Immunohistochemical analysis of autoptic lung tissue from a patient with COVID-19. (A) Immunostaining for angiopoietin-2 (20×): immunohistochemistry shows immunoreactivity in the endothelia, pneumocytes, and macrophages. Scale bar, 6.7 μm. (B) Immunostaining for Tie2 (20×): the same field as in Figure 1. Tie2 showed the same immunoreactivity observed for angiopoietin-2 immunoreaction. (C) Hematoxylin and eosin staining (20×): diffuse alveolar damage in COVID-19, interstitial inflammation, and dilated capillaries in the alveolar wall. The alveolar spaces show fibrin and macrophages. (D-F) The brown DAB signal for angiopoietin-2 shown in panel A was converted into a blue color and that for Tie2 shown in panel B was converted into a red color. The images in panels D and E were merged, resulting in a violet color, where the angiopoietin-2 and Tie2 colocalize. (G-I) Higher magnification (63×) of the inset in panel A highlights angiopoietin2 and tie2 immunoreactivity both in endothelia (red arrowheads) and in pneumocytes (yellow arrows). Scale bar, 1.7 μm. (J-L) The brown DAB signal for angiopoietin-2 shown in panel G was converted into a blue color and that for Tie2 shown in panel H was converted into a red color. The images in panels J and K were merged, resulting in a violet color, where the angiopoietin-2 and Tie2 colocalize.