Human Th1 and Th17 cells exhibit epigenetic stability at signature cytokine and transcription factor loci.

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-γ, 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-γ 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.

Placenta-specific expression of the interleukin-2 (IL-2) receptor ß subunit from an endogenous retroviral promoter.

The long terminal repeat (LTR) sequences of endogenous retroviruses and retroelements contain promoter elements and are known to form chimeric transcripts with nearby cellular genes. Here we show that an LTR of the THE1D retroelement family has been domesticated as an alternative promoter of human IL2RB, the gene encoding the ß subunit of the IL-2 receptor. The LTR promoter confers expression specifically in the placental trophoblast as opposed to its native transcription in the hematopoietic system. Rather than sequence-specific determinants, DNA methylation was found to regulate transcription initiation and splicing efficiency in a tissue-specific manner. Furthermore, we detected the cytoplasmic signaling domain of the IL-2Rß protein in the placenta, suggesting that IL-2Rß undergoes preferential proteolytic cleavage in this tissue. These findings implicate novel functions for this cytokine receptor subunit in the villous trophoblast and reveal an intriguing example of ancient LTR exaptation to drive tissue-specific gene expression.