Pathology and Pathogenesis of COVID-19 

Wun-Ju Shieh, MD, MPH, PhD 

The ongoing global pandemic of coronavirus disease (COVID-19), caused by severe  acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was identified in Wuhan, Hubei  Province, China, and has spread rapidly around the world since January 2020. This  emerging virus has posed a major threat to global health. Coronaviruses are enveloped,  positive-stranded RNA viruses that infect many animals; human-adapted viruses likely are  introduced through zoonotic transmission from animal reservoirs. Most known human  coronaviruses are associated with mild upper respiratory illness. SARS-CoV-2 belongs to  the group of betacoronaviruses that includes severe acute respiratory syndrome  coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV),  which can infect the lower respiratory tract and cause a severe and fatal respiratory  syndrome in humans. SARS-CoV-2 has >79.6% similarity in genetic sequence to SARS CoV. SARS-CoV-2 is highly transmissible among humans; fatality rates for COVID-19 vary  and are higher among the elderly and persons with underlying conditions or  immunosuppression. 

A specific etiologic diagnosis is particularly important during the outbreak because  of the impact on hospital infection control and other public health measures. Since clinical  features of COVID-19 are similar to many other respiratory infections, a definitive  diagnosis can be made only by laboratory confirmation. Laboratory assays currently used  for SARS-CoV-2 diagnosis include virus isolation, PCR, and serology. Pathology-based  methods, such as immunohistochemistry (IHC), in-situ hybridization (ISH), and electron  microscopy (EM) offer the advantage of interpreting the test results within a morphologic  context and can also be used for diagnosis if appropriate tissue specimens are available.  Furthermore, pathologic evaluation and determination of virus distribution and cellular  localization within tissues is crucial to elucidating the pathogenesis of these fatal 

infections and can help guide development of therapeutic and preventive  countermeasures. The current knowledge about COVID-19 pathogenesis and pathology  in fatalities is based on a small number of described cases and extrapolations from what  is known about other similar coronaviruses, such as SARS-CoV and MERS-CoV. 

The clinicopathologic, immunohistochemical, and electron microscopic findings in  tissues from fatal laboratory-confirmed cases of SARS-CoV-2 infection in the United  States have been studied and reported by Infectious Diseases Pathology Branch, CDC. In  these case-patients, histopathologic evaluations performed on autopsy tissues consistently  showed mild to moderate tracheobronchitis and characterized by mononuclear inflammation, with  epithelial denudation and submucosal congestion. The predominant lung pathology was diffuse  alveolar damage (DAD) at various phases, depending on the clinical course of the fatal cases. Desquamation of pneumocytes and the presence of hyaline membranes, alveolar edema and fibrin  deposits, type II pneumocyte hyperplasia, and alveolar infiltrates, including increased alveolar  macrophages, were seen. Other less common findings included squamous metaplasia, atypical  pneumocytes, intra-alveolar hemorrhage, mucus aspiration, emphysema, microthrombi, and rare  multinucleated cells. Sinus histiocytosis and hemophagocytosis in subcapsular sinuses were frequently observed in adjacent lymph nodes. Respiratory viral and bacterial co-infections  were identified in some cases. Notable pathologic findings in extrapulmonary tissues included  evidence of chronic renal disease, acute renal tubular injury, hepatic steatosis and cirrhosis, and  focal myocardial fibrosis. No myocarditis, encephalitis, or prominent histopathologic changes in  the intestine were seen in limited cases.  

SARS-CoV-2 antigens were observed in upper airway and bronchiolar epithelium,  submucosal gland epithelium, and in type I and type II pneumocytes, alveolar macrophages, and  hyaline membranes in the lung by IHC assay. Upper airways and lung tissues were also positive  by SARS-CoV-2 RT-PCR. Double staining of IHC assay with surfactant showed colocalization of  SARS-CoV-2 antigen with type II pneumocytes, and double staining with CD-163 showed viral  antigen colocalization with macrophages. SARS-CoV-2 antigens were not detected by IHC in  heart, liver, kidney, spleen, or intestine from limited fatal cases in the study. EM examination of  respiratory tissues showed virions with prominent surface projections (spikes) characteristic of the  family Coronaviridae. In the lung, extracellular virions free in the alveolar space were around 105 

nm in diameter, including surface projections. In upper airways, virions were seen extracellularly  among the cilia and within the cytoplasm of respiratory epithelial cells. Intracellular virions in type  II pneumocytes and in cytoplasmic vesicles or phagosomes of alveolar macrophages were, on  average, 75 nm in diameter and lacked prominent spikes. Viral particles were also found associated  with fibrin or hyaline membranes within alveolar spaces. These findings were similar to those  ultrastructural features previously seen in SARS-CoV infection. 

Together, the histopathologic, IHC, and EM findings provide insight into SARS-CoV-2  pathogenesis. IHC testing, including double staining with surfactant, and EM confirmed viral  tropism for pulmonary II pneumocytes. Viral antigen was also seen in respiratory epithelium of  conducting airways (trachea, bronchi, and bronchioles) and occasionally in alveolar macrophages;  infection of these cell types may be key in viral replication and trafficking. In addition to direct  viral effects on tissues, the immune response to viral infection likely plays a major role in  determining clinical outcome, and acute decline in COVID-19 patients has been linked to an  immune-mediated cytokine storm. Preliminary evaluation of immune cell populations in the  respiratory tissues revealed abundant T lymphocytes in the upper airways and lung parenchyma,  with B lymphocytes in smaller numbers and predominating in areas of lymphoid aggregates.  Further investigation into the roles of these cell populations in COVID-19 is needed. SARS-CoV 2 uses the angiotensin-converting enzyme 2 (ACE2) receptor to facilitate viral entry into target  cells. ACE2 is expressed in multiple tissues throughout the body, including type II pneumocytes,  myocardial cells, cholangiocytes, enterocytes, and oral mucosal epithelium. However, SARS CoV-2 antigens were not detected in extrapulmonary tissues besides hilar lymph node, and  pathologic findings in other tissues were attributable to other underlying concurrent conditions.  Some of the underlying conditions in these case-patients (e.g., hypertension, COPD) are associated  with upregulation of ACE2 receptors; possible correlation of these conditions with COVID-19  severity warrants further exploration. In summary, pathologic studies on tissue samples from fatal  COVID-19 cases represent a crucial step forward in understanding the pathogenesis of SARS CoV-2 infection and provides some insights relevant to the development of targeted therapeutic  and preventive measures to combat COVID-19.