Epigenetic regulation of monocyte heterogeneity

Human monocytes are a heterogeneous cell population consisting of three subsets: classical CD14++CD16-, intermediate CD14++CD16+ and nonclassical CD14+CD16++ monocytes. Intermediate monocytes contribute to inflammation, and their circulating cell counts are increased in many chronic inflammatory diseases, even though the underlying pathways are poorly characterized. We tested in how far epigenetic mechanisms regulate human monocytes heterogeneity in chronic kidney disease, which is a proinflammatory condition of substantial epidemiological importance. By applying next-generation Methyl-Sequencing (Methyl-Seq) we characterized genome-wide DNA methylation within the three monocyte subsets and analyzed the impact of uremia on DNA methylation in differentiating monocytes. We found that each monocyte subset displays a unique phenotype with regards to DNA methylation. Genes with differentially methylated promoter regions in intermediate monocytes were linked to distinct immunological processes, which is in line with results from recent gene expression analyses. In vitro, uremia induced a dysregulation of DNA methylation in differentiating monocytes which affected several transcription factors important for monocyte differentiation (e.g. FLT3, HDAC1, MNT) and led to enhanced generation of intermediate monocytes. As a potential mediator, the uremic toxin and DNA methylation inhibitor S-Adenosylhomocysteine induced shifts in monocyte subsets in vitro, and associated with monocyte subset counts in vivo. In summary, our data support the concept of monocyte trichotomy and the distinct role of intermediate monocytes in human immunity. The shift in monocyte subsets which occurs in chronic kidney disease, a proinflammatory condition of substantial epidemiological impact, may be induced by the accumulation of specific uremic toxins that mediate epigenetic dysregulation. Overall design: Seven healthy donors were recruited for DNA methylation analysis of the three monocyte subsets; DNA methylation analysis of differentiating monocytes was performed in 5 independent experiments