ABSTRACT
Background: SARS-CoV-2 infection results in acute respiratory distress and multiple organ failure, but the pathogenesis of the disease is poorly understood. Recent data suggest that viral RNA may be found in mast cells, but patients with asthma who have hyperplasia of mucosal mast cells do not experience asthma exacerbations during infection and do not suffer from increased mortality due to COVID-19-associated lung damage. Mast cells from bone marrow do not express the ACE2 receptor used for SARS-CoV-2 entry into human cells, and patients with mast cell activation disorders such as systemic mastocytosis do not show mast cell activation during infection. Because activated mast cells can release potent inflammatory mediators including IL-6 and have been implicated in fibrotic lung damage, the purpose of this study was to investigate the role of mast cells in COVID-19 fatal lung disease. Design: We evaluated 19 autopsies and post-mortem biopsies performed on patients who died of COVID-19 in April and May 2020. Representative sections of lung tissue with typical histological changes of diffuse alveolar damage (DAD;both acute and organizing) were selected. Mast cells were identified by immunohistochemistry for KIT (CD117), tryptase, and chymase. Mean values for mast cells were obtained by counting 3 different areas each with the highest and lowest density of CD117/tryptasepositive cells with 40x magnification. Results: Patients in this cohort were mostly obese with systemic hypertension or diabetes and had elevated CRP and IL-6. All mast cells in COVID-19 lung autopsies were positive for tryptase and chymase, indicating a connective tissue phenotype (Figure 1). While both acute and organizing forms of non-COVID-19-related lung injury showed a 3-5-fold increase in mast cell numbers between low and high-density areas, acute COVID-19 showed a <2-fold increase. In contrast, organizing DAD in COVID-19 showed a 3-fold increase in mast cells between low and high-density areas (Figure 2). Few mast cells were co-localized with SARS-CoV-2 mRNA. Conclusions: During the early phase of DAD in SARS-CoV-2 infection, mast cells are suppressed, potentially due to an interferon surge, which is reversed during the organizing DAD phase of infection. Viral RNA is rarely present in mast cells, consistent with the reported lack of ACE2 receptors in bone marrow mast cells.
ABSTRACT
Angiotensin converting enzyme 2 (ACE2) is the putative functional receptor for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Current literature on the abundance and distribution of ACE2 protein in the human respiratory tract is controversial. We examined the effect of age and lung injury on ACE2 protein expression in rodent and non-human primate (NHP) models. We also examined ACE2 expression in human tissues with and without coronavirus disease 19 (COVID-19). ACE2 expression was detected at very low levels in preterm, but was absent in full-term and adult NHP lung homogenates. This pattern of ACE2 expression contrasted with that of transmembrane protease serine type 2 (TMPRSS2), which was significantly increased in full-term newborn and adult NHP lungs compared to preterm NHP lungs. ACE2 expression was not detected in NHP lungs with cigarette smoke-induced airway disease or bronchopulmonary dysplasia. Murine lungs lacked basal ACE2 immunoreactivity, but responded to hyperoxia, bacterial infection, and allergen exposure with new ACE2 expression in bronchial epithelial cells. In human specimens, robust ACE2 immunoreactivity was detected in ciliated epithelial cells in paranasal sinus specimens, while ACE2 expression was detected only in rare type 2 alveolar epithelial cells in control lungs. In autopsy specimens from patients with COVID-19 pneumonia, ACE2 was detected in rare ciliated epithelial and endothelial cells in the trachea, but not in the lung. There was robust expression of ACE2 expression in F344/N rat nasal mucosa and lung specimens, which authentically recapitulated the ACE2 expression pattern in human paranasal sinus specimens. Thus, ACE2 protein expression demonstrates a significant gradient between upper and lower respiratory tract in humans and is scarce in the lung. This pattern of ACE2 expression supports the notion of sinonasal epithelium being the main entry site for SARS-CoV-2 but raises further questions on the pathogenesis and cellular targets of SARS-CoV-2 in COVID-19 pneumonia.
ABSTRACT
Objective: To report neuropathological findings and quantify SARS-CoV-2 viral burden for 18 consecutive coronavirus disease 2019 (COVID-19) autopsies. Background: COVID-19 is a respiratory disease caused by SARS-CoV-2, a virus known to infect lung epithelial cells, yet data about SARS-CoV-2 neuropathology in human brain autopsies is limited. Design/Methods: Brain tissue specimens were sampled from 18 subjects (10 standard areas), fixed in formalin, and stained with hematoxylin and eosin for histopathological analysis. SARSCoV-2 immunohistochemistry and reverse transcription quantitative polymerase chain reaction (RT-qPCR) were performed on 10 brain sections from 2 subjects and 2 sections (medulla and frontal lobe with olfactory nerve) from the remaining 16 subjects. Results: Median age was 62 years (interquartile range, 53 to 75), and 14 patients (78%) were men. Presenting neurologic symptoms were myalgia (n=3), headache (n=2), and decreased taste (n=1);11 received mechanical ventilation. Acute hypoxic injury was detected in cerebrum, hippocampus, and cerebellum in all patients;rare foci of perivascular lymphocytes (n=2) or focal leptomeningeal inflammation (n=1) were also detected. RT-qPCR showed limited evidence of viral RNA. In 10 unique specimens from two subjects, results were equivocal (viral load <5.0 copies/mm3) in 4 and 5 sections, respectively. In the remaining 16 patients, 3 medulla sections and 3 frontal lobe and olfactory sections were positive (5.0 to 59.4 copies/mm3) while the rest were equivocal or negative. SARS-CoV-2 viral load did not correlate with the interval between the onset of symptoms and death or histopathological findings. Immunohistochemical staining for SARS-CoV-2 nucleocapsid protein was negative in neurons, glia, endothelium, and immune cells. Conclusions: Histopathology of brain specimens revealed hypoxia with limited evidence of direct viral damage, including no viral protein. Concordantly, although SARS-CoV-2 was detected by RT-qPCR in some sections, viral load was low and did not correlate with other pathological features.