The major limitation of adenovirus is its association with induction of an inflammatory response and relatively short-term production of the gene therapy transgene product. Adeno-associated virus (AAV) is a 4.68-kb single-strand DNA virus that contains ITRs for viral replication and a packaging signal, and also has been engineered to contain therapeutic genes up to 5 kb in length. Transduction of recombinant AAV (rAAV) results in low inflammatory response and long-term expression. We have cloned a low-immunogenic form of human sTNFRI (sTNFRI2.6D) into AAV (rAAVsTNFRI). This vector was analyzed for its ability to transfect and neutralize the effect of TNF-α on primary rheumatoid arthritis synovial fibroblast (RASFs). The rAAVsTNFRI was transduced into the cells at 1.8 × 10¹, 1.8 × 10², and 1.8 × 10³ viral particles per cell. There was greater than 90% neutralization of TNF-α at 1.8 × 10³ viral particles/cell. There was a significant decrease in the synovial cell hyperplasia and cartilage and bone destruction in human TNF-α transgenic mice treated intraarticularly with rAAVsTNFRI. These results indicate that the low-immunogenic and long-term expressing vector, rAAVsTNFRI, can be used to deliver the soluble TNF-α in vitro and in vivo and effectively reduce the severity of arthritis.