Thymocyte precursors in early-thymectomized Xenopus: migration into and differentiation in allogenic thymus grafts

In an anuran amphibian, Xenopus laevis, thymectomy of 4-day-old larvae abrogates T-cell dependent immune responsiveness. When such early-thymectomized (TX) diploid frogs were implanted with histocompatible triploid thymuses and grafted 8 weeks later with skin from third-party donors, the grafts were rejected relatively normally in 20-27 days. Microspectrophotometric determination of ploidy 3-5 months after thymus reconstitution revealed that most thymocytes were donor-derived. In contrast, when TX frogs received allogeneic triploid thymuses, they rejected skin grafts from a third-party donor relatively slowly (48-92 days) but did not reject skin from the thymus donor. Most thymocytes in such animals were of host origin. Host thymocytes were present 4 weeks after thymus implantation and became dominant population by 12 weeks. Few thymus implant-derived donor cells were detectable in the host spleen. These data suggest that existence of precursor cells in TX Xenopus that can functionally differentiate along a T-cell pathway as a result of microenvironment provided by the thymus implant.