CCL21 expression in secondary lymphoid organs is instrumental in mediating L-selectin+ CCR7+ naive T-cell and B-cell recruitment from the bloodstream via PNAd+ high endothelial venules (HEV). CCL21 is also constitutively produced by lymphatic vessels functioning as a recruiting factor for CCR7+ mature dendritic cells from peripheral tissues to regional lymphoid organs. The same factor participates to the inflammatory cascade being associated with lymphoid neogenetic events and being upregulated in peripheral lymphatic vessels, increasing the magnitude of T-cell response by favouring dendritic cell recruitment to draining lymph nodes.
In this study we analysed CCL21 protein and mRNA expression in rheumatoid synovium and its relationship with the organizational features of the inflammatory infiltrate and the vascular system (blood and lymphatic vessels).
Thirty-one rheumatoid synovial samples characterized by a variable degree of inflammation and aggregational tendency were analysed by immunohistochemistry and in situ hybridization for CCL21. Molecular features of synovial vessels were analysed by immunohistochemistry for pan-vascular (CD31), HEV (PNAd) and lymphatic (LYVE-1) markers.
Two distinct patterns were recognized: vascular and non-vascular. Non-vascular CCL21-producing cells were specifically localized within lymphoid aggregates, frequently surrounding PNAd+ HEV. This pattern was demonstrated to be similar to human secondary lymphoid organs where, different from the mouse, no CCL21 mRNA was detected directly in HEV. In contrast, CCL21+ vessels did not show a specific association with lymphoid organization, lacking PNAd expression and being recognized inside but also outside perivascular aggregates. Serial section analysis demonstrated the colocalization of CCL21+ vessels with LYVE-1.
This study emphasizes a differential regulation of CCL21 in the context of synovial lymphoid organization. The anatomical relationship of CCL21-producing cells with synovial PNAd+ HEV suggests its involvement in cellular homing although, contrary to the mouse, CCL21 is actually produced perivascularly. Furthermore, CCL21 colocalization with LYVE-1+ lymphatic vessels suggests multiple functions of CCL21 in mediating CCR7+ leucocyte synovial homing, intratissue clusterization and promoting exit via lymphatics to draining lymph nodes. These results define CCL21 potential roles in the pathogenetic cascade of rheumatoid synovitis.