Cellular and airway remodelling changes in asthma and COPD overlap (ACO)

Background: Asthma and chronic obstructive pulmonary disease (COPD) are two prevalent heterogeneous airway diseases affecting millions of people worldwide. They are characterized as distinct entities, despite the fact that their underlying pathophysiological mechanisms are numerous, meaning that they comprise a variety of disease subtypes, including the overlapping features, known as asthma COPD overlap (ACO). ACO is not a disease, but a condition describing patients with asthma who have persistent airflow limitation or COPD patients exhibiting variable airflow limitation with airway inflammation. Critical components of asthma and COPD diseases include chronic but variable inflammation throughout the airway and airway wall remodelling. Therefore, it is quite possible that these two components play an active role in the ACO. Patients with ACO are considered to be more vulnerable than those suffering from asthma or COPD alone, therefore, if not managed adequately, the burden on the healthcare system is huge. Despite the considerable clinical implication, it is surprising that no consensus on a universally accepted definition or a clear diagnostic criterion for ACO exists. Our understanding of ACO is at a very preliminary stage due to a dearth of original research. Therefore, it is vital to have a deeper understanding of the pathological mechanisms behind this critical condition in order to comprehend the natural course and best therapy options for this patient population. I have taken up this research to generate vital evidence in relation to airway remodelling and cell populations from the large airway endobronchial biopsy (EBB) tissues of patients with ACO and compared them with asthma, COPD ex-smokers (ES), and current smokers (CS), normal lung function smokers (NLFS), and healthy controls (HC). Furthermore, I have also evaluated the epithelial-to-mesenchymal transition (EMT) in these groups.
Methods: For the assessment of large airway morphometric remodelling changes, I performed histochemical staining for EBB tissues using the Masson trichome. Following staining, I have captured images using lower (10X) and higher (40X) magnifications. I have then assessed the changes in epithelium, goblet cells, reticular basement membrane (RBM), cellularity, lamina propria (LP), and smooth muscle (SM). For the evaluation of the inflammatory cell populations, I have used immunohistochemistry as the method of detection. I have stained EBB tissues with macrophages, mast cells, eosinophils, neutrophils, CD4+, and CD8+ T-cells biomarkers and captured bright filed images of stained tissues using higher magnification (40X). Then, I evaluated the images for inflammatory cells in the epithelium, RBM, and LP up to 120 µM deep. For assessment of EMT, I have done immunohistochemical staining with E and N-cadherin, vimentin, S100A4, and collagen IV. After capturing bright filed stained tissue images using higher magnification (40X), I assessed the percent expression (epithelial E and N-Cadherins) and marker (vimentin, and S100A4) positive cells in epithelium (basal cells) and RBM, and collagen IV positive vessels in the epithelium, RBM, and LP. Additionally, I evaluated the degree of RBM fragmentations. I performed image analysis using the software Image-Pro Plus 7.0. Following the data distribution check using D'Agostino & Pearson test, I assessed intra- and inter-group variances using one-way ANOVA and Kruskal–Wallis with multiple comparisons using uncorrected Dunn's test. I explored the correlation between parameters and assessed the effect of ICS on the morphometric parameters, cell population, and EMT markers using a nonparametric test. I used GraphPad Prism v9 (San Diego, CA, USA) for statistical analysis
Results:
Morphometric remodelling changes- A limited change was noted in the ACO epithelium compared with other pathological groups. RBM was substantially thicker in ACO than in HC and tended to be thicker than in patients with asthma and NLFS. The total RBM cells were significantly higher in ACO than in the HC, COPD-CS, -ES, and NLFS, but did not differ from patients with asthma. Goblet cells were substantially higher in the ACO than in the HC and COPD-ES. The total LP cells in ACO appeared to be higher than in HC, COPD-CS, and NLFS but appeared to be lower than in patients with asthma. Finally, the SM area was significantly lower in the ACO than in patients with asthma, COPD-CS, and NLFS
Inflammatory cell populations- Compared to asthma and COPD, ACO had higher macrophages and lower neutrophils among the inflammatory cells evaluated. Macrophages substantially increased in the epithelium, RBM, and LP of ACO than HC, COPD-CS, and -ES. Compared to NLFS, macrophages were remarkably higher only in ACO RBM. Neutrophils were low in ACO RBM and LP than HC, COPD-CS, -ES, and NLFS. Neutrophils in the epithelium were unchanged except in HC, with significantly higher cells than ACO. While CD8+ cells in epithelium and LP were unchanged among groups, it increased significantly in ACO RBM than HC, ES, and NLFS. Similarly, CD4+ cells were unchanged among groups except with significantly higher cells in ACO RBM than in asthma, COPD-CS, ES, and NLFS.
EMT- In the epithelium, as compared to HC, ACO had a substantially decreased percent expression of E-cadherin and a statistically insignificant increase of N-cadherin percent expression. Both vimentin and S100A4 positive basal cells tended to be higher than the HC. In the RBM, S100A4 positive cells were markedly high in ACO than HC. Vimentin-positive cells were elevated in the RBM of ACO than HC without statistical significance. I noticed the greatest RBM fragmentation in ACO which was more substantial than the HC. The number of vessels in ACO was higher in the RBM compared to HC; however, was lower in the LP.
Conclusion: There is an urgent unmet need to characterize the ACO phenotype pathologically, especially compared with the contributing pathological groups of asthma and COPD. My research on ACO, is the first to report such a detailed analysis of large airway morphometric changes in patients, inflammatory cellular composition, and EMT in these patients. The findings suggest that patients with ACO have differential airway remodelling changes that appeared to be more severe in some aspects than asthma and COPD. ACO airway tissue inflammatory cellular profile was different from the contributing diseases of asthma and COPD with a predominance of macrophages, and finally an active EMT process in the large airways of patients with ACO.