To determine whether interleukin‐6 (IL‐6) trans‐signaling directs the expression of pre–B cell colony‐enhancing factor (PBEF) in vitro and in vivo.

Complementary DNA from rheumatoid arthritis (RA) synovial fibroblasts treated with IL‐6 and soluble IL‐6 receptor (sIL‐6R) was used to probe a cytokine microarray. PBEF regulation by the IL‐6–related cytokines, IL‐6, sIL‐6R, oncostatin M (OSM), IL‐11, and leukemia inhibitory factor (LIF) was determined by reverse transcription–polymerase chain reaction analysis. IL‐6–mediated STAT‐3 regulation of PBEF was determined using a cell‐permeable STAT‐3 inhibitor peptide. Antigen‐induced arthritis (AIA) was induced in wild‐type (IL‐6^+/+) and IL‐6–deficient (IL‐6^−/−) mice. PBEF and STAT were detected by immunohistochemistry, immunoblotting, and electrophoretic mobility shift assay. Synovial levels of PBEF were quantified by enzyme immunoassay.

IL‐6 trans‐signaling regulated PBEF in a STAT‐3–dependent manner. In addition, PBEF was regulated by the IL‐6–related cytokine OSM, but not IL‐11 or LIF. Flow cytometric analysis of the IL‐6–related cognate receptors suggested that OSM regulates PBEF via its OSM receptor β and not its LIF receptor. The involvement of PBEF in arthritis progression was confirmed in vivo, where induction of AIA resulted in a 4‐fold increase in the synovial expression of PBEF. In contrast, little or no change was observed in IL‐6^−/− mice, in which the inflammatory infiltrate was markedly reduced and synovial STAT‐1/3 activity was also impaired. Analysis of human RA synovial tissue confirmed that PBEF immunolocalized in apical synovial membrane cells, endothelial cells, adipocytes, and lymphoid aggregates. Synovial fluid levels of PBEF were significantly higher in RA patients than in osteoarthritis patients.

Experiments presented herein demonstrate that PBEF is regulated via IL‐6 trans‐signaling and the IL‐6–related cytokine OSM. PBEF is also actively expressed during arthritis. Although these data confirm an involvement of PBEF in disease progression, the consequence of its action remains to be determined.