Role of mast cells in the pathogenesis of severe lung damage in COVID-19 patients

Abstract

Background: There is still insufficient knowledge with regard to the potential involvement of mast cells (MCs) and their mediators in the pathology of coronavirus disease-2019 (COVID-19). Therefore, our study aimed to investigate the role of MCs, their activation and protease profiles in the pathogenesis of early and late lung damage in COVID-19 patients.

Methods: Formalin-fixed and paraffin embedded lung specimens from 30 patients who died from COVID-19 and 9 controls were used for histological detection of MCs and their proteases (tryptase, chymase) followed by morphometric quantification.

Results: Our results demonstrated increased numbers of MCs at early stage and further augmentation of MCs number during the late stage of alveolar damage in COVID-19 patients, as compared to the control group. Importantly, the percentage of degranulated (activated) MCs was higher during both stages of alveolar lesions in comparison to the controls. While there was no prominent alteration in the profile of tryptase-positive MCs, our data revealed a significant elevation in the number of chymase-positive MCs in the lungs of COVID-19 patients, compared to the controls.

Conclusions: MCs are characterized by dysregulated accumulation and increased activation in the lungs of patients suffering from COVID-19. However, future profound studies are needed for precise analysis of the role of these immune cells in the context of novel coronavirus disease.

Keywords: COVID-19; Cytokine storms; Inflammation; Mast cells; SARS-CoV-2.

Fig. 1

Microscopic changes in the lung tissues of patients who have died from COVID-19 (staining with hematoxylin and eosin). A Diffuse alveolar damage in the acute stage; the walls of the alveoli are filled with dense eosinophilic masses of fibrin, hyaline membranes (arrows); in the lumen of the alveoli, you can observe individual filaments of fibrin. B Diffuse alveolar damage in the acute stage, the lumen of the alveoli is filled with fibrin filaments, single erythrocytes, cells of the respiratory epithelium, and polymorphonuclear leukocytes. C Diffuse alveolar damage in the acute stage, round and oval cells almost completely filling the lumen of the alveoli. D Proliferation of type II alveolocytes leads to the formation of bizarre epithelial structures. Scale bars A, C, D—100 μm, B—50 μm

Fig. 2

Late injury pattern in the lung tissue. Pronounced phenomena of the organization. Fixation: 10% neutral formalin. Technique: staining with hematoxylin and eosin (A, B) and picro Mallory (C) and PCNA was detected using immunolabeling with primary rabbit monoclonal antibodies Anti-PCNA antibody (Epitomics #AC-0087RUO, clone EP91, dilution 1:1000, USA), (D). A A late stage diffuse alveolar damage; the walls of the alveoli are thickened, hemorrhages and desquamation of the alveolar epithelium are observed, the focus of inflammatory infiltration is represented by lymphocytes. B A late stage diffuse alveolar damage; pronounced fibrosis of the alveolar septa, desquamation of the alveolar epithelium. C A late stage diffuse alveolar damage; special staining reveals the connective tissue fibers (blue color), focal hemorrhages, desquamation of the alveolar epithelium. D Expression of PCNA in type II alveolocytes (brown staining). Scale bars A–C—100 μm, D—50 μm

Fig. 3

Mast cells in the lung tissues of COVID-19 patients. A–C Immunohistochemical staining with mouse monoclonal antibodies to tryptase (Abcam, # ab2378, dilution 1:4000) in accordance with standard protocol; D–F Immunohistochemical staining with mouse monoclonal antibodies to chymase (Abcam, #ab233103, dilution 1:1000). The peroxidase label on antibodies was visualized using DAB as a substrate (Vector Laboratories, Burlingame, CA, USA). The slices contrasted with Mayer’s hematoxylin. A, B Early diffuse alveolar injury, mast cell tryptase expression; mast cells are unevenly distributed in the alveolar septa. C Early diffuse alveolar damage, active degranulation of MCs, arrow indicates mast cell at the end of degranulation. D, E Uneven distribution of MCs with chymase expression. F Pronounced degranulation of mast cell chymase (arrows) with alveolar damage in the late stage. Scale bars A—100 μm, others—50 μm