Tumor necrosis factor a-induced protein 6 (Tnfip6), the product of TNFa-stimulated gene 6 (TSG-6), is a hyaluronan(HA)-binding protein that is secreted in response to pro-inflammatory stimuli, and has been detected in synovial fluid and tissue from patients with rheumatoid arthritis (IRA). Treatment of mice with recombinant Tnfip6 has been shown to inhibit inflammatory leukocyte infiltration, and to ameliorate joint swelling in murine models of RA. Tnfip6 also provides protection against cartilage degradation through potentiation of the protease inhibitor activity of serum anti-a trypsin inhibitor (lal). These findings clearly suggest that Tnfip6 antagonizes leukocyte influx as well as inflammatory tissue damage. However, the exact mechanisms by which these therapeutic effects of Tnfip6 are achieved in vivo, remain unknown. Our preliminary in vitro studies show that Tnfip6 interacts with the cell surface HA receptor, CD44, and modulates the CD44-dependent rolling of leukocytes on HA. Rolling of CD44-expressing cells on HA-covered microvascular endothelium is a critical step in the process of leukocyte recruitment by which effector cells of the innate and adaptive immune system gain entry into sites of inflammation. Modulation of cell rolling by Tnfip6 in vivo, therefore, could have a negative impact on leukocyte extravasation, leading to resolution of inflammation. We hypothesize that Tnfip6, in particular when administered in pharmacological doses, inhibits leukocyte influx into inflamed synovial joints through interactions with cell surface CD44. We will test this hypothesis by conducting real-time intravital videomicroscopy on the synovial microcirculation of BALB/c mice with proteoglycan-induced arthritis (PGIA), following treatment with recombinant Tnfip6 (Aim 1). We will record and analyze the rolling and adhesion interactions of leukocytes with endothelium in the synovial microvessels of the ankle, a distal joint which is afflicted with inflammation in both PGIA and RA, but has not been accessible to real-time investigations thus far. Requirement for endogenous Tnfip6 and CD44 for down-regulation of inflammatory leukocyte recruitment will be studied using BALB/c mice deficient in Tnfip6 and CD44, respectively (Aims 2 and 3). To validate the in vivo approach, the results of intravital microscopy experiments will be correlated with the morphological features of arthritis. Tnfip6 also participates in the transcriptional regulation of two serine protease inhibitors, distinct from lal, as suggested by the gene expression profile of in vitro-stimulated Tnfip6-deficient cells. One of these antiproteases, secretory leukocyte protease inhibitor (Slpi), has anti-inflammatory properties. We will investigate the regulatory relationship between Tnfip6 and Slpi, and the role of SIpi in arthritis suppression (Aim 4). In summary, Tnfip6 is an arthritis-associated protein, induced by, and antagonistic to, inflammatory processes, Elucidation of the multiple functions of Tnfip6 will help unravel the contribution of this protein to a negative regulatory loop that curtails joint inflammation.
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