Compensatory proliferation accounts for near-normal CCR7/CCR9 DKO thymic cellularity. (A) Sorted ETPs from WT or CCR7/CCR9 DKO mice were cultured on OP9-DL1 stromal layers. WT cells were cultured in triplicate wells, whereas CCR7/CCR9 DKO cells were cultured in a single well. After 2 weeks, cultured cells were analyzed by flow cytometry for expression of CD25 and Thy1. Representative FACS plots of cells previously gated to be CD45⁺ are shown. (B) Graphs show the relative numbers of cells obtained from cultures described in panel A. Shown is the mean ± SEM for WT and CCR7/CCR9 DKO cultures. (C) Lin⁻Flt3^hi cells (3 × 10³) sorted from WT BM (CD45.1) were intrathymically injected into WT or CCR7/CCR9 DKO recipients (both CD45.2). After 15 days, recipient thymi were analyzed for donor-derived DN3 cells (left column) or DP cells (right column). Shown are representative donor/host FACS plots. (D) Graphs show the total numbers of donor DN3 cells (left panel) and donor DP cells (right panel) found in recipient thymi for the experiment described in panel C. Shown is the mean ± SEM for 5 WT recipients and 3 CCR7/CCR9 DKO recipients. **P < .01 compared with WT controls.