Rheumatoid arthritis (RA) is a prototype inflammatory disease characterized by leukocyte infiltration, which is in large part mediated by chemokines and cellular adhesion molecules. Angiogenesis, or new blood vessel growth, is integral to the development of the inflamed RA synovial tissue (ST) pannus. Without angiogenesis, leukocyte infiltration could not occur. A novel cytokine, interleukin (IL)-I8 has recently been identified. This cytokine stimulates T helper 1 (Thl) cytokine production by T cells. The functional role of this cytokine in RA is still unclear. In this proposal we hypothesize that IL-18 contributes to RA joint inflammation and angiogenesls. We plan to determine the mechanism by which IL-18 serves as an inducer of chemokine production in RA ST fibroblasts and whether this chemokine production occurs via G proteins, src family kinases, mitogen activated kinases, or PI3 kinases. Besides chemokines, cellular adhesion molecules are needed for leukocyte ingress into inflamed STs. We will examine whether IL-18 induces leukocyte-endothelial adhesion molecule expression and the mechanism of this expression. Finally, leukocyte ingress would not take place without angiogenesis. We will ascertain the mechanism by which IL-18 mediates angiogenesis in RA in terms of the endothelial signaling pathways, which are activated. Finally, we will study IL-18 gene-deficient mice to determine if they have impaired angiogenesis. We will employ these gene deficient animals bred on an arthritis-susceptible strain to examine whether arthritis associated angiogenesis in the joints is decreased. Several years ago cytokine modulation as a therapy for RA was simply a hope. Currently, cytokine modulation aimed at ablating tumor necrosis factor-alpha is one of the best treatments available for RA and IL-1 targeting is beginning to be used therapeutically. IL-18, by virtue of its ability to induce chemokine release, adhesion molecule expression, and angiogenesis, appears to be a very critical cytokine to target. Our proposal should answer some key questions, which may determine the ability of IL-18 to be a therapeutic target in RA.
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