In vivo administration of interleukin 2 stimulates mitosis in thymus and bone marrow.

We have used short-term, high-density cultures to demonstrate that interleukin 2 (IL 2) in picomolar amounts causes entry of approximately 2% of thymocytes from 3-month-old rats into mitosis. Newborn and fetal animals show a higher response reflecting a greater proportion of cells which have been shown to express IL 2 receptors at this age. In vivo administration of nanogrammes of IL 2 or injection of rats with syngeneic spleen cells which had been stimulated in vitro with concanavalin A to release IL 2 were also shown to increase the proliferation of both thymus and bone marrow cells. This suggests that IL 2, in amounts which could be produced by peripheral lymphoid tissue during immune responses, could act to increase the turnover of lymphocytes in bone marrow and thymus.