BACKGROUND
Progressive respiratory failure is the primary cause of death in the coronavirus disease 2019 (Covid-19) pandemic. Despite widespread interest in the pathophysiology of the disease, relatively little is known about the associated morphologic and molecular changes in the peripheral lung of patients who die from Covid-19.
METHODS
We examined 7 lungs obtained during autopsy from patients who died from Covid-19 and compared them with 7 lungs obtained during autopsy from patients who died from acute respiratory distress syndrome (ARDS) secondary to influenza A(H1N1) infection and 10 age-matched, uninfected control lungs. The lungs were studied with the use of seven-color immunohistochemical analysis, micro–computed tomographic imaging, scanning electron microscopy, corrosion casting, and direct multiplexed measurement of gene expression.
RESULTS
In patients who died from Covid-19–associated or influenza-associated respiratory failure, the histologic pattern in the peripheral lung was diffuse alveolar damage with perivascular T-cell infiltration. The lungs from patients with Covid-19 also showed distinctive vascular features, consisting of severe endothelial injury associated with the presence of intracellular virus and disrupted cell membranes. Histologic analysis of pulmonary vessels in patients with Covid-19 showed widespread thrombosis with microangiopathy. Alveolar capillary microthrombi were 9 times as prevalent in patients with Covid-19 as in patients with influenza (P<0.001). In lungs from patients with Covid-19, the amount of new vessel growth — predominantly through a mechanism of intussusceptive angiogenesis — was 2.7 times as high as that in the lungs from patients with influenza (P<0.001).
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CONCLUSIONS
In our small series, vascular angiogenesis distinguished the pulmonary pathobiology of Covid-19 from that of equally severe influenza virus infection. The universality and clinical implications of our observations require further research to define
In this study, we examined the morphologic and molecular features of seven lungs obtained during autopsy from patients who died from SARS-CoV-2 infection. The lungs from these patients were compared with those obtained during autopsy from patients who had died from ARDS secondary to influenza A(H1N1) infection and from uninfected controls. The lungs from the patients with Covid-19 and the patients with influenza shared a common morphologic pattern of diffuse alveolar damage and infiltrating perivascular lymphocytes. There were three distinctive angiocentric features of Covid-19. The first feature was severe endothelial injury associated with intracellular SARS-CoV-2 virus and disrupted endothelial cell membranes. Second, the lungs from patients with Covid-19 had widespread vascular thrombosis with microangiopathy and occlusion of alveolar capillaries.Third, the lungs from patients with Covid-19 had significant new vessel growth through a mechanism of intussusceptive angiogenesis. Although our sample was small, the vascular features we identified are consistent with the presence of distinctive pulmonary vascular pathobiologic features in some cases of Covid-19.
Our finding of enhanced intussusceptive angiogenesis in the lungs from patients with Covid-19 as compared with the lungs from patients with influenza was unexpected. New vessel growth can occur by conventional sprouting or intussusceptive (nonsprouting) angiogenesis. The characteristic feature of intussusceptive angiogenesis is the presence of a pillar or post spanning the lumen of the vessel.Typically referred to as an intussusceptive pillar, this endothelial-lined intravascular structure is not seen by light microscopy but is readily identifiable by corrosion casting and scanning electron microscopy.Although tissue hypoxia was probably a common feature in the lungs from both these groups of patients, we speculate that the greater degree of endothelialitis and thrombosis in the lungs from patients with Covid-19 may contribute to the relative frequency of sprouting and intussusceptive angiogenesis observed in these patients. The relationship of these findings to the clinical course of Covid-19 requires further research to elucidate.
A major limitation of our study is that the sample was small; we studied only 7 patients among the more than 320,000 people who have died from Covid-19, and the autopsy data also represent static information. On the basis of the available data, we cannot reconstruct the timing of death in the context of an evolving disease process. Moreover, there could be other factors that account for the differences we observed between patients with Covid-19 and those with influenza. For example, none of the patients in our study who died from Covid-19 had been treated with standard mechanical ventilation, whereas five of the seven patients who died from influenza had received pressure-controlled ventilation. Similarly, it is possible that differences in detectable intussusceptive angiogenesis could be due to the different time courses of Covid-19 and influenza. These and other unknown factors must be considered when evaluating our data.22 Nonetheless, our analysis suggests that this possibility is unlikely, particularly since the degree of intussusceptive angiogenesis in the patients with Covid-19 increased significantly with increasing length of hospitalization, whereas in the patients with influenza it remained stable at a significantly lower level. Moreover, we have shown intussusceptive angiogenesis to be the predominant angiogenic mechanism even in late stages of chronic lung injury.
ACE2 is an integral membrane protein that appears to be the host-cell receptor for SARS-CoV-2.23,24 Our data showed significantly greater numbers of ACE2-positive cells in the lungs from patients with Covid-19 and from patients with influenza than in those from uninfected controls. We found greater numbers of ACE2-positive endothelial cells and significant changes in endothelial morphology, a finding consistent with a central role of endothelial cells in the vascular phase of Covid-19. Endothelial cells in the specimens from patients with Covid-19 showed disruption of intercellular junctions, cell swelling, and a loss of contact with the basal membrane. The presence of SARS-CoV-2 virus within the endothelial cells, a finding consistent with other studies,suggests that direct viral effects as well as perivascular inflammation may contribute to the endothelial injury.
We report the presence of pulmonary intussusceptive angiogenesis and other pulmonary vascular features in the lungs of seven patients who died from Covid-19. Additional work is needed to relate our findings to the clinical course in these patients. To aid others in their research, our full data set is available on the Vivli platform
Reference & source information : https://www.nejm.org/
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