Extracellular matrix remodeling of lung alveolar walls in three dimensional space identified using second harmonic generation and multiphoton excitation fluorescence

The structural reorganization of extracellular matrix (ECM) is an important feature of peripheral lung tissue remodeling in chronic obstructive pulmonary disease (COPD). Ordered ECM macromolecules such as the fibril-forming collagens produce second harmonic generation (SHG) signal without the need for any exogenous label, while ECM macromolecules such as the elastin fibers generate MPEF signal due to their endogenous fluorescence characteristics. Both these signals can be captured simultaneously to provide spatially resolved 3D structural reorganization of ECM matrix. In this study, SHG and MPEF microscopy methods were used to examine structural remodeling of the ECM matrix in human lung alveolar walls undergoing severe emphysematous destruction. Flash frozen lung samples removed from two patients undergoing lung transplantation for severe COPD (n=4) were compared to similar samples from an unused donor lung (n=2) that served as a control. The imaging operations were performed directly on these tissue sections at least three different areas. The generated spatially resolved 3D images showed the distribution of collagen and elastin in the alveolar walls. In the case of the control, we found well ordered alveolar walls with a composite type structure made up of collagen bands and relatively fine elastic fibers. In contrast, lung tissues undergoing emphysematous destruction were highly disorganized with significantly increased alveolar wall thickness compared to the control. We conclude that these non-invasive imaging modalities provide spatially resolved 3D images with spectral specificities that are sensitive enough to identity the ECM structural changes associated with emphysematous destruction.