The stimulatory effect of serum thymic factor (FTS) on spontaneous DNA synthesis of mouse thymocytes.

Synthetic serum thymic factor (FTS), a T-cell differentiating factor, was studied for its ability to modify the spontaneous DNA synthetic activity of immature or immunocompetent T cells in vitro. In serum free RPMI 1640 medium, FTS stimulated [3H]thymidine incorporation into the DNA of young adult mouse thymocytes in a time and concentration dependent manner. One to 100 pg/ml FTS were the effective concentrations, and the stimulation appeared 3 h after the treatment. A comparable stimulation was noticed, but to a lesser extent, on cultured bone marrow cells. In contrast, no effect could be detected on spleen cells whatever time or concentrations were studied. Neither thymocytes treated in vitro with hydroxyurea nor the medullary, cortisol-resistant thymocytes were sensitive to FTS treatment. In contrast, thymocytes bearing receptor to peanut agglutinin (PNA+) were significantly stimulated with FTS, which suggested that the target population of FTS belongs to the maturing, non-immunocompetent thymocyte compartment.