Expression of molecules involved in B lymphocyte survival and differentiation by synovial fibroblasts
The synovitis of rheumatoid arthritis (RA) is one of few pathological lesions in which B lymphocyte accumulation progresses to the extent of germinal centre formation. The present study was designed to assess the ability of synovial fibroblasts to express molecules implicated in B lymphocyte survival and differentiation, both in vivo, and in response to cytokines in vitro. Normal and diseased synovia were examined by indirect immunofluorescence. In all tissues synovial intimal fibroblasts showed co‐expression of vascular cell adhesion molecule‐1 (VCAM‐1) and complement decay‐accelerating factor (DAF) comparable to that of follicular dendritic cells (FDC), but not complement receptor 2 (CR2). In rheumatoid synovia, subintimal cells showed variable expression of VCAM‐1 and DAF, with bright co‐expression of VCAM‐1, DAF and CR2 in lymphoid follicle centres. B lymphocytes, some of which were proliferating cell nuclear antigen‐positive, were present in contact with subintimal cells expressing VCAM‐1 with or without DAF or CR2. B lymphocytes were rarely present in the intimal layer, and, where present, showed fragmentation. In vitro, synovial fibroblasts exposed to tumour necrosis factor‐alpha (TNF‐α) in combination with interferon‐gamma (IFN‐γ) showed enhanced expression of VCAM‐1, in comparison with fibroblasts from skin and lung and, unlike skin and lung fibroblasts, also expressed DAF and CR2. These findings support the hypothesis that synovial targeting in RA involves an enhanced ability of synovial fibroblasts to support B lymphocyte survival. This appears to be dependent, not on the constitutive expression of VCAM‐1 and DAF on intimal cells, but on the increased ability of subintimal cells to respond to proinflammatory cytokines, perhaps critically in the expression of VCAM‐1.