Poly-N-acetyllactosamine structures on murine cell surface T200 glycoprotein participate in natural killer cell binding to YAC-1 targets

Cell-surface murine T200 glycoprotein has been implicated in the binding of NK cells to certain susceptible tumor targets. The existence of poly-N-acetyllactosamine structures on T200 glycoprotein and the ability of lactosamine-type oligosaccharides to inhibit NK cell-mediated cytotoxicity suggest that these structures may also be important in NK-target binding. To further identify and characterize these structures, relevant saccharides and reconstituted membrane liposomes containing fractionated effector cell membrane proteins were tested for their ability to block conjugate formation. Under base line conditions, the majority of plastic-non-adherent, Percoll-fractionated, NK-enriched splenocytes that formed conjugates with NK-susceptible YAC-1 targets functioned as lytic effectors in a single-cell cytotoxicity assay. These effectors were blocked in their ability to bind to YAC-1 targets by the addition of N-acetyllactosamine Gal(beta 1,4)-GlcNAc] and chitobiose GlcNAc(beta 1,4)GlcNAc], but not by saccharides lacking lactosamine-type linkages. Liposomes prepared from octyl-beta-D-glucopyranoside-extracted YAC-1 and NK-enriched effector cell membranes interfered with conjugate formation, whereas liposomes prepared from NK-insensitive P815 cells were inconsequential. Surface radiolabeled effector cell membrane proteins were fractionated by tomato lectin-Sepharose 4B (poly-N-acetyllactosamine-specific) column chromatography. Tomato lectin-bound material was enriched in a glycoprotein identical with T200, which, when incorporated into liposomes, was a potent inhibitor of effector-target binding. This inhibitory capacity was abrogated by treatment of liposomes with Ly-5 mAb (T200 mAb) or the lactosamine-specific enzyme endo-beta-galactosidase. When T200 was purified by mAb affinity chromatography and incorporated into liposomes, it was a potent inhibitor of conjugate formation, an effect that was blocked by pretreatment of T200-containing liposomes with Ly-5 mAb or endo-beta-galactosidase. These data provide additional evidence that T200 can mediate binding of NK cells to YAC-1 targets, and that poly-N-acetyllactosamine-type structures on NK cell surface T200 glycoprotein are important in the binding process.