Thymus Ontogeny in Frogs: T-Cell Renewal at Metamorphosis

Metamorphosis in amphibians presents a unique problem for the developing immunesystem. Because tadpoles are free-living, they need an immune system to protect againstpotential pathogens. However, at metamorphosis, they acquire a variety of new adultspecificmolecules to which the tadpole immune system must become tolerant. Wehypothesized that Xenopus laevis tadpoles may avoid potentially destructive antiselfresponses by largely discarding the larval immune system at metamorphosis andacquiring a new one. By implanting triploid (3N) thymuses into diploid (2N) hosts, weexamined the influx and expansion of host T-cell precursors in the donor thymus ofnormally metamorphosing and metamorphosis-inhibited frogs. We observed that donorthymocytes are replaced by host-derived cells during metamorphosis, but inhibition ofmetamorphosis does not prevent this exchange of cells. The implanted thymuses exportT cells to the spleen. This donor-derived pool of cells declines after metamorphosis innormally developing frogs but is retained to a greater extent if metamorphosis isinhibited. These studies confirm previous observations of a metamorphosis-associatedwave of expansion of T cells and demonstrate that it is not dependent on the relativelyhigh concentrations of thyroid hormones required for metamorphosis. Although somelarval T cells persist through metamorphosis, others may be destroyed or the larvalpopulation is significantly diluted by the expanding adult population.