Demonstration of cytokeratin in endothelial cells of the synovial microvasculature in situ and in vitro.

We have used a panel of anti-cytokeratin antibodies and immunofluorescence microscopy to examine synovial tissue from a variety of large and small joints in patients with various rheumatic conditions, including RA, OA, AS, pigmented villonodular synovitis (PVNS) and tenosynovitis (TS). In every case we regularly found blood vessels with endothelia which express cytokeratin. Positive staining was obtained with a guinea pig anti-keratin antibody, with monoclonal antibody 8.13 and with a monoclonal antibody specific for keratin 18. Staining of endothelial cells was confirmed by double labelling with antibodies to cytokeratin and factor VIII/von Willebrand factor. We also detected a polypeptide corresponding to cytokeratin 18 (MW 45,000) by Western blotting of synovial tissue. In addition we have isolated pure populations of synovial endothelial cells in culture and demonstrate an extensive cytokeratin intermediate filament network which co-localizes with vimentin filaments. An understanding of the role of the cytokeratin 18 network in synovial endothelium may be important for our understanding of endothelial changes in synovial disease.