Ihh produced by thymocytes signals to negatively regulate differentiation from DN to DP in the adult. (A) Excision of the Ihh gene as assessed by PCR from CD2Cre− (lane 1) and CD2Cre+ Ihhfl/fl (lane 2) thymocytes. (B) Relative thymocyte number (relative to the mean of WT littermates) of 4- to 6-week-old CD2Cre− and vavCre+ Ihhfl/fl littermates showed a significant difference in SD, by F test (P = .001). Mean thymocyte number: litter 1, CD2Cre−Ihhfl/fl 3.77 × 108, CD2Cre+Ihhfl/fl 4.225 × 108; litter 2, CD2Cre−Ihhfl/fl 1.03 × 108, CD2Cre+Ihhfl/fl 1.245 × 108; litter 3, CD2Cre−Ihhfl/fl 2.84 × 108, CD2Cre+Ihhfl/fl 3.575 × 108; litter 4, CD2Cre−Ihhfl/fl 1.08 × 108, CD2Cre+Ihhfl/fl1.34 × 108; litter 5, CD2Cre−Ihhfl/fl 2.67 × 108, CD2Cre+Ihhfl/fl 3.31 × 108. (C) FACS analysis of CD44 and CD25 expression on CD4−8−3− thymocytes from CD2Cre−Ihhfl/fl and CD2Cre+Ihhfl/fl mice. (D) Histogram shows the percentage of DN4 cells in the DN population, relative to the mean percentage in Cre− littermates for CD2Cre−Ihhfl/fl (n = 13) and CD2Cre+Ihhfl/fl knockout mice (n = 6). The differences were not significant by Student t test. (E) Model of functions of Shh and Ihh signaling in the control of thymocyte ho-meostasis.
