Reply to: Comment on “Bronchiolitis Obliterans and Pulmonary Fibrosis after Sulfur Mustard Inhalation in Rats”

From the Authors:

We thank Dr. Aliannejad for his comments on our recent article in the Journal (1). We are honored to hear that our article has reached clinicians who care for patients with sulfur mustard (SM) exposure, and we hope that our translational study results can be found applicable to improving patient care. Dr. Aliannejad’s comments highlight some flaws with retrospective and cross-sectional clinical studies as well as some of the inconsistencies and controversies associated with the reported incidences of long-term respiratory complications associated with SM inhalation exposure.

In our article, the term lung fibrosis was used to indicate fibrosis found anywhere within the lungs, airway, or parenchyma. When discussing more specific areas of fibrosis, the term bronchiolitis obliterans was used to indicate bronchial fibrosis, and the term pulmonary fibrosis or parenchymal fibrosis was used to indicate parenchymal (interstitial) fibrosis. Other stenotic lesions of the trachea and subglottis, also within the lung, are all included in the lung fibrosis terminology. Thus, when stating that 75% of survivors after SM inhalation have some degree of lung fibrosis, we refer to any of those locations of fibrosis. To add to this, Ghanei and Harandi, in their scientific book publication titled Mustard Lung (2), also reported on high incidences of lung fibrosis, sometimes greater than 75%. Air trapping was noted on high-resolution computed tomographic scans in 55–67% of persons after SM inhalation, thought to be due to bronchiolitis obliterans, and alveolar septal thickening (interstitial lung disease) was seen in 26% of patients studied. Pulmonary function test findings late after SM exposure showed up to 55% obstructive and 24% restrictive patterns. Dl_CO has been noted to be below 60% after SM exposure in many individuals (3, 4), although it was not decreased in some others in articles discussed by Dr. Aliannejad. Although high-resolution computed tomographic, pulmonary function test, and Dl_CO results are all indirect findings for lung fibrosis, they pointed clinicians who reported on these patients toward a lung fibrosis type of diagnosis. Nevertheless, we recognize that without lung biopsy confirmation, all reported incidences of lung fibrosis (any type) can be viewed only as indirect evidence of disease. We also note that Dr. Aliannejad incorrectly classified these patients as having “idiopathic pulmonary fibrosis” in his letter. Patients in this study had a known SM exposure; thus, the correct terminology would be “SM-induced interstitial pulmonary fibrosis or parenchymal fibrosis,” as noted in our article (1) as well as within the referenced articles (5).

As clearly stated in multiple published reports (2, 3), parenchymal pulmonary fibrosis complication is an established late complication after SM inhalation. In our study, we aimed to model bronchiolitis obliterans and interstitial pulmonary fibrosis sequelae after SM exposure within a time frame that is logistically sound for conducting laboratory experiments (28 d). The inhaled dose of SM has been reported to have a direct correlation to the severity of late complications (2, 6). We agree that our rat model of late effects after SM shows a higher incidence of interstitial pulmonary fibrosis than noted in humans, and this is most likely due to a higher dose of SM inhaled by these animals. A lower dose of SM will decrease lethality and will increase time to fibrosis in our rats. The high scrubbing ability of upper airways (nose and sinuses) will decrease the amount of SM that reaches the alveoli in both humans and rats. Rats, however, have a higher surface area than humans for nasal scrubbing of agents (7). Indeed, this high degree of nasal scrubbing prevented lung injury in our early modeling studies when SM was given via the nasal inhalation route to rats. Therefore, we decided to use an intubated model (1, 8) to obtain a more targeted injury within the lung and to create a 28-day model for future drug efficacy testing. Therefore, we agree with Dr. Aliannejad that the rats we studied likely received a higher dose of SM than a human patient with SM lung fibrosis who initially presents 10 years after exposure.

In a previous publication (8), our group reported on the regional differences in SM epithelial injury in rats between the central airways, specifically the mainstem bronchi and proximal bronchioles, as well as the more distal lung parenchyma and terminal bronchioles. The most significant direct injury occurred to the mainstem bronchi and proximal bronchioles, where there was loss of epithelial integrity and complete epithelial denudation one day after exposure. Distal parenchymal and terminal bronchiolar architectures were preserved without epithelial denudation, suggesting less direct SM injury. The rats were intubated to the midtrachea during exposure, so it would seem unnecessary to report pathological injury to the central airways above the carina, as suggested by Dr. Aliannejad, because injury to those areas was prevented by intubation exposure. Our goal for these studies was not to model subglottic or upper tracheal stenosis, but rather to model bronchiolitis obliterans and parenchymal interstitial fibrosis.

Most bronchiolitis obliterans lesions and parenchymal fibrosis lesions of any etiology are heterogeneous in distribution, and SM-related fibrosis lesions are no different. We do not know the cause of the heterogeneity of lung disease after SM inhalation, but we suspect a systemic reaction may be involved, because no acute alveolar injury was seen, and yet interstitial fibrosis still developed. Inflammation and infection were both evaluated in our model, unfortunately only after submission of the manuscript for publication, with noted abnormalities that related both to time as well as to length of survival. Details of these findings will be reported in a follow-up paper.