9421349 Rollins-Smith Metamorphosis in the South African clawed frog, Xenopus laevis, results in significant changes in the immune system. It is characterized by a striking involution of the thymus and spleen followed by significant lymphocyte expansion in the postmetamorphic period. Thymus involution is observed seasonally in some other amphibian and reptilian species, and with aging in a number of vertebrate species. In most of these examples of thymus involution, the causes are not well defined. Corticosteroid hormones (CH) have been implicated, but careful studies with physiological concentrations of natural hormones and specific hormone- receptor antagonists have not been done. To determine whether CH are responsible for thymus involution and the decline in numbers of lymphocytes at metamorphosis in Xenopus, two specific CH receptor antagonists will be tested. In vitro studies will establish the effectiveness of these compounds in this system and determine effective concentration ranges. In vivo studies are designed to inhibit binding of CH to their receptors and thus inhibit possible steroid-induced cell death. The likely mechanism of CH-induced cell death is programmed cell death or apoptosis. To examine this question, the rates of spontaneous apoptosis in the thymus and spleen cell populations of control frogs and frogs treated with CH- antagonists will be determined at several stages before, during, and after metamorphosis. If CH-induced apoptosis is a mechanism of cell decline, it should be inhibited by the CH-antagonists. If the decline in thymocyte and splenocyte numbers in not due to CH-induced apoptosis, it may be due to a decreased rate of precursor immigration and expansion during metamorphic climax. To examine this question, the rate of immigration and expansion of diploid (2N) precursors into an implanted triploid (3N) thymus will be studied at climax and compared with that observed at premetamorphic and postmetamorphic stages. C ollectively, these studies will more precisely define the dynamics of cell death and replacement in the thymus at metamorphosis. Survival of young postmetamorphic frogs might depend on their ability to mount a memory response to a pathogen encountered as a tadpole. Therefore, we would like to know to what extent tadpole lymphocyted persist in the postmetamorphic period. Tadpole T cells differ phenotypically from T cells because they do not express class II major histocompatibility complex (MHC) antigens. Several experiments are designed to learn whether tadpole T cells persist through metamorphosis and retain their tadpole phenotype or become adult-like and express class II MHC. %%% These experiments will provide information about the regulation of the immune system during development as well as the role of corticosterid hormones in programmed cell death ***
