Bim deficiency leads to exacerbation and prolongation of joint inflammation in experimental arthritis

Objective: Rheumatoid arthritis (RA) is characterized by hyperplasia of the synovial lining, inflammation, and destruction of cartilage and bone. Since there are only a few detectable cells undergoing apoptosis in the joint, it is possible that a defect in apoptosis may contribute to synovial hyperplasia. This study sought to identify and characterize the direct role of apoptotic regulators in a mouse model of inflammatory arthritis. Methods Using a serum transfer model, experimental arthritis was induced in mice lacking the proapoptotic Bcl‐2 family genes Bak (Bak −/− ), Bax (Bax −/− ), or Bim (Bim −/− ), as compared with wild‐type (WT) control mice. Physical examination for edema of the ankles and histopathologic analysis of ankle sections were used to determine the severity of arthritis. The serum and ankles were examined for production of chemokines and cytokines using enzyme‐linked immunosorbent or Luminex‐based assays. Results Bim −/− mice displayed increased severity and prolongation of arthritis. In contrast, Bak −/− and Bax −/− mice showed no difference in the severity of arthritis as compared with WT mice. In addition, Bim −/− mice had elevated levels of proinflammatory chemokines and cytokines, decreased joint and serum production of antiinflammatory cytokines, fewer TUNEL‐positive cells, and reduced levels of active caspase 3 as compared with WT mice. Conclusion These studies are the first to demonstrate a role for the proapoptotic Bcl‐2 protein Bim in the effector phase of RA. The findings indicate that Bim potentially functions to repress the effector phase of arthritis by regulating the milieu of the joint and serum, and by inducing apoptosis.