The immune response pattern of tadpoles of the South African clawed frog, Xenopus laevis, is distinct from that of the adult. Metamorphosis is a transitional period that separates the two patterns. The objectives of this research are to examine the hypothesis that development of the adult immune system requires metamorphosis and is characterized by the loss of a major population of larval lymphocytes and replacement with new, adult- type lymphocytes. To prove or disprove the hypothesis, larval thymus or spleen cell populations will be cytogenetically labelled, and their fate during and after metamorphosis will be followed. By implanting triploid thymus or spleen into diploid hosts and blocking metamorphosis in some of them, it will be determined whether renewal of T and B cell populations at metamorphosis is dependent on thyroid hormones. Because metamorphosis is regulated by thyroid hormones whose effects may be modulated by corticosteroid hormones, the direct effects of thyroid hormones and corticosteroid hormones in promoting metamorphosis-dependent changes in the immune system will be analyzed. These studies will contribute to a better understanding of the ontogeny of the immune system of amphibians, the mechanisms that regulate production of lymphocytes during ontogeny, the neuroendocrine regulation of development of the immune system, and how immunological tolerance to "self" develops.
