Conditional SCL activation induces the reversible accumulation of small CD8⁺TCRβ^low thymocytes. (A) Thymic flow cytometric analysis of tamoxifen (TAM)–treated (fed for 3 weeks) lck-ER^T2-SCL and littermate control mice. (B) Analysis of CD44 expression on CD8⁺TCRβ^low cells. TAM feed for 6 weeks; wild-type, n = 4; lck-ER^T2-SCL, n = 4; representative plots are shown; dashed lines represent corresponding isotype control staining. (C) Absolute cell numbers were calculated for total thymocytes and different subsets. Bars represent means plus or minus SD; wild-type no TAM, n = 10; lck-ER^T2-SCL no TAM, n = 5; wild-type + TAM, n = 10; lck-ER^T2-SCL + TAM, n = 13; TAM feed for 2-6 weeks. (D) CD8⁺TCRβ^low cells were detected in peripheral blood and within spleens of TAM-treated lck-ER^T2-SCL mice. Representative plots of at least 3 mice per group are shown; TAM feed for 3 to 4 weeks. The percentage of CD8⁺TCRβ^low cells per total nucleated cells is displayed. (E) After the cessation of TAM treatment, thymic CD8⁺TCRβ^low cells disappeared and thymic cellularity returned back to normal levels. A cohort of mice (n = 13) was injected with TAM (1 mg intraperitoneally daily) for 14 days. Flow cytometric analysis of the thymus was carried out on days 1, 5, 8, and 15 after cessation of TAM administration (at least n = 3 mice per time point). The proportion of CD8⁺TCRβ^low cells per total nucleated thymocytes (left) and the total thymic cellularity (right) was calculated for each time point. Black squares represent mean (± SD). FSC indicates forward scatter; and PI, propidium iodide.