Restricted human CD45 isoglycoforms serve as functional E-selectin ligands and delineate hematopoietic maturity

CD45 is the most abundant glycoprotein on the surface of all nucleated hematopoietic-lineage cells, comprising multiple isoforms generated by alternative splicing of three exons ("A," "B," and "C"), which are exquisitely restricted across hematopoietic development. Despite the ubiquitous expression of CD45 on hematopoietic cells, its function(s) remain rather obscure. Here, we report that discrete CD45 isoforms expressed uniquely by immature human hematopoietic cells are distinguished as functional glycoforms ("isoglycoforms") that bind E-selectin. Moreover, our studies indicate that "CD45RA," a marker of human acute myeloid leukemia (AML) cells, identifies a distinct isoglycoform of CD45 containing all splice exon-encoded peptides. This isoglycoform, "CD45RABC-E," is directly upregulated by AML cells and demarcates these malignant cells from mature human leukocytes and the native human hematopoietic stem cell. Further analyses revealed that treatment-resistant AML highly expresses CD45RABC-E. Our findings thus unveil heretofore unrecognized functions of CD45 and define a novel CD45 isoglycoform that delineates AML cells from life-sustaining human hematopoietic cells.