Rapid activation‐independent shedding of leukocyte L‐selectin induced by cross‐linking of the surface antigen

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Abstract
L-selectin (also termed LAM-1, Leu-8, TQ-1, gp90MEL-14, peripheral lymph node homing receptor and LECAM-1) is an adhesion protein thought to be important in leukocyte entry into lymphoid tissues and sites of inflammation. We, as well as others, have shown that leukocyte activation by chemotactic factors results in rapid shedding (release) of L-selectin from the cell surface. Here we have used flow cytometry, enzyme-linked immunosorbent assay, and SDS-PAGE/Western blot analysis to determine whether cross-linking of L-selectin in the absence of activation causes shedding. We found that rapid loss of leukocyte L-selectin expression (down-regulation) could be induced by treating cells with a chemical cross-linker [bis (sulfosuccinimidyl) suberate]. L-selectin down-regulation via cross-linking could occur at 4°C and in the absence of detectable cell activation (increased expression of CD11b/18). The loss of L-selectin expression was due to shedding of the molecule from the leukocyte cell surface. Cross-linking of L-selectin with specific monoclonal antibodies also caused loss of surface expression of L-selectin at 37°C. Finally, shed L-selectin was detected in the plasma of healthy adults whose peripheral blood leukocytes demonstrated no obvious signs of activation. Our results suggest that activation-independent shedding of leukocyte L-selectin may occur in vivo and a possible mechanism could involve cross-linking of leukocyte L-selectin. This provides a novel mechanism for rapid regulation of expression of a leukocyte-endothelial cell adhesion receptor.