Thymus-specific CCL21 deficiency decreases RTE frequency in WT peripheral tissues. (A) Schematic of the experimental approach used to measure thymic output from Ccl21a-deficient thymus. Freshly isolated E17 CD45.2⁺Ccl21a^+/− or Ccl21a^−/− thymic lobes were grafted under the kidney capsule of CD45.1⁺ WT hosts. Spleens were harvested from host mice 7 days after surgery. (B) Quantitation of total thymocyte numbers, and the number and proportion of CD4⁺CD8⁺ double-positive (DP) thymocytes in E17 Ccl21a^+/− (n = 10) or Ccl21a^−/− (n = 11) thymic lobes before transplantation. Flow cytometric detection and quantitation of donor thymus–derived CD45.2⁺TCRβ^hi T cells (C) and CD45.2⁺TCRβ^hi cSP4 and CD45.2⁺TCRβ^hi SP8 T cells (D) in the spleens of WT mice that received either Ccl21a^+/− (n = 8; blue bars) or Ccl21a^−/− (n = 8; red bars) grafts. Error bars represent mean ± SEM. Flow cytometric data representative of at least 3 independent experiments. *P < .05, **P < .01.