Quantification of the progenitors for thymic T cells in various organs

Partial characterization and frequency determination of the progenitors for thymic T cells in various organs were made by transferring cells directly into the thymus (intrathymically, i.t.). B10. Thy-1.1 (H-2b, Thy-1.1) mice were used as the donor, and C57BL/6 (H-2b, Thy-1.2) mice that had been whole body irradiated with 800 rads and reconstituted with 10(7) syngeneic (B6) bone marrow (BM) cells were used as the recipient. BM cells, spleen cells and thymus cells from young adults and fetal liver cells (day 14 of gestation) were treated with anti-Thy-1.1 antibody plus complement, and transferred i.t. The generation of Thy-1.1+ donor type cells in the recipient's thymus was investigated by using flow cytometry. From the time course of generation, it was shown that the progenitor cells in the thymus were distinct from those in other organs. After the transfer of thymic non-T cells, donor-type cells began to generate on the 4th day, the proportion of donor-type T cells increased quickly thereafter, and the progenitors in this organ ceased to produce T cells by day 21. On the other hand, a latent period of about 10 days was required for progenitor cells in the BM, spleen or fetal liver to generate T cells, and T cell producing-activity of the progenitor cells in these organs lasted as long as 7 weeks. The frequency of progenitor cells was analyzed by transferring serial dilutions of anti-Thy-1.1 plus complement-treated cells i.t. and investigating the generation of donor-type T cells in the recipient's thymus on day 11 in the case transferred with thymic cells and on day 21 in other cases. The proportion of negative recipients which did not contain detectable levels of donor-type cells was plotted on a logarithmic scale against the number of cells transferred on a linear scale. The progenitor cell frequencies in BM, spleen, thymus and fetal liver were estimated to be 12.5 x 10(-5), 6.25 x 10(-5), 0.22 x 10(-5) and 0.14 x 10(-5), respectively. The reliability of the frequency determination was supported by the finding that when a limited number (10(3)) of a 1:1 mixture of BM cells from two mutually identifiable donors was transferred i.t., most of the recipients were negative for donor-type T cells and 3 out of 4 positive recipients were seeded by progenitor cells of a single donor.