Background Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease in which an unknown autoantigen is presented to T cells. In RA synovial tissue, fully differentiated dendritic cells (DC) are found in close association with T cells. DC are thought to be the major antigen presenting cells in RA. It has previously been shown that human monocytes differentiate into DC when stimulated with GM-CSF and IL-4 with terminal differentiation promoted by TNFa. We have shown that circulating monocytes and synovial macrophages are stimulated to differentiate into functional osteoclasts in the presence of RANKL, M-CSF and dexamethasone (Dex).

Objectives We aimed to determine whether synovial macrophages preferentially differentiate into DC or functional osteoclasts in the presence of specific combinations of cytokines/growth factors which are known to be involved in the pathogenesis of RA and other inflammatory bone conditions.

Methods Synovial macrophages were isolated from the knee and hip of RA and OA patients. The cells were seeded onto dentine slices and coverslips and cultured for 1, 7 and 14 days in the presence/absence of different combinations of the following factors: RANKL (30 ng/ml), Dex (10–8M), MCSF (0–50 ng/ml), GM-CSF (0–50 ng/ml), IL-4 (10 ng/ml), TNFa (10 ng/ml). Macrophage-osteoclast differentiation was quantified by counting the number of multinucleated, tartrate-resistant acid phosphatase (TRAP) cells present, and by measuring their ability to carry out lacunar resorption on dentine slices. Mature DC were identified by the expression of the marker CD83.

Results In the presence of RANKL, M-CSF and Dex, RA and OA synovial macrophages were maximally stimulated to form osteoclasts; a few cells in these cultures were CD83 positive. The majority were TRAP positive and capable of forming resorption pits on dentine. GM-CSF, IL-4 and TNFa stimulated maximum expression of the mature DC marker CD83 in the cultures; these cultures were TRAP negative and incapable of resorbing dentine. Synovial macrophages cultured in the presence of RANKL and GM-CSF alone resulted in the formation of large multinucleated cells which expressed the marker CD83; a few of these large multinucleated cells were also TRAP positive. Cell cultures treated with GM-CSF were capable of significantly less dentine resorption than cultures treated with RANKL and M-CSF. The addition of RANKL did not modify CD83 expression in any of the above cytokine/growth factor combinations. The presence of Dex significantly inhibited the expression of this DC marker in all cultures.

Conclusion This study demonstrates that synovial tissue macrophages have the capacity to differentiate into either functional osteoclasts or mature dendritic cells depending on the presence or absence of specific humoral factors.

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