Following intratumoral injection of the anti-PD-L1-loaded hydrogel into melanoma-bearing C57BL/6 mice, the levels of ROS and IDO in the TME can be reduced due to the anti-oxidation effect of LMet residues and IDO inhibiting effect of D-1MT that was gradually released by the degradation of the polypeptide segment (Fig. 11). This evidence concerns the gene IDO1 and melanoma.