Spontaneous reversion of the R allele in aged <i>R/-; CreERt2</i> mice.

<p>Images show representative immunohistochemical analysis of carrier-only injected 6 month-old <i>R/−; CreERt2</i> mice analyzed one month after injection. (<b>A</b>) Carrier-only injected mice exhibited a thymus structure containing both cortical and medullary regions (cytokeratin 14 (K14) and CDR1 indicate medullary and cortical TEC respectively, while pan cytokeratin identifies all TEC). Scale bar: 150 um. <b>B</b>, the thymic region of a carrier-only injected mouse, showing a series of small thymic lobes. Scale bar: 300 µm. (<b>C,D</b>) Plots show analysis of thymocytes for CD4 and CD8 expression after gating on live CD45⁺ cells for (<b>C</b>) two randomly selected, untreated 11 month old <i>R/−; CreERt2</i> mice and (<b>D</b>) two untreated <i>R/−</i> mice (7 m and 11 m old, respectively). Each of the untreated <i>R/−; CreERt2</i> mice contained DP cells within the CD45⁺ population (33.9% and 85.6%, respectively) while the <i>R/−</i> mice contained no DP cells (mean±SD: Untreated <i>R/−; CreERt2</i> mice, 6 months old, 48.20%±40.88%, n = 5; 11 m old, 61.78%±39.34%, n = 2. <i>R/−</i> controls 1.09%±1.55%, n = 2). Of note is that the CD4⁺ and CD8⁺ single positive populations present in the aged <i>R/−</i> mice are also commonly observed in aged <i>nu/nu</i> mice, and are thought to arise by homeostatic expansion of extrathymically-generated T cells <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114842#pone.0114842-GuyGrand1" target="_blank">[30]</a>. (<b>E</b>) Representative images of thymic rudiments from 5 month-old <i>R/−</i> mice after immunohistochemical analysis with the markers shown.