The aim of this study was to investigate in situ the expression of the classic vitronectin (VN) receptor consisting of the alpha v and beta 3 subunits in synovial lining cells (SLC) of chronic synovitis occurring in osteoarthritis (OA) and in rheumatoid arthritis (RA). The expression and function of alpha v and beta 3 as VN receptor in cultured fibroblast-like synoviocytes (FBS) derived from patients with OA and RA was also compared.
Expression of alpha v and beta 3 was examined immunohistochemically in normal synovial tissue and in synovial tissue from patients with OA and RA. The effect of proinflammatory cytokines and of a synovial fluid of a patient with RA on the expression of the alpha v and beta 3 subunits of cultured FBS was determined by flow cytometry. Binding of OA and RA-FBS to VN was quantified using adhesion assays and the effect of interleukin 1 beta (IL1 beta) and tumour necrosis factor alpha (TNF alpha) on adhesion was measured. The specificity of the adhesion was tested by inhibition studies using monoclonal antibodies to integrin subunits.
In in situ studies normal SLC showed a parallel distribution of alpha v and beta 3 subunits. OA-SLC strongly and uniformly expressed alpha v whereas RA-SLC showed heterogeneous expression of alpha v. In situ both OA-SLC and RA-SLC lacked the expression of the integrin subunit beta 3. In in vitro studies, OA-FBS and RA-FBS did not differ as regards expression of alpha v and beta 3, and VN attachment. Binding of RA-FBS to VN was partially blocked by antibodies against alpha v, beta 1, and beta 3 subunits, whereas only antibodies against alpha v and beta 3 inhibited the binding of OA-FBS to VN. The proinflammatory cytokines TNF alpha and IL1 beta increased the expression of alpha v and beta 3, and the VN binding of OA-FBS, whereas alpha v and beta 3 expression, and VN binding were downregulated in RA-FBS. Similar effects were found when the synovial fluid of an RA patient was used.
The integrin subunit beta 3 seems to be one partner but not the major one with which the subunit alpha v forms functional vitronectin receptors in OA-FBS and RA-FBS. The interaction between synovial cells and inflammatory cytokines seems to be different for OA and RA; the basis for this difference, however, remains to be established.