INTRODUCTION
Cystic Fibrosis (CF) is the most frequent fatal genetic disease among Caucasian populations, with over 80,000 patients worldwide [1]. CF is an autosomal recessive disorder caused by one of more than 2,000 variants in the gene encoding the CF transmembrane conductance regulator (CFTR) anion channel. As such, newborn screening is now being widely implemented to diagnose children shortly after birth [2]. While multiple organs are affected in patients with CF, the majority of morbidity and mortality is due to airway disease [3]. Mutations in the CFTR protein, which is mainly expressed in epithelial cells, disrupts ion flow, resulting in early and sustained airway inflammation and mucus abnormalities [4]. This environment is also highly susceptible to chronic infection by opportunistic pathogens [5].
While recently approved CFTR-targeted modulator therapies are effective at restoring function of mutant channels, the control of ongoing inflammation remains a challenge [6]. Therefore, understanding and tracking early events in CF airway inflammation is essential for improving patient outcomes. In prior studies, extracellular neutrophil elastase (NE) activity measured in bronchoalveolar lavage (BAL) fluid at 3 months of age was found to be more effective than infection status and mucus obstruction at predicting lung damage at 1 and 3 years of age [7, 8]. Most of the extracellular NE activity in CF airway fluid stems from the active exocytosis of NE-rich granules by neutrophils acquiring the GRIM (granule releasing, immunomodulatory, and metabolically active) phenotype upon recruitment to CF airways [9, 10]. Live GRIM neutrophils are identifiable in BAL of young CF children and correlate positively with structural lung damage [11], as quantified by chest computed tomography (CT) scans [12].
While BAL is the gold standard for assessing CF airway inflammation [13], it is an invasive procedure that requires sedation and bronchoscopy, which often limits its use to clinically indicated procedures [14]. To better understand early events in CF airway inflammation, more frequent longitudinal sampling of young children using minimally invasive techniques would be desirable. One such option is induced sputum (IS), which has been used successfully to collect airway samples from children [15] and adults [9, 16] with CF. An important difference is that BAL primarily samples material from distal airways, while IS samples material from both distal and proximal airways. Recently, a study assessing the ability of IS to yield a relevant picture of lower airway microbiology in young children and adolescents with CF found similarities to BAL, but also notable differences [17]. With regard to inflammation, IS in adults with CF has shown similarities to BAL based on immune mediator levels, total leukocyte count and subset frequencies [16]. Critically, IS has not yet been used to study molecular and cellular biomarkers of inflammation in early CF lung disease.
To address this gap, we conducted a cross-sectional, single-center study of 2-year-olds with CF with sequential IS, BAL, and blood collections at the same visit. At this age, most patients have minimal structural lung damage, yet some have already developed signs of inflammation [18]. For BAL, we collected samples from the right middle lobe (RML) as a consistent measurement, and a separate sample collected from the lingula (LIN) [19, 20]. Via the analysis of soluble immune mediators and NE, leukocyte subsets and phenotype, and assessment of structural damage by chest CT scans, we demonstrate that IS can be used to detect early CF airway inflammation and lung damage, and document similarities and discrepancies with BAL fractions. Some of these results have been previously reported in the form of an abstract [21].