Discussion
Several recent studies have associated cigarette smoking in the organ donor with adverse outcomes in the lung transplant recipient. For example, in a prospective cohort study of 1255 lung transplant recipients by the Lung Transplant Outcomes Group, donor smoking was an independent risk factor for primary graft dysfunction after lung transplantation. In another study of 1295 lung transplant recipients in the UK Transplant registry, those who received lungs from donors who were smokers had significantly lower 3-year survival. The current study was designed to investigate potential mechanisms for these reported relationships between donor smoking and short- and long-term adverse outcomes in lung transplant recipients.
In lungs recovered from 298 donors, we found that current or ever smokers had significantly higher recovered lung weights, suggestive of increased pulmonary edema. This finding was associated with poorer donor oxygenation during the donor management period. To determine whether the increases in pulmonary edema as estimated by lung weight were due to lung epithelial dysfunction, we measured the rate of alveolar epithelial fluid clearance in recovered lungs. Although there were no significant differences in mean rates of net AFC between smokers and nonsmokers, donors with the highest cigarette smoke exposure (≥20 pack years) had slower rates of AFC, suggesting that detrimental effects of cigarette smoke on alveolar epithelial fluid transport function may be dose related. Although we have previously reported that AFC rates are impaired in lung transplant recipients with primary graft dysfunction, this is the first study, to our knowledge, to systematically measure the rate of AFC in a large number of donor lungs. Since intact AFC mechanisms are critical to the resolution of both acute lung injury and primary graft dysfunction, the finding of an inverse association between pack years of smoking and rates of AFC suggests one potential mechanism to explain the reported association between donor smoking and primary graft dysfunction in lung transplant recipients.
Levels of SP-D, a biomarker of alveolar epithelial type II injury, were lower in the BAL in current or ever smokers. Decreased levels of SP-D in the pulmonary edema fluid have been previously reported as a marker of lung epithelial injury in the ARDS. In addition, in one study of 110 healthy volunteers, BAL SP-D levels were lower in smokers compared to nonsmokers. Interestingly, the median SP-D levels in the BAL were substantially higher (~600 ng/mL in nonsmokers) in that study compared to the levels that we report in donor lungs. Although these differences could be due to different immunoassay and BAL methods, another potential explanation is that lower levels in the BAL reflect lung epithelial injury even in nonsmoking donors perhaps due to mechanical ventilation, critical illness or the underlying insult leading to brain death.
Levels of the proinflammatory chemokine IL-8, a chemokine that is produced abundantly by activated lung epithelium, and induced in the lung epithelium by cigarette smoke were increased in the BAL from smokers. This finding is in contrast to several prior reports of BAL IL-8 levels in healthy volunteers that found no significant differences perhaps due to the small numbers of patients enrolled (n = 18–39). Kuschner et al did report higher BAL IL-8 levels in smokers (n = 16) compared to nonsmokers (n = 14). Of note, in all prior studies where actual IL-8 levels are available, the levels in the BAL were substantially lower than levels measured in the current study with median levels in the 30 pg/mL range compared to medians of 834 pg/mL in noncurrent smokers and 1888 pg/mL in current smokers in this study. Although the higher IL-8 levels in the current study could be due to methodologic differences in immunoassays and BAL, the high levels are comparable to the levels measured in normal volunteers after lipopolysaccharide challenge and may reflect brain death-related or ventilator-induced lung inflammation in the critically ill donor population. There was no difference in the mean time from brain death to organ procurement between current smokers and current nonsmokers (data not shown), indicating that differences in IL-8 were not due to the timing of organ procurement with relation to the early proinflammatory and late immunosuppressive effects of brain death. When taken together, the findings of higher IL-8 and lower SP-D in the BAL of smokers suggest that donor smoking is associated with significant lung epithelial dysfunction and release of proinflammatory mediators that could contribute both to pretransplant lung dysfunction as manifested by pulmonary edema and to posttransplant lung dysfunction including primary graft dysfunction and long-term graft and recipient survival. Furthermore, the high BAL IL-8 levels in smokers may have contributed to the lack of efficacy of albuterol in the parent BOLD trial of albuterol versus placebo since we have recently reported that IL-8 can impair beta-adrenergic agonist stimulated up-regulation of AFC.
Chronic alcohol ingestion has also been associated with adverse effects on the lung including reduced antioxidant capacity, propensity to develop acute lung injury, and lung epithelial dysfunction including alterations in alveolar epithelial barrier properties, ion transport and fluid clearance. Chronic alcohol consumption was common in the donor population studied and was significantly more common in current or ever smokers. This finding raised the concern that alcohol use might be confounding the association between smoking and increased pulmonary edema, decreased oxygenation, and decreased SP-D and increased IL-8 in the BAL. When chronic alcohol users were compared to nonusers, there were no significant differences in lung weight, oxygenation, AFC rates or BAL IL-8 levels. However, BAL SP-D levels were lower in alcohol users consistent with more severe epithelial injury in this group; the lowest BAL SP-D levels were observed in donors who both smoked and drank alcohol, suggesting a possible additive effect of cigarette smoke exposure and alcohol use on the lung epithelium. These findings are important, in that studies of the impact of cigarette smoking on lung function do not typically take into account possible confounding by alcohol use. In addition, alcohol use can be difficult to quantify without the use of standardized validated questionnaires. Future prospective studies of the impact of cigarette smoking on donor and lung transplant recipient outcomes should aim to collect quantitative measures of alcohol use.
One question that arises from the current study is whether the findings support a limitation on the use of lungs from donors with cigarette smoke exposure. A detailed analysis of outcomes in the UK Registry study suggested that limiting the use of lungs from smokers would lead to increased death on the waiting list for lung transplantation that would not be offset by improved survival in lung transplant recipients. Our study focused on the effect of donor smoking on lung epithelial function in order to understand mechanisms of disease, and by necessity did not include clinical outcomes in transplant recipients; therefore, drawing conclusions related to clinical practice would be premature. Rather than limit the use of lungs from donors that smoked, one potential benefit of the current study is to provide targets for potential therapeutic interventions that might be used to improved outcomes in recipients who received lungs from donors that smoked. The current findings suggest that therapies that target the lung epithelium and/or the proinflammatory response might be helpful.
This study has several strengths. First, to our knowledge, it is one of the only studies to date to quantify the effects of long-term cigarette smoke exposure on physiologic and biochemical indices of lung epithelial injury in the explanted human lung. Close to 300 explanted human lungs were studied, providing a robust sample size for analysis. Second, the study includes predominantly young and otherwise healthy organ donors without chronic lung disease, making it likely that the observed changes are related to cigarette smoke exposure and less likely that they are due to advanced cigarette smoke-related lung disease. Indeed, only a small minority of the donors studied had any history of chronic lung disease. Finally, the experimental model, which includes measurement of rates of AFC in the isolated perfused human lung is a novel feature of this study that has not previously been applied to such a large number of human lung explants.
This study also has some limitations. First, both smoking history and alcohol history were obtained from the donor social history. The donor social history is usually obtained by the organ procurement organization from the closest available relative of the brain dead organ donor and may not be completely accurate. In addition, quantitative exposure estimates, particularly for alcohol, but also for duration and number of cigarettes smoked are likely to be inaccurate. Moreover, exposure to secondhand cigarette smoke, which might also be harmful, is not captured in the social history. In future studies, measurement of biomarkers of cigarette smoke exposure in organ donors such as serum cotinine or urine NNAL (4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol) could provide better quantification of cigarette smoke exposure. A second limitation is that all the lungs that were studied were deemed to be not suitable for transplantation. It was not possible in this study to obtain samples of lungs that were utilized for transplantation, and thus, it is not possible to determine if any of the observed changes in the lungs that were recovered for this study might have had an impact on lung function if transplanted. A third limitation is that the measure used to assess pulmonary edema, total lung weight, may not be as quantitative as gravimetric methods. However, in initial studies in the BOLD cohort, we observed that the lung wet-to-dry weight ratio was highly variable and inconsistent, likely reflecting heterogeneity in distribution of excess lung water, particularly between dependent and nondependent lung regions. By contrast, total lung weight was highly correlated with the extent of radiographic infiltrates as scored on the anterior-posterior chest radiograph. For this reason, it is likely that the total lung weight is actually more accurate as a global index of pulmonary edema than the lung wet-to-dry weight ratio. In addition, these lungs have very little intravascular blood volume so this means that the wet weight measurement should primarily reflect extravascular lung water. A final limitation is that certain variables in the study could not be controlled; for example, the lungs were not flushed at the time of resection, and retained blood volume may have been variable. Likewise, the cold ischemic time prior to reperfusion for measurement of AFC was also variable.
In conclusion, chronic exposure to cigarette smoke results in more pulmonary edema (as estimated by lung weight) and worse oxygenation in the potential organ donor. Mechanistically, these findings may be explained in part by more alveolar inflammation (elevated IL-8) and a dysregulated alveolar epithelium (impaired alveolar epithelial fluid clearance and reduced levels of SP-D), findings that may be exacerbated by chronic alcohol use. These abnormalities in lung fluid balance, gas exchange and alveolar epithelial function could be important determinants of the risk of acute and chronic lung dysfunction following lung transplantation in donor lungs exposed to cigarette smoke.