A depletion approach to analyze thymocytopoiesis under competitive conditions with near-normal precursor ratios. (A) Outline of the experimental approach. For details see “Results.” (B) Postsort analysis of lin⁻ BM cells depleted of populations expressing various surface markers by FACS sorting (right panels). (Left panels) Sorted lin⁻ BM cells. Numbers in gates and quadrants indicate the percentage of cells. (C) T-cell precursors are confined to CD27- and CD135-expressing cells under competitive conditions. Lin⁻ BM cells from B6 CD45.1 mice (test) were depleted of CD27⁺ cells, CD135⁺ cells, or both, mixed with lin⁻ BM cells from B6 CD45.2 mice (competitor), and transferred intravenously into Il7ra-deficient hosts. Ctl indicates mixture of nondepleted B6 CD45.1 lin⁻ BM cells with B6 CD45.2 lin⁻ BM cells. Donor-derived thymocytopoiesis was analyzed flow cytometrically 2 and 4 weeks after transfer. Representative FACS plots of test versus competitor ratios 2 weeks (top panels) and 4 weeks (bottom panels) after transfer. Numbers in gates indicate the percentage of cells. Combined analysis of 2 independent experiments with 2 mice per group.