Participant characteristics and environmental exposures
The Cincinnati Childhood Allergy and Air Pollution Study is comprised of 54.3% males, 76.6% Caucasians, 19.8% African-Americans, and 3.8% are bi-racial or other. Since the genotype frequencies of several studied single-nucleotide polymorphisms (SNPs) were significantly different between Caucasians and African-Americans, only Caucasians were evaluated as power was too low for other racial groups.
describes the eczema status and skin prick test (SPT) results among Caucasian children at ages 1–3. Eczema was present in 9.7% of children and increased to 29.1, and 26.7% at ages 2 and 3, respectively. The proportion that was SPT+ for one or more allergens were 28.1, 39.7, and 42.0% at ages 1, 2 and 3, respectively. The percentage of children who were SPT+ to aeroallergens alone increased over time, 13.9, 31.1, and 36.0%, while those who were SPT+ to food antigens alone decreased, 9.7, 2.5, and 1.2%, respectively. The percentage did not change appreciably for children who were SPT+ to both aero- and food allergens, with 4.5, 6.1, and 4.8% positive at ages 1, 2 and 3, respectively (data not shown).
Summary of eczema and SPT results of Caucasians in the CCAAPS cohort at ages 1–3.
A summary of environmental exposures is displayed in . At enrollment, almost 30% were exposed to environmental tobacco smoke in the home. Dog(s) were present in 38% of the homes, while 28% reported having at least one cat. During the first year of life, almost 60% of homes had visible mold, with 6% in the high mold category. Diesel exhaust particles (DEPs) exposure ranged from 0.23 μg m−3 to 0.88 μg m−3; high DEP was defined as the top quartile. Endotoxin levels obtained from home dust samples ranged from 6.0 to 800.0 EU mg−1, and the geometric mean was 73.2 EU mg−1.
Summary of environmental exposure status among CCAAPS cohort Caucasians
Environmental exposures, SPT results, and eczema
The associations between SPT results, environmental exposures, and eczema outcomes are displayed in . Children who were SPT+ to at least one aeroallergen during the first three years of life were over two times more likely to have eczema by age 3 (OR 2.69, 95% confidence interval (CI): 1.80–4.01, P < 0.001) and almost nine times more likely to have eczema at both ages 2 and 3 (OR 8.78, 95% CI: 3.87–19.92, P < 0.001). Children with persistent aeroallergen sensitization were over five times more likely to have eczema at both ages 2 and 3 (OR 5.25, 95% CI: 3.00–9.19, P < 0.001). A sub-analysis of children who were persistently positive to aeroallergens only and negative to food (n = 470) yielded similar results (OR 5.50, 95% CI: 2.73–11.05; data not shown). Children sensitized to foods were significantly more likely to develop eczema at age 1, by age 3, and at both ages 2 and 3 (P < 0.001).
Unadjusted associations between eczema, SPT testing, and environmental exposures among CCAAPS Caucasians
Children whose parents reported having a pet dog at enrollment were over 50% less likely to have eczema at age one (OR = 0.49, 95% CI: 0.25–0.95, P = 0.04). Children who had a pet dog had nearly a two-fold decreased risk of eczema at both ages 2 and 3 (OR = 0.54, 95% CI: 0.30–0.97, P = 0.04). To ensure there was no bias in dog exposures related to parental history of eczema, we examined this relationship and found no effect. There was an increased risk for eczema at age 1 with high DEP exposure (OR = 1.89, CI: 1.05–3.04, P = 0.03). For 95% all other environmental exposures, no significant association was observed with eczema during the first three years of life.
Association of CD14 and IL-4Rα with eczema
Seven SNPs were genotyped in four atopy-related genes. These genes were chosen because more than 10 independent studies (Ober and Hoffjan, 2006
) have shown associations with atopy or asthma phenotypes, and are biologically relevant by altering gene expression or function.
The associations of the CD14 C-159T and IL4Rα I75V SNPs with eczema and SPT+ are summarized in . The genotype CC of the SNP CD14 C-159T significantly increased the risk of eczema at both ages 2 and 3 (OR = 3.01 95% CI: 1.04–8.68, P = 0.01), and this trend was also observed for eczema at age 1 and eczema development by age 3. An increased risk of eczema by age 3 was also observed with the IV genotype of IL4Rα I75V (OR 1.77, 95% CI: 1.07–2.96, P = 0.03). This same trend was observed for all three eczema outcomes for both the IV and VV genotypes of the IL4Rα SNP, so these two genotypes were then combined. Children who carried both the CC and IV/VV variants (n = 64) were over two times more likely to develop eczema by age 3 (OR = 2.24, 95% CI: 1.27–3.94, P < 0.01), and were over three times more likely to have eczema at both ages 2 and 3 (OR = 3.44 95% CI: 1.56–7.57, P < 0.01). The association of the combination of these two SNPs with eczema remained significant even after Bonferroni correction. There were no significant associations observed with any of the other SNPs and eczema. The association of these SNPs with a positive SPT response was also examined. The combination of the two SNPs (CD14-159C/T and IL4Rα I75V) was not significantly associated with SPT+, but showed similar trend as observed with eczema (26.1% vs 17.3%, P = 0.10; data not shown).
Associations of CD14 C-159T and IL4Rα I75V SNPs with Eczema among CCAAPS Caucasians
Since allergen sensitization plays a critical role in eczema development, we evaluated the subgroup of children who were SPT+ and had eczema. Strikingly, children SPT+ and with eczema by age 3 were over three times more likely to have the genotype combination of CD14 C-159T CC and IL4Rα (IV + VV) compared with children without eczema who were SPT− (OR = 3.01, 95% CI: 1.37–6.63, P < 0.01; data not shown). The increased prevalence of this genotype combination between CD14 and IL4Rα among the SPT+ children with eczema supports the potential of combined genetic susceptibility to allergic disease and eczema.
CD14-159C/T and dog exposure association with eczema and SPT+
The associations of CD14 C-159T and dog exposure are displayed in . Of the 148 Caucasian children who had eczema at age 2, about half, 72 (48.6%), also had eczema at age 3. We evaluated the individual and combined effects of the CT and TT genotypes with dog exposure since they both confer protection in our data (see and ). The CT/TT genotype of CD14-159C/T was significantly associated with a 38% reduction in risk of eczema by age 3 (OR = 0.62, 95% CI: 0.40–0.97, P = 0.03) and an even stronger reduced risk (57%) of eczema at both ages 2 and 3 (OR = 0.43, 95% CI: 0.23–0.82, P = 0.01). Dog exposure also reduced the risk of eczema at=age 1 (OR = 0.49, 95% CI: 0.25–0.95, P = 0.04) as well as eczema at both ages 2 and 3 (OR 0.54, 95% CI: 0.30–0.97), P = 0.04). This same protective trend was observed with eczema development by age 3 (P = 0.10). When genetic and environmental factors that conferred decreased eczema risk were combined, an even stronger effect was observed. Children with the CD14 C-159T CT or TT genotype who lived in homes with dog(s) were almost 60% less likely to have eczema by age 3 (OR 0.56, 95% CI: 0.33–0.96, P = 0.04) and eczema at both ages 2 and 3 (OR 0.36, 95% CI: 0.14–0.89, P = 0.03) compared with all other children (children with the CC genotype and/or no dog exposure; data not shown). This association was even stronger for atopic eczema at both ages 2 and 3 (OR = 0.33, 95% CI 0.12–0.89, P = 0.03; data not shown). Thus, dog exposure in early life and the CT or TT genotype of CD14-159C/T SNP both provide protection from the development of eczema in young children. There were no significant associations between genetics and DEP exposure with eczema risk.
C-589T, dog exposure and eczema. Unadjusted associations between eczema and CD14 C-589T (a) and dog exposure (b) among CCAAPS Caucasians. *denotes P < 0.05.
Similarly, an analysis with persistent SPT+ as the outcome was evaluated. Of the 207 Caucasian children who were SPT+ at age 2, 126 (60.9%) remained SPT+ at age 3. There were no significant associations found between CD14 C-589T and dog exposure with persistent allergen sensitization.
Adjusted models of eczema
We also evaluated models of the three eczema outcomes including dog exposure, persistent aeroallergen sensitization, food sensitization, and the CD14 C-159T and IL4Rα I75V SNPs, adjusted for gender and parental history of eczema (data not shown). Persistent aeroallergen sensitization was the most significant predictor of eczema at both ages 2 and 3 (OR = 14.39, 95% CI 4.40–47.10, P < 0.001), and there was a significant interaction between persistent aeroallergen sensitization and the CD14-159T CC genotype (OR 4.66, 95% CI 1.41–15.38, P = 0.01). This indicates that children who carry the SNP and are persistently sensitized to aeroallergens are at the greatest risk for persistent eczema, suggesting a role for innate immunity. We observed the same significant associations in the model of eczema development by age 3. When stratified by pet dog ownership, the interaction between persistent aeroallergen sensitization and the CD14 CC genotype remains significant only for children without a pet dog presumably because this was a protective factor. Food sensitization was the most significant predictor of eczema at age 1 (OR = 2.73, 95% CI: 1.03–7.23, P = 0.04).