The effect of thymectomy at different stages of larval development on the immune response of the clawed toad to sheep erythrocytes.

The role of the thymus during the first three weeks of larval life in the maturation of humoral immunity in the amphibian, Xenopus laevis, is examined. Thymectomy throughout this period dramatically affects the heterologous red cell response of young adults. Sheep erythrocyte (SRBC) administration elicited haemolytic antibody production in the blood and in the spleen (measured by the appearance of plaque forming cells) of control animals, but failed to do so in all thymectomized toadlets. Moreover, use of the immunocyto-adherence assay, which proved to be a sensitive test for SRBC reactivity in control Xenopus, indicated a complete absence of induced responsiveness to this antigen in the spleens of thymectomized toadlets, even in those animals thymectomized as late as 22 days of age. In contrast to allograft immunity in Xenopus, Which is suppressed only by thymic ablation during the first two weeks of larval life, the development of reactivity to heterologous erythrocytes therefore requires an additional period of thymic influence during ontogeny. These experiments support a concept of thymus-dependent cell heterogeneity in amphibians.