Lack of expression of the T-cell marker XTLA-1 in Xenopus tropicalis: exploitation in thymus restoration studies.

Flow cytometric studies employing the monoclonal antibody XT-1 reveal that, in contrast to other strains and species of Xenopus examined, thymocytes and splenocytes from X. tropicalis do not express the T lineage-specific antigen XTLA-1. Thymus dependence of XTLA-1 expression in splenocytes is confirmed in X. laevis by early thymectomy experiments. When X. tropicalis larval thymus is implanted to thymectomized X. borealis larvae, histological studies reveal extensive colonisation by host-derived (quinacrine-positive) cells following metamorphosis. Flow cytometry of propidium iodide-stained nuclei shows that within 3 months of xenothymus implantation, 90% of cells within the implant originate from the recipient; donor cells were undetectable in recipient spleens at this time. The emergence of near normal levels and distribution of XTLA-1 positive cells within xenogeneic thymus implants within 3 months of implantation is illustrated by flow cytometric observations and through immunofluorescence analysis of cryostat sections. These kinetic studies indicate that immigrant host cells require sojourn within the foreign thymus environment before they express the T-cell marker.