The lung samples from the mice were immobilized with OCT (Sakura Finetek Europe B.V.) at −80°C for ≥24 h. The frozen samples were cut on a cryostat microtome, and the 7-µm sections were placed on polylysine-coated glass slides. The tissue sections were fixed in 4°C acetone and rehydrated in PBS. The slides were gently washed in PBS for three times and then blocked with 5% goat serum (cat. no. 16210-064; Gibco; Thermo Fisher Scientific, Inc.; 1:500 in PBS) at 37°C to reduce non-specific binding. After 30 min, the sections were washed with PBS and stained with Histone H3 (citrulline R2+R8+R17) antibody (cat. no. ab5103; Abcam; 1:300 diluted) (23 (link)) overnight at 4°C in the dark. Subsequently, the sections were gently washed in PBS and then stained with the fluorescent-conjugated secondary antibody [Goat anti-Rabbit IgG (H+L) Alexa Fluor® 555 conjugate, cat. no. A-21428; Invitrogen; Thermo Fisher Scientific, Inc.] at 37°C for 60 min in the dark. After the unbound fluorescent antibody was removed, the sections were incubated with SYTOX Green diluted in PBS (1:2,000) at 37°C for 15 min. The sections were washed again, and then observed and recorded using confocal microscopy (magnification, ×100; CarlZeiss LSM710; Zeiss AG).