Paired synovium and lymph nodes from rheumatoid arthritis patients differ in dendritic cell and chemokine expression

The aim of this study was to compare the cellular composition and organization of rheumatoid (RA) synovium, which has several of the characteristics of lymphoid organs, with lymph nodes. To clarify further whether RA synovium can be classified as an ectopic lymphoid organ, paired RA synovium and lymph node (LN) tissues from 11 patients were compared in terms of T-cell-B-cell and germinal centre (GC) organization, dendritic cell (DC) subsets, and chemokine expression. Tonsil, a normal secondary lymphoid organ, was used as a tissue control. In paired RA LN and synovium, more follicular DC-positive GCs were observed in LN, but when observed in synovium, they shared the same T-cell-B-cell organization and mean GC size. In LN, a predominance of mature DC-LAMP-positive DCs of myeloid (CD11c-positive) or lymphoid (CD123-positive) origin was observed, whereas paired RA synovium was characterized by the relative accumulation of immature CD1a-positive DCs. In the same way, CCL19-CCL21/CCR7, a chemokine/chemokine receptor complex involved in mature DC migration, was more frequently seen in LN than in paired RA synovium. In synovium, such expression was associated with lymphoid follicle formation, with or without a GC. Conversely, CCL20, a chemokine involved in immature DC migration, was expressed in RA synovium and tonsils but not in paired LNs. In conclusion, although similarities were observed, this study, using paired samples, indicates that the RA synovium lacks some of the features that are characteristic of a lymphoid organ.