Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the gene for CFTR, a chloride channel that regulates salt and water transport across epithelial cells (1). CFTR dysfunction results in dehydrated airway secretions in the lungs of people with CF (PwCF), which promotes infection, inflammation, and premature death from respiratory failure (2, 3). The pathophysiology of CF includes episodes of worsened respiratory and/or constitutional signs and symptoms called pulmonary exacerbations (PEXs). PEXs undermine health-related quality of life (HRQoL) (4, 5), accelerate loss of lung function (6, 7), increase risk of death or lung transplant (7), and are costly, as they usually require hospitalization (8). In 2018, one-third of the 30,775 PwCF in the Cystic Fibrosis Foundation Patient Registry (CFFPR) had PEXs treated with intravenous (IV) antibiotics (9), underscoring the prevalence of these events and the substantial resources required for their treatment.
For reasons that remain incompletely understood, there is marked heterogeneity to outcomes of guideline-driven PEX treatment (10) in the U.S. CF population. Registry-based (11), single-center (12), and multicenter (13) studies have shown that 15-35% of patients do not regain at least 90% of their baseline lung function (percent-predicted forced expiratory volume in one second, FEV1%). Female sex, malnutrition, CF-related diabetes (CFRD), lung infection by multidrug-resistant pathogens, and large declines in lung function prior to treatment have been associated with suboptimal outcomes, including incomplete recovery of FEV1% (11-13). Studies have also determined that the magnitude of FEV1% recovery is higher when PEXs are treated in the hospital rather than in the outpatient setting (14, 15) and that 30-day risk of re-treatment with IV antibiotics is higher if patients are never treated in the hospital (16). Because clinical practices vary among, and even within, CF care centers (13, 17, 18), and PEX treatment outcomes remain heterogeneous, there is an unmet need to characterize patient phenotypes and correlative biochemical indices that clinicians might use to improve the efficiency and accuracy with which they diagnose and treat PEXs and to personalize therapies (19, 20). Trends in these indices could complement serial symptom scores in individuals with poor baseline lung function who are less likely to have significant treatment-related increases in FEV1% (13), thus providing additional objective evidence of recovery that might be leveraged to reduce overtreatment.
Inflammation is a pathophysiologic hallmark of CF (21), but limited information exists about correlations between inflammatory mediator levels and lung function (22-24) or HRQoL (22) during PEXs because few studies have monitored PwCF throughout a complete PEX cycle (i.e., from baseline health to PEX onset and serially during PEX treatment to recovery). Accordingly, there remains a need to identify biochemical tests that are easily performed in clinical laboratories and are able to identify groups of PwCF with different physiologic and symptomatic responses to PEX treatment. Concentrations of certain cytokines and protein effectors of inflammation in blood and sputum from PwCF measured before, during, and after PEXs could have diagnostic and prognostic utility (12, 24-27). With the exceptions of calprotectin and C-reactive protein, clinical laboratories do not routinely quantify these substances in biological samples, which is a barrier to their widespread application as tools to monitor PEX treatment responses (28, 29).
Interleukin-6 (IL-6), a pro-inflammatory cytokine found at high concentrations in blood and sputum from PwCF (30), stimulates the liver to produce hepcidin-25 (31), a hormone that reduces blood iron levels by attenuating gastrointestinal iron absorption (32) and triggering mononuclear cells to sequester iron (33). In adults with CF, we found that serum iron levels were lower and sputum iron, serum IL-6, and serum hepcidin-25 levels were higher immediately before PEX treatment (34), a pattern consistent with the mechanisms by which IL-6 and hepcidin-25 link iron homeostasis to inflammation (35). A limitation of our previous work (34) was the omission of serial assessments of lung function and HRQoL to compare to biomarkers of iron homeostasis.
The current study had a master aim of phenotyping cohorts of adults with CF with respect to HRQoL, FEV1% and other clinical metrics, and laboratory indices of iron homeostasis measured at pre-specified points during a complete PEX cycle. Assuming, as the literature would suggest (11-13), that our study participants would heterogeneously benefit from PEX treatment, we tested a primary hypothesis that those with non-sustained improvements in HRQoL would have non-sustained improvements in lung function and those with sustained improvements in HRQoL would have sustained improvements in lung function, thus establishing two cohorts with distinct clinical responses. We then tested a secondary hypothesis that biomarkers of iron homeostasis in the cohort with non-sustained improvements in HRQoL and lung function would reflect an unresolved inflammatory state at the end of PEX treatment characterized by higher levels of serum IL-6, serum hepcidin-25, and sputum iron, and lower levels of serum iron than those found in the cohort with sustained HRQoL and lung function responses.