The clinical pathology of severe acute respiratory syndrome (SARS): a report from China

Severe acute respiratory syndrome: an update

PURPOSE OF REVIEW

An international outbreak of severe acute respiratory syndrome, a recently recognized syndrome caused by the newly identified severe acute respiratory syndrome-associated coronavirus, began in November 2002 and ended in July 2003. Since then, a large body of research on the syndrome has been published; the most updated developments are summarized here.

RECENT FINDINGS

Recent findings suggest that animal severe acute respiratory syndrome-like coronaviruses may have been transmitted to humans without detection for years before the recent outbreak, and that such transmission may be continuing today. The 2002-2003 outbreak probably originated from similar animal-to-human transmission, but selection and purification of the animal severe acute respiratory syndrome-like virus appears to have occurred, creating the more virulent severe acute respiratory syndrome-associated coronavirus. Recent studies have documented that severe acute respiratory syndrome-associated coronavirus is primarily transmitted via contact and/or respiratory droplets and that the combination of standard, contact, and droplet precautions is generally effective for its control. It has been shown that severe acute respiratory syndrome-associated coronavirus is typically relatively inefficiently transmitted, with the notable exception of transmission during superspreading events. Insights into the pathogenesis of severe acute respiratory syndrome have been made: one study suggests that human leukocyte antigen HLA-B*4601 is a possible risk factor for more severe disease, while another identifies angiotensin-converting enzyme 2 as a cellular receptor for severe acute respiratory syndrome-associated coronavirus. Promising treatments have been identified, including interferons, an anti-spike monoclonal antibody, and fusion inhibitors. In addition, many promising vaccines are currently in development.

SUMMARY

New findings regarding severe acute respiratory syndrome are continuing to be discovered at an unprecedented pace, permitting a better understanding of the disease and enabling better preparation for its possible return.

Exploring the pathogenesis of severe acute respiratory syndrome (SARS): the tissue distribution of the coronavirus (SARS-CoV) and its putative receptor, angiotensin-converting enzyme 2 (ACE2)

Severe acute respiratory syndrome (SARS) is an emerging infectious disease associated with a new coronavirus, SARS-CoV. Pulmonary involvement is the dominant clinical feature but extra-pulmonary manifestations are also common. Factors that account for the wide spectrum of organ system involvement and disease severity are poorly understood and the pathogenesis of SARS-CoV infection remains unclear. Angiotensin converting enzyme 2 (ACE2) has recently been identified as the functional cellular receptor for SARS-CoV. Studies of the tissue and cellular distribution of SARS-CoV, and ACE2 protein expression, reveal new insights into the pathogenesis of this deadly disease. ACE2 is expressed at high level in the primary target cells of SARS-CoV, namely pneumocytes and surface enterocytes of the small intestine. Despite the fact that SARS-CoV can infect the lung and intestine, the tissue responses in these two organs are different. All other tissues and cell types expressing ACE2 may be potential targets of SARS-CoV infection. Remarkably, endothelial cells, which express ACE2 to a high level, have not been shown to be infected by SARS-CoV. There is also evidence that cell types without detectable ACE2 expression may also be infected by the virus. Furthermore, studies in a new human cell culture model have indicated that the presence of ACE2 alone is not sufficient for maintaining viral infection. Therefore, other virus receptors or co-receptors may be required in different tissues. Moreover, the interaction between SARS-CoV and the immunological or lymphoid system remains to be defined. It is clear that we are only at the dawn of our understanding of the pathogenesis of SARS. As our knowledge of the pathogenic mechanisms improves, a more rational approach to therapeutic and vaccine development can be designed in order to combat this new and fatal human disease.

Severe acute respiratory syndrome: prognostic implications of chest radiographic findings in 52 patients

PURPOSE

To retrospectively assess prognostic implications of radiographic findings in severe acute respiratory syndrome (SARS).

MATERIALS AND METHODS

Radiographic findings were reviewed by two radiologists for 52 patients with SARS. On each radiograph, each lung was separated into upper, middle, and lower zones. A four-point scale was used to score extent of SARS-related lesions in each zone; points from all zones were added for a cumulative score. Patient sex, age, comorbidities, duration of developing lesions, lesion score for each radiograph, need for mechanical ventilation, and percentage of lung affected were compared between patients who died (n = 20) and survivors (n = 32). Continuous and categorical variables were analyzed with Mann-Whitney test and Fisher exact or chi(2) test, respectively.

RESULTS

Survival and mortality groups showed no significant differences with respect to patient sex, duration of SARS-related lesions, development of lesion shifting, and acute respiratory distress syndrome. Patients who died were significantly older (mean +/- standard deviation, 56.9 years +/- 17.2 vs 40.4 years +/- 16.6; P =.002) and had higher frequency of comorbid lung illnesses (nine of 20 vs two of 32, P =.001), maximal lesion extent score of 7 or higher (20 of 20 vs five of 32, P <.001), involvement of four or more lung zones (17 of 20 vs four of 32, P <.001), bilateral lung involvement (19 of 20 vs 14 of 32, P <.001), need for mechanical ventilation (18 of 20 vs two of 32, P <.001), and higher percentage of affected areas (41.5% +/- 8.6 vs 16.4% +/- 10.0, P <.001) than those of survivors.

CONCLUSION

On chest radiographs, maximal SARS-related lesion extent score of 7 or higher is a strong predictor of mortality, especially in patients with comorbid lung illnesses and involvement of four or more lung zones.

Organ distribution of severe acute respiratory syndrome (SARS) associated coronavirus (SARS-CoV) in SARS patients: implications for pathogenesis and virus transmission pathways

We previously identified the major pathological changes in the respiratory and immune systems of patients who died of severe acute respiratory syndrome (SARS) but gained little information on the organ distribution of SARS‐associated coronavirus (SARS‐CoV). In the present study, we used a murine monoclonal antibody specific for SARS‐CoV nucleoprotein, and probes specific for a SARS‐CoV RNA polymerase gene fragment, for immunohistochemistry and in situ hybridization, respectively, to detect SARS‐CoV systematically in tissues from patients who died of SARS. SARS‐CoV was found in lung, trachea/bronchus, stomach, small intestine, distal convoluted renal tubule, sweat gland, parathyroid, pituitary, pancreas, adrenal gland, liver and cerebrum, but was not detected in oesophagus, spleen, lymph node, bone marrow, heart, aorta, cerebellum, thyroid, testis, ovary, uterus or muscle. These results suggest that, in addition to the respiratory system, the gastrointestinal tract and other organs with detectable SARS‐CoV may also be targets of SARS‐CoV infection. The pathological changes in these organs may be caused directly by the cytopathic effect mediated by local replication of the SARS‐CoV; or indirectly as a result of systemic responses to respiratory failure or the harmful immune response induced by viral infection. In addition to viral spread through a respiratory route, SARS‐CoV in the intestinal tract, kidney and sweat glands may be excreted via faeces, urine and sweat, thereby leading to virus transmission. This study provides important information for understanding the pathogenesis of SARS‐CoV infection and sheds light on possible virus transmission pathways. This data will be useful for designing new strategies for prevention and treatment of SARS. Copyright © 2004 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

Severe acute respiratory syndrome (SARS) is an acute infectious disease that spreads mainly via the respiratory route. A distinct coronavirus (SARS-CoV) has been identified as the aetiological agent of SARS. Recently, a metallopeptidase named angiotensin-converting enzyme 2 (ACE2) has been identified as the functional receptor for SARS-CoV. Although ACE2 mRNA is known to be present in virtually all organs, its protein expression is largely unknown. Since identifying the possible route of infection has major implications for understanding the pathogenesis and future treatment strategies for SARS, the present study investigated the localization of ACE2 protein in various human organs (oral and nasal mucosa, nasopharynx, lung, stomach, small intestine, colon, skin, lymph nodes, thymus, bone marrow, spleen, liver, kidney, and brain). The most remarkable finding was the surface expression of ACE2 protein on lung alveolar epithelial cells and enterocytes of the small intestine. Furthermore, ACE2 was present in arterial and venous endothelial cells and arterial smooth muscle cells in all organs studied. In conclusion, ACE2 is abundantly present in humans in the epithelia of the lung and small intestine, which might provide possible routes of entry for the SARS-CoV. This epithelial expression, together with the presence of ACE2 in vascular endothelium, also provides a first step in understanding the pathogenesis of the main SARS disease manifestations.

S protein of severe acute respiratory syndrome-associated coronavirus mediates entry into hepatoma cell lines and is targeted by neutralizing antibodies in infected patients

The severe acute respiratory syndrome-associated coronavirus (SARS-CoV) causes severe pneumonia with a fatal outcome in approximately 10% of patients. SARS-CoV is not closely related to other coronaviruses but shares a similar genome organization. Entry of coronaviruses into target cells is mediated by the viral S protein. We functionally analyzed SARS-CoV S using pseudotyped lentiviral particles (pseudotypes). The SARS-CoV S protein was found to be expressed at the cell surface upon transient transfection. Coexpression of SARS-CoV S with human immunodeficiency virus-based reporter constructs yielded viruses that were infectious for a range of cell lines. Most notably, viral pseudotypes harboring SARS-CoV S infected hepatoma cell lines but not T- and B-cell lines. Infection of the hepatoma cell line Huh-7 was also observed with replication-competent SARS-CoV, indicating that hepatocytes might be targeted by SARS-CoV in vivo. Inhibition of vacuolar acidification impaired infection by SARS-CoV S-bearing pseudotypes, indicating that S-mediated entry requires low pH. Finally, infection by SARS-CoV S pseudotypes but not by vesicular stomatitis virus G pseudotypes was efficiently inhibited by a rabbit serum raised against SARS-CoV particles and by sera from SARS patients, demonstrating that SARS-CoV S is a target for neutralizing antibodies and that such antibodies are generated in SARS-CoV-infected patients. Our results show that viral pseudotyping can be employed for the analysis of SARS-CoV S function. Moreover, we provide evidence that SARS-CoV infection might not be limited to lung tissue and can be inhibited by the humoral immune response in infected patients.

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

Severe acute respiratory syndrome (SARS) is an acute infectious disease that spreads mainly via the respiratory route. A distinct coronavirus (SARS-CoV) has been identified as the aetiological agent of SARS. Recently, a metallopeptidase named angiotensin-converting enzyme 2 (ACE2) has been identified as the functional receptor for SARS-CoV. Although ACE2 mRNA is known to be present in virtually all organs, its protein expression is largely unknown. Since identifying the possible route of infection has major implications for understanding the pathogenesis and future treatment strategies for SARS, the present study investigated the localization of ACE2 protein in various human organs (oral and nasal mucosa, nasopharynx, lung, stomach, small intestine, colon, skin, lymph nodes, thymus, bone marrow, spleen, liver, kidney, and brain). The most remarkable finding was the surface expression of ACE2 protein on lung alveolar epithelial cells and enterocytes of the small intestine. Furthermore, ACE2 was present in arterial and venous endothelial cells and arterial smooth muscle cells in all organs studied. In conclusion, ACE2 is abundantly present in humans in the epithelia of the lung and small intestine, which might provide possible routes of entry for the SARS-CoV. This epithelial expression, together with the presence of ACE2 in vascular endothelium, also provides a first step in understanding the pathogenesis of the main SARS disease manifestations.

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

Severe acute respiratory syndrome (SARS) is an acute infectious disease that spreads mainly via the respiratory route. A distinct coronavirus (SARS-CoV) has been identified as the aetiological agent of SARS. Recently, a metallopeptidase named angiotensin-converting enzyme 2 (ACE2) has been identified as the functional receptor for SARS-CoV. Although ACE2 mRNA is known to be present in virtually all organs, its protein expression is largely unknown. Since identifying the possible route of infection has major implications for understanding the pathogenesis and future treatment strategies for SARS, the present study investigated the localization of ACE2 protein in various human organs (oral and nasal mucosa, nasopharynx, lung, stomach, small intestine, colon, skin, lymph nodes, thymus, bone marrow, spleen, liver, kidney, and brain). The most remarkable finding was the surface expression of ACE2 protein on lung alveolar epithelial cells and enterocytes of the small intestine. Furthermore, ACE2 was present in arterial and venous endothelial cells and arterial smooth muscle cells in all organs studied. In conclusion, ACE2 is abundantly present in humans in the epithelia of the lung and small intestine, which might provide possible routes of entry for the SARS-CoV. This epithelial expression, together with the presence of ACE2 in vascular endothelium, also provides a first step in understanding the pathogenesis of the main SARS disease manifestations.

SARS: clinical virology and pathogenesis

Severe acute respiratory syndrome (SARS) is caused by a novel coronavirus, called the SARS coronavirus (SARS‐CoV). Over 95% of well characterized cohorts of SARS have evidence of recent SARS‐CoV infection. The genome of SARS‐CoV has been sequenced and it is not related to any of the previously known human or animal coronaviruses. It is probable that SARS‐CoV was an animal virus that adapted to human‐human transmission in the recent past. The virus can be found in nasopharyngeal aspirate, urine and stools of SARS patients. Second generation reverse transcriptase polymerase chain reaction assays are able to detect SARS‐CoV in nasopharyngeal aspirates of approximately 80% of patients with SARS within the first 3 days of illness. Seroconversion for SARS‐CoV using immunofluorescence on infected cells is an excellent method of confirming the diagnosis, but antibody responses only appear around day 10 of the illness. Within the first 10 days the histological picture is that of acute phase diffuse alveolar damage (DAD) with a mixture of inflammatory infiltrate, oedema and hyaline membrane formation. Desquamation of pneumocytes is prominent and consistent. After 10 days of illness the picture changes to one of organizing DAD with increased fibrosis, squamous metaplasia and multinucleated giant cells. The role of cytokines in the pathogenesis of SARS is still unclear.

Sputum cytology of patients with severe acute respiratory syndrome (SARS)

BACKGROUND

Severe acute respiratory syndrome (SARS) is a newly described form of atypical pneumonia linked to a novel coronavirus.

AIMS

To review the sputum cytology of 15 patients who fulfilled the World Health Organisation clinical criteria for SARS in an attempt to evaluate whether early diagnosis is feasible with routine sputum examination.

METHODS

All sputum samples from patients with SARS from the four major hospitals in Hong Kong were reviewed; abnormalities were sought in the cellular component, including abnormal numbers and morphology of the component cells compared with those from age matched controls taken over the same period one year ago.

RESULTS

Fifteen sputum samples from patients were compared with 25 control samples. In the patients with SARS, loose aggregates of macrophages were seen more frequently in the sputum. These macrophages frequently showed morphological changes, such as cytoplasmic foaminess, vacuole formation, and nuclear changes (including multinucleation and a ground glass appearance) when compared with the control samples.

CONCLUSIONS

The cytological features of SARS are non-specific, but the observation of any of the described features should prompt further investigations, especially in patients with suspicious clinical features.

Pulmonary pathological features in coronavirus associated severe acute respiratory syndrome (SARS)

BACKGROUND

Severe acute respiratory syndrome (SARS) became a worldwide outbreak with a mortality of 9.2%. This new human emergent infectious disease is dominated by severe lower respiratory illness and is aetiologically linked to a new coronavirus (SARS-CoV).

METHODS

Pulmonary pathology and clinical correlates were investigated in seven patients who died of SARS in whom there was a strong epidemiological link. Investigations include a review of clinical features, morphological assessment, histochemical and immunohistochemical stainings, ultrastructural study, and virological investigations in postmortem tissue.

RESULTS

Positive viral culture for coronavirus was detected in most premortem nasopharyngeal aspirate specimens (five of six) and postmortem lung tissues (two of seven). Viral particles, consistent with coronavirus, could be detected in lung pneumocytes in most of the patients. These features suggested that pneumocytes are probably the primary target of infection. The pathological features were dominated by diffuse alveolar damage, with the presence of multinucleated pneumocytes. Fibrogranulation tissue proliferation in small airways and airspaces (bronchiolitis obliterans organising pneumonia-like lesions) in subpleural locations was also seen in some patients.

CONCLUSIONS

Viable SARS-CoV could be isolated from postmortem tissues. Postmortem examination allows tissue to be sampled for virological investigations and ultrastructural examination, and when coupled with the appropriate lung morphological changes, is valuable to confirm the diagnosis of SARS-CoV, particularly in clinically unapparent or suspicious but unconfirmed cases.

Tissue and cellular tropism of the coronavirus associated with severe acute respiratory syndrome: an in-situ hybridization study of fatal cases

Severe acute respiratory syndrome (SARS) is a new human infectious disease with significant morbidity and mortality. The disease has been shown to be associated with a new coronavirus (SARS‐CoV). The clinical and epidemiological aspects of SARS have been described. Moreover, the viral genome of SARS‐CoV has been fully sequenced. However, much of the biological behaviour of the virus is not known and data on the tissue and cellular tropism of SARS‐CoV are limited. In this study, six fatal cases of SARS were investigated for the tissue and cellular tropism of SARS‐CoV using an in‐situ hybridization (ISH) technique. Among all the tissues studied, positive signals were seen in pneumocytes in the lungs and surface enterocytes in the small bowel. Infected pneumocytes were further confirmed by immunofluorescence–fluorescence in‐situ hybridization (FISH) analysis. These results provide important information concerning the tissue tropism of SARS‐CoV, which is distinct from previously identified human coronaviruses, and suggest the possible involvement of novel receptors in this infection. Whereas the lung pathology was dominated by diffuse alveolar damage, the gut was relatively intact. These findings indicated that tissue responses to SARS‐CoV infection are distinct in different organs. Copyright © 2004 John Wiley & Sons, Ltd.

Ultrastructural characterization of SARS coronavirus

Severe acute respiratory syndrome (SARS) was first described during a 2002-2003 global outbreak of severe pneumonia associated with human deaths and person-to-person disease transmission. The etiologic agent was initially identified as a coronavirus by thin-section electron microscopic examination of a virus isolate. Virions were spherical, 78 nm in mean diameter, and composed of a helical nucleocapsid within an envelope with surface projections. We show that infection with the SARS-associated coronavirus resulted in distinct ultrastructural features: double-membrane vesicles, nucleocapsid inclusions, and large granular areas of cytoplasm. These three structures and the coronavirus particles were shown to be positive for viral proteins and RNA by using ultrastructural immunogold and in situ hybridization assays. In addition, ultrastructural examination of a bronchiolar lavage specimen from a SARS patient showed numerous coronavirus-infected cells with features similar to those in infected culture cells. Electron microscopic studies were critical in identifying the etiologic agent of the SARS outbreak and in guiding subsequent laboratory and epidemiologic investigations.

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Severe acute respiratory syndrome: global initiatives for disease diagnosis

We present a retrospective analysis of the available articles on severe acute respiratory syndrome (SARS) published since the outbreak of the disease. SARS is a new infectious disease caused by a novel coronavirus. Originating in Guangdong, Southern China, at the end of 2002, it has spread to regions all over the world, affecting more than 8000 people. With high morbidity and mortality, SARS is an important respiratory disease which may be encountered world-wide. The causative virus was identified by a WHO-led network of laboratories, which identified the genome sequence and developed the first molecular assays for diagnosis. For the respiratory physician, detecting SARS in its earliest stages, identifying pathways of transmission, and implementing preventive and therapeutic strategies are all important. The WHO and the CDC have published helpful definitions of 'suspected' and 'probable' cases. However, the symptoms of the disease may change, and laboratory tests and definitions are still limited. Even in a situation of no new cases of infection, SARS remains a major respiratory health hazard. As with influenza virus outbreaks, new epidemics may arise at the end of each year.