Mast Cells in COVID-19-Associated Lung Damage

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.