RANK and RANKL expression in RA synovium and lymph nodes as markers of dendritic cell-T cell interactions

The receptor activator of NF-κB/receptor activator of NF-κB ligand (RANK/RANKL) pathway is critical in osteoclastogenesis and bone erosion and has been implicated in the process of focal bone erosion in rheumatoid arthritis (RA). However, its involvement in the immune response during chronic inflammation remains to be clarified since deficient mice show major lymph node (LN) abnormalities.

We investigated by immunohistochemistry the RANK and RANKL expression pattern of dendritic cell (DC) and T cell subsets in paired RA synovium-LN and also in normal peripheral blood mononuclear cells (PBMC) stimulated with phorbol miristate acetate/phytohemagglutin during 4, 24, and 48 hours. In RA synovium, RANKL+ cells were detected in the lining layer and the lymphocytic infiltrates, whereas RANK expression was restricted to perivascular infiltrates. In LN, RANK+ and RANKL+ cells were diffusely expressed in both the T-cell zone and germinal centers. Double staining with anti-RANK or anti-RANKL and anti-CD1a or anti-DC-LAMP antibodies showed that some immature CD1a+ DC expressed RANK and RANKL, whereas some mature DC-LAMP+ DC only expressed RANK. Double staining with the CD3, CD4 T-cell markers and the interferon gamma and IL-17 Th1 cell markers showed that some CD3+, CD4+, interferon gamma+, and IL-17+ cells produced RANKL whereas none of them express RANK. The same pattern was observed on activated PBMC. RANK+ cells were detected in unstimulated PBMC and identified as CD14+ monocytes.

This study showed the involvement of RANK/RANKL in DC–T cell interactions as found during the inflammatory process, and makes RANK and RANKL potential therapeutic targets outside the bone field.