Regulation of T cell differentiation: in vitro induction of 20 alpha-hydroxysteroid dehydrogenase in splenic lymphocytes from athymic mice by a unique lymphokine.

The enzyme 20 alpha-hydroxysteroid dehydrogenase (20 alpha SDH) has previously been shown to be a specific enzyme marker of mature T cells. In vivo, the expression of 20 alpha SDH is thymus dependent, in that splenic lymphocytes from athymic mice have only low levels of activity, although the levels of enzyme activity increase gradually with age. In vitro, 20 alpha SDH can be induced in splenic lymphocytes from nu/nu mice by conditioned media from mitogen- or alloantigen-stimulated normal lymphocytes. Induction is rapid in vitro. Beginning after a lag period of approximately 6 hr, enzyme activity increases linearly for approximately 20 to 30 hr resulting in a 5- to 10-fold increase in activity. Induction is blocked by mitomycin C, suggesting a requirement for cell proliferation. The phenotype of both the precursor and the induced lymphocyte populations is Thy 1.2-, Lyt 1-, 2-, suggesting that induction of 20 alpha SDH expression is an early step in T cell differentiation. The factor responsible for 20 alpha SDH induction has been partially purified and is distinct from other known lymphokines in both its biochemical and functional properties. The term interleukin 3 is proposed for this factor.