Glycoengineering of HCELL, the Human Bone Marrow Homing Receptor: Sweetly Programming Cell Migration

The successful clinical implementation of adoptive cell therapeutics, including bone marrow transplantation and other stem cell-based treatments, depends critically on the ability to deliver cells to sites where they are needed. E-selectin, an endothelial C-type lectin, binds sialofucosylated carbohydrate determinants on its pertinent ligands. This molecule is expressed in a constitutive manner on bone marrow and dermal microvascular endothelium, and inducibly on post-capillary venules at all sites of tissue injury. Engagement of E-selectin with relevant ligand(s) expressed on circulating cells mediates initial "tethering/rolling" endothelial adhesive interactions prerequisite for extravasation of blood-borne cells at any target tissue. Most mammalian cells express high levels of a transmembrane glycoprotein known as CD44. A specialized glycoform of CD44 called "Hematopoietic Cell E-/L-selectin Ligand" (HCELL) is a potent E-selectin ligand expressed on human cells. Under native conditions, HCELL expression is restricted to human hematopoietic stem/progenitor cells. We have developed a technology called "Glycosyltransferase-Programmed Stereosubstitution" (GPS) for custom-modifying CD44 glycans to create HCELL on the surface of living cells. GPS-based glycoengineering of HCELL endows cell migration to endothelial beds expressing E-selectin. Enforced HCELL expression targets human mesenchymal stem cell homing to marrow, licensing transendothelial migration without chemokine signaling via a VLA-4/VCAM-1-dependent "Step 2-bypass pathway." This review presents an historical framework of the homing receptor concept, and will describe the discovery of HCELL, its function as the bone marrow homing receptor, and how enforced expression of this molecule via chemical engineering of CD44 glycans could enable stem cell-based regenerative medicine and other adoptive cell therapeutics.