CXCL12γ isoform is expressed on endothelial and dendritic cells in rheumatoid arthritis synovium and regulates T cell activation

Objective CXCL12 gamma is an alternative splicing isoform of CXCL12 with enhanced affinity for heparan sulfate (HS) proteoglycans. This study was undertaken to investigate the distribution and potential function of CXCL12 gamma in rheumatoid arthritis (RA) synovium and normal lymphoid tissue, where its immobilization to HS may be relevant in pathologic or homeostatic immune cell migration and activation. Methods. Expression of CXCL12 or CXCL12 gamma was immunodetected in RA and normal synovium, lymphoid tissue, and cultured cells with anti-pan-CXCL12 or anti-CXCL12 gamma-specific monoclonal antibodies. CXCL12 alpha and CXCL12 gamma messenger RNA expression was analyzed by quantitative reverse transcription-polymerase chain reaction. Binding of wild-type CXCL12 isoforms or their HS binding-defective mutants to monocyte-derived dendritic cells (DCs) was analyzed by flow cytometry. The effect of DC-bound CXCL12 alpha and CXCL12 gamma on T cell activation was analyzed in DC/T cell allogeneic cultures. Results. CXCL12 gamma expression was increased in RA compared to normal synovium and preferentially located in endothelia and DC-SIGN-positive cells. This distribution was also observed in lymphoid organs. Surface-bound CXCL12 gamma was detected in a fraction of freshly isolated DCs. Monocyte-derived DCs, but not monocytes, showed a high capacity to bind CXCL12 gamma in an HS-dependent manner. Surface-bound CXCL12 gamma and CXCL12 gamma on monocyte-derived DCs were potent inhibitors of allogeneic T cell activation, in contrast to the T cell-stimulatory effects of soluble CXCL12 proteins. Conclusion. CXCL12 gamma shows a specific and similar distribution in RA synovium and lymphoid tissue, consistent with its higher HS binding affinity. Presentation of CXCL12 to T cells on membrane HS in DCs can play a distinct regulatory role in T cell activation.