BMP signals: Mediated by stroma or thymocytes?

This is an invited comment on the paper by Hager-Theodorides AL, et al. Cell Cycle 2014; 13:324–33; PMID:24240189; http://dx.doi.org/10.4161/cc.27118 which is published in the same journa

Repression of hedgehog signal transduction in T-lineage cells increases TCR-induced activation and proliferation

Hedgehog proteins signal for differentiation, survival and proliferation of the earliest thymocyte progenitors, but their functions at later stages of thymocyte development and in peripheral T-cell function are controversial. Here we show that repression of Hedgehog (Hh) pathway activation in T-lineage cells, by expression of a transgenic repressor form of Gli2 (Gli2DeltaC2), increased T-cell differentiation and activationin response to TCR signalling. Expression of the Gli2DeltaC2 transgene increased differentiation from CD4(+)CD8(+) to single positive thymocyte, and increased peripheral T cell populations. Gli2DeltaC2 T-cells were hyper-responsive to activation by ligation of CD3 and CD28: they expressed cell surface activation markers CD69 and CD25 more quickly, and proliferated more than wild-type T-cells. These data show that Hedgehog pathway activation in thymocytes and T-cells negatively regulates TCR-dependent differentiation and proliferation. Thus, as negative regulators of TCR-dependent events, Hh proteins provide an environmental influence on T-cell fate

The transcription factor Gli3 regulates differentiation of fetal CD4- CD8- double-negative thymocytes.

Glioblastoma 3 (Gli3) is a transcription factor involved in patterning and oncogenesis. Here, we demonstrate a role for Gli3 in thymocyte development. Gli3 is differentially expressed in fetal CD4- CD8- double-negative (DN) thymocytes and is most highly expressed at the CD44+ CD25- DN (DN1) and CD44- CD25- (DN4) stages of development but was not detected in adult thymocytes. Analysis of null mutants showed that Gli3 is involved at the transitions from DN1 to CD44+ CD25+ DN (DN2) cell and from DN to CD4+ CD8+ double-positive (DP) cell. Gli3 is required for differentiation from DN to DP thymocyte, after pre-T-cell receptor (TCR) signaling but is not necessary for pre-TCR-induced proliferation or survival. The effect of Gli3 was dose dependent, suggesting its direct involvement in the transcriptional regulation of genes controlling T-cell differentiation during fetal development

The Gli3 transcription factor expressed in the thymus stroma controls thymocyte negative selection via Hedgehog-dependent and -independent mechanisms

The Hedgehog (Hh) responsive transcription factor Gli3 is required for efficient thymocyte development in the fetus. In this study we show that Gli3, not detected in adult thymocytes, is expressed in the murine fetal and adult thymus stroma. PCR array analysis revealed Cxcl9, Rbp1, and Nos2 as novel target genes of Gli3. We show that Gli3 positively regulates the expression of these genes, most likely by suppressing an intermediate repressor. Deletion of autoreactive thymocytes depends on their interactions with the thymus stroma. Repression of the proapoptotic gene Nos2 in Gli3 mutants coincides with reduced apoptosis of double positive thymocytes undergoing negative selection in vitro and in vivo, and the production of autoreactive thymocytes. Taken together these data indicate that Gli3 controls thymocyte apoptosis and negative selection possibly via the regulation of Nos2. Defective Gli3 expression in the thymus stroma also resulted in decreased CD5 expression on mature thymocytes and inappropriate production of MHC class I-selected CD4+ cells, both consistent with reduced TCR signal strength. Overall our data indicate that Gli3 expressed in the thymus stroma regulates negative selection and TCR signal strength via Hh-dependent and -independent mechanisms, with implications for autoimmunity

Indian hedgehog (Ihh) both promotes and restricts thymocyte differentiation.

We show that Indian Hedgehog (Ihh) regulates T-cell development and homeostasis in both fetal and adult thymus, controlling thymocyte number. Fetal Ihh(-/-) thymi had reduced differentiation to double-positive (DP) cell and reduced cell numbers compared with wild-type littermates. Surprisingly, fetal Ihh(+/-) thymi had increased thymocyte numbers and proportion of DP cells relative to wild type, indicating that Ihh also negatively regulates thymocyte development. In vitro treatment of thymus explants with exogenous recombinant Hedgehog protein promoted thymocyte development in Ihh(-/-) thymi but inhibited thymocyte development in Ihh(+/-), confirming both positive and negative regulatory functions of Ihh. Analysis of Rag(-/-)Ihh(+/-) thymi showed that Ihh promotes T-cell development before pre-T-cell receptor (pre-TCR) signaling, but negatively regulates T-cell development only after pre-TCR signaling has taken place. We show that Ihh is most highly expressed by the DP population and that Ihh produced by DP cells feeds back to negatively regulate the differentiation and proliferation of their double-negative progenitors. Thus, differentiation from double-negative to DP cell, and hence the size of the DP population, is dependent on the concentration of Ihh in the thymus. Analysis of Ihh conditional knockout and heterozygote adult mice showed that Ihh also influences thymocyte number in the adult