Ontogeny of Immunity in Frogs/Metamorphic Changes

南非爪蟾（Xenopus laevis）的变形会导致免疫系统发生重大变化。其特点是胸腺和脾脏明显退化，变质后淋巴细胞显著扩增。在其他一些两栖动物和爬行动物物种中，胸腺退化是季节性的，在许多脊椎动物物种中，胸腺退化是随年龄增长而发生的。在大多数胸腺复旧的例子中，病因并不明确。皮质类固醇激素（CH）已被提及，但对天然激素和特异性激素受体拮抗剂的生理浓度的仔细研究尚未完成。为了确定CH是否与非洲爪蟾胸腺退化和变态时淋巴细胞数量下降有关，将测试两种特异性CH受体拮抗剂。体外研究将确定这些化合物在该系统中的有效性，并确定有效浓度范围。体内研究旨在抑制CH与其受体的结合，从而抑制可能的类固醇诱导的细胞死亡。ch诱导细胞死亡的可能机制是程序性细胞死亡或凋亡。为了研究这个问题，对照青蛙和使用CH-拮抗剂的青蛙的胸腺和脾脏细胞群的自发凋亡率将在变态前、变态中和变态后的几个阶段进行测定。如果ch诱导的细胞凋亡是细胞衰退的一种机制，那么它应该被ch拮抗剂所抑制。如果胸腺细胞和脾细胞数量的下降不是由于ch诱导的凋亡，则可能是由于变质高潮时前体迁移和扩张速度的降低。为了研究这个问题，我们将研究在高潮阶段二倍体（2N）前体向植入的三倍体（3N）胸腺的迁移和扩张速度，并将其与变质前和变质后阶段的观察结果进行比较。总的来说，这些研究将更精确地定义胸腺蜕变过程中细胞死亡和替换的动力学。幼年变形蛙的存活可能取决于它们对蝌蚪时期遇到的病原体产生记忆反应的能力。因此，我们想知道蝌蚪淋巴细胞在变质后的持续程度。蝌蚪T细胞在表型上与T细胞不同，因为它们不表达ⅱ类主要组织相容性复合体（MHC）抗原。几个实验旨在了解蝌蚪T细胞是通过蜕变持续存在并保留其蝌蚪表型，还是变成成体样并表达II类MHC。这些实验将提供有关免疫系统在发育过程中的调节以及皮质类固醇激素在程序性细胞死亡中的作用的信息

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 ***