American Journal of Respiratory and Critical Care Medicine

Rationale: Idiopathic pulmonary fibrosis (IPF) is a progressive and inevitably fatal condition for which there are a lack of effective biomarkers to guide therapeutic decision making.

Objectives: To determine the relationship between serum concentrations of the cytokeratin fragment CYFRA 21-1 and disease progression and mortality in individuals with IPF enrolled in the Prospective Observation of Fibrosis in the Lung Clinical Endpoints (PROFILE) study.

Methods: CYFRA 21-1 was identified by immunohistochemistry in samples of human lung obtained at surgery. Concentrations of CYFRA 21-1 were measured using an ELISA-based assay in serum samples collected at baseline, 1 month, and 3 months from 491 individuals with an incident diagnosis of IPF who were enrolled in the PROFILE study and from 100 control subjects at baseline. Study subjects were followed for a minimum of 3 years after their first blood draw.

Measurements and Main Results: CYFRA 21-1 localizes to hyperplastic epithelium in IPF lung tissue. Peripheral CYFRA 21-1 concentrations were significantly higher in subjects with IPF than in healthy control subjects in both the discovery (n = 132) (control: 0.96 ± 0.81 ng/ml; vs. IPF: 2.34 ± 2.15 ng/ml; P < 0.0001) and validation (n = 359) (control: 2.21 ± 1.54 ng/ml; and IPF: 4.13 ± 2.77 ng/ml; P < 0.0001) cohorts. Baseline concentrations of CYFRA 21-1 were able to distinguish individuals at risk of 12-month disease progression (C-statistic, 0.70; 95% confidence interval, 0.61–0.79; P < 0.0001) and were predictive of overall mortality (hazard ratio, 1.12 [95% confidence interval, 1.06–1.19] per 1 ng/ml increase in CYFRA 21-1; P = 0.0001). Furthermore, 3-month change in concentrations of CYFRA 21-1 separately predicted 12-month and overall survival in both the discovery and validation cohorts.

Conclusions: CYFRA 21-1, a marker of epithelial damage and turnover, has the potential to be an important prognostic and therapeutic biomarker in individuals with IPF.

Scientific Knowledge on the Subject

Idiopathic pulmonary fibrosis (IPF) is a progressive and invariably fatal condition. There is a lack of validated prognostic or theragnostic biomarkers to guide treatment in IPF. Measurements of physiological parameters, including FVC and DlCO, are currently the best determinants of prognosis and disease behavior for patients with IPF. However, both baseline and short-term serial measures of FVC and DlCO are poor determinants of subsequent progression and death in individual patients with IPF.

What This Study Adds to the Field

CYFRA 21-1 is a cytokeratin fragment released by proteolytic degradation after apoptosis or necrosis of a wide range of lung epithelial cells. Serum concentrations of CYFRA 21-1 distinguish subjects with IPF from healthy control subjects. Baseline serum concentrations of CYFRA 21-1 significantly differ between subjects with IPF with progressive disease over the subsequent 12 months and those with more stable disease. Serial change in CYFRA 21-1 over 3 months identifies those individuals with IPF at greatest risk of death.

Idiopathic pulmonary fibrosis (IPF) is a progressive, life-shortening, fibroproliferative disease. Current clinical tools permit only limited assessment of IPF-related prognosis, fail to define an individual’s response to treatment, and necessitate large and lengthy clinical trials to determine the effectiveness of novel therapies (1).

IPF arises as the consequence of an aberrant wound-healing response driven by persistent or repeated microinjuries to the alveolar epithelium in individuals rendered susceptible by ageing or genetic predisposition (2). We and others have highlighted the biomarker potential of measures of epithelial damage in individuals with IPF (35).

Keratins are the principal structural elements of the cytoskeleton of epithelial cells. Cytokeratin-19 is expressed by airway epithelial cells; its proteolytic degradation results in release of the soluble Cytokeratin-19 fragment CYFRA 21-1, which is detectable in both airways and the systemic circulation (6, 7). Accelerated CYFRA 21-1 degradation occurs in rapidly proliferating and neoplastic epithelium (8). Serum concentrations of CYFRA 21-1 are elevated in individuals with lung cancer and systemic sclerosis–associated interstitial lung disease (8, 9). In IPF, concentrations of CYFRA 21-1 measured in airway lining fluid obtained at bronchoscopy are elevated compared with healthy individuals, and higher concentrations associate with poorer prognosis (10).

In this study, we assessed the prognostic potential of serum CYFRA 21-1 in individuals with IPF recruited into the Prospective Observation of Fibrosis in the Lung Clinical Endpoints (PROFILE) study.

Subject Recruitment and Sampling

Incident cases of IPF, diagnosed according to international guidelines, were recruited prospectively as part of the PROFILE study through two coordinating centers in the United Kingdom: Nottingham University Hospitals, Nottingham (NCT01134822) and Royal Brompton Hospital, London, United Kingdom (NCT01110694) (11). As previously described, subjects were assessed at baseline, 1 month, 3 months, 6 months, and annually for 3 years (12). Serum was collected at serial timepoints from PROFILE participants, and single samples were collected from healthy control subjects. Samples were processed within 2 hours of collection and stored at −80°C until assayed.

FVC and DlCO were recorded at baseline, 6 months, and 12 months. The discovery cohort included the first 132 subjects recruited into PROFILE and 50 healthy control subjects. A validation cohort consisted of the next 359 individuals from the PROFILE cohort and a further 50 healthy control subjects.

CYFRA 21-1 Assay

Analysis of serum CYFRA 21-1 concentrations was performed using a competitive sandwich ELISA (DRG Diagnostics, catalogue no. EIA-5070) according to manufacturer instructions. A single aliquot for each subject at each timepoint was used for all assays to avoid repeated freeze/thaw. The discovery and validation cohorts were analyzed as two separate batches. Samples were analyzed blinded to associated clinical data.

Immunohistochemistry

Immunohistochemical staining was performed on formalin-fixed, paraffin-embedded sections of human lung tissue, obtained from Royal Brompton biobank (ethics approval number; 15-SC-0101), using the avidin–biotin antibody complexing method. After rehydration and antigen retrieval (conditioning agent, Ventana Ltd.) CYFRA-positive cells were detected using primary antibodies against CYFRA 21-1 (1:500 concentration, catalog no. orb81975, Biorbyt Ltd.). Staining was shown to be absent in sections using NCL-L-TTF-1 isotype-matched control antibody (Leica Biosystems).

Statistical Analysis

Disease progression was defined as all-cause mortality or a decline of >10% in FVC within the first year after the baseline visit. For patients without 12-month FVC data, progression was deemed to have occurred if a >10% decline was observed at any time within 6 months. Where no lung function data were available beyond baseline, cases were adjudicated, after case note review, by the local principal investigator blinded to biomarker results. A total of nine subjects were adjudicated for progression: one subject was adjudicated to have progressed and eight were adjudicated as stable (13). Missing lung function data were not imputed. Before analysis, biomarker values were log10 transformed to maintain the assumption of normality. A general linear model was used to evaluate the association between group (IPF/healthy) and biomarker. A repeated-measures, mixed-effects model was used, with biomarker value as the dependent variable and progression status, visit, and visit by progression status as explanatory variables. All analyses were adjusted for age, sex, site, baseline lung function, and smoking status. The covariance structure was constrained for measurements that were repeated within participant, with visit as a repeated effect, and with an unstructured covariance matrix. A proportional hazards model was used to assess the association between time to death or censoring and baseline CYFRA 21-1 concentrations. The censor date was June 1, 2020 for both cohorts. To identify whether CYFRA 21-1 concentrations changed dynamically in response to progression of fibrosis, cohorts were dichotomized into those with either rising concentrations or stable or falling concentrations over 3 months on the basis of paired samples at 0 months and 3 months. The gradient of CYFRA 21-1 over 3 months was first treated as a continuous explanatory variable; a separate analysis used a binary version of the gradient dichotomized by whether this was rising (>0) or falling (⩽0).

Immunohistochemical Localization of Cytokeratin-19

Localization of cytokeratin-19 was assessed in paraffin-embedded sections from 20 IPF surgical lung biopsies and 10 control samples from histologically normal lung obtained at tumor resection. Consistent with previous observations (8), there was minimal expression of cytokeratin-19 in normal lung tissue (Figure 1A). In sections from subjects with IPF, there was only limited staining in areas of histologically normal lung tissue, with small numbers of cytokeratin-19–positive cells in the alveolar epithelium as well as in macrophages (Figure 1B). There was a marked increase in cytokeratin-19 staining in fibrotic areas, with positive cells appearing to be bronchial epithelium and hyperplastic epithelial cells lining honeycomb cysts (Figure 1C).

CYFRA 21-1 Distinguishes Healthy Control Subjects from Subjects with IPF

The discovery cohort comprised 132 subjects with IPF and 50 healthy control subjects; the validation cohort consisted of 359 individuals with IPF and a further 50 healthy control subjects who were on average slightly younger than the IPF population (Table 1). Both IPF cohorts were predominantly male (78% and 77%), with mean ages of 69.8 ± 8.2 and 70.9 ± 8.1 years. Median follow-up was 1,005 and 1,370 days in the discovery and validation cohorts, respectively.

Table 1. Demographics

 DiscoveryValidation
IPF (n = 132)Control (n = 50)IPF (n = 359)Control (n = 50)
Age, yr69.8 ± 8.255 ± 9.770.9 ± 8.163.4 ± 7.9
Male sex103 (78)35 (70)277 (77)40 (80%)
Smoker, current or former94 (72)NA249 (69)NA
FVC % predicted76.5 ± 20NA78.8 ± 18NA
DlCO % predicted41.6 ± 14NA41.6 ± 16NA
Progressive disease at 12 mo77 (58)NA151 (42%)NA

Definition of abbreviations: IPF = idiopathic pulmonary fibrosis; NA = not available.

Data presented represent number (%) or mean ± SD.

Baseline concentrations of CYFRA 21-1 were significantly different between healthy control subjects and individuals with IPF in both the discovery (control: mean ± SD, 0.96 ± 0.81 ng/ml; vs. IPF: 2.34 ± 2.15 ng/ml; P < 0.0001) and validation (control: 2.21 ± 1.54 ng/ml; and IPF: 4.13 ± 2.77 ng/ml; P < 0.0001) cohorts (Figures 2A and 2B). A general linear model was used to evaluate age- and sex-adjusted differences in CYFRA 21-1 serum concentrations between healthy control subjects and subjects with IPF. This demonstrated that mean concentrations were two- to threefold higher across the discovery and validation cohorts in the serum of subjects with IPF.

Relationship of CYFRA 21-1 to Disease Progression

To assess the relationship, in the discovery cohort, between baseline biomarker concentrations and subsequent outcome, subjects with IPF were dichotomized into those with stable (n = 53) or progressive disease (n = 75) at 12 months. CYFRA 21-1 was able to distinguish patients with IPF with progressive disease (Figure 2C) (P < 0.001). This distinction between groups remained evident at 1, 3, and 6 months. The same relationship between stable (n = 164) and progressive (n = 149) subjects was observed in the validation cohort (P < 0.001) (Figure 2D). Analysis of the capacity for baseline serum CYFRA 21-1 concentrations to distinguish progressive from stable subjects disclosed a C-statistic in the discovery cohort of 0.70 (95% confidence interval [CI], 0.61–0.79; P < 0.0001) and in the validation cohort of 0.65 (95% CI, 0.59–0.71; P < 0.0001).

The ability of CYFRA 21-1 to predict progression status was compared with the GAP index, a clinical score that predicts IPF progression. In the discovery cohort, GAP predicted disease progression at 12 months with a C-statistic of 0.66 (95% CI, 0.57–0.75; P < 0.0003) and in the validation cohort of 0.60 (95% CI, 0.53–0.66; P < 0.003). CYFRA 21-1 alone was able to outperform this scoring system in our cohort and when combined with the GAP score improved the C-statistic in both the discovery (0.71; 95% CI, 0.59–0.71; P < 0.0001) and the validation cohort (0.65; 95% CI, 0.59–0.71; P < 0.0001).

Baseline CYFRA 21-1 and Mortality

In the discovery cohort, baseline CYFRA 21-1 concentrations, corrected for age, sex, recruitment site, and smoking status, demonstrated an association with both 12-month (hazard ratio [HR], 1.26 [95% CI, 1.12–1.41]; P < 0.0001) and overall mortality (HR, 1.28 [95% CI, 1.17–1.41]; P < 0.0001). The HR reflects the risk associated with a 1-ng/ml change in CYFRA 21-1 concentration. The findings from the discovery cohort were successfully replicated in the validation cohort for both 12-month (HR, 1.17 [95% CI, 1.12–1.23]; P < 0.0001) and overall mortality (HR, 1.16 [95% CI, 1.11–1.21]; P < 0.0001). The relationship between baseline CYFRA 21-1 concentration and mortality is clearly illustrated when separating subjects in both the discovery and validation cohorts into tertiles of baseline serum CYFRA 21-1 values. For the discovery cohort, the tertile with highest baseline values had a median survival of 670 days, compared with 1,039 days and 1,646 days for the middle and lowest tertiles, respectively (Figure 3A). In the validation cohort, the median survival for the highest cohort was 728 days, compared with 1,366 days and 1,892 days in the middle and lowest tertiles (Figure 3B). Baseline FVC and DlCO have both previously been identified as predictors of survival in individuals with IPF. Therefore, the multivariate analysis was repeated incorporating age, sex, site, smoking status, and baseline lung function parameters. In this model, CYFRA 21-1 continued to demonstrate an association with overall mortality in both the discovery (HR, 1.13 [95% CI, 0.02–1.25]; P = 0.023) and validation cohorts (HR, 1.12 [95% CI, 1.06–1.19]; P = 0.0001).

Longitudinal Change in CYFRA 21-1 and Disease Progression

The relationship between changing concentrations of biomarker and mortality were then assessed by calculating the rate of change in CYFRA 21-1 for each subject between baseline and 1 month and then from baseline through to month 3. In the discovery cohort, application of a proportional hazards model adjusted for age, sex, site, and smoking status to the continuous values demonstrated that changing CYFRA 21-1 concentrations over 1 month were not predictive of either 12-month (HR, 1.06 [95% CI, 0.72–1.56]; P = 0.7) or overall survival (HR, 1.22 [95% CI, 0.94–1.59]; P = 0.14), but slope from baseline to 3 months was predictive of both 12-month (HR, 1.81 [95% CI, 1.16–2.83]; P = 0.010) and overall survival (HR, 1.94 [95% CI, 1.41–2.66]; P < 0.0001). These findings were mirrored by the results in the validation cohort. Change in CYFRA 21-1 over 1 month failed to predict 12-month (HR, 1.08 [95% CI, 0.93–1.24]; P = 0.31) or overall survival (HR, 1.07 [95% CI, 0.98–1.17]; P = 0.11). However, 3-month change in CYFRA 21-1 was predictive of both 12-month (HR, 1.25 [95% CI, 1.06–1.49]; P = 0.01) and overall survival (HR, 1.18 [95% CI, 1.02–1.38]; P = 0.03).

To further assess the effect of changing concentrations of CYFRA 21-1, subjects were dichotomized into those with either a rising (slope >0) or a stable (slope ⩽ 0) CYFRA 21-1 concentration over 1 month and 3 months. In both discovery (Figure 4A) and validation (Figure 4B) cohorts, subjects with a rising concentration of CYFRA 21-1 over 3 months had a worse prognosis (discovery HR, 2.15 [95% CI, 1.38–3.35]; P = 0.0007; validation HR, 1.35 [95% CI, 1.01–1.81]; P = 0.045). A rising concentration of CYFRA 21-1 over 1 month was predictive of a poorer prognosis in the discovery (HR, 1.82 [95% CI, 1.19–2.76]; P = 0.0054) but not the validation (HR, 1.21 [95% CI, 0.88–1.68]; P = 0.24) cohort.

The 3-month CYFRA 21-1 slope analysis was repeated using a proportional hazards models incorporating age, sex, site, smoking status, and lung function parameters. In the discovery cohort, 3-month slope in CYFRA 21-1 remained predictive of overall survival when corrected for baseline percent predicted FVC (HR, 2.17 [95% CI, 1.37–3.44]; P = 0.001) and percent predicted DlCO (HR, 2.07 [95% CI, 1.27–3.36]; P = 0.003). Assessment of the 3-month CYFRA 21-1 slope in the validation cohort yielded similar results for both percent predicted FVC (HR, 1.44 [95% CI, 1.07–1.95]; P = 0.017) and percent predicted DlCO (HR, 1.39 [95% CI, 1.02–1.91]; P = 0.040).

We have demonstrated that CYFRA 21-1, a cleavage fragment of cytokeratin-19, is present in greater abundance in the lung tissue of subjects with IPF than in healthy control subjects and that it predominantly localizes to hyperplastic and bronchiolized epithelial cells. CYFRA 21-1 is measurable in the serum of both healthy individuals and those with IPF but is present in significantly higher concentrations in IPF. In those subjects with IPF, baseline concentrations of CYFRA 21-1 identify individuals most likely to progress over the next 12 months and are predictive of overall survival even after accounting for baseline disease severity. Concentrations remain elevated over time in subjects with disease progression. Rising concentrations of CYFRA 21-1 over 3 months correlate with overall survival, and this effect is strengthened by correcting for baseline disease severity as measured by FVC and DlCO.

Although fibroblasts and myofibroblasts have long been recognized to be key effector cells in the pathogenesis of IPF, the role of the epithelium has tended to be overlooked. Recent data have highlighted the importance of epithelial abnormalities in the development of pulmonary fibrosis (14, 15). Many of the genetic polymorphisms associated with risk of developing IPF relate to either proteins involved in maintaining epithelial integrity or epithelial secreted proteins (such as surfactant protein C and MUC5B) (16, 17). It is now believed that premature senescence and thus replicative exhaustion of epithelial stem cells, which replenish damaged alveolar epithelium, is the primary event triggering the development of progressive fibrosis after repeated alveolar injury (18). The evolution from normal alveoli lined by pneumocytes to fibrotic honeycomb lung lined by hyperplastic epithelium likely has its origins, at least in part, in failure of reepithelialization of denuded alveoli. These hyperplastic epithelial cells are highly secretory and produce a multitude of growth factors. They are also highly susceptible to apoptosis, which leads to release into the circulation of proteins related to cell degradation (19). Given the pathological role played by the epithelium in IPF, peripheral biomarkers of epithelial turnover represent an attractive target for defining disease activity and potential response to therapy in fibrotic lung disease (3).

We and others have identified a number of circulating proteins that both distinguish individuals with IPF from healthy, age-matched control subjects and predict prognosis (3, 5, 12, 20, 21). The earliest such biomarkers to be identified were related to matrix deposition or turnover. These include MMP-7 (matrix metalloproteinase 7), osteopontin, and collagen synthesis and degradation fragments (3, 12, 13, 21, 22). Although MMP-7 and osteopontin have prognostic value at baseline, they do not appear to change in concentration as disease progresses. Collagen synthesis and degradation fragments also provide baseline prognostic information, but, importantly, serial change in these markers identifies individuals with worsening pulmonary fibrosis independent of lung function (12). More recently, we identified that the classic tumor biomarkers CA125 (an epithelial secreted mucin, MUC16) and CA19-9 (a cell surface tetrasaccharide), both of which provide an indirect measure of epithelial damage and turnover, are similarly predictive of outcome at baseline and over time in individuals with IPF (3). Following on from this observation, we subsequently showed in the INMARK trial, a phase 4 randomized placebo-controlled biomarker discovery trial, that nintedanib therapy results in an approximate 20% population-wide reduction in serum CA125 within 4 weeks of initiation of treatment (20, 23).

Cytokeratins are structural intracytoplasmic proteins found in epithelial cells. Cytokeratin-19 is a 40-kD type 1 keratin protein, which, according to the IPF cell atlas (www.ipfcellatlas.com), is widely expressed in multiple epithelial cell types in the IPF lung, including aberrant basaloid, basal, ciliated, club, goblet, and alveolar type 1 and 2 epithelial cells (24, 25). CYFRA 21-1 detects fragments of cytokeratin-19. Although first described as a prognostic marker for non-small cell lung cancer, more recent studies have identified elevation of CYFRA 21-1 in the bronchoalveolar lavage fluid of individuals with pulmonary fibrosis and in the serum of subjects with scleroderma-associated interstitial lung disease (ILD) (6, 9, 10). Our data extend these previous observations and demonstrate a potentially important role for CYFRA 21-1 as a prognostic biomarker in IPF, both at baseline and when measured serially. Importantly, baseline concentrations of CYFRA 21-1 appear to provide a more clear-cut separation between prognostic groups than do other epithelial and matrix markers. Based on analysis of the IPF Cell Atlas, CYFRA 21-1 is expressed by a wider range of epithelial cell types in the IPF lung than either CA125 or CA19-9. Furthermore, CYFRA 21-1 is expressed in epithelium overlying fibroblastic foci and may therefore provide an indirect marker of profusion of fibroblastic foci; a measure previously linked to prognosis (26). As such, CYFRA 21-1 has the potential to be a more informative serial biomarker in IPF than other markers thus far identified.

There remains a vital need for biomarkers in IPF and other forms of fibrotic lung disease. Although both baseline and serial measures of lung function (FVC and DlCO) provide important prognostic information in fibrotic lung disease, they fail to define treatment response in individual patients, and serial measurements only really become informative at 12 months (27, 28). The lack of biomarkers of treatment response in IPF has important implications when it comes to judging the success or failure of antifibrotic therapy in any given individual. Serum biomarkers that show short-term (i.e., <3 mo), prognostically relevant change have the potential to be measures of therapeutic success. Identifying biomarkers of treatment response in fibrotic lung disease could shorten clinical trials and enable the design of treatment failure trials, something that is likely to be necessary should further drugs be approved for the treatment of IPF. Although further validation is required, CYFRA 21-1 displays the characteristics that are expected of a biomarker that might indicate treatment response.

The observation that baseline and serial CYFRA 21-1 concentrations are prognostically informative suggests that this biomarker could be used in clinical practice to identify individuals at high risk of future disease progression (and thus in need of early aggressive treatment and/or referral for transplant). Furthermore, with the recent approval of nintedanib for progressive pulmonary fibrosis of any cause, there is also an urgent need to better identify individuals at risk of progressive fibrosis before irreversible loss of lung tissue occurs (29, 30). CYFRA 21-1 should be assessed in a broader population of individuals with fibrotic ILD to see whether it might identify individuals at risk of subsequent progression of fibrosis (so that treatment could be initiated in advance of the development of irreversible damage).

The current study has a number of strengths and weaknesses. The PROFILE study is the largest prospective longitudinal study performed in IPF. Importantly, subjects were recruited before the advent of antifibrotic therapy, and therefore baseline biomarker values are unaffected by antifibrotic treatment. Long-term outcome data are available on all subjects, ensuring that there was no need to impute missing data in any of the analyses. The subjects were recruited within 6 months of diagnosis and have outcomes typical of untreated cohorts of patients with IPF, with a median survival of approximately 3 years (31). In terms of weaknesses, the assay used to measure CYFRA 21-1 was a commercially available ELISA. Although efforts were made to ensure consistency of measures, there were batch-by-batch variations (all samples in each of the discovery and validation cohorts were measured in a single batch). This variation accounts for the differences observed in mean CYFRA 21-1 concentrations when comparing the discovery and validation cohorts. Further work will therefore be required to validate specific risk thresholds in the clinical setting. A proportion of the PROFILE subjects received antifibrotic therapy from some point beyond 2 years after enrollment into the study, and this may have influenced longer-term survival outcomes, especially in the validation cohort (who represent later recruits into the PROFILE study).

In conclusion, as a marker of epithelial turnover and cell death, serum CYFRA 21-1 concentrations link directly to biologically important processes underpinning the development and progression of pulmonary fibrosis. CYFRA 21-1 is strongly predictive of prognosis when measured at baseline in subjects with IPF. Change in serial measurements over 3 months further identifies individuals with a poor prognosis. Further research is needed to understand whether changes in CYFRA 21-1 might be used to identify treatment response after the initiation of antifibrotic therapy in IPF and other fibrotic lung diseases. Nonetheless, CYFRA 21-1 represents part of a growing repertoire of serum biomarkers that hold promise for transforming delivery of care for individuals with life-shortening pulmonary fibrosis.

The authors thank all patients for their participation in the PROFILE study and the Brompton/Imperial National Institute for Health Research Biomedical Research Unit for infrastructure support in the collection and processing of samples.

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Correspondence and requests for reprints should be addressed to Toby M. Maher, M.D., Ph.D., Keck School of Medicine, University of Southern California, 1510 San Pablo Street, Los Angeles, CA 90033. E-mail: .

*These authors contributed equally to this work.

The PROFILE study was funded by Medical Research Council grant G0901226 and GlaxoSmithKline R&D grant CRT114316 and was sponsored by Nottingham University and Royal Brompton and Harefield NHS Foundation Trust. Additional support comes from National Institute for Health Research Clinician Scientist Fellowship CS-2013-13-017 (T.M.M.); a British Lung Foundation Chair in Respiratory Research C17-3 (T.M.M.); Medical Research Council MRC Industry Collaboration Agreement grant G0901226 (R.G.J.); Joan Bending, Evelyn Bending, Mervyn Stephens, and Olive Stephens Memorial Fellowship AUK-SNF-2017-381 (A.J.B.); and an Action for Pulmonary Fibrosis Mike Bray fellowship (P.L.M.).

Author Contributions: T.M.M., R.G.J., P.L.M., E.O., and W.A.F. designed the study. T.M.M. and R.G.J. secured funding for the study. A.J.B., G.A., and A.G.N. undertook and interpreted the immunohistochemistry. P.L.M., T.M.M., R.B., P.S., R.T., F.C., E.A.R., A.U.W., G.S., and R.G.J. identified and recruited study subjects. Y.K., T.M.M., W.A.F., and P.L.M. planned, undertook, and interpreted the statistical analyses. T.M.M. and P.L.M. wrote the first draft of the manuscript, which was revised and approved by all authors.

This article has a related editorial.

Originally Published in Press as DOI: 10.1164/rccm.202107-1769OC on April 1, 2022

Author disclosures are available with the text of this article at www.atsjournals.org.

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