The cumulative effect of inflammation and infection on structural lung disease in early cystic fibrosis

Structural lung disease

The variability in structural lung disease extent between patients seen in this study is consistent with the heterogeneous nature of CF lung disease. Nevertheless, as expected, an overall progression over time was observed, with %Disease increasing by around 0.5 per year. Furthermore, to this end we have presented z-score equations for all PRAGMA-CF outcomes. Although this provides a useful estimate for future studies in this age group and may inform clinical trial design, all children in this study were part of a standardised, extensive early surveillance programme. Therefore, these results may not be directly comparable to other centres with different monitoring and treatment regiments.

The repeatability of %Bronchiectasis in this study was somewhat lower than originally reported in the PRAGMA-CF publication [10]. This may be the result of the longer time period over which CT scans were collected, and thus a greater variety in CT settings and image quality. Because bronchiectasis is rare in young children compared with other abnormalities such as bronchial wall thickening, its extent tends to be quite low (in this study 0.37% of the lung) and thus it is subject to greater variability. In young children with small airways, determining the airway/vessel ratio (and thus bronchiectasis) can be a matter of one or two pixels on the CT scan. Nevertheless, because the scans were allocated randomly and scorers were blinded to patient details, clinical status or BAL results, variability in %Bronchiectasis is likely to add noise to our measurements but not bias the results.

Relationships between CT and inflammation/infection

As expected, we found that all CT outcomes were associated with detectable free NE activity, IL-8 concentration and infection presence in cross-sectional analyses, other than between %Bronchiectasis and infection. We also found that CT outcomes increased in the 1-year interval prior to the detection of NE activity or infection. These results are consistent with previous studies relating CT to lower airway samples [2, 10, 16, 17]. The most important finding from this study was that structural lung disease extent was associated with the number of previous visits with BAL-detected NE activity, but not pulmonary infection. There is currently no consensus on whether pulmonary inflammation in young children is a transient effect with little long-term consequence or a clinically important finding that warrants treatment. The results of this study seem to suggest the latter, although prospective clinical trials are needed to confirm whether structural lung disease can be prevented with targeted interventions.

Comparison between inflammation and infection

The results of this study show that inflammation, particularly free NE activity, has a stronger correlation with and larger effect on structural lung disease than infection. Therefore, treating lung inflammation has additional potential for reducing structural lung disease compared with using anti-infective therapies alone. Despite this, detection of NE activity or another marker of lung inflammation is rarely acted upon, whereas pulmonary infection with, for example, P. aeruginosa is often considered to be a clinically significant finding necessitating eradication therapy [18, 19]. Numerous studies have shown that eradication of this organism when first detected leads to delayed onset of chronic infection and improved clinical outcomes throughout childhood [20–24], and there is mounting evidence that eradication of other organisms such as S. aureus may also be indicated [25]. However, aggressive Pseudomonas eradication strategies fail to prevent the high prevalence of bronchiectasis seen at 5 years of age [26], suggesting that chronic inflammation, regardless of the triggers, is a critical factor in the development of structural lung damage [27]. Prophylactic anti-inflammatory therapy is rarely included in routine management early in life, although in children aged >6 years and in adults there is evidence for clinical benefits and its use is included in some clinical guidelines [28, 29]. However, although the side-effects profile of ibuprofen limits its use in newborns and infants, there are significant opportunities for repurposing a number of alternative anti-inflammatory molecules. The prevention of bronchiectasis in infants with CF study (COMBAT-CF; ClinicalTrials.gov identifier NCT01270074) of prophylactic azithromycin from diagnosis provides a template for studies of repurposed molecules to prevent lung disease. Such an approach could result in the relatively expeditious implementation of new anti-inflammatory interventions for young children with CF.

Reference equations

The reference equations in this study are CF specific, in that they capture the “usual” trajectory of lung disease in CF in a population undergoing “standard” treatment. These values may add clarity when comparing consecutive CT scans, to differentiate between natural disease progression and a potentially clinically significant change. Additionally, these values may aid in clinical trial design, e.g. when determining a clinically relevant treatment effect. However, it is well recognised that there exists significant variability in patient outcomes between countries, centres and even individual clinicians. The equations presented are only directly applicable to the centres in this study; other centres should use them only as a guide, ideally developing their own using the methods in this study. With the advent of CF transmembrane conductance regulator modulators and other disease-modifying medications, none of which were available for young children during the period of this study, consideration of their use should be taken into account when using these reference ranges.

Limitations

Although both NE activity and IL-8 are important indicators, their absence does not imply an absence of pulmonary inflammation. In addition, IL-8 was present in the majority of BALs and thus the number of times it is detected is not a particularly informative measure. The binary classification of NE activity into present/absent may mask the effects of inflammation severity on structural lung disease. A comprehensive survey of biochemical measures and cell counts would provide a more holistic view of lung inflammation. Furthermore, BAL samples are taken from individual lobes and may not be representative of the entire lung. Nevertheless, free NE activity in particular is a known risk factor for structural lung disease in early life CF [2] and its repeated detection from BAL provides a simple yet meaningful summary of inflammatory history.

Infection was detected using culture methods, as per routine clinical practice. Because bacterial cultures are not as sensitive to detect pathological organisms as other methods, e.g. gene sequencing or mass spectrometry [30], it is likely that the bacterial infection was underestimated using this study. The prohibitive cost and lack of availability of these techniques preclude their use in this retrospective analysis. However, the clinical impact of these infections that are unable to be cultured is not yet known, whereas culture of bacteria from BAL is a known prognostic indicator, and thus these results still provide a compelling argument for the importance of early life infection and inflammation in the pathogenesis of structural lung disease.

Our data were acquired as part of annual surveillance snapshots and thus may not accurately reflect the course of CF lung disease in the intervening year between visits. However, the strength of this approach is that the full spectrum of CF lung disease is represented, unlike studies recruited based upon episodes of acute disease. Because visits were scheduled at regular intervals while the children were clinically well, selection bias is unlikely to affect the results presented.

Due to differences in clinical protocol, CT scans were only performed every second year in Melbourne, but annually in Perth. As a result, the associations reported will be more heavily weighted towards data from the Perth centre, with the 1-year interval data exclusively from Perth. Both centres performed BAL annually, so the cumulative inflammation and infection results are not affected, and therefore although there is a Perth bias, this does not affect the interpretation of our results.

Finally, there were a number of visits where inflammation and/or infection measurements were not available, either through failure to collect adequate sample or nonattendance at the scheduled BAL. Missed visits will result in an underestimation of inflammatory/infective history, particularly if the visit was missed due to the patient being too unwell. However, the number of missed visits was small and unlikely to influence the results substantially.

Conclusions

We have shown that repeated detection of neutrophilic inflammation plays an important and ongoing role in the trajectory of structural lung disease throughout the first 6 years of life. Although infection is generally treated aggressively, inflammation is not routinely measured nor acted upon. These data present compelling evidence that pulmonary inflammation, particularly NE activity, throughout early life is a key prognostic indicator. Therefore, early-life anti-inflammatory prophylaxis could be a useful strategy to improve long-term outcomes and further trials aimed at preventing lung disease are urgently needed in this young age group.