To the Editors:
Whereas tobacco smoking is a well-established risk factor for the development and progression of chronic obstructive pulmonary disease (COPD), the molecular basis for individual predisposition and disease progression remains largely unknown. Immunological processes and especially autoreactive immune processes have recently been implicated, with anti-elastin antibodies and a T-helper type-1 lymphocyte response identified in patients with COPD/emphysema 1. Immune mechanisms are also implicated in interstitial lung disease and pulmonary fibrosis 2, and smoking-induced autoimmunity has been demonstrated in rheumatoid arthritis 3.
We explored whether an autoimmune process implicating anti-elastin autoantibodies and facilitated by tobacco smoking may take place in patients with combined pulmonary fibrosis and emphysema (CPFE), a distinct entity recently defined on the basis of characteristic features of chest imaging, pulmonary function and outcome 4.
Patients with CPFE, diagnosed within the previous 5 yrs using published criteria based on computed tomography of the chest 4, were studied. Patients with connective tissue disease at the time of diagnosis were excluded. Control patients were selected from within the same department of respiratory medicine and had various diagnoses: idiopathic interstitial pneumonia (n = 13); pre-capillary pulmonary hypertension (n = 12); eosinophilic lung disease (n = 7); sarcoidosis (n = 2); Wegener’s granulomatosis (n = 2); and other diagnoses (n = 6). Patient consent was obtained for the analysis of auto-immunity.
Anti-elastin antibodies were quantified using modified ELISA as previously described 1. Microplates were coated with human lung elastin peptides (Elastin Products Company, Owensville, MO, USA) or with PBS (control wells) overnight, then washed and blocked. After washing, diluted human serum samples were incubated for 2 h, then washed, incubated with peroxidase-conjugated goat anti-human immunoglobulin G and revealed with O-phenylenediamine dihydrochloride. Optical density (OD) was determined at 490/620 nm. The specific binding of antibodies was evaluated by calculating the difference between the average OD of antigen-coated wells and that of control wells (using 2×sd above the mean of 100 sera from blood donors as a cut-off). A ratio (OD of sample/OD of cut-off value) above 1 arbitrary unit·mL−1 was considered positive.
We studied 42 patients with CPFE (37 males, five females), all smokers, with a mean age of 66.4±10.2 yrs, and 42 controls (26 males, 16 females) with a mean age of 53.2±22.8 yrs. Anti-elastin antibodies were detected in three out of 42 patients with CPFE, and three out of 44 control patients (who had Wegener's granulomatosis, Churg–Strauss syndrome and pulmonary hypertension, respectively; p = 1.0, Fisher's exact test). The titre of anti-elastin antibody was not elevated in patients with CPFE compared with controls (0.61±0.04 relative units versus 0.58±0.07; p = 0.74, unpaired t-test; fig. 1⇓).
Quantification of anti-elastin antibodies using ELISA in patients with combined pulmonary fibrosis and emphysema (CPFE) and controls. Results >1 are considered positive. –––––: median.
According to the model of Lee et al. 1, exposure to tobacco smoking might increase secretion of proteolytic enzymes in the lung from cells of the innate immune system, thereby liberating lung elastin fragments and initiating elastin-targeted T- and B-cell mediated immunity in susceptible individuals. Our results do not support the idea that either emphysema or fibrosis in CPFE may be mediated by an autoimmune process directed against elastin. Since all patients with CPFE had emphysema on imaging (mostly paraseptal emphysema, and also centrolobular emphysema), our results also fail to confirm the presence of circulating anti-elastin antibodies in patients with emphysema (at least when associated with fibrosis).
Future studies in CPFE will explore putative alternative immune processes described in other conditions. For instance, experimental autoimmune emphysema in mice may be caused by the development of antibodies against endothelial cells 5 or against vascular endothelial growth factor (VEGF) receptors 6, VEGF being a cytokine also implicated in lung fibrogenesis 7. Autoantibodies against pulmonary epithelial cells have been reported in COPD 8, and might conceivably contribute to alveolar epithelial cell injury and activation and, furthermore, to pulmonary fibrosis, a process similar to abnormal wound repair 9.
Further work is needed to address the intriguing coexistence of pulmonary fibrosis and emphysema within the lung, two processes with distinct, if not opposite, pathogenesis.
Support statement
Donations have been received from patients for research on orphan lung diseases.
Statement of interest
None declared.
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