a, Splenocytes from 8–10-month-old Vav-iCre+/−;Ercc1−/fl, Vav-iCre+/− controls, or 2-year-old wild-type mice were injected retro-orbitally into 3–4-month-old p16Ink4+/Luc senescence reporter mice (n = 2 donor mice per genotype) as described in Fig. 4. Tissues were collected from recipient mice 2 weeks after the final imaging and the expression of p21 was measured by qRT–PCR. Data are mean ± s.d. *P < 0.05, ∞P < 0.01, one-way ANOVA with Tukey’s test. b, Splenocytes (5 × 106 cells) from 9–10-month-old Vav-iCre+/−;Ercc1−/fl and Vav-iCre+/− mice were injected retro-orbitally into 3–4-month-old p16Luc/Luc senescence reporter mice (n = 2 donor mice per genotype; n = 4 p16Luc/Luc recipient mice for Vav-iCre+/−;Ercc1−/fl splenocytes; n = 3 receiving Vav-iCre+/− splenocytes). Weekly measurements of luminescence in recipient reporter mice. Data are mean ± s.d. *P < 0.05, ∞P < 0.01, two-tailed unpaired Student’s t-test. c, Splenocytes from 7- or 26-month-old male mice were injected retro-orbitally into female mice to track distribution of the transplanted cells (n = 2 donor mice and n = 3 recipient mice per age group; n = 2 uninjected controls). Tissues were collected 24 h after injection. Expression of the Sry gene on the Y chromosome measured by qRT–PCR in RNA isolated from tissues of recipient mice was used to track homing of immune cells to various recipient mouse organs. There was little difference in immune cell homing if the donor mice were young or old.