Requirement for intron structures in activating the Cd8a locus

The presence of an intron relieves gene repression caused by promoter-proximal four-bp specific sequences in yeast

Introns can enhance gene expression in eukaryotic cells in a process called intron-mediated enhancement (IME). The levels of enhancement are affected not only by the intron sequence but also by coding sequences (CDSs). However, the parts of CDSs responsible for mediating IME have not yet been identified. In this study, we identified an IME-mediating sequence by analyzing three pairs of IME-sensitive and -insensitive CDSs in Saccharomyces cerevisiae. Expression of the CDSs yCLuc, yRoGLU1, and KmBGA1 was enhanced by the presence of an intron (i.e., they were IME sensitive), but the expression of each corresponding codon-changed CDS, which encoded the identical amino acid sequence, was not enhanced (i.e., they were IME insensitive). Interestingly, the IME-insensitive CDSs showed higher expression levels that were like intron-enhanced expression of IME-sensitive CDSs, suggesting that expression of IME-sensitive CDSs was repressed. A four-nucleotide sequence (TCTT) located in the promoter-proximal position of either the untranslated or coding region was found to be responsible for repression in IME-sensitive CDSs, and repression caused by the TCTT sequence was relieved by the presence of an intron. Further, it was found that the expression of intron-containing yeast-native genes, UBC4 and MPT5, was repressed by TCTT in the CDS but relieved by the introns. These results indicate that TCTT sequences in promoter-proximal positions repress gene expression and that introns play a role in relieving gene repression caused by sequences such as TCTT.

Constitutive CD8 expression drives innate CD8+ T-cell differentiation via induction of iNKT2 cells

Temporal down-regulation of the CD8 co-receptor after receiving positive-selection signals has been proposed to serve as an important determinant to segregate helper versus cytotoxic lineages by generating differences in the duration of TCR signaling between MHC-I and MHC-II selected thymocytes. By contrast, little is known about whether CD8 also modulates TCR signaling engaged by the non-classical MHC-I-like molecule, CD1d, during development of invariant natural killer T (iNKT) cells. Here, we show that constitutive transgenic CD8 expression resulted in enhanced differentiation of innate memory-like CD8⁺ thymocytes in both a cell-intrinsic and cell-extrinsic manner, the latter being accomplished by an increase in the IL-4-producing iNKT2 subset. Skewed iNKT2 differentiation requires cysteine residues in the intracellular domain of CD8α that are essential for transmitting cellular signaling. Collectively, these findings shed a new light on the relevance of CD8 down-regulation in shaping the balance of iNKT-cell subsets by modulating TCR signaling.

Differential Requirement of Cd8 Enhancers E8I and E8VI in Cytotoxic Lineage T Cells and in Intestinal Intraepithelial Lymphocytes

CD8 expression in T lymphocytes is tightly regulated by the activity of at least six Cd8 enhancers (E8_I-E8_VI), however their complex developmental stage-, subset-, and lineage-specific interplays are incompletely understood. Here we analyzed ATAC-seq data on the Immunological Genome Project database and identified a similar developmental regulation of chromatin accessibility of a subregion of E8_I, designated E8_I-core, and of E8_VI. Loss of E8_I-core led to a similar reduction in CD8 expression in naïve CD8⁺ T cells and in IELs as observed in E8 _I ^-/- mice, demonstrating that we identified the core enhancer region of E8_I. While E8 _VI ^-/- mice displayed a mild reduction in CD8 expression levels on CD8SP thymocytes and peripheral CD8⁺ T cells, CD8 levels were further reduced upon combined deletion of E8_I-core and E8_VI. Moreover, activated E8 _I -core^-/- E8 _VI ^-/- CD8⁺ T cells lost CD8 expression to a greater degree than E8 _I -core^-/- and E8 _VI ^-/- CD8⁺ T cells, suggesting that the combined activity of both enhancers is required for establishment and maintenance of CD8 expression before and after TCR activation. Finally, we observed a severe reduction of CD4 CTLs among the TCRβ⁺CD4⁺ IEL population in E8 _I -core^-/- but not E8 _VI ^-/- mice. Such a reduction was not observed in Cd8a ^-/- mice, indicating that E8_I-core controls the generation of CD4 CTLs independently of its role in Cd8a gene regulation. Further, the combined deletion of E8_I-core and E8_VI restored CD4 CTL subsets, suggesting an antagonistic function of E8_VI in the generation of CD4 CTLs. Together, our study demonstrates a complex utilization and interplay of E8_I-core and E8_VI in regulating CD8 expression in cytotoxic lineage T cells and in IELs. Moreover, we revealed a novel E8_I-mediated regulatory mechanism controlling the generation of intestinal CD4 CTLs.