Low-grade disease activity in early life precedes childhood asthma and allergy

Asthma and allergies are today the most common chronic diseases in children and the leading causes of school absences, chronic medication usage, emergency department visits and hospitalizations, which affect all members of the family and represent a significant societal and scientific challenge. These highly prevalent disorders are thought to originate from immune distortion in early childhood, but the etiology and heterogeneity of the disease mechanisms are not understood, which hampers preventive initiatives and makes treatment inadequate. The objective of this thesis is to investigate the presence of an early life disease activity prior to clinical symptoms to understand the anteceding pathophysiological steps towards childhood asthma and allergy. The thesis is built on seven studies from the Copenhagen Prospective Studies on Asthma in Childhood (COPSAC2000) birth cohort examining biomarkers of disease activity in 411 asymptomatic neonates in cord blood (I-II), urine (III), exhaled breath (IV-V) and infant lung function (VI-VII) in relation to the subsequent development of asthma and allergy during the first seven years of life. In papers I-II, we studied cord blood chemokines and 25(OH)-vitamin D, which represent a proxy of the inborn immature immune system, the intrauterine milieu, and the maternal immune health during pregnancy. High levels of the Th2-related chemokine CCL22 and high CCL22/CXCL11 ratio were positively correlated with total IgE level during preschool age (II). This suggests an inborn Th2 skewing of the immune system in healthy newborns subsequently developing elevated total IgE antibodies, which is considered to increase the risk of asthma and allergies later in life. Additionally, deficient cord blood 25(OH)-vitamin D levels were associated with a 2.7-fold increased risk of recurrent wheeze at age 0-7 years (I). Together, these findings support the concept that early life immune programming in the pre-symptomatic era plays an essential role for promotion of or protection against asthma and allergies. Therefore, preventive initiatives to restore immune health, such as vitamin D supplementation, should be directed to the fetus and the earliest postnatal life. The eosinophil granulocyte has a major role in the allergic inflammatory cascade and eosinophilia is considered a hallmark of many allergic phenotypes. In paper III, we examined neonatal urinary biomarkers including eosinophil protein X (u-EPX), which is contained in the eosinophil granules. Elevated u-EPX in asymptomatic neonates was associated with development of allergic sensitization and nasal eosinophilia, but not with wheezing or asthma (III). These findings suggest the presence of an ongoing low-grade disease process in early life characterized by eosinophil activation prior to appearance of allergy-related conditions. In papers IV-V, we investigated perinatal and genetic predictors of neonatal fractional exhaled nitric oxide (FeNO) and the relationship between neonatal FeNO and wheezing later in child-hood. The a priori selected determinants encompassed asthma genetic risk variants, anthropometrics, demographics, socioeconomics, parental asthma and allergy, maternal smoking, paracetamol and antibiotic usage during pregnancy, and neonatal bacterial airway colonization. Among those, only the DENND1B risk allele and paternal history of asthma and allergy were associated with increased FeNO values (V) suggesting that raised FeNO in neonatal life is primarily an inherited trait. The neonatal FeNO levels were widely dispersed (1-67 ppb) and children with values in the upper quartile were at increased risk of recurrent wheezing in early childhood, but not persistent wheezing, reduced lung function or allergy-related endpoints (IV). This suggests that elevated neonatal FeNO represents an early asymptomatic low-grade disease process other than congenitally small airway calibre contributing to a transient wheezing phenotype. Reduced lung function in neonates is associated with wheezing and asthma proneness, but it is unknown if such host factor also confers a risk of acute bronchiolitis, which is considered an index event of asthma persisting into school age. In paper VI, we investigated neonatal forced flow, volume, and responsiveness to methacholine in relation to occurrence of acute severe bronchiolitis at age 0-2 years. Children developing bronchiolitis had a 2.5-fold increased bronchial responsiveness as neonates (VI) suggesting a preexisting joint propensity of the airways to react adversely to common respiratory viruses and to develop asthma. This finding proposes airway hyperresponsiveness as yet another marker of low-grade disease activity among asymptomatic neonates on a trajectory towards childhood asthma. In paper VII, we examined whether neonates with impaired pulmonary capacity also had signs of systemic inflammation prior to clinical symptoms. Reduced FEV0.5 was significantly associated with elevated serum hs-CRP and other blood inflammatory markers (VII) suggesting presence of systemic low-grade inflammation from the beginning of life. Chronic low-grade inflammation is a common nominator of virtually all the major non-communicable welfare diseases (NCDs) of modernity whereof asthma and allergies are the earliest debuting disorders. The novel finding of systemic low-grade inflammation among neonates at increased risk of asthma and allergy, therefore implies that exploring the origins of asthma and allergy may also unravel disease mechanisms involved in other NCDs. In conclusion, the series of papers presented in this thesis (I-VII) evidence the presence of a pre-symptomatic disease process measurable in several body compartments, which supports the notion of low-grade disease activity in early life as a generic trait among neonates developing asthma and allergy. This hypothesis piggybacking on single biomarker assessments could be enforced and refined by applying novel global omics approaches. In particular, metabolomic analyses of serum, urine, and airway lining fluid from neonates as well as neonatal VOC profiling of exhaled breath may facilitate a broader understanding of the early low-grade disease activity preceding clinical symptoms. Disentangling the introductory pathophysiological mechanisms and underlying endotypes of disease is paramount for generating successful preventive measures to alleviate the major global burden of asthma, allergy, and other NCDs of modern time.