Meeting Abstract
Anuran metamorphosis is modulated by the synergistic actions of thyroid (TH) and glucocorticoid hormones. Metamorphosis is accompanied by immune system remodeling as larval antibodies are replaced by a new adult repertoire. Although glucocorticoids have been shown to induce thymus lymphocyte cell death, the influence of TH on thymus remodeling remains unknown. Here we profile changes in thymus gland cell proliferation (immunoreactivity against phosphohistone H3; PH3) and apoptosis (immunoreactivity against active caspase-3) during natural metamorphosis, and also following treatment with TH (5 nM T3) and/or dexamethasone (Dex, 2 uM) for 48 hours. Natural metamorphosis was accompanied by a doubling of thymus size from late prometamorphosis (NF57) through the end of metamorphic climax (NF66). Peaks in caspase and PH3 immunoreactivity occurred at early (NF 60) and late climax (NF 62), respectively. Treatments of premetamorphic (NF 50) or prometamorphic (NF 56) tadpoles with either TH or Dex alone increased caspase immunoreactivity, with Dex+TH in combination producing the highest response. Compared with prometamorphic controls, Dex treatment alone or in combination with T3 doubled PH3 immunoreactivity, whereas T3 alone quadrupled the amount of signal. Taken together, our findings suggest that both glucocorticoids and TH each contribute to thymus cell proliferation and apoptosis during metamorphosis. However, whereas glucocorticoids suppress both larval and adult thymocytes, we hypothesize that T3 functions to suppress larval thymocytes, but promotes the proliferation of adult thymocytes.
