Epigenetic Stability at Signature Cytokine Human Th1 and Th17 Cells Exhibit

The linear model of Th cell lineage commitment is being revised due to reports that mature Th cells can trans-differentiate into alternate lineages. This ability of Th cells to reprogram is thought to be regulated by epigenetic mechanisms that control expression of transcription factors characteristic of opposing lineages. It is unclear, however, to what extent this new model of Th cell plasticity holds true in human Th cell subsets that develop under physiological conditions in vivo. We isolated in vivo-differentiated human Th1 and Th17 cells, as well as intermediate Th1/17 cells, and identified distinct epigenetic signatures at cytokine (IFNG and IL17A) and transcription factor (TBX21, RORC, and RORA) loci. We also examined the phenotypic and epigenetic stability of human Th17 cells exposed to Th1-polarizing conditions and found that although they could upregulate TBX21 and IFN-g, this occurred without loss of IL-17 or RORC expression, and resulted in cells with a Th1/17 phenotype. Similarly, Th1 cells could upregulate IL-17 upon enforced expression of RORC2, but did not lose expression of IFN-g or TBX21. Despite alterations in expression of these signature genes, epigenetic modifications were remarkably stable aside from the acquisition of active histone methylation marks at cytokine gene promoters. The limited capacity of human Th17 and Th1 cells to undergo complete lineage conversion suggests that the bipotent Th1/17 cells may arise from Th1 and/or Th17 cells. These data also question the broad applicability of the new model of Th cell lineage plasticity to in vivo-polarized human Th cell subsets. C D4 + Th cells are divided into functionally distinct subsets , which initiate and maintain diverse immune responses by producing distinct cytokines. For example, Th1 cells express T-BET, produce IFN-g, and regulate cellular immunity, whereas Th17 cells express RORC2 and RORad, produce IL-17, and provide host defense against extracellular pathogens (1–6). The development of these distinct lineages of Th cells involves heritable epigenetic changes in chromatin structure (7–10) that control expression of the lineage-defining transcription factors and signature cytokines. How epigenetic modifications regulate the differentiation of human Th1 and Th2 cells has been extensively studied and is typified by modifications to the IFNG locus, a region of ∼100 kb surrounding the gene (8). In Th1 cells, regulatory sequences in the IFNG locus undergo DNA demethylation, an epigenetic modification typically associated with active promoters, and are enriched with active histone modifications such as histone H3 acetylation and trimethyl-histone H3 lysine 4 (H3K4me3). …