Our data indicate that bronchial epithelium remains structurally and functionally abnormal in patients with COPD for years after smoking cessation and that the extent of structural changes to the epithelium in both large and small airways correlates with disease severity. We found a strong relationship between epithelial abnormalities, localized deficiency of SIgA on the bronchial mucosal surface, and the presence of CD4+
lymphocytes. In addition, we found that mucosal SIgA deficiency is associated with latent or persistent herpesvirus infection in small conducting airways, submucosal thickening, fibrotic remodeling of the airway walls, and severity of airway obstruction. These results extend previous reports demonstrating associations of airflow limitation with immune and inflammatory cell infiltration (24
) and mural fibrotic remodeling of small airways in COPD (19
) by linking these changes with epithelial remodeling and deficiency of SIgA on the epithelial surface. In cultured airway epithelial cells, we found that complete differentiation to a pseudostratified structure was necessary for IgA transcytosis. Together, our findings point to a possible role for epithelial structural abnormalities and impaired mucosal immunity in persistent airway inflammation and functional decline in former smokers with COPD.
We showed that SIgA-mediated bronchial mucosal host defense is closely associated with bronchial epithelial cell differentiation and that only normal-appearing pseudostratified ciliated epithelium is able to express pIgR and transport pIgA to the bronchial mucosal surface. In areas of goblet cell hyperplasia, we found limited SIgA delivery to the epithelial surface despite pIgR expression in ciliated cells, suggesting that reduced SIgA delivery in these areas results from a reduction in the number of epithelial cells responsible for IgA transport rather than alterations in molecular mechanisms of delivery. More severe epithelial structural disorders with incomplete or altered epithelial cell differentiation were characterized by a lack of pIgR expression in epithelial cells and striking SIgA deficiency on the luminal surface. Because only a small portion of the bronchial mucosa in tissue specimens from patients with severe COPD was covered by normal differentiated bronchial epithelium, our data suggest widespread bronchial mucosal SIgA deficiency in these patients. This concept was further supported by identification of reduced SIgA in BAL from patients with COPD.
Because the pIgR makes only one passage across epithelial cells before being cleaved (8
), one molecule of pIgR must be produced by the epithelial cell for every pIgA transported across the epithelial layer. Therefore, diminution of pIgR expression results in a proportional reduction in pIgA delivery to the epithelial surface. In airway and intestinal mucosa, pIgR expression can be transcriptionally up-regulated by proinflammatory cytokines (29
). Thus, our finding that pIgR is reduced in the inflammatory environment of the COPD airway is paradoxical. Whereas the molecular mechanisms responsible for down-regulation of pIgR in COPD airways requires further investigation, we speculate that the reduction in pIgR expression results from altered transcriptional profiles in epithelial cells that are not completely differentiated. This idea is supported by our finding of reduced PIGR
mRNA expression in airway epithelium from patients with COPD and incompletely differentiated epithelium in vitro
. Alternatively, it is possible that reduction in pIgR results from increased protein degradation, given the prior report that neutrophil serine proteases can cleave pIgR (33
); however, if this were the case it would be hard to explain the precise correlation between pIgR expression and epithelial structure in the same airway.
Although surface SIgA deficiency occurs in both large and small airways, there are differences in the pathophysiologic mechanisms at these different anatomic sites. In small airways, SIgA secretion is possible only by local transcytosis via a pIgR-mediated mechanism (6
). Progressive epithelial remodeling and the associated reduction in pIgR expression subsequently result in profound surface SIgA deficiency. However, in large airways, SIgA may be secreted from serous cells of submucosal glands as an alternative mechanism to local transcytosis (6
). Although hyperplasia of submucosal glands in COPD might suggest increased SIgA secretion and abundance in large airways, we demonstrate that SIgA deficiency develops on the mucosal surface in large airways despite the presence of submucosal IgA-producing plasma cells and pIgR expression in submucosal glandular cells. Although the cause is not clear from our data, it is possible that damage to the bronchial cilia apparatus and decreased mucociliary clearance limit mucosal distribution of SIgA secreted by submucosal glands. Furthermore, dehydration of periciliary liquid on the epithelial surface, frequently seen in patients with COPD (34
), may preclude adequate storage and maintenance of surface SIgA. Interestingly, we found reduced SIgA, but not total IgA, in BAL fluid from patients with COPD. Although the reason for this discrepancy is not entirely clear, this imbalance could be caused by increased passive leakage of serum IgA into airways in patients with COPD instead of secretion via pIgR-dependent mechanisms.
Recent studies have demonstrated an important role for viral and bacterial pathogens in COPD exacerbations (12
). The most common viral pathogens implicated are rhinovirus, influenza, parainfluenza, and respiratory syncytial virus (12
). Respiratory syncytial virus and adenovirus have been detected by RT-PCR in a high percentage of patients with stable COPD, raising the possibility that latent or low-grade viral infection may have implications in the pathogenesis of COPD (12
). In this study, we detected EBV and CMV antigens in bronchial epithelial cells almost exclusively in SIgA-deficient small airways. These herpesviruses were chosen for evaluation because they can establish long-standing latency in host tissues (38
). In addition, EBV and CMV infections have been previously demonstrated in patients with COPD (14
) and idiopathic pulmonary fibrosis (40
). Because one of the host defense functions of surface SIgA is direct intraepithelial viral clearance (42
), reduced pIgR expression and impaired pIgA transcytosis may be critical factors for persistent herpesvirus infection of bronchial epithelium in COPD.
cytotoxic T lymphocytes predominate in the inflammatory cell infiltrates of small airways in patients with COPD (22
). The mechanisms by which these cells are recruited to airways in COPD are not fully understood, but one possibility is the immune response to respiratory viral infection (22
). Because CD8+
cells play a substantial role in clearance of respiratory virus via contact-dependent effector functions (47
), chronic colonization or recurrent or persistent viral infections may be responsible for T cell–mediated airway cell and tissue injury in patients with COPD. Chronic inflammatory responses also seem to induce subepithelial fibrosis that characterizes small airways in COPD and that contributes to airflow limitation (19
The strong correlations between small airway SIgA deficiency, viral infection, increased CD8+
T lymphocyte infiltration, airway wall remodeling, and clinical status demonstrated in this study suggest that mucosal SIgA deficiency may be an important pathogenetic factor in the development of progression of COPD. In addition to malfunction of the mucociliary clearance mechanism secondary to bronchial epithelial remodeling (49
), concomitant surface SIgA deficiency likely contributes to derangement of mucosal immune host defense in COPD. We speculate that acquired surface SIgA deficiency increases the risk of chronic or repeated airway infection and enhances exposure of the bronchial mucosa to inhaled antigens. The resulting antigen deluge fuels persistent activation of adaptive immune responses and local accumulation of immune and inflammatory effector cells. In turn, chronic airway inflammation drives further epithelial remodeling and progressive fibrotic changes in COPD airways. This pathologic sequence may help to explain the persistence of airway inflammation and bronchial epithelial remodeling seen in patients with COPD years after smoking cessation. Importantly, the pathologic cycle proposed previously may be susceptible to therapeutic measures designed to alter the progression of disease by restoring mucosal immunity and therefore curtailing ongoing airway inflammation and fibrotic remodeling.