Rebuttal from Stephen M. Stick and André Schultz

Figueira and colleauges provide a very useful summary regarding cystic fibrosis transmembrane regulator (CFTR) function and pH homeostasis in the body including elements not directly relevant to the lung. However, the arguments they present do not address the fundamental proposition that ‘mucosal acidification drives early progressive lung disease in cystic fibrosis’. In fact, the authors use data from the cystic fibrosis (CF) pig model (Pezzulo et al. 2012) to argue a mechanism that is not supported by human data, namely ‘CFTR‐mediated anion secretion initiates chronic lung infections’. Crucial to this debate is the accumulating strong evidence from human studies that inflammation is likely to precede infection in the first months of life (Sly et al. 2009, 2013). Sly and colleagues reported that 85% of infants with CF diagnosed following newborn screening had evidence of lung disease at 3 months of age (Sly et al. 2009). Few of these infants had evidence of infection with pathogens in bronchoalveolar lavage (BAL) fluid using standard microbiological techniques. Recent studies using sensitive RNA sequencing techniques suggest a transition from an essentially sterile airway microenvironment to one that is dominated by recognised CF lung pathogens at a time when airway inflammation is already established (Muhlebach et al. 2018), although the associations of microbiota with inflammation and lung damage are complex (Frayman et al. 2017). Nonetheless it seems highly unlikely that pathogen colonisation that is undetectable by BAL and conventional culture techniques can be responsible for the extent of early lung disease that is identified by computed tomography and BAL (Sly et al. 2009). On the contrary, the most convincing data from animal models, supported by carefully acquired human data, suggest that the earliest inflammatory responses can be independent of infection. In this regard, the overexpressed β‐subunit of the epithelial sodium channel (βENaC) mouse model has proven particularly informative. Mall and colleagues demonstrated that these neonatal mice exhibit mucus plugging, goblet cell hyperplasia and neutrophilic inflammation (Mall et al. 2004) associated with reduced airway surface liquid (ASL) volume (Mall et al. 2010) due to excess epithelial sodium absorption. These features are present in the absence of spontaneous bacterial infection but are associated with reduced bacterial clearance when pathogens are instilled postnatally. Furthermore, this group later demonstrated that local tissue hypoxia secondary to mucus plugging is a potential mechanism for sterile inflammation (Fritzsching et al. 2015). Data from in vivo human studies supported by animal studies therefore suggest that the basic CFTR defect which results in ASL dehydration, mucus plugging and hypoxia can drive early disease and inflammation in the absence of infection.

The main argument proffered by Figueira and colleagues to support the assertion that ‘mucosal acidification drives early progressive lung disease’ assumes that infection precedes inflammation in infants and children, and therefore, in terms of this debate is both mis‐directed and highly questionable. However, the question of whether abnormal apical epithelial bicarbonate transport initiates progressive airways disease in CF is worthy of further discussion. We agree with Figueira that abnormal CFTR‐related bicarbonate transport has been well demonstrated in CF. However, abnormal bicarbonate transport does not necessarily equate to reduced ASL pH that drives airway disease. Previous studies that demonstrated reduced ASL pH in vitro were conducted after relatively large volumes of unphysiological solutions were added to the apical surface (Coakley et al., 2003; Garland et al. 2013). In contrast, our in vitro experiments that indicate normal ASL pH were undertaken in stable conditions, in a precisely controlled temperature and CO₂ environment and in different laboratories with two different types of pH probes and different types of primary cell cultures (Schultz et al. 2017). These experiments indicated that potential trans‐epithelial bicarbonate gradients, due to defective apical bicarbonate transport, are effectively shunted out by paracellular pathways. Consequently, when ASL pH is measured, using probes specifically designed to operate optimally and with precision in a mucoid, liquid environment, pH is not reduced (Schultz et al. 2017) in the lower airways of young children with CF: ergo mucosal acidification does not drive early progressive lung disease.

Call for comments

Readers are invited to give their views on this and the accompanying CrossTalk articles in this issue by submitting a brief (250 word) comment. Comments may be submitted up to 6 weeks after publication of the article, at which point the discussion will close and the CrossTalk authors will be invited to submit a ‘LastWord’. Please email your comment, including a title and a declaration of interest, to jphysiol@physoc.org. Comments will be moderated and accepted comments will be published online only as ‘supporting information’ to the original debate articles once discussion has closed.

Additional information

Competing interests

None declared.

Author contributions

Both authors have read and approved the final version of this manuscript and agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.