IDO1 and head and neck squamous cell carcinoma: The probable effect of immunotherapy on head and neck squamous cell carcinomas (HNSCC) patients was determined using an in vitro 3D microfluidic chip, which loaded different ICIs, Indoleamine 2, 3-dioxygenase 1 (IDO1) inhibitors, and PD-L1 antibodies (Figure 4A) (Al-Samadi et al., 2019).