Modulation of mouse natural killer cell activity by the serum thymic factor

After mice had been treated with serum thymic factor (FTS), we observed an increase or a decrease in the NK activity, depending on the duration of treatment and mouse strain: an increase of C57BL/6, BALB/c, A, and nu/nu, and a decrease for NZB and CBA mice. The fact that treatment of C57BL/6 mice with FTS produces the same effects as thymectomy--that is, an increase--on the NK activity and returns the augmented NK activity of thymectomized mice to normal levels suggests that FTS has multiple effects with different results depending on the level of differentiation of the target cells. Different mechanisms of action of FTS on NK cells are discussed.