Is IL-2 regulated by a serum inhibitor?

Abstract Normal murine serum inhibits the proliferation of cloned cytotoxic T lymphocytes driven by pure interleukin 2 (IL-2), indicating that a component of normal murine serum is directly inhibitory to IL-2-dependent proliferation. However, the effect is not specific to such cells, since an IL-2-independent variant cell, and a number of lymphoid tumor cell lines are similarly inhibited. Addition of purified IL-2 does not overcome the inhibition, although its degree is reduced. Fractionation of murine serum showed that there are at least two inhibitory activities, which migrate with globular proteins of molecular weights ⩾ 10 6 and 4 × 10 4 , respectively, on gel chromatography. Neither of the activities was specific for IL-2-dependent cells. Furthermore, murine IL-2 is stable in murine serum in vitro , although it disappears rapidly from the circulation after intravenous injection. It is therefore unlikely that serum inhibitor of IL-2 is an important immunoregulator in vivo .