Sonic Hedgehog Regulates Early Human Thymocyte Differentiation by Counteracting the IL-7-Induced Development of CD34+ Precursor Cells

Abstract

The Hedgehog (Hh) family of signaling molecules normally functions in the development of numerous tissues by regulating cellular differentiation and proliferation. Recent results have demonstrated that the different components of the Hh signaling pathway are expressed in the human thymus. In this study, we investigate the potential role of Sonic hedgehog (Shh) in human intrathymic T cell maturation. Results show that the expression of the two components of the Hh receptor, Patched and Smoothened, is mostly restricted to CD34+ precursor cells that are committing to the T cell lineage. Shh significantly increased the viability of CD34+ T cell precursors modulating bcl-2 and bax protein expression, and also inhibited their proliferation. The treatment of chimeric human-mouse fetal thymus organ cultures with Shh resulted in an arrested thymocyte differentiation and an accumulation of CD34+ progenitor cells. This effect was mainly attributed to the ability of Shh to counteract the IL-7-induced proliferation and differentiation of CD34+ cells. Shh down-regulated in the precursor cell population the expression of IL-7R as well as stromal-derived factor-1 chemokine receptor, CXCR4, and inhibited IL-7-dependent STAT5 phosphorylation. Therefore, Shh may function as a maintenance factor for intrathymic CD34+ precursor cells.

Sonic hedgehog negatively regulates pre-TCR–induced differentiation by a Gli2-dependent mechanism

“…6,21,24 Differences in the molecular regulation of thymocyte development between adult and fetus might account for this discrepancy. We therefore investigated the impact of Hh signaling on the production of DP cells in the adult thymus.…”

“…1,6,[18][19][20][21][22][23] Analysis of mice mutant for Shh, Gli3, and Smo have shown that Hh signaling is necessary for efficient survival, proliferation, and differentiation of thymocytes at the transition from DN1 to DN2, 18,21,22 but the role of Hh signaling at the transition from DN to DP thymocytes is controversial. Different experimental approaches have produced conflicting results that have led to 3 different interpretations: that Hh is a negative regulator of the pre-TCR signal and differentiation to DP 6,21,24 ; that Shh is a positive regulator of the DN to DP transition 18 ; or that Hh signaling does not influence thymocyte differentiation after the DN2 stage. 22 In vitro studies first demonstrated that Hh signaling influences thymocyte development 6,24 and suggested that Hh signaling was a negative regulator of differentiation to DP cell.…”

“…Different experimental approaches have produced conflicting results that have led to 3 different interpretations: that Hh is a negative regulator of the pre-TCR signal and differentiation to DP 6,21,24 ; that Shh is a positive regulator of the DN to DP transition 18 ; or that Hh signaling does not influence thymocyte differentiation after the DN2 stage. 22 In vitro studies first demonstrated that Hh signaling influences thymocyte development 6,24 and suggested that Hh signaling was a negative regulator of differentiation to DP cell. Treatment of mouse fetal thymus organ cultures (FTOCs) with recombinant Shh protein arrested thymocyte development at the DN3 stage after TCR␤ chain rearrangement, whereas neutralization of endogenous Hh signaling by anti-Hh antibody treatment increased DP production.…”

The Gli3 Transcription Factor Expressed in the Thymus Stroma Controls Thymocyte Negative Selection Via Hedgehog-Dependent and -Independent Mechanisms

“…These signaling proteins are required for efficient maturation and differentiation of the earliest double negative (DN) populations (3,4,6,7,10,12). Shh also influences TCR repertoire selection, most likely by modulating TCR signal strength (11,29).…”

“…Hedgehog (Hh) 3 proteins secreted by the thymus stroma and the CD4 ϩ CD8 ϩ double positive (DP) thymocytes have important regulatory functions in both mouse thymus (3)(4)(5)(6)(7)(8)(9)(10)(11) and human thymus (12,13). The Hh family of secreted signaling proteins is essential for embryonic development and homeostasis of adult renewable tissues (14 -17), and inappropriate Hh pathway activation is involved in many cancers (18,19).…”