QUANTITATION OF L-SELECTIN DISTRIBUTION ON HUMAN-LEUKOCYTE MICROVILLIBY IMMUNOGOLD LABELING AND ELECTRON-MICROSCOPY

L-Selectin is a leukocyte cell adhesion receptor that contributes to n eutrophil (PMN) rolling on activated endothelium at sites of inflammat ion and mediates lymphocyte attachment to high endothelial venules in peripheral lymph nodes, Localization of this receptor to the tips of P MN and lymphocyte microvilli has been demonstrated. However, its distr ibution on these cells has not been quantified, and its localization o n other leukocytes and the morphometry of microvilli on different leuk ocyte subpopulations have not been previously examined. In this study, PMN and mononuclear leukocytes were isolated from anticoagulated bloo d by dextran sedimentation and density centrifugation, fixed in 2% par aformaldehyde and 0.05% glutaraldehyde, immunogold-labeled for L-selec tin, and embedded in Epon resin. The distribution of L-selectin was de termined by counting gold I;articles on the plasma membrane of section ed cells, and the surface microstructure of these cells was surveyed o n two-dimensional transmission electron micrographs, On average, 78% o f PMN, 72% of monocyte, and 71% of lymphocyte L-selectin was observed on the microvilli, with more variance on lymphocytes than the other ce ll 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 m imic microvascular environments.