To study the capacity of macrophage migration inhibitory factor (MIF) to regulate proliferation, apoptosis, and p53 in an animal model of rheumatoid arthritis (RA) and in fibroblast‐like synoviocytes (FLS) from humans with RA.

Antigen‐induced arthritis (AIA) was induced in MIF^–/– mice and littermate controls. FLS were obtained from patients with RA. Western blotting and immunohistochemistry were used to measure p53 in cells and tissues. Apoptosis was detected in cells by flow cytometry using TUNEL and annexin V/propidium iodide labeling. Apoptosis in tissue was detected using TUNEL. Proliferation was assessed in cultured cells and tissue by ³H‐thymidine incorporation and Ki‐67 immunostaining, respectively.

MIF inhibited p53 expression in human RA FLS. Levels of p53 were correspondingly increased in MIF^–/– mouse tissues and cells. Spontaneous and sodium nitroprusside–induced apoptosis were significantly increased in MIF^–/– cells. In vitro exposure of FLS to MIF reduced apoptosis and significantly induced FLS proliferation. Synoviocyte proliferation in MIF^–/– mice was correspondingly reduced. A decrease in the severity of AIA in MIF^–/– mice was associated with an increase in p53 and apoptosis in synovium. Evidence of in situ proliferation was scant in this model, and no difference in in situ proliferation was detectable in MIF^–/– mice compared with wild‐type mice.

These results indicate a role for MIF in the regulation of p53 expression and p53‐mediated events in the inflamed synovium and support the hypothesis that MIF is of critical importance in the pathogenesis of RA.