The role of H3K27me3 dosage for thymus development and function

The thymus provides the physiological environment for the development of the majority of T lymphocytes. Hence, its function is critical for the successful establishment and maintenance of the immune system's capacity to distinguish between vital self and injurious non-self. Thymocyte maturation and selection are primarily instructed by thymic epithelial cells (TEC), which can be classified into cortical (cTEC) and medullary TEC (mTEC) based on their respective location and phenotype. However, the genetic and epigenetic mechanisms that control thymus formation, homeostatic maintenance, and function are only incompletely understood to date despite the essential role of the thymus for immunological competence and hence an individual’s health. The importance of the repressive histone mark H3K27me3 (i.e., tri-methylation at histone 3 lysine 27) for TEC development and function is, for example, demonstrated in mice unable to generate this specific mark. As a result, these animals display a severe decrease in mTEC and thymocyte cellularity and exhibit T cell lymphopenia and a reduced TCR diversity. Importantly, these and comparable studies have relied on the complete absence of this particular epigenetic mark due to the functional loss of the polycomb repressive complex 2 (PRC2), a complex catalysing the deposition of me3 at H3K27. For this study, I used a mouse model (designated TECK27M) expressing a dominant negative histone H3 variant that replaces the lysine at position 27 with a methionine (generating an H3K27M mutant). This mutant cannot be methylated by the PCR2 complex, leading to a TEC-specific decrease in H3K27me3 marks. I investigated how a reduction (but not an absence) in H3K27me marks impacts TEC maturation and function. Here I show that TEC from TECK27M mice display a changed epigenetic landscape marked by H3K27-specific alterations, which include an increase in H3K27 acetylation and mono methylation. These alterations increase TEC cellularity and impact on subset composition secondary to a decrease in apoptotic signalling within specific TEC subsets. Both the intrinsic and the extrinsic apoptotic pathway are differentially regulated in TECK27M mice with BCL-2 and TRAIL being suggested as primary effectors, while proliferation of TEC is unchanged. The epigenetic changes affect the capacity of TEC to instruct positive thymocyte selection resulting in a decrease in the frequency of mature thymocytes. Central T cell tolerance induction is also partially affected as older TECK27M mice display lymphocytic infiltrates in peripheral organs, despite regular tissue-restricted antigen (TRA) expression profiles and regular negative selection. In summary, my studies confirm that a reduced placement of H3K27me3 affects TEC differentiation and function differentially when compared to mice displaying a complete loss of this specific mark. Therefore, it can be concluded that the correct dosage of this repressive mark is required for the regulation of TEC cellularity, apoptosis, and function.