Early-life regional and temporal variation in filaggrin-derived natural moisturizing factor, filaggrin-processing enzyme activity, corneocyte phenotypes and plasmin activity: implications for atopic dermatitis

Abstract

Background: Filaggrin is central to the pathogenesis of atopic dermatitis (AD). The cheeks are a common initiation site of infantile AD. Regional and temporal expression of levels of filaggrin degradation products [natural moisturizing factors (NMFs)], activities of filaggrin-processing enzymes [bleomycin hydrolase (BH) and calpain-1 (C-1)] and plasmin, and corneocyte envelope (CE) maturity in early life are largely unknown.

Objectives: We conducted a cross-sectional, observational study investigating regional and age-dependent variations in NMF levels, activity of proteases and CE maturity in stratum corneum (SC) from infants to determine whether these factors could explain the observed predilection sites for AD in early life.

Methods: We measured NMF using a tape-stripping method at seven sites in the SC of 129 children (aged < 12 months to 72 months) and in three sites in 56 neonates and infants (< 48 h to 3 months). In 37 of these neonates and infants, corneocyte size, maturity, BH, C-1 and plasmin activities were determined.

Results: NMF levels are low at birth and increase with age. Cheek SC, compared with elbow flexure and nasal tip, has the lowest NMF in the first year of life and is the slowest to reach stable levels. Cheek corneocytes remain immature. Plasmin, BH and C-1 activities are all elevated by 1 month of age in exposed cheek skin, but not in elbow skin.

Conclusions: Regional and temporal differences in NMF levels, CE maturity and protease activities may explain the predilection for AD to affect the cheeks initially and are supportive of this site as key for allergen priming in early childhood. These observations will help design early intervention and treatment strategies for AD.

Figure 1

Levels of total natural moisturizing factor (NMF), histidine (His), pyrrolidone carboxylic acid (PCA) and sum of trans‐urocanic acid (UCA) and cis‐UCA in the stratum corneum of children (median with interquartile range) in different age groups [< 48 h (n = 18), 48 h – 4 weeks (n = 18), 1–11 months (n = 41), 12–35 months (n = 25) and 36–72 months (n = 30)] on three body regions. Differences between age groups were determined by Kruskal–Wallis test followed by Dunn's multiple comparisons test. ***P < 0.001, **P < 0.01 and *P < 0.05.

Figure 2

Ratio of histidine (His) to urocanic acid (UCA) (median with interquartile range) (a–c) and linear regression between ratio of His to UCA and age of children [(d–f) up to 3 months of age; (g–i) from 3 to 72 months of age] on three body regions. For regression analysis the values were log‐transformed. Age groups: < 48 h (n = 18), 48 h – 4 weeks (n = 18), 1–3 months (n = 21), 4–11 months (n = 20), 12–35 months (n = 25), 36–72 months (n = 30). ***P < 0.001, **P < 0.01 and *P < 0.05 as determined by Kruskal–Wallis test followed by Dunn's multiple comparisons test. b, slope of the regression line; CI, confidence interval.

Figure 3

Ratio of cis‐urocanic acid (UCA) to total UCA (cis‐UCA + trans‐UCA) (median with interquartile range). Age groups: < 48 h (n = 18), 48 h – 4 weeks (n = 18), 1–3 months (n = 21), 4–11 months (n = 20), 12–23 months (n = 1), 24–35 months (n = 14), 36–47 months (n = 12) and 48–72 months (n = 18). Differences between age groups were determined by Kruskal–Wallis test followed by Dunn's multiple comparisons test. ***P < 0.001, **P < 0.01 and *P < 0.05.

Figure 4

(a–c) Corneocyte envelope (CE) maturity, stratum corneum (SC) cohesion, cornecyte surface area and activity of (d–f) bleomycin hydrolase (BH), calpain‐1 (C‐1) and plasmin in the SC of children up to 11 months of age (n = 37). Difference between cheek region and elbow flexure region were determined by two‐tailed paired t‐test (SC cohesion and CE maturity) or by two‐tailed Wilcoxon matched‐pairs signed‐rank test (corneocyte surface area, BH, C‐1 and plasmin). ****P < 0.0001, **P < 0.01.

Figure 5

Linear regression plasmin activity vs. ratio of cis‐urocanic acid (UCA) to total UCA (cis‐UCA + trans‐UCA) on two body sites. The values were log‐transformed. b, slope of the regression line; CI, confidence interval.

Figure 6

Corneocyte envelope (CE) maturity, stratum corneum (SC) cohesion and cornecyte surface area in the SC of children (median with interquartile range) in three age groups across two body regions. Age groups: < 48 h (n = 10); 48 h – 4 weeks (n = 10) and 1–11 months (n = 16). Differences between age groups were determined by one‐way anova followed by Tukey's multiple comparison test (for CE maturity and cornecyte surface area on cheek region Kruskal–Wallis test followed by Dunn's multiple comparisons test was used). *P < 0.05.

Figure 7

Cheek skin is slow to mature with respect to natural moisturzing factor (NMF) levels and corneocyte maturity. While other exposed sites, such as the nasal tip, rapidly reach steady‐state maturity in the early months of life, the cheek skin starts life with the lowest NMF and only reaches steady‐state NMF levels at approximately 7 years of age. NMF levels are graphically represented by depth of blue colour.