Preleukemic Sur-TCR-Tg mice demonstrate perturbed thymopoiesis, with increased cycling of DN2 and DN3 thymocytes and expansion of the DN4 and CD8 SP subsets. (A) Preleukemic 6 weeks (6w) Sur-TCR-Tg mice have abnormally small thymi, but thymic size significantly increases by 4 months of age. Absolute thymocytes counts in Sur-TCR-Tg mice and WT mice at 6 weeks and 4 months of age are shown. Each shape represents 1 individual mouse. Experiment was conducted 3 times with similar results. (B) Thymocyte subset analyses as revealed via flow cytometry in preleukemic 6w Sur-TCR-Tg mice vs age-matched WT mice. Sur-TCR-Tg mice have increased frequencies (top) and absolute numbers (bottom) of CD4–CD8– thymocytes and CD8⁺CD4⁻ SP thymocytes compared with WT mice, in contrast to diminished numbers of CD4⁺CD8⁺ double-positive and CD8⁻CD4⁺ SP thymocytes. This experiment was conducted >3 times with similar results. (C) Using CD44 and CD25 to stratify the DN thymocyte subset, 6w Sur-TCR-Tg mice show a selective increase in the frequency of the DN4 subset, whereas other subsets are reduced compared with WT mice. This experiments was conducted 3 times with similar results. (D) BrdU incorporation demonstrates increased cycling of the DN2 (CD25⁺CD44⁺), DN3 (CD25⁺CD44⁻), and CD8⁺CD4⁻ subsets in preleukemic Sur-TCR-Tg mice compared with WT controls. BrdU was injected intraperitoneally (IP) to 6w Sur-TCR-Tg and control mice. Eighteen hours later, thymi were harvested and BrdU incorporation was analyzed using flow cytometry. Fluorescence-activated cell sorter (FACS) plots from 1 representative Sur-TCR-Tg are shown (top) as well as summative data from 7 mice (bottom). This experiment was performed twice with similar results.