Osteoclast formation from peripheral blood mononuclear cells (PBMC) is promoted by the enhanced expression of C/EBPβ in fibroblast-like synoviocytes from the synovium of rheumatoid arthritis patients (RA-FLS). (A) Co-cultures of PBMCs and RA-FLS transfected with adenovirus vectors expressing C/EBPβ-LAP, −LIP or LacZ control for 72 hours. Number of TRAP-positive multinucleated cells recognized in the co-culture system. Osteoclasts were identified as TRAP-positive multinucleated cells that contained more than three nuclei. Original magnification, 100×. *P <0.05 versus control using the Mann-Whitney U-test. (B) Negative control for the co-culture experiments. PBMC or RA-FLS over-expressing C/EBPβ-LAP, −LIP or LacZ control, respectively, were cultured for 72 hours. No TRAP-positive multinucleated cells were observed.