Hedgehog-interacting protein (Hhip) as a candidate Foxn1 target in the thymus

The thymus is a primary lymphoid organ that supports the development of functional T cells through its unique stromal architecture. The most important and abundant component of this microenvironment are thymic epithelial cells, or TECs. TEC development, function and maintenance are critically dependent on the expression of the master transcription factor Foxn1. The Hedgehog-Interacting Protein (Hhip) was identified as a novel candidate target of Foxn1-mediated gene expression. Hhip is an inhibitor of Hedgehog (Hh) signalling, an embryonic developmental pathway which is also vital for normal T cell development. I first provided evidence that Foxn1 indirectly modulated the Hh signalling pathway via hhip. To identify a role for Hhip in thymus biology, I investigated the consequences of a constitutive deficiency for hhip expression in embryonic and neonatal mice, and demonstrated that loss of hhip resulted in an upregulation of Hh signalling via the coreceptors Gas1 and Boc, affecting the relative frequencies of cortical and medullary TECs in a dose-dependent manner and favouring medullary TEC development. I also generated a novel transgenic mouse model with a targeted loss of hhip in Foxn1-expressing cells, to specifically delineate the role of Hhip in TECs. This approach additionally circumvented the limitation of neonatal lethality in constitutive hhip-deficient mice. TEC-targeted loss of hhip expression not only impacted TEC sublineage decisions, but also affected the ability of TECs to perform positive thymocyte selection, resulting in the generation of T cells with reduced TCR signal strength which were less responsive to mitogenic stimuli. In aggregate, the experimental data here presents the first evidence that Hhip plays an important role in regulating TEC differentiation and function.