L-selectin, the peripheral lymph node “homing receptor,” is an adhesion protein that mediates lymphocyte binding to lymph node high endothelial venules. Ligands for this protein have been identified only on endothelial cells, and recent murine studies indicate that CD34 on endothelial cells is an L-selectin ligand. To investigate whether CD34 expressed on hematopoietic cells functions as an L-selectin ligand, we used an in vitro binding assay to examine lymphocyte adherence to KG1a, a CD34+ human hematopoietic progenitor cell line. We observed specific L-selectin-mediated adherence of lymphocytes to KG1a: the binding was calcium-dependent, was strictly inhibited by anti-L-selectin antibodies and by carbohydrate ligands of L-selectin, and was abrogated by induction of L-selectin shedding from the lymphocyte membrane by treatment with phorbol esters. However, blocking studies using anti- CD34 antibodies, and experiments using KG1a cells sorted for CD34 expression and COS-7 cells transfected with full-length CD34 cDNA indicate that the ligand on KG1a is not CD34; moreover, RPMI 8402, a CD34+ cell line, does not support lymphocyte adherence in the binding assay. Treatment of KG1a with the enzymes neuraminidase, chymotrypsin, and bromelain abrogated lymphocyte binding to the cells, indicating that the ligand is a glycoprotein. These experiments show that CD34 on hematopoietic cells is not an L-selectin ligand and provide the first evidence of a ligand for L-selectin present on a non-endothelial cell.

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