Abstract. L-selectin expressed by granulocytes, lymphocytes, and monocytes is responsible for initial leukocyte attachment to inflamed endothelium and high endothelial venules of peripheral lymph nodes. After leukocyte activation in vitro, L-selectin is rapidly shed from the cell surface. In this study, shed L-selectin (sL-selectin) from both lymphocytes and neutrophils was demonstrated to be present in high levels in human plasma by Western blot analysis and using a quantitative ELISA. In serum from normal human blood donors, a mean sL-selectin level of 1.6 ___ 0.8 #g/ml (n = 63) was found by ELISA. In addition, semipurified sL-selectin from plasma inhibited L-selectinspecific attachment of lymphocytes to cytokineactivated endothelium in a dose-dependent manner. L-selectin-dependent leukocyte attachment was completely inhibited at sL-selectin concentrations of 8-15 /zg/ml, while physiological concentrations of sLselectin caused a small but consistent inhibition of lymphocyte attachment, sL-selectin in plasma also inhibited anti-L-selectin mAb (2-5 #g/ml) binding to the surface of leukocytes. Interestingly, one epitope present within the EGF-like domain of L-selectin was lost in sL-selectin, suggesting a conformational change in the structure of the receptor after shedding. The presence of serum sL-selectin with functional activity indicates a potential role for sL-selectin in the regulation of leukocyte attachment to endothelium.T HE ability of leukocytes to leave the circulation and to migrate into tissues is a critical feature of the immune response. Several adhesion molecules are involved in the process of adhesion and transmigration of leukocytes through vascular endothelium at sites of inflammation (45). One molecule responsible for the initial attachment of leukocytes to endothelium is L-selectin (Leukocyte Adhesion Molecule-1 [LAM-1] t MEL-14) (24,30,43,44). L-selectin is a member of the selectin family of adhesion molecules (3,11,27,36,37,49) that includes E-selectin (Endothelial-Leukocyte Adhesion Molecule-1 [ELAM-1]) (1, 2, 31, 32) and CD62 (P-selectin, PADGEM, GMP-140) (13,18,25,26). The selectins are derived from evolutionarily related genes (7,9,18,33,52), and are characterized by a NH2-terminal, Ca-dependent lectin domain, an epidermal growth factor (EGF)-like domain followed by multiple short consensus repeat (SCR) domains, a transmembrane region, and a cytoplasmic tail.L-selectin is expressed on the surface of most leukocytes,

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A central feature of host defence is the ability of leukocytes to enter tissues in response to immune or inflammatory stimuli. The leukocyte adhesion molecule-1 (LAM-1) regulates the migration of human leukocytes by mediating the binding both of lymphocytes to high endothelial venules of peripheral lymph nodes and of neutrophils to endothelium at inflammatory sites. As lymphocytes and neutrophils express the same LAM-1 protein, it is not clear how lineage-specific differences in leukocyte migration are controlled. We now report that the affinity of LAM-1 for a carbohydrate-based ligand, PPME, is dramatically increased following lymphocyte and neutrophil activation by lineage-specific stimuli. In addition, activation of lymphocytes by physiological stimuli enhanced LAM-1-dependent binding to high endothelial venules. Thus, transient changes in LAM-1 affinity after leukocyte stimulation probably directly influence leukocyte migration.