Narrow-band UVB radiation triggers diverse changes in the gene expression and induces the accumulation of M1 macrophages in human skin

Background: The underlying molecular mechanisms that determine the biological effects of UVB radiation exposure on human skin are still only partially comprehended. Objectives: Our goal is to examine the human skin transcriptome and related molecular mechanisms following a single exposure to UVB in the morning versus evening. Methods: We exposed 20 volunteer females to four-fold standard erythema doses (SED4) of narrow-band UVB (309–313 nm) in the morning or evening and studied skin transcriptome 24 h after the exposure. We performed enrichment analyses of gene pathways, predicted changes in skin cell composition using cellular deconvolution, and correlated cell proportions with gene expression. Results: In the skin transcriptome, UVB exposure yielded 1384 differentially expressed genes (DEGs) in the morning and 1295 DEGs in the evening, of which the most statistically significant DEGs enhanced proteasome and spliceosome pathways. Unexposed control samples showed difference by 321 DEGs in the morning vs evening, which was related to differences in genes associated with the circadian rhythm. After the UVB exposure, the fraction of proinflammatory M1 macrophages was significantly increased at both timepoints, and this increase was positively correlated with pathways on Myc targets and mTORC1 signaling. In the evening, the skin clinical erythema was more severe and had stronger positive correlation with the number of M1 macrophages than in the morning after UVB exposure. The fractions of myeloid and plasmacytoid dendritic cells and CD8 T cells were significantly decreased in the morning but not in the evening. Conclusions: NB-UVB-exposure causes changes in skin transcriptome, inhibiting cell division, and promoting proteasome activity and repair responses, both in the morning and in the evening. Inflammatory M1 macrophages may drive the UV-induced skin responses by exacerbating inflammation and erythema. These findings highlight how the same UVB exposure influences skin responses differently in morning versus evening and presents a possible explanation to the differences in gene expression in the skin after UVB irradiation at these two timepoints.