a, Gross pathology and histopathology of lungs from HB-01-infected wild-type mice (3 dpi), mock-treated hACE2 mice (3 dpi) and HB-01-infected hACE2 mice (3 and 5 dpi). Post-mortem examinations showed focal dark-red lesions (red arrow) throughout the dorsal region of the right middle lobe of the lung at 3 dpi. The lesions progressed into multifocally scattered dark-reddish-purple areas and palpable nodules (red arrow) throughout the right lobe of the lung at 5 dpi. Histopathological observations indicated that moderate interstitial pneumonia with thickened alveolar septa (black arrows) and infiltration of lymphocytes (red frames, 1,000× magnification). The swollen and degenerative mononuclear cells (green frames, 1,000× magnification) are scattered within the alveolar cavities at 3 and 5 dpi. b, IHC examination of the lungs of each mouse group. The sequential sections were stained by haematoxylin and eosin (H&E) or IHC. Viral antigens were observed in the mononuclear cells (green arrows), alveolar epithelia (blue arrows) and bronchial epithelial cells that were degenerative and being desquamated (black arrows). The black frames in the top-right corners are a magnification of the region in the respective black box. Scale bars, 100 μm (black), 50 μm (red). Data in a, b are representative of three independent experiments.