IDO1 and head and neck squamous cell carcinoma: Methods: Pretreatment formalin-fixed paraffin-embedded biopsies of 82 anti-PD-1 treated recurrent/metastatic SCCHN patients were studied for key immune resistance markers, namely, PD-L1 (tumor, stroma, PD-1/PD-L1 interaction), Myeloid suppressor cells (CD11b/IDO1/HLA-DR/ARG1) and regulatory T cells (CD4/CD8/CD25/FOXP3/Ki67) by combining multiplexed immunofluorescence (IF) and novel quantitative spatial imaging algorithms via AQUA® (Automated Quantitation Analysis) technology.