Fibroblast-like synoviocytes support B-cell pseudoemperipolesis via a stromal cell–derived factor-1– and CD106 (VCAM-1)–dependent mechanism

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Abstract
B-cell accumulation and formation of ectopic germinal centers are characteristic changes in the diseased joints of patients with rheumatoid arthritis (RA). Earlier studies suggested that interactions between B lymphocytes and specialized synovial “nurse-like” cells peculiar to the RA synovium may be responsible for the homing and sustained survival of B cells in the synovium. However, in this study, we found that B cells spontaneously migrate beneath ordinary fibroblast-like synoviocytes (FLSs) and then experience prolonged survival. FLSs isolated from joints of patients with osteoarthritis also supported this activity, termed B-cell pseudoemperipolesis. We found that FLSs constitutively expressed the chemokine stromal cell–derived factor-1 (SDF-1), and that pertussis toxin or antibodies to the SDF-1 receptor (CXCR4) could inhibit B-cell pseudoemperipolesis. However, expression of SDF-1 is not sufficient, as dermal fibroblasts also expressed this chemokine but were unable to support B-cell pseudoemperipolesis unless previously stimulated with IL-4 to express CD106 (VCAM-1), a ligand for the α4β1 integrin, very-late-antigen-4 (VLA-4 or CD49d). Furthermore, mAb’s specific for CD49d and CD106, or the synthetic CS1 fibronectin peptide, could inhibit B-cell pseudoemperipolesis. We conclude that ordinary FLSs can support B-cell pseudoemperipolesis via a mechanism dependent upon fibroblast expression of SDF-1 and CD106.