Histopathology.
Lung tissues were fixed in neutral-buffered formalin and embedded in paraffin. Lung sections were cut at 4 μm, dewaxed, rehydrated, and stained with hematoxylin and eosin. Various lung pathologic features including acute or organizing diffuse alveolar damage (DAD), alveolar hemorrhage, thrombosis, and fibrotic remodeling described earlier in COVID-19 patients38-40 were evaluated by an experienced pathologist (AK) in a blinded manner. Pathological lesions of acute DAD were characterized by disrupted alveolar epithelium, epithelial sloughing, formation of hyaline membranes, necrotizing bronchiolitis, interstitial inflammation, oedema, fibrin deposition, vasculitis, pulmonary hemorrhage, and microvascular thrombosis. The organizing form of DAD was defined as loosely organized fibrosis, interstitial fibrotic changes, and abnormal remodeling of epithelium with hyperplasic type II pneumocytes and proliferative airway epithelium. Fibrotic changes were defined as extensive and dense interstitial fibrosis replacing lung parenchyma and disintegrating alveolar architecture. The histopathological grading was 0-absent, 1-mild, 2-moderate, and 3-severe. Lung sections were stained with Verhoff van Gieson (VVG) and orcein for detecting elastin fibers and Masson’s trichrome for identification of collagen depositions.