Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease. Neutrophils are found in large numbers in rheumatoid synovium and have been suggested to be involved in clinical signs of inflammation and pain associated with RA. Neutrophil activation during host defense and inflammation is mediated by G-protein coupled receptors (GPCRs) for chemoattractants. Leukotriene B4 (LTB4), a potent chemoattractant for neutrophils activates its receptor (BLT-1) to mediate diverse physiological effects in neutrophils. A second LTB4 receptor (BLT-2) with distinct antagonist specificity and tissue distribution was recently described. GPCRs are regulated by receptor phosphorylation leading to desensitization as well as down regulation. We will test the hypothesis that LTB4 acting through the high affinity receptor BLT-1 mediates its effects on neutrophils in RA, while LTB4 acting through BLT-2 modulates T-lymphocyte activation and function in RA. The goal of the current studies is to develop comprehensive in vivo and in vitro models to determine the role of LTB4 and the relative contributions of BLT-1 and BLT-2 in the development of murine RA. In specific aim 1 we will define the role of BLT-1 in the development and progression of collagen induced arthritis in the BLT-1 deficient mice we have already generated by targeted gene disruption. In specific aim 2 we will use the well-established RBL-2H3 cell model to determine the differences in signaling, desensitization, internalization and antagonist specificity of BLT-1 and BLT-2. These studies take advantage of the novel video microscopy and live cell imaging methods we have recently developed. Leukotrienes are involved in the pathophysiology of many acute and chronic inflammatory diseases such as systemic anaphylaxis, atherosclerosis, RA and asthma. Understanding the precise function of distinct LTB4 receptors in mice deficient in specific receptors and defining the role of these receptors in RA will identify novel targets for therapeutic intervention of RA.