Quantitation of L-selectin distribution on human leukocyte microvilli by immunogold labeling and electron microscopy.

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Abstract
L-Selectin is a leukocyte cell adhesion receptor that contributes to neutrophil (PMN) rolling on activated endothelium at sites of inflammation and mediates lymphocyte attachment to high endothelial venules in peripheral lymph nodes. Localization of this receptor to the tips of PMN and lymphocyte microvilli has been demonstrated. However, its distribution on these cells has not been quantified, and its localization on other leukocytes and the morphometry of microvilli on different leukocyte subpopulations have not been previously examined. In this study, PMN and mononuclear leukocytes were isolated from anticoagulated blood by dextran sedimentation and density centrifugation, fixed in 2% paraformaldehyde and 0.05% glutaraldehyde, immunogold-labeled for L-selectin, and embedded in Epon resin. The distribution of L-selectin was determined by counting gold particles on the plasma membrane of sectioned cells, and the surface microstructure of these cells was surveyed on two-dimensional transmission electron micrographs. On average, 78% of PMN, 72% of monocyte, and 71% of lymphocyte L-selectin was observed on the microvilli, with more variance on lymphocytes than the other cell types. Typical PMN and monocyte sections had 26 microvilli, whereas typical lymphocyte sections had 23. Quantitation of the distribution of L-selectin and leukocyte surface topology offers a foundation from which to study the requirement of microvilli or microvillus-localized L-selectin for leukocyte tethering and rolling in model systems that mimic microvascular environments.

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