Clinical translation of gene-based therapies for arthritis could be accelerated by vectors capable of efficient intra-articular gene delivery and long-term transgene expression. Previously, we have shown that lentiviral vectors transduce rat synovium efficiently in vivo. Here, we evaluated the functional capacity of transgene expression provided by lentiviral-mediated gene delivery to the joint. To do this, we measured the ability of a lentiviral vector containing the cDNA for human interleukin-1 receptor antagonist (LV-hIL-1Ra) to suppress intra-articular responses to IL-1beta. Groups of rats were injected in one knee with 5 x 10(7) infectious units of LV-IL-1Ra. After 24 h, a range of doses of fibroblasts (3 x 10(3), 10(4), 3 x 10(4), or 10(5) cells) genetically modified to overexpress IL-1beta was injected into both knees. Intra-articular delivery of LV-hIL-1Ra strongly prevented swelling in all treated knees, even in those receiving the greatest dose of IL-1beta(+) cells. Cellular infiltration, cartilage erosion, and invasiveness of inflamed synovium were effectively prevented in LV-hIL-1Ra-treated knees and were significantly inhibited in contralateral joints. Beneficial effects were also observed systemically in the lentivirus-treated animals. Interestingly, intra-articular expression of the IL-1Ra transgene was found to increase in relation to the number of IL-1beta(+) cells injected. Further experiments using GFP suggest this is due to the proliferation of cells, stably modified by the integrative lentivirus, in response to inflammatory stimulation.