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Distinct Airway Involvement in Subtypes of End-Stage Fibrotic Pulmonary Sarcoidosis

Published:January 09, 2021DOI:https://doi.org/10.1016/j.chest.2021.01.003

      Background

      Sarcoidosis is a systemic granulomatous disease that in most patients affects the lung. Pulmonary fibrotic sarcoidosis is clinically, radiologically, and pathologically a heterogeneous condition. Although substantial indirect evidence suggests small airways involvement, direct evidence currently is lacking.

      Research Question

      What is the role of the (small) airways in fibrotic sarcoidosis?

      Study Design and Methods

      Airway morphologic features were investigated in seven explant lungs with end-stage fibrotic sarcoidosis using a combination of CT scanning (large airways), micro-CT scanning (small airways), and histologic examination and compared with seven unused donor lungs as controls with specific attention focused on different radiologically defined sarcoidosis subtypes.

      Results

      Patients with central bronchial distortion (n = 3), diffuse bronchiectasis (n = 3), and usual interstitial pneumonia pattern (n = 1) were identified based on CT scan, showing a decrease and narrowing of large airways, a similar airway number and increased airway diameter of more distal airways, or an increase in airway number and airway diameter, respectively, compared with control participants. The number of terminal bronchioles per milliliter and the total number of terminal bronchioles were decreased in all forms of fibrotic sarcoidosis. Interestingly, the number of terminal bronchioles was inversely correlated with the degree of fibrosis. Furthermore, we identified granulomatous remodeling as a cause of small airways loss using serial micro-CT scanning and histologic examination.

      Interpretation

      The large airways are involved differentially in subtypes of sarcoidosis, but the terminal bronchioles universally are lost. This suggests that small airways loss forms an important aspect in the pathophysiologic features of fibrotic pulmonary sarcoidosis.

      Key Words

      Abbreviations:

      IPF (idiopathic pulmonary fibrosis), UIP (usual interstitial pneumonia)
      Sarcoidosis is a relatively rare systemic disease of unknown cause that can involve almost all organs, but frequently affects the lung. The disease mostly resolves spontaneously and responds well to corticosteroids, cytotoxic therapy, or both, but about one-third of patients do not improve after pharmacologic treatment or experience relapse of the disease.
      • Valeyre D.
      • Prasse A.
      • Nunes H.
      • Uzunhan Y.
      • Brillet P.-Y.
      • Müller-Quernheim J.
      Sarcoidosis.
      These patients are most likely to demonstrate progressive pulmonary fibrosis secondary to sarcoidosis. Histologically, the disease is characterized by granuloma formation with a preferable lymphatic distribution. These granulomas typically are well formed and noncaseating. They are believed to result from monocyte activation resulting from an unknown antigen, leading to CD4+ T-cell overstimulation, which promotes further macrophage activation and organization.
      • Baughman R.P.
      • Culver D.A.
      • Judson M.A.
      A concise review of pulmonary sarcoidosis.
      Why transition from an inflammatory to a fibrotic state occurs in some patients remains unknown.
      • Bonham C.A.
      • Strek M.E.
      • Patterson K.C.
      From granuloma to fibrosis: sarcoidosis associated pulmonary fibrosis.
      Fibrotic sarcoidosis remains ill-investigated because of its low prevalence. It forms a heterogeneous condition, with potential life-threatening symptoms. Systematic radiologic assessment demonstrated three distinct radiologic patterns: a central bronchial distortion, peripheral honeycombing, and a diffuse linear pattern, found in 47%, 29%, and 24%, respectively, of patients with fibrotic sarcoidosis, which may correlate with the degree of pulmonary impairment.
      • Abehsera M.
      • Valeyre D.
      • Grenier P.
      • Jaillet H.
      • Battesti J.P.
      • Brauner M.W.
      Sarcoidosis with pulmonary fibrosis: CT patterns and correlation with pulmonary function.
      Indeed, bronchial distortion correlated with decreased expiratory flow rate, honeycombing with restriction, and the linear pattern with less functional impairment. Nishino et al
      • Nishino M.
      • Kuroki M.
      • Roberts D.H.
      • Mori Y.
      • Boiselle P.M.
      • Hatabu H.
      Bronchomalacia in sarcoidosis.
      suggested that the small airways also might be affected in sarcoidosis: 11 of 18 sarcoidosis patients showed evidence of bronchomalacia (dynamic narrowing or collapse of the bronchi in end expiration) that was associated with increased evidence of air trapping. Indeed, air trapping, which indicates small airways disease, is a common finding in different radiologic studies concerning sarcoidosis and is considered supportive of diagnosis.
      • Nishino M.
      • Lee K.S.
      • Itoh H.
      • Hatabu H.
      The spectrum of pulmonary sarcoidosis: variations of high-resolution CT findings and clues for specific diagnosis.
      However, the pathologic features of fibrotic sarcoidosis remain mostly elusive. Transbronchial biopsies are an important tool to diagnose pulmonary sarcoidosis, but in fibrotic pulmonary sarcoidosis, transbronchial biopsy often is unfeasible technically and if obtained, tissue samples are small. Therefore, most of the knowledge about the histopathologic description of fibrotic sarcoidosis is limited to end-stage specimens obtained at transplantation or autopsy. These studies again demonstrate that fibrotic sarcoidosis can present in different ways. Remarkably, a small proportion of lungs also are reported to show a usual interstitial pneumonia (UIP) pattern that frequently is associated with idiopathic pulmonary fibrosis (IPF). Further interesting findings include interstitial lymphocytic infiltrates,
      • Shigemitsu H.
      • Oblad J.M.
      • Sharma O.P.
      • Koss M.N.
      Chronic interstitial pneumonitis in end-stage sarcoidosis.
      central bronchial dilatation, and bronchiectasis.
      • Zhang C.
      • Chan K.M.
      • Schmidt L.A.
      • Myers J.L.
      Histopathology of explanted lungs from patients with a diagnosis of pulmonary sarcoidosis.
      Given that both radiologic and pathologic studies consistently demonstrate the existence of multiple presentations of fibrotic sarcoidosis with a potential differential involvement of the airways, we aimed to perform an in-depth investigation of the role of the airways in lungs of patients with end-stage fibrotic sarcoidosis undergoing lung transplantation using a combination of CT scanning, micro-CT scanning, and histopathologic analysis, as used previously in other interstitial lung diseases such as IPF
      • Verleden S.E.
      • Tanabe N.
      • McDonough J.E.
      • et al.
      Small airways pathology in idiopathic pulmonary fibrosis: a retrospective cohort study.
      and lymphangioleiomyomatosis.
      • Verleden S.E.
      • Vanstapel A.
      • De Sadeleer L.
      • et al.
      Quantitative analysis of airway obstruction in lymphangioleiomyomatosis.

      Methods

       Patient Selection

      Explant lungs were collected from patients undergoing lung transplantation for fibrotic sarcoidosis at University Hospitals, Leuven, Belgium. All patients had biopsy-proven sarcoidosis in their clinical history showing typical granulomas. Treatment regimens included the following: methylprednisolone (n = 7), azathioprine (n = 3), and methotrexate (n = 2). Unused donor lungs were matched according to age, sex, and smoking history and were included as control subjects. All patients provided written informed consent, and this study was approved by the hospitals’ ethics committee (Identifier: S52174). Donor lungs were collected following Belgian legislation that states that declined donor lungs can be offered for research after formal ethical approval (Identifiers: S59648 and S61653). The seven control lungs consisted of four lungs declined for non-allograft-related reasons (logistics, bacterial meningitis, single lung transplant, and kidney tumor) and three lungs declined for allograft-related reasons (emphysema and persistent microthrombi). All lungs showed morphologically inconspicuous lung parenchyma. Preoperative inspiratory chest CT scans from the sarcoidosis patients were scored by a single experienced radiologist (A. D.) to estimate the percentages of airway dilation, consolidation, ground-glass opacities, reticular pattern, volume loss, and pleural thickening per lung lobe from 0 to 3 according to the Fleishner Society guidelines.
      • Hansell D.M.
      • Bankier A.A.
      • MacMahon H.
      • McLoud T.C.
      • Müller N.L.
      • Remy J.
      Fleischner Society: glossary of terms for thoracic imaging.
      These scores were recalculated to percentages. Expiratory CT scans to assess air trapping were not available.

       Lung Processing for Ex Vivo CT and Whole Lung Micro-CT Scanning

      Lungs were processed as described previously.
      • Verleden S.E.
      • Tanabe N.
      • McDonough J.E.
      • et al.
      Small airways pathology in idiopathic pulmonary fibrosis: a retrospective cohort study.
      Briefly, one of the explant lungs was air inflated at 30 cm H2O and after deflation to 10 cm H2O, lungs were fixed in the fumes of liquid nitrogen at constant airway pressure, preserved at -80°C, and scanned with ex vivo CT imaging in frozen condition (Siemens Somatom, 1.0-mm slice thickness, B60F window) as described previously.
      • Verleden S.E.
      • Tanabe N.
      • McDonough J.E.
      • et al.
      Small airways pathology in idiopathic pulmonary fibrosis: a retrospective cohort study.
      ,
      • Verleden S.E.
      • Vanstapel A.
      • De Sadeleer L.
      • et al.
      Quantitative analysis of airway obstruction in lymphangioleiomyomatosis.
      This CT was used to segment the airways semiautomatically using ITK-SNAP software.
      • Yushkevich P.A.
      • Piven J.
      • Hazlett H.C.
      • et al.
      User-guided 3D active contour segmentation of anatomical structures: significantly improved efficiency and reliability.
      NEURONstudio software (Icahn School of Medicine at Mount Sinai) was used to assess the number of airways per generation, mean airway diameters, and total airway numbers.
      • Rodriguez A.
      • Ehlenberger D.B.
      • Dickstein D.L.
      • Hof P.R.
      • Wearne S.L.
      Automated three-dimensional detection and shape classification of dendritic spines from fluorescence microscopy images.
      Additionally, lung volume, lung density, and lung weight were determined by manual segmentation using Horos (open-source code software program available at Horosproject.org and sponsored by Nimble Co, LLC, d/b/a Purview). The contralateral lung was used for in-depth histopathologic examination according to the standard hospital practice.

       Micro-CT Scanning and Histologic Examination

      After CT scanning, lungs were sliced using a band saw from apex to base and systematically sampled using a core bore or power drill resulting in specimen cores (2-cm height, 1.4-cm diameter). We randomly selected four cores (two apical and two basal) per lung, which were scanned in frozen condition (-30°C) using micro-CT imaging (SkyScan 1272, Bruker) at a voxel size of 10 μm. This subsample micro-CT scan was used further to count the number of terminal bronchioles manually (defined anatomically as the last conducting airway). CTan software (Bruker) was used to measure surface density, tissue volume, and tissue percentage. After frozen micro-CT scanning, a selected sample was fixed in 6% paraformaldehyde, dehydrated using ethanol, embedded in paraffin, sectioned at 10-μm slice thickness, and stained with hematoxylin-eosin according to conventional protocols.

       Statistical Analysis

      Results are expressed as median and interquartile range or mean ± SD. The number of terminal bronchioles, surface density, and percentage of tissue were averaged over the entire lung. Differences between the sarcoidosis and control group were compared using the Wilcoxon signed rank test. The number of airways per generation and airway diameters were compared using a two-way analysis of variance with Tukey’s post hoc test (GraphPad Prism version 8.0 software; GraphPad). A P value of less than .05 was considered significant.

      Results

      Seven patients with fibrotic sarcoidosis underwent transplantation between 2009 and 2020 at the University Hospitals Leuven (six men and one woman) with a median age of 60 years. Time between first diagnosis of sarcoidosis and lung transplantation was highly variable (range, 2-39 years). Pulmonary function was consistent with end-stage disease necessitating treatment with lung transplantation. The control group was well matched according to age, sex, and smoking history (Table 1).
      Table 1Patient Characteristics
      CharacteristicControl Group (n = 7)Sarcoidosis Group (n = 7)
      Age, y56 (56-66)60 (48-64)
      Sex, male/female6/16/1
      Smoking, none/smoking history4/32/5
      Pack-years of smokingNA/10/124/5/8/10/35
      BMI, kg/m225 (24-28)25 (22-31)
      Time since diagnosis, yNA14 (7-30)
      FEV1......
       LNA1.14 (0.72-1.47)
       % predictedNA36 (28-49)
      FVC......
       LNA1.90 (1.16-2.93)
       % predictedNA49 (27-68)
      FEV1 to FVCNA0.74 (0.37-0.83)
      FEF25%-75%......
       L/sNA0.78 (0.44-1.27)
       % predictedNA34 (10-39)
      Data are presented as No. or median (interquartile range). FEF25%-75% = forced expiratory flow at 25% to 75% of FVC; NA = not available.

       Histopathologic Findings in Fibrotic Sarcoidosis

      The contralateral lung that was used for conventional histopathologic assessment showed severe lung remodelling with abundant interstitial fibrosis and evidence of chronic lymphocytic inflammation in all investigated cases. Clear granulomas with a predominant centrilobular and subpleural distribution were found in five of seven cases. One lung showed multifocal interstitial fibrosis with subpleural and peribronchial fibrosis following a lymphatic distribution, consistent with sarcoidosis. The last lung showed diffuse areas of fibrosis with the presence of mature fibrosis, immature fibrosis with fibroblast foci, and the presence of honeycombing areas consistent with UIP, despite transbronchial biopsy showing clear granulomas previously and initial positive response to corticosteroids.

       Radiologic Findings in Fibrotic Sarcoidosis

      Analysis of the last available in vivo CT scan, obtained at a median time of 528 days (interquartile range, 274-625 days) before transplantation, demonstrated severe bronchial dilatation, pleural thickening, volume loss or distortion, and reticulation (Table 2). Four lungs showed evidence of severe bronchial dilatation (> 70%), whereas the others showed no or very mild bronchial dilatation. Ex vivo CT scanning analysis demonstrated higher lung weight (P = .038), lower lung volume (P = .0023), and higher lung density (P = .0006) in sarcoidosis lungs vs control lungs (Table 2).
      Table 2Overview of In Vivo CT Scanning, Ex Vivo CT Scanning, and Micro-CT Scanning Measurements
      VariableControl Group (n = 7)Sarcoidosis Group (n = 7)P Value
      In vivo CT scoring, %
       Bronchial dilatationNA72 (11-83)
       NodulesNA17 (0-44)
       Airway wall thickeningNA11 (0-17)
       ConsolidationNA33 (0-61)
       Ground glass opacitiesNA6 (0-17)
       Pleural thickeningNA50 (33-61)
       ReticulationNA56 (33-89)
       Volume lossNA72 (67-72)
      Ex vivo CT calculation
       Lung volume, L3.5 (3.3-3.9)2.0 (1.4-2.5).0023
       Lung mass, g327 (268-365)554 (345-559).038
       Lung density, g/L99 (83-125)303 (194-414).006
      Micro-CT calculation
       Terminal bronchiole/mL3.89 (3.60-4.28)1.18 (0.68-1.42).026
       Terminal bronchiole/lung14,099 (11,693-16,034)2,188 (1,595-3,490).0012
       Surface density, 1/μm0.0106 (0.0100-0.0118)0.0052 (0.0047-0.0054).006
      Data are presented as median (interquartile range), unless otherwise indicated.

       Airways in Sarcoidosis Subtypes

      An overview of the airway segmentation and representative CT scanning images of different sarcoidosis lungs is presented in Figure 1. Three lungs showed central airway distortion characterized by a central mass affecting both the upper and lower lobe in two of three lungs and only the upper lobe in one lung. The four other sarcoidosis lungs showed pronounced bronchiectasis with a more diffuse and peripheral (subpleural) fibrosis distribution (consistent with the previous in vivo CT scan scoring by the radiologist). This is illustrated further and quantified in Figure 2, where a clear separation in distinct groups is seen based on the number of airways per generation, the total number of airways, and the mean airway diameter per generation: the central airway distortion group showed decreased airway counts and decreased airway diameters (n = 3; P < .0001 vs control lungs), the diffuse bronchiectasis group demonstrated seemingly normal airway counts, but increased airway diameter in more distal airway generations (n = 3), and the lung with a UIP pattern displayed the highest airway numbers and increased airway diameters in more distal generations. Of interest, although FEV1 is comparable, measures of small airways dysfunction such as FEV1 to FVC ratio and forced expiratory flow at 25% to 75% of FVC were decreased in the central airway distortion group compared with the diffuse bronchiectasis group, consistent with increased airway obstruction.
      Figure thumbnail gr1
      Figure 1CT scan-based overview of included sarcoidosis lungs (n = 7). The CT-based airway segmentation and coronal view in a central area are shown side by side. A clear distinction can be seen between lungs with primarily central airway distortion (n = 3), diffuse bronchiectasis (n = 3), and UIP pattern (n = 1). UIP = usual interstitial pneumonia.
      Figure thumbnail gr2
      Figure 2A-C, Graphs showing CT scan measures of airway numbers. A, Stratification of the airway numbers of the sarcoidosis lungs in the central airway distortion, diffuse bronchiectasis, and UIP group shows pronounced differences in the number of airways per generation in the central airways distortion group, a similar number of airways in the diffuse bronchiectasis group, and an increased number of airways in the UIP group. B, Total numbers of airways per generation is not different comparing all sarcoidosis lungs with control lungs; however, the different subtypes show a pronounced difference. C, Mean airway diameter per airway generation in the central airway distortion, diffuse bronchiectasis, and UIP group shows pronounced differences. UIP = usual interstitial pneumonia.

       Micro-CT Scanning Assessment

      Micro-CT scanning (resolution, 10 μm) was performed on the extracted cores to assess the small airways and parenchyma systematically. Terminal bronchiole numbers were reduced significantly in sarcoidosis compared with control lungs, both when assessed as number of terminal bronchioles per milliliter (P = .026) or as total number of terminal bronchioles per lung (P = .0012) (Fig 3A, 3B ). Surface density, reflecting parenchymal remodelling, was significantly lower in sarcoidosis compared with control lungs (P = .0006) (Fig 3C). Of note, the heterogeneity in the number of terminal bronchioles was lower compared with the previously described differences in CT scan-based (large) airway counts. Only one sarcoidosis lung (ie, the lung with a UIP pattern on histopathologic analysis and the highest airway numbers on CT imaging) showed a higher count of terminal bronchioles per milliliter compared with the other sarcoidosis lungs. Interestingly, a positive association also was found between the number of terminal bronchioles per milliliter and the surface density (P < .0001; R2 = 0.69) (Fig 3D), indicating that more severe remodelling was associated with a further loss in terminal bronchioles.
      Figure thumbnail gr3
      Figure 3A-D, Graphs showing micro-CT scan-based measures of the small airways and parenchyma. A, Number of terminal bronchioles per milliliter is lower in the sarcoidosis group compared with the control group. B, Total number of terminal bronchioles per lung is lower in sarcoidosis compared with control lungs. C, Surface density is lower in sarcoidosis compared with control lungs. D, A positive association between the number of terminal bronchioles per milliliter and surface density analyzing the sarcoidosis cores alone is shown (n = 28). Dashed line indicates the mean number of terminal bronchioles per milliliter and mean surface density in the control lungs. TB = terminal bronchiole.

       Serial Micro-CT Scanning

      Because micro-CT scanning allows in-depth 3-dimensional investigation of tissue specimens, it therefore facilitates the interpretation of the sequence of events evolving from mild or moderate to advanced fibrosis (Fig 4). Micro-CT scan assessment in the central bronchial distortion group suggested more centrilobular fibrosis gradually compressing the airway lumen and advancing to complete fibrosis. In the diffuse bronchiectasis group, the fibrosis seems to originate from the septa bordering the secondary pulmonary lobulus, dilating the airways advancing to severe bronchiectasis with surrounding matrix remodelling. In UIP, more diffuse fibrosis exists where the airways are dilated gradually, but the terminal bronchioles are more spared.
      Figure thumbnail gr4
      Figure 4A-C, Micro-CT scan-based evolution patterns of sarcoidosis subtypes. A, In the central airway distortion group, initial mild septal thickening and centrilobular granulomas and fibrosis are seen, followed by centrilobular fibrosis, gradually compressing the airways until in the advanced stage, when complete remodelling with loss of bronchial structures is visible. B, In the diffuse bronchiectasis group, less centrilobular remodelling and similar septal thickening appear. Subsequently, further distortion of the alveoli appears, starting from the interlobular septa, until ultimately pronounced airway dilation appears, resulting in a honeycomb appearance with surrounding loss of matrix and small airways. C, UIP pattern initially shows more diffuse parenchymal fibrosis, where gradually the airways become dilated, but the terminal bronchioles are spared, ultimately resulting in a completely remodelled matrix with less involvement of the small airways.

       Serial Histologic Examination and Micro-CT Scanning

      To illustrate further the site and nature of small airways loss, serial micro-CT images were aligned with serial histologic images in a representative example (Fig 5). Following an apparently normal airway to more distal airway branches demonstrates a granuloma surrounding the airway systematically compressing the lumen until the lumen is no longer discernible, and only remodelled lung parenchyma with diffuse granulomas and fibrosis are seen.
      Figure thumbnail gr5
      Figure 5A-C, Serial micro-CT scans (left column) and photomicrographs (middle and right columns) showing histologic results demonstrating airway loss. A1-3, Conducting airway with accompanying artery surrounded by fibrotic matrix. B1-3, Conducting airway is bifurcating, surrounded by a fibrotic matrix with granulomas. C1-3, Narrowed airway lumen and compression of the airway by a granuloma (arrow). D1-3, Complete airway luminal loss without sprouting to the terminal and respiratory bronchioles.

      Discussion

      In this study, we investigated the different presentations of fibrotic sarcoidosis with a specific emphasis on airway morphologic features. We validated previously reported central airway distortion, diffuse bronchiectasis, and UIP pattern group in end-stage fibrotic sarcoidosis lungs. Although the larger airways displayed considerable variation on CT scanning, micro-CT scanning showed consistent and equal loss of terminal bronchioles among the different subtypes. We further proposed different evolution patterns and validated that loss of small airways can occur as a result of compression of the airways by granulomas. To our knowledge, this is the first report extensively validating different radiologic patterns of fibrotic sarcoidosis with a particular focus on the role of the airways.
      Although the central airway distortion and the diffuse bronchiectasis and honeycombing groups are well established based on radiologic studies (next to a linear pattern, but this shows only limited pulmonary impairment), this has not been validated further and investigated in depth.
      • Polychronopoulos V.S.
      • Prakash U.B.S.
      Airway involvement in sarcoidosis.
      In this study, we performed an airway-based evaluation of these subtypes.
      In the central airways distortion group, a great loss of both large and small airways was found, whereas in the diffuse bronchiectasis group, a normal number of larger airways was seen, although their diameter was increased in more distal generations. By extensive serial histologic analysis of a selected lung, we were able to demonstrate that the granulomas directly compress and remodel the small airways and thereby can be responsible for the loss of the small airways. Moreover, the small airways were lost in both presentations of sarcoidosis, suggesting that small airways loss is a universal problem in fibrotic sarcoidosis. Whether the small airways should be therapeutically targeted further remains questionable because the granulomas seem to be involved in the airway loss. However, given the severe obstruction, especially in the central bronchial distortion group, bronchial dilatation may be of value for this specific subgroup of patients.
      Because we were limited by the use of only end-stage specimens, we cannot be sure when exactly the small airways are lost in the process of progressive fibrotic sarcoidosis. Given that air trapping and bronchomalacia often are observed in the course of the disease,
      • Miller W.T.
      • Chatzkel J.
      • Hewitt M.G.
      Expiratory air trapping on thoracic computed tomography. A diagnostic subclassification.
      ,
      • Nunes H.
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      • Gille T.
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      Imaging of sarcoidosis of the airways and lung parenchyma and correlation with lung function.
      this may be already in the early stage of fibrotic sarcoidosis. This is corroborated by historic reports showing that small airways disease is common in early sarcoidosis and is found independently of a restrictive defect
      • Argyropoulou P.K.
      • Patakas D.A.
      • Louridas G.E.
      Airway function in stage I and stage II pulmonary sarcoidosis.
      and is confirmed by studies showing a high number of patients with a FEV1 to FVC ratio consistent with an obstructive pulmonary function defect, also in never smokers.
      • Harrison B.D.W.
      • Shaylor J.M.
      • Stokes T.C.
      • Wilkes A.R.
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      ,
      • Baughman R.P.
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      • et al.
      Clinical characteristics of patients in a case control study of sarcoidosis.
      This indeed suggests small airways disease or obstruction, which was demonstrated using histologic examination and micro-CT scanning in this study. However, the relatively strong correlation between the decrease in surface density (as a measure of disease severity) and the declining number of terminal bronchioles (ie, with increasing fibrosis, more terminal bronchioles are lost) (Fig 3D) opposes the hypothesis that the airways are lost before the matrix remodelling. In fact, it also remains possible that small airways loss and fibrotic parenchymal remodelling are simultaneous and not sequential processes, which cannot be evaluated further in these end-stage explant specimens.
      Whether these different subtypes of sarcoidosis reflect a continuous process with a possible evolution from one subtype to another remains to be investigated. Abehsera et al
      • Abehsera M.
      • Valeyre D.
      • Grenier P.
      • Jaillet H.
      • Battesti J.P.
      • Brauner M.W.
      Sarcoidosis with pulmonary fibrosis: CT patterns and correlation with pulmonary function.
      demonstrated that the bronchial distortion pattern was found in patients with a longer duration of disease compared with the diffuse bronchiectasis group. This suggests either a possible evolution from diffuse bronchiectasis to central bronchial distortion or that the disease is more aggressive in the diffuse bronchiectasis group. However, this is not reflected in our (small) population of lungs, in which the mean disease duration is 5, 12, and 23 years in the central bronchial distortion group and 23, 30, and 39 years in the diffuse bronchiectasis group.
      Only five of seven explant lungs showed clear evidence of granulomas in the explant lung, despite prior diagnostic biopsy-proven granulomas. Although this may seem surprising, Aisner and Albin
      • Aisner S.C.
      • Albin R.J.
      Diffuse interstitial pneumonitis and fibrosis in sarcoidosis.
      already reported that interstitial pneumonitis and secondary fibrosis can occur independent of active granuloma formation in sarcoidosis. This also was validated by Shigemitsu et al,
      • Shigemitsu H.
      • Oblad J.M.
      • Sharma O.P.
      • Koss M.N.
      Chronic interstitial pneumonitis in end-stage sarcoidosis.
      who found granulomas in only four of seven included cases of fibrotic sarcoidosis, whereas two cases showed an UIP pattern. Xu et al
      • Xu L.
      • Kligerman S.
      • Burke A.
      End-stage sarcoid lung disease is distinct from usual interstitial pneumonia.
      also could not find evidence of granulomas in two of nine cases. Comparing end-stage sarcoidosis lungs with end-stage UIP lungs suggested that those lungs with sarcoidosis showing a UIP pattern harbored a fibrotic lung disease superimposed on the sarcoidosis.
      • Zhang C.
      • Chan K.M.
      • Schmidt L.A.
      • Myers J.L.
      Histopathology of explanted lungs from patients with a diagnosis of pulmonary sarcoidosis.
      Based on our findings, we concur with the idea that the UIP pattern seems to be separate from other pulmonary sarcoidosis patterns. Next to a large increase in discernible airways on CT scanning (resulting from airway wall thickening and dilatation of the airway lumen), the terminal bronchioles largely were spared. The medical history of this patient with UIP was completely consistent with the diagnosis of sarcoidosis (typical well-formed nonnecrotizing granulomas detected on biopsies on two different occasions, initial response to corticosteroids, and disease relapse when steroids were weaned, familial sarcoidosis history, and abdominal adenopathy). Remarkably, we previously investigated small airways disease in UIP and IPF and found a 57% loss in terminal bronchioles,
      • Verleden S.E.
      • Tanabe N.
      • McDonough J.E.
      • et al.
      Small airways pathology in idiopathic pulmonary fibrosis: a retrospective cohort study.
      which is a much larger decrease than what is reported here, indicating that UIP in sarcoidosis may not completely represent UIP and IPF, either. However, we acknowledge that we cannot overinterpret these findings, which are based on a single patient.
      Limitations of this study include the relatively low number of sarcoidosis lungs that were investigated, limiting the generalizability and opening the possibility for potential confounding factors. However, only 2.4% of the lung transplantations performed worldwide are performed as ultimate treatment for sarcoidosis, making this a relatively rare procedure even in high-volume lung transplantation centers.
      • Chambers D.C.
      • Cherikh W.S.
      • Harhay M.O.
      • et al.
      The International Thoracic Organ Transplant Registry of the International Society for Heart and Lung Transplantation: thirty-sixth adult lung and heart-lung transplantation report-2019. Focus theme: donor and recipient size match.
      Also, the number of included patients is consistent with previous reports of histopathologic findings in end-stage sarcoidosis. The different presentations of sarcoidosis proposed on radiologic studies all are present in this small cohort study, making this a representative population. Moreover, we were limited to the investigation of end-stage specimens and did not have longitudinal data (ie, serial CT imaging), and therefore cannot investigate airway involvement in early disease mechanisms. Indeed, although micro-CT imaging forms an excellent tool to investigate spatial heterogeneity, we cannot make assumptions about the potential temporal sequence based on our study.
      In conclusion, we validated previously postulated subtypes of central airway distortion, diffuse bronchiectasis, and UIP in end-stage fibrotic sarcoidosis. Moreover, we demonstrated an important loss of small airways in end-stage sarcoidosis, in which granulomas directly compress the small airways. Preventing small airways loss could be an important future therapeutic target.

      Acknowledgments

      Author contributions: S. E. V. takes responsibility for and is the guarantor of the content of the manuscript, including the data and analysis. S. E. V. had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis, including and especially any adverse effects. S. E. V., A. V., L. D., A. D., T. G., I. G., V. G., J. K., D. E. V. R., L. J. C., J. Y., R. V., B. V., B. W., J. V., and W. A. W. contributed substantially to the study design, data analysis and interpretation, and the writing of the manuscript.
      Financial/nonfinancial disclosures: None declared.
      Role of sponsors: The sponsors had no influence in the concept and design of the study and had no access to study results.

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