The RANK/RANKL pathway is critical in bone destruction in conditions such as rheumatoid arthritis (RA). Since RANK/RANKL-deficient mice show major lymph node (LN) abnormalities, undertook this study to investigate the expression of RANK/RANKL in paired samples of synovium and LNs from RA patients.
Using immunohistochemistry, RANK/RANKL expression by dendritic cell (DC) and T cell subsets was studied in this unique set of samples and in RA synoviocytes stimulated with interleukin-1β (IL-1β), tumor necrosis factor α (TNFα), and IL-17.
In RA synovium, RANKL+ cells were detected in the lining layer and the lymphocytic infiltrates, whereas RANK expression was restricted to the perivascular infiltrates. In LNs, RANK+ and RANKL+ cells were diffusely expressed in the T cell zone and in germinal centers. Double staining of paired RA synovium and LN sections showed that some immature CD1a+ DCs expressed RANK and RANKL, while some mature DC-LAMP+ DCs expressed only RANK. Some CD3+, CD4+, interferon-γ+, and IL-17+ cells expressed RANKL, while none expressed RANK. Treatment of synoviocytes with TNFα or IL-1β in combination with IL-17 was particularly potent at inducing RANKL expression.
This study shows the involvement of RANK/RANKL in DC–T cell interactions during an inflammatory process. RANK expression appears to be limited to the sites of immune reaction, both in the synovium and in LNs. Therapeutic control of these targets may have both positive and negative consequences for the immune system.