A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan-dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PS GL-1, a ligand for all selectins, Using a shear-based adhesion assay, a hem atopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoi etic cell line KG1a and on normal human hematopoietic progenitors was previ ously identified. To characterize the structural biology of HCLL and to def ine its relationship to PSGL-1, the effects of chemical and enzymatic treat ments on HCLL activity of KG1a cells and membrane preparations were analyze d. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein, Glycosidase diges tions of membrane protein preparations and metabolic treatments of KG1a cel ls with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex- type N-glycans. Adhesion assays and biochemical studies showed that this gl ycoprotein is also expressed on circulating blasts in native acute leukemia s. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 ex pression had equivalent HCLL activity; (2) anti-PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias w ith low expression of PSGL-1 and CD34 display high HCLL activity; and (4) d espite high level expression of PSGL-1, HCLL activity was absent on HL60 ce lls. These data provide first evidence of a naturally expressed membrane L- selectin ligand expressing binding determinant(s) on an N-linked glycoconju gate. This novel ligand may help mediate L-selectin-dependent cell-cell adh esive interactions within the cytoarchitecture of the bone marrow microenvi ronment, (C) 2000 by The American Society of Hematology.