T-cell and natural killer cell development in thymectomized Xenopus

The Xenopus early-thymectomy model system is used to investigate the extent to which the thymus controls T-cell development and to probe the evolution of natural killer (NK) cells. Loss of T-cell function following thymectomy , together with the paucity of cells expressing monoclonal antibody-defined T-cell surface markers, and greatly reduced expression of T-cell receptor beta transcripts in spleen, liver and intestine, indicate that T-cell devel opment is minimal in the absence of the thymus. Our findings therefore miti gate against the idea that a substantial extrathymic pathway of T-cell deve lopment exists in early vertebrate evolution. Rather, they suggest that in this amphibian representative T cells are predominately thymus dependent. I n vitro studies with control and thymectomized Xenopus splenocytes reveal t hat a non-T/non-B population and also two T-cell subsets all display natura l cytotoxicity towards allogeneic thymus lymphoid tumour cells (which are d eficient in MHC antigen expression). Since Xenopus thymectomized early in l arval development are permanently deficient in T cells, they may provide a useful phylogenetic model for the study of NK cells.