Immunohistochemical analysis of IDO1 and Ki67

Immunohistochemistry analysis was performed as described previously (Yang et al., 2019 (link); Du et al., 2021 (link)). Samples were fixed in 4% paraformaldehyde (PFA), paraffin-embedded and cut into 4–6 μm slides. All slides were dehydrated in gradient ethanol. Following the antigen retrieval done in 10 mM citrate buffer pH 6.0 (Na3H6H5O7, Beyotime, China), blocking was done using PBS with 10% normal goat serum (Sigma-Aldrich, United States of America) for 40 min. Slides were incubated overnight at 4°C with primary antibodies of IDO1 (ab211017, Abcam) or Ki67 (sc-23900, Santa Cruz). Slides were then incubated with goat anti-rabbit secondary antibody (G-21234, Thermo Fisher Scientific) or goat anti-mouse secondary antibody (G-21040, Thermo Fisher Scientific), stained using 3,3-diaminobenzidine solution and counterstained with hematoxylin.