Thymic epithelial cells amplify epigenetic noise to promote immune tolerance

Cellular plasticity is a principal feature of vertebrate adaptation, tissue repair and tumorigenesis1,2. However, the mechanisms that regulate the stability of somatic cell fates remain unclear. Here, we use the somatic plasticity of thymic epithelial cells, which facilitates the selection of a self-discriminating T cell repertoire3, as a physiological model system to show that fluctuations in background chromatin accessibility in nucleosome-dense regions are amplified during thymic epithelial maturation for the ectopic expression of genes restricted to other specialized cell types. This chromatin destabilization was not dependent on AIRE-induced transcription but was preceded by repression of the tumour suppressor p53. Augmenting p53 activity indirectly stabilized chromatin, inhibited ectopic transcription, limited cellular plasticity and caused multi-organ autoimmunity. Genomic regions with heightened chromatin accessibility noise were selectively enriched for nucleosome-destabilizing polymeric AT tracts and were associated with elevated baseline DNA damage and transcriptional initiation. Taken together, our findings define molecular levers that modulate cell fate integrity and are used by thymic epithelial cells for immunological tolerance. The activity of the tumour-suppressor protein p53 is repressed in the thymus to augment fluctuations in background chromatin accessibility as a means of mediating ectopic gene expression and immune tolerance.