The lining cell layer of the synovium proliferates strongly in rheumatoid arthritis. It has been suggested that it has a central role in the destruction of cartilage. We have analyzed the structure of the extracellular matrix and the adhesion molecules of normal, osteoarthritic and rheumatoid lining cell layer.
We localized the alpha v integrin subunit and its 4 putative partner beta subunits in synovial samples by using indirect immunofluorescence. The specimens were also analyzed by confocal microscopy. Indirect immunofluorescence was also used to analyze the ligands of alpha v integrins, namely fibronectin and vitronectin.
The alpha v integrin was abundant in the lining cell layer of normal and osteoarthritic synovia, whereas it was not expressed in the proliferating rheumatoid lining cell layers. A similar expression pattern was found for beta 5 subunit, suggesting that it is the major partner for alpha v. However, also some alpha v beta 1 and alpha v beta 3 heterodimers may be present. The confocal microscopy revealed the presence of both alpha v beta 5 positive and negative lining cells. The putative ligands for alpha v integrins, namely fibronectin and vitronectin were found in the lining cell layer of all the synovial specimens.
In spite of the proliferation of the lining cell layer in rheumatoid inflammation, the extracellular matrix stays very similar to that in normal and osteoarthritic synovium, whereas the pattern of the adhesion receptors is completely altered.