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The intestinal mucosal biopsies sampled from 11 patients at three timepoints were processed and embedded in paraffin as described above. A total of 33 5-μm transverse sections were obtained and stained with haematoxylin and eosin (H&E). The sections were imaged and evaluated through microscopy (Danjier, China) with 3DHISTECH’s Slide Converter and CaseViewer 2.4 (scale bar = 1mm).

Mice were euthanized, and their lungs were slowly inflated by intratracheally instilling 1 mL of 4% formaldehyde. The trachea was tightened, and the lungs were fixed in formalin for 48 h at 4 °C and embedded in paraffin. Lung tissue sections (5 mm) were stained with hematoxylin and eosin for histological analysis. Lung histological scores were determined based on a 0–4 point scale of combined scores for alveolar structure, inflammatory cell infiltration, aggregation in the alveolar space and septa, bronchiolitis, and lung edema by a pathologist blind to the study. A score of 0 indicated no damage, 1 indicated mild damage, 2 indicated moderate damage, 3 indicated severe damage, and 4 indicated very severe histological changes; increments of 0.5 were used if the levels of inflammation fell between two integers. Three random areas in each tissue sample were scored, and the mean value was calculated. The histological score was the median of values from three mice. For immunohistochemical studies, sections were stained with rabbit anti-human ACE2 polyclonal antibody (ab15348; Abcam, UK) overnight at 4 °C, then incubated with horseradish peroxidase-conjugated goat anti-rabbit secondary antibody (ab6721; Abcam, UK) for 30 min, and counterstained with hematoxylin. Images were obtained using a Slide Converter (3DHISTECH, Hungary).