Competition between thymus-intrinsic and -extrinsic T cell progenitors is regulated by cytokine receptors. (A) Newborn thymus lobes were grafted into the indicated recipients. After 9 wk, cells of donor thymus and host origin were analyzed for CD4 and CD8 expression. Numbers are percentages of cells in each quadrant. Numbers of analyzed recipients are displayed in B. (B) Summary of the proportions of persistent (filled bars) versus exhausted (open bars) T cell development in thymus grafts. Exhausted refers to recipients in which both grafted lobes contained only circulating SP T cells but no DP thymocytes, whereas persistent indicates recipients with ongoing T cell development (presence of DP and SP cells) in at least one lobe. Number (n) indicates the total number of recipients. Data in B were obtained 9 to 11 wk after transplantation.