Scalable synovial fibroblast sources enable reproducible basal synovial organoid formation: an in vitro platform...

This study aimed to identify new and scalable sources of cells for forming synovial organoids that align with the pauci-immune synovial pathotype and characterise both structure and cellular organisation of such organoids. The study modified a previously published method for forming synovial organoids using Matrigel, and tested it with combinations of immortalised synovial fibroblasts derived from synovium or synovial fluid together with human umbilical vein endothelial cells or the EA.hy926 cell line. Healthy synovium and synovial fluid derived fibroblasts with human umbilical vein endothelial cells resulted in formation of synovial organoids. Organoid diameter, area and cell count showed no significant differences (p > 0.05) in organoids formed with the different fibroblasts. Median CD31 signal intensity was 213 (184-218)) in the vascular-like area, 58 (55-60) in the lining-like area, and 62 (52-72) in the stroma-like area. Median podoplanin signal intensity was 971 (880-1052) in the lining-like area, 341 (310-560) in the vascular-like area, and 342 (281-356) in the stromal-like area. Here, we present novel sources of synovial fibroblasts for successfully forming of pauci-immune-aligned basal synovial organoids. These synovial organoids showed spatial expression patterns of PDPN and CD90 consistent with in vivo synovial fibroblast phenotypes and thereby showed potential as a reproducible basal synovial organoid platform, providing a structural foundation for future mechanistic and translational extensions with additional work needed to establish a fully validated disease model.