Introduction The synovium is primarily built by fibroblast-like-synoviocytes (FLS). In the healthy synovium FLS form a complex tissue network via long-distance intercellular connexions (nanotubes). Using a 3D synovial micromass culture system, our previous research demonstrated that FLS exchange cytoplasmic cargo, i.e. organelles like mitochondria, via transfer through direct cell-to-cell connexions. The adaptive synovial tissue response to inflammation (i.e. to TNFa) likely depends upon the concerted communication between FLS.

Objectives To determine how the cellular organisation of the synovial tissue affects intercellular cargo exchange and how it changes under inflammatory conditions.

Methods Human FLS were isolated from joint tissues after synovectomies; passaged FLS were used to generate 3D micromass cultures using Matrigel (BD). For ultrastructural 3D reconstruction ultrathin sections of fixed micromasses were cut with an ATUMtom for FE-SEM scanning, followed by alignment and reconstruction of individual cells with Fiji TrackEM. To compare unstimulated and TNF-stimulated micromass cultures, cells were dyed with Cell- and Mitotracker dyes (TF). Using confocal microscopy, micromass cultures were analysed for cellular organisation within the 3D sphere, cell volume of individual cells, cell-to-cell interconnectivity and cargo transfer between cells. Analyses of the 3D confocal imaging data were done in Bitplane Imaris.

Results SEM 3D reconstruction of synovial micromass tissue revealed that FLS are compartmentalised, and thus, much larger and extended than expected. Thin membrane tubes not only connect different cells but also compartments of the same cell. Additionally, one nanotube may provide a link to several other cells and these connexions are in general separated by membranes. Treatment of micromasses with TNFa resulted in cellular condensation, reduced cell volume as well as diminished cellular connectivitiy when compared to unstimulated micromasses. In particular, the abundance of interconnecting nanotubes was significantly attenuated. The mitochondrial transfer rate increased as FLS clustered upon TNFa stimulation.

Conclusions TNFa directs FLS cellular re-organisation that is associated with increased intercellular transfer of mitochondria. These studies may provide insight into the concerted cooperation of FLS that is likely critical for the adaptive synovial response to inflammation.

Disclosure of interest None declared