Congenitally thymusless nude mice that lacked functional T cells were reconstituted with H-2-compatible or -incompatible thymus grafts taken from either fetal, newborn, or adult mice and transplanted under the kidney capsule or subcutaneously. Transplantation with unirradiated fetal (15--17 d) or newborn thymus grafts reconstituted the nude mice as assessed by their subsequent generation of virus-specific cytotoxic T cells in vivo or alloreactive T cells in vitro. The restriction specificity of T cells from homozygous mice was exclusively for the nude host H-2, as shown by direct cytolysis or by cold target competitive inhibition assays. irrespective of whether nude mice were reconstituted with H-2-compatible, semiallogeneic, or H-2-incompatible, unirradiated newborn or fetal thymus grafts (in order of decreasing efficiency of reconstitution). The restriction specificity for the nonhost H-2 of the thymus could not be demonstrated even after primary or secondary sensitization in an infected appropriate F1 environment. These nude mice reconstituted with fetal or newborn grafts were tolerant to the H-2 of the thymus donors. Nude mice transplanted with irradiated adult thymus grafts were reconstituted functionally with syngeneic or semisyngeneic but not with allogeneic thymus grafts. In homozygous nu/nu irradiated heterozygous recipients of F1 thymus grafts, the restriction specificity for the nonhost thymic H-2 could not be elicited upon adoptive sensitization in irradiated and infected F1 heterozygote stimulator mice; in fact, these chimeras' lymphocytes were not tolerant to the nonhost H-2. The discrepancy between the restorative capacity of unirradiated vs. irradiated thymus grafts suggests that precursors of T cells in nude mice can acquire restriction specificity and immunocompetence independently of a conventional, functioning H-2-compatible thymus if exposed to an allogeneic fetal or a newborn thymus that contains functioning thymocytes of donor type but not if reconstituted with an irradiated adult allogeneic thymus.

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