Effects of thyroxine-driven precocious metamorphosis on maturation of adult-type allograft rejection responses in early thyroidectomized frogs.

In contrast to the adult pattern of allograft reactivity, larval South African clawed frogs (Xenopus) either become tolerant of adult major histocompatibility complex (MHC) disparate skin grafts or reject them slowly. Larvae fail to reject grafts that are incompatible at minor histocompatibility (H) loci (MHC-identical); rather, they become immunologically tolerant. We report here that early thyroidectomized (thyroidx) larvae, forced to metamorphose precociously on a regimen of thyroxine (T4) treatment, showed larva-like responses in that their ability to reject skin allografts that differed by minor H loci or one MHC haplotype was significantly impaired relative to age-matched intact postmetamorphic controls. During the normal ontogeny of Xenopus, lymphocyte numbers increase in larval life, decrease during metamorphosis, and increase to adult levels at 6-12 months of age. Thyroidectomy and a low-dose regimen of thyroxine treatment limited the number of lymphocytes that developed in larval organs and very dramatically reduced those populations below control levels during metamorphosis. Because these changes were stage-dependent rather than age-dependent, the second wave of lymphopoiesis in thyroidx frogs treated with low T4 may occur out of synchrony with the expression of Class I MHC antigens. Impaired allograft rejection, therefore, may reflect the absence of a population of cells that can effectively recognize minor H alloantigens and/or self and allo Class I antigens.