Homeodomain-interacting protein kinase 2, a novel autoimmune regulator interaction partner, modulates promiscuous gene expression in medullary thymic epithelial cells.

Promiscuous expression of a plethora of tissue-restricted Ags (TRAs) by medullary thymic epithelial cells (mTECs) plays an essential role in T cell tolerance. Although the cellular mechanisms by which promiscuous gene expression (pGE) imposes T cell tolerance have been well characterized, the underlying molecular mechanisms remain poorly understood. The autoimmune regulator (AIRE) is to date the only validated molecule known to regulate pGE. AIRE is part of higher-order multiprotein complexes, which promote transcription, elongation, and splicing of a wide range of target genes. How AIRE and its partners mediate these various effects at the molecular level is still largely unclear. Using a yeast two-hybrid screen, we searched for novel AIRE-interacting proteins and identified the homeodomain-interacting protein kinase 2 (HIPK2) as a novel partner. HIPK2 partially colocalized with AIRE in nuclear bodies upon cotransfection and in human mTECs in situ. Moreover, HIPK2 phosphorylated AIRE in vitro and suppressed the coactivator activity of AIRE in a kinase-dependent manner. To evaluate the role of Hipk2 in modulating the function of AIRE in vivo, we compared whole-genome gene signatures of purified mTEC subsets from TEC-specific Hipk2 knockout mice with control mice and identified a small set of differentially expressed genes. Unexpectedly, most differentially expressed genes were confined to the CD80(lo) mTEC subset and preferentially included AIRE-independent TRAs. Thus, although it modulates gene expression in mTECs and in addition affects the size of the medullary compartment, TEC-specific HIPK2 deletion only mildly affects AIRE-directed pGE in vivo.

Epigenetic regulation of promiscuous gene expression in thymic medullary epithelial cells.

Thymic central tolerance comprehensively imprints the T-cell receptor repertoire before T cells seed the periphery. Medullary thymic epithelial cells (mTECs) play a pivotal role in this process by virtue of promiscuous expression of tissue-restricted autoantigens. The molecular regulation of this unusual gene expression, in particular the involvement of epigenetic mechanisms is only poorly understood. By studying promiscuous expression of the mouse casein locus, we report that transcription of this locus proceeds from a delimited region ("entry site") to increasingly complex patterns along with mTEC maturation. Transcription of this region is preceded by promoter demethylation in immature mTECs followed upon mTEC maturation by acquisition of active histone marks and local locus decontraction. Moreover, analysis of two additional gene loci showed that promiscuous expression is transient in single mTECs. Transient gene expression could conceivably add to the local diversity of self-antigen display thus enhancing the efficacy of central tolerance.