Identification of thymocyte progenitors in hemopoietic tissues of the rat. I. A quantitative assay system for thymocyte regeneration.

A quantitative adoptive transfer system is described which can be used to precisely monitor the generation of thymocytes and peripheral T cell subsets by precursor cells in hemopoietic tissues of rats. This assay utilizes the rat pan-T cell alloantigens, A.R.T.-1a and A.R.T.-1b, and the fluorescence-activated cell sorter (FACS) to directly enumerate donor- and host-origin thymocytes and T cells in irradiated, histo-compatible recipients. The assay has the advantage over related systems in that it detects more than 95% of total newly-formed thymocytes and T cells at all stages of differentiation; that it is highly sensitive (as few as 1% of the appropriate A.R.T.-1 bearing cells can be detected in cell mixtures); and that it permits selective recovery of donor-and/or host-origin cells for further characterization. The results show that, after a lag period of 10-12 days, the regenerative kinetics of donor-origin thymocytes are linear with respect to time and cell dose, over a range of 2.5 to 50 X 10(6) bone marrow cells. Above a threshold of approximately 400R, the regenerative kinetics of donor-origin thymocytes are independent of irradiation dose, but are inversely related to the age of the recipient. On a per cell basis, bone marrow cells are more efficient than spleen cells at regenerating the thymus. By 4 months after bone marrow cell transfer, permanent chimeras are established in which the proportion of donor-origin T cells approximates that of donor-origin thymocytes, and in which the ratio of presumptive helper (W3/25+) and suppressor/cytotoxic (OX8+) donor-origin T cells is normal. The assay therefore appears to be suitable for following both the purification of prothymocytes from rat hemopoietic tissues and the proliferation and differentiation of their progeny in thymus and peripheral lymphoid tissues.