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 LN, RANK+ and RANKL+ cells were diffusely expressed in both T-cell zone and germinal centres. Double staining with anti-RANK or anti-RANKL and anti-CD1a or anti-DC-LAMP antibodies revealed that, in paired RA synovium-LN sections, some immature CD1a+ DC express RANK and RANKL whereas some mature DC-LAMP+ DC expressed only RANK. Double staining with the CD3, CD4 T-cell markers and the IFN-γ and IL-17 Th1 cell markers showed that some of CD3+, CD4+, IFN-γ+ and IL-17+ cells expressed RANKL, whereas none of them expressed RANK. The same pattern was observed on activated PBMC. The RANK+ cells, detected in unstimulated PBMC, were identified as CD14+ monocytes.