Histological Analysis of SARS-CoV-2 Infection in Mice

The tissue of one-year-old female BALB/c mice intranasally infected with 10³ PFU of mouse-adapted SARS-CoV-2 (MA10) or mock-infected [16 (link)] were stored in 10% phosphate-buffered formalin fixative and processed for paraffin embedding. After paraffin embedding, paraffin sections (5 μm in thickness) were used for hematoxylin and eosin (H&E) stains to identify morphological changes in brains. Slides were scanned with a digital slide scanner (Zeiss Axio Scan; Zeiss, White Plains, NY, USA). Representative photomicrographs at 20× magnification were acquired from whole scanned coronal sections using the Aperio Image Scope software (version 12.3.2.8013, Leica, Buffalo Grove, IL, USA). The number of perivascular cuffing sites was counted from all the mice in each group (uninfected control n = 5; SARS-CoV-2 (MA10)-infected n = 5) and was quantified by a pathological score. Whole specimens were examined by using Aperio Image Scope software to establish a histopathological score in each case.