Figure 1. The cell surface bioengineering tool box.

Schematic of key bioengineering methodologies to therapeutically modify the surface of live mammalian cells. (a) Direct conjugation of therapeutic materials (TM) functionalized with reactive groups which covalently bond to amine (−NH₂) or thiol (−SH) groups, intrinsic to cell membrane proteins. (b) Covalent attachment of biotin anchors to membrane proteins by reacting N-Hydroxysuccinimide (NHS)-activated biotin with primary amine groups in membrane proteins, followed by streptavidin-biotin linkage of TM. (c) Exogenous insertion of recombinant GPI-anchored proteins into the outer membrane leaflet (d) Exogenous insertion of palmitate-conjugated protein A or G into the cell membrane to subsequently immobilize antibodies or recombinant Fcγ-fusion proteins. (e) Anchoring ligand-functionalized TM to membrane receptors naturally present on the cell surface. (f) Metabolic labeling of surface glycans by the biosynthetic introduction of unnatural sugars containing unique functional groups, which serve as reactive sites for the attachment of TM. (g) Targeting TM site-specifically to surface proteins genetically fused to the BirA biotinylation enzyme acceptor peptide (AP) sequence. (h) Nonspecific electrostatic adsorption of cationic TM to the negatively charged cell membrane.