This study examined the ability of E2F decoy oligodeoxynucleotides (ODN) to inhibit proliferation of synovial fibroblasts derived from patients with rheumatoid arthritis (RA). The effect of E2F decoy ODN on cartilage invasion by RA synovium in a murine model of human RA was also investigated. E2F decoy ODN were introduced into synovial tissue and synovial fibroblasts derived from patients with RA using hemagglutinating virus of Japan (HVJ)-liposomes. The effect of E2F decoy ODN on synovial fibroblast proliferation was evaluated by MTT assay and by RT-PCR for the cell cycle regulatory genes proliferating-cell nuclear antigen (PCNA) and cyclin-dependent kinase 2 (cdk2). Changes in production of inflammatory mediators by RA synovial tissue following transfection with E2F decoy ODN were assessed by ELISA. Human cartilage and RA synovial tissue transfected with E2F decoy ODN were co-transplanted in severe combined immunodeficient (SCID) mice. After 4 weeks, the mice were sacrificed and the implants histologically examined for inhibition of cartilage damage by E2F decoy ODN. E2F decoy ODN resulted in significant inhibition of synovial fibroblast proliferation, corresponding with reduced expression of PCNA and cdk2 mRNA in synovial fibroblasts. The production of interleukin-1beta (IL-1beta), IL-6 and matrix metalloproteinase (MMP)-1 by synovial tissue was also significantly inhibited by the introduction of E2F decoy ODN. Further, in an in vivo model, cartilage that was co-implanted with RA synovial tissue transfected with E2F decoy ODN exhibited no invasive and progressive cartilage degradation. These data demonstrate that transfection of E2F decoy ODN prevents cartilage destruction by inhibition of synovial cell proliferation, and suggest that transfection of E2F decoy ODN may provide a useful therapeutic approach for the treatment of joint destruction in arthritis.