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.