3-DIMENSIONAL DISTRIBUTION OF INTERCELLULAR-ADHESION MOLECULE-1 ON LYMPHOCYTES IN THE HIGH ENDOTHELIAL VENULE ANALYZED BY BACKSCATTER ELECTRON IMAGING

The intercellular adhesion molecule-1 (ICAM-1) is a ligand for the bet a 2 integrin. It is present on the vascular endothelium and plays an i mportant role in cell to cell interaction in lymphocyte recirculation. ICAM-1 is expressed not only on the endothelium but also on other cel ls, especially lymphocytes adhering to the endothelium, B cells and mi togen-stimulated T cells. To define the functional role of ICAM-1 in l ymphocytes, the three-dimensional distribution of ICAM-1 on the surfac e of lymphocytes in the rat mesenteric lymph node HEV was investigated by immunoscanning electron microscopy. ICAM-1 expression was closely associated with variations in surface structures of lymphocytes, which showed a cap-like structure, membrane ruffles and slender microvilli. ICAM-1 was preferentially localized on the cell surface including mem brane ruffles (about 90% of all counted gold particles), but rarely on the microvilli (10%). The results suggest that activated lymphocytes with variations in surface structures demonstrate moderate expression and preferential localization of ICAM-1. This seems to enable a three- dimensional interaction of the microvilli of one lymphocyte with the s urface membrane of another cell and might be a prerequisite for cluste r formation between them, as seen in antigen presentation.