P-selectin is a cell adhesion molecule that resides in the storage granules of platelets and endothelial cells. Upon cell stimulation, the protein is translocated to the plasma membrane where it functions as a leukocyte receptor for PSGL-1 on neutrophils and monocytes. Having discovered P-selectin about 15 years ago, this laboratory continues to study the biology of P-selectin and PSGL-1. Since PSGL-1 has been shown to bind to all selectins, the kinetic and equilibrium binding of soluble PSGL-1 to soluble P-selectin, E-selectin and L-selectin is being analyzed by fluorescence spectroscopy. We have prepared a PSGL-1 deficient mouse by homologous recombination and have completed the initial characterization. Cells from this mouse are being used to establish the physiologic role of PSGL-1 in selectin function by comparing the interaction of PSGL-1 (-/-), (+/-) and (+/+) leukocytes with P-selectin, E-selectin and L-selectin under rolling conditions in a parallel plate ex vivo assay using neutrophils and T lymphocytes. The PSGL-1 deficient mouse and double knockout mice including PSGL-1 null/P-selectin null mice, PSGL-1 null/E-selectin null mice and PSGL-1 null/L-selectin null mice are being employed in model systems to determine the physiologic function of PSGL-1. Pathologic processes studied include models of non-immune mediated and T-cell mediated skin inflammation, leukocyte rolling following trauma and TNF, experimental glomerulonephritis, chemical peritonitis, bacterial pneumonitis, thrombosis, atherosclerosis, wound healing and platelet rolling. To understand the molecular basis of signal transduction and effector function induced by platelet activation or P-selectin binding to the P-selectin ligand on leukocytes, the induction of Ca2+ flux in platelets by PSGL-1 via P-selectin is being analyzed. These studies will contribute to our understanding of the physiologically relevant receptors and counterreceptors that define cell-cell interaction during inflammation.