THE LANCET Infectious Diseases
Summary
Background
Methods
Findings
Interpretation
Funding
Introduction
Since the earliest reports of cases in China in December, 2019, countries around the world have faced outbreaks of COVID-19, the disease caused by the novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Italy was the first country in Europe to document a large number of cases of COVID-19, and Lombardy in particular was severely affected, with a total of 17 713 people testing positive for SARS-CoV-2 and 1593 admitted to intensive care units between Feb 20 and March 18, 2020.
Luigi Sacco Hospital in Milan and Papa Giovanni XXIII Hospital in Bergamo were the first hospitals in Lombardy faced with managing the epidemic crisis.
SARS-CoV-2 is the seventh member of the coronavirus family identified to cause disease in humans. Coronaviruses are enveloped, positive-sense, single-stranded RNA viruses.
Two other members of this family, severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV), cause acute diffuse alveolar damage, pneumocyte hyperplasia, and interstitial pneumonia.
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At the mild end of the clinical spectrum, COVID-19 can be asymptomatic or manifest as mild upper respiratory disease with fever and cough, while severe cases can result in pneumonia, leading to acute respiratory distress syndrome (ARDS) in around 15% of hospitalised patients.
To the date of our last literature review (May 3, 2020), the only published reports of the pathological features of COVID-19 lung involvement were from small case series or isolated cases, both from China and other countries. In those reports, the main histological features comprised exudative diffuse alveolar damage with massive capillary congestion, often accompanied by microthrombi.
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Research in context
Methods
Patient samples
Autopsies and tissue processing
Autopsies were done in airborne infection isolation autopsy rooms, with personnel using personal protective equipment in accordance with the Italian recommendations.
A team of pathologists (LC, AS, AN, RSR, AP, PZ, AG, and MN) with extensive experience in the field of infectious diseases was involved in the autopsy procedures in both hospitals.
Histopathological evaluation
Histological evaluation was done independently by two pathologists from each hospital, from the same team involved with the autopsy, who were masked to patient characteristics, symptoms, and diagnoses. Each pathologist analysed all the slides from both hospitals, and any discrepant results were jointly reviewed. The histological features of cellular and interstitial damage were described and graded on a semiquantitative scale on the basis of the percentage of tissue involved: absent (0%), rare (<5%), focal (5–25%), multifocal (26–50%), plurifocal (51–75%), or diffuse (>75%).
To quantify pulmonary intracapillary megakaryocytes, each tissue sample was scanned at low magnification to identify the hotspot area in which megakaryocytes were most easily recognisable, and CD61-positive cells in these areas were counted. A high value was defined as the presence of more than four CD61-positive cells per 25 high-power fields, which is considered to be the average number of intracapillary megakaryocytes in the lungs of people without diffuse alveolar damage.
Role of the funding source
Results
Upon macroscopic examination, the lungs of all patients were heavy, congested, and oedematous, with patchy involvement. In all cases, histological examination revealed features corresponding to the exudative and early or intermediate proliferative phases of diffuse alveolar damage (figure 1). These features were also focally associated with patterns of interstitial pneumonia (presence of inflammatory lymphomonocytic infiltrate along the slightly thickened interalveolar septa), organising pneumonia (alveolar loose plugs of fibroblastic tissue), and acute fibrinous organising pneumonia (some alveolar spaces containing granulocytes and fibrin, with the formation of balloon structures). Features indicative of the fibrotic phase of diffuse alveolar damage, such as mural fibrosis and microcystic honeycombing, were observed to be focal, suggesting that none of the patients had progressed to the fibrotic phase, possibly because of the short duration of the disease.
Morphological findings and their corresponding semiquantitative grades are reported in the table. The predominant histological pattern of the exudative phase of diffuse alveolar damage, which was observed in all cases, included capillary congestion, interstitial and intra-alveolar oedema, dilated alveolar ducts and collapsed alveoli, hyaline membranes composed of serum proteins and condensed fibrin, and loss of pneumocytes (figure 1). Platelet–fibrin thrombi in small arterial vessels (<1 mm diameter) were found in 33 (87%) cases. Moreover, type 2 pneumocyte hyperplasia, showing various aspects of cellular atypia, was present to some extent in all patients; interstitial myofibroblastic reaction was observed in 25 (66%) cases and alveolar granulation tissue in 22 cases (58%), whereas septal collagen deposition was found in 15 (39%) cases and alveolar loose plugs of fibroblastic tissue in 11 (29%). Mural fibrosis was sometimes observed (24 [63%] cases), as was microcystic honeycombing (15 [39%] cases, most often with a focal pattern of distribution). Although no clinical history of pre-existing fibrosing interstitial lung diseases was found in patients with mural fibrosis and microcystic honeycombing, the presence of mild fibrotic alterations not in continuum with other pre-fibrotic features (myofibroblastic proliferation or organising pneumonia), as expected in a context of disease progression, might suggest that fibrosing interstitial lung diseases were pre-existing in some cases.
Absent | Rare | Focal | Multifocal | Plurifocal | Diffuse | |
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Main morphological aspects | ||||||
Capillary congestion | 0 | 0 | 0 | 24 (63%) | 1 (3%) | 13 (34%) |
Interstitial and intra-alveolar oedema | 1 (3%) | 0 | 19 (50%) | 10 (26%) | 5 (13%) | 3 (8%) |
Alveolar haemorrhage | 5 (13%) | 1 (3%) | 20 (53%) | 8 (21%) | 2 (5%) | 2 (5%) |
Hyaline membranes | 5 (13%) | 1 (3%) | 19 (50%) | 5 (13%) | 3 (8%) | 5 (13%) |
Dilated alveolar ducts plus collapsed alveoli | 2 (5%) | 0 | 16 (42%) | 18 (47%) | 2 (5%) | 0 |
Endothelial necrosis | 9 (24%) | 1 (3%) | 7 (18%) | 21 (55%) | 0 | 0 |
Increased megakaryocytes | 5 (13%) | 0 | 25 (66%) | 4 (11%) | 3 (8%) | 1 (3%) |
Alveolar granulocytes | 6 (16%) | 1 (3%) | 14 (37%) | 14 (37%) | 2 (5%) | 1 (3%) |
Loss of pneumocytes | 0 | 0 | 11 (29%) | 20 (53%) | 3 (8%) | 4 (11%) |
Platelet–fibrin thrombi | 5 (13%) | 0 | 16 (42%) | 4 (11%) | 13 (34%) | 0 |
Type 2 pneumocyte hyperplasia with epithelial atypia | 0 | 0 | 14 (37%) | 9 (24%) | 8 (21%) | 7 (18%) |
Squamous metaplasia with atypia | 17 (45%) | 1 (3%) | 12 (32%) | 7 (18%) | 1 (3%) | 0 |
Interstitial myofibroblast reaction | 13 (34%) | 0 | 18 (47%) | 6 (16%) | 1 (3%) | 0 |
Alveolar granulation tissue | 16 (42%) | 1 (3%) | 13 (34%) | 3 (8%) | 4 (11%) | 1 (3%) |
Septal collagen deposition | 23 (61%) | 0 | 13 (34%) | 1 (3%) | 1 (3%) | 0 |
Alveolar loose plugs of fibroblastic tissue | 27 (71%) | 1 (3%) | 7 (18%) | 3 (8%) | 0 | 0 |
Capillary proliferation | 20 (53%) | 0 | 14 (37%) | 3 (8%) | 0 | 1 (3%) |
Organised alveoli plus dilated alveolar ducts | 29 (76%) | 0 | 6 (16%) | 3 (8%) | 0 | 0 |
Pleural involvement | 38 (100%) | 0 | 0 | 0 | 0 | 0 |
Mural fibrosis | 14 (37%) | 0 | 12 (32%) | 10 (26%) | 1 (3%) | 1 (3%) |
Microcystic honeycombing | 23 (61%) | 0 | 9 (24%) | 6 (16%) | 0 | 0 |
Further associated lesions | ||||||
Interstitial inflammatory infiltrate | 7 (18%) | 0 | 5 (13%) | 12 (32%) | 10 (26%) | 4 (11%) |
Alveolar inflammatory infiltrate (macrophages) | 14 (37%) | 1 (3%) | 13 (34%) | 8 (21%) | 0 | 2 (5%) |
Alveolar multinucleated giant cells | 19 (50%) | 6 (16%) | 9 (24%) | 1 (3%) | 1 (3%) | 2 (5%) |
Ultrastructural examination (figure 2) revealed particles suggestive of viral infection in nine (90%) of the ten cases analysed. The particles had a mean diameter of about 82 nm and projection of about 13 nm in length. The particles, assumed to be virions, were mainly localised along plasmalemmal membranes and within cytoplasmic vacuoles, as described for other coronaviruses.
Infected cells were type 1 and type 2 pneumocytes; however, in two cases, particles were observed in alveolar macrophages, albeit scarcely. No particles resembling viruses were observed in multinucleated cells. Ultrastructural analyses of alveolar capillaries frequently showed platelet and fibrin plugs within the lumina, but no particles resembling virions were detected in endothelial cells.
Discussion
SARS-CoV, MERS-CoV, and SARS-CoV-2 infections show many similarities in clinical presentation.
SARS-CoV and MERS-CoV particles have been observed and described in pneumocytes, macrophages, and lung interstitial cells by electron microscopy, immunohistochemistry, and in situ hybridisation.
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In two autopsy studies of patients who died from SARS (eight cases from Singapore
and 20 cases from Toronto),
the predominant pattern of lung injury was diffuse alveolar damage, including the exudative and proliferative phases. Inflammatory infiltrate, oedema, pneumocyte hyperplasia, fibrinous exudate, and organisation were found. The Toronto case series
included a comparison group of matched control patients who presented with respiratory symptoms and signs, died in the same period as those with SARS, and were negative for SARS-CoV. Compared with non-SARS lung lesions, SARS lesions were distinguishable by a prominence of vascular endothelial injury and extensive acute lung injury in varying stages of exudation and organisation.
Autopsy studies of patients who died from MERS are limited. In the only complete report available,
the authors described lung injury characterised by exudative diffuse alveolar damage, pneumocyte hyperplasia, and septal inflammatory infiltrate.
Despite the relevance of lung involvement in patients with COVID-19, few data regarding lung pathology are available. In a case report of a patient who died from COVID-19 in China, the histological findings in the lungs included desquamation of pneumocytes, diffuse alveolar damage, and oedema.
In addition, Tian and colleagues
described the pulmonary pathology of early-phase COVID-19 in two patients with lung carcinoma; both patients showed signs of the exudative phase of diffuse alveolar damage.
In our study, fibrin thrombi in the small arterial vessels (<1 mm diameter) were observed in 87% of cases, around half of which had involvement of more than 25% of the lung tissue as well as high levels of D-dimers in the blood. These findings might explain the severe hypoxaemia that characterises ARDS in patients with COVID-19.
Vascular microthrombi are often identified in areas of diffuse alveolar damage, and are associated with diffuse endothelial damage. These features, although not pathognomonic, were frequent in our series, widespread in the lung samples of the patients examined, and the predominant distinctive vascular component.
Our data support the hypothesis proposed in clinical studies
that COVID-19 is complicated by coagulopathy and thrombosis. Furthermore, D-dimer values of greater than 1 μg/mL have been associated with fatal outcomes in patients with COVID-19.
For these reasons, the use of anticoagulants has been suggested to be potentially beneficial in patients with severe COVID-19, owing also to their anti-inflammatory properties, although their efficacy and safety are being closely monitored.
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In this study, we looked for virions in a subset of patients, and found particles resembling virions to be present, albeit rarely, in the cytoplasm of pneumocytes and macrophages. The morphology of the observed particles (about 80 nm in diameter, enveloped, with spike-like projections, and an electron-lucent core with peripheral electron-dense granules of the sectioned nucleocapsid) and their intravacuolar cytoplasmic location are consistent with the reported ultrastructural features of coronaviruses, including SARS-CoV-2.
Despite the low number of cases assessed, these findings might suggest that the virus remains in the lung tissue for many days, even if in small quantities, and might trigger the mechanism that leads to lung damage and causes it to progress. Further histological and molecular analyses and extension of the case series are ongoing to better define the cellular and tissue distribution of the virus as well as inflammatory responses in different organs.
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Figures
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Figure 1Haematoxylin and eosin-stained sections from representative areas of lung parenchyma with diffuse alveolar damage
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Figure 2Electron microscopy of a representative case
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