AbstractLI is a cell surface glycoprotein of the immunoglobulin superfamily which was initially shown to mediate adhesion between neural cells. Recently we have reported that LI is expressed by bone marrow cells and the majority of mature lymphocytes (Kowitz et al., Eur. J. Immunol. 1992.22: 1199‐1205). To analyze the function of LI on leukocytes we studied its regulation following cell activation.In vitroactivation of B lymphocytes with lipopolysaccharide or T lymphocytes with phorbol 12‐myristate 13‐acetate/Ca2+ionophore, concanavalin A or anti‐CD3 monoclonal antibody as well asin vivoactivation of Vβ8+T cells with staphylococcal enterotoxin B (SEB) revealed a down‐regulation of LI within 48 h. A rapid loss of LI expression was seen when mouse neutrophils were activated with PMA alone. This rapid loss paralleled the shedding of L‐selectin.We also studied a possible role of LI in the binding of leukocytes to endothelial cells. ESb‐MP lymphoma cells with a high expression of L1 (L1hi) could bind to bend3 endothelioma cells without prior activation with inflammatory cytokines. The interaction was inhibited by anti‐L1 antibodies. In contrast, ESb‐MP cells with low LI expression (L110) were only marginally bound. Latex beads coated with affinity‐isolated LI antigen were also able to bind to the endothelioma cells in a specific fashion. The binding of ESb‐MP lymphoma cells required Ca2+and Mg2+ions and was sensitive to cold temperature. Since the endothelioma cells did not express LI the binding mechanism studied here is distinct from the established L1‐L1 homotypic interaction. It is possible that the novel L1‐mediated adhesion pathway involves an unidentified ligand and could play a r
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