Thyroid function and immune reactivity during metamorphosis in Xenopus laevis, the South African clawed toad

The Hypothalamic-Pituitary-Thyroid (HPT) Axis in Frogs and Its Role in Frog Development and Reproduction

Metamorphosis of the amphibian tadpole is a thyroid hormone (TH)-dependent developmental process. For this reason, the tadpole is considered to be an ideal bioassay system to identify disruption of thyroid function by environmental contaminants. Here we provide an in-depth review of the amphibian thyroid system with particular focus on the role that TH plays in metamorphosis. The amphibian thyroid system is similar to that of mammals and other tetrapods. We review the amphibian hypothalamic-pituitary-thyroid (HPT) axis, focusing on thyroid hormone synthesis, transport, and metabolism. We also discuss the molecular mechanisms of TH action, including the role of TH receptors, the actions of TH on organogenesis, and the mechanisms that underlie the pleiotropic actions of THs. Finally, we discuss methods for evaluating thyroid disruption in frogs, including potential sites of action, relevant endpoints, candidate protocols for measuring thyroid axis disruption, and current gaps in our knowledge. The utility of amphibian metamorphosis as a model for evaluating thyroid axis disruption has recently led to the development of a bioassay using Xenopus laevis.

Amphibian virology

Amphibians are a diverse group of species; much work remains to be done to elucidate the viruses of amphibians. Viral diseases may play an important role in wild and captive amphibian populations. Iridoviruses are a leading cause of wild amphibian mortality events in the United States and are a common cause of amphibian mortality events worldwide. In addition to frank viral diseases, viral infections may play a role in the establishment of bacterial, fungal, and parasitic diseases and are an underlying cause of neoplasia. It is important for the amphibian clinician to recognize disease syndromes and pathology that are consistent with viral etiology.

Histological examination of the effects of corticosterone in larvae of the western toad, Bufo boreas (Anura: Bufonidae), and the Oriental fire-bellied toad, Bombina orientalis (Anura: Discoglossidae)

The effects of corticosterone (CORT)-treatment on various tissues were examined in two species of anuran larvae, the discoglossid Bombina orientalis, and the bufonid Bufo boreas. Corticosterone was administered directly into aquarium water for 15 days. After treatment, histological analyses were conducted on skin, gut, spleen, thymus, and neural and muscle tissue. Corticosterone treatment prevented sloughing of the skin, which resulted in a build-up of stratum corneum, and inhibited the development of gland nests and the subsequent formation of dermal granular and mucous glands in both species. Corticosterone treatment also decreased epithelial folding in the gut and caused vesiculation of the gut epithelial cells. The thymus of CORT-treated animals was significantly reduced in size (P < .05) and cell density (P < .05), and the spleen of CORT-treated animals was completely involuted. The brain and pituitary of CORT-treated animals had a decreased cell density (P < .05) and many pyknotic cells. An examination of muscle revealed that muscle fibers of CORT-treated animals had a decreased cross-sectional area (P < .05). The dose of CORT used (1.1 microM) was within the range used in other studies in the literature and resulted in tissue levels within the range experienced by larvae at metamorphic climax. Thus, this study is appropriate to address the histological effects of CORT in experimental manipulations and the role of increasing CORT at metamorphic climax. The data suggest that increasing endogenous CORT at metamorphosis may be involved in degeneration of larval tissue, prior to regeneration, which is stimulated by thyroid hormones.

The development of peripheral TNP-tolerance and suppressor function in Xenopus laevis, the South African clawed toad

In adult Xenopus laevis, inducer- and effector-suppressor functions are located in the spleen. These peripheral suppressor functions must be established at this location near the end of metamorphosis, since both functions are in the thymus in premetamorphic and in developmentally-blocked metamorphosing larvae. This study examined whether TNP-conjugated self-antigens resulting from exposure to trinitrobenzene sulfonic acid (TNBS), will stimulate TNP-tolerance in premetamorphic, metamorphic, and in developmentally-blocked metamorphosing larvae. Premetamorphic and developmentally-blocked larvae produce little TNP-tolerance or peripheral suppressor function. However, when TNBS exposure includes the late stages of the metamorphic period, both TNP-tolerance and splenic anti-hapten suppressor function are demonstrable. Removal of suppressor function with cyclophosphamide prevents expression of tolerance, thus, they are functionally related. Suppressor function and tolerance both differentiate during the late metamorphic stages when new adult antigens are being expressed and incorporated into a library of self.

Apoptosis in the thymus of developing Xenopus laevis

Metamorphosis in Xenopus laevis is a time when thyroxine and glucocorticoid levels rise, dramatic morphological and physiological changes take place, and tolerance is established to newly expressed adult antigens. In vitro exposure of thymocytes tested at different metamorphic stages, to the T-cell lectin, phytohemagglutinin (PHA), stimulates increased apoptosis, but incubation with the synthetic glucocorticoid, dexamethasone (DEX), fails in this regard. Altered-self antigenicity, following trinitrobenzene sulfonic acid (TNBS) treatment, increases apoptosis only in the late stages of metamorphosis. Developmentally blocked metamorphosing larvae demonstrate low thymic apoptotic rates that are also unaffected by in vitro exposure to DEX or by in vivo exposure to thyroxine, but are increased by PHA and in some individuals by TNBS. When released from blockade, their thymic apoptotic rates rise as progress through metamorphosis is renewed. Larval thymic apoptosis is glucocorticocoid- and thyroxine insensitive, but is lectin and altered-self antigen activated, particularly during postclimax stages.

An examination of the immune system of the duck (Anas platyrhynchos) for factors resembling some defined mammalian cytokines

Duck lymphoblasts generated by phytohaemagglutinin (PHA) did not respond to recombinant or Jurkat cell line human interleukin (IL)-2 or possess surface antigens resembling mammalian IL-2 receptors or IL-1 beta. Supernatant fluids from normal and PHA-stimulated duck lymphocyte cultures, and normal and lipopolysaccharide (LPS)-stimulated monocytes, gave negative results in a range of assays for biological activity and immunochemical presence of factors resembling mammalian IL-1 and IL-2. However, supernatant fluids from LPS-stimulated duck monocytes contained IL-6-like activity (up to 35 units/mL) assessed on the 7TD-1 murine cell line. We were unable to demonstrate mRNA that would hybridize to cDNA probes for human IL-1 beta, IL-6, and tumour necrosis factor (TNF) in extracts of blood and lymphoid organs from normal and antigen-stimulated ducks. Because homologous serum or plasma is essential for duck lymphocytes and macrophages to respond to mitogens in vitro, we asked whether this growth-factor-like activity might be caused by substances resembling mammalian cytokines. Serum and plasma were examined for activity consistent with IL-1 and IL-6 on mammalian target cells. None was detected. Instead, both serum and plasma contained inhibitors of human IL-1 beta and IL-6, detected at dilutions up to 1:100. Inhibition by serum was heat (56 degrees C, 30 min) labile but inhibition by plasma was heat stable. The identities and biological functions of these inhibitors remain to be defined.

Impaired T cell functions during amphibian metamorphosis: IL-2 receptor expression and endogenous ligand production

T cell functions are impaired during defined developmental stages of amphibian metamorphosis (Marx et al., 1987). Here we show, using a fluorescent anti-human IL-2 receptor antibody and flow cytometry, that during these stages, the splenocytes of Xenopus laevis, the South African clawed toad, have a progressively diminished capacity to express IL-2 receptors (IL-2R), after in vitro lectin stimulation. Preincubation with human rIL-2 specifically blocks binding of the anti-IL-2R antibody. Separation of an endogenous ligand bound to the IL-2R leads to a substantial increase in available epitope recognized by the anti-IL-2R antibody when pre- and postmetamorphic splenocytes are employed, but not when splenocytes of the prometamorphic stages are treated similarly. Thus, the cells from the prometamorphic stages are not producing significant quantities of the ligand. Finally, we demonstrate that human rIL-2 is not by itself mitogenic in the toad, but it can act as a co-stimulator of antigen-induced mitogenesis. Thus, an absence of an endogenous ligand (autologous IL-2?), coupled with a reduced capacity to express IL-2 receptors may be responsible for impaired T cell clonal expansion in metamorphosing Xenopus. Inhibition of T cell functions during this period is vital, since adult cells forming within the larval body bear surface proteins not found on larval cells (Flajnik et al., 1986).

Thymus-replacing activity from the metamorphic spleen of Xenopus laevis

Anuran metamorphosis offers an interesting vertebrate immunological paradigm, for adult cells that arise within the immunocompetent larval body are MHC Class I disparate from those of the larva. The animals, in order to avoid immune self-destruction during this transition period, are made unresponsive to these modified-self cells by an impairment of T-cell functions. However, it remains to be discovered how an animal with compromised T-cell functions can protect itself from those environmental pathogens protection from which is thymus dependent. During metamorphosis, larval and adult immunocytes capable of reacting to each other coexist within the animal. Their interaction might stimulate the secretion of a cytokine capable of circumventing T-cell functions by acting directly on B cells. Here, we report that such an activity is released in vitro by metamorphic and not by adult splenocyte suspensions. This activity will amplify in vitro anti-hapten responses by immunized, but not carrier-primed, adult splenocytes. The activity is unaffected by dexamethasone and, since it will amplify anti-hapten responses in T-cell-depleted immunized adult splenocyte suspensions, the antibody-producing (B) cell population may be affected directly. Two radiolabeled protein peaks of 65 kD and 40 kD were obtained by SDS-PAGE analysis from secreting, metamorphosing, but not from adult, splenocytes.

A monoclonal antibody against the human IL-2 receptor binds to paraformaldehyde-fixed but not viable frog (Xenopus) splenocytes

Others have reported that a monoclonal anti-human IL-2 receptor antibody (anti-CD25) specifically binds a membrane receptor on Xenopus laevis PHA-induced and paraformaldehyde-fixed splenic blasts. In this paper, we present evidence suggesting that this binding is an artifact of membrane damage. Specifically, significant binding of anti-CD25 could only be achieved if the lymphoblasts were acid-washed and/or paraformaldehyde-fixed prior to being incubated with the fluoresceinated antibody. For example, in a representative experiment 95% of paraformaldehyde-fixed blasts, about 19% of acid-washed but not fixed blasts, but fewer than 2% of viable (untreated) blasts were positive for the CD25 epitope. Paraformaldehyde is known to alter membrane permeability. The DNA dye propidium iodide (PI) was used to demonstrate that the acid washing procedure also causes membranes to become permeable. Flow cytometric analyses of acid-washed PHA-induced splenic blasts doubly stained with the anti-CD25 antibody and PI showed that only 1.5% of the cells that were positive for CD25 did not stain with PI. Additionally, the anti-CD25 antibody, which immunoprecipitated a molecule from human lymphoblasts of between 50 and 60 kDa, did not immunoprecipitate any surface molecules from 125I-labeled Xenopus splenic blasts. Since binding of anti-CD25 to Xenopus splenic blasts appears to occur only after membrane damage, the antibody may be recognizing a cross-reactive internal epitope that is not involved in ligand binding on the cell surface.