Histopathological and Immunohistochemical Findings in Postmortem Lungs from Mexican Patients with Severe COVID-19.

During the COVID-19 pandemic, SARS-CoV-2 quickly spread all over the world in a pattern of waves. In Mexico, the first wave was from March 2020 to September 2020, and during this time autopsies were forbidden. After that, the postmortem lung samples allowed us to identify histological alterations because of COVID-19. Moreover, SARS-CoV-2 infections are characterized by the manifestation of cytopathic effects like inclusion bodies, and multinucleated cells in alveolar spaces and alveolar walls. Additionally, atypical, enlarged cells, presence of macrophages in alveolar spaces, and congestion of vascular vessels were the other histopathologic alterations of the lung. Our study covered the analysis of nine postmortem lung samples from patients with severe COVID-19 diagnosed by qRT-PCR. The samples were stained with Hematoxylin-Eosin to identify the histological alterations related to lung architecture and cell populations and were subjected to immunohistochemistry for the SARS-CoV-2 Spike and Nucleocapsid proteins. All samples showed alterations associated with diffuse alveolar damage and 1/9 presented no alveolar space, 5/9 presented different levels of pleural fibrosis, and 4/9 presented distention of the small capillaries. Immunohistochemistry results revealed that 4/9 samples showed Spike-positive cytoplasmic inclusion bodies in type I pneumocytes and 2/9 Spike-positive nuclear inclusion bodies in type I pneumocytes. These inclusion bodies were found to be eosinophilic with H&E stains. The H&E results suggest tissue alterations that may contribute to the signs and symptoms of severe COVID-19, as well as the Spike protein expression, as its distribution suggests its participation in pathophysiology.