CCR7 mediates thymic settling by T-lineage progenitors. (A) Thymi from WT and CCR7^−/− mice were analyzed by flow cytometry to calculate absolute numbers of ETPs. Absolute numbers were obtained by multiplying the frequency of ETPs among live singlets by total thymic cellularity. Shown is the mean ± SEM for 21 WT (8 males, 13 females) and 18 CCR7^−/− (7 males, 11 females) mice all between the ages of 4 and 9 weeks. These data were pooled from 4 separate experiments, but for each experiment the mice were age and sex matched. *P < .05 compared with the WT control. (B) Mixed BM chimeras were generated using CD45.2 WT BM and CD45.1 WT BM mixtures as controls (top panel), CD45.2 CCR7^−/− BM and CD45.1 WT BM mixtures (middle panel), or CD45.2 CCR9^−/− BM and CD45.1 WT BM mixtures (bottom panel). Chimeras were analyzed by flow cytometry after 10 weeks using antibodies to CD45.1 and CD45.2 to determine donor chimerism. Shown is the mean CD45.2 donor chimerism ± SEM for each indicated population. *P < .05 and **P < .01 for the CD45.2 donor chimerism of the indicated population compared with HSC CD45.2 donor chimerism. (C) For adoptive transfer experiments, BM from either WT or CCR7^−/− mice (both CD45.2) was administered intravenously (left 2 panels) or intrathymically (right panel) to unirradiated WT recipient mice (CD45.1). After 2 weeks (intrathymic transfer) or 3 weeks (intravenous transfer), donor-derived cells were quantified by flow cytometry. Results show the mean ± SEM for 5 recipients in each group. *P < .05 compared with the WT control.