The effects of hexose 6-O-sulfate esters on human natural killer cell lytic function

Natural cell-mediated cytotoxicity (NCMC) is inhibited by some neutral hexoses and hexose phosphates at 25 to 100 mM concentrations. In this study we describe the effects of hexose 6-O-sulfate esters on NCMC against K-562 target cells. Mannose 6-sulfate, galactose 6-sulfate, N-acetylglucosamine 6-sulfate, and N-acetylgalactosamine 6-sulfate inhibit NCMC in a dose-dependent manner at concentrations of 10 mM and below. Inhibitory effects of mannose 6-sulfate and galactose 6-sulfate were evident at concentrations as low as 1.25 mM. The neutral forms of these sugars, glucose and glucose 6-sulfate, did not inhibit NCMC over this range of concentrations. Comparison of the inhibitory effects of sulfated and phosphorylated forms of mannose and galactose indicated that the sulfated forms are much more potent inhibitors. Formation of effector cell:target cell conjugates was unaffected by the presence of sugar sulfates. Calcium pulse experiments demonstrated that inhibitory effects of sugar sulfates were exerted after the Ca++-dependent triggering step in the NK lytic process. Kinetic studies showed that addition of sugars as long as 60 min after initiation of cultures yielded potent inhibitory effects. Sugar sulfates were not toxic for effector cell populations and effectors were not refractory for lytic function after removal of sugars. Sugar sulfates were inhibitory against multiple tumor types in both human and murine NK lytic assays. These results suggest that the sugar sulfates inhibit NK cells at a postconjugation, posttriggering step involving lectin-like receptors or lectin-like molecules.