Differential regulation of leucocyte L-selectin (CD62L) expression in normal lymphoid and inflamed extralymphoid tissues.

AIMS: To study tissue expression of L-selectin, a leucocyte cell surface molecule that is considered to be involved in adhesion to certain endothelia, particularly in peripheral lymph nodes and during inflammation, and is shed upon leucocyte activation. METHODS: Leucocytes were examined by immunohistochemistry and double immunofluorescence staining in various lymphoid sites and normal and inflamed extralymphoid tissues. RESULTS: L-selectin was present on mantle zone B lymphocytes in different lymphoid sites, including in intestinal lymphoid tissue, but was absent on germinal centre B cells. Splenic white pulp B cells also expressed L-selectin. The proportion of T lymphocytes expressing L-selectin depended on the site under study, being greatest in peripheral lymph nodes (mean 48% of T cells positive), and lower in mucosal lymphoid sites and spleen (9 and 11% positive, respectively). Non-lymphocytic L-selectin staining was observed on follicular dendritic cells in tonsils and on macrophages in thymus. L-selectin positive leucocytes were rare in normal extralymphoid tissues, and relatively few were seen in most inflammatory settings. However, in rejecting renal transplants, a higher proportion (30%) of leucocytes expressed L-selectin. CONCLUSIONS: Overall, the results indicate how the degree of L-selectin expression by leucocytes in particular tissues may reflect a requirement for L-selectin expression for entry into those tissues and the activation state of leucocytes once localised there.