: Human FLS were prepared from synovial tissues obtained as discarded specimens following joint arthroplasty. Cells were labeled with cell tracker dyes or specific organelle dyes and cultured in spherical matrigel micromasses. For selected experiments, micromasses were challenged with TNF or IFN-gamma. Data was acquired by confocal live cell imaging. Analysis of the resulting 4D movies was done using Imaris® software.

Results: To examine whether FLS transfer cytoplasmic cargo, we labeled 50% of FLS with green cell tracker dye and the other 50 % with Mitotracker. Over time, red labeled organelles accumulated in green labeled cells with a transfer rate of 10 % of newly affected cells/day. Confocal live cell imaging revealed that FLS indeed use their long-distance intercellular connections for transfer of organelles. When micromasses were stimulated with TNF (10 ng/ml) the transfer rate increased by 2-fold when compared to control. By contrast, IFN-gamma-stimulation (100 U/ml) resulted in decreased organelle transfer. The combined treatment of micromasses with TNF and IFN-gamma, however, increased the transfer rate to a level beyond stimulation with TNF alone.

Conclusion: Our experiments suggest transfer of cytoplasmic cargo, including organelles such as mitochondria between FLS. As transfer is distinctly regulated by the cytokine milieu, organelle transfer seems to be part of the adaptive synovial response to inflammation. These studies may provide insight into how synoviocytes orchestrate their activity. Further studies will demonstrate the significance of directional cytoplasmic cargo exchange for the function of the normal as well as the diseased synovium.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/share-the-fate-fibroblast-like-synoviocyte-cell-to-cell-organelle-transfer-is-directed-by-the-inflammatory-microenvironment/