Allograft rejection in Xenopus laevis following larval thymectomy.

Xenopus laevis thymectomized at stages 41 through 49 accept first set allografts, while animals thymectomized at stage 51 or older reject allografts in times similar to intact animals. However, thymectomy at progressively earlier stages results in a greater proportion of animals unable to reject second set grafts. In some animals, the allograft response remains deficient even after multiple challenges. The results indicate that alloreactive cells are thymus dependent, and suggest that the thymus processes precursor thymocytes starting upon its formation at around stage 41. The processed cells, competent to respond to alloantigens, are released to the periphery almost immediately. While an increasing pool of processed T cells accumulates during stages 41-49, the persistent defective allograft response displayed by animals thymectomized during these stages suggests that early thymectomy may leave a population of alloreactive cells qualitatively defective in some subpopulation necessary for normal allograft responses, or that any residual cells processed prior to thymectomy are capable of only limited clonal expansion.