Reduced thymocyte proliferation and impaired peripheral homeostatic CD4 T-cell proliferation in adult mice double deficient in Fl and Il7r expression. (A) Mice were injected with BrdU and after 12 hours proportion of BrdU incorporating SP CD4⁺, SP CD8⁺, and DP CD4⁺CD8⁺ thymocytes was determined. Data represent mean (SD) values of 6 to 8 age-matched mice of each genotype (2 thymi pooled in 3 replicate analyses). *P < .01 comparing Il7r^−/− and Fl^−/−Il7r^−/− mice; **P < .01 comparing Fl^−/− and WT mice. (B) Representative FACS profiles of CD44 and CD62L expression within SP CD8⁺ TCRβ^hi thymocytes (gated on viable CD8⁺CD4⁻ TCRβ^hi) with frequency of activated/memory (CD44⁺) T cells indicated by gate (numbers show mean percentages of 5–8 mice of each genotype). (C,D) Nonirradiated 15-week-old Rag-1^−/− and Fl^−/−Il7r^−/− mice (CD45.2) received a transplant of 0.5 × 10⁶ FACS-purified naive CD4⁺CD44^low T cells from 8- to 10-week-old WT CD45.1 mice, and 14 to 17 days after transfer the total number of donor-derived CD4⁺ T cells was established (C). (D) Representative FACS profiles of CD44 and CD62L expression within donor-derived CD4⁺ cells at 14 to 17 days after transfer (gated on viable donor CD4⁺ cells). Data represent mean (SD) values of 7 age-matched mice of each genotype. *P < .01.