Mannose 6-, fructose 1-, and fructose 6-phosphates inhibit human natural cell-mediated cytotoxicity.

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Abstract
In vitro human natural cell-mediated cytotoxicity (NCMC) to K-562 leukemia, Molt-4 T lymphocyte and F-265 lymphoid cells is inhibited in a dose-dependent manner by mannose 6-phosphate [M6P], fructose 1-phosphate [F1P] and fructose 6-phosphate [F6P]. This inhibition is not observed with mannose, glucose, fucose, glucose 6-phosphate, mannose 1-phosphate, galactose 1-phosphate or galactose 6-phosphate. Preincubation of the effector cells, obtained from fresh whole blood, with M-6-P, F-1-P or F-6-P did not inhibit cytotoxicity, which indicated that these hexose phosphates are not nonspecifically toxic towards the effector lymphocytes. M6P and the stereochemically similar F1P are more potent inhibitors than F6P in terms of concentration required and time of onset of effect. Inhibition of cytotoxicity by M6P varied with target cell type: F-265 is protected at much lower concentrations of M6P (< 1 mM) than is either Molt-4 or K-562. The inhibition of NCMC is also observed with the inhibitors of lysosomal function, NH4Cl and chloroquine. The presence of a functional M6P receptor on target cells was demonstrated: gelonin, a seed protein that inactivates the eukaryotic ribosome but is nontoxic to intact cells, was covalently linked to monophosphopentamannose, and this conjugate was toxic to both K-562 and F-265 target cells, the latter being by far the more sensitive; and chloroquine, NH4Cl and M6P all inhibited the toxicity of gelonin-monophosphopentamannose. These results suggest either that a cytolytic contains a hexose phosphate residue and may be taken up by target cells through the lysosomal/M6P pathway or that such a residue is involved in target cell-effector cell recognition.