Pulmonary pathology of COVID-19

Currently, animal-to-human transmission of SARS-CoV-2 (COVID-19) has not yet been confirmed, whereas the main mode of transmission is human-to-human. Droplets are the main route of human-to-human transmission, whereas aerosols could be another route in addition to stool-based transmission. Currently, no evidence is available to indicate intrauterine vertical transmission of SARS-CoV-2 (COVID-19) in pregnant women. In the host, the life cycle of coronavirus consists of 5 steps: 1) attachment, 2) penetration, 3) biosynthesis, 4) maturation, and 5) release. Once viruses bind to host receptors (attachment), they enter host cells, particularly type II pneumocytes via endocytosis or membrane fusion (penetration). Once viral contents are released inside the host cells, viral RNA enters the host’s nucleus for replication and making viral proteins (biosynthesis). New viral particles are produced (maturation) and released. Spike protein of coronaviruses which determines the diversity of coronaviruses and host tropism is composed of a transmembrane trimetric glycoprotein protruding from the viral surface. Structural and functional studies demonstrated that the spike protein the of coronaviruses can bind to angiotensin converting enzyme 2 (ACE2), a functional receptor for SARS-CoV. ACE2 expression is high in lung (high expression on lung epithelial cells), heart, ileum, and kidney. The lungs of severe COVID-19 patients demonstrate infiltration of a large number of inflammatory cells. Due to high ACE2 expression on the apical side of lung epithelial cells in the alveolar space, SARS-CoV-2 (COVID-19) can enter and destroy lung epithelial cells. Significant ACE2 expression on innate lymphoid cells (ILC)2, ILC3, and endothelial cells is also demonstrated. Pulmonary endothelial cells represent one third of the lung cells. Endothelial function includes promotion of anti-aggregation, fibrinolysis, and vasodilatation. Due to a significant role playing in thrombotic regulation, hypercoagulable profiles that are demonstrated in severe COVID-19 patients likely suggest significant endothelial injury. Pulmonary thrombosis and embolism accompanying elevation of d-dimer and fibrinogen levels have been demonstrated in severe COVID-19. In conclusion, whether these histopathological lesions are direct consequences of sepsis, SARS-CoV-2 (C)OVID-19), and /or multiple organ failure is difficult to conclude. Further studies on understanding the roles of ILC1, ILC2, ILC3, including the difference in response to SARS-CoV-2 (COVID-19) infection between children and adults are urgently needed to develop efficient targeted therapies.