In vitro restoration by thymulin of NK activity of cells from old mice.

Old mice show a reduced natural killer (NK) cell activity. Among the causes proposed to explain this defect, both intrinsic failure of NK cells or age-related alteration of microenvironmental factors relevant for NK function, have been taken into consideration. The findings reported in the present paper, demonstrate that thymic peptides, and in particular the facteur timique serique (FTS), more recently called thymulin (ZnFTS) in its zinc-bound form, whose production and activity is generally reduced in old age, is able, when administered in vitro, to restore the crippled NK cytotoxicity of spleen cells from old mice. Neither the zinc-unbound form of the hormone (FTS) nor zinc ions alone are effective. The action is exerted on the basal NK activity and not on IFN-boosted NK cytotoxicity, at variance with the restoration obtained in similar experimental conditions with thyroid hormones, thus suggesting that different mechanisms or maturative steps of NK cells are involved.