Late thymectomy in Xenopus tadpoles reveals a population of T cells that persists through metamorphosis.

To investigate the persistence of larval T lymphocytes in the adult period, tadpoles of the South African clawed frog, Xenopus laevis, were allowed to develop to prometamorphic stages 57-58 and thymectomized (Tx). Thymectomy at this stage allows for maximal expansion of the larval T cell population but prevents emergence of the adult T cell population. Using a T cell-specific monoclonal antibody (mAb) which recognizes the XTLA-1 determinant, we examined the absolute numbers of thymic and splenic T cells expressing XTLA-1 in normal tadpoles, postmetamorphic Tx frogs, and intact age-matched adult frogs. A small, but measurable, number of larvally-derived XTLA-1+ cells persists through metamorphosis. By simultaneously staining with a mAb specific for class II major histocompatibility (MHC) antigens, we determined the phenotype of the persisting XTLA-1+ cells in the Tx frogs. Like XTLA-1+ splenocytes in intact adult controls which are predominantly class II+, most XTLA-1+ cells in Tx adults also express class II. In contrast, most XTLA-1+ cells in the tadpole are class II-. This suggests that a small population of class II+ larval T cells survives metamorphic transition to become a long-lived population in the adult. Alternatively, some class II- larval T cells may express class II in the adult period.