Factors associated with preschool wheeze developing into school-age asthma

Abstract

Preschool wheeze affects one third of all toddlers up to the age of three years and half of the children before six years of age. Approximately one third of these children will develop asthma in school age, and several risk factors have been proposed. However, as of today, it is not possible to reliably predict which children, with preschool wheeze, will develop asthma. All four studies in this thesis are based on the Gene Expression in Wheezing and Asthmatic Children (GEWAC) study, a longitudinal case-control study in which 156 cases and 102 healthy controls, ages 6-48 months, were included. The cases were recruited from the pediatric emergency department at Astrid Lindgren’s Children’s Hospital in Stockholm, when seeking care for an episode of acute wheeze. They came to a revisit after approximately 3 months and were followed annually up to the age of 7 years and a follow-up at age 11. The age-matched healthy controls were recruited from the same hospital at the surgical day-care ward and came to a follow-up at ages 7 and 11 years. The study protocol included nasopharyngeal swabs for viral detection at inclusion, blood sampling, questionnaires, physical examination, measurements of lung function and fractional exhaled nitric oxide (FeNO).

Study I consisted of 113 children with an episode of preschool wheeze (cases) and 52 healthy controls who came to the 7-year follow-up. The prevalence of asthma at age 7 was 70.8 % among cases and 1.9 % in healthy controls. Rhinovirus-induced preschool wheeze was more common among cases with asthma in comparison to cases without asthma at age 7 years (48.1 % vs. 21.9 %, p = 0.011; OR 3.3, 95 % CI 1.3-8.5) and this association remained after adjustment for infection with other viruses (OR 3.8, 95 % CI 1.4-10.5). Cases with asthma at age 7 years were admitted to hospital more often because of respiratory difficulties (p = 0.024) and spent more time hospitalized (p = 0.01) during the year after inclusion in the study. In study II we evaluated 107 cases and 46 healthy controls at the 11-year follow-up. We found that 62.6 % of cases and 13.0 % of healthy controls had asthma at age 11 years. Early life factors associated with asthma at age 11 years, among cases, were rhinovirus-induced wheeze (OR 2.4, 95 % CI 1.02-5.6) and allergic sensitization at 2 years of age (OR 2.9, 95 % CI 1.05-8.1). However, in multivariate logistic regression only allergic sensitization at age 2 years (adjusted OR 3.0, 95 % CI 1.02-8.7) and parental heredity for asthma and/or allergy (adjusted OR 3.4, 95 % CI 1.1-9.9) were associated with asthma at age 11 years. Cases with both rhinovirus-induced wheeze at inclusion and allergic sensitization at age 7 years had a higher prevalence of asthma at age 11 years, in comparison to cases with rhinovirus-induced wheeze at inclusion but without allergic sensitization at age 7 years (92.9 % vs. 57.1 %, p = 0.03). In study III we measured 92 inflammatory-related plasma proteins during an episode of acute preschool wheeze in 145 cases and compared them to 101 healthy controls. With unsupervised clustering we found that the ten most differentially expressed inflammatory-related proteins could almost entirely separate cases from healthy controls. Seven proteins exhibited a higher expression in cases (OSM, IL-10, IL-6, CXCL10, FGF21, AXIN1 and SIRT2) and three proteins had a lower expression (TNFSF11, TNF-b and CASP8), in comparison to healthy controls. These proteins are implicated to be involved in airway epithelial dysfunction, airway remodelling, viral defence, and type 2 inflammation. Among the ten proteins, three (FGF21, SIRT2 and IL-10) were still differentially expressed between cases and controls at the revisit 3 month later. Finally, in study IV we investigated sensitization to multiple allergen molecules longitudinally and its relation to asthma development at 7 years using the multiplex ImmunoCAP ISAC measuring 112 allergen molecules. In this study 72 cases were included and 43 healthy controls. Sensitization to each additional allergen molecule from preschool age to 7 years was associated with asthma at 7 years (OR 1.2; 95 % CI 1.01-1.5). The median number of sensitizing molecules increased from 3 (1-14) at inclusion to 10.5 (1-21) at 7 years of age among sensitized cases with asthma at 7 years of age (p = 0.038). No significant increase was seen in cases without asthma (p = 0.26). Lastly, the number of sensitizing allergen molecules at 7 years was associated with asthma at the same age (OR 1.2; 95 % CI 1.02-1.42).

In summary, we found rhinovirus-induced wheeze to be associated with asthma at both 7 and 11 years of age, although probably acting as an unveiling factor in children already predisposed to asthma development. We highlighted the importance of allergic sensitization in asthma development with molecular spreading and polysensitization being involved in disease development. Finally, we found ten inflammatory-related plasma proteins that could contribute to the understanding of why preschool wheeze is a risk factor for asthma development.