Objectives: Ablative fractional CO 2 laser (AFL) therapy is an effective intervention to induce dermal remodeling. AFL treatment of the skin triggers the recruitment of immune cells, with neutrophils dominating the early phase. However, the role of recruited neutrophils in AFL-induced microinjuries and their subsequent dermal remodeling capacity remains elusive. Materials and Methods: A mouse model of AFL-induced dermal remodeling was established. RNA sequencing was used to identify the prominent features of AFL-treated tissues. Histological analysis, including H&E and Masson staining, ultrastructure observation by transmission microscopy, immunofluorescence, and quantitative real-time polymerase chain reaction were used for dermal remodeling analysis. Moreover, AFL-treated mice were intraperitoneally injected with antimouse Ly6G antibodies to deplete neutrophils. Neutrophil extracellular traps (NETs) were explored using immunofluorescence, transmission microscopy, and in vitro coculture experiments. Results: Dermal remodeling, characterized by an increased number of CD31-positve vessels and elevated messenger RNA (mRNA) expression of genes encoding transforming growth factor-β (TGF-β), collagen I, and collagen III, was observed at 15 days after AFL treatment. In the AFL-induced inflammation phase, RNA sequencing identified neutrophil chemotaxis, and degranulation genes were significantly enriched. Histology and immunofluorescence staining of human and mouse tissues harvested at Day 1 after AFL treatment revealed significant neutrophil infiltration surrounding thermal-induced microinjuries. Neutrophil depletion decreased the expression of stressrelated genes such as S100A8 and S100A9 in the early phase following AFL treatment. Importantly, neutrophil depletion enhanced dermal remodeling at Day 15, as reflected by enrichment of the extracellular matrix and collagen biosynthesis genes based on RNA sequencing. Moreover, increased collagen I, collagen III, and TGF-β mRNA expression, increased cell proliferation, and vascularity were observed. Interestingly, NETs, which could be induced by AFL-treated fibroblasts in vitro, were identified in both human and mouse tissues on Day 1 after AFL treatment. Conclusions: AFL-treated human and mouse skin recruited a large number of neutrophils. The neutrophil surge impaired dermal remodeling in mice. The microenvironment and fibroblast functional modulation mediated by neutrophil degranulation and NET formation were determined to be the underlying mechanisms. Our results indicate that modification of infiltrated neutrophil activity might be a potential therapeutic target for AFL-induced dermal remodeling.

Background: Although it is known that epidermal biophysical properties vary with age and gender, the changes in epidermal biophysical properties over the time from baby to adolescence have not been elucidated yet. In the present study, we assessed the trend of changes in transepidermal water loss rates (TEWL), stratum corneum hydration, and skin surface pH in Chinese children. Participants and methods:A total of 780 boys and 610 girls, aged 1 month to 17-year old, were enrolled in this study. TEWL and stratum corneum hydration on the forearm and the shin were measured with GPSkin Barrier, whereas skin surface pH was measured with portable skin pH meter.Results: Overall, TEWL and stratum corneum hydration levels decreased, whereas skin surface pH increased in children from 1-month old to 17-year old. Significant decline in TEWL was observed on both the forearm and the shin of girls, and the shin of boys aged 13-17-year old. Similarly, marked decline in stratum corneum hydration levels started at ages of 6-12-year old. In contrast, decline in skin surface pH was observed in both girls and boys aged one to 12-month old except on the forearm of boys. Afterward, skin surface pH remained either stable or slight increase except on the shin of boys aged >12 months to 3-year old.Conclusions: These results demonstrate that both TEWL and stratum corneum hydration levels decrease, whereas skin surface pH increases in children aged 1 month to 17-year old. The changes in these biophysical properties vary with age, gender, and body site.