THE EFFECTS OF CORTICOSTEROID HORMONES AND THYROID-HORMONES ON LYMPHOCYTE VIABILITY AND PROLIFERATION DURING DEVELOPMENT AND METAMORPHOSIS OF XENOPUS-LAEVIS

Metamorphosis in the South African clawed frog, Xenopus laevis, is cha racterized by a striking loss of lymphocytes in the thymus, liver, and spleen. Changes in the proliferative responses of splenocytes and thy mocytes to T cell mitogens and semi-allogeneic cells are also observed at metamorphosis. Because the levels of circulating thyroid hormones (TH) and corticosteroid hormones (CH) increase dramatically during the climax of metamorphosis, we have investigated the possible role of TH and CH as mediators of the changes in lymphocyte numbers or lymphocyt e function. Here we report on the in vitro effects of CH and TH on lym phocyte viability and on phytohemagglutinin-P (PHA)-stimulated lymphoc yte proliferation at prometamorphosis and climax of metamorphosis. We have observed consistently significant inhibition of proliferation by corticosterone. In contrast, we have observed inconsistent inhibition of proliferation by both thyroxine (T4) and triiodothyronine (T3), In short-term studies, the viability of thymocytes and splenocytes was re duced in the presence of CH but not TH. These observations are consist ent with a hypothesis that loss of larval lymphocytes and changes of l ymphocyte function at metamorphosis may be due to elevated concentrati ons of CH rather than TH. Because CH have been shown to enhance TH-ind uced effects during metamorphosis, we looked at the combined effects o f these agents on PHA-stimulated lymphocyte proliferation. While each agent was inhibitory in several experiments, there was no significantl y greater inhibition when splenic lymphocytes were cultured with both.