Gli3 in fetal thymic epithelial cells promotes thymocyte positive selection and differentiation by repression of Shh

Gli3 is a Hedgehog (Hh)-responsive transcription factor that can
function as a transcriptional repressor or activator. We show that Gli3
activity in mouse thymic epithelial cells (TECs) promotes positive
selection and differentiation from CD4+ CD8+ to CD4+ CD8− singlepositive
(SP4) cells in the fetal thymus and that Gli3 represses Shh.
Constitutive deletion of Gli3, and conditional deletion of Gli3 from TECs,
reduced differentiation to SP4,whereas conditional deletion of Gli3 from
thymocytes did not. Conditional deletion of Shh from TECs increased
differentiation to SP4, and expression of Shh was upregulated in the
Gli3-deficient thymus. Use of a transgenic Hh reporter showed that the
Hh pathway was active in thymocytes, and increased in the Gli3-
deficient fetal thymus. Neutralisation of endogenous Hh proteins
in the Gli3−/− thymus restored SP4 differentiation, indicating that Gli3
in TECs promotes SP4 differentiation by repression of Shh.
Transcriptome analysis showed that Hh-mediated transcription was
increased whereas TCR-mediated transcription was decreased in
Gli3−/− thymocytes compared with wild type.