Ascorbic Acid Promotes

Variegated expression of killer immunoglobulin-like receptors (KIR) in human natural killer (NK) cells is a stochastic process exclusive to subsets of mature NK cells and CD8(+) T cells. Allele-specific KIR expression is maintained by DNA methylation within the proximal promoter regions. Because KIR genes are densely methylated in NK cell progenitors, there is an implied stage of human NK cell development where DNA demethylation takes place to allow for active transcription. When and how this process occurs is unknown. Here, we show that KIR proximal promoters are densely methylated in less mature CD56(bright) NK cells and are progressively demethylated in CD56(dim) NK cells as they mature and acquire KIR. We hypothesized that ten-eleven translocation (TET) enzymes, which oxidize 5mC on DNA could mediate KIR promoter demethylation. The catalytic efficiency of TET enzymes is known to be enhanced by ascorbic acid. We found that the addition of ascorbic acid to ex vivo culture of sorted CD56(bright) NK cells increased the frequency of KIR expression in a dose-dependent manner and facilitated demethylation of proximal promoters. A marked enrichment of the transcription factor Runx3 as well as TET2 and TET3 was observed within proximal KIR promoters in CD56(bright) NK cells cultured with ascorbic acid. Additionally, overexpression of TET3 and Runx3 promoted KIR expression in CD56(bright) NK cells and NK-92 cells. Our results show that KIR promoter demethylation can be induced in CD56(bright), and this process is facilitated by ascorbic acid.